pid

[moneyfreddie]

can any one tell me how to keep my pid from going back to the last value recorded in auto mode, when i switch form manuel to auto.

 

[Steve Bailey]

Not without more information. Nobody here is looking at your system. Nor are we mind readers. Based on what you've told us, the best answer you can expect is, "Configure your controller so it doesn't do that". Try searching your documentation for "bumpless transfer".

 

[Peter Nachtwey]

A general case

Set the SP to the PV so there is no error and load the integrator control signal with the current control signal.

If you must leave the SP alone then the integrator must be modified as follows:

IntgratorControl=LastControl-Kp(Sp-PV)

I am assuming the IntegratorControl is in control output units. Otherwise you need to adjust the accumulated error.

IntgratorControl=Kp*Δt/τ_i*Σerrors
So
(Kp*Δt/τ_i)*Σerrors=LastControl-Kp(Sp-PV)

or
Σerrors=(LastControl-Kp(Sp-PV))*τ_i/Kp*Δt
Easy

 

[Ron Beaufort]

can you post your program file? ... (note that you'll have to ZIP it first) ...

at this point, we don't even know what brand of PLC you're using (or if you've even using a PLC at all) ... it's impossible to give you detailed answers without more information ...

we'll be glad to try to help - but you've got to give us a lot more to work with than you've posted so far ...

and welcome to the forum ...

 

[Paul B]

Of course you want bumpless both ways - Auto to Manual and then Manual to Auto. It depends upon the controller...

With a GE or Siemens PLC my program turns on the Manual bit of the PID loop. When manual is on the manual setpoint is moved into the PID loop. When manual is switched off the PID loop takes off from there - naturally bumpless. Then when in Auto (not manual), the program moves the pid output to the Manual setpoint, making it bumpless the other way.

 

[moneyfreddie]

thanks for replying,it is a slc 5/04 and making it bumpless is what i was told to do but how.i have looked up pid control bumpless transfer and still do not understand.

 

[Ron Beaufort]

once again ...

can you post your program file? ... (note that you'll have to ZIP it first) ...

 

[Paullys50]

Just responded in your other thread, please don't post the same topic twice.