Analog in module measurement reference (not ground).

[GeneL]

I wish to measure analog voltages greater than 10 vdc.  Am I allowed to reference a UIA-08 input module to 10 vdc and measure up to 20 vdc?  If not why not or better yet, how can I measure values greater than 10 volts?  I have used voltage dividers but they tend to amplify error. I am looking for a "differential amplifier" approach.  Admittedly, I'm new to Unitronics hardware, but I haven't seen a differential input device.

Gene

[Joe Tauser]

1 hour ago, GeneL said:

I have used voltage dividers but they tend to amplify error.

But do you really need to care?  Think it through.

You're really not amplifying error, you're reducing sensitivity.  Your circuit would be calibrated for a fixed input anyway, and once the divider was installed it would not change its response appreciably.

The accuracy on a 10V input module is 0.3%, which is not as good as a modern multimeter.  Your uncertainty (call it error if you want) is 30 mV.  If you set up your divider to measure 20 VDC then your uncertainty is 60 mV.  Pretty crappy.

Here's a DC "differential amplifier":

https://www.acromag.com/catalog/875/signal-conditioners-converters/signal-converters-transmitters/dc-voltage-current-input

$185.00 each.

Accuracy - 0.05% of span

This will improve your uncertainty but you're still stuck with the PLC's relatively sloppy 30 mV. 

Bottom line - a PLC analog input is NOT a multimeter.  I have tried to make electrical measurements like this in the past and did not have good long term results, thanks to the uncompensated thermal drift of all components involved creating slightly different readings every other day for the same calibration signal.

If all you're trying to do is get a close-ish measurement the PLC is fine.  But your initial comment about error tells me you want something better.

You need to get a real voltage measuring  instrument that talks Modbus and let it do the work.  Or get an Agilent 34401 and use the Message composer to talk to it with SCPI commands over the RS 232 port.  I have a customer who has done this.

Joe T.

[GeneL]

Thanks Joe, I certainly understand the limits on the AI unit but I was hoping for an easy fix.  Our measurements can be critical in the 10s of millivolts so working with "real voltage measuring devices is likely the path to success. The TT238 is a nice unit, but it would be a bit too expensive for 30 channels perhaps in conjunction with a multiplexer or a physical switching device???  BTW I agree the divider network is better described as less sensitive  - reducing resolution as opposed to "amplifying error"

Thanks for your response I will keep the forum posted.  

GeneL

[Joe Tauser]

30 channels of precise measurements is not going to be cheap.  I'd look at a Keysight 34970a with a pair of 20 channel cards.  You can talk to it with the PLC via RS232 or get yourself a copy of LabView.

Do you need control or just data acquisition?  Many moons ago I did a 48 channel hybrid system for Emerson Motor that combined a PC app talking to the above instrument and also talking to a V350 that performed UL life tests with temperature monitoring on new motor designs.

Not cheap (about $25K).  But good.  Very good.

There is a new family of UniStream Ethernet I/O that sports 16 bit four channel analog modules that may suit your needs.  Ask your supplier about it.

Joe T.

[GeneL]

Hi Joe

to update this is exactly what I've done I have used this equipment previously with a 20 cell stack fuel cell.  Now I'm trying to interface the switcher with a Unitronics USP-104-B10

I would like to sent it SCPI commands from the PLC's USB port via a USB/GPIB 82357B interface.  I'm not the PLC programmer, I just want to see if my request is feasible.  I just want to include the 34970 at the test stand.  I don't even know if it is possible to integrate the Two systems or even if it is necessary at this point.  What I used before was Visual Studio (Visual Basic) and it worked quite well.  I am guessing that I can just program the switcher to scan through a list of channels and arm an alarm output based on the voltages measured.

Gene

[Joe Tauser]

SCPI commands are just another kind of serial protocol, so the commands themselves aren't a problem.  The question is whether or not the USB to GPIB converter will work as a stand-alone gateway or if a driver is needed.

Joe T.

Edit - They 34970A has an RS-232 port.  I would lean toward getting an RS232 module for the UniStream and communicate to the instrument that way.