Induced Master Motion in Force-Reflecting Teleoperation

[+] Author and Article Information
Katherine J. Kuchenbecker

Telerobotics Lab, Mechanical Engineering Department, Stanford University, Stanford, CA 94309katherin@stanfordalumni.org

Günter Niemeyer

Telerobotics Lab, Mechanical Engineering Department, Stanford University, Stanford, CA 94309gunter.niemeyer@stanford.edu

J. Dyn. Sys., Meas., Control 128(4), 800-810 (Apr 01, 2006) (11 pages) doi:10.1115/1.2364011 History: Received October 19, 2004; Revised April 01, 2006

Telerobotic systems have persistently struggled to provide users with realistic force feedback; high-frequency contact transients convey important information about the remote environment but are typically attenuated to avoid the contact instability they incite. This undesirable behavior can be traced to high-frequency induced master motion, movement of the master device that is caused by force feedback rather than user intention. Such motion is interpreted as a position command to the slave, closing an internal control loop that is unstable under high gain. This paper examines the phenomenon of induced master motion in position-force teleoperation, presenting a new approach for achieving stable, high-gain force reflection using model-based cancellation. Requirements for the model of the induced motion dynamics and methods for its characterization are described, focusing on successive isolation of inertial and connecting elements. The sixth-order nonlinear model obtained for a one-degree-of-freedom user-master system is validated and then tested in a cancellation controller. Canceling high-frequency induced master motion during teleoperation is shown to improve the stability of impacts, allowing significantly higher force reflection levels and a more authentic user experience.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

A telerobotic system connects the user to the environment via master and slave robots

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Figure 2

The master’s long dynamic chain connects the user to the controller

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Figure 3

In position-force control, the slave is commanded to track the master’s position, and the master mechanism recreates tip interaction forces for the user

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Figure 4

Display of a scaled, prerecorded force profile to a user executing a constant motion with a 1-dof master. Increasing the force scale λ generates motor movement that is not intended by the user.

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Figure 5

Motion induced by the force feedback xmi superimposes with the user’s voluntary motion xmv at the master motor. The electrical, mechanical, and biomechanical dynamics of the user-master system Gi govern the pathway of induced master motion, and the user’s cognitive processes Gv determine the voluntary motion.

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Figure 6

Position-force control with three compensation options: local derivative feedback on master position via b(d∕dt), position command filter Kμ, and force feedback filter Kλ

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Figure 7

High-frequency induced master motion can be canceled using the model Ĝi,hf

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Figure 8

Successive isolation of the user-master system progresses from the force command through the motor, cables, drum, linkage, handle, and user

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Figure 9

Single-axis position-force telerobotic testbed

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Figure 10

Nonlinear lumped-parameter model of the user-master system. The κ parameters signify nonlinear stiffness relationships, and the C parameters indicate Coulomb friction.

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Figure 11

When held by a user, the system exhibits a well-damped resonance at 70Hz, in which the handle vibrates against the flesh of the user’s hand

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Figure 12

The full model accurately predicts induced master motion during open-loop display of a prerecorded force profile, providing a smooth estimate of user intention, x̂mv

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Figure 13

Force feedback Ff and slave position command xc with and without cancellation for a range of λ values, keeping μ=1. Contacts are stabilized by canceling induced master motion from the measured master position xm, using the model’s real-time estimate x̂mi.

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Figure 14

Cancellation of induced master motion is implemented via C5 in the general architecture for teleoperation




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