AB-Rockwell CompactLogix PID loop software

[Robot_Man]

Hi Guys,

I have a requirement to contol a pump speed with a VSD to achieve a steady temperature in a process environment. Hot water is being pumped and the actual temperature in the environment is measured and input via an analog card. The VSD is controlled via ethernet linked to the 1769-L32E Processor.

The setpoint temperature is operator controlled via HMI and ideally I would like to be able to vary the deadband too.

One final point is that if the tempertaure control loop wants to take the pump VSD below 25Hz, the VSD has to be switched off since that is the minimum speed for which the pump is rated.

Once the upper limit of the deadband is exceeded, the control loop would (hopefully!) command a frequency a few Hz above 25Hz such that the pump runs a decent amount of time rather than stop-start all the time.

I see the instruction set has a PID function, but I've not used it before so could do with some help please!

Many thanks in advance!

 

[Robot_Man]

...any takers.... ?

 

[kamenges]

So.....what's the question?

What is a PID?
Can I use a PID for theis application?
How do I set up a PID controller?
How do I set up the PID controller specific to the Rockwell Logix platform?

Any or all of the above or something completely different?

As a forum we do better with specific, direct questions. Nature of the universe; not so much.

Keith

 

[blamb]

I would reference the RSLogix 5000 instruction help for the PID instruction. It should help explain the different bits and registers associated with the PID block.

Obviously the temperature would be your process variable (.PV). The speed reference for the VSD would be the control variable (.CV) and the output of the PID to your analog output. You can make the setpoint (.SP) and deadband (.DB) registers that can be user adjusted via the HMI. I usully prefer to make these separate registers and then move them into the PID registers.

As for the 25Hz minimum speed, this is usually a setting available in the VSD. I believe you can also limit the output of the PID instruction. Either that or you can monitor the actual analog output and use a clamp to ensure that is doesn't go below 25Hz.


Hopefully I have my PID information correct, I don't have the software in front of me and haven't used a PID for a while. Lot's of options here, hopefully this will help get you started? As kamenges said, you need to be more specific with your questions.

 

[Peter Nachtwey]

My alarms are have been set off

I would start with the design first. My first question are for the people that picked the pump, did you optimize the pump size so that the pump would spend little or no time below 25Hz?

When the PID output is in the range that is below 25Hz then set the output to 0 and turn off the integrator. When the PID requests an output corresponding to 28 Hz turn the integrator back on and don't zero out the control output.

This is simple FOPDT, first order plus dead time, system. You should be able to get by with just a PI control. There is a similar heat exchanger application on www.controlguru.com.

 

[Robot_Man]

Guys,

Many thanks for your help and answers. Sprry I was not more specific - if I knew what specific questions to ask I would have done so...

Anyway, with tips, all solved and after tweaking around it does as required. Sure it could be optimised some but it's fine.

As Blamb suggested SP and DB are settable using the HMI and I used the PID config. to specify a minimum output of 24Hz and trapped anything below 25Hz to turn off the inverter.

Thanks again!

 

[Bobbias]

Well, hopefully next time if you have a question to ask, you'll have a better idea of how to be specific. I still have trouble knowing what to ask these guys when I need help.