The on-off control method is the least expensive closed loop system to use due to its simplicity. The on-off control method does however tend to cause more wear on its equipment due to the excessive amount of on-off cycles. It is also the hardest control system to maintain a consistent process variable because the tolerances are expanded to avoid starting and stopping of the load device. A good example of an on-off system is a conventional oven. The heating element is the load which has a controller that operates off of feedback from a thermocouple.
Proportional Control method is a way to meter the output by monitoring the measured variable. The controller varies the output according to the error signal produced between the setpoint and the measured variable to mitigate error between them. For instance, the larger the error the larger the output command. Let’s say there is a water tank that opens a valve to drain the tank at a rate of 30 GPM. The proportional control method is used to adjust the output (pump) to match the flow into the tank as what is flowing out of the tank. So, the output increases to equal the 30 GPM.
Proportional-Integral method is the same as a proportional control with an inputted integral change to ensure that offset is minimized. The Proportional portion controls the margin of error but does not make up for the offset between the control variable and the setpoint. The integral control continuously adjusts the output reference to eliminate offset. Continuing on the water tank example in the proportional control description, while the output has adjusted to match the flow of water entering as what is exiting, there was an offset created due to the slow increase of inflow. The tank is 30 gallons under its setpoint by volume. The integral mode is used to increase the output so that offset of the tank level is at zero (back at setpoint pertaining to level).
Proportional-Integral-Derivative method is a way to produce even tighter tolerances between the control variable and setpoint. This method is to keep the controller from overshooting by using the derivative portion to sense the control variable’s change relative to the setpoint. This enables the controller to respond more appropriately to ensure that the measured variable does not stray too far from setpoint by responding to more sudden process changes. A “boost” is given to the output to allow the proportional method to take control quicker so that there is less of opportunity for error. This portion can be included in the previous water tank process where the emptying valve opens up at a drain rate of 30 GPM. The derivative can instantly ensure that the output drives at a higher frequency so that there is less error for the proportional method and less offset in the integral method.
References:
Bartelt, T. L. (2011). Industrial Automated Systems: Instrumentation and Motion Control. Cengage Limited. https://ecpi.vitalsource.com/books/9781305474277
