What are air-open valves, air-close valves, and PID control?

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When there is a fault in the air supply for a pneumatic control valve (i.e., the pressure is 0), the valve remains in the fully open position; such a valve is called a fail-open valve (FO). When there is a fault in the air supply for a pneumatic control valve (i.e., the pressure is 0), the valve remains in its fully closed position; such a valve is called a fail-closed valve (FC). Repost: In process control, the PID controller (also known as a PID regulator), which operates based on proportional (P), integral (I), and derivative (D) actions, is the most widely used type of automatic controller. It has the advantages of simple principle, easy implementation, wide applicability, independent control parameters, and straightforward parameter selection ； Moreover, it can be theoretically proven that for the typical objects in process control – namely those with \"first-order lag + pure lag\" and \"second-order lag + pure lag\" – the PID controller represents an optimal control strategy. Parameter selection: ① The impact of the proportional coefficient P on system performance: An increase in the proportional coefficient makes the system respond more quickly, increases its speed, and reduces the steady-state error ； P is high, the number of oscillations increases, and the adjustment time lengthens ； When P is too large, the system tends to become unstable ； If P is too small, it will also slow down the operation of the system. P can take negative values, which is mainly determined by the characteristics of the actuator, sensors, and the controlled object. If the sign of P is chosen improperly, the measured value of the object will move further and further away from the set value of the control target; in such a case, the sign of P must be reversed. It should also be noted that the P parameter of the PID control block in the force-controlled strategy controller represents the gain in PID control. 　　②Effect of integral control I on system performance: The integral action reduces the stability of the system; a small value of I (strong integral action) makes the system unstable, but it eliminates steady-state error and improves the control accuracy of the system. 　　③Impact of differential control D on system performance: The differential action can improve dynamic characteristics; when D is large, the overshoot is greater and the settling time is shorter ； When D is small, the overshoot is also large, and the settling time is longer as well ； Only D is suitable to reduce overshoot and shorten the settling time. Proportional (P) control: Proportional control is the simplest form of control. The output of its controller is proportional to the input error signal. When only proportional control is used, there is a steady-state error in the system output. Integral (I) control: In integral control, the output of the controller is proportional to the integral of the input error signal. For an automatic control system, if there is a steady-state error after it reaches steady state, then such a control system is referred to as a system with steady-state error, or simply a system with error. To eliminate steady-state error, an “integral term” must be introduced into the controller. The integral term represents the time-dependent integration of the error, and as time increases, this integral term grows. In this way, even if the error is small, the integral term increases over time; it drives the output of the controller to increase, thereby further reducing the steady-state error until it becomes zero. Therefore, a proportional-plus-integral (PI) controller enables the system to have no steady-state error after reaching steady state. Differential (D) control: In differential control, the output of the controller is proportional to the derivative of the input error signal (i.e., the rate of change of the error). Automatic control systems may experience oscillations or even instability during the process of correcting errors. The reason is the presence of components with high inertia or components that introduce delay, which have the effect of suppressing errors; their changes always lag behind those of the errors. The solution is to make the change in the error-suppression effect \"proactive\", that is, when the error approaches zero, the error-suppression effect should be zero. In other words, it is often insufficient to include only a \"proportional\" term in the controller; the function of this proportional term is merely to amplify the magnitude of the error. What is needed now is a \"derivative\" term, which can predict the trend of error changes. Thus, a controller with both proportional and derivative elements can bring the control action to eliminate the error to zero, or even to a negative value, thereby preventing severe overshoot of the controlled variable. Therefore, for controlled objects with high inertia or lag, a proportional-plus-differential (PD) controller can improve the dynamic characteristics of the system during the regulation process. 5. Parameter tuning of the PID controller: The parameter tuning of the PID controller is a core aspect of control system design. It determines the values of the proportional gain, integral time, and derivative time of the PID controller based on the characteristics of the process under control. There are many methods for tuning PID controller parameters, which can be broadly divided into two categories: one is the theoretical calculation tuning method. It primarily determines the controller parameters through theoretical calculations based on the system’s mathematical model. The calculation data obtained using this method may not be directly usable; it must still be adjusted and modified based on actual engineering conditions. The second is the engineering tuning method, which relies primarily on engineering experience and is carried out directly during the testing of control systems. It is a simple method that is easy to master, and it is widely used in practical engineering applications. The engineering tuning methods for PID controller parameters mainly include the critical ratio method, the response curve method, and the attenuation method. The three methods each have their own characteristics, and what they all have in common is that experiments are conducted first, followed by the tuning of controller parameters using engineering experience formulas. However, the controller parameters obtained using either method require final adjustment and refinement during actual operation. The critical ratio method is generally used nowadays. The steps for tuning the PID controller parameters using this method are as follows: (1) First, pre-select a sufficiently short sampling period for the system to operate ； (2) Only a proportional control element is added, until the system exhibits critical oscillation in its step response to the input; at that point, the proportional gain and the critical oscillation period are recorded ； (3) The parameters of the PID controller are calculated using formulas under a certain degree of control. Setting of PID parameters: This is done based on experience and familiarity with the manufacturing process, by referring to the measured values and the setpoint curve in order to adjust the values of P, I, and D. For the engineering tuning of PID controller parameters, the following are some empirical values for P.I.D parameters in various control systems: For temperature T: P=20~60%, T=180~600s, D=3-180s; for pressure P: P=30~70%, T=24~180s; for liquid level L: P=20~80%, T=60~300s; for flow rate L: P=40~100%, T=6~60s. Common mnemonic from the book: To determine the optimal parameter settings, check in ascending order; start with the proportional term, then the integral term, and finally add the derivative term. If the curve oscillates frequently, increase the value of the proportional gain. If the curve wanders around in large swings, reduce the value of the proportional gain. If the curve takes a long time to return to its normal position, decrease the integral time. If the curve has long fluctuation cycles, extend the integral time further. If the oscillation frequency of the curve is high, first reduce the value of the derivative term. Large errors lead to slow fluctuations. The differential time should be increased. For the ideal curve, there are two waves – higher at the beginning and lower at the end, in a 4-to-1 ratio. Observe carefully, make adjustments as needed, and conduct thorough analysis; this way, the quality of the adjustments will not be poor

Wind on, wind off – that is, open or closed in the event of an accident; in other words, normally open for air shutdown and normally closed for air activation； A normally open air-shut valve is normally open and can only be closed when there is air ； PID, proportional, integral, and derivative control of control valves ；

Wind on, wind off – that is, open or closed in the event of an accident; in other words, normally open for air shutdown and normally closed for air activation； A normally open air-shut valve is normally open and can only be closed when there is air ； PID, proportional, integral, and derivative control of control valves ；

I’ve learned it, thank you all, experts. . . . .