The Effect of Transmission Design on Force Controlled Manipulator Performance

1. The Effect of Transmission Design on Force Controlled Manipulator Performance William T. Townsend Presented by: Sean Verret

2. Transmission? • Transmission is defined as the the mechanical hardware which transforms an actuator into a contact force against the environment CMPUT 610 - Jagersand

3. Goal of his paper? • To identify a set of mechanical design requirements for force-controlled manipulators • To analyze the effect of transmission properties such as stiffness, transmission ratio and nonlinear friction disturbances • To propose transmission-design strategies for improving performance CMPUT 610 - Jagersand

4. Quick intro to control schemes • No Force Feedback • Where explicit force measurement is absent the force control is considered to be “open-loop” control • Force Feedback • Where manipulators measure forces by measuring the strain of some mechanical element in the system or environment . This usage is called “closed-loop” control CMPUT 610 - Jagersand

5. Problems Implementing Force Control • Disturbances between the sensor and the environment are not attenuated. • Stability is difficult to maintain. • Dry friction CMPUT 610 - Jagersand

6. Examples of Teleoperators and Manipulators Designed for Good Force Control • French MA-22 • French MA-23 • JPL Universal Reflecting Hand Controller • Asada Direct Drive Arm CMPUT 610 - Jagersand

7. Requisites for Good Performance • Large dynamic range of force controllability • Robustness • High Bandwidth • High-aspect-ratio links • High efficiency • Good backdrivability CMPUT 610 - Jagersand

8. Spectrum of tasks based on certainty • Two extremes • Hard-automation devices • Unfamiliar environments CMPUT 610 - Jagersand

9. Whole Arm Manipulation (WAM) • Designed to contact and interact with the environment using any of its surfaces • Examples… CMPUT 610 - Jagersand

10. Large Dynamic Range of Force Controllability • Definition: maximum controllable force divided by the minimum controllable force (strength over accuracy) CMPUT 610 - Jagersand

11. Robustness • Definition: The ability of the manipulator to perform its tasks reliably and without suffering damage • Dynamically Stable • Mechanically Durable CMPUT 610 - Jagersand

12. High Bandwidth • High bandwidth of force and position control is important in manipulators used for assembly, where the cycle time of tasks is critical CMPUT 610 - Jagersand

13. High Aspect Ratio • Definition: The length divided by the width of the link • Increasing the aspect ratio increases the unobstructed workspace CMPUT 610 - Jagersand

14. High Efficiency • Definition: The output power at the joint divided by the input power at the motor shaft CMPUT 610 - Jagersand

15. Good Backdrivability • Acceleration dependent • Velocity dependent • Good backdrivability causes the manipulator to behave desirably without dependence on closed-loop control CMPUT 610 - Jagersand

16. The effect of transmission design on bandwidth • A high performance manipulator must respond quickly to both commands and disturbances, therefore its bandwidth must be large. CMPUT 610 - Jagersand

17. Open- and Closed-Loop Bandwidth • Open-loop: measure of the quickness with which the transmission mechanism communicates position and force from the actuators to the output • Closed-loop: open-loop system with explicit force feedback CMPUT 610 - Jagersand

18. The Speed-Reducer • Included in the transmission to boost the actuator torque capacity. CMPUT 610 - Jagersand

19. Open-Loop Position Bandwidth • When speed reduction is used in a manipulator transmission, most of the time the reducer is placed at the motor rather than a joint. • How does this affect position bandwidth? CMPUT 610 - Jagersand

20. Bandwidth Design Guidelines • Best to locate and speed reduction at a joint rather than at the motor. CMPUT 610 - Jagersand

21. The Effect of Dry Friction and Compliance • Friction is present in most mechanisms • Commonly modeled as linear-viscous damping, Coulomb friction, stiction or a combination… CMPUT 610 - Jagersand

22. Friction Design Guidelines • Coulomb friction is bad and can be limited by limiting GI using the Routh Criteria (as most of us geers learned in our undergrad controls courses) • Most friction can also be limited through material selection. CMPUT 610 - Jagersand

23. The Efficiency Limit of Tension-Element Drives • Belt and cable drives cannot approach perfect efficiency • Chain sprocket drives, however, may not be limited to this efficiency CMPUT 610 - Jagersand

24. Transmission Design Guidelines • Reduction of tension difference by using high speed elements • Minimize the number of transition stages • Position and velocity estimators to improve accuracy CMPUT 610 - Jagersand

25. Conclusions • This paper basically went through the best way to design an armed robot in order to maximize its efficiency and effectiveness • Now how can we use all of this information to model a human arm? CMPUT 610 - Jagersand