4-20mA scaling or offsetting ??????

I am working on a project where I have 1 DC drive which outputs a 4-20mA signal to a VFD, I want the VFD to follow the DC drive which we have
currently accomplished, however I want to be able to scale the VFD. That is if the DC drive is at say 50% then the VFD would be at 50%, but, in some cases I want the VFD to be say 45%. I have gone through the programming and found that the DC drive has no means of scaling the 4-20mA signal and the VFD would be too complicated for my production people to program each time they wanted to change scaling. Is it possible to adjust the 4-20mA signal in some way, say with a potentiometer or some device. I would like to do it this way so as to make it simple for my operators. This setup is basically a machine which is driven by a DC drive and a colour feeder which is driven by a VFD which sends a pigment into the machine, when we push the output up or down on the machine the colour must follow, but as you may have guessed the colours are fed at differentpercentages thus the need to offset or scale the 4-20mA signal. My whole idea is to simply bump the 4-20mA signal down and the pot would be a nice solution (cheap and reliable)if it would work. I do understand that using a pot may bump the bottom end down below 4 mA, but would this have any detrimental effect to either drives. Thanks to all whom reply. Paul
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Paul,
You are quite right in concluding that a simple pot isn't quite right. Unfortunately the 4-20 mA system, for all its advantages, can't be scaled that way. Better would be one of the Action Pak instruments as listed in http://www.actionio.com/products/actionpak/signalcond/index.html . By putting a 250 Ohm resistor in parallel with the input of a voltage to 4-20mA converter you should be able to get what you want.
Walter.

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The company I work for, Rogue Engineering Inc. is actually getting ready to release an I/O module which includes both 4-20mA inputs and controllable 4-20mA outputs. Essentially, it is a mini-PLC. It would be a simple matter for us to modify one to do any scaling you wish on the output signal. The module also has an RS-485 port that communicates Modbus protocol for setup, so you could change the scaling on the fly if you wished. I don't know the exact pricing since I'm not in charge of sales stuff, but I'm guessing it will be less than $200. Send an email to snipped-for-privacy@rogue-engr.com if you're interested.
Tim Kirk snipped-for-privacy@rogue-engr.com Electronics Design Engineer Rogue Engineering Inc.
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This equipment you are using, is it located reasonbly close to any PLCs with 1 unused analog input and 1 unused analog output?
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