Electrohydraulic servosystems for position, velocity and force control

The paper presents three experimental electrohydraulic servomechanisms made with the intention of conducting research and for education’s purpose in the field of conventional, proportional and servo hydraulics and shows some typical experimental results. All three systems have the characteristics of general industrial plants and also include the necessary measuring equipment for realization of control algorithms. The first system is a module for translational motion control of hydraulic actuator. This system consists of a main cylinder which is controlled by using a proportional directional control valve, while the reaction force (i.e. disturbance) is generated by using a load cylinder and a pressure control valve. The rods of these cylinders are coupled by a steel joint. The experimental test rig can also be used to demonstrate the working principle of a conventional hydraulic system. In that case the proportional valve should be replaced with a directional control valve using flexible hydraulic hoses. The second system is a module for the control of force, i.e. a hydraulic press where it is possible to regulate the pressing force. To hold pressure in the cylinder’s chamber and thereby the cylinder’s force to a desired value, a pressure type servo valve is used. The third system is a module for rotational motion control of hydraulic actuator. In this system a hydraulic motor as a control object is used, while the disturbance is generated by using a hydraulic pump whose output pressure is adjusted with a proportional pressure valve. The rods of motor and load pump are tightly connected. The hydraulic motor speed can be controlled both by using a servo valve (throttling control principle) or by controlling the speed of induction motor and supply pump by means of a frequency converter (volumetric control principle). For the implementation of the control process various control devices were used, such as CompactRIO system programmed using graphical program LabVIEW, PLC (programmable logic controller) and a control computer with DAQ card using Matlab/Simulink/RTW program.