Acceleration measurement with NI9234

[bon2b]

I'm measuring Acceleration and analysis the frequency of the signal. I'm using the following in my test:

• measurement device: NI9234 on  cDAQ9172 chassis
• measurement software: LabView SignalExpress 2009 with Sound and Vibration Suite
• accelerometer: ADXL330

The accelerometer is externally powered by a 3V power supply, so it has a 0-3V output range. The sensitivity is 300mV/g.

When I used the attached project written in Signal Express to measure a 1 g signal, the measurement is around 5.5. I attached a screen shot of this measurement.

Where did the bias come from? What did I do wrong in configuring the test?

Thanks a lot!

[Sean_F]

Hey Bon,

I ran your code using my 9234 with 300mV wired to the input and it showed up correctly as a 1 g reading. This indicates a problem with your accelerometer/connection. If you have a DMM verify that your accelerometer is in fact generating reasonable voltage levels. 

Good luck!

[bon2b]

Hi Sean,

Thank you for helping!

the accelerometer(ADXL330) used in my application has a 0-3V output, 300mV/g sensitivity and a zero g bias level at 1.5V, so I have the calculations below:

accelerometer output: 1.5V---> 0g

accelerometer output: 1.8V--> 1g ( not 300mV)

accelerometer output: 3.0V---> 5g

I set the parameters in the DAQmx Acquire step according to the calculations above, as shown in the attached screenshot in my previous post.

When I measuring the 1g signal, the input voltage to NI9234 is 1.8V, so the accelerometer and connections should be OK. I believe something's wrong with my parameters but I don't where I did wrong.

Thanks again!

[Sean_F]

Hi Bon,

In this case you're going to have to use a custom scale. In MAX, right click on  Scales  » create new  »  NI-DAQmx Scale  »  Linear.  For your scale, the pre-scaled units should be g and set your scaled units to be g. Your slope should be 1, and the y-intercept should be -5. This will shift your scale down by 1.5 V (5g).

In your acceleration task, select your scale under Custom Scaling.

This should sort you out.

Happy Holidays!

[bon2b]

Hi Sean,

Thank you so much for the detailed information!

I modified my test using customized scale as you suggested(I totally agree with your calculation). Then I used this test to measure two known signals:

1) cDAQMod6_ai0: 1.8V

2) cDAQMod5_ai3: 1.5V

My accelerometer's output range is 0-3V, with 1.5V at 0g and 300mV sensitivity, so I expected to see a 1g signal on Mod6 channel 0 and a 0g signal on Mod5 channel 3.

The results shows a 450mg acceleration on Mod6 channel 0 and a -450mg signal on Mod5 channel 3. I attached a screenshot with this post, along with my modified SignalExpress project.

I have no clue what went wrong...my NI9234 cards are newly purchased last year. Shall I calibrate the cards? I did notice that in your last post, you have a "Calibration" option on your channel setup page which I don't have. I'm using SignalExpress2009 with Sound and Vibration Suite by the way.

Thank you again for your time!

P.S. So when configuring a channel, for the "Signal Input Range", I should enter the min and max value of my input signal in whatever unit stated in "Scaled Units" block, am I correct here?

[bon2b]

Attachments

[Ali M]

Hello bon2b,

Can you create a Custom Voltage with Excitation task in MAX and verify whether your values are correct?

Best regards,

Ali M

Applications Engineer

National Instruments

www.ni.com/support

[bon2b]

Hi Ali,

I have to say I'm not quite sure how to create a custom voltage with Excitation task in Max.

I configue my data acquisition tests in SignalExpress DAQmx Aquire step most of the time, and only use MAX accationally to verify that my devices are there.

Can we do the verification you suggested in SignalExpress? Or if not, could you please show me how to do it in Max? By the way, I tried to create an analog input voltage siganl(DC, 1.8V) in SignalExpress and got the signal exactly the way I wanted.

Thanks

[Sean_F]

Hey,

So if I understand this correctly you applied 1.8 V to the 9234 and you verified that the 9234 is in fact reading 1.8 V in the Test Panel? If this is the case, then there's a problem with the way you're applying your scale. Using your SE project and my scale I applied 1.8 V and read 1 g,  as expected. I attached my scale (just remove the .txt extension) so that you can import it into MAX (File -> Import).

Best of luck.