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

March 27, 2001CMPUT Jagersand2 Transmission? Transmission is defined as the the mechanical hardware which transforms an actuator into a contact force against the environment

March 27, 2001CMPUT Jagersand3 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

March 27, 2001CMPUT Jagersand4 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

March 27, 2001CMPUT Jagersand5 Problems Implementing Force Control Disturbances between the sensor and the environment are not attenuated. Stability is difficult to maintain. Dry friction

March 27, 2001CMPUT Jagersand6 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

March 27, 2001CMPUT Jagersand7 Requisites for Good Performance Large dynamic range of force controllability Robustness High Bandwidth High-aspect-ratio links High efficiency Good backdrivability

March 27, 2001CMPUT Jagersand8 Spectrum of tasks based on certainty Two extremes Hard-automation devices Unfamiliar environments

March 27, 2001CMPUT Jagersand9 Whole Arm Manipulation (WAM) Designed to contact and interact with the environment using any of its surfaces Examples…

March 27, 2001CMPUT Jagersand10 Large Dynamic Range of Force Controllability Definition: maximum controllable force divided by the minimum controllable force (strength over accuracy)

March 27, 2001CMPUT Jagersand11 Robustness Definition: The ability of the manipulator to perform its tasks reliably and without suffering damage Dynamically Stable Mechanically Durable

March 27, 2001CMPUT Jagersand12 High Bandwidth High bandwidth of force and position control is important in manipulators used for assembly, where the cycle time of tasks is critical

March 27, 2001CMPUT Jagersand13 High Aspect Ratio Definition: The length divided by the width of the link Increasing the aspect ratio increases the unobstructed workspace

March 27, 2001CMPUT Jagersand14 High Efficiency Definition: The output power at the joint divided by the input power at the motor shaft

March 27, 2001CMPUT Jagersand15 Good Backdrivability Acceleration dependent Velocity dependent Good backdrivability causes the manipulator to behave desirably without dependence on closed-loop control

March 27, 2001CMPUT Jagersand16 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.

March 27, 2001CMPUT Jagersand17 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

March 27, 2001CMPUT Jagersand18 The Speed-Reducer Included in the transmission to boost the actuator torque capacity.

March 27, 2001CMPUT Jagersand19 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?

March 27, 2001CMPUT Jagersand20 Bandwidth Design Guidelines Best to locate and speed reduction at a joint rather than at the motor.

March 27, 2001CMPUT Jagersand21 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…

March 27, 2001CMPUT Jagersand22 Friction Design Guidelines Coulomb friction is bad and can be limited by limiting G I using the Routh Criteria (as most of us geers learned in our undergrad controls courses) Most friction can also be limited through material selection.

March 27, 2001CMPUT Jagersand23 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

March 27, 2001CMPUT Jagersand24 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

March 27, 2001CMPUT Jagersand25 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?