Principle of Cascade PID Control
The typical structure of the cascade computer control system is shown in the following figure. There are two PID controllers in the system. Gc2 (s) is called the transfer function of the sub-regulator, and the inner loop surrounding Gc2 (s) is called the sub-loop. Gc1 (s) is called the transfer function of the main regulator, and the outer ring surrounding Gc1 (s) is called the main loop. The output control quantity u1 of the main regulator is used as the given amount R2 (s) of the secondary circuit.
The computer sequence of the cascade control system is the main loop (PID1) first and then the secondary loop (PID2). There are two control methods: one is asynchronous sampling control, that is, the sampling control period T1 of the main loop is an integer multiple of the sampling control period T2 of the sub-loop. This is because the response speed of the main control object is slow and the response speed of the sub-control object is fast in the general cascade control system. The other is the same frequency sampling control, that is, the sampling control cycle of the main and auxiliary loops is the same. At this time, the sampling period should be selected according to the secondary loop, because the response speed of the controlled object of the secondary loop is faster.
The main advantages of cascade control:
1. Add interference to the secondary circuit, and suppress it by the secondary circuit control;
2. The changes of the parameters in the secondary circuit are controlled by the secondary circuit, and the influence on the controlled variable Gc1 is greatly reduced;
3. The inertia of the secondary loop is adjusted by the secondary loop, thus increasing the response speed of the entire system.
The secondary circuit is the key to the design of the cascade system. There are many ways to design the secondary circuit. The following describes the design method of designing the secondary regulator according to the expected closed-loop characteristics.
From the block diagram of the secondary circuit, the transfer function of the closed-loop system of the secondary circuit is:
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