Force control and force observer design of series elastic actuator based on its dynamic characteristics

A series elastic actuator (SEA) is a promising actuation method in robotics and mechatronics applications that intelligently interact with environments, including humans. The SEA is characterized by a spring placed between the load and an actuator, which is an electric motor in most SEA systems. Since the spring plays the role of a transducer between a position (i.e., spring deflection) and a force, it is able to control the output force (torque) precisely by utilizing typical position control methods. Also, realization of the precise force (torque) control implies that the SEA exhibits zero mechanical impedance, which renders its superior advantage in human-interactive systems. In many applications, however, the dynamic characteristics of an SEA have not been considered in controller design process, and mostly PID (Proportional-Integral-Derivative) control has been applied to as controllers for SEA. However, PID controller has various drawbacks including wind-ups and thus there are rooms to be improved for SEA control. This paper, taking into considerations this, proposes a dynamic model based control design of SEA. The dynamic characteristics of SEA are analyzed theoretically and experimentally, and feedback controllers based on the derived dynamics is proposed. Comparisons with conventional control methods verify the advantage of the proposed dynamic model based controller on control performance and force observation.