Evaluation of a Pneumatic Cylinder for Ankle Exoskeleton Actuation

Abstract

Ankle exoskeletons are used to reduce the effort of walking. This research aims to design and test an actuator that takes advantage of new joint angle prediction methods. First, a Maxon EC motor was tested on an exoskeleton emulator using a position and velocity cascaded controller to follow a recorded ankle angle profile. It performed well and met requirements but also showed shortcomings in speed, safety, and fault tolerance. A study was done on alternative actuation technologies resulting in pneumatics being selected for prototyping. A pneumatic cylinder has advantages in this application such as speed, force to weight, and compliance to the wearer. For this study servo valves were used to control a prototype pneumatic actuator. The prototype actuator system was tested using data from human motion capture for velocity profile movement control and derivative based force output control in fixed and loaded cases. We evaluated the performance of the controllers for stability, movement speed, force, response delay, and accuracy based on delayed percent RMSE. The design met performance goals but was not as accurate as expected due to the lack of direct position feedback control needed to maintain stability at high speeds. The controller tracked the velocity profile of the test subjects but with a large position bias. Force control using ankle moment data was also highly inaccurate. Overall, this system shows promise for applications with simpler control needs or lower actuation speeds with high force.