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Stephen J. BallDepartment of Electrical and Computer

Engineering,Queen’s University,

Kingston, ON, K7L 3N6, Canadae-mail: [email protected]

Ian E. BrownDepartment of Anatomy and Cell Biology,

Centre for Neuroscience Studies,Queen’s University,


Stephen H. Scott1

Department of Anatomy and Cell Biology,and Department of Medicine,

Centre for Neuroscience Studies,Queen’s University,


Performance Evaluation of aPlanar 3DOF Robotic Exoskeletonfor Motor AssessmentA new planar robotic exoskeleton for upper-limb motor assessment has been developed.It provides independent control of a user’s shoulder, elbow, and wrist joints in the hori-zontal plane. The lightweight backdriveable robot is based on a novel cable-drivencurved track and carriage system that enables the entire mechanism to be located under-neath the user’s arm. It has been designed to extend the assessment capabilities of anexisting planar robotic exoskeleton. This paper presents the design and performance ofthe new robot. �DOI: 10.1115/1.3131727�

IntroductionRobotic technology is becoming an important tool for quanti-

ying and manipulating sensorimotor performance �1–4�. Robotsave the ability to provide sensitive and inherently objectiveuantitative assessments of both the kinematics and the kinetics ofovement. Moreover, robots excel at making repetitive controlledovements, and they are easy to incorporate into virtual reality

nvironments. Not only does this have value in basic research ofensorimotor performance as a whole, but robotics is also beingsed as a tool for assessing and treating motor impairments �5,6�.

Robots designed for the human upper-limb are differentiatedrimarily by the means of interfacing with the human and as such,hey can be classified into one of two categories. End-point typeobots �sometimes called hand-held type� are devices in which theser grasps the end-effector of the robot using a handle or otherttachment. Some examples include the MIT-Manus �7� �nownown as InMotion2 by Interactive Motion Technologies, Inc.,ambridge, MA� and MIME �8�, both of which are intended torovide movement training for stroke patients. The end-effector ofhe robot is the only point of attachment between the user and theobot, and is used to track the position of the user’s hand and alsoo apply forces. These robots are simple and versatile, and canasily be used for both planar and three-dimensional movements.owever, the robot knows only about the hand kinematics, so it isot possible to quantify the individual joint kinematics of theimb. Moreover, torques cannot be applied directly to the joints ofhe user’s limb.

Exoskeleton robots make up the second category. An exoskel-ton robot is designed to align its joints with the joints of theser’s limb, allowing the distinct ability to monitor and controlimb joints independently. Examples of this type of robot include-WREX �9� �now known as Armeo® by Hocoma, Volketswil,witzerland�, ARMin II �10�, and KINARM™ �11� �BKIN Tech-ologies Ltd., Kingston, ON, Canada�. These robots must be at-ached to the user’s limb at multiple points, and they must bedjustable to accommodate users of different size. As a result of

1Corresponding author.Manuscript received November 13, 2008; final manuscript received April 4, 2009;

ublished online May 27, 2009. Review conducted by Vijay Goel. Paper presented athe 2007 IEEE International Conference on Engineering in Medicine and Biology
ociety.
ournal of Medical Devices Copyright © 20

om: http://medicaldevices.asmedigitalcollection.asme.org/ on 10/30/2015 T

this close interaction with the user, exoskeleton robots tend to bemore complex, and thus more expensive. Nevertheless, exoskel-eton robots can provide substantial insight into the mechanics oflimb motion and therefore are particularly well-suited to motorassessment.

The existing KINARM robotic design permits analysis of pla-nar limb movements involving flexion and extension movementsof the shoulder and elbow joints and has been essential for uncov-ering many novel features of motor function in humans �3,12–14�and in nonhuman primates �15–17�. However, there are severalquestions that it cannot address. In particular, the present designcannot address questions related to motor redundancy since thespatial and motor degrees of freedom are identical. The additionof a third joint at the wrist would allow subjects to make a reach-ing movement using a combination of three joints. Moreover, theaddition of a third mechanical degree of freedom at the wrist alsohas practical value for quantifying sensorimotor impairments instroke subjects. It is known that there is a proximodistal gradientin motor impairments following stroke with greater deficits tend-ing to occur at more distal joints �18–20�. The addition of thewrist joint would greatly improve the ability of the KINARMrobotic system to quantify this variation in motor impairmentsalong the limb.

KINARM is driven by equipment placed vertically along theshoulder joint axis, but above the shoulder because the torso pre-vents equipment from being placed underneath. The mechanismthen extends around the arm to provide support for the limb fromthe underside. A consequence of this design is that the equipmentmust be placed beside the user’s head, which can cause users tofeel claustrophobic in the system. Also, this design restricts accessto the limb for clinicians. Therefore, in terms of user comfort andclinical appeal, it would be desirable to move all equipment un-derneath the arm. More importantly, however, the current designdoes not work in bilateral situations in which the distance betweenshoulders is small because there is not enough physical space forboth exoskeletons. This problem is particularly apparent whenstudying upper-limb motion of children. In this context, it wouldbe beneficial to move all of the equipment away from the user’shead so that there is no interference.

With these ideas in mind, it was decided to build a new exosk-eleton robot that could serve as a possible revision to KINARM.
The design of this new device, called Planar MEDARM™, has
JUNE 2009, Vol. 3 / 021002-109 by ASME

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een introduced previously �21�. The present paper describes theesign of Planar MEDARM in more detail and discusses the per-ormance of this new robot.

Design ObjectivesThe fundamental goal for Planar MEDARM is to develop a

ew exoskeleton design that can be used to study 3DOF planarimb movements versus the 2DOF available with KINARM. Add-ng this third DOF will permit both the measurement of motion atnd the application of loads to the shoulder, elbow, and wrist inhe horizontal plane. Secondary goals for Planar MEDARM are tovoid placing equipment beside head as per the present KINARMechanism and to improve vertical out-of-plane compliance.These goals should be achieved without a significant compro-ise in performance relative to KINARM. Planar MEDARM

hould also be able to perform the same types of assessment tasksor which KINARM was designed. First of all, Planar MEDARMust be able to actuate each joint independently to permit appli-

ation of a variety of joint-based loads. For example, this capabil-ty allows the robot to quantify the ability of a subject to compen-ate for perturbations applied to a joint during a whole-arm task14�. In order to probe other aspects of limb motor performance,lanar MEDARM must be able to impose other more complexorce fields, including stable viscous loads, on the subject’s limburing motion. The second and more challenging design feature iso minimize the influence of Planar MEDARM on natural limb

ovements. Therefore, the system should be backdriveable, andhe friction should be as low as possible. Moreover, the inertia ofhe exoskeleton should be low and proportional to the upper-limbtself so that the user can adapt more easily when moving with theevice. For all assessment tests, it is important that PlanarEDARM provides an accurate measure of the actual limb posi-

ion. Any compliance in the robot will introduce a discrepancyetween the measured position and the actual position of the limb,nd thus the robot should at least as stiff as KINARM to minimizehis discrepancy.

Finally, Planar MEDARM should be able to accommodate us-rs of a wide range of size. To fit most adult users, the length ofhe mechanism should have a range of approximately.26–0.37 m and 0.20–0.29 m for the upper arm and forearmimb segments, respectively �22,23�. Likewise, the handle position

ust also be adjustable from 0.06–0.11 m in order to accommo-ate different size hands.

