unit ii solved question bank - robotics engineering -

VMKV ENGINEERING COLLEGE, SALEM

AERONAUTICAL ENGINEERING COURSE

ELECTIVE – ROBOTICS ENGINEERING – QUESTION BANK 2012 REGULATION

UNIT II - END EFFECTORS AND CONTROL SYSTEMS

End effectors - classification - mechanical, magnetic, vacuum and adhesive gripper - gripper force analysis and

design. Robot control - unit control system concept - servo and non-servo control of robot joints, adaptive and

optimal control.

PART A

1. What is an end effector?

Ans. End effector is a device that is attached to the end of the wrist arm to perform specific task.

2. Classify end effectors with examples.

Ans. Gripper, Tools, Welding equipments, End of arm Tooling (EOAT)

3. What do you mean by gripper?

Ans. Gripper is the End effector which can hold or grasp the object.

4. Differentiate between internal grippers and external grippers.

Ans. In internal grippers, the finger pads are mounted on the inside of the fingers. This mounting

allows the pads to fit into the inside diameter of the part it must lift. The pads are pressed against the inside

wall of the part.

An external gripper is designed so that the finger pads press against the outside of the

component. Grips the exterior surface of the objects with closed fingers.

5. Define base and tool Coordinate system.

Ans. A tool coordinates definition system capable of easily obtaining a transformation matrix for

defining a tool coordinates system of a robot. The tool coordinates system at the 0° position of the

robot is rotated around each axis so that the tool coordinates system becomes parallel to a basecoordinates

system.

6. What are the types of Mechanical Grippers?

Ans. (i) Linkage actuation gripper (ii) Gear and rack actuation gripper

(iii) Cam actuated gripper (iv) Screw actuated gripper

7. List any two limitations of magnetic grippers.

Ans. (i) Residual magnetism (ii) Side slippage

(iii) More than one sheet will be lifted by the magnet from a stack

8. Give some examples of tool as robot End effector.

Ans. (i) Spot Welding Tools (ii) Arc welding Torch (iii) Spray painting nozzle (iv) Water jet cutting tool

9. List any four important factors to be considered in the selection and design of grippers.

Ans. (i) The gripper must have the ability to reach the surface of a work part.

(ii) The change in work part size must be accounted for providing accurate positioning.

(iii) During machining operations, there will be a change in the work part size. As a result, the

gripper must be designed to hold a work part even when the size is varied.

(iv) The gripper must not create any sort of distort and scratch in the fragile work parts.

10. List out the types of Drive systems used in Robots.

Ans. (i) Electric motors like: Servomotors, Stepper motors

(ii) Hydraulic actuators

(iii) Pneumatic actuators

11. What is a rotation matrix? Write basic rotation matrices.

12. Name some feedback devices used in robotics.

Ans. (i) Potentiometer (ii) Resolver (iii) Encoder

13. Write the characteristics of actuating systems.

Ans. (i) Weight (ii) Power-to-weight ratio (iii) Operating Pressure (iv) Stiffness Vs. Compliance

14. List any two unique features of a stepper motor.

Ans. (i) Moves in known angle of rotation. (ii) Position feedback is not necessary.

(iii) Rotation of the shaft by rotation of the magnetic field.

15. What are the types of encoders?

Ans. (a) Linear encoder

(b) Rotary encoder

(i) Absolute encoder

(ii) Incremental encoder

16. What do you mean by a composite rotation matrix?

Ans. Basic rotation matrices can be multiplied together to represent a sequence of finite rotations about

the principal axes of the OXYZ coordinate systems to obtain a resultant composite 3 X 3 rotation matrix.

17. State the Euler angles used in rotation matrix.

Ans. φ, θ, and ψ are the Euler angles used in rotation matrix in described in Eulerian three systems.

18. Write basic homogenous rotation matrix.

19. Write basic homogenous translation matrix.

20. State Denavit – Hartenberg parameter.

Ans. To describe the translational and rotational relationships between adjacent links, Denavit and

Hartenberg (1955) proposed a matrix method of systematically establishing a coordinate system (body

attached frame) to each link of an articulated chain. The D-H representation results in a 4 X 4 homogenous

transformation matrix representing each link’s coordinate system at the joint with respect to the previous

link’s coordinate system.

PART B

1. Describe five types of mechanical joints for robots with neat sketches.

2. Describe five common body-and-arm configurations with neat sketches.

Ans. Common Body-and-Arm configurations

3. Describe PUMA 560 manipulator and obtain link transformation matrix from D-H table.

4. Describe in detail the drive systems used to actuate robotic joints.

Ans. One common method of classifying robots is the type of drive required by the actuators.

• Electrical actuators use electric power.

• Pneumatic actuators use pneumatic (air) power.

