Scaled for PID Question 1769-IT6

Thor Hammer · Feb 12, 2020

Hi everyone. New to RSLogix5000 and learning as much as I can. I'm much more familiar with Omron.

Is it required I select "scaled for pid"(0-16383) as data format for my type j t/c that will be my PV for a PID loop? Or can I select engineering units which would be the range of the thermocouple at -3460F to 21920F?

PLC: L33ER
Card: 1769-IT6

Application:
Nitrogen fed heating element to a tank where parts are dried after processing. (Semi industry)

My max allowed setpoint is 572 degrees F. Would this be the value I put in the CV Max at 100% in the scaling tab? Wouldnt the output need to be driven to 100% when the temp is at 0? Seems backwards.

Thanks for any input!

Thor Hammer · Feb 12, 2020

One more question. The output of my PID controller needs to go to a discrete output that connects to my Solid State Relay. Not an analog output like would typically be utilized.

When we set this up in our Omron software we create our own PID loop with a series of instructions and turn the discrete bit on and off based upon the current temperature.

How could i accomplish this with the PID instruction output? Hmmm, going to have to do some more research.

Thor Hammer · Feb 12, 2020

edit. wont work the way I thought

Bit_Bucket_07 · Feb 12, 2020

Thor Hammer said:
Hi everyone. New to RSLogix5000 and learning as much as I can. I'm much more familiar with Omron.

Is it required I select "scaled for pid"(0-16383) as data format for my type j t/c that will be my PV for a PID loop? Or can I select engineering units which would be the range of the thermocouple at -3460F to 21920F?

It used to matter that you used the maximum resolution available for all of your PID terms back when your analog I/O data (and sometimes the PID function as well) was data type integer. You sacrificed granularity by using scaled to EU units. Now that you have the option of FP math available for scaling your analog I/O points at the hardware configuration level, it doesn't really make much difference.

I would say that it probably is important to work within the limited band of the thermocouple's range that you're likely to actually encounter within your application. A one degree change of temperature is an insignificant PV change as a percentage of FS when you're measuring a range of 25381 degrees. A one degree change would register as a more significant change of temperature if you were only looking at, for example, 0 to 1000 degrees.

There are many threads here regarding PWM and other methods for converting a PID CV to discrete control for heating applications.

Thor Hammer · Feb 12, 2020

Bit_Bucket_07 said:
It used to matter that you used the maximum resolution available for all of your PID terms back when your analog I/O data (and sometimes the PID function as well) was data type integer. You sacrificed granularity by using scaled to EU units. Now that you have the option of FP math available for scaling your analog I/O points at the hardware configuration level, it doesn't really make much difference.

I would say that it probably is important to work within the limited band of the thermocouple's range that you're likely to actually encounter within your application. A one degree change of temperature is an insignificant PV change as a percentage of FS when you're measuring a range of 25381 degrees. A one degree change would register as a more significant change of temperature if you were only looking at, for example, 0 to 1000 degrees.

There are many threads here regarding PWM and other methods for converting a PID CV to discrete control for heating applications.

The range I posted is Eng unitsx1. The actual range is 346 to 2192 degrees F.

For the discrete outputs i found the time proportional control example in the SLC manual. So that may be an option.

I also came across a solution using an SCP instruction that scales the PID output to a 10 second time base. then compare the scaled output of the SCP to the timers .ACC value and connect that to an OTE.

Thor Hammer · Feb 13, 2020

Anyone else have any input?

Scaled for PID Question 1769-IT6

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