25.2 CONTROL OF LOGICAL ACTUATOR SYSTEMS

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Many continuous systems will be controlled with logical actuators. Common examples include building HVAC (Heating, Ventilation and Air Conditioning) systems. The system setpoint is entered on a thermostat. The controller will then attempt to keep the temperature within a few degrees as shown in Figure 385. If the temperature is below the bottom limit the heater is turned on. When it passes the upper limit it is turned off, and it will stay off until if passes the lower limit. If the gap between the upper and lower the boundaries is larger, the heater will turn on less often, but be on for longer, and the temperature will vary more. This technique is not exact, and time lags will often lead to overshoot above and below the temperature limits.

Figure 385 Continuous Control with a Logical Actuator

Figure 386 shows a controller that will keep the temperature between 72 and 74 (degrees presumably). The temperature will be read and stored in temp, and the output to turn the heater on is connected to heater.

Figure 386 A Ladder Logic Controller for a Logical Actuator