Operation of the Control Loop
There now follows a description of the action taken by the controller.
The algorithm which controls the on-off control loop which operates
the heater has the following steps.
- The temperature of the water is measured.
- It is then compared with the desired temperature.
- If the actual temperature is too cold then the heater is turned on.
- If the actual temperature is too hot then the heater is turned off.
If this procedure is used then it would lead to
the situation where the heater was
switched on/off as soon as the water was either too hot or too cold. The
result would be reasonably tight control around
the setpoint but the heater would be switched on and off
rapidly. This is unsatisfactory as the switch is sure to wear out.
One way to get round this disadvantage is to build hystersis into
the control algorithm i.e. a deviation from the setpoint known as the
dead zone.
Thus the operation of the control system now has the following steps.
- The temperature of the water in the tank is measured.
- It is then compared with the desired temperature.
- If the actual temperature is below the sepoint by a specified amount
then the heater is turned on.
- If the actual temperature is above the setpoint by a specified
amount then the heater is turned off.
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