Planar MEDARM DescriptionPlanar MEDARM �Fig. 1� is a 3DOF exoskeleton robot that

rovides independent measurement and control of motion at thehoulder, elbow, and wrist in the horizontal plane. The entireechanism is located underneath the user’s arm, and all joints are

ctuated by a cable-drive system. All three joint axes are parallel,nd the distances between the axes are adjustable to accommodatesers with different arm lengths

3.1 Mechanical Design. The joint layout of PlanarEDARM provides a simple solution for multiple mechanical

esign issues. Figure 2 takes a step-by-step approach to describehe logic behind the final design, and begins by considering thehree joint system that the exoskeleton aims to mimic. A cable-riven 3DOF mechanism consisting of a standard serial connec-ion of links with adjustable length, as shown in Fig. 2�a�, wouldun into three design issues.

Cable tension. A concern with the design of Fig. 2�a� is that theables do not permit free link length adjustments when the actua-ors are not applying tension. The cables are routed along the linksrom joint to joint, so changing the length of the links will changehe distance between the joints and hence length of cable requiredo pass along the link. A single link length change can be up to

10 cm, so the cable will be as much as �20 cm too short or
oo long. This is more than sufficient for all of the cables to fall
21002-2 / Vol. 3, JUNE 2009


off the pulleys entirely if one or both links are shortened. A solu-tion is to guide the cables along passive linkages between eachjoint, as shown in Fig. 2�b�. Now, if the limb segment lengths areadjusted to accommodate different size users, the total distancebetween joint axes does not change.

Gravity support and body interference. There are two additionaldrawbacks for both Figs. 2�a� and 2�b�. The first is that the weightof both the exoskeleton and the limb must be supported at theshoulder joint axis. Providing support only at this point means thata significant bending moment would be applied to the shoulderjoint axis at all times. Not only does this put significant force onthe shoulder joint bearings, but also, it invariably leads to out-of-plane compliance. The second problem is that all equipment mustbe attached to or passed across the shoulder joint axis. The torsoprevents equipment from being placed directly under the shoulderjoint, so all equipment must be placed above the shoulder, result-ing in a substantial amount of equipment directly beside the user’shead. Not only can this be claustrophobic for the user, but it alsolimits the amount and kind of equipment that can be used. Forexample, for bilateral systems, the presence of this equipment willprevent use with small children because each exoskeleton willinterfere with each other.

A solution to both of these problems is to introduce a virtualfour-bar linkage into the design. First, a circular track on which alow friction carriage can freely move is placed underneath theupper arm so that it is centered on the shoulder joint �Fig. 2�c��.The carriage contains four wheels that roll along the v-shapededge of the curved track. Second, all equipment on the shoulderjoint axis are moved back and away from the shoulder joint axis.

(a)

(b)

(c)

Fig. 1 CAD drawings of Planar MEDARM: „a… setup with auser, „b… top view, and „c… side view. Actuation cables are notshown for clarity.

In this case, the passive linkage of the upper arm becomes an

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ctive linkage that drives the carriage along the curved track. Theombined weight of the exoskeleton and limb is now supported byhe carriage �see center of mass in Fig. 2�d��, and all equipmentre moved away from the user. The resulting motion is identical tofour-bar linkage, but there are no physical structures near the

houlder joint axis as indicated by the dashed lines in Fig. 2�c�. Inther words, it is a virtual four-bar linkage. A computer-aidedesign �CAD� view of the final mechanism is shown in Fig. 2�d�.

In existing cable-driven curved track designs that are used toreate a virtual joint axis �for example, as used by CADEN 7 �24�r ARMin II �10��, the driving joint axis is attached to the car-iage. This means that the actuator must move along with thearriage, adding significant inertia and bulk to the mechanism. For

(a)

(b)

(c)

(d)centreof mass

ig. 2 Design „a… offers no way to maintain cable length whendjusting the link lengths, provides support only at the shoul-er joint axis, and interferes with the user’s body. „b… The pas-ive triangular linkages permit adjustment of the limb segment

engths „in gray with arrows… without the cables falling off. „c…he curved track and carriage underneath the arm ensure that

he exoskeleton and the user’s limb are well-supported. Theirtual four-bar linkage „dashed lines… that allows all equipmento be moved away from the user. „d… A CAD drawing of the final

echanism is shown without cables for clarity. The center ofass of the exoskeleton is indicated.

lanar MEDARM’s curved track system, the driving joint axis is

ournal of Medical Devices


fixed relative to the track and therefore does not move with thecarriage. The actuator is connected to the carriage by a lightweightlinkage, greatly reducing the inertia of the system.

Planar MEDARM’s structure is similar to KINARM in thatboth can be described as a four-bar linkage �see Fig. 3�. Thedifference is that Planar MEDARM does not require any physicalstructures on the shoulder joint axis, whereas KINARM is sup-ported entirely at the shoulder joint axis. An advantage of this newdesign is that all equipment are moved away from the user, andbecause there is no longer any interference with the user’s body,the equipment can be placed underneath the arm entirely. An ad-ditional benefit of the new design is reduced vertical compliancebecause the weight of the arm is directly supported by the carriagenear the elbow joint axis.

Planar MEDARM’s design also allows an actuated DOF at thewrist, which would be a challenge to incorporate into the currentKINARM design. In the KINARM design, the actuators aremounted above the user’s shoulder, beside their head, and are bothcoupled to the shoulder joint axis. Torques are transmitted to thejoints of the limb by vertical linkages that are routed from abovethe shoulder to underneath the limb. The advantage of this designis that the linkages are lightweight. Adding a wrist joint wouldrequire additional linkages and transmission components, whichwould diminish the lightweight advantage that the currentKINARM design offers. Furthermore, the shoulder joint is alreadyquite complex because both motors act through this axis, and itserves as the main support for the exoskeleton. Adding anotheractuator would complicate the system further and would placeeven more equipment beside the user’s head. In contrast, for Pla-nar MEDARM, adding the third DOF is simply a matter of addinganother pulley to each of the existing joint axes. The motors arelocated behind the user, and thus additional motors have no effecton the user.

The shoulder joint ��s� is indirectly actuated by the shoulderdriving joint ��sd�, so it is necessary to quantify their relationship.

shoulder joint

elbowjoint

no wristjoint

shoulderjoint

elbowjoint

wristjoint

shoulderdrivingjoint

(a)

(b)

Fig. 3 Top view schematics of „a… Planar MEDARM and „b…KINARM. Planar MEDARM provides planar 3DOF motion„shoulder, elbow, and wrist…, and is driven by a joint that isoffset from the shoulder joint axis and that is part of a virtualfour-bar linkage. KINARM provides planar 2DOF motion „shoul-der and elbow… and is driven directly through the shoulder jointaxis, where the elbow is driven by a four-bar linkage „parallelo-gram…. The shoulder joint axis is the only support point forKINARM, while Planar MEDARM is supported by its curvedtrack.

