Selector Control

Selector control can be viewed as the inverse of split range control. In split range there is one measured signal and several actuators. In selector control there are many measured signals and only one actuator. A selector is a static device with many inputs and one output. There are two types of selectors: maximum and minimum. For a maximum selector the output is the largest of the input signals.

There are situations where there are several controlled process variables that must be taken into account. One variable is the primary controlled variable, but it is also required that other process variables remain within given ranges. Selector control can be used to achieve this. The idea is to use several controllers and to have a selector that chooses the controller that is most appropriate. One example of use is where the primary controlled variable is temperature and we must ensure that pressure does not exceed a certain range for safety reasons.

The principle of selector control is illustrated in Figure 7.18. The primary controlled variable is the process output y. There is an auxiliary measured variable z that should be kept within the limits z_min and z_max. The primary controller C has process variable y, setpoint y_sp, and output u_n. There are also secondary controllers with measured process variables that are the auxiliary variable z and with setpoints that are bounds of the variable z. The outputs of these controllers are u_h and u_l. The controller C is an ordinary PI or PID controller that gives good control under normal circumstances. The output of the minimum selector is the smallest of the input signals; the output of the maximum selector is the largest of the inputs.

Under normal circumstances the auxiliary variable is larger than the minimum value z_min and smaller than the maximum value z_max. This means that the output u_h is large and the output u_l is small. The maximum selector, therefore, selects u_n and the minimum selector also selects u_n. The system acts as if the maximum and minimum controller were not present. If the variable z reaches its upper limit, the variable u_h becomes small and is selected by the minimum selector. This means that the control system now attempts to control the

Figure 7.18 Selector control.

variable z and drive it towards its limit. A similar situation occurs if the variable z becomes smaller than z_min.

In a system with selectors, only one control loop at a time is in operation. The controllers can be tuned in the same way as single-loop controllers. There may be some difficulties with conditions when the controller switches. With controllers having integral action, it is also necessary to track the integral states of those controllers that are not in operation. Selector control is very common in order to guarantee that variables remain within constraints. The technique is commonly used in the power industry for control in boilers, power systems, and nuclear reactors. The advantage is that it is built up of simple nonlinear components and PI and PID controllers. An alternative to selector control is to make a combination of ordinary controllers and logic. The following example illustrates the use of selector control.

EXAMPLE 7.7 Air-fuel control

In Example 7.6 we discussed air-fuel control. Ratio control has two disadvantages. When the power demand is increased, there may be lack of air because the setpoint of the air controller increases first when the dual controller has increased the oil flow. The system cannot compensate for perturbations in the air channel. A much improved system uses selectors, such as is shown in Figure 7.19. The system uses one minimum and one maximum selector. There is one PI controller for fuel flow and one PI controller for the air flow. The setpoint for the air controller is the larger of the command signal and the fuel flow.

Figure 7.19 Air-fuel controller based on selectors. Compare with the ratio controller for the same system in Figure 7.16.

This means that the air flow will increase as soon as more energy is demanded. Similarly, the setpoint to the fuel flow is the smaller of the demand signal and the air flow. This means that when demand is decreased, the setpoint to the dual flow controller will immediately be decreased, but the setpoint to the air controller will remain high until the oil flow has actually decreased. The system thus ensures that there will always be an excess of air. It is important to maintain good air quality. It is particularly important in ship boilers because captains may pay heavy penalties if there are smoke puffs coming out of the stacks when in port.

Median Selectors

A median selector is a device with many inputs and many outputs. Its output selects the input that represents the current median of the input signals. A special case is the two-out-of-three selector, commonly used for highly sensitive systems. To achieve high reliability it is possible to use redundant sensors and controllers. By inserting median selectors it is possible to have a system that will continue to function even if several components fail.