PID Control Loop Feedback System for Accurate and Precise Dosing Control

PID is a control loop feedback system used by Process Instruments, comprised of 3 components: Proportional, Integral and Derivative. PID is mostly used in industrial control systems, to continuously and automatically modulate a control output to achieve a desired setpoint.

Proportional (P)

This is the most commonly used component in system control, as it suits most applications without the need for Integral and Derivative control components. ‘P’ control is most effective when the process water is recirculated such as with a swimming pool. The main concept with proportional is that the further away the measured value is from the set point, the larger the controller output. This helps to achieve the setpoint and maintain the dosing throughout the water treatment process. However, in some processes where the measured parameter is lost to the process e.g. chlorine from a pool, the proportional control may approach the setpoint but won’t quite reach it. The difference between the setpoint and the achievable value is known as ‘droop’.

Integral (I)

Integral is used to eradicate ‘droop’ with a proportional controller or when the process water isn’t recirculated, such as on a water treatment plant. When Integral is used, the controller output is determined by both magnitude and duration of error. So small errors that occur over longer periods of time will trigger large responses. ‘I’ control therefore maintains the setpoint where there is a requirement to continually dose to reach the setpoint and maintain it.

Derivative (D)

Derivative uses the rate of change of deviation from the setpoint to predict future errors. As Proportional and Integral are extremely effective in process control, Derivative is rarely used.

(The amount of time that the measurement parameter has been above or below the setpoint. ‘I’ is used when controlling process that require a steady setpoint, but dosing is never ‘off’.)

CRIUS®4.0 and CRONOS® controllers from Pi are often used to control chemical dosing in water treatment processes. This is because dosing via PID, as opposed to standard manual dosing or on/off relays based on thresholds can: