CASCADE CONTROL TECHNOLOGY
When one steps beyond basic feedback control, cascade and ratio control are probably the first of the so-called advanced regulatory control techniques one encounters. In a cascade control system, one feedback controller adjusts the set point of another feedback controller. The upper-level controller is called the “primary,” while the lower level is called the “secondary.” A typical application of cascade control is a temperature controller cascaded to a flow controller.
Figure 9-1 depicts a cascade control system, using both ISA (Ref. 9-1) and SAMA symbols (Ref 9-2).
Figure 9-1. Symbolic Representations of a Cascade Control System 7,&
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Before investigating the finer points of cascade control systems, let us first consider a situation that illustrates the motivation for using cascade control. In Figure 9-2a the process outlet tem-perature of a heat exchanger is sensed. The temtem-perature controller then adjusts the set point of the steam-flow controller to maintain the outlet temperature at set point. An alternative control strategy would be for the temperature controller to directly manipulate the control valve, as shown in Figure 9-2b. To compare these two schemes, we need to consider the disturbances to the process. We will consider several sources of disturbance, both with and without the cas-cade control system.
First, suppose there is no cascade, as shown in Figure 9-2b, and suppose also that the process flow rate increases. This increased load on the heat exchanger will cause the outlet temperature to drop. The temperature controller will react by increasing the signal to the valve. The new valve position will in turn cause an increase in steam flow. The effect of this change in steam flow must then pass through the heat exchanger before its corrective effect is felt by the tem-perature sensor.
Now, suppose that the temperature controller cascades a steam-flow controller, as shown in Figure 9-2a. Suppose that there is the same increase in the process flow rate as before. Again, there will be a drop in outlet temperature. The temperature controller will react by increasing the set point of the steam-flow controller, which, in turn, will increase the signal to the valve.
With the increased valve position, the flow will quickly respond to the increased demand from the temperature controller. The effect of this change in steam flow must then pass through the heat exchanger before its corrective effect is felt by the temperature sensor.
These two scenarios illustrate the fact that with this particular disturbance, the cascade control loop provides little or no difference in response. Consider two other scenarios. We begin as we did before, with no cascade—the temperature controller is connected directly to the valve.
This time, the assumed disturbance is a drop in the steam header pressure. At the current valve position, this will cause a drop in steam flow; after the effect passes through the heat exchanger, this results in a drop in outlet temperature. From there on, the events are the same as in the first scenario. The temperature controller reacts by increasing the valve position. This restores the steam flow, but the effect must then pass through the heat exchanger before the corrective effect is felt by the temperature sensor. The response of the temperature loop to this disturbance is approximately the same as it was to the increase in process flow.
One last scenario. We return to the cascade control configuration and suppose there is a drop in steam header pressure. The steam-flow rate drops, but this is sensed by the flow transmitter.
The flow controller quickly responds by increasing the valve position, restoring the flow to the set point demanded by the temperature controller. The net effect is that the steam flow is dis-turbed only momentarily. Given that this momentary disturbance must pass through the heat exchanger whose dynamics will tend to filter the effect, the process outlet temperature will show very little of the effect of this disturbance.
With this example in mind, look at the block diagram of a generic cascade control loop shown in Figure 9-3. This figure shows an inner loop and an outer loop. The inner loop is comprised
Figure 9-2. A Temperature Control System, With and Without Cascade Control )7
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of a secondary controller and a secondary process. The primary controller and the primary pro-cess are components of the outer loop. The inner loop is also a component of the outer loop, since the primary controller sets the set point of the secondary controller. Also, the output of the secondary process is an input to the primary process.
In the example shown in Figure 9-2, the primary controller is the temperature controller, the secondary controller is the flow controller, the primary process is the heat exchanger itself, and the secondary process is the flow-control valve. The output of the secondary process (steam flow) is the input to the primary process (heat exchanger). It is also measured and becomes the process variable for the flow controller.
The block diagram shows two disturbances, one into the primary process and one into the sec-ondary process. The primary process disturbance is analogous to a change in process flow, whereas the secondary disturbance is analogous to changes in the steam header pressure.
The results of analyzing the example can be extended to the more general case. For distur-bances that enter the primary loop but are outside the secondary loop, the cascade is of little benefit. For disturbances that enter the inner loop, the secondary controller is very beneficial in compensating for them and preventing the disturbances from affecting the outer loop. If the secondary controller were not present then only the outer loop would exist. A disturbance to the secondary process would inherently be a disturbance in the outer loop, and we would have to pay the feedback penalty at the primary controller to compensate for it. When the secondary loop is present, it provides the necessary control action without paying the feedback penalty in the primary loop.
One requirement for cascade control that we have alluded to but not explicitly stated is that the inner (secondary) loop should be significantly faster than the outer (primary) loop. There is no precise definition of “significantly faster.” A good rule of thumb is that the frequency of oscil-lation of the secondary loop, when well tuned, should be at least three times that of the primary loop, also when well tuned. If the primary loop’s speed of response is near that of the second-ary loop, adverse interaction (“fighting”) can occur between the primsecond-ary and secondsecond-ary con-Figure 9-3. Block Diagram of a Cascade Control System
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necessary in the first place, since a single feedback controller can cope with equal ease to dis-turbances to the secondary or primary process.
Normally, the loops’ relative speed of response is not a problem, since with most common applications of cascade, such as temperature cascaded to flow, the difference in speed of response will be greater than the rule of thumb requires. If, for some reason, the primary and secondary loops have a similar speed of response, then the primary loop can be dampened by reducing the controller gain and lengthening the integral time.