Variable Position and Force Control of a Pneumatically Actuated Knee Joint

Pneumatically powered prostheses have the ability to restore function and improve the quality of life by providing external power, thus decreasing human effort. However, such prostheses are expensive because they use high performance servo valves and complex control systems. To overcome this limitation, a pneumatic actuator is retrofitted with cheap solenoid valves and controlled by pulse width modulation for continuous control. High fidelity control is achieved by using a negative displacement configuration in which pressure is released on one side to create motion. The performance of the system is demonstrated using a series of position, force control experiments and ability to withstand external impulses. The pneumatic system is cheap, has high repeatability, and accuracy. The main limitation is that the speed of response is much slower than a positive displacement system but better design of the solenoid valve and use of predictive control has the potential to alleviate this issue.