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DESIGN, ACTUATION, AND CONTROL
OF A COMPLEX HAND MECHANISM
by
Jason Dean Potratz
A thesis submitted in partial fulfillment of the requirements for the Master of Science degree
in Mechanical Engineering in the Graduate College of The University of Iowa
July 2005
Thesis Supervisor: Professor Karim Abdel-Malek
Copyright by
JASON DEAN POTRATZ
2005
All Rights Reserved
Graduate College The University of Iowa
Iowa City, Iowa
CERTIFICATE OF APPROVAL
_________________________
MASTER’S THESIS
_____________
This is to certify that the Master’s thesis of
Jason Dean Potratz
has been approved by the Examining Committee for the thesis requirement for the Master of Science degree in Mechanical Engineering at the July 2005 graduation.
Thesis Committee: ___________________________________________________
Karim Abdel-Malek, Thesis Supervisor
___________________________________________________
Jasbir Arora
___________________________________________________
Shaoping Xiao
ii
“Sorry” doesn’t put thumbs on the hand, Marge.
- Homer J. Simpson
iii
ACKNOWLEDGMENTS
I would like to that Professor Karim Abdel-Malek for supervising this research
and serving on my thesis committee. I would like to thank Professors Jasbir Arora and
Shaoping Xiao for also serving on my thesis committee. I would like to thank Dr.
Timothy Marler for thesis editing and expert writing advice. I would like to thank Dr.
Jingzhou Yang for collaborating on this work. Finally I would like to thank my friends
and family for their support.
iv
TABLE OF CONTENTS
LIST OF TABLES v
LIST OF FIGURES vi
CHAPTER
I. INTRODUCTION 1
1.1 Literature review 3 1.2 Motivation and objectives 7 1.3 Overview of thesis 9
II. MECHANICAL DESIGN 10
2.1 Design of the spring deflection joint mechanism 10 2.2 Design of the actuation system 19 2.3 Finger deflection 25 2.4 Aesthetics 29
III. ELECTRICAL DESIGN 31
3.1 Performance requirements 31 3.2 Stepper motors versus servomotors 34 3.3 Stepper motor theory and operation 36 3.4 Stepper motor, drive, and power supply setup 45
IV. MOTION CONTROL 48
4.1 Motion control setup 48 4.2 Programming 55
V. APPLICATIONS AND RESULTS 60
5.1 Applications 60 5.2 Results 65
VI. CONCLUSION 70
6.1 Contributions 70 6.2 Shortcomings and future work 73
REFERENCES 77
v
LIST OF TABLES
Table 2.1 Spring properties 18
Table 2.2 Joint range of motion 27
Table 3.1 Force and torque requirements for each joint 33
vi
LIST OF FIGURES
Figure 2.1 Finger segment with cable and conduit 11
Figure 2.2 Degrees of freedom of fingers 12
Figure 2.3 Completed finger assembly 13
Figure 2.4 Cable routing 14
Figure 2.5 Hand anatomy 15
Figure 2.6 Entire hand assembly 16
Figure 2.7 Actuation system 19
Figure 2.8 Pulley assembly with shaft extension, coupler, pulleys, bearing, and nut 22
Figure 2.9 Fixture assembly with motors, motor drives, and pulley assemblies 24
Figure 2.10 Trajectory and range of motion experimental setup 25
Figure 2.11 Fingertip trajectory and orientation 28
Figure 2.12 Artificial and human hand, palm 30
Figure 2.13 Artificial and human hand, back of hand 30
Figure 3.1 Unipolar stepper motor 38
Figure 3.2 Bipolar stepper motor 39
Figure 3.3 Torque versus position for a single winding and two winding motors 41
Figure 3.4 Torque versus position for two windings using micro stepping 41
Figure 3.5 Current through winding versus time 42
Figure 3.6 Torque versus speed 43
Figure 3.7 Current versus time with and without current limiting 45
Figure 3.8 Manufacturer’s torque rating (oz-in) of motor versus speed (revolutions/second) 46
vii
Figure 4.1 Actuation and control setup 50
Figure 4.2 Wiring diagram 52
Figure 4.3 State activation chart 56
Figure 5.1 Cylindrical grasp without glove 66
Figure 5.2 Spherical grasp with glove, one 66
Figure 5.3 Spherical grasp with glove, two 67
Figure 5.4 Pinch grasp with glove, one 67
Figure 5.5 Pinch grasp with glove, two 68
1
CHAPTER I
INTRODUCTION
An artificial human-like hand based on a new design would fulfill different needs.
There are numerous robotics applications that would benefit from an anthropometrically
shaped devise, a “gripper”, capable of grasping and manipulating objects. Other grippers,
often as simple as a one degree of freedom (DOF) pinching device, are used successfully
in many applications, but a gripper that accurately reproduces human motion and
grasping postures is novel and would be beneficial. For instance, in cases where the
environment prevented or inhibited a human from completing a task, a robotic hand could
be used as part of a system to complete the task. If humanoid robots were ever to become
a reality, then a human-like hand would become a necessity. These robots would interact
in the same environment as humans and therefore manipulate the same objects that would
be designed with the intention of being manipulable by human hands.
Another central need that would be fulfilled by a new artificial human-like hand
would be advanced prosthetics. The ideal replacement for a missing human hand that
accurately mimics a human hand in appearance, dexterity, tactile feedback to the user,
and simplicity of use, remains far from a reality. Currently available myoelectric robotic
prosthetic hands, which typically only have a few DOFs fall far short of this ideal
prosthetic hand in terms of appearance, functionality, and easy of use. Surveys
conducted on user satisfaction of myoelectric hands show that 30-50 percent of users do
not use their hands regularly. The reasons for this lack of use are: low functionality,
unsatisfactory cosmetics partly caused by unnatural motion, and low controllability.
2
From a mechanics perspective, even these relatively simple hands are not natural to
control; users have to exert an enormous amount of concentration for even simple tasks
(Massa et al, 2002).
In this work we developed a five-fingered hand with 15 DOFs that mimics a
human hand in terms of appearance, motion, and grasping ability. One key feature that
makes this hand more advanced than any other multi-fingered hands is the unique
mechanism used for the joints. This newly developed mechanism is based on the loading
of a compression spring in both the axial and transverse directions and offers several
advantages over a hand mechanism based on rigid links and revolute joints. Since the
structure of each finger is primarily made up of compression springs, the majority of the
volume of each finger is occupied by empty space. Thus, the potential for constructing a
very light hand exists if the proper materials are used along with an efficient design. A
lightweight design is a critical aspect of a comfortable, wearable prosthetic devise and
offers obvious advantages for robotics applications, such as smaller actuators and less
power consumption. The unique mechanism also allows the hand to be inherently
compliant, which enables the fingers to naturally conform to the shape of the object the
hand is grasping, making for a more secure grip without the necessity of deliberately
adjusting the defection of each joint to achieve the same shape. The joint mechanisms
are actuated by a series of cables and conduits that allow the motors that actuate the joints
to be located off the hand, also increasing the potential for a very light weight hand.
3
1.1 Literature review
Some of the first mechanical hands were part of body-powered artificial limbs
with a mechanical hook or claw type end effecter. Powered mechanical hands have seen
some significant advances since these hands were considered state of the art. These
advances have occurred primarily in prosthetics applications. An ideal solution for
replacing a human hand is still far from being realized. Possibly one of the reasons this is
true is that the two primary design criteria for prosthetic hands are often conflicting ones.
Because the hand is part of the body, it is unique to each person. Likewise, the ideal
replacement for the human hand would also be unique to each user. On the other hand,
any prototype device has to be a solution that is well accepted by enough users to warrant
developing it into a marketable product (Kyberd et al, 2003). Although there are many
advanced hands under development, none of them incorporates all of the advancements
that research has produced. Many of them implement advanced control schemes and
sensors to achieve a significant level of automation, but many of the most advanced
hands still do not have five powered fingers (see Butterfass et al, 1998, Tura et al, 1998,
Carrozza et al, 2003, Zecca et al, 2003, Kyberd and Chappell, 1994), and Kyberd et al,
2001). None of them has as many fully actuated DOFs as the human hand, an attribute
which of course, an ideal replacement for the human hand would have. These
deficiencies are probably due to compromises made to save weight and to reduce the
complexity of controlling a greater number of DOFs.
Many commercially available hands, such as the Otto Bock SensorHandTM, only
have three digits, usually the thumb, index, and middle fingers, and have two or three
4
DOFs, which are coupled together, and actuated by one motor. Hands such as these are
only capable of being used for a precision or power grasp and have only limited
functional use (Yang et al, 2005).
One of the projects that has been under development the longest is the
Southampton Hand at the University of Manchester Institute of Science and Technology.
The Southampton Hand generically refers to the concepts that have been implemented in
a series of hands, but the original was built in 1969 (Kyberd and Chappell, 1994). The
earliest designs were quite large and relied on very simple electronics. Great potential for
advancement in robotic hands came with the advent of the microprocessor. This was the
key to solving many of the problems that prevented early hands from being very
practical: size, weight, and power consumption. In the current state the Southampton
Hand has two independent degrees of freedom, flexion of the first two fingers and flexion
of the thumb. Combined force and slip sensors in the tips of the fingers along with a
force sensor in the palm are used to aid the user in grasping and holding an object with
minimal user input. The idea of minimal user input is one of the key concepts being
developed as part of the Southampton Hand project, or more specifically that of
hierarchical control. The user only has to provide higher-level commands like “open”,
“hold”, or “squeeze”, and the movements of individual finger joints are coordinated by
microprocessors using feedback from the sensors. This reduces the mental load on the
user, which is a common cause of rejection of a device by the user (Kyberd, 2000).
Another research project that has been under development since the mid 1980’s
and has been a test bed for many other research projects to develop control strategies is
the Utah/MIT Dexterous Hand. This hand has three fingers and a thumb with four
5
degrees of freedom each for a total of sixteen degrees of freedom. The joints are each
powered by opposing extensor and flexor tendons, which are actuated by pneumatic
actuators that are located in a remote actuator pack. There are Hall effect sensors to
provide angular position feedback and the tendons pass over force sensors in the wrist to
provide force feedback. The control system for the Utah/MIT hand “attempts to control
forces between the fingers and an object by recursive refinement of trajectories and by
varying stiffnesses of the joints. Ultimately, controlled compliance of the fingers
combined with a regulated position error produces appropriate forces to maintain the grip
and to move the object within the grasp” (Speeter, 1990). This enables the hand to grasp
an object and manipulate it within its grasp. Although this hand is capable of complex
manipulation and has served for a very effective test bed for developing other
components and control schemes the bulky apparatus required to control it, including the
pneumatic actuators, preclude it from being used as a prosthetic and give it a rather
limited potential for use in practical robotic applications.
A group in Italy is responsible for the development of a three fingered, two-
degree of freedom, underactuated prosthetic hand. The key feature of this hand is the
capability of an adaptive grasp due to the underactuation. Underactuation refers to a
mechanism that has fewer actuators than it has degrees of freedom. The other degrees of
freedom are then “actuated” by elastic elements, usually springs, and mechanical stops.
