Method for determining the positive/negative action of PID regulator

A method for determining the positive/negative action of a regulator

Automatic adjustment of the control system: After the control scheme is determined and the positive/negative effect of the regulator is judged, the most critical thing is how to adjust the P, I, and D parameters. Based on many years of field work experience, let's talk about how to adjust the P, I, and D parameters of the adjustment system. Please refer to it in your project.

Before adjusting the P, I, and D parameters of the control system, it is necessary to ensure that the closed-loop control system must be negative feedback, that is, Ko*Kv*Kc ＞0.

Adjustment object Ko:

If the valve and actuator are opened wide and the measured PV increases, Ko>0; otherwise, Ko<0;

Regulating valve Kv:

If the valve is in positive action (gas-open, electric-open), Kv>0; if the valve is in negative action (gas-close, electric-close), Kv<0;

The positive and negative values ​​of Ko and Kv are determined by the process object and production safety. According to the positive and negative values ​​of Ko and Kv and Ko*Kv*Kc ＞0, we can determine the positive and negative value of Kc.

Regulator Kc:

If Kc＞0, the regulator is in reverse action; if Kc＜0, the regulator is in direct action; the software configuration must be set correctly. Before debugging and starting the device and adjusting the P, I, and D parameters, the positive/reverse action of the regulator must be checked and correct.

1. Before adjusting the P, I, and D parameters of the control system, ensure that the measurement is accurate and the valve action is flexible;

2. When adjusting the P, I, and D parameters of the adjustment system, make a good call and require the user's process operation to pay close attention to the production operation status to ensure safe production;

3. When adjusting the P, I, and D parameters of the adjustment system, first use the automatic control and then the cascade control, first use the secondary loop and then the main loop, with the secondary loop being coarse and the main loop being fine. On the CRT of the operating station, open the adjustment window of the adjustment screen of the regulator, change the given value SP or the output value OP, give a step signal allowed by the process, observe the change of the measured value PV and the trend chart, and constantly modify the PID parameters, often repeatedly several times, until stable control is achieved. In practice, it is generally sufficient to achieve the first-order characteristics that satisfy the process.

2. Preset of empirical PID tuning parameters

For fluid (gas, liquid) media, the empirical PID tuning parameters are as follows (it is best to set them in the software configuration before leaving the plant, and then fine-tune or leave them unchanged on site):

1. Flow rate adjustment (F):

Generally P=120～200%, I=50～100S, D=0S;

For anti-surge system: generally P=120～200%, I=20～40S, D=15～40S;

2. Pressure adjustment (P):

Generally P=120～180%, I=50～100S, D=0S;

For venting system: generally P=80～160%, I=20～60S, D=15～40S;

3. Liquid level adjustment (L):

1] Large container (tower tank with a diameter of 4 meters and a height of more than 2 meters): generally P = 80-120%, I = 200-900S, D = 0S;

2], Medium container (diameter 2-4 meters, height 1.5-2 meters tower tank): generally P = 100-160%, I = 80-400S, D = 0S;

3] Small container (tower tank with a diameter of 2 meters and a height of less than 1.5 meters): generally P = 120 ~ 300%, I = 60 ~ 200S, D = 0S;

4. Temperature adjustment (T):

Generally P = 120 ~ 260%, I = 50 ~ 200S, D = 20 ~ 60S;

The above parameters are empirical and not absolute. In addition, in practice, sometimes a process object or valve (positioner) of a regulation system has a problem, which can be overcome by changing the PID parameters to enable automatic operation. Automatic operation requires patient observation and continuous correction. In practice, the key to automatic operation is that the valve (positioner) and actuator are easy to use and flexible in action.

In a cascade control system (for example, with two regulators), the entire inner loop (sub-regulator, with Ko1*Kv1*Kc1 > 0) is equivalent to the Kv of the main loop, which is always positive. The result of PID parameter tuning: observe the curve, which is generally a first-order characteristic (of course, in theory, a second-order attenuation characteristic).

