The use of haptic interfaces to control mobile hydraulic machinery has several enhancing features over traditional human-machine interfaces comprised of joysticks/levers. This paper presents pressure and flow control schemes designed for coordinated haptic control. Typical of many small backhoes and excavators, the hydraulic system used on the test-bed is comprised of constant displacement pumps and proportional directional control valves. In this type of system, a main pressure regulator is needed to supply the other closed-centre valves with pressure and dumps the additional flow from the pump to tank. Using these valves for haptic applications requires accurate closed-loop control. The focus of this paper is on the flow control scheme. Performance of the proportional directional control valves are compared with and without pressure compensation. A velocity feedback loop is present in order to improve the accuracy of the control system. A small input dead-zone function is proposed in order to prevent a limit cycle around zero velocity caused by the valve dead-band. Coordinated motion with multiple cylinders is demonstrated.

Keywords: haptics, excavators, backhoes, coordinated motion, flow control, pressure compensation