Trajectory tracking of multi-link systems with input delay via simultaneous modulation of torque and stiffness

The control of multi-link nonlinear mechanical systems through the selection of both torque control inputs and stiffness modulation at each joint is considered. Such control problems, motivated by the use of electrical stimulation of muscle to obtain locomotion in paralyzed individuals, involve systems having significant actuator delays as well as constraints on the magnitude of actuator inputs. A combination of feedback linearization techniques (applied to a reference model to obtain feedforward control), with standard feedback compensation and output prediction is explored. The effectiveness of this combination for tracking in the presence of unknown but bounded disturbances is demonstrated.