PLC5 to CLX PID conversion

Join Date
Mar 2018
Location
Columbus Ohio
Posts
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Has anyone ever converted a the PID instruction from a PLC5 to a CLX platform?

The issue I am facing concerns the difference between a PID instruction containing the PD control block and a PID instruction containing the Integer control block. PDxx vs Nx:x.

The Integer control block PID in the PLC5 seems to have a parameter called "Feed Forward" that does not seem to have an equivalent parameter in the CLX platform. I did find something called "Output Bias" in the CLX that might be what I'm looking for but am not sure how it was converted. possibly scaling but I cant seem to replicate with the math used in the scaling tabs.

See attachment for a screenshot of how I think it was converted.
thanks!

Capture.PNG
 
According to the manual for PLC5

The value is either between -4095 and 4095 or -100% and 100%. I'm not really sure how to see which is which on your screenshot.

Either way, this will be related to the output signal range either by percentage or absolute terms.

In the PLC5 manual, Feed forward is also called output bias. Look for 1785-RM001_-en-p.pdf online.
 
The CLX is on the left side of the screenshot the PLC 5 is on the right.

If it was a conversion of -4095 to 4095 converted to -100 to 100% I would expect the conversion for a feed forward = 54 to look like this:
(54/x)=(4095/100)
X= 1.3
but it got converted to 0.6 in the translation.

Should I change it to 1.3 so the math makes sense?
 
I seem to recall that under certain circumstances, the PLC automatically puts a value into the Feed Forward value when transitioning from Manual to Auto mode to create a bumpless transfer. This could explain the variance in data values. My memory is a bit cloudy on this but maybe some else would be able to verify.

In the meantime, can you post a screen shot of your ladder rung containing the PID? This may help and it certainly won't hurt.

By the way, welcome to the forum Joshua!
 
Last edited:
That pic may be worth 1000 words. I'm pretty certain that having a 0 in your tieback register forces a value into your feed forward register to provide bumpless transfer when switching from manual to auto. The one's I've seen have the Control Variable address used for the Tieback, so the FF/Output Bias value stays at zero unless manipulated by the programmer.

I'll attach an example of a PLC5 PID which was recently converted to CLX. Note that the setup screen is a bit different because a PD control block was used instead of an N.

PLC5 PD example - status.JPG PLC5 PD example.JPG
 
From Knowledgebase ID44873:

Note: Both manual modes will alter the output bias if the integral term is 0. This achieves a bumpless transfer when the PID is commanded back into auto mode. If this is not desired, make sure the No Bias Calculation checkbox in the Configuration tab of the PID setup screen is checked. Be aware that checking the No Bias Calculation can cause a bump in the output when going back into auto mode.
 
I seem to recall that under certain circumstances, the PLC automatically puts a value into the Feed Forward value when transitioning from Manual to Auto mode to create a bumpless transfer.

The circumstance which uses the Feedforward/Bias term for bumpless transfer is when the Integral gain is zero. (If using the PD block this can be disabled by the Bias Back Calculation configuration setting.)

You should probably just start with zero bias since it will be "absorbed" by the loop after it starts controlling in Auto.
 
See attached ladder logic


Put all your logic on one rung for best timing accuracy...


TON : XIC T.DN : PID : RES T


Better still, put the PID instruction (only) in a periodic task set to 20 seconds (that seems a long time for a PID update, no harm going down to 1 second, is there?).


Also, make sure the PID.UPD parameter is set to the same time interval that you execute the PID instruction. Code it in so it can't be overwritten by anyone.
 

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