Micrologix 1400 Timed Proportioned Output PID Tuning

Paul Begley

Join Date
Jan 2014
Canton, Michigan
Greetings everyone, hoping to gain some knowledge on fine tuning PID controllers specifically for proportional timed outputs.

I will be controlling an industrial immersion heater which is broken into 8 cicuits. Normally for my companys application we would energize the immersion heater with a simple mercury contactor using an output either off the PLC or a relay card.

However in this certain case our customer has requested we replace the mercury contactors with SCR's and mechanical contactors.

From what I understand about SCR's they should not be powered constantly rather they like to be turned on/off gradually until everythings is up to speed so to speak. Thus we have implemented the Timed Proportioned PID Controller.

I have a Micrologix 1400 1766-L32AWAA, in slot # 2 I have a 24VDC SSR card part number 1762-OB8 which has 8 channels dedicated to the 8 SCR's which energizes the 8 circuits of the immersion heater. There is a type J thermocouple taking a temperature reading converted through a signal conditioner into an analog input card, which is then used as the PID PV.

When the unit is first turned on and the temp. probe is reading ambient temperature it seems as though the PID calculates a very large error in turn the PID is energizing the SCR's for to long and too frequent, my question and concern is what can be done to smooth out the "ramping effect" we're seeing?

Any advice will be very much so appreciated.

Thank you in advanced for your help!
Last edited:
The control output should increase rapidly if there is a large error. Often the control output will jump to 100%. This means the SCRs will be active 100% of each update. That is expected. As the temperature increases the control output should start to drop to its steady state level if the system is tuned right. The SCR duty cycle will drop too. This is normal if the system is tuned right.

If the PV increases too quickly then you need to ramp the SP from the current PV to the desired PV. One can use a low pass filter on the command SP so the target SP doesn't change instantly. If the time constant for the low pass filter is longer than that of the tuned system the PV should be able to track the target SP very closely.

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