Benefits of Cascade Control

We know very well that closed loop control provides increased performance over open loop control. So it would be rational to expect benefits from adding inner control loop around plant items that have degrading overall performance.

The Figure (a) below illustrates a typical example of a cascade control:

In Figure (a) above, the outer loop controller becomes the setpoint for the inner controller. Any problems in the inner loop like disturbances, nonlinearities, phase lag, etc. will be handled by the inner controller thereby improving the overall performance of the outer loop. This whole arrangement is called cascade control.

It is clear that to apply cascade control, there must be some intermediate process variable that can be measured in this case PV_IN as shown in Figure (a) and some actuation point that can be used to control it.

Benefits of Cascade Control

Cascade control adds several benefits to the process:

• The secondary (inner) controller deals with the disturbances before they can affect the outer loop.
• Phase shift within the inner loop is reduced leading to increased stability and speed of response in the outer loop.
• Devices with inherent integral action like a motorized valve can introduce an inherent -90° integral phase lag. This can be removed by adding a valve positioner in cascade.
• Cascade control also reduces the effect of nonlinearities e.g. nonlinear gain, backlash, etc. in the inner loop.

However there are a few precautions that need to be taken. Our analysis above does not take into account the fact the components in the system saturate and stability problems may arise when the loop saturates. This kind of problem can be overcome by limiting the demands the outer loop controller places on the inner loop i.e. by ensuring that the outer loop controller saturates first or by providing a signal from the inner to the outer controller which inhibits the integral term when the loop is saturated.

The application of cascade control requires an intermediate process variable and control action point and ought to include if possible, the plant item with the shortest time constant. Generally high gain proportional only control will often be enough for the inner loop, any offset is of little concern as it will be removed by the outer controller. For stability the inner loop must always be faster than outer loop.

Tuning a system with cascade control requires systematic approach. The inner loop must be tuned first with the outer loop in manual control. Once the inner loop is tuned satisfactorily, the outer loop can be tuned as usual.

Once tuned, the cascade control system must be observed to ensure that, the inner loop does not saturate which can lead to instability or excessive overshoot to the outer loop. If saturation is observed, limits must be placed on the output of the outer loop controller or a signal provided to prevent integral wind up.

You may also read:

• Ratio Control
• What is Sequential Control System?
• Feedforward Control
• What is Process Control?
• Feedback Control System