Scaled physical units for a loadcell

[maurlind]

Hi!

I have a load cell that i want to measure the weight of an object in the unit gram (g). I have a Ni 9237 connected with my load cell. I'm aware that the NI 9237 displays the signal in mV/V. I want to scale this unit to gram. In my labview code I use the DAQ assistans and i know that I have to set my specific values from my load cell certificate in here. But i don't really see where all of my values from my load cell should be inserted. I will attach a picture of my certificate if that will help you understand it better.

I have tried a custom scaling with scaling parameters. The values for slope and intercept I had to calculate by measureing wieghts of 10g 20g 30g and so on , to see what values of mV/V they generated. Then i made a regression and took that slope and intercept, but that didn't work. I gett errors like, 200077" Requested value is not a supported value for this property. The property value may be invalid because it conflicts with another property", and it says that I need to select Newtons instead of mV/V.

I have searched on the forum, but I haven't found a direct guide to this. I know there is but seems that I'm not very good at searching info. 

To be noted I have the student edition of Labview 2013

Best Regards Maurlind

[GerdW]

Hi maurlind,

that certificate doesn't provide the details of relation between weight and output. All you see is dependence of output signal vs. supply voltage (as a typical value of full/wheatstone resistor bridges).

For your error message: we cannot help as long as you don't attach your VI. (Saved for LV2011 would be fine for me.)

Which polynomial regression values did you use?

Ah, I see you're using that resistor bridge module. What excitation voltage do you use?

What's wrong with Newton outputs? You are measuring a force (aka load)!?

[maurlind]

Hi!

I use 10V as excitation voltage. There is nothing wrong with newtons as outputs, I can always convert it later in my code. I tried to change to it but it didn't' work.

I used a linear regression withe the formula y=0,0362x-0,0187 where x is in gram and y in mV/V. the values 0,0362 and -0,0187 where inputted in the custom scaling parameters. Se the attached print screens. For max I inputted 3000 which is the maximum load for my load cell and min -3000, in grams.

My error comes from the DAQ assistans when I'm done with my settings and want to exit. It has nothing to do with my code. 

Best Regards Maurlind

[GerdW]

Hi,

which output values do you get without applying a custom scale?

[maurlind]

Hi!

With output values do you mean the values from the signal?. It is calibrated so it should give out zero, or around it offcourse. 

With out applying a custom scale I use the default setting except for the vex value.

Best regards Maurlind

[GerdW]

Hi maurlind,

With out applying a custom scale I use the default setting except for the vex value.

Did you read the comments on V_excitation in the manual of your NI9237?

With output values do you mean the values from the signal?

Yes: which signal value do you get at 3kg, at 2kg, at 1kg, at zero load?

[maurlind]

Hi!

I have read about the NI 9237 from this link http://www.ni.com/pdf/manuals/374186e.pdf
What I understand is that the NI 9237 uses an internal voltage , 2.5, 3.3, 5 and 10V since my load cell need 5-12V according to my certificate I can use 10V from the NI 9237 and use then use the Vex value as 10V, or I'm I wrong? I don't know what exact voltage I must use or how I can adjust it since I haven't found any info of it. I don't have any physical manuals for my instruments at least I'm aware of, maybe in a box somewhere in my Lab.

Here are the output values that I have noted:

g	mV/V
10	0,35
20	0,7
30	1,05
40	1,425
50	1,8
60	2,16
70	2,52
80	2,875
90	3,22
100	3,6
1000	35,28
2000	70,53
3000	105,78

[GerdW · Accepted solution]

Hi maurlind,

What I understand is that the NI 9237 uses an internal voltage…

That manual also mentions some power limits. Did you read the manual?

Here are the output values that I have noted…

The output signal of a bridge is of unit "mV" - not "mV/V"!

You get about 35µV/g (with current excitation voltage). All you need to do is to multiply your signal with this number (signal * 1g/35µV) to get your load. Either create a DAQmxScale to do this or use simple math on your signal…

[maurlind]

Hi ! 

It worked with multiplying with that factor.

I also read that the power limit is at 150 mW. According to the pdf file i can use a full bridge of 350 ohm with 3.3V. Then I should set the Vex value to 3.3V ? Don't i have to set my NI 9237 to the same voltage as well, if so how do you do that, didn't read anything about that in the manual.

Best Regards Marulind