Cascade Control Principle

What is Cascade Control?

In single-loop control, an operator sets the controller’s set point, and the controller’s output drives a final control element. For instance, a level controller may operate a control valve to maintain a predetermined level.

There are two (or more) controllers in a cascade control setup, with one controller’s output driving the set point of the other. A level controller, for example, could drive the set point of a flow controller to keep the level at its set point. The flow controller then controls a control valve to match the flow to the setpoint requested by the level controller.

The major, outer, or master controller is the one that drives the set point (in this case, the level controller). The secondary, inner, or slave controller is the one that receives the setpoint (in this case, the flow controller). When either (1) disturbances affect a measurable intermediate or secondary process output that directly affects the primary process output that we want to control; or (2) the gain of the secondary process, including the actuator, is nonlinear, cascade control can improve control system performance over single-loop control. A cascade control system can restrict the influence of the first situation.

In the second situation, a cascade control system can reduce the impact of changes in actuator or secondary process gain on control system performance. Modifications in operating points as a result of setpoint changes or prolonged disturbances are the most common causes of such gain variations.

Cascade control’s components

A cascade control system with two controllers, two sensors, and one actuator functioning in series on two processes is depicted in the Cascade Control Block Diagram. The setpoint for a secondary or slave controller is generated by a primary or master controller. The actuator, in turn, is used by the controller to direct the control effort to the secondary process. After that, the secondary process generates a secondary process variable that serves as the primary process’s control effort.

The geometry of the block diagram defines an inner loop involving the secondary controller and an outer loop involving the primary controller. The internal loop is comparable to the typical feedback control system since it has a setpoint, a process variable, and a process controller using an actuator. The external loop operates in the same way as the inner loop, except that the entire inner loop is used as its actuator.

The tank temperature controller is primary in the water heater example because it specifies the setpoint that the steam flow controller must attain. The primary process, primary process variable, secondary process, and secondary process variable, respectively, would be the water in the tank, the tank temperature, the steam, and the steam flow rate (refer to the Cascade Control Block Diagram). The actuator, which works directly on the secondary process and indirectly on the primary process, is the valve that the steam flow controller employs to keep the steam flow rate constant.

Requirements

Naturally, a cascade control system cannot address all feedback control problems, but under the correct situations, it can be beneficial:

The outer loop is controlled by the inner loop. The secondary controller’s actions must have a predictable and repeatable effect on the major process variable; otherwise, the primary controller will be unable to influence its own process.

The inner loop beats the outer loop in speed. The secondary process must respond to the secondary controller’s efforts at least three or four times faster than the primary process. This provides adequate time for the secondary controller to adapt for inner loop disturbances before they affect the primary operation.

Disturbances in the inner loop are less severe than those in the outer loop. Otherwise, the secondary controller will be unable to apply consistent corrective efforts to the primary process since it will be continuously correcting for secondary process disturbances.

Because raising or lowering the steam flow rate raises or lowers the tank temperature without any additional actuators, a valve can manipulate a steam flow rate almost instantly in comparison to the slow rate at which steam can heat the water in a large tank, and disturbances to the steam supply pressure a valve can manipulate a steam flow rate almost instantly in comparison to the slow rate at which steam can heat the water in a large tank, stream-fed water heaters like the example are particularly amenable to cascade control.

When Should Cascade Control be Used?

If you have a process with relatively slow dynamics (such as level, temperature, composition, or humidity) and you need to manage a liquid or gas flow, or some other relatively quick process to control the slow process, you should always employ a cascade control. Adjusting the flow rate of cooling water to control condenser pressure (vacuum), or changing the flow rate of steam to control the heat exchanger outlet. Flow control loops should be employed as inner loops in cascade systems in both circumstances.

Does Cascade Control Have any Disadvantages?

There are three drawbacks to cascade control. To begin with, it necessitates an additional measurement (typically flow rate) in order to function. Two, there’s another controller that needs to be fine-tuned. Third, the control method is more difficult to understand — for both engineers and operators. To determine if cascade control should be used, these disadvantages must be evaluated against the benefits of the predicted improvement in control.

When Should Cascade Control Not be Used?

Only if the inner loop’s dynamics are faster than the outer loop’s is cascade control advantageous. If the inner loop is not at least three times quicker than the outer loop, cascade control should be avoided since the improved performance may not justify the added complexity.

When the inner loop is not considerably faster than the outer loop, the benefits of cascade control are reduced, and there is a possibility of interaction between the two loops, which could lead to instability – especially if the inner loop is adjusted very aggressively.

How Should Cascade Controls be Tuned?

Starting with the innermost loop, a cascade configuration should be tuned. After that one is tuned, it is put into cascade control, or external set point mode and the loop driving its set point is tuned as well. When tuning control loops in a cascade structure, quarter-amplitude-damping tuning rules (such as the unaltered Ziegler-Nichols and Cohen-Coon rules) should not be used since they can cause instability.