The basic mechanism to control an underactuated system is a differential gear system.
The mechanism employed in this hand is based on a tendon-like transmission system.
The tendons are actuated by electric motors. The tendons then wrap around a series of
three pulleys, one pulley being located at each of the three finger joints. The tension in
6
the tendon is then transformed into a moment that is proportional to the radius of each
pulley and actuates the corresponding joint in the finger. The three joints in each finger
are also fitted with springs to “actuate” the remaining degrees of freedom. The kinematic
relationship between each link in the fingers can be modified by changing either pulley
size or spring stiffness. This arrangement allows for an adaptive grasp that adjusts the
shape of each finger to conform to the shape of the object being grasped (Massa, et al,
2002). This concept of adaptive grasping has shown much potential for grasping tasks in
prosthetic applications but is not capable of manipulation tasks and is therefore unsuitable
for most robotics applications. It will also have to be abandoned in order to someday
achieve the perfect solution for a human hand replacement, which of course, would not
be capable of just performing grasping tasks but also performing manipulation tasks.
This would, of course, only be possible provided that a user interface advanced enough to
be capable of interpreting the users intentions with minimal conscious effort is
developed.
The TUAT/Karlsruhe Humanoid Hand, which is being developed with both
prosthetics and robotics applications in mind, is a five-fingered hand with twenty-one
degrees of freedom that are driven by one actuator. This hand has a unique link
mechanism to couple all of the joints together to allow for an adaptive grasp. Basically,
all of the fingers begin to move as the actuator begins to move and the grasp begins to
close. As one finger comes into contact with the grasped object that finger stops moving
and the fingers not in contact with the object yet, continue to displace until they too come
into contact with the object. In this way the individual fingers conform to the shape of
the object, contact force is balanced between the fingers, and a truly adaptive grasp is
7
achieved. The fingers even tend to automatically readjust if the object inadvertently
shifts within the grasp. Because of this, the researchers plan not to rely on any sort of
sensors for feedback (Fukaya, et al, 2000). Again, using a hand with an adaptive grasp
offers an advantage in the fact that it can achieve a stable grasp without the use of a
complex control system, but also has a major disadvantage in the fact that this kind of
hand can only ever be used for grasping and is not capable of performing any kind of
manipulation tasks.
1.2 Motivation and objectives
There remains a need to have a much more human-like artificial hand for many
applications. Patients who lack part of an upper limb desire to not only restore the
functionality of that limb but just as importantly any device used should be life-like and
aesthetically pleasing. Currently available prosthetic devices fall short in many ways.
They are hard to control and do not result in a natural motion. Due in part to their limited
number of degrees of freedom, currently available prosthetic hands have limited
practicality. They cannot perform a variety of tasks that would normally be completed
using a human hand. Lastly, they are not aesthetically pleasing. Ideally the device would
perfectly mimic the human hand in strength, dexterity, appearance, and easy of use. It
should also be so natural to operate that the user would not have to put any more
conscious effort into it than a real hand. This idealization is a far from being realized and
there is much room for improvement in models currently available and under
development. Even when considering projects under development, compromises have
8
been made in every prototype. Many still do not have five digits. Some are relatively
simple to operate due to their adaptive grasp designs, but these can only be used in
grasping type applications and cannot be used in any sort of application that requires
complex manipulation. Currently available prosthetic devices lack degrees of freedom
and powered digits. They cannot perform the same manipulation tasks and grasping
postures as a human hand. They are cumbersome and impractical because they require a
substantial amount of concentration from the user to operate and can only be used for a
limited number of basic tasks. There is also much room to improve the aesthetics of
these devises, which is an important part of overall comfort for the user.
There is also a need to have a very human like manipulation ability in many
robotics applications. For instance, in the remote operation of a robotic hand in situations
where the environment either inhibits a human to do the same task unaided or prevents it
all together. One specific example of this is the ADAH Project that involves developing
a robotic hand to assist astronauts during extra vehicular activities in space who are
normally hindered by the need to wear pressurized gloves (Carrozza, et al, 2002).
Another instance would be in the eventual development of a humanoid robot which
would likely be required to interact in the same environments as humans do on a regular
basis, and therefore manipulate the same objects that humans do. The most logical end
effecter for these kinds of tasks would, of course, be a hand that mimics the human hand.
The objectives to be demonstrated as fulfilled by this research are:
1. Design and produce a five-fingered, fifteen degree-of-freedom prototype hand
that implements a novel joint mechanism based on the deflection of a
9
compression spring. Mimic a human hand in size, appearance, and motion.
Design the hand to accommodate a cosmetic glove.
2. Create a bench-top actuation and motion control system to explore, develop, and
demonstrate the capabilities of the hand.
3. Develop software for a user interface to coordinate hand movement.
1.3 Overview of thesis
Chapter 2 covers the mechanical design of the hand, including the following:
design of the spring deflection joint mechanism, design of the actuation system, results of
experimentation, and overall esthetics. Chapter 3 discusses the electrical design,
including: performance requirements, how the stepper motors used in this application
fulfill those requirements and general theory of operation of stepper motors, and will
discuss the other electronic components used. Chapter 4 discusses computer control of
the hand including the motion controller card used and the program developed for control
of the hand and simulation of a user interface scheme under development that uses
myoelectric signals to control a prosthetic device. Chapter 5 discusses use of the hand.
The first section in this chapter covers potential applications of the hand, such as an
improvement over current designs in prosthetic hands or the possibility of its use in
robotics applications. The second section will discuss examples of what the hand is
capable of. Chapter 6 provides a summary of contributions and a discussion of potential
future work.
10
CHAPTER II
MECHANICAL DESIGN
Chapter two will cover aspects of the mechanical design of the hand and the
system that actuates it. More specifically, it covers the novel flexing elements used to
make up the joints of the hand, the actuation system, experimentation on the hand to
determine the kinematic relationship between the orientation of the motor rotor and the
position of the end effecter, and how the components were designed to be esthetically
pleasing. These flexible joint elements are the basis for hand motion and at the same
time make up the majority of the structure of the fingers. The purpose of the actuation
system is to move the joints of the hand in response to computer-generated commands. It
includes five stepper motors, fifteen flexible cable and conduit sets to transmit motion
from the motors to the joints of the hand, and five pulley assemblies.
2.1 Design of the spring deflection joint mechanism
The key feature of this hand mechanism that makes it unique is its joint
mechanism used for the finger joints. Unlike all other mechanical hands, which employ
solid links with revolute joints or another conventional mechanism, this one makes use of
a flexible element made up primarily of a compression spring. Most of the structure for
each finger segment is made up by this spring, Figure 2.1. It is also this flexible element
that allows for the motion for the flexion and extension DOF of the finger, Figure 2.2.
Each finger is made up of a series of three springs connected in series that are held in
11
place by short aluminum finger segment links. Each segment, in the current design, has
one DOF and is actuated by a single cable. Although each joint currently has only one
DOF to replicate the extension and flexion of the human finger, more cables could be
added at appropriate orientations around certain finger segments to add additional DOF’s;
the DOF’s would replicate the abduction and adduction motion of the human finger.
Figure 2.1 depicts a CAD model of a typical joint segment that would be used to
make up a finger. This segment includes a spring and the cable and conduit used to
actuate the segment. Two aluminum finger segment links at either end are used to
connect the spring to the springs in the preceding and following segments and to retain
the ends of the cable and conduit. The finger segment links have a thread-like structure
Figure 2.1 Finger segment with cable and conduit
12
at each end that screw into the first coil of the compression spring, holding the spring
securely.
The joint segment also includes a rectangular rubber block, or stiffener, oriented
parallel to the axial direction of the compression spring, located at on the edge of the
segment opposite of the cable. The finger segment structure offers very little resistance
to lateral deflection but offers a much higher resistance to axial compression, which
ensures that segment will deflect laterally in the direction of the palm (flexion of the
finger) with very little resistance. The finger segment will require much more applied
force to deflect in the direction of the back of the hand (extension of the finger). In other
Figure 2.2 Degrees of freedom of fingers
_________________________________________________________________________ Source: Peña Pitarch, E., Yang, J., Abdel-Malek, K., (2005) “SANTOSTM Hand: A 25 Degree-of-Freedom Model” Proceedings of the Society of Automotive Engineers Digital Human Modeling for Design and Engineering Symposium, June 14-16, Iowa City, Iowa
13
words, it makes the finger segment stiffer when bent in one direction as apposed to the
other.
Figure 2.3 shows one entire finger unit with three finger segments and three
cable-and-conduit sets to actuate all the segments. The tip of the thumb is located
towards the right of the Figure 2.3. At the left of the Figure 2.3 is the thumb base, which
slips into the aluminum hand body fixture. The other four fingers are constructed in a
similar fashion.
The cables and conduits run from the finger segment that they actuate, through the
empty space in the center of the preceding elements, continue through the hollow finger
bases, and exit though the bottom of the hand body fixture, Figure 2.4 and Figure 2.5.
This arrangement allows for a single segment to be actuated without affecting the other
two segments located in that finger. Figure 2.4 shows how a set of cables and conduits
would be routed through a series of finger segments.
Figure 2.3 Completed finger assembly
14
The hand body is also constructed of aluminum and designed to resemble the
shape of a hand. The fingers are arranged in a configuration that resembles human
anthropometry and is suitable for grasping.
Fifteen finger segments similar to one shown in Figure 2.1 were developed to
comprise an entire five-fingered hand with three segments in each of the four fingers and
the thumb to make a total of 15 DOF’s. The thumb is capable of bending in extension
and flexion at the carpometacarpal (CMC), the metacarpophangeal (MCP), and the
interphangeal (IP) joints. Each finger is capable of bending in flexion and extension at
Figure 2.4 Cable routing
15
the metacarpophalangeal (MCP), the proximal interphalangeal (PIP), and the distal
interphalangeal (DIP) joints, Figure 2.5.
Each finger can be added to and removed from the rest of the hand as one unit,
which allows the entire hand to be assembled inside a cosmetic glove, which will be
discussed more in section 2.4. Figure 2.6 shows the complete hand with three DOF’s for
each of the five fingers, giving it 15 DOF’s.
Figure 2.5 Hand anatomy ______________________________________________________
Source: Peña Pitarch, E., Yang, J., Abdel-Malek, K., (2005) “SANTOSTM Hand: A 25 Degree-of-Freedom Model” Proceedings of the Society of Automotive Engineers Digital Human Modeling for Design and Engineering Symposium, June 14-16, Iowa City, Iowa
16
One of the most important objectives when considering the design of the entire
hand as a system is the ability to grip an object. The inclusion of a rubber stiffener in the
finger segment is the key to making it operate in a way that makes gripping possible.
