Measure a sensor's voltage

[Hijawi]

Hello all!

I am trying to measure a strain gauge pressure sensor voltage output, it has three pins ground, +5v and a signal pin. The +5v bias voltage is coming from a battery source, however, when I try to scope the signal pin by connecting it with Myrio's analog input pin and the ground pin to Myrio's analog ground, the values I'm getting are not constant and are definitely wrong. I scoped the voltage using a multimeter and it shows 1.6v which makes a lot of sense. I am using Myrio 1900 and I tried to look for a solution with no luck. Your help is much appreciated. 

Regards,

Hijawi

[TimothyDug]

Hi Hijawi,

How do you have the connection configured? i.e. Referenced Single Ended (RSE), Non-Referenced Single Ended (NRSE), or Differential. This behavior definitely sounds like a connection configuration issue. You can read more about that here:

http://www.ni.com/white-paper/3344/en/

How are you running your code? Is it running in LabVIEW? Did you use an example?

Thanks,

[Hijawi]

Hello Timothy, 

Thanks a lot for your reply, I have a ground pin in the sensor which i assume means that I do have a reference?

I read that white paper but I couldn't find a similar problem.

I am using my custom FPGA vi and reading using fifo registers in rt.

I tried using the express analog in Vis with the same result.

Any suggestions on how to connect?

Regards,

Hijawi

[TimothyDug]

Hey Hijawi,

So, I think I know the issue. Tell me if I'm wrong. I think your signal pin is connected to the AI 0+ or AI 0- pin, and then the ground pin is connected to the AI GND pin, is that correct? This connection is differential which means it will compare the + and the -, so you would want your ground from your sensor to connect to the AI 0- connection, and then the signal to connect to the AI 0+ connection.

If this isn't your resolution, what specific pins are you connecting to on the MyRIO? On one side of it, there are specific pins like AI 0+, AI 0-, AI 1+, AI 1-, etc. On the other side, there are 34-pin connections. Which are you using, specifically?

Thanks!

[Hijawi]

Hello Timothy,

Thanks a lot for your reply, connecting the ground to AI - terminal doesn't solve the problem, I am using connector C (MXP) for scoping so I have access to the differential input pins, if I want to do differential input using connector A or B (MSP) should I subtract the values that I'm getting from say AI1 and AI2 (positive signal - negative signal)?

Regards,

Hijawi

[TimothyDug]

Hey Hijawi,

1. The A and B connectors are designed for Referenced Single Ended (RSE--thus, not Differential). It would be possible to set it up to work like differential, but I would not recommend it.

2. If you are connecting to connector A and B Analog Input pins, your sensor should not be grounded, or else there will potentially be a ground current loop.

3. If you are connecting to the differential pins on connector C, and you have ground connected to AI 0- and the signal connected to AI 0+... If you have these connected, and your signal is incorrect, then that is a problem, and there may be damage to the board.

Let me know what the situation is! I hope we can get this resolved!

Thanks 🙂

[Hijawi]

Hello Timothy,

Once again I really appreciate your effort, I have succeeded in pulling the signal using the differential connector C pins, by connecting resistors to the signal and ground pins on the sensor and connecting the resistor to AGND while having the +/- pins connected as you said. I am glad it is solved, but I have 3 more sensors in the system, using connector C will not be enough, I need a way to do this differential input on connectors A and B.

Regards, 

Hijawi