【Daily Question 20090405】Feedforward Control Systems and Feedback Control Systems
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The control loop systems that people are usually exposed to are generally feedback control systems, which calculate the control output based on the difference between the measured value and the set value. I have some examples of where feedforward control systems have been used in practice, which I’d like to share with everyone. Original contributions from everyone are welcome; the more detailed the examples, the better. It would be ideal to include detailed explanations of the control methods and the underlying principles. This post was last edited by chmyh on 2009-4-5 11:06]
A system that is controlled based on the magnitude of the disturbance is called a feedforward control system. The main characteristics of a feedforward control system are as follows: 1. Feedforward control operates on the principle of invariance, and it is more timely and effective than feedback control ； 2. Feedforward control belongs to the “open-loop” control system ； 3. Feedforward control uses a “specialized” controller determined by the characteristics of the object, also known as a feedforward compensation device ； 4. A feedforward action can only overcome one type of interference. Feedforward control is mainly used in the following situations: 1. When the amplitude of disturbances is large and frequent, causing significant effects on the controlled variable, and simple feedback control is insufficient to meet the requirements ； 2. The main interference is a measurable but uncontrollable variable ； (3) When the control channel of the object has a large lag, feedback control is not timely, and the control quality is poor, a feedforward-feedback control system can be employed to improve the control quality.
The inlet-outlet control of the mixing tank in oilfield wastewater treatment plants adopts a pH value feedforward-feedback control scheme.
There are also typical heat exchanger enthalpy control and three-transistor control for the boiler drum liquid level, among others.
The heat exchange control in the synthesis tower is a typical example of feedforward control
For boiler drum water level control, steam flow serves as the primary feedforward element and the secondary control mechanism