Throttling and Volumetric Control Principle to an Electrohydraulic Velocity Servomechanism

The main principles for motion control in modern machinery design that use hydraulic energy are throttling principle by using constant displacement pump and proportional or servo valve in line with the actuator, and volumetric control principle by using constant-speed electric motor in combination with a variable displacement pump or variable-speed induction motor in combination with a constant displacement pump. By using a directional control valve in the hydraulic circuit for controlling the flow to and from the hydraulic actuator a constant quantity of oil is always required which leads to the oil being more heated and more electrically energy is consumed. Thus, regularly occur problems with heating oil and usually a separate heat exchanger is used in the hydraulic system. This conventional control principle is associated with significant loss of energy ; however a good dynamic behavior of the control system can be achieved. Increased use of volumetrically controlled hydraulic energy where speed of the induction motor is adjusted by the frequency converter in a closed loop control is motivated by its large range of the system overall efficiency and its favorable price, which is enabled by using modern microprocessor technology and digital control theory. In the case of frequency controlled drive only the amount of fluid required to achieve the desired speed has to be supplied, and this smaller flow of fluid means lower energy consumption, which makes an example of modern solution in electro-hydraulic technology application. The aim of this article is the comparison of these two control concepts on the example of hydraulic motor speed control. The actuator speed can be controlled both by using a servo valve and by controlling the speed of the induction motor and pump by means of the frequency converter and control computer with digital electronic card. The control strategies will be experimentally validated on a rotary hydraulic system, which is designed at our Laboratory. The controller is tuned according to damping optimum method in order to achieve precise speed control of the hydraulic motor. The control algorithms and monitoring process are implemented on a real-time hardware board that is programmed in Matlab/Simulink environment using Real-Time Workshop program. The experimental results show that the displacement-controlled system can produce satisfactory response characteristics and yield an effective trade-off between control performance and energy efficiency of the control system.