PID control in various automatic control systems

Proportion first, integration later； From smallest to largest ratio (from largest to smallest scale), integral time from largest to smallest, and finally add a suitable amount of differentiation. Adjust the range a bit smaller, and experiment gradually while combining it with theory. Experience is gained through accumulation, reflection, and understanding; no matter how much others say, it’s useless if one does not put it into practice.

Agree with the comment above: use PI for flow, pressure, and liquid level, and PID for temperature and analysis

A common mnemonic for PID tuning: To find the optimal settings, start by checking values from low to high. Begin with the proportional term, then add the integral term, and finally the derivative term. If the curve oscillates frequently, increase the proportional gain; if the curve wanders around in large swings, decrease the proportional gain. If the curve takes a long time to return to its normal level, reduce the integral time. If the fluctuations in the curve have a long cycle, extend the integral time further. If the oscillation frequency is high, first reduce the derivative term. If there is significant steady-state error and slow response, increase the derivative time. An ideal curve should have two peaks, with the first peak being higher than the second one, in a 4:1 ratio. Observe carefully, make adjustments as needed, and analyze thoroughly – this way, the tuning quality will be excellent.

In production automation systems, the three control mechanisms of proportional, integral, and derivative action are often combined to form proportional-integral-derivative control, also known as PID control. In its input-output mathematical expressions, there are three characteristic constants: the proportional band δ(P), the integration time TI, and the derivative time TD, which are used to represent the tuning characteristics of proportional-integral-derivative control. The proportional band is the reciprocal of the proportion, expressed as a percentage; therefore, the larger the proportional band, the smaller the proportion. 7 H- k! N; D2 b When a step disturbance is applied to the input of a proportional-integral-differential controller, the control signal output by the controller is equal to the sum of the outputs from the proportional, integral, and differential actions. The output of proportional action is proportional to the magnitude of the error ； The output of the integral action is proportional to the rate of change of the deviation. The magnitude of the output change depends not only on the size of the deviation but also on how long the deviation persists. As long as there is an input deviation, the output continues to change; it is only when the input deviation is zero that the output stops changing. Thus, the integral action can eliminate deviations ； The output of the differential action is proportional to the rate of change of the deviation; it adjusts according to the speed at which the deviation changes. This is known as lead action – adjustment takes place immediately as soon as any change in the deviation occurs. When there is no change in the deviation, differential adjustment has no effect. The order of action of the three characteristic parameters is as follows: when a deviation is present, the differential action takes effect immediately, causing a sudden and significant change in the regulator’s output, after which this output gradually decreases. The proportional action also activates at the same time, reducing the deviation. Subsequently, the integral action comes into play; over time, it becomes increasingly dominant until the deviation is finally eliminated. Let’s use an example of filling a water tank to illustrate the above relationship. ! U’ R, x$ Q R* e! F- N7 x# When the water level in the tank is lower than expected, our brain immediately thinks of opening the valve wider to add more water to the tank. The amount of water added will be large; if it is anticipated that opening the valve too wide will make it difficult to control the water level, which could quickly exceed the desired level, then the valve is gradually closed to regulate the amount of water added. This is the principle of differential action ； At the same time, opening or closing the valve to different degrees determines different water supply amounts, and the water level rises in proportion; this is the proportional action ； Whether the valve is fully open or partially closed, the time required to reach the desired water level varies; the greater the degree of opening of the valve, the shorter the time needed, while the smaller the degree of opening, the longer the time required. Normally, over time, the water level gradually reaches the desired value, after which we close the valves; however, even while closing the valves, water continues to flow in, causing the water level to exceed the desired value. To better control the water level, we gradually reduce the valve opening and lower the water supply volume until the water level reaches the desired value at which the valve is fully closed. This process takes some time; the smaller the reduction in valve opening, the longer it takes, and vice versa. This is what is known as integral control.