Control of Two Industrial Manipulators Rigidly Holding an Egg - IEEE Control Systems Magazine

ontrol of cooperating multiple robots has been studied extensively in the past decade [ 11-[ 121. The special feature of multi-arm systems compared to single-arm systems is that closed kinematic loops exist which present a challenge for both motion control C and internal force control. On the other hand, single-arm constrained robots also require motion control and constraint force control. However, control of cooperating multiple robots and control of single-arm constrained robots are still two different areas. The first question is: can we establish a general framework within which both cooperating multiple robots and single-arm constrained robots are unified so that most control approaches developed so far for one area can be directly extended to the other? Most applications employ unilateral constraints which require not only the control of constrained motion but also the control of approach motion, i.e., motion from free space toward contact with the constraints. Mills 1131 studied the stability issue related to the transition phases. Tam, et. al. [ 141, used a switching control law to stabilize the system during transition. However, because of control delay, switching based control laws may result in bouncing phenomena during transition phases. The second question is: can we use a unified no-switching control law to control both constrained motion and approach motion? Dexterous robot hands can perform many delicate operations, such as holding an egg [ 151. Can we use two big heavy-duty industrial robots to do the same thing, that is, is the up-to-date robot force control accuracy high enough to perform very delicate operations? The aforementioned three questions are answered in this paper. Firstly, a framework of general constrained robots [ 161 is used to deal with cooperating multiple robots. Secondly, an impedance-like no-switching adaptive control law with strong real-time capability is proposed to control both constrained motion and approach motion. Thirdly, an experiment with two big industrial robots,