PID Controllers, 2nd Edition

Chapter 7.2.6 - Control Paradigms: Some Applications

Some Applications

Cascade control is a convenient way to use extra measurements to improve control performance. The following examples illustrate some applications.

EXAMPLE 7.2 Valve positioners

Control loops with pneumatic valves is a very common application. In this case the inner loop is a feedback around the valve itself where the valve position is measured. The inner loop reduces the influences of pressure variations and various nonlinearities in the pneumatic system. ?

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Figure 7.4 Block diagram of a system for position control. The system has three cascaded loops with a current controller (CC) with feedback from current (I), a velocity controller (VC) with feedback from velocity (v), and a position controller (PC) with feedback from position (y).

EXAMPLE 7.3 Motor control

Figure 7.4 is a block diagram of a typical motor control system. This system has three cascaded loops. The innermost loop is a current loop where the current is measured. The next loop is the velocity loop, which is based on measurement of the velocity. The outer loop is a position loop. In this case integral action in the velocity loop is equivalent to proportional action in the position loop. Furthermore, it is clear that the derivative action in the position loop is equivalent with proportional action in the velocity loop. From this it follows directly that there is no reason to introduce integral action in the velocity controller or derivative action in the position controller.

EXAMPLE 7.4 Heat exchanger

A schematic diagram of a heat exchanger is shown in Figure 7.5. The purpose of the control system is to control the outlet temperature on the secondary side by changing the valve on the primary side. The control system shown uses cascade control. The secondary loop is a flow control system around the valve. The control variable of the primary loop is the setpoint of the flow controller. The effect of nonlinearities in the valve, as well as flow and pressure disturbances, are thus reduced by the secondary controller.

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