1. Feedforward control is an open-loop control, and feedback control is a closed-loop control with negative feedback.
- The general fixed value control system is adjusted according to the deviation obtained by comparing the measured value with the given value, which belongs to the closed-loop negative feedback adjustment. Its characteristic is to adjust after the controlled variable has a deviation; if the disturbance has occurred without deviation, the regulator will not work. Therefore, the adjustment effect of the feedback control method lags behind the disturbance effect.
- Feedforward regulation is adjusted according to the disturbance effect. The feed-forward control measures the disturbance and directly introduces it into the adjustment device, and the overcoming of the disturbance is more timely than the feedback control.
Now take the heat exchanger control scheme as an example to intuitively explain the feedforward control and feedback control:
2. Measure the disturbance in the feedforward control system, and measure the controlled variable in the feedback control system
In a pure feedforward control system, the controlled variable is not measured, and in a pure feedback control system, no disturbance is measured.
3. Feedforward control requires a special regulator, and feedback control generally uses a general-purpose PID regulator
Feedback regulation conforms to PID regulation law, and it is usually realized by general-purpose PID regulator, DCS, etc. or PLC control system. The regulator used in feedforward regulation is a feedforward regulator that determines the regulation law according to the characteristics of the controlled object.
4. Feedforward control can only overcome the measured disturbance, while feedback control can overcome all disturbances
If the disturbance cannot be measured in the feedforward control system, it is impossible for the feedforward to overcome it. In the feedback control system, any disturbance, as long as it affects the controlled variable, can be overcome to a certain extent.
5. Feedforward control can theoretically have no difference, but feedback control must have difference.
Feedback adjustment makes the system achieve dynamic stability, allowing the adjusted parameter to stabilize and change dynamically near the given value, but it cannot stabilize the adjusted parameter at the given value. Feed-forward regulation can realize no-difference regulation in theory.
6. Limitations of feedforward control
A. In the production and application, the characteristics of various links change with the load, and the diversity of the dynamic characteristics of the object is difficult to accurately measure, which may easily cause over-compensation or under-compensation. In order to compensate for the inaccuracy of feedforward regulation, feedforward and feedback control systems are usually combined to form a feedforward feedback control system.
B. There are multiple disturbances in industrial objects. If feed-forward controllers are set for all of them, the equipment investment will be high and the workload will be heavy.
C. Many feedforward compensation results have no detection means under the existing technical conditions.
D. Feedforward control is limited by the accuracy of the feedforward control model.
E , feed-forward control algorithm, often do approximate processing.
Feedforward control selection principle
1. When there are disturbances with high frequency, large amplitude, measurable but uncontrollable disturbance in the system, feedforward control can be selected.
2. When the lag time of the control system control channel is long and the feedback control cannot obtain good results, the feedforward control can be selected.
3. The selection of feedforward control should conform to the principle of economy.
4. After the feedforward control scheme is decided, if the static feedforward can meet the process requirements, then the dynamic feedforward is not selected.
Advantages of Feedforward-Feedback Control System
1. From the perspective of feedforward control, due to the addition of feedback control, the requirements for the accuracy of the feedforward control model are reduced, and the disturbance of the unmeasured interference signal can be corrected.
2. From the perspective of feedback control, the feedforward control function can make rough adjustments to the main disturbances in time, greatly reducing the burden of feedback control
Feedforward-Feedback Control Application Example
Now take two heat exchanger control schemes as an example to illustrate the feedforward-feedback control intuitively:
1. Heat exchanger feedforward feedback control scheme 1
2. Heat exchanger feedforward feedback control scheme 2
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