• Hydraulic actuators use hydraulic (fluid) power.

Electric Drive

Three types of motors are commonly used for electric actuator drives:

AC servo motors, DC servo motors, and Stepper Motors.

Both AC and DC servomotors have built-in methods for controlling exact position. Many newer

robots use servo motors rather than hydraulic or pneumatic ones. Small and medium-size robots commonly

use DC servo motors. Because of their high torque capabilities, AC servo motors are found in heavy-duty

robots.

A stepper motor is an incrementally controlled dc motor. Stepper motors are rarely used in

commercial industrial robots, but are commonly found in educational robots.

Conventional, electric-drive motors are quiet, simple, and can be used in clean-air environments.

Robots that use electric actuator drives require less floor space, and their energy source is readily available.

However, the conventionally geared drive causes problems of backlash, friction, compliance, and wear.

These problems cause inaccuracy, poor dynamic response, need for regular maintenance, poor torque

control capability, and limited maximum speed on longer moves. Loads that are heavy enough to stall

(stop) the motor can cause damage.

Conventional electric-drive motors also have poor output power compared to their weight. This

means that a larger, heavier motor must be mounted on the robot arm when a large amount of torque is

needed.

Hydraulic Drive

Many earlier robots were driven by hydraulic actuator drives. A hydraulic drive system uses fluid

and consists of a pump connected to a reservoir tank, control valves, and a hydraulic actuator. Hydraulic

drive systems provide both linear and rotary motion using a much simpler arrangement than conventional

electric-drive systems. The storage tank supplies a large amount of instant power, which is not available

from electric-drive systems.

Hydraulic actuator drives have several advantages. They provide precise motion control over a wide

range of speeds. They can handle heavy loads on the end of the manipulator arm, can be used around

highly explosive materials, and are not easily damaged when quickly stopped while carrying a heavy load.

However, they are expensive to purchase and maintain and are not energy efficient.

Hydraulic actuator drivers are also noisier than electric-drive actuators and are not recommended

for clean-room environments due to the possibility of hydraulic fluid leaks.

Pneumatic Drive

Pneumatic drive systems make use of air-driven actuators. Since air is also a fluid, many of the

same principles that apply to hydraulic systems are applicable to pneumatic systems. Pneumatic and

hydraulic motors and cylinders are very similar. Since most industrial plants have a compressed air system

running throughout assembly areas, air is an economical and readily available energy source. This makes

the installation of robots that use pneumatic actuator drives easier and less costly than that of hydraulic

robots. For lightweight pick-and-place applications that require both speed and accuracy, a pneumatic robot

is potentially a good choice.

Pneumatic actuator drives work at high speeds and are most useful for small-to-medium loads.

They are economical to operate and maintain and can be used in explosive atmospheres. However, since air

is compressible, precise placement and positioning require additional components to achieve the smooth

control possible with a hydraulic system.

It is also difficult to keep the air as clean and dry as the control system requires. Robots that use

pneumatic actuator drives are noisy and vibrate as the air cylinders and motors stop.

5. Describe in detail the different types of possible control for joint movements.

Ans. Types of possible control for joint movements are as follows:

6. (i)Derive a 3 X 3 rotation matrixRx,αabout the OX axis with ‘α’ angle.Given two points auvw= (4,3,2)T

buvw= (6,2,4)T

with respect to therotated OUVWcoordinate system, determine the corresponding points axyz,

bxyzwith respect to the reference coordinate system if it has been rotated 600 about the OZ axis.

(ii)Derive a 3 X 3 rotation matrix Rx,αabout the OY axis with ‘φ’ angle. axyz= (4,3,2)T

and bxyz= (6,2,4)T

are the coordinates with respect to the reference coordinate system, determine the corresponding points

auvwand buvwwith respect to therotated OUVWcoordinate system if it has been rotated 600 about the OZ

7. Describe SCARA manipulator and obtain link transformation matrix from D-H table.

8. Derive a 3 X 3 rotation matrix Rφ,θ,Ψ with Euler angles representation.If the OU, OV, andOWcoordinate

axes were rotated with ‘α’ angle about the OX axis, what would the representation of the coordinate axes of

the reference frame be in terms of the rotated coordinate system OUVW.

Ans. Express Rφ,θ,Ψ with respect to System I and System II as derived in class.

9. Determine a transformation ‘T’ matrix that represents a rotation of ‘α’ angle about the OX axis, followed

by a translation of ‘b’ units along the rotated OV axis.

10. Find a homogenous transformation ‘T’ matrix that represents a rotation of ‘α’ angle about the OX axis,

followed by a translation of ‘a’ units along the OX axis, followed by a translation of ‘d’ units along the OZ

axis, followed by a rotation of ‘θ’ angle about the OZ axis.

unit ii solved question bank - robotics engineering -

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