How to choose feedforward or cascade control

If the main factors causing changes in the modulated variable can be effectively measured, feedforward control can be used.

This post was last edited by pH7 on 2010-8-4 21:02. Before explaining when to use cascade and when to use feedforward, let's first clarify their differences. Cascade and feedforward are two completely different adjustment methods. Cascade is two feedback loops, or two closed loops. And feedforward is open loop. Therefore, whether it is a cascade or a single loop, the system does not start to adjust until the adjusted device deviates from the given value. Feedforward means that the system starts to adjust before the interference channel starts to change and affects the adjusted one. For example, feedback or feedback is equivalent to people being in fashion * * If you risk the environment and catch a cold, go to the doctor for treatment. And feedforward is that although we are in the popular * * Risk the environment and take the initiative to take some medicine to prevent the occurrence of colds. Let’s take a heat exchanger as an example. Usually we need to control the temperature of the outlet material, and often measure the temperature of the outlet material to use the measured value to control the steam valve. This is a single circuit. Sometimes the material flow fluctuates greatly, so we add a material flow loop as an inner loop to form a cascade (usually we don’t do this, but use steam flow as the inner loop. This is said to facilitate comparison and illustrate the problem). However, regardless of single loop or cascade, the system does not start to adjust until the fluctuation of material flow causes the outlet material temperature to deviate from a given value. Of course, with cascade, the impact of material flow fluctuations on the outlet material temperature is much smaller. However, if we know that material flow fluctuation is the main interference and know the characteristics of the interference channel, or the mathematical model, we can use this characteristic to make compensation in the system. The input of the compensation is the flow rate of the material, and the output is the input signal of the steam regulating valve, which constitutes a feedforward regulation. In this loop, as soon as the flow rate of the material changes, the amount of steam will immediately change, and the temperature of the outlet material will not be disturbed. This is an open loop. From the above discussion, we can conclude: Feedforward regulation can be used when the main disturbance is known and its characteristics are known. If there are many interferences but the main interference cannot be found, or the characteristics of the main interference cannot be determined, we use feedback, which can be single loop or cascade. If feedforward is used well, it will achieve better results than feedback. However, if the characteristics of the interference channel are uncertain, or there are other powerful interferences, it is better to use feedback. Of course, if conditions permit, multi-variable feedforward can be used for multiple interference channels. In addition, feedforward can also be used in combination with feedback, cascade, etc. I won’t write much anymore. If I continue to write, it will be a thesis.

HOHO, those two ×× are influenza and cold

Reply 3# pH7 In petrochemical enterprises, can you give some examples of the application of feedforward control or which unit controls are suitable for feedforward control.

In fact, the three-impulse adjustment of boiler water level is a typical feedforward plus cascade adjustment system. Fluctuations in steam flow are added as feedforward quantities to the cascade system consisting of water level and inlet water flow. There's a post here that goes into great detail about the three-impulse system, I just looked at it. http://bbs.hcbbs.com/thread-441760-1-1.html In petrochemicals, I have the impression that the heat enthalpy control (UOP technology) of the xylene (maybe toluene) distillation tower is a feed forward. It’s been 10 years since I left Petrochemical and I really can’t remember. If the moderator has friends who work in a xylene workshop, you can ask. I also want to return * Just for a moment. As for which units can use feedforward, this is a topic worthy of discussion. I hope everyone can discuss it together

Generally, feedforward control is not used alone.

What was said on the third floor is quite good. The three-impulse adjustment of the boiler is a very typical example.