This can be done by analyzing the mechanics of the closed-loop

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hain �see Fig. 4�. Knowing the link lengths �lu, lo, lg, and lsd�, theffset angle ��o�, and any one other angle in the four-bar linkage,he remaining angles can be related. As the shoulder driving joints actuated �i.e., known�, the shoulder angle ��s� and guide linkagengle ��g1� can be determined. The relationships are given by Eqs.1�–�4�

�s = arctan� y − lg sin��g1�x − lg cos��g1�� 1�

here

�g1 = arctan� y

x� − arccos� lg

2 − lu2 + x2 + y2

2lg�x2 + y2 � �2�

x = lo cos��o� + lsd cos��sd� �3�

y = lo sin��o� + lsd sin��sd� �4�Similarly, a torque at the shoulder is created by applying a

orque at the shoulder driving joint using an appropriate scalingactor

�sd = � lsd sin��g1 − �sd�lu sin��g1 − �s�

��s �5�

Equation �5� assumes that no torques are applied at the elbowr guide pulley joints, and thus describes the simplest case inhich only shoulder torque is applied.

3.2 User Attachment and Alignment. The user alignmentnd attachment design is similar to KINARM, and is illustrated inig. 5. It is first necessary to align the user’s shoulder joint centerglenohumeral joint� with the robot. For the current prototype, thiss achieved through adjustment of the chair position. The upperrm and forearm lengths can be independently adjusted to accom-odate users of different size. Upper-arm length can be adjusted

y sliding the elbow joint relative to the carriage. A single quick-

elbow

joint

guide pulley joint

shoulder

joint

shoulder

driving

joint

ig. 4 A labeled schematic of the virtual four-bar linkage. �o−20 deg, lo=0.125 m, lsd=0.20 m, and lg=0.25 m. lu is the

ength of the user’s upper arm, and is a fixed value between.26m and 0.37m. �sd is the shoulder driving joint angle, �s ishe shoulder joint angle, and �g1 is the guide pulley angle. �sd ishe torque applied at the shoulder driving joint, and �s is theorque that appears at the shoulder joint. Positive angle is de-ned as counterclockwise from the x-axis.

elease clamp is used to lock the joint in place as shown in Fig.

21002-4 / Vol. 3, JUNE 2009


5�b�. Forearm length can be adjusted by positioning the wrist jointusing a telescopic linkage, which is then clamped using thumb-screws �see Fig. 5�a��.

The user is aligned with the mechanism at the upper arm andforearm using molded fiberglass arm troughs, which can be ad-justed along the linkages as needed �see Fig. 5�a��. The system isdesigned to allow the arm troughs to be easily swapped for dif-ferent sizes to accommodate a wide range of users. Currently, thesubject grasps a handle. The location of the arm troughs and thehandle can be fixed with a single thumbscrew clamp, as shown inFig. 5�a�.

All links are custom machined aluminum to keep the mass andinertial properties low in order to minimize the exoskeleton’s in-fluence on natural limb motion. Each joint has built-in mechanicaljoint limits to ensure that the robot does not extend the user’s armbeyond physiological limits.

3.3 Actuation System. The core of Planar MEDARM’s ac-tuation system is an open-ended cable-drive transmission and isillustrated in Fig. 6. A cable-drive system was chosen because themotors can be located remotely from the joints. This choice hasseveral benefits. First, cable-drive systems add minimal weight tothe structure of the robot, substantially increasing the power-to-weight ratio of the actuation system, and reducing the apparentinertia of the robot as seen by the user. Second, with no motors onthe linkages, there is less chance of mechanical interference withthe user. Finally, the transmission offers significant design flex-ibility in how the cables can be routed along the mechanism.

The choice of cable routing scheme has a significant effect onthe performance of the device. There are five unique cable routingschemes for a 3DOF system �25�. The schemes were analyzed tofind the choice that has the best compromise between having bothminimal antagonism between cables �and hence the most even

(a)

(b)

UT FT H

FL

UL

opposite

view in (b)

Fig. 5 „a… A side view of the exoskeleton showing the segmentlength adjustments for the exoskeleton. The labels are as fol-lows: UL—upper-arm length, FL—forearm length, UT—upper-arm trough, FT—forearm trough, and H=handle. „b… A close-upview of the carriage „opposite side view dashed rectangle in„a……, showing the quick-release clamp for the upper-arm lengthadjustment.

distribution of forces across the cables� and minimal peak forces.



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igure 6 illustrates the routing scheme chosen for this robot.Open-ended cable-drive systems are not new, and the relation-

hip between joint motion and cable motion is well understood25,26�. However, Planar MEDARM’s novel virtual four-barechanism requires special modifications to account for the fact

hat the cables are routed along one edge of the four-bar mecha-ism. In actual fact, the four cables are routed across four jointsrather than three� because the guide pulley on the four-bar link-ge that drives the shoulder also contributes to the motion. Theesult is that the cables that reach the elbow and wrist jointshange length when the shoulder moves. Fortunately, the guideulley angle is not independent because it is a function of thether angles of the four-bar linkage. The cables also pass around aecond guide pulley on the forearm triangular linkage, but thisinkage is fixed and thus does not affect the system. Therefore,fter some modifications to account for the four-bar linkage, theable displacement, s, and the change in joint angle, ��, can beelated using Eq. �6�

s1

s2

s3

s4

= − rsd re − rw − 1

rsd − re − rw 1

− rsd − re rw 1

rsd re rw − 1

��sd

��e

��w

��4bar

�6�

here ��4bar is the added term

��4bar = rg1��sd − re��s + �re − rg��g1 �7�

hese relationships are illustrated in Fig. 6�b�. Note that �s referso the actual shoulder joint, and that �sd refers to the shoulderriving joint, which is offset from the shoulder joint axis. Both �snd �g1 can be calculated from �sd using Eqs. �1� and �2�,espectively.

Changes in cable length are achieved by winding up or unwind-ng cable from driving pulleys. A cable is clamped to each drivingulley, which is driven by a slotless brushless dc motor �Compu-otor SM Series, Parker Hannifin Corporation, Rohnert Park,A� using a timing belt. Thus, a rotation of the motor either windsp or unwinds the cable from the driving pulley. Each motor hasbuilt-in optical encoder, which is used to calculate the cable

ength changes. The motor encoders are capable of measuring

(a)

(b)

ShoulderDriving Joint

WristJoint

ElbowJoint

GuidePulley #1

GuidePulley #2

ig. 6 „a… A CAD drawing and „b… a simplified planar schematicepresentation of the original cable routing structure. Each ofhe four cables is denoted by a different line type. Symbols s, �,, �, and � represent cable displacement, cable force, pulleyadius, joint torque, and joint angle, respectively.

oint angle in increments of 0.006 deg ��0.02 deg accuracy�. All



motors are located behind the user, as shown in Fig. 1. In addition,secondary optical encoders �Mercury I and II Series, Micro-E Sys-tems, Natick, MA� are mounted to each of the three joints toobtain direct measures of the joint kinematics. The elbow andwrist joint encoders each employ a high-resolution rotary glassscale, and the angles can be measured in increments of 0.0001 degand 0.0002 deg, respectively with an accuracy of �0.005 deg. Theshoulder joint encoder measures shoulder angle directly using atape scale mounted to the outer diameter of the curved track. Thelarge diameter permits measurement of shoulder angle in incre-ments of 0.00004 deg ��0.00002 deg accuracy�. Using such high-resolution secondary encoders permits a much more smooth cal-culation of joint velocity and acceleration.

