Active Compliance and Damping in Telemanipulator ControlNASA's Jet Propulsion Laboratory, Pasadena, California
An experimental telemanipulator system of the force-reflecting-hand-controller type provides for active compliance and damping in the remote, robotic manipulator hand. Active compliance can help to soften collisions between the manipulator and objects. The compliance parameters can be adjusted to suit the task at hand. In addition, active compliance becomes essential to successful telemanipulator control when the communication delay between the master- control and remote-manipulator stations exceeds 1 s. In a typical current industrial force-reflecting telemanipulator, the force of co ntact between the robot hand and an object is, strictly speaking, neither sensed nor controlled. Instead, the remote manipulator is activated according to hand position commands, while the contact force fed back through the hand controller to the human operator is made to increase with the feedback signal that indicates the error between the actual and the commanded position of the manipulator. In such a system, the manipulator hand is prone to hard bumps against objects, and small errors in the execution of position commands can give rise to undesired large contact forces and torques. Such errors, forces, and torques increase with the communication delay; in practice, they render conventional (noncomp!iant) force-reflecting hand control unusable at delays greater than about 1 s. In the experimental telemanipulator system, the control task is shared by subsystems in both the control station and the remote-manipulator station. The robot hand is equipped with force and torque sensors. A control subsystem at the remote-manipulator station implements active complianceQin effect, it simulates springs and shock absorbers by lowpass filtering the outputs of the force and torque sensors and using these signals to alter the position commands.
The active-compliance feature is semi- autonomous, in that it resides in the remotemanipulator station but can be programmed from time to time by commands from, the human operator at the master control station. From a software menu, the operator can select the force-feedback gains and the damping time constraints of the hybrid position/force-feedback subsystem to obtain the desired spring stiffness (or its reciprocal, compliance) and damping. The experimental telemanipulator system provides for various combinatiors of conventional force-reflecting hand control and/or active compliance. It accommodates a spectrum of operations that range from full manual through shared manual and automatic (in the sense of remote autonomous) to full automatic control. Shared manual and automatic control is Implemented by freezing the output of the master controller in some task-space coordinates that are selectable by the operator from a menu. Motion about the set point in the frozen task-space coordinates can then be controlled by a computer algorithm that can be referenced to force-, moment-, or proximity-sensor information. Control by active compliance alone (that is, without force reflection), can be extremely useful when the communication delay is long. This was demonstrated by using the experimental telemanipulator system to insert a peg in a hole. The task proved impossible without active compliance when the delay exceeded 0.5 s. With active compliance but no delay, it took almost a minute. With a delay of 4 s, it took 3 min. With a delay of 8 s, it took 7 min.
Point of Contact:
Antal K. Bejczy
Mail Stop 198-219
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena CA 91109
818-354-4568
bejczy@telerobotics.jpl.nasa.gov
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Last updated: May 10, 1996