Force Control of Motors Based on Second Derivative of Force Using Observers Which Compensate Environment Variations

Abstract

We propose a robust force control strategy based on observers which compensate disturbance and parameter variations of motor and environment. We present two methods: one is a 1-observer system (using force-based disturbance observer), the other is a 2-observer system (using position-based disturbance observer and environment observer). The 1-observer system is simpler than the 2-observer system and does not need position sensor. However, by using 2-observer system we can determine the sensitivity to actuator disturbance and environment disturbance independently, and can achieve higher precision control. Second derivative of force can be controlled because the nominalized motor and environment are realized by the observers. Robust and fast force control is realized by this strategy. Moreover, an adaptive control is introduced to obtain better performance for wide parameter variation. The parameters of the force controller and the observers are adjusted by an identified stiffness obtained from a least square estimation. By using the identified value of stiffness as a nominal value, the response of the force becomes closer to the force command. We also examine influence of resolution of the position sensor on the performance of the observers. We show the effectiveness of the proposed method by simulations and experiments.