help with a micrologix 1100 talking to vfd

ianavina

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Join Date
Nov 2013
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long beach,ca
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hi guys, im trying to make my vfd automatically adjust speed based on the liquid level of a tank. if the tank level keeps rising to speed up my pump using my vfd. im using a vegason 61 sonar sensor to read my level and sending that data to a red lion cub5p and AB micrologix 1100 with a AB 2ch input / 2ch (1762) output analog module added to it. i honestly have no idea on how about doing this. any help would be greatly appreciated
 
Hello, if 1 of the analogue cards AO is spare, use it to drive the vsd speed reference. From a programming perspective, if you only need coarse level control, just apply a proportional band so the vsd reference increases min - max speed between appropriate levels using a SCP instruction.
 
Sure, will help if I can... Is the coarse approach going to work for you ?? If you need to hold at a specific level, a pid of some description is a better fit...

R u familiar with using the analogue in / out and scaling with the plc to meaningful values ??
 
The program I did before this I used scp geq and leq to turn the pump on at a specific level and turn it off at another level, but ou still had to manually control the vfd speed, yes u would like to hold the fluid at a specific level

Some of our units use pid but no I'm not to familiar with it
 
If the process is happy between those tank limits, I would stick to the proportional band approach, just add another SCP within your start and stop range which generates the vsd reference. Ie, start @ 70%, stop @ 30%. Max speed at 65%, min speed @ 35%. In practise the level will end up being a function of the input flow...

A pid for a level application can actually be a pain to setup... If you don't need, practise KISS
 
Yes first program works great in my opinion lol but my boss likes to tinker with everything and this was his new request for today lol so a pid for level control is what I would need to figure out then?
 
Yes first program works great in my opinion lol but my boss likes to tinker with everything and this was his new request for today lol so a pid for level control is what I would need to figure out then?


Then the best place to start is with the instruction manual, see link chapter 19.

Read thru it a couple of times, give it a try and then come back with your questions.

http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1763-rm001_-en-p.pdf
 
Like Mickey said, have a look through the manual first up. The instruction help in Logix 500 isn't too bad by its self..

Bear in mind the PID instruction isn't a magic bullet for process control... Any loop where the PV responds quickly is usually easily tuned, but add delays from CV movement to PV response, and changing process conditions on top, tuning starts to get more complicated.

If your tank is big volume for small mm change, you are more than likely going to end up with tuning issues, especially from start-up.. If its a little tank (only a few minutes pumped full to empty) shouldn't be as much of a problem.

If your boss is looking for the pump to continue running rather than start / stop, the proportional band approach will still be easiest to achieve... Have seen this done without the PLC in the middle, straight level 4-20 into the VSD direct, and the analogue in setup to suit...

Cheers Chris
 
ok guys ive been reading ch 19. based on that it says the setpoint and the pv have a range of 0-16383, why? my signal from my level sensor will be 4-20ma does that matter?
 
The PID instruction scales the PV internally from a raw value of 0-16383. If you look at your analogue input config, you will see an optio for "scaled for PID" which does just that.
I usually scale this externally thou to get a more appropriate PV range.

Just scale from the level transmitter input your valid level range (0-100% ??) to 0-16383 and use it for the PV. It is important to put bounds protection in place as if the SCP goes high / low it will fault the processor...

In the PID there are 2 parameters which allow the scaling of the PV input to what ever EU you want. Is decimal only, so add dp by factors of 10 as required..

The CV is also 0-16383, so you need to scale this to whatever is required to drive your VSD. I usually do this twice, first SCP to an internal EU which makes sense to look at (0-100 % FP) and then to the AO...
 
Hey guys sorry I haven't responded, I was on leave because just had baby!!! but now that im back this project was waiting for me lol can you guys explain how I would do a proportional band since a pid seems excessive for what im doing
 

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