Control modes in process control are the methods in which a controller influences the output of a control system to match a desired set point. These modes describe how a controller responds to changes in the error value, which itself is the difference between the set point and the actual process variable. The standard types of control modes include: Proportional (P), Integral (I), Derivative (D), and combination thereof, usually termed as PID control.
Proportional Control (P): The proportional control mode adjusts the output in proportion to the current error value. The calculation involves multiplying the error by a constant known as the gain. A high proportional gain results in a large change in output for a given change in error. While a P controller alone reduces the overall error, it may not eliminate it completely. There might still exist a residual steady state error referred to as the offset.
Integral Control (I): The Integral control mode adjusts the output based on the cumulative sum of past errors over time, hence it is able to drive the steady state error to zero. The integral term is calculated by accumulating the error over time and then by multiplying it by the integral gain (Ki). The I controller speeds up the response but can also cause overshoot if not properly tuned.
Derivative Control (D): The Derivative control mode adjusts the output in proportion to the rate of change of the error. It "predicts" the error in the future based on its current rate of change, thus the D control mode provides damping or resistance to control signal changes. It is calculated by determining the slope of the error over time and multiplying this rate of change by the derivative gain (Kd). The D controller contributes by reducing the overshoot and dampening the system response.
PID Control: These modes are often used in combination (PID control) to balance the strengths and weaknesses of each. PID controllers are often used to adjust a control system's performance to best meet criteria like responsiveness, overshoot, settling time, etc.
Each of these control modes contribute uniquely to the overall system. P-control provides a robust, if somewhat inaccurate, initial response. I-control ensures that persistent errors are handled. D-control helps in mitigating overshoot and in achieving a quick response. Combined as a PID controller, they help in creating systems that can be finely tuned for a wide array of dynamic performances.