To apply a set of torques to the joints, calculations must bemade to determine what force to apply in each of the cables. Therelationship between joint torque and cable force in an open-ended cable-drive system has been described previously �25,26�,but once again, special considerations must be made to accountfor the contributions from Planar MEDARM’s virtual four-barlinkage. The reason for the modification is that when a torque isapplied to the elbow joint, a torque of the same magnitude is alsoapplied at the guide pulley joint. This occurs because the cablesare routed around the guide pulley in the same manner as theelbow joint, and therefore a torque is simultaneously applied tothe four-bar mechanism. The result is an unwanted torque aboutthe shoulder joint. To correct this problem, an additional torque,�4bar, must be applied to the shoulder driving joint whenever anelbow torque is applied, in addition to the properly scaled shoul-der torque, �s. This relation is defined by Eq. �8�

�sd = � lsd sin��g1 − �s�lu sin��g1 − �sd�

��s + �4bar �8�

where �4bar is given by:

�4bar = �1 −lsd sin��g1 − �s�lu sin��g1 − �sd�

��e �9�

Note that in Eqs. �8� and �9�, the denominator will not go tozero. The link lengths of the four-bar linkage were chosen suchthat �g1��sd for the entire range of motion for all upper-armlengths. In fact, the linkages in this mechanism conform to therequirements for a Grashof linkage �27�, which mean that thelinks connected to the smallest link �in this case, the fixed link�can both rotate 360 deg without reaching singularity. Therefore,applying a torque at the shoulder driving joint will always producea torque at the shoulder joint.

The cable force, �, and joint torque, �, can then be related usingEq. �10�:

�sd

�e

�w = − rsd rsd − rsd rsd

re − re − re re

− rw − rw rw rw

�1

�2

�3

�4

�10�

where �sd is the total torque to be commanded to the shoulderdriving joint, as defined by Eq. �8�. The scaling factor in Eq. �8� isdetermined from Eq. �5�. It should be noted that with an elbowtorque command of zero, �4bar becomes zero, and therefore �sdreduces to a single term that describes the applied shoulder torque.

Thus, Eq. �10� must be used with Eqs. �8� and �9� to determinewhat force to apply in each of the cables to apply torques at PlanarMEDARM’s joints. However, because the system is overactuated,there are an infinite number of solutions for a given set of jointtorques. A practical solution that ensures that the cables are al-ways under tension and that positive force is always applied canbe obtained using the torque resolver technique �25�. This solutionincludes a constant pretension force in the cables that prevents thecables from becoming slack, even when the mechanism is movedpassively without any joint torques applied. As such, the motors
must always be supplying at least this constant torque. To prevent
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he cables from becoming slack when the system is turned off oruring a power failure, each motor is equipped with an electricrake.

3.4 Electronics and Control. The present motion control sys-em used for this prototype is basic. Each motor is powered by aervo drive amplifier �Compumotor Gemini GV Series, Parkerannifin Corporation, Rohnert Park, CA� operating in torqueode such that the motors provide a constant torque proportional

o a �10 V command. The drives are wired with a single switchhat disables all drives in the event of an emergency. Additionally,he drives route the power to the motor brakes so that the brakesngage when the drives are disabled or a fault is detected. Asentioned in Sec. 3.2, each joint contains mechanical joint limits

o prevent the robot from pushing the limb beyond safe physi-logical limits.

The heart of the system is a motion control card �PMAC, Deltaau Data Systems Inc., Chatsworth, CA� mounted in a computer.he motion control card receives the quadrature encoder signals

rom the four motor encoders and three secondary encoders asnput. The output is simply four �10 V analog torque commandignals, which are sent to the four servo drives. Presently, theotion control card is programmed directly with all instructions

ecessary to calculate joint kinematics and to apply joint torques.hile basic control algorithms have been hard-coded into the mo-

ion control card, a custom software package is required to applyore advanced control and data handling. The DEXTERIT-E soft-are package �BKIN Technologies, Kingston, ON, Canada� was

dapted to allow simple data collection from Planar MEDARM.

3.5 Dynamic Model and Simulation. A dynamic model wasreated for Planar MEDARM in MATLAB based on the robot tool-ox �28�. The model was defined as a standard rigid-body manipu-ator with negligible cable dynamics. Dynamic parameters of thexoskeleton were estimates from CAD drawings, and upper-limbarameters were calculated from anthropometric data tables basedn user height and weight �22,23�. The model first calculated theoint torques required to achieve a given trajectory. The cableorces required to generate these joint torques were then calcu-ated using the torque resolver technique �25�. All forces and non-xial moments at each joint were also calculated.

Simulations were performed for various reaching movementsith a peak end-point velocity of 1.0 m/s �29�. Movements in-

luded single-joint motion through each joint’s full range, and aariety of multijoint reaching movements. The simulations weresed to determine the range of forces/torques that would occururing these movements, which were in turn used to select appro-riate motors, gear ratios, cables and joint bearings. Peak jointorques of �5 Nm, �2 Nm, and �0.5 Nm for the shoulder, elbow,nd wrist, respectively, were found to be sufficient for movementsf this type. These torques are much smaller than would be re-uired for movements out of the horizontal plane because PlanarEDARM does not need to overcome gravity, and thus the risk of

njury is significantly reduced.

Performance EvaluationA prototype of Planar MEDARM has been fully assembled

Fig. 7�. Before any tests were performed, the secondary encoderst the joints were used to confirm that the joint angle calculationsrom the motor encoders were correct. Also, the motors and servomplifiers were calibrated to ensure that the torque output woulde as expected and consistent across all four motors. Calibrationf motor output is particularly important in this type of robot ashe four cables require constant pretension to be applied by the

otors. Small differences between the motors lead to the applica-ion of a small torque at one or more of the joints.

Several fundamental performance parameters of the prototypeave been measured including: joint friction, inertia as seen at theoints, joint compliance, and vertical �out-of-plane� compliance.
hese parameters are related to one another, thus compromises
21002-6 / Vol. 3, JUNE 2009


must be made. Planar MEDARM is primarily an assessment toolfor upper-limb motion, and as such, it must influence natural mo-tion as little as possible. Therefore, for this application, achievingboth low friction and inertia is more important than achieving lowcompliance. In fact, little can be done to reduce the total compli-ance in the system because the interface between the limb and therobot is inherently compliant due to the layers of soft tissue, andcan dominate the overall compliance. In terms of the exoskeletonitself, as long as bending of the linkages is not the dominantsource of compliance, the secondary encoders at the joints willreduce much of the problem associated with compliance becausethe position of the robot will be more precisely known. Data col-lection has also been tested by collecting several samples of ex-perimental data using Planar MEDARM. A goal of PlanarMEDARM is to extend the capabilities of KINARM, so the per-formance measures are compared directly to those of theKINARM where possible. The following is a brief description ofthe testing parameters and the methods used to obtain measure-ments.

4.1 Measured Parameters. Joint friction. Friction in a ro-botic system can have both beneficial and detrimental effects onperformance. Friction can provide a level of damping that helps tostabilize the system under position control. However, a highamount of friction affects natural passive motion of the user,which can interfere with unaided natural reaching movements.The main application of this robot is measuring motor perfor-mance of a user. This application demands smooth and effortlessoperation of the device and therefore, low friction is desirable.

