Enhancing motor torque control by implementing H-infinity controller and compensating electronics nonlinearities

Motor torque control is a key feature in force centered robotic applications such as haptics, force amplification and telemanipulation with force feedback. Most of the actually applied control schemes for this type of applications are based on 3 levels cascade control: high level force/torque loop, inner velocity loop, inner current loop. The two inner loops are respectively of P and PI type. The main goal of the two inner loops is to limit the effect of the counter-electromotive force and linearize the torque transfer by squeezing mechatronics nonlinearities. This paper presents another approach that explicitly deals with the nonlinearities and avoids the use of a velocity loop. This approach consists in adding a lag compensator to the PI current loop to enhance the torque control bandwidth, respectively the controllability of the system from the higher level controller viewpoint. The PI-Lag compensator parametrization based on fixed structure H_∞ synthesis is illustrated. In addition, the performances of the implemented control law are discussed in comparison with other approaches.