Hydraulic Linear Actuator Velocity Control Using a Feedforward-plus-pid Control

A practical approach to design a feedforward-plus-proportional-integral-derivative (FPID) controller for accurate and smooth velocity control on a hydraulic linear actuator is presented. The integrated controller consists of a feedforward loop and a PID loop. The feedforward loop is designed to compensate for the nonlinearity of the electrohydraulic system, including the deadband of the system and the nonlinear flow gain of the control valve. A feedforward gain schedule determines the basic control input based on the command velocity. The PID loop complements the feedforward control via velocity tracking error compensation. Also presented are comparisons of the experimental results from an open-loop feedforward controller, a conventional PID controller, and an integrated feedforward-PID (FPID) controller. In each experiment, the controllers were tuned to provide optimal responses. Results demonstrate that the FPID controller is capable of improving the accuracy and dynamic performance of velocity control of a hydraulic linear actuator. INTRODUCTION Hydraulic actuators are widely used on mobile equipment and robots, due to their high power density, environment tolerance, and compact size (Backe, 1993). One of the fundamental tasks in designing hydraulic actuating systems is the development of effective velocity control of the actuator using a control valve (Burrows, 1994). The adoption of electrohydraulic (E/H) proportional valves, which are usually 4-way infinite position directional control valves, increased the efficiency and performance of hydraulic actuating systems (Caputo, 1994). Most E/H proportional valves have undesirable inherent characteristics including high nonlinearity, asymmetric flow gain, and hysteresis. The nonlinear behavior may vary with changes in system load and operating conditions. Nonlinear effects can cause performance deterioration in terms of response speed, control accuracy, and stability of the system (Anderson, 1988). Nonlinear characteristics make it difficult to adequately control an E/H actuating system using a classical proportional-integral-derivative (PID) controller (Zhang et al. 1996). Many researchers attempted to develop adequate controls for accurate and efficient control of nonlinear systems. Edge and Figueredo (1987) developed a model reference adaptive control (MRAC) for a hydraulic servosystem utilizing a linear model. The MRAC algorithm modified the controller parameters to make the plant track the system model. Researchers have successfully developed various neural networkbased controllers for E/H systems (Newton, 1994; Watton and Kwon, 1996; Qian et al. 1998). Neural network-based control is capable of offering good nonlinear control and an ability to learn. Fuzzy logic control is another capable control technology for handling the high nonlinearity inherent in hydraulic actuating systems (Tessier and Kinsner, 1995; Wang and Chang, 1998; Zhang et al. 1999). Zheng et al. (1998) applied an adaptive learning approach to control the position of a hydraulic linear actuator using an E/H proportional valve. The system is capable of compensating for major nonlinearities, including deadband, saturation, and friction, to achieve satisfactory performance. This paper presents a practical approach to design a feedforward-plus-proportional-integralderivative (FPID) controller for accurate and smooth velocity control on a hydraulic linear actuator. The integrated FPID controller consists of a feedforward loop and a PID loop. The feedforward loop is designed to compensate for the nonlinearity of the electrohydraulic system, including the deadband of the system and the nonlinear flow gain of the control valve. A feedforward gain schedule determines the basic control input based on the command velocity. The PID loop complements the feedforward control via the velocity tracking error compensation. DESCRIPTION OF THE HYDRAULIC LINEAR ACTUATOR SYSTEM Hydraulic cylinder actuators are widely used in many fluid power systems, such as robots, aircraft, construction machinery, and agricultural machinery. To develop an adequate velocity controller for hydraulic cylinder actuators without loss of generality, a hardware-in-the-loop E/H linear actuating system simulator was developed. This interactive simulator consists of a computer-based controller, a pulse width modulation (PWM) valve control driver, an electrohydraulic proportional directional control valve, a hydraulic linear actuator, and a load adjustable cylinder (Fig.1). The load adjustable cylinder is linked to the actuator and allowed a positive load to resist the motion of the actuator. The linear actuator is a single-rod, double-acting hydraulic cylinder; its velocity is determined by the flow rate supplied to the cylinder. A Packer-Hannifin (Elyria, OH) four-way proportional E/H directional control valve is used to regulate the flow rate (Fig. 2). In this system, the orifice area of the cylinder-to-tank (C-T) port is always larger than that of the pump-to-cylinder (P-C) port, and a positive load is always applied to the actuator. It is reasonable to assume that the actuator velocity is controlled solely by the P-C orifice. Based on flow continuity theory, the actuator extending motion can be described using a flow continuity theory when the friction and leakage are neglected. The system momentum can be determined by actuating force, system load, and system mass. The actuator retracting motion can be modeled in a similar manner to the extending motion. dt dP V dt dy A QS 1 1 1 β + = (1) F dt y d m A P A P + = − 2 2