To obtain an idea of the levels of friction that a user would seeat each joint of the exoskeleton, static friction was measured.Static friction can be measured by determining the minimumtorque required to create a movement at the joint. Friction is ahighly variable parameter that is influenced by many aspects ofthe system, therefore the friction was measured for several con-figurations across the range of motion of the joint, and an averagefriction torque measurement was obtained. The accuracy was de-termined by finding the maximum and minimum friction valuesfor the entire joint range of motion. It was determined that thestatic friction measurements are accurate to within �0.05 Nm.Torque ripple created by the servo drives was the dominant sourceof error.

Inertia. The inertia of the exoskeleton, as seen by the joints ofthe user, also has a significant impact on the performance of thesystem. In this paper, the phrase “inertia as seen by a joint of theuser” refers to the inertia of all components of the exoskeleton andmotor system a user feels when they move that joint with otherjoints fixed. For example, the “inertia as seen by the elbow” in-cludes the inertia of all distal components of the exoskeleton �i.e.,the forearm, wrist, and hand components�, and the inertia of thepulleys, gears, and motors that move during elbow motion. Ide-

Fig. 7 A photo of the fully constructed Planar MEDARM proto-type in its original configuration. The view was chosen tomatch the CAD view from Fig. 1.

ally, from an experimental point of view, the inertia should be



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ero, but of course this is not possible. More practically, the massnd inertia of the exoskeleton should be kept as low as possible,nd the joint inertias should be proportional to the joint inertias ofhe human upper-limb. Otherwise, the device will have a signifi-ant effect on the natural intersegmental dynamics of the limb.

Minimum and maximum inertias were estimated from the de-ailed CAD drawings used to manufacture the prototype for the

inimum and maximum segment lengths, as indicated in Sec. 2.he gear ratio and the number of motors driving the joint were

ncorporated into the calculations. All calculations specified ahoulder angle of 45 deg, an elbow angle of 90 deg, and a wristngle of 0 deg, corresponding to a typical starting point for aeaching task. Considering that manufacturing processes typicallyroduce parts with a tolerance of �0.1 mm �at most�, it is ex-ected that the actual inertia values are within approximately 1%f the CAD estimates. Another source of error arises from ne-lecting cable mass in the estimates.

Compliance. Compliance has an impact on the overall perfor-ance of a robot. With high compliance, tight position control is

hallenging, and it is not possible to accurately measure true jointngles. This is a result of the elasticity of various components ofhe system. Sources of compliance include elasticity of cables andelts, and bending of shafts and linkages.

In-plane joint compliance was measured by reading the changen position noted by the motor encoders while commanding a jointorque to the system with all joints locked in place �using-clamps and/or screws�. The effective torsional spring constanti.e., stiffness and hence compliance� could then be easily calcu-ated from Hooke’s Law. The joint torque was applied in 0.5 N mncrements up to the maximum output of the motors. Note that forlanar MEDARM, the joint compliance can be different for posi-

ive and negative rotations due to the distribution of the cables,ut in this paper, the average is presented. The compliance valuesresented here are based on measurements of cable length changemotor encoder readings�, torque, and pulley radii, each of whichas an associated measurement error. Propagating these errorshrough the calculations leads to a compliance accuracy of �9%.he largest source of error comes from the cable displacementeasurements, which are limited by the accuracy of the motor

ncoders.Vertical compliance was measured by placing known masses at

he wrist joint and observing the resulting vertical displacement.he system was configured with a shoulder angle of 45 deg, anlbow angle of 90 deg, and a wrist angle of 0 deg, with an upper-rm length of 0.30 m and a forearm/hand length of 0.25 m �PlanarEDARM’s wrist joint was locked because KINARM does not

ave a wrist joint�. Once again, this configuration corresponds to aypical starting point for a reaching task, so it is a reasonable pointf comparison. Accuracy of the vertical compliance measurementss �1%. The main source of error is the vertical distance

easurement.Reaching task. To assess its ability to monitor limb motion,

lanar MEDARM was set up to perform one of the basic taskshat is performed in motor control research—the center-out reach-ng task �30,31�. A simple 2D virtual display was built to projectisual targets in the plane of the robot �see Fig. 8�. The display isapable of presenting to the user any one of eight peripheral tar-ets equally spaced around a 10 cm radius circle, as shown in Fig.�c�. Several normal, healthy volunteers were recruited to performhis experiment to test out the robot’s measuring abilities.

The system was calibrated so that the starting point for the usercentral target� was located such that the handle of the exoskeletonorresponded to the limb configuration in which the shoulder andandle were aligned at x=0 and that the elbow was 90 deg and therist was 0 deg �Fig. 8�c��. The subject was directed to move asuickly and as accurately as possible from the central target to theeripheral targets as they appeared one at a time in the display.fter reaching the target, the subject moved back to the central

arget. Planar MEDARM recorded the three joint angles during



the outward movement. Each target was displayed a total of tentimes, in random order. DEXTERIT-E �BKIN Technologies,Kingston, ON, Canada� was used to develop the experimentalprotocol and to collect the data.

Viscous load stability. To demonstrate the ability of PlanarMEDARM to apply joint torques, a viscous loading test was per-formed. A viscous load is a simple load that opposes motion at ajoint with a magnitude proportional to the velocity of the joint, as

(a)

(b)

(c) 1

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6

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LED Panel

Semi-

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LED

Mirror

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Fig. 8 „a… Planar MEDARM’s virtual display includes a basicstructure that holds a semitransparent mirror and a set of LEDlights above the workspace. „b… The mirror is placed equidis-tant from the working plane of the robot and the LEDs so thatthe user can see their limb and an image of the LED in the sameplane. This system allows unrestricted access to the work-space. „c… The visual display system was used to present thesubject with a reaching task which involved making move-ments from the center target „black circle… to one of the eightoutlying targets „hollow circle, 10 cm distance….

given by

JUNE 2009, Vol. 3 / 021002-7


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�i = − Kv�̇i �11�

here �i is the torque applied to joint i, Kv is the velocity feed-

ack gain, and �̇i is the joint angular velocity of joint i.Robots tend to become unstable when velocity feedback loads

xceed a certain magnitude. The stability is influenced by severalspects of the mechanical design, such as the resolution of thencoders �i.e., quality of the velocity signal�, and flexibility �in-lane compliance� in the robot. Increasing the velocity feedbackain amplifies this problem, and ultimately can cause unstablescillatory motion. In general, friction will help to dampen anyscillations, but because this type of robot is designed to have lowriction, these robots are often limited in the magnitude of viscousoads that can be applied. This has far-reaching consequences thatimit not only the maximum magnitude of viscous loads, but alsohe ability to apply other force fields or position control.

To test the stability of Planar MEDARM under viscous loadingonditions, velocity feedback torque was applied to each jointndividually using Eq. �11�, while moving the robot around thentire workspace using both smooth and abrupt movements. Theighest velocity feedback gain, Kv, that could be applied withoutny amount of instability was recorded. Instability involved oscil-atory motion of the motors, and would typically first manifest asiny vibrations, which could be felt at the robot’s handle. The testas performed using the velocity signal, as calculated from theotor encoders, and again using the secondary encoders. In both

ases, the velocity signal was filtered using a second-order But-erworth filter with a 10 Hz cut-off frequency for comparison withxisting KINARM data.