Although the performance of the flexing element would be greatly reduced, it would be
possible to use it without the stiffener. Without the stiffener, the stiffness of the
compression spring itself represents an important tradeoff in the design of the finger
segment. The maximum normal force that can be generated between a finger and an
object grasped between the fingers and the palm or between the fingers and the thumb is
proportional to the stiffness of the springs in that finger. When the hand closes to grasp
the object, the cables shorten and pull the upper link towards the lower link, causing the
finger segments to bend and curl towards the palm. After a finger first comes into
contact with an object and more tension is placed on the cable, normal force is developed
Figure 2.6 Entire hand assembly
17
between the finger and the surface of the object. Given that the grasped object exerts a
force on the finger, there is a corresponding equal and opposite force for the force that the
finger exerts on the grasped object. This force will cause the flexing elements to
compress on the backside, opposite of the cable, causing the flexing element to
simultaneously straighten and shorten. This effect produces undesirable and unrealistic
motion and grasping postures and does not allow for sufficient force to be placed on the
grasped object. Increasing the stiffness of the springs will increase the maximum amount
of normal force that the finger is able to produce and also the maximum grasping force of
the entire hand. Greater tension for actuating the hand would be required and therefore
necessitate the use of large and more powerful motors for actuation. Using a rubber
stiffener to increase the stiffness of the compression spring in only one direction is an
innovative solution that simultaneously takes advantage of using both a stiffer spring
which can create greater grasping force and a softer spring which is easier to actuate.
This is because the material in between each link of the spring makes it much harder for
that side of the spring to compress. This design still allows the fingers to react naturally
to applied external forces.
More than one set of springs was tested on the hand. The first set proved to be
too stiff and would require using larger motors. A second set of springs that were softer
was tested. This set proved to be much more practical. They are sufficiently stiff enough
to provide structure for the finger and to produce normal force for gripping, at the same
time soft enough to require a reasonable size motor for actuation. Their dimensions and
spring constants are included in Table 2.1. The spring constant, k, is calculated from the
dimensions and the number of active coils, or the number of coils which separate the two
18
finger segment links, not the entire number of coils in the compression springs. The
spring constants are calculated according to the equation 4
38
Gdk
nD= , where G is the shear
modulus of steel which is approximately 611.6 10× pounds per square inch. In Table 2.1,
the individual joints are labeled with an i – j convention where i refers to the finger and j
refers to the joint. The thumb corresponds to i = 1 and the index finger to i = 2 and so
forth. The joint closest to the fingertip of each finger corresponds to j = 1, the middle
joint to j = 2, and the joint closest to the hand corresponds to j = 3.
Table 2.1 Spring properties
19
2.2 Design of the actuation system
The design of the hand mechanism calls for the joints to be actuated by cable and
conduit sets. Since the hand was designed to be incorporated into either a robotic or
prosthetic system, the most likely method of doing the work required for actuation is
using electric motors. The actuation system, Figure 2.7, links the motors to the cable
conduit sets and the individual motors to each other, in such a way that individual cables
can be displaced while holding the conduit in place.
Once the hand mechanism itself was developed, an actuation system was
necessary to explore its capabilities and refine its design. For the research and
Figure 2.7 Actuation system
20
development stage of the hand prototype, a bench-top actuation system serves well for
experimentation and further refinement. This bench-top system is designed to be simple,
cost effective, and configurable. It is not optimized to fit in a small, ergonomic package
similar to what would be necessary for integration into a robotics or prosthetics
application.
Included in the actuation system are the cables that actuate each finger segment
and the conduit that houses and routes the cables. As tension is applied to the free end of
the cable, an equal and opposite force must be applied to the same end of the
corresponding conduit to prevent movement of the entire hand. This force is transmitted
down the length of the conduit to the other end of the conduit and to the finger segment
where it is constrained. The equal and opposite force restricts the tension in a cable to
actuating only the intended finger segment.
The actuation system also contains five stepper motors which each turn three
pulleys; each pulley actuates one cable each. The rest of the actuation system consists of
assorted hardware to connect three pulleys to each motor and to connect one cable to
each pulley (Figure 2.8), and a fixture to hold all of the motors and pulleys (Figure 2.9).
Finally, 15 cable adjusters individually adjust the pretension in each cable in order for all
three cables in one finger to begin to displace all the finger segments simultaneously.
The cable adjusters screw in or out of the fixture, in effect lengthening or shortening the
conduit so the that the tension in a each cable can be adjusted with respect to the tension
of the other two cables for that finger. In this way the finger segments for a given finger
can all begin to move at a certain orientation of the stepper motor and can reach full
displacement at the same time at a second orientation of the motor. The cables are 3/64
21
of an inch diameter steel “wire rope” or “aircraft cable”. At first 1/16 of an inch diameter
cable was used but it was determined that it was too stiff to wrap around such small
diameter pulleys. Switching to 3/64 diameter cable solves this problem well. Currently
enough cable and conduit was included to locate the hand approximately three feet away
from the rest of the actuation system.
To reduce cost and complexity for research and development stage, the joints are
coupled to reduce the DOF’s of the actuation system for the hand. Joint coupling also
reduces the number of motors that would be potentially required. This is a big advantage
when considering that the hand mechanism is meant to be part of a robotic or prosthetic
system. The coupling is achieved by actuating all three DOF’s for each finger with one
stepper motor that rotates three pulleys. The circumferences of the pulleys were varied
with respect to each other according to the maximum displacement of the cable necessary
to draw the corresponding finger segment to its maximum defection. This way, the three
finger segments in a given finger would be at the same percentage of full deflection at all
times. All three segments would begin to move simultaneously. They would also reach
their maximum displacement simultaneously at one orientation of the motor. Coupling
the joints in this manner provides a motion that is similar to natural human flexion and
extension of the fingers because human DIP and PIP joints move in a coupled manner as
well.
The pulley system assembly is shown in Figure 2.8. This system incorporates
timing pulleys to wrap up the cables. Timing are simple, cogged pulleys that are
typically available in various sizes, but are generally relatively small and are designed to
be used with cogged belts that transfer power between two or more axes without slip.
22
This application is much different than the intended one but the pulleys perform well and
were a convenient choice. This is because they were readily available in the correct
sizes, were economical, and required little modification to work with the other
components.
To turn three pulleys with each motor, it was necessary to create an assembly to
lengthen the motor shafts. This was done by producing a shaft that was the same
diameter as the motor shaft, had a flat notch for a setscrew and was threaded at the other
end. This was connected to the shaft of the stepper motor by use of a coupler with two
setscrews to engage both the motor shaft and the shaft extension. This allowed room for
three pulleys, a bearing, and sufficient threads still exposed to hold a nut on top while
Figure 2.8 Pulley assembly with shaft extension, coupler, pulleys, bearing, and nut
23
leaving room in between the pulleys to pinch three cables. The pulleys are kept from
slipping around the surface of the shaft by use of a setscrew that sets on to same notch as
the coupler. The cables are gripped by the pulleys by means of a notch cut into the hubs
of the pulleys. A cable retainer, which is essentially a ring, then fits around the hub to
keep the cable from slipping out of the notch. A space is then left in between each pulley
and the pulley above it or in the case of the upper most pulley the space is left in between
the pulley and the bearing above it. This space allows the cable to be wrapped around the
shaft after it is run through the notch so that when the nut on top is tightened all three
cables are pinched and the cables are locked firmly to the pulleys.
A bench top fixture was constructed to hold several components including the
stepper motors, pulley assemblies and cable adjusters and performs several other
functions necessary to actuate the hand. The fixture also houses the stepper motor drives
and two switches all of that will be discussed in more detail in chapter three. See Figure
2.9. This fixture may not be a practical enough means of housing all the components to
be suitable for robotics or prosthetics applications, but serves its purpose well for the
developmental stage.
One of the other functions fulfilled by the fixture is to give the conduits
something to push against as the cables are tensioned allowing the actuation of the fingers
to occur. Recall, that as a finger is actuated the conduits push on one finger segment link
as the cables pull on another. The fixture also has a plate that can be slid horizontally and
then held in place with two bolts, adjusting its distance from the five stepper motors.
This plate provides support to the free end of the motor shafts by providing a surface to
support the normal forces that are transmitted through the bearings on the ends of the
24
shafts as the cables are tensioned. Transmitting forces through the bearing to the motor
fixture lessens the bending moment being applied to the end of the motor shaft by
changing the loading conditions from those similar to a cantilever beam to those of a
simply supported beam.
Figure 2.9 Fixture assembly with motors, motor drives, and pulley assemblies
25
2.3 Finger deflection
The change in shape of the fingers as they are deflected is important to understand
when planning motions that will be executed by a control system. The deformed shape
and fingertip trajectory determine where the finger will come into contact with an object
as it is grasped. Fingertip orientation as the finger is flexed towards full flexion is also
important to understand. This is so that the points on the fingers that contact an object as
it is grasped can be predicted. A series of experiments to measure these characteristics is
presented below (Figure 2.10).
Figure 2.10 Trajectory and range of motion experimental setup
26
Each finger was measured individually. They were removed from the rest of the
hand and fitted with an end effecter (needle) at the fingertip. The base of the finger was
then fixed in a clamp. The initial position and the orientation of the end effecter were
recorded on paper behind the finger. The finger was initially positioned at full extension
and then was stepped in small increments, 20 steps apart (see Chapter 3), until full
flexion was reached. Measurements were taken to determine the position of the end
effecter in the flexion / extension plane and orientation of the end effecter in the same
plane (Figure 2.11). The joint angles of each segment were measured at full extension
and full flexion to determine range of motion (Table 2.2). Joint angles are defined as the
angle of deflection from one end of the spring to the other. They are determined by the
orientation of one finger segment link with respect to the previous one.
These experiments point out that range of motion of the fingers is one area where
there is room for improvement in the design of the hand. It is suggested that the total
range of motion for the fingers of the human hand is 215 degrees for the thumb and
between 270 and 300 degrees for the index, middle, ring, and little fingers (Peña Pitarch
et al, 2005). Because of this deficiency, the hand may have trouble grasping small
objects in certain grasp types, for instance grabbing a pencil in a cylindrical grasp.
Results from these experiments, specifically the maximum range of motion, were
used to develop the motion control program discussed in Chapter 4. The maximum
rotation of the motor must be observed so that the motors do not try to move the fingers
past their maximum deflection point.
These experiments point out that range of motion of the fingers is one area where
there is room for improvement in the design of the hand. It is suggested that the total
27
range of motion for the fingers of the human hand is 215 degrees for the thumb and
between 270 and 300 degrees for the index, middle, ring, and little fingers (Peña Pitarch
et al, 2005). Because of this deficiency, the hand may have trouble grasping small
objects in certain grasp types, for instance grabbing a pencil in a cylindrical grasp.
Results from these experiments, specifically the maximum range of motion, were
used to develop the motion control program discussed in Chapter 4. The maximum
rotation of the motor must be observed so that the motors do not try to move the fingers
past their maximum deflection point.
