A Generalized-Compliant-Motion Primitive

A computer program that bridges the gap between planning and execution of compliant robotic motions has been developed and installed in the control system of a telerobot at NASA's Jet Propulsion Laboratory. The program, called the "generalized-compliant-motion primitive," is one of several task-execution-primitive computer programs, which receive commands from higher-level task-planning systems and execute those commands by generating the required trajectories and applying the appropriate control laws.

The program comprises four parts that correspond to nominal motion, compliant motion, ending motion, and monitoring, respectively. Nominal motion is motion of the robot along the specified trajectory. Compliant motion is perturbation of the nominal motion due to sensory inputs, e.g., force control, teleoperation, and joint and Cartesian space virtual forces. Ending motion begins at the end of nominal motion and continues during a specified time or until specified ending conditions (e.g., attainment of a desired position and contact force) are satisfied. Monitoring involves verifying that the commanded task or motion is being executed safely and stopping the motion if an anomalous condition (e.g., excessive contact force or motion beyond a limit position) is detected.

Commands from higher-level systems are received in the form of parameters: the generalized-compliant-motion-primitive computer program provides a rich set of such input parameters, which specify the details of a variety of compliant-motion tasks - for example, opening a door, turning a crank (see figure), seating and turning a bolt, pushing, sliding, leveling, and insertion or removal of a pin. The parameterization scheme provides for general compliant motion, nominal motion, force control, and sensing; separate nominal-motion, compliant-motion, and sensor reference frames for control and safety monitoring; combined position and force control in all six degrees of freedom of the control reference frame; and testing for, and responses to, ending conditions.

The program implements a split-rate force-control technique. Force and torque data are read from a six-axis force sensor. Contact forces are computed by subtracting, from sensed forces, the contributions of the weight of the manipulated object in the given robot pose. The program is written in the C language, with utilities from the RCCL robot prog ramming and control software system.

More details can be found in:

Paul G. Backes. "Generalized Compliant Motion With Sensor Fusion ." In Proceedings 1991 ICAR: Fifth International Conference on Advanced Robotics, Robots in Unstructured Environments, pages 1281-1286, Pisa, Italy, June 19-22, 1991.

Point of Contact:
Paul G. Backes,
Mail Stop 198-219
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109
818-354-3850
Paul.G.Backes@jpl.nasa.gov

Telerobotics Program page

Please email the site webmaster with any comments, criticisms or corrections for this page.
Maintained by: Dave Lavery
Last updated: May 10, 1996