Hammerstein Adaptive Impedance Controller for Bionic Wrist Joint Actuated by Pneumatic Muscles

For the nonlinear, time-variable and unpredictable environment (such as the manipulator workspace that has many random and moveable obstacles), a Hammerstein adaptive (HA) impedance controller based on a Hammerstein impedance model is proposed in this paper. The model consists of a static nonlinear element and a linear dynamic element, which can describe the nonlinearity of the environment. Then, an adaptive law is designed by a Lyapunov function, which can adjust the parameters of the static nonlinear element, so that the model has good adaptability and robustness for the environment. By introducing a parameters self-adjust model-free adaptive controller as the inner position controller, the HA impedance control system is built, which does not require the knowledge of the robot dynamics. In order to verify the force-control performance of the proposed control scheme, some simulations and experiments are done on a bionic wrist joint actuated by pneumatic muscles. Compared with the conventional impedance controller with constant parameters, experimental results demonstrate that HA impedance controller has better force tracking characteristic and can restrict the contact force effectively, no matter how the external environment changes, which increases the safety between robots and humans or operation objects.