Problem using NI 9237 + NI 9171

[syed.raza]

Hi Guys,

I hope that you are doing fine.

I am facing this measurement problem since quite a while and its time to resolve it with your help.

INTRODUCTION

I am using 120 ohm strain gauges to measure strain on my sample. I have connected strain gauge to N1 9944, then using RJ50 cable, connected the strain gauge to NI9237. This NI9237 is plugged into NI9171 chaises, which then further connected to computer's USB port using USB cable.

Upon connecting to USB port, the READY green light become ON normally.

And when I start the VI, for strain gauge measurements, ACTIVE green light turn on, and stay ON.

Using labview 2014 SP1 32 bit, on a dell computer windows 7 enterprise SP1 64 bit computer

PROBLEM

My strain gauge measurements are usually takes weeks, and after running VI, say 8 hrs, the ACTIVE green light ON and OFF with few second gap. Due to this reason, my strain gauge reading goes up and down and does not make any sense.

WHAT I NORMALLY DO

I normally stop the VI and start it again to avoid this problem. But I can not do it for a whole week (DAY/NIGHT).

But when I stop/start again, the measurements of stain gauge reading lock them self to a totally different baseline, although the trend of the data remains same. so I have to manually shift that baseline to previous one.

All I want to run this program continuously with out any problem.

I am attaching my VI, and I normally use VI, as exe file makes condition even worse.

Please note that I am measuring other things also, but getting problem only in strain gauge reading.

Please help me out.

Thanks

Syed

[OA1]

Haven't checked your VI yet.

But you're saying you have a changing  offset? If I understand what you're saying correctly? The way I solve this is to read the first value in my strain gauge gives me and subtract it (stored within a shift register). You can then regularly check it is within an accepted range and make any changes if necessary.

[syed.raza]

Yeah I do the subtracting thing. But what if it on and off after every 10 seconds? Creates only data full of noise.

if the NI 9171 always remain active, then I will not face any problem. So after 8 hrs of run, it start doing active light on and off, every 10 sec or so.

btw, thanks for reply 🙂

[OA1]

Hi Syed,

Check your usb power settings in device manager and make sure "Allow the computer to turn off this device to save power" is deselected.

[syed.raza]

Hi QA1,

thanks for the reply.

I already changed these USB settings from device manager long time ago. Even I change computer once because of this reason, but problem still there. USB is always powered up, I even switch to different ports.

it may be because of sampling rate or something ?

[syed.raza]

Hi,

How are you ?

I am still getting spikes and noise in my strain data. It is may be that NI 9237 is not providing enough power to the strain gauges.

I recently found a document on NI9237 which provides the detail for power consumption for the bridge circuit. I try to work on it, but it will be better if someone can assist me on this.

I am using 3 strain gauges of 120ohms on my sample in quarter bridge configuration, and this document will help to identify the power usage. So that we can decide to put an external source of power or not.

Please if you can read the page 14 and 15 of the attached document, and let me know if I can required external power source on not ? currently I use 2.5V of internal power.

Kind regards

Syed

[syed.raza]

Hi guys,

Recently I have added a 4 Volt external power to NI 9237. I will update you with the results.

[Kyle97330]

I see that you're using the "DAQ assistant" express VIs.

I've heard that these are often unreliable.  You may get better results if you create your own code using DAQmx VIs.

[syed.raza]

Thanks Kyle for the reply.

Point noted.