4.2 Testing Configurations. In order to get a better idea howhe above performance measures are affected by the componentsnd specific design features of Planar MEDARM’s actuation andransmission system, the parameters were measured for severalariations in the actuation system. Starting with the original de-ign �as described in Sec. 3�, changes were made one at a time,ut each consecutive variant maintained the changes from the pre-ious systems. So in the end, the final system consisted of allhanges. Each modification was chosen to upgrade the perfor-ance of the system without requiring any structural changes or

hanges to the prototype’s overall mechanical design.System a: original. The original design was tested first. Theain parameters included a gear ratio of 6 �3 from belts and 2

rom cable pulleys�, a cable routing structure, as in Fig. 6�b�, aulley diameter of 60 mm, and a cable diameter of 1.19 mm.

System b: gear ratio. The goal of this first change was to reduceoth the friction and inertia of the motor system. The overall gearatio was reduced to 2 �3 from belts and 2/3 from cable pulleys�y adding an adapter to the cable-windup system. The adapterrovided a 90 mm diameter pulley to wind up the cable, whereashe original system used a 30 mm diameter pulley.

System c: cable routing. The goal of the second change was tourther reduce the overall friction and inertia about the wrist andlbow joints. An alternate cable routing structure was imple-ented to drive the joints �Fig. 9�a��. While the original structure

ad the lowest and most even force distribution, it required allour cables to span all three joints. In contrast, the new structureeeded only two cables for the wrist, three for the elbow and fouror the shoulder.

System d: thicker cable. The primary goal of this third changeas to decrease the compliance by increasing the thickness of the

ables. Doubling the cable diameter from 1.19 mm to 2.38 mm,ncreased the cross-sectional area of the cable by a factor of 4, andherefore reduced compliance of the cable by a factor of 4.

System e: 2DOF. The fourth change was to convert the systemo a 2DOF robot to allow a more direct comparison withINARM. The wrist joint was still present �but locked�, but it wasot actuated because the cables were removed from the wrist jointntirely. Note that the secondary encoder at the wrist could still
easure wrist position, even though the joint was not actuated.
21002-8 / Vol. 3, JUNE 2009


Only three cables were required to drive the shoulder and elbowjoints, and therefore the cable structure was updated using theonly possible structure for a 2DOF system with three cables, asshown in Fig. 9�b�.

System f: KINARM. As a base of comparison, the same param-eters were measured for KINARM. No special changes weremade to KINARM, so all testing parameters were measured forboth the shoulder and elbow.

Although the motors themselves were not changed, the changesmade to the actuation system had a direct effect on the magnitudeof torque that can be applied to each joint. Table 1 summarizes thepeak torque that can be produced at each joint for each testingconfiguration.

4.3 Results. Measurements of joint friction are shown in Fig.10�a� for all systems. Friction was initially several times largerthan KINARM, but it is clear that the changes provided a substan-tial reduction in friction. The biggest improvement occurred forsystem b when the gear ratio was changed. The friction was nearlyhalved for all three joints. This makes intuitive sense becausereducing the gear ratio by a factor of 3, reduces the friction of themotor system �the motors themselves and the timing belts�, asseen by the joints by a factor of 3. Since all four motors areconnected to all three joints, this is a substantial reduction. An-other substantial reduction in friction occurred for the elbow andwrist when switching the cable routing structure �system c�. This

(a)

(b)


WristJoint

ElbowJoint

GuidePulley #1

GuidePulley #2


WristJoint

ElbowJoint

GuidePulley #1

GuidePulley #2

Fig. 9 Planar schematic representation of „a… the alternatecable routing structure of system c and „b… the 2DOF structureof system e. In „a…, there are four cables at the shoulder, threeat the elbow, and two at the wrist. In „b…, there are three cablesat the shoulder, two at the elbow, and zero at the wrist „free torotate….

Table 1 A summary of the peak joint torque that can be ap-plied to the joints in the various configurations of PlanarMEDARM and KINARM

Configuration

Peak torque �Nm�

Shoulder Elbow Wrist

a-original �37 �37 �37b-gear ratio �11.5 �11.5 �11.5c-cable routing �5.75 +5.75 +5.75

�12.5 �2.85d-thicker cable �5.75 +5.75 +5.75

�12.5 �2.85e-2DOF +5.75 +6.15 N/A

�12.5 �5.75f-KINARM �12 �12 N/A



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eduction cut the friction at the wrist in half simply by removingwo cables �including pulleys, belts and motors� entirely from therist drive system. Similarly, the elbow was reduced by a quarterecause one of the four cables was removed. For system e, therist joint friction was not measurable using our approach as thenly possible source of friction is a pair of bearings on the wristoint shaft. It should be noted that moving from system a to sys-em c, the friction was reduced by similar magnitudes at eachoint. However, the reduction in friction of the wrist joint is mosterceptible by the user.

For systems c–e, the friction at the elbow was reduced to a

a - Originalb - Gear Ratio d - Thicker Cable

c - Cable Routing e - 2 DOFf - KINARM

0

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ig. 10 Overall „a… joint friction, „b… inertia as seen by theoints, and „c… compliance at the joints for all variants of therototype. In „a…, the dark and light gray bars for the shoulder

oint indicate the total measured friction with the original andew carriage designs, respectively. In „b…, the dashed lines rep-esent the estimated inertia of limbs corresponding to the mini-um and maximum size of Planar MEDARM. The bottom and

op of the bars indicate the estimated inertia of the exoskeletonhen set up for the smallest and largest arms, respectively. In

c…, the total height of the bars indicates the total measuredompliance of the exoskeleton. The light portion of the barsndicates the estimated contribution of the cables to the mea-ured compliance.

agnitude comparable with KINARM. Similarly, the wrist exhib-



ited low friction. In contrast, the shoulder joint has substantiallyhigher friction. An estimate of the friction in the curved tracksystem indicates that about half of the measured friction �in sys-tems b–e� comes from the curved track and carriage. The tracksystem was designed for heavy-duty industrial use, and the car-riage wheel surfaces are flat to help distribute the forces on thetrack. This design forces the wheels to slide against the tracksurface as they rotate, which increases friction and audible noise.As a subsequent test, a custom carriage that achieved point con-tact with the track was designed to resolve this issue. The newcarriage has reduced strength when compared with the originaldesign, but it remains sufficient for this application. Friction of thenew carriage on the track was reduced by 0.1 N m to 40% of itsoriginal value, and the audible noise level was also reduced. Thisreduction in friction is indicated by the light gray bars in Fig.10�a�. The remaining friction at the shoulder is primarily a resultof the motor system.

Figure 10�b� shows the estimates of the inertia, as seen by thejoints. The length of the bars for each system shows the estimatedrange of inertia for the robot. The bottom of the bar indicates theinertia for the exoskeleton when set up for the smallest arm �cor-responding approximately to a person 1.4 m in height�, and thetop of the bar is for the largest arm �2.0 m in height�. The exosk-eleton can be adjusted anywhere in between this range. Thedashed lines in the plots indicate the estimated inertia of limbscorresponding to the minimum and maximum size of PlanarMEDARM.

