Application of PID control in frequency converter

PID controller is the most widely used industrial controller in electromechanical, metallurgical, mechanical, chemical and other industries. There are two PID control methods through frequency converter: one is the built-in PID control function of frequency converter, the given signal is input through the panel or the terminal of frequency converter, and the feedback signal is fed back to the control end. PID is performed inside the frequency converter. Adjustment to change the output frequency; the other is to use an external PID controller to set the given amount and feedback amount, and output it as a control signal to the control end of the frequency converter.

1. PID control block diagram

2. PID control overview

In automatic control, P (proportional), I (integral), and D (differential) are generally not used alone. PI control, PD control and PID control are often used.

01 PI control

P action control cannot completely eliminate deviation. In order to eliminate residual deviation, PI control is generally used, that is, the sum of P+I operations. When using PI control, the target value and deviation caused by changing can be eliminated.

02 PD control

In order to attenuate the oscillation of P action and stabilize the system, PD control can be used, that is, the sum of P and D operations. In other words, PD control is suitable for loads that do not have a braking effect in the process itself.

03 PID control

PID control is a control that combines the advantages of PI control and PD control. PID operation is the sum of the three operations of P, I and D, which can obtain a control process with no deviation, high precision and stable system.

① Negative action: When the deviation X (target value-measured value) is positive, increase the execution amount (output frequency); if the deviation is negative, reduce the execution amount.

② Positive action: When the deviation X (target value-measured value) is negative, increase the execution amount (output frequency); if the deviation is positive, reduce the execution amount.

3. Feedback signal access method

1) Given input method: When the inverter uses the PID function, the feedback signal measured by the sensor is directly connected to the given signal terminal, and its target signal is given by the keyboard.

2) Independent input method: The inverter is specially configured with an independent feedback signal input terminal. Some inverters are also configured with a power supply for the sensor. Its target value can be given by the keyboard or input by the specified input terminal.

4. Preset of PID adjustment function

01 Preset PID adjustment function

The preset content is whether the PID adjustment function of the inverter is effective. After the PID adjustment function of the inverter is effective, its acceleration/deceleration process will completely depend on the dynamic response process determined by P, I, and D data, and the originally preset "acceleration time" and "deceleration time" will no longer work.

02 Preset of target value

The fundamental basis of PID adjustment is the result of the comparison between the feedback amount and the target amount. Therefore, it is very important to accurately preset the target value, mainly in the following two aspects.

① Panel input type: just enter the target value through the keyboard. The target value is usually a percentage of the actual size of the measurement to the sensor range. For example, the pressure (target pressure) required by the air compressor is 6MPa, and the range of the pressure gauge used is 0~10Mpa, then the target value is 60%.

② External given type: preset by an external potentiometer, which is more convenient to adjust.

5. Characteristics of the inverter when it operates according to the P, I, D regulation rules

1) The input frequency of the inverter is adjusted only according to the comparison result between the actual value and the target value, and there is no corresponding relationship between it and the controlled quantity.

2) The output frequency of the inverter is always in the adjustment state, and its value is often unstable.