transducer controlling an vfd through a plc

hi guys
i need your help again, i have been asked to fit an transducer to control a pump via a plc the pump is already running on a vfd with the speed set from a hmi ,all they want me to come up with is to get the transducer to control the speed via the plc when the pressure adjusts.they would like to set the speed on the hmi until we have got the right running pressure and the press a button to switch over to the transducer to keep the pressure constant? can anyone help.thanks stu

stu,
Sounds like you will need to program a PID for this.
Search the forum for PID and Ron Beaufort or go here and enjoy the light reading.

http://www.ronbeaufort.com/sample_lessons.htm

I don't know what PLC you are using but this should get you started.

Hope this helps

Well, I do this sort of thing all the time, and I still consider doing it RIGHT to be a fairly good sized project.

The format I would consider is to have the pressure SET-POINT displayed and controlled by the HMI. The HMI just changes a register value in the PLC as the operator presses the up or down buttons, and displays the desired pressure (the "set-point") and the "actual pressure".

The register in the PLC is used as your set-point so when the operation is dialed in and working properly the operator just STOPS doing anything and the operation continues.

You have to use the information from the transducer analog input channel ALL the time.

With some PLC brands you can use a built-in PID instruction. I often just write something of my own. PID is a science all unto its own.

The object is to check the pressure as measured by the transducer every so often, compare it to the set-point, and add to the "analog out" register that is in control of the VFD if the pressure is too low, or subtract from the analog out register if the pressure is too high.

NOW..... fine TUNING those operations.... THAT's where the science comes in. I've spent several years developing and improving the system that I use.

There are BOOKS and VOLUMES that deal with the building and tuning of PID loops. Some devices now have an "Auto-tune" feature that makes the whole process a lot easier.

What stage are you at? What PLC do you wish to use? what are YOUR thoughts on format?

Stationmaster

What stage are you at? What PLC do you wish to use? what are YOUR thoughts on format?

Stationmaster[/quote]

hi
the stage i am at is i have got the machine working using the hmi-plc and vfd,so the pump is being controlled via the hmi (speeds/stop/start) now i have got to write a program to incorporate the transducer. i am using a mirologix 1200, and i am new to plc programing but i am willing to do what it takes to make this work.thamks stu

Have you ever worked with a PID loop before?

the short answer is no??

stu said:

the short answer is no??

Click to expand...

Then the next step for YOU is to learn all about PID. The CONCEPT, the details, the terminology.... The practical application....

Search this forum, Google it, Look on "patchin.com".....

pressure -->analog IN -->PID -->analog OUT -->VFD speed -->pump speed -->pressure

Stationmaster

hi have got a tranducer on order and i have fitted a analogue input card for the ml1200, i will have to read up about the pid control.

Is that an AB PLC with PIDE instruction
What is the make/model of your HMI
Which software do you use to configure PLC/HMI
I have a faceplate built for RSLogix5000, RSView Studio if it will help
Roy

hi i am new to plcs, the plc is a mirologix 1200 (1762 series c)the analogue input/ output card is-1762-if20f2.
the software i am using is rslogix 500. also the hmi is an automation direct 8" ea7-t8c thanks stu

what kind of drive do you have, and how do you communicate with it?

Chances are that it can control the PID for you.
Chances are that you don't even need PID control.

Here are some links to get you started though:

What is P in PID:
http://www.plctalk.net/qanda/showthread.php?t=13459

What is I in PID:
http://www.plctalk.net/qanda/showthread.php?t=11242

What is D in PID:
http://www.plctalk.net/qanda/showthread.php?s=&threadid=11969

hi guys
what is the beast way to control the vfd with a transducer, is is using a pid ?
thanks
stu

If you've got a VFD with an external PID controller built in, all you would really need is an analog signal from your PLC to the VFD to establish the setpoint. You can then display the setpoint on the HMI.

The drive will do all the loop control work involved with keeping the pressure constant under varying flow conditions. All you need to give it is the pressure setpoint.

hi it looks like the vfd hasnt got a pid?
thats why i was going to do the controls through the plc + they would like to set the pressure on the hmi,