Figure 1: Operator filling a tank with water.
An example of a typical open-loop system in industry would be an operator filling a tank with water. Figure 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.
