Decoupled Control for the Bicycling UGent Knee Rig: Design, Implementation, and Validation

This paper presents the design and implementation of a new control strategy for Ghent University, Ghent, Belgium (UGent) knee rig, which is capable of imposing bicycle motions on cadaver knee specimens in order to investigate knee biomechanics and the impact of newly designed knee implants. An electromechanical description of the system with its instrumentation and limitations is given. Via system identification, a dynamical model of the multiple-input/multiple-output system is obtained on which the control strategy design is based. This control strategy combines position control and force control. Dynamical analysis of the system suggests the need for a Proportional-Integral-Derivative (PID) control strategy with gain adaptation. In order to fulfill the performance specifications, a feed-forward action and decouplers are added to the control strategy, and their advantages are shown via simulations and experiments. The complete strategy is implemented on the real system and the output signals are measured for analysis. The results indicate that the identified model fits well to the measured data and that the designed control strategy is able to accurately control the system. The measurements show that the predefined performance specifications have been achieved for a bicycle motion with a period of 10 s; the error on the position is smaller than 2 mm and the error on the force is smaller than 10 N.