Pneumatic servosystems realised with digital valves piloted using modulation techniques suitable for regulating airflow have numerous advantages in terms of economy, reliability and simplicity of installation and use. On the downside, an important technological limit is set by the need to use valves with very low response times and consequently small sizes. This limitation can even make it difficult to obtain satisfactory dynamic performance in servosystems characterised by the presence of large chambers. To avoid having to use to high a number a digital valves, which can be the cause of excessive costs and risks of failure, it is possible to hypothesise the use alongside small fast valves of valves of greater size, characterised by response times of 10/20 ms, which can be used to handle high flows when necessary. The creation of a servosystem of this kind requires the development of a precise control strategy which makes it possible to guarantee both the precision of the controller and the speed of response simultaneously. This paper develops some considerations aimed at defining criteria for selecting valves and the experimental results obtained in the case study of an application involving the pressure control of a tank.
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