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Using "Custom Voltage with Excitation"-Task on DAQPad-6259

We use fullbridge based load cells to measure torque. A custom made amplifier is also used. I think the "Custom Voltage with Excitation"-Task in the DAQ-Assistant is best suited for this challenge. Unfortunately the "Connection Diagram" in the DAQ Assistant is unable to generate a pin assignments for the DAQPad as the accessory is not supported... Well, our 6259 has a screw terminal...
Which are the relevant AIs to connect the signals (Vex + Vsig) of our load cell?
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Message 1 of 11

if you have a full fullbridge system then you have only two connections which go out.
Then you can connect this two connectors to two inputs like AI0 and AI8, or AI1 and AI9, or AI2 and AI10,....
You can find additional help to fullbridges in the manuel of the SCXI-1520 on page 2-6 and following:

Greetings from Munich
National Instruments Germany
Application Engineer
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Message 2 of 11

Hi Plamen,

My question was not related to SCXI as we have no SCXI frontend. As mentioned in my post we use a DAQPad-6259 with screw terminals. The torque sensor has 6 connections :

  • Vex-
  • Vex+
  • Vex-sense-
  • Vex-sense+
  • Vsig-
  • Vsig+

Vex+/- will be connected to an external supply or the DAQPad +5V terminals. The question is now if the "Custom Voltage with Excitation" setting in the DAQ assistant can be used to measure Vex-sense and Vsig in on turn and let the DAQ function do the excitation-compensation (without SCXI modules).

Of coarse I can measure thes two voltages myself and do the compensation in my software. But maybe this is already builtin into the DAQ Assistant.

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Message 3 of 11
when you create your task you say which channels to use.
so you must only connect the bridge on this channels and you will get the correct value.
National Instruments Germany
Application Engineer
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Message 4 of 11
OK, I chose the channel, in my example ai20, but then this is only the channel for the Vsig+/- signals. Where do I choose the channel for the Vex-sense+/- lines (Erregungsquelle - Extern)?
With ai20 in differential mode the Vsig+/- signals are connected to AI20/AI28 (terminal 49/50). To which terminal do I have to connect Vex-sense+/-?
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Message 5 of 11
can you upload your circuit diagram?
National Instruments Germany
Application Engineer
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Message 6 of 11
Öhm, no. There is no circuit diagram so far. I just use Test button in the DAQ Assistant to view the measurements.
Is ist possible that we are talking past each other? The simple question is:
"Is ist possible to measure signals from sensors with excitation including excitation compensation? How is this done using a USB-6259 with screw terminals?"
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Message 7 of 11
National Instruments Germany
Application Engineer
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Message 8 of 11
I mean your hardware circuit diagram.

I did not understand which connectors are Vex-sense+/-.
You have from a fullbridge 2 outputs.
In fullbridge_1.jpg you can see the outputs. these are Vch+/-.
so when you look at fullbridge_2.jpg you will see the signal S+ and S- which are going into the card.
and this are only 2 outputs which goes into a differential input of the card.

National Instruments Germany
Application Engineer
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Message 9 of 11
Here the fullbridge circuit including the Vex-sense signal. My understanding of the "Mit Erregungssp. skalieren"-setting is that the DAQmx-Task is able to sense the exitation voltage and scale the measurement signal (Vch) at the ratio Vex-sense/Erregungswert. So the result should be always the same even if Vex varies...

Message Edited by beatz on 12-12-2007 08:55 AM

Message Edited by beatz on 12-12-2007 08:57 AM
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Message 10 of 11