Semiactive virtual control of a hydraulic prosthetic knee

The paper considers the problem of controlling a novel hydraulic actuator to be used in a transfemoral (above-knee) prosthesis. The control objective is tracking of knee angle reference data corresponding to able-bodied walking. The forces and velocities appearing in a prosthesis when worn by a person are simulated with a two degree-of-freedom robotic hip emulator which allows the prosthesis to walk on a treadmill. The robot produces vertical hip displacement and thigh angle motions using electromechanical actuators with direct force and torque control servos, respectively. Control inputs for the prosthetic knee are the instantaneous positions of two hydraulic valves. The integrated hip robot and prosthesis is controlled using a semiactive virtual control approach. First, a virtual controller is designed for the integrated model, assuming full control authority over the joints. Then, an online optimization problem is solved to find the valve positions that minimize the difference between the actual moment and the moment demanded by the virtual controller. Simulation studies were conducted to evaluate the feasibility of the approach. Knee angle tracking was possible with a root mean square average of 2.2 degrees (approx. 3% of the full range of knee motion), with ground reaction force magnitudes comparable to able-bodied walking data.