AB PID best way to reset output to specific value

Hi All,

With AB RsLogix PID: What is the best way to reset the output to a specific value. For now i'm overwriting the PID.DATA[0] but i'm not sure it's the right way to go.

I'm using a PID in a Compactlogix to control PUMP speed to maintain a flow setpoint, when PID output = 100%, I start another Pump but I need to reset the PID output to compensatate the additionnal flow with this formula: NewOutput = LastOutput * (NbPump/NbPump+1).

Regards

It's not clear whether both (or all) pumps are driven by a single PID output, though it is implied to be the case. One option is to put the PID in software manual (SWM). Start the additional pump, delay for ramp up, change set output (SO) according to the scaling equation, delay to settle, put the loop back into automatic. It will probably be a bumpy transition, depending on the delays.

mrmick22 said:

when PID output = 100%, I start another Pump but I need to reset the PID output to compensatate the additionnal flow with this formula: NewOutput = LastOutput * (NbPump/NbPump+1).

Regards

Click to expand...

If you only have 2 pumps enable the second pump at 50%??? If you have 10 pumps enable every 10% etc.

How many pump stages do you have? Are they variable speed or fixed speed?

Hi,
Fixed and modulating pump, thats why I need to reset PID when I start a fixed speed pump. Its one PID controlling all modulating pump.
With Delta controller I only reset the bias but the AB PID not allowed this feature directly (that why I reset the .DATA[0])

What processor?

The "right" way to set PID output value in AB PLC is to set the PID to Manual mode then overwrite the control variable. This way, when you turn the PID back to Auto mode it will be bumpless.

If you have multiple fixed stages and one modulating stage then think about this as an example - 9 fixed stages, 1 modulating stage

Output from PID 0-100% > 0-10% modulates, >10% 1st fixed stage 10-20% modulates, >20% 2 fixed stages 20-30 modulates etc etc It is straightforward to do with a mod instruction and some compares - I had an example on here showing this for heating (though that was with RSLogix500 but the scheme applies)

Makes sense?

If you really want to go down the road you are on and are using CLX then look at the PIDE instruction where you can ONS a value into CVn-1 (CVprevious) - if you look at the equation in the help file it shows you. Basically CV = CVn-1 +Pterm +Iterm +Dterm

As already asked what processor are you using? I don't know any AB PID instruction that doesn't have BIAS?? Though Im often wrong

I am not sure about the processor. I thought PID vs PIDE is limited by version type, standard vs Pro/ Enterprise.

What do you mean when you say you can't write to the bias directly? Have you tried .BIAS?

Ok, now I can see OP is talking about the Contrologix line based on the mentioning of PID.DATA[0]

Typically, you would put the PID block into SWM (software manual mode), then the output will be set to the same as .SO (Set Output).

However, not sure if this is the best control strategy for OP's setup and I can't really comment on that without seeing the P&ID.

Controler is a compactlogix 1769 L32.
The BIAS available in the configuration is more like an fixed offset. I talked about the bias calculation value(I think it is the value in the PID.DATA[0]).

I will try the SWM and .SO method to be safe because i'm not sure if overwriting .data[0] will cause problem.

Thanks everyone