Effect of level-flow cascade control

Liquid level – flow rate cascade control: You need to understand that each control system has a setpoint (SV), a process variable value (PV), and an output (MV). In a simple flow control system, for example, with a setpoint of SV=2 m3/h, if the actual value PV is only 1 m3/h, then the output MV represents the valve opening degree; MV=60% means that the control valve is open to 60% of its full capacity. Similarly, level-flow cascade control is just a cascade system with an additional level control step; the level output is used as the setpoint for flow control. Level SV — Level PV — Level MV (flow setting) — Flow PV — Flow MV

Cascade control is managed by two regulators; the output of the main regulator serves as the setpoint for the secondary regulator. In level control, the level is used as the input for the main regulator, while flow rate is used as the input for the secondary regulator. When the level or flow rate changes, the changes in each component are not necessarily fixed, and this depends on the positive and negative feedback settings of each control element.

It should be like this: the objective of control is to maintain a certain liquid level. To keep this level stable at a specific height, flow rate is controlled indirectly; flow rate constitutes the second control parameter, or the secondary control variable, and its output is the valve opening degree mv. To control the liquid level, you can directly adjust the opening degree of the valve; however, this will cause fluctuations in flow rate, which is not suitable for certain chemical production processes that require a stable material flow rate. If the valve is located before the system, that is, it is the control valve leading to the tank, then as the liquid level rises, the flow rate set at that point decreases, which in turn results in a smaller opening degree of the valve. If the valve is located after the system, that is, it is the control valve that regulates the flow out of the tank, then as the liquid level rises, the flow rate set at that point increases, which in turn causes the control valve to open more widely.

If it is only a change in flow rate, this will cause the opening degree of the control valve to change, allowing it to return to the set flow rate

What are the advantages of level-flow cascade control over simple level control? Is level-flow cascade mainly used for controlling the liquid level? The final result is that the liquid level remains constant, with only slight fluctuations in flow rate? :)

If you want to maintain a stable liquid level, set the liquid level value as the primary controlled variable and the flow rate measurement as the secondary controlled variable. If the industrial process is unstable and there are slight fluctuations in the flow rate, the secondary circuit will adjust first, preventing any fluctuations in the liquid level. However, if the flow rate fluctuates significantly, the liquid level will also fluctuate; at this point, the regulator in the primary circuit will send out signals. For the secondary circuit, both the setpoint and the feedback value change, which causes the regulator in that circuit to increase its output and thus accelerate the adjustment process. The advantage is that minor disturbances in the liquid level have little impact; its robustness is greater than that of a simple single-loop liquid level control system, and it recovers to the normal value more quickly in the case of major disturbances

Can level-flow cascade control be used to control flow rate? Is it still mainly for controlling the liquid level?