Table 2.2 Joint range of motion
Finger JointMinimum joint angle (degrees)
Maximum joint angle (degrees)
Range of motion
(degrees)
Maximum roation of motor
(degrees)
1-1, interphangeal 22 49 271-2, metacarpophalangeal 12 44 321-3, carpometacarpal 18 49 31finger range of motion2-1, distal interphalangeal 12 83 712-2, proximal interphalangeal 12 62 502-3, metacarpophalangeal 9 66 57finger range of motion3-1, distal interphalangeal 4 76 723-2, proximal interphalangeal 12 72 603-3, metacarpophalangeal 10 84 74finger range of motion4-1, distal interphalangeal 13 75 624-2, proximal interphalangeal 19 87 684-3, metacarpophalangeal 0 56 56finger range of motion5-1, distal interphalangeal 17 76 595-2, proximal interphalangeal 17 77 605-3, metacarpophalangeal 15 63 48finger range of motion 167
90
178
206
186
Little
Thumb
Index
Middle
Ring
130
200
170
170
200
28
Thumb
0
10
20
30
40
50
60
70
0 20 40 60 80 100
X Position (mm)
Y P
ositi
on (
mm
)
Index
0
10
20
30
40
50
60
70
0 20 40 60 80 100
X Position (mm)
Y P
ositi
on (
mm
)
Middle
0
10
20
30
40
50
60
70
0 20 40 60 80 100
X Position (mm)
Y P
ositi
on (
mm
)
Ring
0
10
20
30
40
50
60
70
0 20 40 60 80 100
X Position (mm)
Y P
ositi
on (
mm
)
Little
0
10
20
30
40
50
60
70
0 20 40 60 80 100
X Position (mm)
Y P
ositi
on (
mm
)
Origin and fingertip location and orientation on finger.
Figure 2.11 Fingertip trajectory and orientation
29
2.4 Aesthetics
The need to have a realistic, esthetically pleasing device is especially necessary in
the field of prosthetics. For many prosthesis users, the aesthetics of a device is just as
important as the functionality. The ideal replacement for the human hand is one that
perfectly mimics not only the function of the human hand but also the appearance
including size, shape, weight, texture, color, and movement. The user of a prosthetic
device has to be very comfortable using it and the ideal comfort level naturally includes
appearance among other things (Kyberd et al, 2003). To help achieve this ideal, this hand
was intentionally designed for use with a cosmetic glove covering it. The glove resembles
a human hand and forearm, and this dictated the dimensions of the majority of the
components used in the construction of the hand. The diameters of the finger segment
links and of the springs were chosen so that they would closely match the inner
dimensions of the cosmetic glove. The lengths of springs were carefully chosen so that
middle of the spring lengthwise, and therefore the middle of the curve of the deflected
finger segment, would coincide with the location of the knuckles of the cosmetic glove.
In its current state, improvements in the hand’s structure could still be made to achieve a
closer resemblance to the shape of a human hand, especially in the case of the hand body.
To achieve the ideal shape for an entire hand assembly would necessitate a close focus on
human hand anthropometry when designing all the components.
Results for the developmental stage of this more realistic mechanical hand have
been satisfactory. Figure 2.12 and Figure 2.13 compare, respectively, the palm side and
the back of the hand of the mechanical hand with a human hand. Each photo shows the
30
human hand on the right, the artificial hand on the left. Although there is some difference
in color between the human and cosmetic glove, which is available in many skin tones,
the shape of artificial hand does mimic the human hand well, especially considering that
improvements could be made with a more anthropometrically driven design.
Figure 2.12 Artificial and human hand, palm
Figure 2.13 Artificial and human hand, back of hand.
31
CHAPTER III
ELECTRICAL DESIGN
This chapter discusses the design of the electrical components of the hand and the
requirements the design must satisfy. The requirements considered include the amount of
tension that must be produced in the cables to fully displace each finger and produce a
sufficient gripping force, the length of cable displacement necessary, and the resolution
of movement necessary to produce precise finger movements. Component selection is
critical to satisfying these requirements and defines the specific challenges that must be
overcome to achieve good performance of the hand. Small DC motors are the best choice
to actuate the hand. This decision leaves two options, servomotors or stepper motors.
The advantages and disadvantages of each as well as the reasoning for choosing stepper
motors to actuate the hand are discussed below. The operating principles and strategies
of stepper motors, five of which form the basis of the electrical system, are also
discussed. Understanding how a stepper motor works is background information but it is
also essential to selecting the proper components, which ultimately determines how the
hand will function. Finally, the entire electrical setup that actuates the hand is detailed.
The actual performance of the entire system is also discussed.
3.1 Performance requirements
The motors that will provide the best solution to actuate the hand, that is provide
the best torque, speed, acceleration, and positional accuracy characteristics, depend on
32
what kind of operating conditions they will experience. Therefore, when selecting
motors and other required electronic components to power the hand, it is necessary to
consider everything that will be required of them when displacing the fingers or grasping
an object. The major requirements considered include the amount of torque required to
fully deflect the fingers and exert gripping force on an object, the speed at which a finger
should move in order to mimic human motion, and positional accuracy of the motor
needed for fine manipulation.
The first requirement to consider is the amount of force it takes to displace the
fingers to full flexion and how much torque is required of the motors to exert this much
force on the cables. This is the minimum force required to produce the closed hand
posture. Additional force will need to be generated in order to produce significant
normal force on the surface of a grasped object. It is also just as important to consider
the pulley radii that will be used with the motors, as this will give a direct relationship
between the force necessary to pull the cables and the torque required to produce this
force. The smaller the radius used for each pulley the less torque required to generate the
same amount of tension in the cable and therefore the smaller motor required. On the
other hand, there is a limit to how small a pulley can be. The outside diameter, the
diameter at which the cable will contact the pulley, must be at least some minimum
amount greater than the bore diameter, or the diameter of the hole in the center of the
pulley that will fit the motor shaft. Therefore, the torque rating of the motor cannot be
considered independently, but must be considered at the same time as size of the pulley
and the diameter of the motor shaft. Table 3.1 lists the force necessary to displace each
joint to full displacement and the total force necessary to displace each finger. Also listed
33
is the required torque for the pulley sizes used. The joints are labeled according to the
same i, j notation as described in Chapter 2.
The sizes of the pulley used, more specifically the radii are based on several
considerations. First is the smallest pulley outside diameter possible to use with the shaft
diameter of the motor. Also it is necessary to have a suitable relationship between the
three pulley radii for one given finger so that the three finger segments will reach full
displacement at the same time. The third consideration is the standard size pulleys
readily available. The pulley radii for each joint are also listed in Table 3.1.
Another consideration to take into account is the maximum and minimum change
in orientation that the motor will be expected to make in one movement. Since the
average displacement required for each joint is less the one inch of cable (listed in Table
3.1) and the displacement for any individual finger joint is less than the circumference of
Table 3.1 Force and torque requirements for each joint
34
the corresponding pulley, no motor will be required to rotate the output shaft of the
gearbox an entire 360° . Therefore the movements that require the highest angular
velocities are very short in duration. Because of this, a motor that can accelerate and
decelerate relatively quickly is required for quick movements, whereas, grasping and
manipulation many times requires very fine movements. Therefore a motor that can be
precisely positioned is also necessary. If no other dedicated device, such as an
electrically triggered brake, is used to maintain cable tension in static loading situations,
an additional requirement of the actuation system is that the motors provide a means to
hold the position of the hand stably.
3.2 Stepper motors versus servomotors
Electric motors are the obvious choice to actuate the fingers of the hand when
considering either robotics or prosthetics applications. No other actuation method, such
as pneumatic or hydraulic, is as reliable or as easy to implement. Production of a
marketable product for either a prosthetics or robotics application will likely require a
device that can run on battery power. This naturally leads to the choice of using motors
that run on direct current (DC) rather than motors that are powered by alternating current
(AC) since power from the battery is already DC. Considering the requirement for a DC
motor, and torque and speed requirements there are two basic motor categories that could
actuate the hand, servomotors and stepper motors. This is a critical decision as it has a
large impact on overall performance and greatly dictates many of the other system
components to be used.
35
Servomotors in a size range suitable for this application are typically DC brush or
brushless motors. They use a position sensor, usually potentiometer based with an
analogue signal or an optical encoder, to provide position feedback for closed loop
control. Using an electronic controller, accurate positioning can be achieved similar to a
stepper motor, but a feedback signal is a necessity. Servomotors generally have a higher
rpm range than stepper motors and produce more torque at higher velocities than at lower
ones.
Stepper motors are electric DC motors with no commutators. In most other
electric motors the commutators switch the electromagnetic poles in the motor so that the
rotor is constantly made to turn. In the case of stepper motors all the commutation is
handled by external circuitry generally referred to as a stepper motor drive. The drive
will energize the correct magnetic pole or poles to advance the motor to the next “step”,
hence the term “stepper motor”. The windings are then energized in the proper sequence
and rate to make the motor rotate the desired direction and at the desired rate.
A stepper motor with the correct power supply and drive can be rotated at
relatively high speeds, although generally a stepper motor cannot easily be made to rotate
as fast as a servomotor. But stepper motors can accelerate or decelerate at relatively high
rates and can be turned to a precise position and then “hold” that position. One
advantage that stepper motors have is that in many applications they can be controlled
with open loop control, which doesn’t require any feedback from an encoder. The
encoder reports the position of the motor to the controller so that errors can be corrected.
As long as the stepper motor doesn’t “slip” or fail to advance to the next step when the
drive commands it to, then the stepper motor can be controlled in an open loop. This is
36
possible in applications where the loads and accelerations placed on the motor do not
exceed its maximum torque (Jones, 2004).
3.3 Stepper motor theory and operation
This section discusses the three basic types of stepper motors, permanent magnet,
variable reluctance, and hybrid, which is a cross between the previous two, advantages
and disadvantages of each, strategies for achieving the greatest performance from a
stepper motor, and the physics behind these concepts.
The basic difference has to do with the construction of their rotors and the
arrangement of their windings. The permanent magnet stepping motor relies on the
electromagnetic interaction of an energized stator winding and a permanent magnet rotor.
One thing that differs between a permanent magnet or a hybrid stepping motor and a
variable reluctance stepping motor is that fact that the permanent magnet and variable
reluctance motors maintain a fraction of their holding torque even the windings are not
energized. This is because the poles of the permanent magnet in either one of these
motors are attracted to the stator poles. The torque present when no windings are
energized is often referred to as detent torque. A permanent magnet motor can be
operated constructed as either a unipolar or a bipolar motor, both of which will be
explained shortly (Jones, 2004).
The variable reluctance stepping motor has no permanent magnet rotor, and
therefore relies on the “principle of minimizing the reluctance along the path of the
applied magnetic field”. The stator has a magnetic core and is constructed with a stack of
37
steel laminations and the rotor, which has teeth and slots, is made of soft steel that is not
magnetized (National Instruments, 2005). Because the variable reluctance stepper motor
does not have a magnet rotor it has no detent torque and will rotate freely when no
current is supplied to the windings.
