Abstract— The paper presents an idea of an open-loop control of a pneumatic positioning system consisting of a pneumatic cylinder, inexpensive measurement system and four on-off switching valves. A strategy of four pulses on- off control trajectory for implementation of a determined piston displacement without an overshoot is presented. Parameters for the control are determined by the application of artificial neural nets that have to be trained before the application. The strategy is nearly time-optimum one and results in a short control time, a quiet valves action and a significant reduction of the compressed air consumption in the comparison to algo-rithms used in pneumatic positioning systems. This way of control was investigated in many simulation experi-ments and in tests performed in a laboratory as well. It can be a basis for construction and development attrac-tive, low-cost, economic pneumatic positioning systems.68734
Keywords: Energy saving system, low-cost control, open-loop control, pneumatic positioning system.
I. INTRODUCTION
Recent development in electronics and mechanical construc-tion of fluidic cylinders creates a possibility of construction of a quite new automatic control devices – the positioning sys-tems composed of a pneumatic cylinder with a servo-valve, a measurement system and a controller based on a one-card computer [4,5,9]. This construction can cope with a tradition-al electric servomechanism since it has a better force-to-mass ratio and is less expensive than electric construction [5,9]. Moreover the pneumatic systems are clean, can operate in hazardous environment and are attractive for new generations of manipulators [9].
Wide implementation of the fast calculation chips with a large memory on board enables us to implement the advanced digi-tal control algorithms in the pneumatic positioning systems [4,5]. Application of the adaptive state space controllers with on-line observers [1,3], together with very fast proportional valves operating with short sampling interval of 1 ms (or even less), has resulted in the positioning systems which achieve the positioning error below 0.1 mm and movement speed more than 1,5 m/s [4,9]. A structure of such a positioning pneumatic drive is presented in Fig. 1a. One should also men-tion a group of drives, in which the expensive proportional valve is replaced by two/four switching valves (alternatively working) with the PWM-like control technique [10]. This solution however, increases air consumption and due to the noise generated (by PWM modulation) is hard to accept in practical applications.
Among others servo-systems, the positioning pneumatic drive is characterised by a high dynamic, an advantageous power-to-weight ratio and extreme overloading capability [9]. But its traditional structure has a number of drawbacks: leakages of
the proportional servo-valve and a continuous action of the used controller activating the valve (even in the steady state) increase considerably the compressed air consumption [9].
Fig.1a) A structure of a modern pneumatic servo-drive with the 5/3 servo-valve, b) A structure of a development system servo-drive with four on-off switching valves.
The advanced control algorithms require us to have a precise (and expensive) measurement system for the estimation of the speed and acceleration of the piston, used in the state space controller [2,5]. Also the cost of the proportional servo-valve is significant. A specific behaviour of the state space control is presented in Fig.2, where typical transients of the position, speed and control signals [4,5] are plotted. A dynamics of a pneumatic positioning system (composed of a servo-valve and a cylinder) can be approximated by a simplified transfer func-tion (between piston velocity and a control voltage) [5]
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