Energy-Saving Control of Single-Rod Hydraulic Cylinders with Programmable Valves and Improved Working Mode Selection

This paper studies the energy-saving adaptive robust precision motion control of a single-rod hydraulic cylinder through the use of programmable valves. The programmable valves used in this study is a unique combination of five proportional cartridge valves connected in such a way that the meter-in and meter-out flows can be independently controlled by four of the valves as well as a true cross port flow controlled by the fifth valve. The programmable valves decouple the meter-in and meter-out flows providing tremendous flexibility to control the cylinder motion while decreasing the energy usage by utilizing the potential and kinetic energy of the load. This paper investigates the different working conditions of the programmable valves and proposes a simple yet effective way to use the programmable valves based on the desired states and current states. INTRODUCTION The use of hydraulic systems is widespread throughout industry due to the large power to size ratio. Hydraulic systems are used very heavily in the construction and agricultural industries and are well suited for these applications. In recent years, the trend is to replace the mechanical valve with an electrically controlled valve. The use of electro-hydraulic valves means that sophisticated electronic control can be applied to control the system. The control of a hydraulic system is far from trivial, due to the highly nonlinear hydraulic dynamics [9]. In addition, parameters such as the bulk modulus change drastically with changing oil temperature and component wear. In the case of construction and agricultural machinery, the mechanical system driven by the hydraulic cylinder may be highly nonlinear itself. Typically, the parameters of the mechanical linkages may vary drastically and are usually unknown, such as the external payload. In addition, significant uncertain nonlinearities such as external disturbances, leakages and friction are unknown and cannot be modeled accurately [3]. These factors result in significant difficulties in controlling a hydraulic system. The advent of electro-hydraulic valves and the incorporation of complex digital control have significantly improved the performance of hydraulic systems. A system using a conventional four-way directional control valve would be able to meet the high performance specification as shown by Bu and Yao [3], but would not be able to simultaneously provide precise motion control and individual cylinder chamber pressure control for better energy saving. With a typical four-way directional control valve only one of the two cylinder states, (pressures), is completely controllable and there is a onedimensional internal dynamics. Although the onedimensional internal dynamics is shown to be stable [3], it cannot be modified by any control strategy. The control input is uniquely determined once the desired motion is specified, which makes the regulation of individual cylinder chamber pressures impossible for energysaving. The result is that while high performance tracking can be attained, simultaneous high levels of energy saving cannot. The uncontrollable state is due to the fact that the meter-in and meter-out orifices are mechanically linked together in a typical directional control valve. This is a fundamental drawback of typical four-way directional control valves. If this link were to be broken, the flexibility of the valve could be drastically increased, making the way for significant improvements in hydraulic efficiency [6]. The technique of breaking the mechanical linkage between the meter-in and meter-out orifices is well known and has been used in heavy industrial applications for several years. Typically, the spool valve is replaced by four poppet type valves [6]. There are a number of slight variations on this theme throughout the 81