A Control Strategy for Pneumatically Powered Below-Hip Orthosis

Objectives

The role of controllers is inevitable in the design of powered orthosis to achieve ideal gait characteristics. Despite the fact that electrical actuators are preferred for most of the orthoses, a pneumatic actuator proves to have low cost and less weight. In this study, suitable controllers are designed and implemented for the knee and hip joints of a pneumatically actuated orthosis for the afflicted people.

Material and Methods

Different controllers (P, PI and PID) were tested for the position control of the orthosis by proper tuning of gain constants ($K_P$, $K_I$, $K_D$). By using Lagrange Euler Method, the optimal trajectory for the knee and hip joints were determined for the pneumatic system. Particle swarm optimization (PSO) based PID controller was further employed for optimizing the gain constants.

Results

With the healthy gait as reference, the knee and hip reference angles were manually set in the PID controller. The subject was made to walk five times at a distance of 5 m and the average knee and hip angles were calculated based on the gait trials. Knee and hip angles varied 0 to 45° and 0 to 35° for healthy subjects while they varied 0 to 41° and 0 to 45° for the implemented pneumatic leg. The values of gain constants obtained in manual tuning matched with the PSO based controller at 25^th iteration and the best fitness function was chosen with least error (0.7011).

Conclusion

The prototype of the orthosis is fabricated and the response of PID controller was found to be acceptable for a desired pressure (5 bar) with an angular velocity of 3 deg/s. Using a PID controlled pneumatic orthosis, exhibited less oscillation and showed an improved steady-state error when compared to the other controllers, thereby replicating healthy gait. A global best position with minimum error was obtained using PSO to find optimal controller gain constants.