Stability and performance of pneumatic actuators in impedance control

Abstract

Investigating the ability of a pneumatic actuator to exhibit the desired mechanical impedance is the subject of this article. Inertia, stiffness and damping are three components of mechanical impedance, and achieving the desired values of these parameters is restricted. Inappropriate compensation of friction, low bandwidth or inaccurate measurement of the external force and losing the pressure tracking ability of the inner pressure control loop are the main factors that affect quality and range of achievable mechanical impedance in pneumatic actuators. In this article, the limitations induced by pressure tracking characteristics on the range of achievable mechanical impedance in pneumatic actuators are investigated by focusing on the inertia term. The experiments show that the minimum stable achievable target inertia in pneumatic actuators is limited; for low values of target inertia, the system goes toward instability. In this condition, the actuator shows sustained oscillations whose frequency and amplitude depend on the value of target inertia. Also, the value of minimum stable achievable target inertia is a function of actuator stiffness, so that for higher values of stiffness, the minimum stable achievable target inertia increases. In other words, the stability margin decreases. Then, on the basis of the results of the pressure tracking tests and taking into consideration the new mathematical model presented in this article, the experimental results are discussed.