While Cascade Control is generally considered an advanced control strategy, it is only marginally more complex to implement than Single Loop Control. More often than not it is applied to improve the performance of a slow process such as temperature that demonstrates sluggish behavior. Essentially Cascade Control improves a “slow” control loop’s ability to respond to disturbances by capitalizing on the dynamics of a faster one – something quicker like a flow or a pressure loop. The faster loop provides an early warning variable that facilitates disturbance rejection, helping to maintain steady performance of the slower primary loop.
To better understand the architecture and benefits of Cascade Control it can help to consider it in the context of an industrial process. Shown on the right is a tank system. A shared header supporting multiple lines allows liquid to flow into the tank. Liquid simultaneously exits through a port at the bottom. Using Single Loop Control the tank level is controlled by adjusting a valve and either increasing or decreasing the rate of fluid that flows into the tank. Whereas the exit stream is predictable, the inlet stream from the header can vary dramatically due to changes in pressure associated with demand from other lines. Due to the process’ dynamics the level controller may be unable to respond adequately to such changes in liquid feed. The slow response can result in a level – whether too high or too low – that is either inefficient or even dangerous.
Now consider a similar tank system that employs the Cascade Control architecture. As before the control objective is to maintain level within the tank. However, a second control loop is effectively “nested” within the architecture outlined above to improve control. Here a secondary flow controller is added that uses the Controller Output of the level controller as its Set Point. As level shifts within the tank the slower level controller establishes a new Set Point for the faster responding flow controller. Because the flow loop is closer to the disturbance it both experiences and rejects any pressure disturbances before they can have an appreciable impact on the tank’s level.
Figure 1- A performance comparison highlights the benefits of Cascade Control vs. Single Loop Control. With the help of a faster secondary loop Cascade Control improves upon the response to disturbances.
In order to implement Cascade Control it is necessary for the process to have access to a secondary control loop that directly influences the primary loop. What’s more the dynamics of that secondary loop must be notably faster than the primary loop – a minimum of 3-5 times faster to be precise. The direct influence and faster speed assures that the secondary loop can readily apply a corrective action capable of minimizing the effects of a disturbance. In the example provided the liquid header flow satisfied both criteria.
While the Cascade Control architecture involves only a single Final Control Element, it does require use of a second sensor and a second PID controller. The added investment in those assets along with the time to configure and tune the controllers represent the sum of the costs. On the other side of the ledger, the benefits are measured in terms of performance gains and the associated economic value.
