On feedforward feedback and cascade control
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When the medium flow rate changes, the solution is to use feedforward and feedback control. In this example, TC and FC are unrelated; is it possible to connect TC and FC in series, similar to cascade control? What does it depend on whether two controllers are connected in series or not? Which one is more effective?
The system shown in the figure should adopt a cascade control system. The control system shown in the above image is quite rare.
This is the combination of feedforward and feedback in Li Yuming’s version of Chemical Process Instruments and Automation. Even if TC and FC are connected in series, it is not cascade control, because the flow loop is open-loop.
If connected in series, it resembles cascade control, but in reality it is a feedforward-feedback control system. The two controllers, TC and FC, operate in a cascade configuration, with the output of TC serving as the setpoint for FC. It resembles cascade control, but there is no secondary loop – only one feedback loop. The output of the actuator cannot change the flow rate of the material, so it cannot be considered a cascade control system. It is a non-standard form of static feedforward-feedback control. This is the answer found online. In any case, it’s definitely not cascade control, whether there is series connection or not. What I want to know is what the differences are in control performance when there is series connection versus when there isn’t
Since the flow rate of the process medium can be measured but not controlled, or it remains stable and thus no control is necessary, the material parameter that needs to be controlled is the outlet temperature. The direct factor affecting the outlet material temperature is the amount of steam added. If the temperature is high, the amount of steam added is reduced; this constitutes feedback control. The flow transmitter detects any changes in the amount of process medium being added, and the controller adjusts its output accordingly to modify the amount of steam added – this adjustment process is known as feedforward control. The advantages of this control system are minimal investment and a simple structure. If the controller function formula is set appropriately, the control performance will still be good.
The topic raised by the original poster is quite interesting. In theory, under ideal conditions – such as when the heat exchange efficiency of the heat exchanger remains constant over a wide range, the steam temperature and pressure stay fixed, the characteristics of the control valves are stable, and there are data available on how the ratio of medium flow rate to steam flow affects the temperature – it is possible to implement cascade control. The steam control valve can be regulated using the medium flow rate (through simple proportional control, which requires prior knowledge of the relationship between changes in the valve position and flow rate). By following a predetermined formula, it is possible to achieve stable control under ideal conditions. The proportionality coefficient in this control system can also be adjusted based on temperature; of course, this requires a lot of experimental data. Haha, cascade control is thus possible, but reality is reality – it takes tremendous effort to control temperature, and there’s no simple or straightforward solution available
I believe that in practical applications, it is relatively easier to tune the parameters for feedforward-feedback control (as shown in the original poster’s diagram). In my opinion, tuning complex circuits is truly difficult, especially when dealing with hazardous media.
This loop is one that uses flow rate as a feedforward element along with temperature feedback; it is not the traditional cascade loop with temperature as the primary loop and flow rate as the secondary loop. The difference lies in the different process objectives that this loop is designed to achieve. Such a loop design is typically used in situations where there are significant variations in flow rate – for example, in batch processing, the feed rate follows a certain curve, and since this rate is constantly changing, using flow rate as a feedforward element helps to overcome the disadvantages associated with delayed feedback
The control parameters need to be taken into account. If temperature is used as the control parameter, installing the steam control valve at the outlet of the heat exchanger can reduce steam consumption, but it will affect the accuracy of temperature control
Cascade control is a type of conventional feedback control, with feedback and feedforward control existing side by side.