Decentralized Adaptive Control for Robots

NASA's Jet Propulsion Laboratory, Pasadena, California

A proposed scheme for the control of a multi-jointed robotic manipulator calls fo r an independent control subsystem for each joint, consisting of a proportional/integral/derivative feedback controller and a position/velocity/ acceleration feedforward controller, both with adjustable gains. The independent joint controllers would compensate for unpredictable effects (e.g., friction, variations in payload, and imprecise knowledge of the dynamics of the manipulator), gravitation, and dynamic coupling between motions of joints, while forcing the joints to track reference trajectories. The scheme is amenable to parallel processing in a distributed computing system wherein each joint would be controlled by a relatively simple algorithm on a dedicated microprocessor. For the purpose of the scheme, it is convenient to view each joint as a subsystem of the entire manipulator system. The subsystems are considered to be interconnected by disturbance torques that represent the inertial coupling, Coriolis, centrifugal, frictional, and gravitational effects. The problem is to d esign the set of independent joint controllers in which the ith controller generates th e joint torque Ti(t) (where t = time) by responding only to the actual joint-angle trajectory
and the reference joint-angle trajectory
and makes
track
. The adaptive independent controller dedicated to the ith joint would be described by

Ti(t) = fi(t) + [
+
]

as shown in Figure 1, where
is the position-tracking error of joint i. The term
represents an auxiliary signal synthesized by the adaptation scheme to improve the tracking performance and partly compensate for the disturbance torques. The term in the first set of brackets represents the adaptive position/velocity feedb ack controller with the adjustable gains
and
acting on the position and velocity tracking errors
and
respectively. The term in the second set of brackets represents the adaptive position/velocity/acceleration feedforward controller wit h the adjustable gains
,
and
operating on the desired position
, velocity
, and acceleration
, respectively. A theorem derived via the theory of model-reference adaptive control provides the necessary controller-adaptation law in the form of specifications for the auxiliary signal, feedback gains and feedforward gains. The resulting independent-joint-control law can be expressed as that of the combination of the proportional/integral/ derivative feedback controller and the proportional/derivative/second-derivative feedforward controller illustrated in Figure 2. The controller-adaptation laws are simple and involve only a few arithmetic operations. The proportional-plus-integral adaptation laws give a large family of adaptation schemes, from which the most suitable scheme for a particular application can be selected. The use of proportional-plus-integral adaptation law s yields improved convergence and increased flexibility in comparison to the conventional integral adaptation laws.

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