Operation of the Cascade Controller

The cascade loop control begins at the master controller. In this example the master controller is continually checking the soup temperature as it leaves the heater to the SP of 180°F. For this example we will assume the two controllers have the soup temperature at SP. When the flow of soup is increased, the additional soup flowing through the heat exchanger causes the soup leaving the heater to become colder than 180°F, and the master controller (temperature controller) begins to increase the amount of output signal that's sent to the slave. The output signal can vary from 0-100%. The slave controller has a setpoint range that's set from 0-50 psi, which is the range of the steam pressure. The master controller treats the SP range as a span of 0% to 100% even though the slave sees this value as 0 to 50 pressure. Since the two controllers are set for cascade mode, the increase in the output signal from the master will cause the SP to increase in the slave.

The slave controller controls the steam pressure like a typical single-loop controller, except its SP is changed only by the master controller rather than by the operator. At the beginning of the example, the output of the slave controller sets the steam valve to a position between 0-100% where the system was holding the soup temperature at 180°F. Since the soup temperature dropped, and the master controller increased its output, the SP of the slave increased, and the slave controller increased its output, which opened the steam valve farther. The increase of steam pressure will cause the temperature of the soup to increase. If the flow of soup slowed down, the soup temperature would rise above the SP, and the master would reduce its output signal, which would reduce the SP of the pressure controller. The pressure controller would then reduce its output signal, which would close down the steam valve and the temperature of the soup would drop.

Since the temperature of the soup changes rather slowly, and the temperature of the jacket which is caused by the steam can change rather quickly, the gain, reset, and rate in the temperature controller can be set for its response. The gain, reset, and rate can be set differently in the pressure controller for its response. In the temperature controller, rate (derivative) action will be useful because it can anticipate that the soup temperature is getting closer to SP and begin to compensate rather than wait for the temperature to reach the SP and then tell the slave to start turning back the steam. If anticipation is not used, the soup temperature will oscillate well above and below the SP because of the difference in response.

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