Force control of series elastic actuators in the presence of motor torque saturation, load side movement and gearbox friction

In many robots that interact with human or the environment, series elastic actuators (SEA) are used because of their inherent compliance and force controllability. Several methods have been proposed for the force control of these actuators. However, the effectiveness of these methods in the presence of limitations such as motor torque saturation, load side movement and gearbox friction has not been evaluated. In this paper, the two common methods of SEAs force control, namely, motor torque-based and motor speed-based control are studied and the effect of above limitations on the stability and bandwidth of the closed loop system is investigated. The results of this research reveal that the motor speed-based control can well overcome disturbances such as gearbox friction and load side movement. However, in the presence of motor torque saturation, there is a possibility that the closed loop system to become unstable. In contrast, the motor torque-based control has lower bandwidth in the presence of load side movement and gearbox friction. Instead, its closed loop system is always stable even in the presence of motor torque saturation.