An improved force controller with low and passive apparent impedance for series elastic actuators

This article presents a force controller for series elastic actuators that are used in gait robots, such as exoskeletons, prostheses, and humanoid robots. Therefore, the controller needs to increase the bandwidth of the actuator, lower its apparent impedance for disturbance rejection or effortless interaction with a human user, and to stably interact with any (dynamic) environment. For gait, these environments are changing discontinuously, thus creating regular impacts. In this article, we propose the use of an inner-loop PD controller to increase the bandwidth of the actuator, alongside an outer-loop disturbance observer (DOB) to lower the apparent impedance of the actuator. To increase the controlled bandwidth of the actuator, we introduce a novel tuning method for the PD controller that allows for independent tuning of bandwidth and damping ratio of the controlled plant. The DOB, which is introduced to reject disturbances by lowering the apparent impedance, causes the apparent impedance to turn nonpassive, resulting in potential contact and coupled instability of the actuator. To enable unconditionally stable interactions with any environment, we scale down the DOB contribution such that it lowers the apparent impedance while remaining passive. The proposed tuning method and DOB adaptation were evaluated on a test setup by identifying the torque controller's transfer behavior and the apparent impedance of the actuator. The results of these tests showed that the proposed tuning method can separately tune bandwidth and damping ratio, whereas the DOB adaptation is able to tradeoff the reduction in the apparent impedance with its passivity.