Open AccessCompressibility of air used as a working medium in pneumatic control systems raise certain difficulties in calculating dynamic characteristics of the pneumatic drive and solving problems of its regulation. These difficulties are due to a number of factors:
- flow of compressed air through the narrow working splits of distributive and throttling devices of pneumatic control;
- filling in and discharging the pneumatic actuator cavities of variable volume (piston and rod cavities of the air-cylinder) under conditions of heat and mass transfer;
- simultaneous filling in and discharging a pneumatic cavity of permanent volume (receiver);
- pneumatic cylinder piston end positioning alteration under variable loading and at the moment of shutdown;
A number of factors have a significant impact on the piston end positioning alteration value, namely an initial positioning of the piston at the moment of its shutdown, which determines the volume of the pneumatic cylinder cavity; a value of the permanent component of the load at the moment the piston shuts down and its change during keeping time period; transmission coefficient of the positioning component of the load; a working area of the air-cylinder piston and also an atmospheric pressure reduction, which can significantly affect the operation of control systems of a small aircraft at high altitudes.
With a view to deepening the problem of calculation and design of pneumatic actuators, it is shown that the relationship between the parameters of compressed air and their changes is determined by the properties of thermodynamic processes under conditions of heat and mass transfer. In pneumatic actuators for general industrial use, the pressure of compressed air does not exceed a value equal to 1 MPa. In this case, the working medium can be regarded as an ideal gas in simplified calculations.
Based on the general equation of thermodynamics, the paper considers the particular cases of a changing gas state process under its constant temperature, pressure, and volume and also in the absolute or partial absence of heat exchange with the surrounding medium. The inconstancy of the compressed air mass during filling in and discharging of pneumatic cavities of variable volume causes non-stationary thermodynamic processes in the pneumatic actuators and requires their specifying in conditions of heat and mass exchange.
The pneumatic actuator piston speed is related to the peculiarities of the working medium movement through the narrow slits of distribution and control devices of pneumatic automation due to dependency of the flow on the outflow parameter, which in turn is related to the criticality of the outflow process (the ratio of the pressure at the outlet of the throttling device and at its inlet).
It is shown that when calculating and designing the pneumatic drives, including closed-loop ones, as well as for evaluating their dynamic characteristics, a system solution of the equations determining the processes of filling and discharging the cavities of the pneumatic cylinder is required, taking into account the peculiarities of compressed air movement through the narrow slits of pneumatic automation devices.
The outflow parameter available in the equations of the pneumatic drive dynamics and determined by the outflow process criticality and by the heat exchange conditions (a poly-tropic index) requires their numerical solution.