A feedback control system that uses the pressure in gas or liquid pipelines or containers as the controlled variable. In many production processes, maintaining a constant pressure or a certain degree of vacuum is often a necessary condition for normal production. Many chemical reactions require constant pressure, and to maintain a constant flow rate, it is often necessary to control the pressure of the main pressure source to be constant. According to different application scenarios, pressure control can adopt different methods. When the control performance requirements are not high, relatively simple control devices such as pressure regulating valves can be used. For high performance requirements or complex production processes, it is advisable to use pressure control systems. The structure of the pressure control system is closed-loop, consisting of pressure sensors, pressure controllers, and controlled objects. The pressure control valve sensor measures the controlled pressure and converts it into signal forms that are easy to use, such as voltage, current, displacement, angle, etc. The comparison device compares the signal reflecting the pressure magnitude with the given pressure value, generating a deviation signal. The deviation signal is applied to the pressure regulating mechanism through the pressure controller, and the pressure at the measured point is adjusted to the desired value in the direction of eliminating the deviation. More complex systems often use methods such as changing the pump speed or reducing the pump efficiency to regulate pressure. In simple pressure control systems, throttle valves are often used to regulate pressure.
The pressure control system, like other feedback control systems, also has the possibility of self-excited oscillation. If the valve opens or closes too quickly corresponding to a certain pressure deviation, self-excited oscillation may occur. This oscillation can be eliminated by limiting feedback. The basic theory for describing the performance and design of pressure control systems is the same as the basic theory for any other feedback control system (see classical control theory, modern control theory). There are many types of pressure control valves. Figure 2 is a schematic diagram of a commonly used pneumatic proportional and differential pressure controller. It is composed of a nozzle baffle pneumatic amplifier that introduces hysteresis negative feedback. The input is the mechanical displacement e of the baffle, and the controlled variable is the output pressure Pc of the pneumatic amplifier. When the input signal e has a step change Δ e, the pneumatic amplifier generates a corresponding pressure change Δ Pc. Due to the hysteresis effect of the throttle valve R, the bellows cannot immediately sense this pressure change, so the pneumatic amplifier will feel the full effect of the baffle displacement in the initial stage. As the pressure at the output end of the throttle valve changes, the feedback bellows expands or contracts, enhancing the negative feedback effect and causing the output pressure to decrease or increase. The change in output pressure includes a component proportional to the input change and a component proportional to the input change rate, thus enabling proportional and differential control. In steady state, the function of the feedback bellows is the same as that of a conventional proportional feedback mechanism.