It is clear that except for the wrist joint in system a, the inertiaof the robot is similar to the inertia of the human limb. Afterreducing the gear ratio of the system and changing the cable rout-ing structure, the apparent wrist inertia dropped dramatically. Insystems c and d, both the elbow and wrist joint inertias lie withinthe lowest range of the human limb. So, in the worst case, for thesmallest person, the inertia of the robot is roughly equal to theinertia of their limb at the elbow and wrist. However, the shoulderjoint inertia is disproportionately higher than the elbow and wristwhen viewed relative to the human limb, and the changes hadlittle effect. This is a result of the heavy carriage used in thecurved track mechanism. The shoulder inertia of PlanarMEDARM is higher than KINARM, while its elbow inertia ismarginally less.

In-plane joint compliance is shown in Fig. 10�c�. The totalheight of the bars is the measured compliance, while the light grayportion indicates the estimated contribution of the cables to themeasured compliance. It is clear that changing the gear ratio andcable routing scheme both increased the compliance of PlanarMEDARM overall. This occurred because the larger pulley wind-ing up the cables causes more cable length change for a givenrotation, and there are fewer cables attached to the elbow andwrist joints.

In system d, upgrading to the thicker �and stiffer� cable helpedto reduce the compliance, but this reduced only the compliancecontributed by the cables themselves. The large proportion of darkgray in the bars for system d indicates that the large majority ofthe compliance ��90%� comes from something other than thecables. One source of compliance is the timing belts, but the ma-jor source is the structure of the mechanism itself. There are sev-eral points in the system that visibly bend when loads are applied.These structural elements include the main support beam, thewrist joint axis, and the elbow joint axis. The cable compliance insystem d is less than KINARM’s total compliance, so there ispotential to reduce the compliance to comparable levels.

One of the advantages of Planar MEDARM’s design overKINARM is the solid support against gravity that the curved trackprovides. Indeed, the vertical compliance measured for PlanarMEDARM is 0.047 mm/N, while for KINARM it is 0.132 mm/N,as shown in Fig. 11. Planar MEDARM is three times stiffer thanKINARM for out-of-plane motion. It was visually apparent that
most of the measured compliance is a result of the elbow joint
JUNE 2009, Vol. 3 / 021002-9


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ending.The reaching experiments were run primarily as a qualitative

est �no numerical analysis was performed� to see how users per-eived Planar MEDARM during use. It is important that the ex-skeleton feels comfortable and natural for the user. The test waserformed to make sure that users were able to make smooth andtraight reaching movements, which are an indication of whetherr not the exoskeleton is unduly influencing normal motion. Whensked about how it feels to use the robot, subjects noted that it wasomfortable, but some subjects pointed out that there is an audibleoise while moving the shoulder �using original carriage design�.his noise is generated by the carriage on the track, and is a resultf the friction in the system. The new carriage design significantlyeduced the audible noise.

Figure 8�c� shows the basic setup for the reaching task. Figure2�a� shows the recorded hand path for all ten trials to each of theight targets for a single subject. As expected from previouseaching experiments, the results exhibit relatively straight trajec-ories with a certain amount of trial-to-trial variability dependingn movement direction �31�. Figure 12�b� shows a sample of theecorded joint motion for a single trial reaching to target 1. Thisonfirms that all three joints are indeed contributing to limbotion.The maximum velocity feedback gain for each joint is shown in

ig. 13. The first note to make is that using the secondary encod-rs �Fig. 13�b�� allowed equal or higher viscous loading for allases when compared with using the motor encoders �Fig. 13�a��.his is expected because the high-resolution secondary encodersllow a much more smooth �less noisy� measure of velocity thanhe motor encoders. Another expected trend that is readily appar-nt is that the maximum viscous loading gain decreases for con-ecutive configuration changes: systems a-c. This coincides withhe reduction in friction at the joints seen in Fig. 10�a�. Thiseduction in viscous loading capability is particularly noticeable athe wrist joint, because the wrist joint friction was reduced sub-tantially. In addition, the shoulder joint exhibits enough frictionnd stiffness that the maximum torque limits of the motors wereeached before instability was apparent. A final note is that theaximum viscous load increased substantially for system d. The

hicker cable increased the stiffness of the system, making it moreifficult to excite oscillatory motion. In the final 3DOF configu-ation �system d�, the performance of Planar MEDARM was simi-ar to KINARM even when the motor encoder signal was used.sing the secondary encoders increased the performance of theDOF configuration �system e� to a level comparable withINARM.

Conclusions and Future WorkPlanar MEDARM is an upper-limb robotic exoskeleton that is

esigned as a possible revision for KINARM. The prototype isased on a novel mechanical design that makes use of a cable-

0

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Planar MEDARM

VerticalCompliance(mm/N)

KINARM

ig. 11 Results of the vertical compliance test, comparing Pla-ar MEDARM with KINARM

riven curved track mechanism which allows all equipment to be

21002-10 / Vol. 3, JUNE 2009


placed underneath the user’s arm. This mechanical design is in-valuable from a clinical perspective because it can accommodate awider range of people, including small children, while providinggreater access to the limb by the clinician. Additionally, PlanarMEDARM includes an actuated wrist joint, which enables thedevice to answer new scientific questions about upper-limb move-ment. Performance testing indicates that Planar MEDARM per-forms as well as KINARM in most respects, despite the addedfunctionality and new mechanical design ideas. All tested configu-rations exceed the specified minimum required torque for eachjoint. While the final configuration of Planar MEDARM has lowerpeak torque capability than KINARM, the motors could easily beupgraded to provide more torque.

Friction and inertia, as seen at the elbow for Planar MEDARM,are comparable to KINARM, and these same parameters for thewrist are equally acceptable. However, Planar MEDARM’s shoul-der joint lags behind. The main area for improvement lies in thecarriage, which is the main source of friction, inertia, and audiblenoise for the shoulder joint, but these shortcomings should not beinsurmountable. The custom carriage design was a substantial firststep to improving the performance of the carriage. Friction wasreduced by 60%, and audible noise was also significantly reduced.The new carriage design did not address the issue of inertia. Highinertia at the shoulder is a direct result of the mass of both thecarriage and the guide pulleys on the driving linkage. With all ofthis weight displaced far from the center of rotation, it is notsurprising that the inertia is high. Removing one of the carriage’sfour wheels �the most massive components�, combining the car-riage and upper-arm linkages into a single piece, and usingsmaller guide pulleys would reduce the shoulder inertia by at least

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Fig. 12 Results of a basic reaching experiment using the Pla-nar MEDARM. „a… The hand path traces for all trials are plotted.„b… The joint angle profile for one of the trials to target 1.

25% bringing the inertia much closer to proportion with the other



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oints and with KINARM. This reduction would not affect thether joints. Also, it is important to note that about 40% of thenertia, as seen at the shoulder, is from the addition of the thirdink and all of the pulleys needed to actuate the wrist joint. Con-idering that Planar MEDARM was designed specifically to add arist joint and to place the support point for the mechanism away

rom the joint axis, it is a substantial achievement to maintainoint inertia on the same level as KINARM. While the inertia canasily be reduced, reducing friction further may be more challeng-ng. Removing one wheel from the carriage will reduce the bear-ng friction by 25%, but this would reduce the friction at thehoulder by only about 10–15% overall.

