A Study on the Design and Control of Energy-efficient Pneumatic Drives
A major criticism for the use of servo pneumatic systems or pneumatics in general is about their low energy efficiency. In the past, major efforts on developing servo pneumatic drives for some advanced applications are largely focused on the control algorithm development whereas the very fundamental knowledge on pneumatic drives has not been much advanced. The low energy efficiency issues with pneumatic drives remain to be an unresolved problem today. This research aims to explore new energy-efficient design and control strategies considering different end-use methods and to understand the dynamic behaviour of energy-efficient pneumatic drives through modelling, simulation and experimental studies. It is observed that the modelling and control of pneumatic drive systems has been limited mostly to those of symmetric cylinders dated back to the foundation work in 195O’s. Therefore, this research has focused firstly on the modelling and analysis of the dynamic behaviour of pneumatic asymmetric-cylinder drives (symmetric cylinder is then seen as a special case). Applying conventional small perturbation techniques, asymmetric cylinder drives are rationally modelled as a two-order or third-order system with regard to the opening condition of the control valve. Based on the model including the factors of ram area ratio and residual volume, some specific relationships have been revealed. The piston position correlating to the minimum stiffness for asymmetric cylinders is identified and a novel analytical solution is offered. It is found that this position is a function of the ram areas, residual volumes, and chamber pressures. The ram area ratio plays a significant role in affecting the distribution of both the natural frequency and damping ratio along the whole working stroke of asymmetric cylinders. The effects of factors, such as ram area ratio, valve opening, viscous coefficient, on the natural frequency and damping ratio are also studied. The position and motion direction dependency of the dynamics of asymmetric cylinder pneumatic drives can serve as a guideline for developing some novel control strategies, such as gain scheduling control, look-up table control and other effective control methods. Secondly, five energy-efficient control strategies have been put forward and studied, namely (i) By-pass valve control; (ii) Rapid motion circuit control; (iii) Semi closed circuit control; (iv) expansion power control and (v) Closed-circuit control. The mathematical models are developed with some basic assumptions and the stability of the energy-efficient control strategies has been studied. Theoretical analysis and simulation studies reveal that the energy-efficient control strategies can offer improved system stability in comparison with traditional control methods. Moreover, the energy efficiency of the five control methods are estimated through comparing the amount of compressed air consumed in the new schemes with that consumed in a traditional control system on similar operating conditions and work cycles. Using this energy efficiency evaluation methodology, the energy efficiency of the five control methods is theoretically analysed and verified with both simulation and experimental means. It is shown that some 12 % to 35 % of energy can be saved. Specific attributes of the five new control methods are analysed and their limitations are discussed. It is anticipated that the results obtained in this research should facilitate a new level of understanding for the design and control of energy-efficient pneumatic servo drives.
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