RSLogix 5000 PID bumpless transfer

Hi I am trying to understand the best method of providing bumpless transfer to the PID function (not PIDE)

The existing code shows

In Manual Mode
Copy ProgCV to PID.SO . ProgCV is a value i am sending to the device by code
Copy ProgCV to OperCV

In Auto Mode
Copy the ProgSP to OperSP
Copy the CV to OperCV

However when the PID is set to auto it is dropping to 0 CV then the CV builds under the PID action

What is the preferred method of achieving bump less transfer from manual to auto mode change

Thanks

Are you using deadband? If yes, is the CV must be a real value.

copy the SP to the PV and ramp to the desired SP from there.
copy the control output to the integrator term. Since the SP and PV are equal there will be no change in the control output until the SP starts changing,

A more important question is what is being "bumped". This is important in motion control systems but not in temperature control systems.

drbitboy said:

Are you using deadband? If yes, is the CV must be a real value.

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What????

Phrog30 said:

What????

Click to expand...

Yeah, whoda thunk?


https://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1756-rm003_-en-p.pdf, PID documentation, p. 685:





I dunno if that relates to OP's issue. There is a fair bit about bumpless transfers in that document though.

First you need to make sure the PID control is installed and running correctly
It should be in a Periodic task to update about every 100 ms faster than that and the PID output may have radical changes it the output. Because the PID can update faster than the hard you are controlling
Always leave it run unconditionally
When you want to enter manual mode
Set the manual Bit (.MO) or the software manual Bit (.SWM)
When you set the PID to manual move the desired output into the output word (.SO)
When the you reset the Manual bit the PID control will control the output
While the PID Manual Bit is set the control will continue to monitor the Set point and the Process variable and calculate the necessary values and track the output, it just will not update the output word

cjd1965 said:

The existing code shows

In Manual Mode
Copy ProgCV to PID.SO . ProgCV is a value i am sending to the device by code
Copy ProgCV to OperCV

In Auto Mode
Copy the ProgSP to OperSP
Copy the CV to OperCV

However when the PID is set to auto it is dropping to 0 CV then the CV builds under the PID action

Click to expand...

What's described above should work as long as one of the following is true:
1. There is a non-zero integral gain, or
2. The Bias Back-calculate bit is set.

This assumes the SP/PV error is not so large to drive the initial control action all the way to zero.

There is often confusion around a manual-to-auto transition "bump" that is really the first control action applied to the "bumpless" starting CV. The last manual SO is automatically copied to the integral term to initialize the pre-auto CV, and if SP is equal to PV, there will be no change to the CV.

The initial SP/PV error can be managed by a one-shot move of the PV to SP upon clearing SWM, or by using the PV Tracking feature.

drbitboy said:

Yeah, whoda thunk?


https://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1756-rm003_-en-p.pdf, PID documentation, p. 685:


View attachment 58156

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That is crazy. Yet there it is in black-and-white, and makes no sense. Must have been some kind of error in the Logix platform implementation because the PLC-5 PID.CV target is always an integer.

At least they call it out in more than just a footnote.

Also a plausible explanation for the OP, especially if this is the initial setup and commissioning of the loop.

Hi all thanks for replies

It all works OK it is an existing system i was just looking to understand it better

Here is a program my brother wrote with a PID coded into a MircroLogix.


The last few rungs of program file 248 (PID_SUBR) implement the bumpless and anti-windup pieces; there might be more related code elsewhere.

Look at the .PVT (process variable tracking) element of the PID instruction. Specifically designed to be used when the PID loop is switched to manual to prevent integral windup, etc, and assist with bumpless transfer.