Analysis of PID parameter self-tuning function of digital display regulator

Various intelligent digital display regulators generally have PID parameter self-tuning function. When the instrument is used for the first time, the best P, I, and D adjustment parameters of the system can be determined through self-tuning to achieve ideal adjustment control. Before the self-tuning is started, because the parameter values ​​adjusted by the system under different set values ​​are not exactly the same, the set value of the instrument should be set to the value to be controlled (if it is a hydropower station or an intermediate value). After starting the self-tuning, the instrument forces the system to generate disturbances, and the self-tuning state ends after 2~3 oscillation cycles. The instrument calculates the best adjustment parameters of the instrument by detecting the transition characteristics of the system from overshoot to steady state (the measured value is consistent with the set value), analyzing the period, amplitude and waveform of the oscillation. The ideal adjustment effect is that the set value should be consistent with the measured value. The system adjustment quality can be evaluated from two aspects: dynamic (set value change or disturbance) and steady state (set value fixed). Through PID parameter self-tuning, most systems can be satisfied. Due to different inertias, different systems have different self-tuning times, ranging from a few minutes to a few hours.

Before the system is put into operation, we perform PID parameter self-tuning on the regulator. First, adjust its set value (SV) to the commonly used process temperature of 90°C. The instrument provides a set of PID parameters:

Proportional band P = 0.1% ~ 999.9%

Integration time I = 1 ~ 6000s

Differential time D = 0 ~ 3600s

Enter the function again and set the P, I, and D parameters according to the empirical values:

P=3.0; I=120; D=30; overshoot suppression coefficient SF=0.4.

After completing the above basic parameter settings and the system forms a closed loop, that is, the instrument input and sensor, output element and load are connected and powered on, enter the function menu to start self-tuning (AT). At this time, the AT indicator light is flashing. When it is close to the set value of 90℃, the OUT indicator light of the instrument is on and off, indicating that the thyristor is on and off, and it has entered the precise temperature control stage. After the self-tuning is completed, the AT light goes out. At this time, you can adjust the function menu to view the PID parameter values ​​after the system self-tuning, which are P=0.6, I=278, D=69, SF=0.4, and the self-tuning time is 18min. After self-tuning, the system works quite stably. The display temperature of the digital display with an accuracy of 0.5 is always 90℃, and the adjustment effect is quite satisfactory. In order to compare the differences in parameters and self-tuning time, we set the set value of the instrument to 45℃, and the self-tuning time this time is 11min. The parameters after self-tuning are:

P=0.8; I=558; D=139; SF=0.4.

After self-tuning, the digital display shows that the temperature is always 45℃, and the adjustment effect is also satisfactory.