Effect of Load Disturbances on Multiplexed Force Control of McKibben Actuators

Abstract

Traditionally in fluid power systems each actuator is provided with its own control valve that is often quite an expensive and weighty component. By using multiplexed control, where a single control valve is shared among a number of actuators, it is possible to achieve cost and weight savings. The control system can be carried out with one pressure regulator and several high-speed on/off valves which form the multiplexer unit. The McKibben actuator has characteristics such as hermetic construction and almost frictionless operation which enable its use especially in force control applications. The muscle can maintain its pressure and thus the generated force for a relatively long time after it is disconnected from the pressure line. It has been proven [Jouppila & Ellman, 2006] that multiplexed force control of pneumatic muscles is an option for applications where multiple actuators have similar tasks with low dynamics. In this paper the effect of different load disturbances on the multiplexed force control system is investigated. The system is basically an open-loop configuration where the pressure regulator controls the system pressure. Thus, the system is sensitive to load variations because there is no feedback loop from the muscles. In the testing procedure the McKibben actuators with similar tasks are controlled with the multiplexing technique. The system behavior is studied when one of the muscles is displaced from its original position. The effects of using different frequencies and amplitudes for the load variation are studied. The purpose is to find the boundaries in which the pressure regulator is able to compensate the disturbance effects. Simulations are first used to investigate how sensitive the system is to the load variations. Also the pressure regulator’s capability for error compensation is studied. Actual measurements are used to verify the simulation results.