Options for -20 to +20mA output & Controllogix?

Does anyone know of any way to generate a +/-20mA signal using a Controllogix PLC system?

In a new installation I will be controlling 2 AC motor drive cabinets, and a PDF file that the drive manufacturer sent indicates that the control/feedback channels are -20mA to +20mA, but some of their support personnel have told me it is +/-10vdc. I am trying to cover my bases and make sure I can handle either scenario, but I am concerned that using a fast analog card (1756-IFX4........ whatever the P/N is) and a signal converter would be too slow. I haven't had much luck in finding signal converters to generate +/-20mA, and the one or two I have found have a pretty slow (125-150ms) response rate that would not work for this application.

I would check the manual myself. You don't normally see those signal levels in most drives. Normally 0-10VDC or 4-20ma.

The 2 page section of the drive interface manual I have does indeed say -20mA to +20mA, but it is totally without context because I do not have the rest of the manual to look at, and it is in German with English translations.

I didn't think it was a common control signal either, and have asked the tech support folks to get me some confirmation, but they don't have all the detailed documents since the drives are made in Europe.

The drive control signals are supposed to be a +/- torque/current command, so fast control is very important

All AI cards are voltage input sensing cards.

Current signals are handled by driving the current through a precision resistor with a known value to produce an IR (voltage) drop that the input amplifier can deal with.

If the AI card can handle bipolar voltages, it can handle bipolar currents, with an appropriate dropping resistor.

The dropping resistor value is pure Ohms Law: E = IR

A 250 ohm resistor give 1.0-5.0 volts when driven with 4.0-20.0mA (0.040 - 0.020A)

±20mA would need an AI with ±5.0Vdc input rnage if used with a 250 ohm resistor.

Or pick another resistor to fit a bipolar range that is available.

I used to work with some servo amplifiers used with a simple analog voltage control of +/-10vdc with the option of +/-20mA, and I think AB makes a card supporting this range, no? The mA signal option is more noise immune and preferable unless you have to add a signal converter to get the range.

I have looked at all the card options for the 1756 product family, and have not found any cards that will read/send +/-20mA without adding external hardware, as danw mentioned above.

Because the PLC will be handling the speed control on these AC motors instead of the drive cabinet, I want my control signals to have the least amount of lag possible.

Edit: I read the manual snippet again, and this is the section that is confusing me a bit:

Input range: +/- 20mA
Input resistor: 250Ω
Output range: +/- 20mA
Max. output voltage: +/-10V

Click to expand...

I communicated with the company, and they confirmed it is a +/-20mA control range. The input on their device has a 250 Ohm resistor built in, so that is what I would be dealing with.

I looked up the specs for an IF4FX card (fast analog I/O) and it says that the input impedance in voltage mode needs to be greater than 1 meg-ohm, so I'm concerned about connecting that to a 250 ohm circuit.

Edit: I did some searching and found a converter by Delta Computer Systems that is adjustable from +/-10mA to +/-100mA in 10mA steps. Response time is 400us for the standard, and 40us for the high speed version, so I think this will fit my needs.

None of logix modules has +/-20mA output and I would not do this with resistors.

Take a look at this converter, it probably will take care of this problem

931S-C4C5D-BC
http://literature.rockwellautomation.com/idc/groups/literature/documents/td/931-td001_-en-p.pdf
http://literature.rockwellautomation.com/idc/groups/literature/documents/pp/931-pp001_-en-p.pdf

With 1ms response time and selectable input it probably can control your drive

Thanks for finding that, I looked through the product catalog page on their website and none of the items they showed had +/- current output, but the PDF files say otherwise. Shame on me for not doing my homework thoroughly.

My simulations are currently running with a 10ms loop, so 1ms response from the module probably is acceptable

I must state that I never used it, never saw it, my recomendation is purely from the reading the PDFs I linked

But I heard very good feedback from 931 product line users in other applications.