Cascade control is a process control strategy that uses two control loops to control a process variable. The primary loop provides the secondary loop with a setpoint, and the secondary loop controls disturbances before they affect the primary control objective. The secondary loop should have faster process dynamics than the primary loop, and flow controllers are commonly used as secondary loops. Cascade control can also involve multiple loops, and several factors must be considered when developing cascade controllers.
Cascade control refers to a process control strategy in which a process variable is controlled by manipulating the setpoint of a related process variable. In this particular strategy, the variable of interest is controlled by two control loops working in tandem. In chemical process industries, a cascade control system is often used to reduce the effect of noise and upheaval on the primary control objective.
A cascade control loop consists of a primary loop and a secondary loop. These cycles may also be referred to as the outer loop and inner loop, respectively. The primary loop provides the secondary loop with a setpoint, or target, for a process related to the primary control objective. The primary loop is sometimes known as the master loop because it provides a setpoint for the secondary loop, or slave loop, to follow.
The primary purpose of using cascade control as a process control strategy is to allow the secondary loop to control disturbances before they affect the primary control objective. For this control system to work effectively, the process dynamics of the secondary loop must be much faster than the dynamics of the primary loop. As a general rule, the secondary loop process dynamics should be at least four times faster than the primary loop process dynamics.
The most common application of cascade control for most chemical processes is to use a flow controller as a secondary loop. Most flow controllers have a very fast response time, making them good candidates for sub-loops. Primary loops tend to focus on variables such as temperature, level, potency levels of hydrogen (pH), or chemical composition.
Cascade control is not necessarily limited to systems based on only two control loops. Multiple cascade arrangements work the same way as a traditional cascade control loop, but have more than two loops. For example, a chemistry analyzer and control loop with a long dead time might rely on a temperature control loop to eliminate process disturbances. The temperature control loop could then rely on an even faster controller, such as a flow controller.
There are several factors that are relevant in developing cascade controllers for a given process. For example, a controls engineer would consider whether a chosen secondary loop had a strong effect on the primary loop of interest. The engineer should also know the process dynamics of the proposed secondary loop. These factors could determine whether the proposed secondary loop can absorb process disturbances and prevent them from affecting the primary loop.
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