What Is PID Temperature Control?

A three term or PID controller is widely considered to be the most effective method of temperature control. During normal operation a PID controller will utilise closed loop control to continually monitor the input (sensor feedback) and adjust the output (SSR, relay or analogue signal) to maintain a steady process value. The adjustment of the output is dependant on the PID parameters which help calculate when the output should turn on/off and for how long. There is a considerable amount of maths involved in the initial and ongoing calculations but it is sufficient to accept that determining the PID parameters is crucial to ensure best performance.

PID Terms Explained

A device marketed as a PID controller implies that all 3 of the below terms will be used to calculate the correct output to maintain a steady process value in relation to the setpoint. It is possible to utilise P, PI or PD control but in the majority of instances a modern controller will function best with full PID control.

Proportional Band (Pb) - proportional control affects the output depending on the error between measured and set values. It is possible to achieve fast response by increasing Pb. If the Pb is too high the system can oscillate out of control

Integral Rate (Ir)  - integral rate concerns the accumulated error. This is the sum of errors between desired and actual temperature over time. Even in the best case scenario once the Pb has been set the process will have minor fluctuations around the setpoint. The function of the Ir is to reduce the steady state error as close as possible to zero.

Derivative Time (Dt) - derivative time relates to the instantaneous rate of change of the error. Dt causes the output to decrease if the process variable is increasing too quickly. Increasing Dt will cause the control system to react more strongly to changes in the error and will increase overall control system response. With a Novus controller we often see very small values of Dt.

What Is PID Tuning?

The tuning process is a critical part in establishing the best values for the above parameters. Therefore it should be carried out on first commissioning of a new device to ensure correct operation. We always recommend tuning a new device and testing operation prior to entering into service proper. Correct tuning will establist the 

P	PROPORTIONAL CORRECTION TO THE ERROR	The correction to be applied to the process must increase in proportion to the progression of the error between the current and the desired value.
I	PROPORTIONAL CORRECTION TO THE PRODUCT ERROR x TIME	Small errors, but ones that have existed for a long time, require more intensive correction.
D	PROPORTIONAL CORRECTION TO THE ERROR RATE OF CHANGE	If the error is varying too fast, this rate of change must be reduced to avoid oscillations.