The Effect of Transmission Design on Force Controlled Manipulator Performance William T. Townsend Presented by: Sean Verret.
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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
March 27, 2001 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
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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?
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