Advanced Closed Loops

In some applications, the variables in the closed-loop system function in a complex manner so that a simple closed-loop controller can't control the loop accurately. For instance, if a product such as soup is heated while it flows to the filler line, a tube and jacket-type steam heater must be used. The soup flows through the inside of the tubes in the heater and steam is injected around the outside of the tubes in the jacket to heat the soup. This system becomes complex for two reasons. First, the flow of soup through the heater is not at a constant rate, so the heater needs a varying amount of steam to heat the product to the same temperature. Second, the steam can change the temperature of the jacket much more quickly than the soup can change temperature. Since the soup and steam change temperature at different rates, a single controller would need two or three different gains, resets, and rates to be able to compensate for the differences in speed. It's easier to control this type of system with two loops that are connected to each other. This type of control is called cascade control.

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