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CLOSED–LOOP CONTROL SYSTEMS

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In a closed-loop control system, the output of the process (controlled variable) is constantly monitored by a sensor, as shown in Figure 1.3(a). The sensor samples the system output and converts this measurement into an electric signal that it passes back to the controller. Because the controller knows what the system is actually doing, it can make any adjustments necessary to keep the output where it belongs. The signal from the controller to the actuator is the forward path, and the signal from the sensor to the controller is the feedback (which " closes" the loop). In Figure 1.3(a), the feedback signal is subtracted from the set point at the comparator (just ahead of the controller). By subtracting the actual position (as reported by the sensor) from the desired position (as defined by the set point), we get the system error.

The error signal represents the difference between " where you are" and " where you want to be." The controller is always working to minimize this error signal. A zero error means that the output is exactly what the set point says it should be. Using a control strategy, which can be simple or complex, the controller minimizes the error. A simple control strategy would enable the controller to turn the actuator on or off—for example, a thermostat cycling a furnace on and off to maintain a certain temperature. A more complex control strategy would let the controller adjust the actuator force to meet the demand of the load, as described in Example 1.2.