A sensor-less approach of a torque controller for pneumatic muscle actuator driven joints

The closer the interaction between humans and robots gets, the more the question of safety and intrinsic compliance arises. Especially in robotic systems that come into contact with humans, pneumatic muscle actuators are increasingly of interest. Many position control approaches for joints driven by pneumatic muscle actuators have been developed and implemented, but only a few for assistive robots. For these robots, like exoskeletons or rehabilitation robots, a torque controller is mostly required. In this paper, a general discussion of a torque controller for joints driven by pneumatic muscle actuators will be given. Subsequently a so called “sensor-less” approach of a torque controller will be implemented and tested. It will be shown that the torque can be controlled without using a force-torque sensor, by substituting the force-torque sensors, with a combination of pressure as well as length measurements and the static force characteristic of pneumatic muscle actuators. Furthermore, it can be seen that a non-model-based, hand-tuned proportional or proportional-integral controller can follow a given torque trajectory up to 1Hz, while using the “sensor-less” approach.