Hybrid motors combine several characteristics of both variable reluctance motors
and permanent magnet motors. They combine a permanent magnet rotor and multi
toothed stator poles made of soft steel and are very similar to a permanent magnet motor
in terms of control (Jones, 2004). The stator of a hybrid motor is very similar to a
variable reluctance motor except for one aspect. In a variable reluctance motor, only one
coil of each phase is wound around each pole. Usually in hybrid motors, which have
what is known as a “bifilar” connection, there are two coils wound around each pole –
one from two different phases. Torque is then created by the magnetic interaction of the
permanent magnet rotor and the stator (National Instruments, 2005).
Permanent magnet and hybrid stepper motors can either be unipolar or bipolar.
The difference is defined by the arrangement of their windings and determines the kind of
drive that must be used to power them. A unipolar motor has two windings or phases.
The windings have a center tap which is usually connected to the positive supply. Then
one or the other of the other two ends is grounded (Figure 3.1). This reverses the current
flow though the winding and the direction of the magnetic field produced by the winding.
Unipolar motors can be operated by energizing either half of one winding at a time, or
half of both windings at a time (Jones, 2004). The advantage with a unipolar motor is that
it requires less sophisticated drive circuitry. The output current from the drive is always
in one direction. The disadvantage with comparison to a bipolar motor is that at any one
38
time, at best only fifty percent of the total windings in the motor are being used. This
results in less torque for a given size motor with comparison to a bipolar motor (Lin
Engineering, 2005).
A bipolar motor is constructed essentially the same as a unipolar motor except
that the windings are simpler. There are no center taps (Figure 3.2). The simpler
winding circuitry leads to a simpler overall motor design. Although the motor itself is
Figure 3.1 Unipolar stepper motor ____________________________________________________________________
Jones, D., “Control of Stepping Motors”, (2004), <http://www.cs.uiowa.edu/~jones/step/> (19 November 2004)
39
simpler, it requires more advanced control circuitry because it is necessary to reverse the
direction of the current through the windings (Jones, 2004). The advantage a bipolar
motor has is that one hundred percent of its windings can be used at any given time. Also
more torque is produced. The amps per phase equals 1 2 of the amps per coil when
two coils are connected in series, as in a bipolar motor. If N represents the number of
turns per coil then N I× is proportional to the torque in a unipolar motor, then
( )2 1 2N I , or 2NI , is proportional to the torque in a bipolar motor, approximately
40% more then a unipolar motor (Lin Engineering, 2005).
Figure 3.2 Bipolar stepper motor ______________________________________________________________
Source: Jones, D., “Control of Stepping Motors”, (2004), <http://www.cs.uiowa.edu/~jones/step/> (19 November 2004)
40
Although a typical hybrid stepping motors, due to the design of their rotors and
windings, have 200 steps per revolution, drive electronics that can utilize more advanced
electromagnetic control strategies can achieve much finer angular resolution. A motor
with 200 steps per revolution is said to have a step angle of 360°/200 or 1.8°. When a
drives divide a step in two, this is commonly referred to half stepping. Anything finer
than half stepping is commonly referred to as micro stepping.
As long as none of the magnetic circuit is saturated, powering both motor
windings simultaneously will produce a torque versus position curve that is the sum of
the torque versus position curves for the two motor windings taken independently (Figure
3.3). The two curves will be S radians out of phase for a two-winding permanent magnet
or hybrid motor, where S is the step angle. If the currents in the two windings are equal,
the peaks and valleys of the sum will be S/2 radians from the peaks of the single winding
curves. This is the key to half-stepping. The two-winding holding torque is the maximum
of the combined torque curve when both windings are carrying their maximum current.
For common two-winding permanent magnet or hybrid stepping motors, the two-winding
holding torque is, again, 2 times the single winding holding torque. This assumes that
the magnetic circuit is not saturated and that the torque versus position curves are ideal
sinusoids.
Micro stepping is an extension of this idea that uses two different current levels
through the two motor windings as in Figure 3.4. Common stepper motor drives divide
steps in to half steps, on quarter steps, and one eight steps. Some stepper motor drives
are capable of even finer precisions.
41
For a two-winding variable reluctance or permanent magnet motor, assuming
nonsaturating magnetic circuits, and making the same assumptions as before, the
following two formulas give the key characteristics of the composite torque curve:
2 2h a b= + and ( )1tan2
Sx b a
π−= . Where: a equals the torque applied by the
winding with equilibrium at 0 radians, b equals the torque applied by the winding with
equilibrium at S radians, h equals holding torque of composite curve, x equals
Figure 3.3 Torque verses position for a single winding and two winding motors ________________________________________________________________
Source: Jones, D., “Control of Stepping Motors”, (2004), <http://www.cs.uiowa.edu/~jones/step/> (19 November 2004)
Figure 3.4 Torque versus position for two windings using micro stepping __________________________________________________________
Source: Jones, D., “Control of Stepping Motors”, (2004), <http://www.cs.uiowa.edu/~jones/step/> (19 November 2004)
42
equilibrium position, in radians, and S again equals step angle, in radians. With no
saturation, the torques a and b are directly proportional to the currents through the
corresponding windings. The two currents are then varied and it is possible to achieve
1/4, 1/8, or smaller step sizes (Jones, 2004).
An important consideration in operating stepper motors at high-speed is the effect
of the inductance of the motor windings. Rise and fall time of the current through the
windings is a factor of inductance of the motor winding. Ideally the current versus time
would be a square-wave but the inductance of the winding causes it to be an exponential,
as illustrated in Figure 3.5
The exact characteristics of the curve of current through each winding versus time
depend as much on the drive circuitry as they do on the motor. These time constants of
these exponentials can easily differ. The rise time is a condition of the drive voltage and
drive circuitry, while the fall time is determined by the circuitry used to dissipate the
stored energy in the motor winding.
Figure 3.5 Current through winding versus time
__________________________________________________ Source: Jones, D., “Control of Stepping Motors”, (2004),
<http://www.cs.uiowa.edu/~jones/step/> (19 November 2004)
43
At relatively low step rates, the rise and fall times have much less effect on the
motor’s torque than at high step rates, as shown in Figure 3.6. This is because the ratios
of rise and fall time to the duration of time a winding is energized is much lower at slow
step rates compared to higher ones. At low step rates the winding is conducting full
current for a greater percentage of the time that the winding is energized. This leads to
greater running torque at low speeds.
The motor's maximum speed is the speed at which the available torque goes to
zero. A curve of torque versus speed for a typical motor and control system can usually
be approximated by a horizontal line at low step rates and a line with negative slope
going to zero over the range of higher step rates. The cutoff speed is defined as the step
rate at which these two regions of the curve meet. A definite cutoff speed is rare,
therefore, statements of a motor's cutoff speed are approximate. The rise and fall times of
Figure 3.6 Torque versus speed
___________________________________________________ Source: Jones, D., “Control of Stepping Motors”, (2004),
<http://www.cs.uiowa.edu/~jones/step/> (19 November 2004)
44
the current through the motor windings occupy a relatively small percent of each step
when the motor is operating at rates less than its cutoff speed. While at the cutoff speed,
the step duration is comparable to the sum of the rise and fall times. The exact torque
versus speed curve depends on the rise and fall times in the motor windings, and these
depend on the motor control system as well as the motor. Therefore, the control system
for a motor also has a large effect on the cutoff speed and maximum speed, not just the
motor itself (Jones, 2004).
One strategy to improve a motor’s cutoff speed, maximum speed, and high-speed
torque is current limiting. Increasing the voltage applied to the windings increases the
current through the windings in a simple V I R= × relationship, where V is the voltage, I
is the current, and R is the resistance of the windings. Increasing the voltage applied to
the winding to a level which results in a current that is significantly higher than the rated
current results in much quicker rise and fall time. Unfortunately current levels this high
result in damage to the motor; usually the thermal breakdown of the insulating material in
the motor windings. The idea behind current limiting is to use a voltage significantly
higher than necessary to achieve the rated current, but to use advanced circuitry to drop
the voltage applied to the windings, once the rated current level is reached. In this way,
the current versus time curve comes much closer to resembling a square wave, as in
Figure 3.7. The advanced circuitry techniques used to achieve current limiting include
resistive current limiters, linear current limiters, open loop current limiters, one-shot
feedback current limiters, or hysteresis feedback current limiting. Current limiting
technology is also required to achieve micro stepping since at times during micro
stepping one of the motor windings is run at less than the rated current (Jones, 2004).
45
3.4 Stepper motor, drive, and power supply setup
For the research and development stage of the hand mechanism, a system that is
easily reconfigurable and expandable has obvious advantages. The overall system that
converts electrical power to mechanical power consists of stepper motors, stepper motor
drives, and a power supply. This section describes how all the separate components that
were selected based on the performance requirements and operating principles discussed
above are combined in one system. It also discusses the performance of the system.
Five NEMA Size-17, bi-polar, hybrid, 1.8° , DC stepper motors equipped with
3.6:1 gear reduction via an offset spur gear provide mechanical power for the hand. Five
RMS Technologies, R208 drives power the motors. They have micro stepping and
current limiting technologies. Two variable voltages linear power supplies from BK
Precision, which run on 110 VAC and provide an output of 0 to 15 VDC at 40 Amps,
provide the electrical power. They are connected in series, providing variable voltage
over the input range of the drives, 12 to 24 Volts.
Figure 3.7 Current versus time with and without current limiting ____________________________________________________
Source: Jones, D., “Control of Stepping Motors”, (2004), <http://www.cs.uiowa.edu/~jones/step/> (19 November 2004)
46
This system provides excellent performance characteristics suitable for actuating
the hand mechanism. The angular resolution of the motors allows for very precise
control of the cable displacement, and therefore, hand posture. With the 3.6 to 1 gear
reduction, this setup is capable of step sizes as small as 0.5° without micro stepping, or
using the finest level of micro stepping, one eighth stepping, the drives are capable of
rotations as small as 0.0625° . Depending on the pulley size for a given joint, these
rotations correspond to cable displacements as small as approximately 0.036 mm and
0.0045 mm, respectively. This particular combination of drives, motors, and gearing
produces running torque of 1.06 N-m (150 oz-in) at low speeds and 0.81 N-m (115 oz-in)
at 2 revolutions per second, as reported by the manufacturer (Figure 3.8).
Figure 3.8 Manufacturer’s torque rating (oz-in) of motor versus speed (revolutions/second) _________________________________________________________________________
Source: Lin Engineering, “Lin Engineering FAQ”, (2005), <http://www.linengineering.com/site/resources/faq.html> (29 March 2005)
47
These torque and velocity figures refer to torque and velocity after gear reduction
and not torque and velocity of the motor itself. Holding torque, or the torque the motor
provides to keep the hand static, is critical to maintaining a grasp. The holding torque
produced when the maximum current is supplied to the windings while the motor is
stationary would be approximately equal to the maximum running torque, but the friction
associated with the gear reduction increases this amount. The amount of torque produced
provides tension on the cables that is quite sufficient for posturing the hand and providing
gripping force.
