Open and Closed-Loop Feedback Systems:
Understanding Gain, Reset, and Rate

The terms gain, reset, and rate are functions that determine how fast the controller will change the output signal. They are also called modes of operation and their functions were developed with the early pneumatic controllers and vacuum-tube controllers and later refined with op amp controllers. These basic modes of controller response were based on mathematical formulas (algorithms) that have been used for many years to solve complex problems. These formulas are derived from the calculus functions proportional, integral, and derivative (PID). Today companies that make process control equipment will use the terms gain, reset, and rate interchangeably with the terms proportional, integral, and derivative. For the person trying to learn process control theory, this tends to make it more difficult. The table below shows the relationship of these terms. The terms gain, reset, and rate originally referred to controller operation. Proportional, integral, and derivative refer to the math functions used to make the controller perform the actions of gain, reset, and rate.

Above: Table 1: Comparison of the PID terms proportional, integral, and derivative to the terms gain, reset, and rate.

Another problem with the terms gain, reset, and rate and the terms proportional, integral, and derivative has occurred with the advent of the microprocessor chip. Since the original calculation in the microprocessor uses formulas, these formulas can be changed slightly to provide different types of controller response when new models are produced each year. This means that you may find several different responses when using the gain function from different brand names of controllers or from different models of the same controller from year to year because a different formula to calculate the gain may have been used.

This may not be a problem for someone learning process control for the first time, but it's definitely a problem for someone who learned how a proportional controller or integral controller operated with op amps or pneumatic controllers. The function of proportional control is rather fixed and limited with the op amp or pneumatic controller and people became familiar with this type of response. In modern microprocessor controllers, the response the manufacturer calls proportional may have enhancements to the calculation the microprocessor performs so that the controller response reacts differently than traditional proportional control. The enhancements were added through the years to make the controller provide better response and control. This means that you only need to understand the basic functions of gain, reset, and rate or proportional, integral, and derivative to determine how this response will react. From these basic functions you will be able to determine the specific detailed differences between brand names of controllers when you start to use them in the field.

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