PID Position, Bumpless is Bumpy

[Mike Nash]

When switching from MAN to AUTO, if the .Bias is greater than 100 (due to high PID gain and error) the .Output will exhibit somewhat random looking pulses of up to 5%. This is happening even though the PV is greater than the SP and there is no Rate (Derivative) action. If I clamp the .Bias to 0.0-100.0% this does not occur. I see this in both SIM and a BX.

Code: [Select]

// Ver(103) PCU(3) Do-more Designer Clip   Format: Do-more Designer 2.00+  DO-MORE Ladder
PLC DM-SIM

// Do not modify SYS_CONFIG block!
#BEGIN SYS_CONFIG
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#END


STR ST1
PIDINIT PID0 2 90 0.2 0

STR C0
PID PID0 0x4 DST511 DST511 DST511 DST511 DST511 """""" DST511 DST511 DST511
NOT
MOVE 75.0 PID0.Output

STR ST1
MATH R0 "PID0.Output - 50.0"

STR ST1
STR ST2
INTEGRAT R0 10 0.1 PID0.PV


#BEGIN MEM_CONFIG
 PID0 PID  0
#END

#BEGIN ELEMENT_DOC
"C0","PID_Auto","",""
#END


Ooh! Post 500 and it had to be a problem. Sorry!

[Controls Guy]

When I write a loop, I not only never let the bias get over 100% (or 50% if center-proportional), I usually clamp it a little UNDER that, even.  Like 40-45% if centered or 90% if not.

[Mike Nash]

Except for the bumps in the output, the code is working as BobO said here: http://forum.hosteng.com/index.php/topic,30.msg3555.html#msg3555

So, you want this at manual to auto transition:
if(PositionMode)
   {
   PID.Bias = PID.Output;
   PID.Bias -= PID.Gain * Error;
   }

But with a PID0.Gain of 90 and an error of 100% (or more), you get a pretty large PID0.Bias.

The autotune had a gain of 191 for the simple test feedback from the INTEGRAT I used, and more aggressive Reset and Sampletime.

I am just making sure I understand what is going to happen when this is controlling a motor that doesn't need to get crazy, ever.

I had to look up that -= code having never seen that. I also won't remember it either since I will probably never use it anyway.

[Mike Nash]

Bump. No pun intended.

I'm hoping to hear from Host if this is something that might get looked into soon.

Clamping the PID.Bias 0-100% outside of the PID instruction seems to prevent or at least limit the PID.Output hiccups. But this giant bias is generated by the instruction after an autotune sets the PID.Gain, .Reset and .Scantime as opposed to me using it in some unintended way.

Is the clamp an acceptable workaround?
Is there a better one?
Is this something that might get a firmware change at some point?
Am I just doing something wrong?
This will be up and running before any firmware updates I'm pretty sure.

Right now, I am just looking for a "we'll look into it." The season is upon us and I hope everyone enjoys the next few weeks.

[BobO]

We are eyeballs deep in new product development. Not a great deal of time to spend on this right now.

There should be no problem with externally limiting the bias. At one point we had discussed internally limiting the bias to 100 to prevent wind up, but opted to implement the bias freeze function that is there now, but of course that only works while the loop is in auto.

[Mike Nash]

Thanks, and Merry Christmas!

[franji1]

When I write a loop, I not only never let the bias get over 100% (or 50% if center-proportional), I usually clamp it a little UNDER that, even.  Like 40-45% if centered or 90% if not.

Do you just use the CLAMP instruction (see attached), or is your algorithm more complex?

[Controls Guy]

I was just speaking generically about how to do PID loops.  I've never written a PID function in DL Classic, the built in one was always fine, and I have yet to do a PID app in Do-More.  When I do, I'll probably start with the built-in one and only do my own if there's something I want that it doesn't do, which I don't anticipate.

I only end up rolling my own if the platform doesn't have one, or if it's overly complex or doesn't do something I need.