The mechanical aspects of the hand have dictated the design of the electrical
system that powers the hand. The most critical aspects were the amount of tension
required in the cables to displace the fingers fully and the corresponding displacement of
cable. Due to these mechanical aspects of the hand, five stepper motors with gear
reduction were determined to be the most suitable devises for converting electrical power
into mechanical movement. With suitable drives, which are capable of current limiting
and micro stepping, the stepper motors provide sufficient torque over the entire angular
velocity range, especially at the low end, and at the same time are capable of extremely
fine movements. Since stepper motors are relatively inexpensive it is wise to plan to size
a motor large enough to provide torque two or three times greater than the maximum
expected required torque. Of course, as the process of optimizing the design for a
prosthetics or robotics application is begun, the use of oversized motors may no longer be
practical. Once the appropriate stepper motors are selected, this decision determines the
other necessary electrical components: stepper motor drives, and a power supply.
48
CHAPTER IV
MOTION CONTROL
This chapter discusses the computerized motion control for the hand, why such
control necessary, various alternatives for achieving it, and specifics of the method used.
A user interface based on a hierarchical control scheme that simulates how the hand
could be controlled in the real world is discussed. New software, which is discussed in
the chapter, was developed to implement this control scheme. Also, further
advancements necessary to achieve a useable level of controllability of the hand are
discussed.
4.1 Motion control setup
To achieve coordinated motion of five DOF’s which mimics human motion,
requires computerized control. At a lower level of control, simple finger motions,
computerized control is necessary to manage simpler requirements such as maintaining
the joint limits of the hand and defining a single move, such as number of steps,
direction, maximum velocity, rate of acceleration, rate of deceleration, etc. At a higher
level of control, computerized control is also necessary for inter-coordination of five
independent DOF’s. Computerized control is especially necessary to implement a
hierarchical control scheme which can simplify the necessary input from a prosthesis user
to a level where managing a high number of DOF’s is comfortable.
49
The term hierarchical control scheme refers to one in which it is possible to
control a high number of degrees of freedom while the user only inputs a smaller number
of DOF’s, in prosthetic control typically one or two. For instance a user may only signal
a higher level command such as “ close grasp” and the control scheme would generate
corresponding lower level commands such as move the first axis 30° counter-clockwise,
move the second axis 45° counter-clockwise, etc. Additional information could also be
collected be external sensors in the hand to help make decisions automatically, such as
stop tightening the grasp when an object comes into contact with one or more fingers.
Since stepper motors have no commutators like other DC motors, all the
commutation, or continuous change in the electromagnetic field to turn the rotor, must be
handled by external electronics. The device that is responsible for this is referred to as a
drive. At the very least the stepper motor drives require two inputs signals to operate,
direction and pulse rate. The direction signal is a simple on - off signal of plus or minus
5V to indicate the direction of travel, while the pulse rate signal is a sine wave, the
frequency of which indicates the pulses per second which is directly proportional to the
number of steps per second. The drives then energize the appropriate windings of the
stepper motor at the appropriate times.
When considering the most basic aspect of control for this application, sending
five direction and pulse signals to five stepper motors drives, the are several categories of
electronics that could possibly perform this task as well as more complex tasks. These
categories include electronics specifically designed for this task such as PCI or ISA bus
motion controller cards that reside in a host PC, a range of stand alone motion controllers
that are an analogue of the PCI or ISA bus versions but operate removed from the PC, as
50
well as more general purpose electronics which could be adapted to perform the task such
as Programmable Logic Controllers (PLC’s), PC-104 based electronics, or a custom
designed electronics board.
For the current state of the project, research and development, a system that is
modular and easily configurable, both in terms of hardware and software, greatly aids in
experimentation and refinement by allowing the easy addition of new hardware
components or programming. After examining all the alternatives, a control system
based on a PCI bus motion controller was determined to be the most practical solution. It
is simple to implement, designed specifically to perform all of the necessary control
tasks, easily configurable in terms of software and hardware, and relatively cost effective.
The motion controller is a DMC-1850 five-axis motion controller that resides in the PCI
Bus of a standard desktop PC. The number of axes refers to the number of DOF’s or
motors that it can control. It has its own microprocessor and memory and performs all of
the motion commands internally without using the computers resources (Figure 4.1).
Figure 4.1 Actuation and control setup
51
The stepper motor drives used are RMS Technologies, R208 bipolar drives. They
have current limiting technology, so they can run on power anywhere from 12 to 24
VDC. They also have a peak current which is adjustable from .35 to 2 Amps peak, to suit
the particular motor and application. They are capable of step sizes of full, half, and 1/4
and 1/8 micro-stepping. Other features include optically isolated step, direction, and
enable/disable inputs, and a current cutback feature that can be disabled. When enabled,
the current cutback feature reduces the holding current to 23% of peak current to reduce
temperature buildup and energy consumption. They have a 9-pin input connection, the
other six that have not been mentioned previously include: main power and ground, a 5V
power and ground for the logic circuits, and two inputs which can be enabled and
disabled in four different combinations to select the step size. In this way the controller
can control all the functions of the drives. A detailed wiring diagram is included in
Figure 4.2.
The interconnect module is basically an extension of the motion controller and
provides terminals to handle all of the input/output for the motion controller. This
includes committed I/O for each axis for pulse rate, direction, and an enable signals
(which activates each drive), along with several digital and analog inputs and outputs,
which allows for a great deal of expandability ideal for this stage of development. The
particular motion controller used actually requires two interconnect modules because
each module can handle up to four axes. This setup has five motors. The interconnect
module terminals included in the figure are only the ones needed for to control the
functions of the drives, there are several more which can be used to add a variety of
hardware.
52
17
18
24
25
27
28
30
31
33
34
66
67
68
69
70
71
72
73
signal ground
+ 5VDC
A axis direction
A axis pulse output
B axis direction output
B axis pulse output
C axis direction
C axis pulse output D axis direction
D axis pulse output
output 1
output 2
output 3
output 4
output 5
output 6
output 7
output 8
37
38
39
40
A axis enable
B axis enable
C axis enable
D axis enable
Power Supply
+12 – 24 VDC ground
33
34
40
E axis direction
E axis pulse output
E axis enable
66output 9
67output 10
Interconnect
Interconnect
1. +12V
2. step
3. step
4. enable
5. direction
6. ground
7. logic ground
8. +5V
9. PWM
B
B
A
A
Drive, A
1. +12V
2. step
3. step
4. enable
5. direction
6. ground
7. logic ground
8. +5V
9. PWM
B
B
A
A
Drive, B
1. +12V
2. step
3. step
4. enable
5. direction
6. ground
7. logic ground
8. +5V
9. PWM
B
B
A
A
Drive, C
1. +12V
2. step
3. step
4. enable
5. direction
6. ground
7. logic ground
8. +5V
9. PWM
B
B
A
A
Drive, D
1. +12V
2. step
3. step
4. enable
5. direction
6. ground
7. logic ground
8. +5V
9. PWM
B
B
A
A
Drive, E
Figure 4.2 Wiring diagram
53
Explicit commands can be sent for execution by the motion controller from an
application running on the PC that is designed specifically for use with this motion
controller. This of course, is a very basic way of controlling the hand and is only useful
for developmental purposes. This method does not allow for the possibility of executing
a coordinated series of motions. The motion controller can also interact with an
executable running on the PC. In this way virtually any level of logic can be
incorporated in the control of the actuation system. The highest-level commands are
generated in the executable running on the PC. Explicit commands, such as specific
relative position moves with a defined acceleration, deceleration, and velocity, can then
be sent to the motion controller card. The motion controller then converts these
commands to the appropriate pulse rate and direction signals for each axis and then sends
them to the motor drives via the interconnect module. The motor drives then use these
pulse rate and direction signals to energize the windings of each stepper motor in the
appropriate sequence to complete the required motion.
The inputs can be used to read nearly any variety of external sensors, which can
then be monitored by the motion controller. The executable can then query the motion
controller for the state of these inputs. In this way, the external sensors can be used to
trigger events that happen in the executable.
For example, as part of a hierarchical control scheme, pressure sensors could be
incorporated into the palm and fingertips of the hand. Myoelectric signals from the user
could be monitored until the appropriate signal was detected to indicate that the user
intends to close the grasp of the hand. The hand would begin to close until a change in
the signal from the pressure sensors indicated that contact with an object had been
54
achieved causing the hand to hold its position after the appropriate pressure has been
applied.
The system also includes a main power switch in the positive wire (Figure 4.2) in
between the power supply and the motor drives that is installed on the motor fixture, a 20
Amp fuse in between the power supply and the switch, and an abort switch. The abort
switch is wired to a designated input, that when activated terminates any motions
currently being executed and prevents other motions from being executed until the switch
is reset.
One addition to the system, which would not only greatly improve performance of
the current system, but would also be a necessary element of the control system in any
form suitable for a consumer product, would be encoders or some other device which
could provide a position feedback signal in order to correct positional errors. Since when
stepper motors move they move a commanded number of steps, which corresponds to a
certain angular position, stepper motors work very well in some applications that do not
include positional feedback. This is true when the inertia of the load and attempted
accelerations do not exceed the available torque of the motor. If the load on the motor
does exceed the available torque, the stepper motor will “slip” or fail to advance to the
next step. In this particular application the end effectors purposely collide with a grasped
object, and therefore the maximum range of motion are, in effect, different for every
object the hand grasps. A positional feedback signal could detect a slip by the stepper
motors, which could signal contact with a grasped object or other cases in which the
motor slips. Without this feedback, once a slip occurs, all knowledge of position is lost.
55
4.2 Programming
This section discusses new software that was developed to implement a
hierarchical control scheme. A generic method for programming the hardware used and
basic capabilities are discussed first.
A software development kit is provided with the motion controller card. It
includes, dynamic link libraries, (DLL’s) which can be used with any Windows
programming environment that interfaces with DLL’s. The basic programming model
includes six steps, described as follows. The first step is to the declare functions. The
second step is to start a communication session between the executable and the controller.
The third step is to download a program. If necessary an entire predefined program can
be downloaded from the computer to the controller and then executed on command. Step
four is to send live commands including axis motions, input activation, and input
querying. The last step after the rest of the program is complete is to close
communication. The software developed to control the hand follows these basic steps.
One of the most difficult aspects of developing a hand mechanism that could
potentially be used as the major component of a prosthetic system with a large number of
DOF’s is a user interface and control scheme. Many currently available prosthetics use
myoelectric signals from the residual muscles to control grasping. However, to
consciously control more than one or two DOF’s individually is too mentally complex for
the user and would make the device too cumbersome to use. Control of a large number
of DOF’s is impossible unless some sort of control scheme can be developed to simplify
the commands the user must provide and coordinate many DOF’s.
