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DAQmx Polynomial Scale: How can I set MIN and MAX values?

Hello, I am trying to understand how I set the MAX & MIN values when I apply a polynomic custom scale to a channel in DAQmx, I found a similar topic but with a linear scale: http://forums.ni.com/t5/LabVIEW/DAQmx-custom-scale-how-do-you-set-MIN-and-MAX-values/td-p/1277028 but the calculations are not the same, I hope somebody can help me.

 

Kind regards.

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Hi,

 

Regarding this issue. I found out that for polynomial equation for second order is y= ax^2+bx+c.

 

The custom scale in NI-DAQmx Scales can be done by My System>>Scales>>NI-DAQmx Scales>> Right Click+creating new NI DAQmx>> Polynomial.

 

From there you can set the coefficient based on the scale you wan. And the cut off point.

 

For example:

a=1

b=10

c=2

 

 

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Hi, yes thats correct but I can create the scale perfectly, the problem comes when I want to apply this one to a DAQmx Task, I have an error like this for example:

 

 

Error -200077

 

Possible reason(s):

Requested value is not a supported value for this property. The property value may be invalid because it conflicts with another property.

Property: AI.Min
Requested Value:  0.0
Value Must Be Greater Than:  11.960000
Value Must Be Less Than:  12.040000

I want to know how these thresolds (11.960000, 12.040000) are calculated by DAQmx, thanks in advance

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Hi,

 

The value is determined by the hardware itself internally. However, can i know what is the range of your max and min. This error can be improve by adjusting the range according to the requested.

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Hello AYanez.

For what I can read, you want to know how to set the input limits in the DAQ's configuration. So, I will share with the following link that explains how to configure the signal input range:

 

How to Configure Signal Input Range for My Task or Virtual Channel When Using a Custom Scale

 

In a few words you have your polynomial equation, for example:

 

2nd order.....y=a+x*b+x^2*c, where y is the scaled signal (i.e. Temperature), x is the pre-scaled signal (voltage acquired by a sensor), a, b and c are the coefficients. For this example a=0.1, b=0.3, c=0.2.

 

Now your DAQ device has the following specifications for the input signal: Range -10 to 10 V
And your sensor outputs a signal from 0 to 5 V.

 

So in this case the scale adjusts the values of the Input Range to 0.1 to 6.6 (Temperature Units) according to the voltage signal acquired by the sensor. Both values will be your input limits, MIN and MAX. Now, I recommend you to set some tolerance in this limits values.

 

In the other hand, I will share with you information related with the Forward & Reverse Polynomials Scale, just for reference:

 

Forward & Reverse Polynomials When Using a Custom Polynomial Scale in Measurement & Automation or th...

 

What Are NI-DAQmx Custom Scales, and How Do I Use Them?

 

 

I hope this information solves the question. I will keep my eye on this subject.

 

Best Regards

 

Antonio S.
Applications Engineer - NI Mexico
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