Why is the lag in level control so large, and why is cascade control of level and flow required?

For level control, it involves water tanks or other volumetric devices. I recall that the mathematical model of a water tank was discussed in university textbooks on automatic control; a single-tank system can be described as a first-order system with time delay. Thus, the control object itself is a system with delay. In single-loop control, it is not possible to eliminate the effect of this time delay. In cascade control, the secondary loop (i.e., the flow control loop) is located closer to the control valve, resulting in a smaller pure delay, which helps to reduce the impact of this delay on the main control variable. That’s about all I can remember; for specific explanations, please refer to the analysis of cascade control in the procedure book! :P

There are two types of cascade control for liquid level and flow rate: one is the cascade control using the incoming material flow and the outlet liquid level, and the other is the cascade control using the outlet liquid level and the outlet flow rate (the use of the incoming material flow to control the liquid level is less common and will not be considered here). Outlet liquid control and outlet flow control are generally not intended to address the issue of lag; rather, they serve to stabilize the outlet flow, preventing large fluctuations in the outlet flow (and thus the flow to downstream equipment) that could result from simple level control. In such cases, it is acceptable for the liquid level to vary significantly. The cascade control between the inlet flow rate (which is uncontrollable) and the outlet liquid level is intended to address the issue of lag in liquid level control; it focuses on stabilizing the liquid level, allowing for significant variations in the outlet flow rate.

It’s not a lag issue; rather, it’s because the container has a large diameter, resulting in a large time constant. Moreover, when flow rate is the main source of disturbance, a cascade control system between the liquid level and flow rate is necessary to improve the stability of the liquid level.

There are many reasons for such lag, and one of them is a problem with the design. For example, with level control based on the liquid level you mentioned, if the container has a large volume but the diameter of the feed valve or discharge valve is small, then even when the valve is fully open or fully closed, the change in liquid level is very slow

1. Doesn’t the liquid level change as long as there is flow entering the device? ——With a stable inflow and a stable outflow, once the balance point is reached, the liquid level will not change. 2. When the flow rate becomes 0, the liquid level also stops changing immediately. ——The feed flow suddenly drops to 0, and the discharge valve doesn’t have time to close, so the liquid level drops rapidly at this point instead of ceasing to change.