56
To demonstrate the capabilities of the hand, a control scheme was developed
based on a control scheme developed for a different application. Knutson et al (2004)
developed a control scheme simulating state activation of a neuroprosthesis using two
myoelectric signals, from the wrist extensor and flexor muscles. Sensors were implanted
in patients’ wrist flexor and extensor muscles, and then both myoelectric signals were
monitored. The flexor myoelectric signal was then used to represent an x-coordinate; the
extensor myoelectric signal was used to represent a y-coordinate, which together
corresponded to a location on a state activation chart (Figure 4.3).
Figure 4.3 State activation chart
__________________________________________________________________ Source: Knutson, J., Hoyen, H., Kilgore, K., Peckham P., (2004), “Simulated Neuroprosthesis State Activation and Hand-Position Control Using Myoelectric Signals from Wrist Muscles”, Journal of Rehabilitation Research & Development, Vol. 41, Issue 3B, P461.
57
The signal space is divided into four regions, “Hold”, “Open”, “Close”, and
“Change Grasp Pattern”. When the Hold command is active, which corresponds to both
sets of muscles being at rest, the hand mechanism stays in its current posture. When the
extensor muscles are excited, the Close command is activated, which causes the hand to
tighten its grasp. Likewise, when the flexor muscles are excited, the Open command is
activated widening the grasp. When both sets of muscles are excited, the Change Grasp
Pattern command is activated; it can be used to toggle between different grasping
postures.
In order to apply this control scheme as a user interface to operate the hand
software was developed to use a joystick to simulate the myoelectric signals. One DOF
of the joystick simulates the extensor myoelectric signal, while the other simulates the
flexor myoelectric signal in the same manner as the above-mentioned neuroprosthesis. A
combination of the two signals determines the location of a control point that exists on a
two-dimensional state activation chart (Figure 4.3).
The joystick, which contains two linear potentiometers that are each manipulated
by one DOF of the joystick, is connected to the interconnect module. The potentiometers
are each connected to one of the analogue inputs and the voltage across the
potentiometers is read. After the program goes through a short calibration routine to find
the minimum and maximum voltages for both DOF’s, the program runs in a loop,
constantly monitoring the voltage of across the potentiometers, until a lack of input
causes the program to time out and terminate. The signals from both potentiometers are
normalized and converted into Cartesian coordinates. The joystick has been physically
modified so that the tension of internal springs cause the at-rest position of the joystick to
58
reside at the lower left corner of the range of motion, similar to the state activation chart.
The command regions of the program are also divided up in a similar manner as the
chart. Moving the control point out of the Hold region by varying one or both DOF’s
activates one of the other commands depending on which command region is entered,
Close, Open, or Change Grasp Pattern.
Proportional control is the ability for the user to determine how far the device
opens or closes by some means. Proportional control is incorporated into the control
scheme by correlating the how far the control point travels into either region before
returning back towards the Hold region to how far the hand opens or closes. Moving the
joystick to its maximum range of motion in a given DOF corresponds to either 100%
closure or opening. Since this is an open loop system, the program monitors position by
tracking the commanded direction and number of steps of each move. Joint limits are
maintained by adjusting any command that calls for a motion beyond the joint limits by
reducing the number of steps in the command so that the motion takes the particular DOF
up to the joint limit. Subsequent motions in the direction of the reached joint limit are
ignored.
The Change Grasp Pattern toggles between different sets of relationships between
the five DOF’s, to achieve different grasp postures. If the adduction / abduction DOF
were included in the fingers the hand would be capable of several kinds of grasps.
However in its current configuration the hand is still sufficiently capable to perform some
grasps types. It is programmed to do a pinch grasp with the thumb and forefinger and a
cylindrical grasp with all five fingers.
59
Use of a PCI 5-axis motion controller allows for computerized motion control
system that is ideal for developing the hand due to the great expandability. Software can
easily be rewritten or hardware added to experiment with new control strategies for the
hand. A user interface that simulates a real world hierarchical control scheme has been
completed. This allows for some rudimentary control of five DOF’s while only requiring
two DOF’s of input from the user. Ideally a level of control would be achieved that more
closely approaches a human hand in terms of intuitive use and performance. It would be
necessary to include encoders for positional feedback as well as additional sensors in the
hand to provide feedback to indicate contact with an object, slip of a grasped object, etc.
60
CHAPTER V
APPLICATIONS AND RESULTS
Previously, in Chapter I, potential applications of the hand mentioned are
mentioned, adaptation of the hand to be used as a prosthetics device and incorporation of
the hand into a robotics system as an anthropometrically based gripper. In addition to
these, there is a third application for which a advanced hand mechanism could be used, as
a physical model to develop a cognitive model that governs the grasping capability of a
digital human and verify its accuracy. Section 5.1 discusses how the hand mechanism
could be used for all three applications and what would have to be done to adapt it for
each. Section 5.2 discusses the results achieved thus far and how suitable the hand would
be for these applications.
5.1 Applications
Prosthetic devices to replace the hand can either be hook shaped or a more
anthropometrically shaped hand with a range of functionality from one or two simple
grasps to purely aesthetic device. Approximately 70 percent of users in the United States
use a hook over a hand devise (Doshi, et al. 1998). The fact that not all users choose to
use a mechanical hand prosthesis leads to the conclusion that the devices commercially
available are either too expensive or are not practical enough for all users to consider
them beneficial. Typically commercially available models only have three active fingers,
which are usually all activated by the same motor and therefore only have one DOF.
61
They are only capable of a cylindrical or pinch type grasp depending on how the hand
comes in to contact with the object.
Integration of the hand mechanism into a prosthetic system would require two
different user interfaces, a physical one and another one that allows the user to send
commands to the device. The purpose of the physical interface would be to connect the
devise to the user’s body. The physical interface would be customized to each user and
would include a socket or cup that would allow the device to join with the user’s residual
limb. The conventional command interface, which allows the user to interact with a
prosthetic devise, uses myoelectric sensors placed on the surface of the skin. Myoelectric
control has three main advantages, accuracy of command selection, intuitiveness of
control, and a quick response time of the system (Englehart and Hudgins, 2003).
Typically myoelectric control can be troublesome because the sensors can move or be
easily placed in slightly different locations every time they are applied which causes
inconsistency in the myoelectric signals, which are inherently weak and noisy (Davalli et
al, 2000). Because of this, many systems require an initialization routine to calibrate the
device to the changes in the signal every time the device is put on. Myoelectric control
would also require additional electronics to amplify and filter the signal.
This hand device, with further advancements such as additional DOF’s and the
appropriate sensors combined with an advanced control system based on a hierarchical
control scheme could be integrated into a prosthetic system. This system would have
several potential advantages over currently available devices and devices currently being
researched. Since the fingers are based on a compression spring and do not contain any
rigid links, complex revolute joint mechanism, or actuators, a considerable percentage of
62
they’re volume is occupied by empty space. This provides the potential for an extremely
lightweight hand if the design for the fingers can be combined with a design for the rest
of the hand that is optimized to be lightweight.
Since the forces to actuate the fingers are transmitted by a cable and conduit pair,
there are several options for the locations of the actuators. This could possibly present a
design trade off between minimization of the perceived weight of the device and
containing it in the smallest package possible. In any case the actuation system would
need to be made more efficient so that smaller, lighter motors could be used and the
entire actuation system could be contained in a smaller, more ergonomic package.
Depending on the size of the motors that are sufficiently powerful enough and therefore
the torque required to actuate the hand, there could be up to three options for the location
of the motors.
The first option would be in the palm of the hand. For this option the motors
would have to be extremely small, since there would still likely be at least five motors.
This would also increase the weight of the hand mechanism and locate the center of mass
of the device further away from the users shoulder, making it more cumbersome and
tiring to use.
The second option would take advantage of fact that the forces are transmitted
through flexible cable conduit pairs and locate the motors completely off of the limb.
The motors could be clustered in an ergonomic package that is worn somewhere else on
the body, possibly in a pack worn around the waist. The cables could then run from the
pack, under the clothes, to the hand. The advantage would be that much of the weight
could be located completely off of the limb, minimizing the burden on the user. The
63
disadvantage would be the addition of another component to the system that the user
would have to wear.
If the amount of residual limb allowed for it, the third option would be a
compromise between the first two options. This would be to locate the motors in the
forearm or wrist space. This would locate the weight further up the user’s limb reducing
the perceived weight of the device compared to locating the motors in the hand itself and
at the same time eliminating the need for an additional pack to be worn.
No matter where the motors are located it would be beneficial to spend effort on
optimizing the entire system so that it requires the smallest motors possible. The design
of the force transmission system could be optimized to reduce losses due to friction. The
design of the joints could be optimized so that a given deflection of the spring takes the
smallest amount of tension in the cable, while at the same time creating the most contact
force at the fingertips. These two design points are key to making the overall system
practical because they lead to reducing the minimum size of the motors necessary, which
is beneficial for several reasons. Smaller motors are easier to locate in an ergonomic
package and leave more options as to their location. Smaller motors would reduce the
total weight of the device, which reduces the amount of effort on the user’s part to utilize
it. They would also require less power, reducing the size of the batteries necessary to
power the device for a given amount of time, additionally reducing weight and bulk.
The hand could also be used as an anthropometrically based end effecter or
gripper for a robotics system. Applications that require this sort of gripper over a more
primitive pincher-like gripper are those that will model their gripping strategies on the
strategies that a human employs. The applications that are likely to use an
64
anthropometrically based gripper are the ones in which the system would be required to
perform operations that were unforeseen and varied in nature and therefore required a
gripper that would be versatile and easily adaptable for many different tasks instead of a
more primitive pincher-like gripper may only performs one kind of task well.
For example, humanoid robots, that one day could be part of everyday life, would
be interacting with the objects found in everyday life. Most of these objects, whether
deliberately or not, are all to one degree or another designed to be manipulated by human
hands. Therefore the most logical end effecter to manipulate these objects is one based
on the human hand.
In the more near future, a complex hand mechanism may be used as part of a
robotic system to complete tasks that would normally be completed by a human but the
task is more difficult or more dangerous to complete due to the environment.
Specifically this is being researched for applications in space (Farry et al, 1996, and
Carrozza et al, 2002). These projects investigate the possibility of allowing astronauts to
complete certain tasks that would normally be extra-vehicular activities by remotely
operating a hand mechanism mounted on a robotics arm system. This would eliminate
the difficulty associated with trying to manipulate objects while wearing a pressurized
glove and the elevated level of danger of an extra-vehicular activity.
Although more of a side effect than a direct application, another field that could
benefit from development of this hand mechanism is digital human modeling. Digital
human modeling tries to replicate what a human does in a virtual environment for the
purpose of studying how a human might interact with products in order to further refine
these products before they are built. Development of a control system that is sufficiently
65
advanced enough to replicate human motion when grasping an object, and do so with a
minimum of effort from the user, would necessarily require an intense study of all the
processes that happen when a human grasps and manipulates an object. Processes that
require better understanding include how the human brain coordinates individual finger
movements and inter-coordination of the fingers to work together without conscious
thought, how tactile feedback plays into this, and how humans unconsciously decided on
a grasping strategy based on the object’s shape, size, and weight, and the task that is to be
accomplished. A better understanding of these things would be important in developing a
control system for the hand. This would benefit the field of digital human modeling,
which also aims to mimic what humans do in real life, but tries to replicate it in a virtual
environment.
