The Effect of Transmission Design on Force Controlled Manipulator

Performance

William T. Townsend

Presented by: Sean Verret

March 27, 2001 CMPUT 610 - Jagersand 2

Transmission?

Transmission is defined as the the mechanical hardware which transforms an actuator into a contact force against the environment

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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

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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

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Problems Implementing Force Control

Disturbances between the sensor and the environment are not attenuated.

Stability is difficult to maintain.

Dry friction

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Examples of Teleoperators and Manipulators Designed for Good Force ControlFrench MA-22

French MA-23

JPL Universal Reflecting Hand Controller

Asada Direct Drive Arm

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Requisites for Good Performance

Large dynamic range of force controllability

Robustness

High Bandwidth

High-aspect-ratio links

High efficiency

Good backdrivability

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Spectrum of tasks based on certainty

Two extremes Hard-automation devices Unfamiliar environments

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Whole Arm Manipulation (WAM)

Designed to contact and interact with the environment using any of its surfaces

Examples…

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Large Dynamic Range of Force Controllability

Definition: maximum controllable force divided by the minimum controllable force (strength over accuracy)

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Robustness

Definition: The ability of the manipulator to perform its tasks reliably and without suffering damage

Dynamically Stable

Mechanically Durable

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High Bandwidth

High bandwidth of force and position control is important in manipulators used for assembly, where the cycle time of tasks is critical

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High Aspect Ratio

Definition: The length divided by the width of the link

Increasing the aspect ratio increases the unobstructed workspace

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High Efficiency

Definition: The output power at the joint divided by the input power at the motor shaft

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Good Backdrivability

Acceleration dependent

Velocity dependent

Good backdrivability causes the manipulator to behave desirably without dependence on closed-loop control

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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.

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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

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The Speed-Reducer

Included in the transmission to boost the actuator torque capacity.

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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?

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Bandwidth Design Guidelines

Best to locate and speed reduction at a joint rather than at the motor.

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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…

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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.

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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

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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

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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?