Active Balancing of Joint Forces in High-Speed Linkages by Redundant Drives and Learning Control

Critical loading and vibrations in high-speed linkages result from external forces at the output link and especially from varying inertia forces of irregular link motion. This paper deals with the active dynamic balancing of joint forces — Joint Force Control. Three possible balancing criterions indicated by joint forces are described: joint-stress, clearance-impact-noise and friction.

Active balancing requires additional drives. The two theoretical cases of arranging actuators are compared: redundantly and with an additional degree of freedom. Using the preferred redundant drives the internal forces in the joints may be varied according to special aims while the normal drive of the given mechanism maintains the motion.

An experimental setup consisting of a crank-and-rocker mechanism is used to study the control aim of a zero joint force at the crankshaft. The speed control of the original servo motor M₁ is combined with the closed loop force control of the two redundant servo motors M₂ and M₃. Assuming a steady-state operation mode the learning control concept is applied successfully and compared with a digital PID force control. By learning control the time delay problem is solved. Furthermore the balancing of only one engine order is presented.