What is Cascade Control?
In single-loop control, the controller’s set point is set by an operator, and its output drives a final control element. For example: a level controller driving a control valve to keep the level at its set point.
In a cascade control arrangement, there are two (or more) controllers of which one controller’s output drives the set point of another controller. For example: a level controller driving the set point of a flow controller to keep the level at its set point. The flow controller, in turn, drives a control valve to match the flow with the set point the level controller is requesting.
The controller driving the set point (the level controller in the example above) is called the primary, outer, or master controller. The controller receiving the set point (flow controller in the example) is called the secondary, inner or slave controller.
What are the Advantages of Cascade Control?
There are several advantages of cascade control, and most of them boil down to isolating a slow control loop from nonlinearities in the final control element. In the example above the relatively slow level control loop is isolated from any control valve problems by having the fast flow control loop deal with these problems.
Imagine that the control valve has a stiction problem (see blog on valve problems.) Without the flow control loop, the level control loop (driving the sticky valve) will continuously oscillate in a stick-slip cycle with a long (slow) period, which will quite likely affect the downstream process. With the fast flow control loop in place, the sticky control valve will cause it to oscillate, but at a much shorter (faster) period due to the inherent fast dynamic behavior of a well-tuned flow loop. It is likely that the fast oscillations will be attenuated by the downstream process without having much of an adverse effect.
Or imagine that the control valve has a nonlinear flow characteristic (see blog on valve problems.) This requires that the control loop driving it be detuned to maintain stability throughout the possible range of flow rates. (Of course there are better ways to deal with nonlinearities, but that is the topic of another blog.) If the level controller directly drives the valve, it must be detuned to maintain stability – possibly resulting in very poor level control. In a cascade control arrangement with a flow control loop driving the valve, the flow loop will be detuned to maintain stability. This will result in relatively poor flow control, but because the flow loop is dynamically so much faster than the level loop, the level control loop is hardly affected.
When Should Cascade Control be Used?
Cascade control should always be used if you have a process with relatively slow dynamics (like level, temperature, composition, humidity) and a liquid or gas flow, or some other relatively-fast process, has to be manipulated to control the slow process. For example: changing cooling water flow rate to control condenser pressure (vacuum), or changing steam flow rate to control heat exchanger outlet temperature. In both cases, flow control loops should be used as inner loops in cascade arrangements.
Does Cascade Control Have any Disadvantages?
Cascade control has three disadvantages. One, it requires an additional measurement (usually flow rate) to work. Two, there is an additional controller that has to be tuned. And three, the control strategy is more complex – for engineers and operators alike. These disadvantages have to be weighed up against the benefits of the expected improvement in control to decide if cascade control should be implemented.
When Should Cascade Control Not be Used?
Cascade control is beneficial only if the dynamics of the inner loop are fast compared to those of the outer loop. Cascade control should generally not be used if the inner loop is not at least three times faster than the outer loop, because the improved performance may not justify the added complexity.
In addition to the diminished benefits of cascade control when the inner loop is not significantly faster than the outer loop, there is also a risk of interaction between the two loops that could result in instability – especially if the inner loop is tuned very aggressively.
How Should Cascade Controls be Tuned?
A cascade arrangement should be tuned starting with the innermost loop. Once that one is tuned, it is placed in cascade control, or external set point mode, and then the loop driving its set point is tuned. Do not use quarter-amplitude-damping tuning rules (such as the unmodified Ziegler-Nichols and Cohen-Coon rules) to tune control loops in a cascade structure because it can cause instability if the process dynamics of the inner and outer loops are similar.
Jacques Smuts – Author of the book Process Control for Practitioners