Friction compensation control of a novel electro-pneumatic adaptable impedance actuator

We present a new decentralised control design involving an adaptive friction compensator for the electro-pneumatic adaptable impedance actuator (EPAIA). EPAIA consists of a brushless direct current (BLDC)-motor and a rotational pneumatic actuator, where the latter serves as an adaptable compliant element in series to the gear train. The decentralised control system consists of two loops and a decoupler. The torque control-loop uses a cascaded/post-compensator architecture to control the interaction torque over the rotational pneumatic spring. The stiffness controller involves a decoupler and continuously adjusts the chamber air pressures to achieve a certain interaction stiffness. Since adverse nonlinear friction effects worsened the performance of the interaction torque control-loop, an adaptive friction observer based on the two-state dynamic friction model with elasto-plasticity (2SEP) has been developed. The proposed approach showed superior performance when compared to the decentralised control strategy without friction compensation in a validated nonlinear model.