Three automatic circuit input precautions

1. Basic principles:

When the device is running, the input of the automatic circuit should ensure the smooth operation of each work section. The main parameters cannot fluctuate greatly, and the pressure, liquid level, temperature and other parameters of other auxiliary equipment cannot fluctuate too much to affect the normal operation of the device.

2. Cooperation with process operators:

The commissioning of the automatic circuit belongs to the commissioning work of the automation transformation project. If our personnel are responsible for commissioning the automatic circuit, when doing this work, we should first explain to the user's operators the content of our work, how the process operators need to cooperate, what the impact is and how to deal with unexpected situations. After the commissioning is completed, the process operators should be notified. Before the automatic circuit is put into use for the first time, the process operators should be required to adjust the working conditions of this part to a relatively stable state as much as possible.

3. Specific precautions for control system:

(1) All automatic circuit configurations should be strictly tested before leaving the station. If the configuration is modified on site, the signal flow and logic correctness of the configuration should be carefully checked before it is put into automatic operation. The timing of the switching logic should be noted in the signal switching part. The configuration should ensure that there is a simple logic part that can be manually intervened at the exit of the automatic circuit to the site, so that in case of configuration errors, the automatic circuit can be manually stopped from affecting the site.

(2) When the PID module is put into automatic operation, the values ​​of the proportional band and integral time of the PID module can be enlarged first, the upper and lower limits of the PID module output can be placed within an allowable range of variation near the current tracking output value of the PID module, and the output change rate of the PID module can be reduced. After the automatic operation is put into automatic operation, observe whether the action direction of the PID module is correct and whether the change of the input deviation of the PID module is within the normal range. After confirmation, release the output limits of the PID module one by one to restore its normal function, and then adjust the parameters of each item of the PID module according to the adjustment quality.

Four PID parameter tuning methods

1. Basic knowledge

In the automatic adjustment system, E=SP-PV. Among them, E is the deviation, SP is the given value, and PV is the measured value. When SP is greater than PV, it is a positive deviation, otherwise it is a negative deviation.

1) The action of proportional regulation is proportional to the size of the deviation; when the proportionality is 100, the output of the proportional action and the deviation act at 1:1 of their respective ranges. When the proportionality is 10, it acts at 10:1. That is, the smaller the proportionality, the stronger the proportional action. Too strong a proportional action will cause oscillation. Too weak a proportional action will cause under-regulation of the proportion, resulting in too many fluctuation cycles in the system convergence process and too small an attenuation ratio. Its function is to stabilize the adjusted parameters.

2) The action of the integral regulation is proportional to the integral of the deviation over time. That is, if there is an integral action on the deviation, there will be an output. It plays a role in eliminating the residual error. If the integral action is too strong, it will cause oscillation, and if it is too weak, the system will have a residual error.

3) The action of differential regulation is proportional to the speed of change of the deviation. Its effect is to prevent all changes in the adjusted parameters and has the effect of leading regulation. It has a good effect on objects with large lags. But it cannot overcome pure lags. It is suitable for temperature regulation. Using differential regulation can shorten the time of the system convergence cycle. Too long differential time will also cause oscillation.

2. Setting method

The empirical method is the most widely used tuning method for simple regulation systems. It is a trial and error method. It is achieved by presetting parameters and repeated trial and error. The preset values ​​of the parameters should be determined according to the characteristics of the object and the range of the instrument. The PID parameters with large instrument ranges should be appropriately strengthened. The general ranges of the four types of adjusted parameters are as follows:

The critical proportionality method is to use pure proportionality to put the system into automatic operation. At this time, the integral time is set to the maximum and the differential time is set to 0. Gradually reduce the proportionality until the system just starts to oscillate with equal amplitude. Note the proportionality Pbc and oscillation period Tc at this time, and then calculate the proportionality and integral time of PID as follows: P=2.2Pbc; T=0.85Tc. The actual situation may exceed this range.

For objects with large pure lag time and time constant, the critical proportionality method should not be used for MACS PID, as it is more difficult to find Pbc.