4 Heat Trace circuits and only 1 PID

Hi,
I’m a new member and I have a question about the PID function in a Micrologix 1400.

I have 4 separate Heat Trace circuits that I need to monitor and control. I have a 1762-IR4 RTD Module attached to a ML 1400.
It’s my understanding that the ML 1400 is limited to two PID functions.
If this is true, is there a way to create a data array and push each data set of values into the PID in a round robin format?
I’m imagining 4 routines that each have a turn using the PID Loop. Where the last PID calculations get moved back into the PID and new calculations are made, re-stored, then on to the next routine.
Maybe put the PID in its own subroutine?
Also The heaters are either on or off.
The typical cycle period for this application is in minutes not seconds so I’m not too worried about the speed.
I have programmed in several languages including assembler but this Ladder stuff is hard for me.
Any thoughts?

Hi! I am not familiar with your PLC, but generally speaking, you don't need a PID for ON/OFF control.

These heat trace elements, are they in the same area or are they separate?

If separate, easiest way is to just make two temperature set-points that turn ON/OFF the elements.

If not separate, you could use a 3-point step controller (if available) that counts up and down an integer from 0 to 4. Assuming the elements all have the same power.

The integer value you could associate with the elements, 1 is group 1, 2 is groups 1 and 2 etc.

Welcome here by the way!

Thanks for your response.
I have read where PID is a little overkill for an application that has a period of about 20 mins and deviation allowance of about 5 degrees F +-.
I'll take your advise here and keep it simple.

Ash Tillotson said:

It’s my understanding that the ML 1400 is limited to two PID functions.

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Where did you learn this? I have not tried to use more than two before, but I do not see that as a limitation. You may be correct, I am just curious myself since I may one day run into this limitation if it exists.

PID may be overkill, but it can also be simpler to set up a PID time proportional control for your heaters if the application fits it well and should be able to exceed your control requirements with less complex logic.

And, welcome to the forum.

I am not so sure that you can only have 2 PID's on a Micrologix 1400. It seems like a pretty arbitrary restriction.

http://literature.rockwellautomation.com/idc/groups/literature/documents/pp/1766-pp001_-en-p.pdf
While I understand that the product brochure isn't the best of information sources, under the PID category all they list is "Yes (limited by loop and stack memory)", whereas with everything else that has a limit they list what limit it has.

Regardless, I do agree with the other posters that with the PID instruction might be a little overkill for your application, but it all depends on your application.

Instead of pid I would use PWM for heating control

Thanks for all of the responses, And you guys are right, I tested with 4 PID loops and no issues. I would have sworn that I read that in the RS pub 1766.
That being the case, I would like to set up a PID time proportional control. If for no other reason than getting my feet wet.

tarik1978 said:

Instead of pid I would use PWM for heating control

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I would love to do this but the chatter from the 30 Amp starters in the small box would cause complaints. But a home brew VFD, now you're talking!
Unfortunately I got no budget for it.

Ash Tillotson said:

I would love to do this but the chatter from the 30 Amp starters in the small box would cause complaints. But a home brew VFD, now you're talking!
Unfortunately I got no budget for it.

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If you are doing timed proportional with contactors, you should use the longest duration time pulses possible to extend their lifespan.

You may want to get some SSRs (and perhaps shunt trip breakers in case an SSR should short) to make the system more durable over the long haul. That will take away much if any limitation on short cycling.

OkiePC said:

If you are doing timed proportional with contactors, you should use the longest duration time pulses possible to extend their lifespan.

You may want to get some SSRs (and perhaps shunt trip breakers in case an SSR should short) to make the system more durable over the long haul. That will take away much if any limitation on short cycling.

Click to expand...

Looks like the SSR's are inline with Contactor pricing.
Seems like a no brainer, I will push this idea.
Thank you!