Plc 5,Slc,Contrologix Pid loops

[plc noob]

What is the best method of temperature controls single loop controlers or pid loops in a plc?

What are the primary differences between the pid loops in the plc 5 ,slc 500 series and contrologix.

Which plc is best at pid loops and why.

What are some best practices for designing pid loops in the plc?

Management wants to put new controls to where the temp control is in the plc so is that better than the single loop standalone controllers?

Which AB plc had the best pid functions?

 

[Paullys50]

What is the best method of temperature controls single loop controlers or pid loops in a plc?

Why buy more hardware if the PLC can already do it?

What are the primary differences between the pid loops in the plc 5 ,slc 500 series and contrologix.

They are all similar and will function essentially the same way. Exception is that the SLC only has the Dependent form of of a PID where as the PLC 5 and Controllogix have both the Independent and Dependent forms, so you need to decide which you want to use.

Also, the CLX line has scaling built into the function, so you don't need separate code to re-scale your PV, CV, SP for the PID.

Which plc is best at pid loops and why.

The one that gets the job done? Of course if you don't have people on site that understand the PID loop as designed within the PLC....PLC brand doesn't matter.

What are some best practices for designing pid loops in the plc?

Depends on how much coding you want to do...I've seen a lot of logic in place to support many PID functions (auto/manual control, sync timing...) and I've seen PIDs only have 1 - 2 rungs associated. I prefer the first method.

Management wants to put new controls to where the temp control is in the plc so is that better than the single loop standalone controllers?

Management shouldn't make controls decisions. Poor tuning will make either system craptastic. Throwing money at a control loop won't solve poor control, unless you are paying for an experienced person to re-tune the system and or correct the logic in the PLC for the PID to perform as designed.

EDIT: Great tuning can't correct a system if the mechanics are poorly designed ie...oversized/undersized pumps, motors, control valves. A great control system can't make management happy if management does not understand the differences in the engineered design of the system versus their expectations of what the they WANT the system to do.

Which AB plc had the best pid functions?

D - all of the above

 

[plc noob]

What are dependant and independant forms. I have been doing some reading on plc pid controls and i have not seen this term yet?

 

[Mickey]

What are dependant and independant forms. I have been doing some reading on plc pid controls and i have not seen this term yet?

http://www.controlguru.com/wp/p77.html

 

[Ron Beaufort]

a very basic explanation:

with the “Dependent” form of the PID equation, the amount of Integral action - and the amount of Derivative action – both DEPEND upon the setting you’ve entered for the Proportional action ...

with the “Independent” form of the PID equation, the various settings (Proportional, Integral, and Derivative) are all INDEPENDENT of each other ...

going further:

first suppose that you’re tuning a loop using the INDEPENDENT form of the PID equation ... suppose that you’ve decided that you need more Proportional action – but you do NOT want to change the existing amount of Integral action or the amount of Derivative action ... no problem, just change the Proportional setting ... the amount of Integral action and the amount of Derivative action will both remain unchanged ...

on the other hand ...

now suppose that you’re tuning a loop using the DEPENDENT form of the PID equation ... once again, suppose that you’ve decided that you need more Proportional action – but you do NOT want to change the existing amount of Integral action or the amount of Derivative action ... this time you COULD have a slight problem ... because whenever you make a change to the Proportional setting, then the amount of Integral action and the amount of Derivative action will both be changed also ... in this case, you’d have to make “compensating” changes to the Integral setting – and to the Derivative setting – in order to accomplish the stated goal ...

finishing up ...

at first glance it seems to be a “no brainer” to go with the INDEPENDENT form (if it’s available) – and keep the tuning task as simple as possible ... I personally won’t enter the debate on which form is “better” ... but here’s something to think about before you make your choice ... MANY controllers ONLY allow the DEPENDENT form ... so ... IF you’ll ever have to work with any of those types of controllers in the future, then consider whether or not you might be “painting yourself into a corner” by starting out with the INDEPENDENT form ... specifically, if you concentrate all of your initial “learning curve” on mastering a feel for tuning the (“somewhat easier”) INDEPENDENT form, then you MIGHT have a problem when you’re eventually forced to deal with the (“more challenging”) DEPENDENT form ...

anticipating your next question:

as long as both forms are properly tuned, then one method doesn’t offer any “better control” for the same process than the other ... specifically, I’ve often had my students tune a system ONE way – and then print out a “test run” of the process ... next we retune the system for the OTHER way – and again print out a “test run” ... when you “stack” the two printouts and hold them up to the light, the traces will essentially be superimposed ...

final thought:

IF (big IF) your process requires a very large amount of Integral action, then the INDEPENDENT form might have a significant advantage ... that’s because with the INDEPENDENT form, the range of values that can be entered for the Integral setting is USUALLY able to crank in more Integral action ...

basic reason: for most controllers using the DEPENDENT form, a LOWER number gives MORE Integral action ... but ... for most controllers using the INDEPENDENT form, a LOWER number gives LESS Integral action ... this has perplexed many people – who quite often “go the wrong way” with their Integral settings – when switching from one form of equation to the other ... (another “gotcha” for the unwary) ...

hope this helps ... please keep us posted on your progress ... printouts of the PID response are always appreciated ...