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 the chemical process industries, a cascade control system is often used to reduce the effect of disturbances and upsets on the primary control objective.
A cascade control loop consists of a primary loop and a secondary loop. These loops might 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 that the secondary loop, or slave loop, must follow.
The primary purpose for using cascade control as a process control strategy is to allow the secondary loop to control disturbances before they can 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 process dynamics of the secondary loop must be at least four times faster than the process dynamics of the primary loop.
The most common application of cascade control for most chemical processes involves the use of a flow controller as the secondary loop. Most flow controllers have a very fast response time, making them suitable candidates for secondary loops. The primary loops tend to focus on variables such as temperature, level, potenz hydrogen (pH) levels or chemical composition.
Cascade control is not necessarily confined to systems based on only two control loops. Multiple cascade arrangements work in the same manner as a traditional cascade control loop but have more than two loops. For example, a chemical composition analyzer and control loop with a long dead time might rely on a temperature control loop to eliminate process upsets. The temperature control loop might then rely on an even faster controller, such as a flow controller.
There are several factors that are relevant in the development of cascade controllers for a given process. For example, a controls engineer would need to consider whether a chosen secondary loop had a strong effect on the primary loop of interest. The engineer would also need to know the process dynamics of the proposed secondary loop. Such factors could determine whether the proposed secondary loop is capable of absorbing process upsets and preventing them from affecting the primary loop.
