Difference between feedforward and cascade control

Ask him to control whatever it is. In short, it’s all rough control. After all this effort to achieve stability within the range of ±4% to ±5%, I guess that’s about as good as it gets? Stability of ±1% – it’s estimated that even if one eats several bowls of feces, it would still be impossible to manage to dig it out. :lol.

Temperature: It can’t be seen over there; it seems to be some kind of temperature compensation

Cascade control is used to regulate the controlled variable so that it does not deviate from the set value, whereas feedforward control is aimed at dealing with disturbances. Feedforward control is generally employed in situations where the variables cannot be controlled. Moreover, a single feedforward path can only address one measurable disturbance; cascade control involves two feedback loops, or in other words, two closed-loop systems. And feedforward is open-loop. If there is a certain mathematical relationship between temperature changes and flow rate, such that an increase in temperature causes the flow rate to increase or decrease immediately, and if the product of the value of another parameter that may cause changes in the value of the controlled object, along with its coefficient of influence, is added to this difference, it becomes possible to predict the potential trends in the behavior of the controlled object in advance and take action accordingly; this is what is known as feedforward control. The main features of a cascade control system are: (1) In terms of its structural layout, it is a double-loop control system composed of two controllers that operate in series ； (2) The purpose of the system is to improve the control quality of the primary variable by introducing secondary variables ； (3) Due to the presence of the secondary loop, there is advanced control over the disturbances that enter this loop, thereby reducing the impact of such disturbances on the primary variable ； (4) The system has a certain degree of adaptability to changes in load.

Is this a control scheme for a distillation tower? I think the person who drew it is an amateur; what he intended to represent was cascade control of temperature and flow rate, but what he ended up drawing looks like nothing of the sort – it has some elements of feedforward control, but that’s not correct either; there’s a SET value, which indicates cascade control, yet there’s no main controller visible

This diagram shows a temperature-flow cascade control system, where temperature is the primary loop and flow is the secondary loop. But there is an error in the diagram: it should be TICA in the main control circuit, which stands for temperature indication, control, and alarm; the letters C and A are missing ； The secondary circuit should be FICA, that is, Flow Indication, Control, Alarm – the letter A is missing.

This should be feedforward control; it’s nothing special, and formula calculations are required. Which one said it’s cascade?

It mainly depends on your process requirements

There is a fundamental difference between feedforward and cascade control in actual control processes. If we only look at the diagram you provided, it seems to indicate cascade control, but a display module is being used for the temperature loop. Whether it is the sensitive plate of the tower or the temperature control of the reactor, temperature cascade control with flow regulation is a very useful approach; with proper adjustment, the temperature fluctuations remain around ±1 degree. The key lies in the design and PID tuning. Feedforward, as the name implies, involves predicting in advance and adding a correction factor; it is generally not used in temperature control loops, but can be applied to the feed of a tower, for example. Taking your diagram as an example, if the feed rate increases by a certain amount, steam HS may need to be increased by 200 kilograms in order to maintain stability in the temperature of the sensitive plate. Feedforward means adding 200 kilograms of steam in advance, before the temperature starts to drop. In the case of a simple cascade control system, it is necessary to wait until the temperature drops before adding steam. Moreover, due to the level of PID control, fluctuations in temperature and in the tower generally occur. If there is anything still unclear, please ask.