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The terms manual mode and auto mode are sometimes used in place of open loop and closed loop. When the system control is placed in manual mode, the loop is in open-loop operation, and when the system control is placed in auto mode, the loop is in closed-loop operation. This means that the switch that controls the feedback signal is also called the auto/manual switch. It may be easier to understand the operation of the loop if it's in manual mode (open loop) when you study an example.
An example of a typical open-loop system in industry would be an operator filling a tank with water. ill. 3 (below) shows an example of this type of system and notice that when the operator manually opens a valve, water starts to fill the tank. When it's determined the water is at the correct depth, the operator turns the valve off. If additional water was needed, the operator would turn the water on again and put more water in the tank. This system is called an open-loop system for several reasons. First, the operator must manually open the water valve if more water is needed in the tank, and close the water valve when the proper amount of water has been added. Second, the operator looks into the tank to determine if more or less water is needed, and then opens or closes the water valve rather than the controller comparing the sensor signal to the setpoint. Open-loop systems tend to be very simple and used frequently in industrial applications because they are inexpensive to operate.
Above:
ill. 3: An example of an open-loop water-level control system. The operator
controls the opening and closing of the water valve when the system
is operated in manual mode. (*NOTE: Operator must visually check water
level when system is in manual control)
This same system can be operated as a closed-loop system if
the sensor signal is sent to the controller and the controller determines
the value for the output signal. The valve that controls the amount of
water flowing in the tank needs to be an electrical or pneumatic valve
that the output signal from the controller can adjust from open to closed.
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