Planar MEDARM’s in-plane compliance is higher thanINARM for all joints. Nevertheless, it is important to note that

he overall compliance of the system is not dominated by theables that drive the system. This means that the compliance cane reduced relatively easily by stiffening a few key structuralomponents, and therefore the high compliance is not due to aimitation of the conceptual design. While already significantlyess than KINARM, the vertical compliance will be further re-uced by these structural upgrades. Once the structural upgradesre implemented, beam bending is reduced, and the remainingompliance will be dominated by the cables and the timing beltssed in the motor system. In this situation, the main problemsssociated with high in-plane compliance are diminished whenecondary encoders are added to the joints because the true jointngles are known. In the case of Planar MEDARM, the secondaryncoders also remove the need to perform some of the four-barinkage calculations during operation. Furthermore, the secondaryncoders increased the stability of the robot while applying

a - Originalb - Gear Ratio d - Thicker Cable

c - Cable Routing e - 2 DOFf - KINARM

Shoulder

VelocityGain(Nm•s/rad)

VelocityGain(Nm•s/rad)

Elbow Wrist

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(b)




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ig. 13 Results of the viscous load stability tests, comparinglanar MEDARM with KINARM. The highest velocity feedbackain that can be applied to each joint for each configurationsing the velocity signal provided by „a… the motor encodersnd „b… the secondary encoders.

eedback-based loads to the joints. With a stiffer structure, the



stability could be further improved. Although these encoders areexpensive, the ability to include them presents a useful advantagein measuring joint kinematics.

Planar MEDARM has successfully achieved its goal to incor-porate a wrist joint into the system. It appears from the initialreaching movement tests that Planar MEDARM does not affectlimb motion in an adverse way. Furthermore, its performancecompares favorably with KINARM, and any problems appear tohave feasible solutions. As well as providing a means of extendingthe capabilities of KINARM, the goal of this prototype was to testthe feasibility of using the curved track design to replicate motionat the shoulder girdle. The sternoclavicular joint of the shouldergirdle is much more challenging to actuate than the glenohumeraljoint of the shoulder because it is centrally located on the body,and even closer to the head. The curved track system would allowthis joint to be actuated from a point offset from the joint itself.The design process is nearing completion for a new 6DOF roboticexoskeleton called MEDARM that will provide independent con-trol of all five major DOFs of the shoulder �including the shouldergirdle�, and 1DOF at the elbow �32,33�. The Planar MEDARMprototype has already proved an invaluable insight into the futuredevelopment of KINARM, and the results presented in this papercan also be transferred directly to the MEDARM design.

AcknowledgmentThe authors would like to thank Luke Harris for machining

parts for Planar MEDARM. His experience and contributions tothe design process had a significant influence on this work.

This work was supported by the Canadian Institutes of HealthResearch �CIHR�, and the Natural Sciences and Engineering Re-search Council of Canada �NSERC�.

Nomenclature�i � torque applied to joint il � length of linkage i

�i � angular position of joint i

�̇i � angular velocity of joint isi � change in length of cable i

��i � change in angular position of joint iri � radius of pulley at joint i�i � force in cable i

Kv � velocity feedback gain for viscous load

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Dynamics During Learning of a Motor Task,” J. Neurosci., 14�5�, pp. 3208–3224.

�2� Gomi, H. and Kawato, M., 1996, “Equilibrium-Point Control Hypothesis Ex-amined by Measured Arm Stiffness During Multijoint Movement,” Science,272�5258�, pp. 117–120.

�3� Nozaki, D., Kurtzer, I., and Scott, S. H., 2006, “Limited Transfer of LearningBetween Unimanual and Bimanual Skills Within the Same Limb,” Nat. Neu-rosci., 9�11�, pp. 1364–1366.

�4� Tcheang, L., Bays, P. M., Ingram, J. N., and Wolpert, D. M., 2007, “Simulta-neous Bimanual Dynamics Are Learned Without Interference,” Exp. BrainRes., 183�1�, pp. 17–25.

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�6� Reinkensmeyer, D. J., Emken, J. L., and Cramer, S. C., 2004, “Robotics,Motor Learning, and Neurologic Recovery,” Annu. Rev. Biomed. Eng., 6, pp.497–525.

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�9� Sanchez, R., Reinkensmeyer, D., Shah, P., Liu, J., Rao, S., Smith, R., Cramer,S., Rahman, T., and Bobrow, J., 2004, “Monitoring Functional Arm Movementfor Home-Based Therapy After Stroke,” Proceedings of the IEEE InternationalConference on Engineering in Medicine and Biology Society �EMBC‘04�, SanFrancisco, CA, Vol. 2, pp. 4787–4790.

�10� Mihelj, M., Nef, T., and Riener, R., 2007, “ARMin II—7 DOF Rehabilitation

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Robot: Mechanics and Kinematics,” Proceedings of the IEEE InternationalConference on Robotics and Automation �ICRA‘07�, Rome, Italy, pp. 4120–4125.

�11� Scott, S. H., 1999, “Apparatus for Measuring and Perturbing Shoulder andElbow Joint Positions and Torques During Reaching,” J. Neurosci. Methods,89�2�, pp. 119–127.

�12� Singh, K. and Scott, S. H., 2003, “A Motor Learning Strategy Reflects NeuralCircuitry for Limb Control,” Nat. Neurosci., 6�4�, pp. 399–403.

�13� Debicki, D. B. and Gribble, P. L., 2004, “Inter-Joint Coupling Strategy DuringAdaptation to Novel Viscous Loads in Human Arm Movement,” J. Neuro-physiol., 92�2�, pp. 754–765.

�14� Kurtzer, I. L., Pruszynski, J. A., and Scott, S. H., 2008, “Long-Latency Re-flexes of the Human Arm Reflect an Internal Model of Limb Dynamics,” Curr.Biol., 18�6�, pp. 449–453.

�15� Gribble, P. L. and Scott, S. H., 2002, “Overlap of Internal Models in MotorCortex for Mechanical Loads During Reaching,” Nature �London�, 417�6892�,pp. 938–941.

�16� Kurtzer, I., Herter, T. M., and Scott, S. H., 2005, “Random Change in CorticalLoad Representation Suggests Distinct Control of Posture and Movement,”Nat. Neurosci., 8�4�, pp. 498–504.

�17� Hatsopoulos, N. G., Xu, Q., and Amit, Y., 2007, “Encoding of MovementFragments in the Motor Cortex,” J. Neurosci., 27�19�, pp. 5105–5114.

�18� Shelton, F. N. and Reding, M. J., 2001, “Effect of Lesion Location on UpperLimb Motor Recovery After Stroke,” Stroke, 32�1�, pp. 107–112.

�19� Jung, H. Y., Yoon, J. S., and Park, B. S., 2002, “Recovery of Proximal andDistal Arm Weakness in the Ipsilateral Upper Limb After Stroke,” NeuroRe-habilitation, 17�2�, pp. 153–159.

�20� Kwakkel, G., Kollen, B., and Twisk, J., 2006, “Impact of Time on Improve-ment of Outcome After Stroke,” Stroke, 37�9�, pp. 2348–2353.

�21� Ball, S. J., Brown, I. E., and Scott, S. H., 2007, “A Planar 3DOF RoboticExoskeleton for Rehabilitation and Assessment,” Proceedings of the IEEE In-ternational Conference on Engineering in Medicine and Biology Society
�EMBC‘07�, Lyon, France, pp. 4024–4027.
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