Near-Minimum-Time Control of a Flexible Manipulator

Near-minimum-time control of flexible manipulators can be accomplished by designing a controller that tracks a reference maneuver. The control method presented here uses a near-minimum-time rigid-link reference maneuver to generate reference control torques and a Liapunov controller to make the flexible manipulator track the reference maneuver while reducing and eventually eliminating flexible motions. The near-minimumtime reference maneuver uses a smoothed bang-bang control plus a term to cancel the nonlinear dynamics of the rigid manipulator. The Liapunov function is a weighted sum of the energies of the elements of the flexible manipulator, and the control law is chosen to make the rate of change of the Liapunov function negative. The Liapunov function is bounded during a maneuver and decays asymptotically after the maneuver ends. RECISE control of a manipulator is an important aspect of robot performance. In the past, this has led to the design of very stiff links for manipulators, which also leads to very heavy links, large motors, and slow response. To avoid these problems, lighter and more flexible manipulators will require advanced control strategies. This work builds on results from the control of rigid robots1 and the control of flexible structures. Fujii and Ishijima2 developed a globally stable Liapunov controller to reorient a structure composed of a rigid hub and a flexible appendage. Junkins et al. 3 extended this control law to track a reference maneuver and showed that the Liapunov function is bounded during the maneuver and decays asymptotically to zero after the maneuver ends. Using a Gibbs phenomenon analysis, Baruh and Tadikonda4 have shown that using smoothed control torque profiles as in Ref. 3 reduces the excitation of the flexible modes during a maneuver. Experimental work in this area is also being done.5'6 In this work, we demonstrate that the control design procedure of Ref. 3 can be developed, using work-energy relations, and we extend it to a multibody case, using a planar two-link manipulator to demonstrate the procedure. A schematic diagram of the manipulator is shown in Fig. 1. The control law developed for the two-link manipulator allows the flexible motions to be stabilized without any finite-dimensional estimation of the flexible modes, thus avoiding any spillover problems.