What Is Lag Time?

The phenomenon that the controlled variable of the controlled object lags behind the interference is called lag. Lag is divided into pure lag and transition lag. In actual objects, pure lag and transition lag coexist at the same time. It is difficult to strictly distinguish them. The two are often collectively called lag time. The existence of the hysteresis is not conducive to control, so when designing and installing the control system, the hysteresis should be minimized as much as possible. ^[1]

Pure lag is also called transfer lag, which is caused by the time required for the material or energy transmission process. For example, in Figure 1, because the inlet valve is far away, after the opening degree of the valve changes, it takes a period of time for the input material to enter the water tank to affect the value of the liquid level.

The transition lag is also called capacity lag. Some objects have two or more capacities, which are called multi-capacity objects, such as jacketed steam heaters and serial level containers. Figure 2 is a dual-capacity object with a resistance element (valve) in communication between its two capacities. When the feed valve suddenly opens, the liquid levels of the two associated containers must change. After a period of time, the liquid levels of the two containers also stabilize again. The dynamic characteristic curve is shown in Figure 3. It can be seen from the figure that the transition process of the first capacity is basically the same as that of the single-capacity object. The controlled variable of the second volume has a special change. There is an inflection point W on the curve. The point on the left of W changes faster, and the change on the right gradually decreases. The tangent of the curve through the point W intersects the original value, and the time interval between the point of intersection and the starting point of the controlled variable starts to change is a transition lag. ^[1]

The larger the capacity coefficient and resistance of the object, the larger the number of capacity, the longer the transition lag time, and the slower the transition process. In automatic control systems, the presence of hysteresis is not conducive to control. Because the interference has occurred, but the detection instrument can not feel it in time, the regulator is still in the original stable state, and it will have a control output after a period of time. After the control signal is sent out, the control effect cannot affect the object in time, and the entire control quality will be seriously affected. Therefore, when designing and installing the control system, the lag should be minimized as much as possible. ^[1]