01-29-2013 01:21 PM
Greetings,
I am working on a pump testing device. I am having issues measuring voltage across a shunt when the voltage it reaches a certain point (~13 Volts or higher).
The reading, as seen in the excel sheet, takes a dive until the voltage on another channel is measured under 13V (it's actually through a 1/5 voltage divider).
Hardware:
NI cDAQ-9184
NI 9211
NI 9402
NI 9207
Shunt R= 0.0005 ohms
I attached the excel file of the data where the errorneous amperage measurement is shown in red. It did this on another pump as well.
When the input voltage was above 13.0 Volts(on the 1st channel of the NI 9211), the shunt voltage differential (on the 2nd channel of the NI 9211) would jump down in value.
This ruins my data collection in ranges above 13.0 volts.
I am looking for advice on how to fix the issue. I am uncertain if it is a hardware issue or a software setting that needs changing.
The volts are read through a voltage divider, but they have been accurate thus far.
Thanks in advance,
-Tom
Solved! Go to Solution.
01-29-2013 01:36 PM
There is a common mode specification to common. On the 9211, it is 1.5V. Common can be 250V from ground. What is the common tied to?
01-29-2013 01:54 PM
My apologizes I said 9211(used for my 2 thermocouples) when I should have said 9207 (Analog Input for 2 voltage and 1 amperage reading).
Voltage Reading 1: measures voltage from the power supply through a 1/5 voltage divider
Voltage Reading 2: measures voltage differential across a R=0.0005 ohm shunt
Amperage Reading 1: measures a 4-20mA signal from a pressure transducer
As for the thought of checking the COM port on the 9207, I don't see where I can find the voltage setting you are referring to.
I power the system off the wall jack.
I power the Vsup through the power supply, which varies.
Does Vsup have to be a fixed value? As long as it was under 2A and 30V, it should be fine according to the manual.
Thanks for the reply.
-Tom
01-29-2013 02:11 PM - edited 01-29-2013 02:11 PM
Ok, for the 9207, each channel has to be within 10.2V of common. If you are using a differential input for your shunt, then the common mode voltage needs to be less than 10.2V. Apparently the D/A gets really screwed up if you go above 13V.9207 Product Manual
Do a search on Digikey for a High Side Shunt amplifier. These are chips specifically made for removing that high common mode voltage so you can use the output in the DAQ.
01-29-2013 02:47 PM
I found this http://www.ti.com/lit/ds/symlink/ina199a1.pdf as a Current Shunt Monitor.
I will be reading it,buying a few, finding out if it shall work, and then doing it.
As for the common thing . . .
The Voltage measurement from the voltage divider comes in as + 2.6 Volts and then is scaled to 13 volts.
The Voltage measurementfrom the shunt comes in as +0.005 Volts and then is scaled to 10 amps.
These are well within the +-10.2 Volts.
However, I do the scaling within the DAQ assistant. Should I do the scaling outside of the assistant?
Should I use a higher resistant shunt so that the incoming measurement is a higher order of magnitude?
Thanks again,
-Tom
01-29-2013 02:56 PM
It is the voltage at the DAQ input that matters with this.
For the differential inputs, it is not the differential voltage that matters. Each input of the differential input has to be less than 10.2V to the common (ground).
02-01-2013 11:23 AM
I recieved the part in the mail, but it was clearly too small for me to work with.
It was designed for an integrated circuit.
I will be trying to get it onto a circuit and use it that way.
Would this be a solution?
http://www.pololu.com/catalog/product/2101
Its an adjustable buck (step-down) voltage regulator that should bring the common to an acceptable range.
Thanks,
-Tom
02-06-2013 04:20 PM
The common ground was the issue, but I wasn't able to solve it using the shunt resistor.
I purchased a small chip that reads current 0 to 30 Amps and outputs a voltage signal.
Thanks for the help.