Coordinated Control of Mobile Robotic Manipulators

A computationally efficient scheme has been developed for on-line coordinated control of both manipulation and mobility of robots that include manipulator arms mounted on mobile bases. The scheme is applicable to a variety of mobile robotic manipulators, including robots that move along tracks (typically, painting and welding robots), robots mounted on gantries and capable of moving in all three dimensions, wheeled robots (see figure), and compound robots (consisting of robots mounted on other robots).

In the past, robots were typically mounted on stationary bases bolted to floors so that they could withstand the forces and torques applied to the bases when the robot arms carried payloads. However, there are significant advantages to placing robot arms on mobile bases. The mobility of a base extends the reach of the manipulator arm and increases the size of the robot workspace substantially at minimal cost. The additional movement of the base complicates the robot-control problem, but the availability of low-cost, high-performance computers makes it possible to achieve real-time control with computationally efficient algorithms like those of the present control scheme.

In some prior approaches taken in the development of control schemes, the additional degrees of freedom in the mobility of the base would have been regarded unfavorably because of the additional complexity that they introduce. In the present approach, the additional degrees of freedom are turned to advantage by using them to accomplish additional tasks specified by the user. Furthermore, the on-line nature of the present method is enhanced by the ability of the user to change the specifications of tasks during operation, as described below.

The theoretical basis of the present method is the configuration-control formalism, which was discussed in several prior articles in NASA Tech Briefs including "Increasing the Dexterity of Redundant Robots" (NPO17801), Vol. 14, No. 10 (October, 1990), page 88; "Redundant Robot Can Avoid Obstacles" (NPO-17852), Vol. 15, No. 10 (October, 1991), page 86; "Configuration-Control Scheme Copes With Singularities" (NPO-18556), Vol. 17, No. 2 (February, 1993), page 81; and "More Uses for Configuration Control of Robots" (NPO18607/NPO18608), Vol. 17, No. 10, (October, 1993), page 120. To recapitulate: A robot has n degrees of freedom. The basic task is to make the end effector (the hand at the tip of the manipulator arm) follow a prescribed trajectory in m-dimensional coordinates (where m < n). The r = n-m redundant degrees of freedom are used simultaneously to perform an additional task.

Additional tasks could include (but are not limited to) reaching around obstacles, avoiding collisions with objects in the workspace, maintaining one or more links of the manipulator arm in a desired pose, and/or optimizing the overall kinematics in both the redundant and nonredundant degrees of freedom to enhance overall manipulability. The additional task is mathematically modeled by a set of kinematic functions that, in effect, define the trajectory in the redundant degrees of freedom. These functions are specified by the user.

In the present method, the degrees of mobility are simply combined with the degrees of manipulation into one set that contains both the redundant and nonredundant degrees of freedom, and all degrees of freedom are treated on an equal footing according to the computationally efficient configurationcontrol formalism. The user can assign weighting factors to individual degrees of mobility or manipulation as well as to each task specification. The user can also change task specifications and weighting factors during operation. Thus, overall, the present method is characterized by conceptual simplicity, computational efficiency, and flexibilityÑall advantageous for online, real-time control.

Point of Contact:
Homayoun Seraji,
Mail Stop 198-219
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109
seraji@telerobotics.jpl.nasa.gov

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Last updated: May 10, 1996