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Abstract: A new kind of volume control hydraulic press that combines the advantages of both hydraulic and SRM (switched reluctance motor) driving technology is developed. Considering that the serious dead zone and time-variant nonlinearity exist in the volume control electro-hydraulic servo system, the ILC (iterative learning control) method is applied to tracking the displacement curve of the hydraulic press slider. In order to improve the convergence speed and precision of ILC, a fuzzy ILC algorithm that utilizes the fuzzy strategy to adaptively adjust the iterative learning gains is put forward. The simulation and experimental researches are carried out to investigate the convergence speed and precision of the fuzzy ILC for hydraulic press slider position tracking. The results show that the fuzzy ILC can raise the iterative learning speed enormously, and realize the tracking control of slider displacement curve with rapid response speed and high control precision. In experiment, the maximum tracking error 0.02 V is achieved through 12 iterations only. Key words: hydraulic press; volume control electro-hydraulic servo; iterative learning control; fuzzy control 40976
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1 Introduction The hydraulic presses are extensively used in different industries because they possess a high power- to-mass ratio, fast response, high stiffness and high load capability. However, the conventional hydraulic presses that adopt the valve-controlled hydraulic system have a lot of drawbacks, such as complex structure, considerable energy consumption, plentiful heat generating, high noise, serious vibration, high precision requirement of oil filtrating and large pressure losing [1]. Recently, with the maturity of the motor speed regulating and servo control techniques, the volume control electro-hydraulic servo system driven by various kinds of servo motor directly has appeared. It radically abandons the working principle and pattern of traditional valve-controlled hydraulic system. The bidirectional fixed volume pump is directly driven by the servo motor. The control of pressure, flow and direction of working liquid is realized through controlling the rotation speed and direction of pump. Thus, the control of the movement speed and direction of actuator is achieved without various servo valves. The volume control system has the property of simpler structure, higher reliability and greater mechanical efficiency compared with traditional valve-controlled electro-hydraulic servo system. Furthermore, the closed volume control system driven by servo motor directly possesses a series of advantages, such as wide governor range, high accuracy, good performance of energy saving, strong anti-pollution capability and easy control by computer. Thus, it has caused wide public concern in recent 20 years [2−5]. However, the volume control system, the same as the other electro-hydraulic servo systems, is seriously nonlinear due to the influence of its intrinsic properties and external disturbance. Therefore, the requirements of dynamic and steady-state indexes cannot be satisfied by using traditional control method [6−7]. ILC, as a branch of non-model control theory, can make the real outputs of the controlled object track the expected outputs by using the error between real outputs and expected outputs to regulate the value of control continuously. Compared with other control method, ILC has a very simple structure. It is especially suitable for solving the uncertainty problems caused by nonlinearity and poor system model. In industry control fields, more and more attention has been paid to ILC. In Refs.[8−9],
ILC was adopted in an electro-hydraulic position servo system. The high positioning precision was achieved, and the dynamic response characteristic of system was improved enormously. Circular trajectory tracking of Pneumatic X-Y table was accomplished by using ILC [10]. Experimental studies on the convergence feature of 5- DOF robot manipulator movement locus tracking were carried out by using the control strategy combining adaptive ILC with classical PD [11]. In order to solve the initial control choice of ILC for a new tracking objective, a new method that made use of the historic data to determine the initial control by fuzzy inference was put forward [12]. The convergence speed was improved greatly with fuzzy sliding mode and variable learning gain ILC method [13−14]. In Ref.[15], a new adaptive switching learning control approach, called adaptive switching learning PD control (ASL-PD), was proposed. The simulation study validated that the convergence rate was faster than that of other iterative learning control method by using the ASL-PD. The SRM servo driven technique is introduced into the field of hydraulic press by combining the latest achievement of electric, microelectronic and hydraulic control technology in this work. The SRM direct-drive volume control system is constructed to substitute the primary valve-controlled system of the hydraulic press. Considering that the serious nonlinearity exists in volume control electro-hydraulic servo system and the molding process is an operation with high repeatability, a new kind of fuzzy ILC method that can adaptively adjust the learning gains based on fuzzy strategies is put forward to carry out the position tracking control for the SRM direct-drive volume control hydraulic press. 2 Working principle of volume control hydraulic press The working principle of the volume control hydraulic press is illustrated in Fig.1. The flow pressure, volume and direction of working medium can be controlled by using SRM to drive the bi-directional axial piston pump directly. So the movement speed and direction of hydraulic press slider is controlled by pump but not with valve. The total system comprises SRM speed control module, volume control module and auxiliary hydraulic loop module. The SRM speed control module, including SRM and servo driver, controls the turning speed, angle, direction and torque of pump. The volume control module is made up of a bi-directionally dischargeable pump and an asymmetric piston cylinder, which controls the movement velocity, pressure and direction of piston by changing the turning speed and direction of pump. The auxiliary hydraulic loop module is composed of two relief valves and two check valves, which are connected with oil tank and the oil pipes of main loop.