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ill. 1 (below) shows the diagram of a cascade system that uses steam to heat a continuous flow of soup. A temperature controller is the primary control for the system. It is also called the outer control or the master and it has a thermocouple as its sensor to provide the PV signal. The thermocouple is identified in the diagram as the temperature element (TE). The output of the temperature controller is sent to the second controller that controls the steam valve, which is called the inner controller or the slave. The steam loop has a pressure sensor called a pressure element (PE) that sends the value of the steam pressure to the controller.
Above: ill. 1: Diagram of a cascaded
control system. The tempera ture controller samples the temperature and sends its output to the pressure controller. The pressure control system
samples the pressure and the amount of error from the temperature control and determines the amount of output signal that's sent to the steam
valve.
To operate the system, the operator enters the SP temperature in the master controller. For this example, the soup is heated to 180°F so the SP in the temperature controller is set to 180°F. Since the system is a cascade system, the operator can't adjust the SP in the pressure controller. The SP for the pressure controller is determined from the output signal from the master (temperature) controller. The output for the slave (pressure) controller is connected directly to the steam valve, which is opened when heat is needed and closed when the temperature is above the SP.
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