5.2 Results
A fifteen DOF hand has been developed. It has all the major DOF’s in the flexion
/ extension plane that a human hand has, which is many more than any commercially
available model and competitive with other complex hand mechanisms currently being
researched. A system to actuate it has been developed that couples the joints together in
sets of thee, reducing the number of motors necessary and therefore simplifying the
required electronics. Motion control and a hierarchical control scheme have also been
developed. Although the hand does not mimic the human hand perfectly, it is capable of
performing a cylindrical grasp, a spherical grasp, and a pinch grasp (Figure 5.1 through
Figure 5.5). In this section we show examples of the hand performing these grasps.
66
Figure 5.2 Spherical grasp with glove, one
Figure 5.1 Cylindrical grasp without glove
67
Figure 5.4 Pinch grasp with glove, one
Figure 5.3 Spherical grasp with glove, two
68
The grasps developed look natural can hold objects securely. The cylinder
shown in Figure 5.1 is a steel bar that weighs approximately 350 grams and is 19 mm in
diameter. The bar has two different surface finishes, a smooth one and a rough one.
When only the smooth surface is gripped, the bar will slip if it is held in the vertical
position. The bar stops slipping as soon as the rougher surface slides in between the
index finger and thumb. However this test was conducted with out the use of the
cosmetic glove, which has a much higher coefficient of friction than the aluminum
surface of the hand.
The device weighs approximately 388 grams. This includes the weight of the
fingers and hand body and does not include the glove, actuation system, or electronics.
The aluminum hand body consists of a large percentage of the weight of the hand,
Finger 5.5 Pinch grasp with glove, two
69
approximately 220 grams or about 57 percent of the total weight. The design of the
fingers gives the hand an advantage in terms of weight. Since most of the volume of the
fingers is empty space they are very lightweight. All five fingers weigh a total of about
118 grams. The hand body could be redesigned with emphasis place on weight savings
and be produced out of a lightweight material. A redesigned hand body and the fingers
would make for a lightweight combination putting the hand on par with or ahead of
competitive models, such as the prosthesis by (Doshi et al, 1998), 203 grams, the Otto
Bock hand, 390 grams, or the APRL hand, 421 grams (Yang et al, 2004).
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CHAPTER VI
CONCLUSION
6.1 Contributions
This work has been focused on developing a complex anthropometrically based
hand mechanism. Such a mechanism has several useful applications. The first
application is use in any robotics system that requires a human like gripper. Such a
gripper has several advantages over simpler designs like simple pinching mechanisms. A
simple mechanism is limited in the number of grasps and manipulations it can perform
and therefore can only be used for a predetermined set of tasks. An anthropometrically
based gripper is highly configurable and adaptable. Its use is only limited in scope to any
grasping or manipulation tasks that a human might perform, and can therefore be used for
unforeseen tasks that arise. This would be an ideal grasping mechanism if humanoid
robots were to ever become part of everyday life. In the more near future, human like
grippers may be used as part of a remotely operated system in environments that make
work difficult or dangerous such as extra vehicular activities in space.
The second is as a prosthetic hand. Commercially available models typically only
have a few active fingers and one DOF. Although they restore some functionality to the
user’s limb, they lack realism and the ability to do more than a few basic grasps. This
leaves the user with less realism and function than a human hand. The ideal replacement
for the human hand would equal the human hand in realism, dexterity, easy of use, and
71
sensory feedback to the point where the user would forget that the hand is not a part of
their own body.
Certain shortcomings of current prosthetic hands, such as lack of realism and
functionality, leave users wanting improvement in their devices. These can be corrected
by adding DOF’s. This hand has fifteen DOF’s, significantly more than current models,
but still fewer than the human hand. This allows it to perform three types of grasps on an
object: cylindrical, spherical, and pinch type grasps.
The contribution that makes this work unique is the development of a novel
flexible bending segment based on a compression spring. Three of these segments make
up a finger that has three DOF’s in the flexion / extension plane. A finger of this type
offers several advantages over a finger made in a more conventional manner using rigid
links and revolute joints. The first advantage is that since the majority of the structure is
comprised of a compression spring, much of the volume is empty space. This gives the
hand the potential to be very lightweight. The other advantage is that the fingers are
inherently compliant. They deflect and conform to the shape of the object that they are
grasping, giving a more secure grip.
An actuation method has been developed that will serve to test and further
develop the hand. The three DOF’s in each finger are coupled together and actuated
through cable and conduit sets by a stepper motor. The stepper motors provide enough
torque to produce significant gripping force, enough to securely hold a 350 gram cylinder
19 mm in diameter without slipping. The stepper motors can be precisely positioned with
displacements of cable as small as 0.0045 mm if micro stepping is used.
72
Computer control is necessary to coordinate the five DOF’s of the hand. A
control system suitable for research and development purposes was developed based on a
motion controller card residing in a desktop PC. An executable can run on the PC and
send commands to be executed by the motion controller card. The motion controller
converts these commands into signals suitable for the stepper motor drives, which
energize the windings in the motor to carry out the motion. There are several available
inputs and outputs that can be used to trigger events in the executable and therefore
provide means to interact with the hand in nearly anyway imaginable. This
configuration works well for research and development because it is easily configurable,
allowing for easy testing of different system configurations.
A simulation of a control scheme was conducted to demonstrate one possibility
for interacting with the hand. The control scheme being simulated uses myoelectric
signals and a hierarchical command structure to return function to the hands of spinal
injury patients. In the actual tests with patients, two myoelectric signals from different
muscle groups in the forearm are monitored. These signals are then used to select
commands to control the hand. The simulation was conducted by using signals from a
two DOF joystick to simulate the two signals from the patients’ muscles. A hierarchical
control scheme such as this one, allows the user to actively control only two DOF’s while
coordinating five. This reduces the mental burden on the user. To consciously control all
the DOF’s in a complex hand mechanism individually would be far too mentally complex
for the user and make the hand completely impractical. An ideal replacement for the
human hand would be completely intuitive to use and require no more conscious thought
than a natural hand.
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This work is by no means completed to the point where the hand is a completed
product ready for integration into one of several applications. It is completed to the point
where the concept of a five fingered anthropometric hand made of fingers with
deformable bending segments is shown to have merit. Thus far, the hand is computer
controlled and remotely actuated. It is capable of using three different grasp styles to
securely hold objects of different shapes and sizes. It has also been developed enough to
discover its weakness and to see areas where further research can be done.
6.2 Shortcomings and future work
The first essential step in developing a hand mechanism based on the concepts
shown in this work to point where it is as capable as a human hand would be to replicate
all the DOF’s that a human hand has, 25 according to some models (Peña Pitarch et al,
2005). All these DOF’s would also need to have the same range of motion that the
human hand does. Only then will a hand mechanism be capable of all the same grasping
styles, gestures, and manipulations that the human hand is capable of. The first step
would be to extend the range of motion of the current DOF’s. This would involve careful
design of the segment links to ensure that the three cables in each finger are all parallel to
the axis of the finger and are coplanar, something that is only approximated in the current
design. These corrections would ensure a motion that is more purely flexion and
extension.
Also the cables and conduits should be retained in the segment links as far away
from center as possible, contacting the inside edge of each coil of the compression
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springs. Designs with the cable located outside of the compression spring should also be
considered. This could dramatically increase the range of motion of the joints, but at the
same time bring about new difficulties. The cable would come into contact with the
cosmetic glove and likely cause tearing. It would also be likely to be the part of the hand
that first come into contact with a grasped object, changing the stability of the grasp.
The next essential modification to increase the number of DOF’s of the hand
mechanism and the number of gestures and grasp styles it is capable of, is to add DOF’s
for the abduction / adduction range of motion in certain finger segments, specifically the
metacarpophangeal joints of the index, middle, ring, and little fingers. This could be
accomplished by adding two more cable and conduit sets to these bending segments and
then locating them at different orientations, ninety degrees either way of the cable used
for flexion of the fingers. A careful study would have to be performed to determine how
to coordinate cable movements of the three cables for both individual DOF movements,
and combined DOF movements of one of these segments.
Opposition of the thumb could possibly be accomplished in the same manner or it
may prove to be more practical to achieve this motion with a more conventional style
joint mechanism. In either case, conventional style joints would likely have to be used to
replicate the remaining DOF’s in the palm of the hand. One model of the human hand
(Peña Pitarch et al, 2005) includes four more DOF’s in the palm, one in the flexion /
extension plane, and one in the abduction / adduction plane at the base of the metatarsal
of both the ring and little finger.
When considering the design of the fingers and the palm for the purpose of adding
new DOF’s, it is not only necessary to consider the realism of the motion achieved, but
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the static shape of the hand as well. Whether developing the hand mechanism for a
robotics or a prosthetic application, overall shape is important. As a gripper for a
humanoid robot a realistic looking hand is more aesthetically pleasing and user friendly
for the people interacting with the robot. As a prosthetic hand a realistic shape is
essential. How the device discreetly blends in with the rest of their body is one of the
primary concerns of prosthetic users.
While additional DOF’s improve the functionality of the hand they also add to the
complexity of other challenges. The control system becomes inherently more complex,
as well as the actuation system. No matter what the application, the actuation system will
likely have to experience a great deal of refinement in order to fit in a small confined
package and to minimize power consumption. This is especially true if the hand is
developed into a prosthetic. Here, the need for the entire device be contained in a small,
ergonomic, lightweight package is probably most relevant. This includes the actuation
system, electronics, and power supply as well as the hand itself.
Perhaps the greatest challenge will be adding some level of intelligence to the
hand so that it will require the least amount of input possible by the user. This is
especially necessary if it is used as a prosthetic device. Reducing the mental burden on
the part of the user is one of the keys to making it an aid in everyday activities. Adding
intelligence to the hand and so it can automatically perform certain functions is one way
to achieve this.
A very robust and sophisticated control system would need to be developed if 25
DOF’s were incorporated into the hand. One strategy for making the control system
more intelligent is to in effect replicate the human sense of touch and use this information
76
to control the hand. For instance, pressure sensors could be integrated into the fingers
and the palm of the hand and could be used to signal the control system when contact has
been made with an object and the magnitude and distribution of force on an object
(Butterfass et al, 1998 and Tura et al 1998). A method to best use this information to
securely hold an object needs to be developed as well.
Another way to make the grasp more reliable is to add accelerometers to the hand
to detect the vibrations from the slip of an object (Tura et al 1998). The idea is that the
hand would grasp an object with a small amount of pressure. If the object begins to slip,
the accelerometers detect the vibration and increase the pressure of the grasp.
All of the improvements suggested have had one central aim, to better mimic the
human hand, in shape, motion, sense of touch, and easy of use. The surest strategy to
achieving the development of the best possible hand mechanism is to strive to mimic the
best possible example, the human hand.
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