what we gets when strain is used as output, voltage or strain?

[M.P.Singh]

sir

i m using scxi1520 with a quarter bridge configuration by setting input as strain, which value i am getting that is not strain which i calculated analytically. somebody told me that which value we gets from here that is voltage not strain, i have to calculate strain from here by the formula.

 i am not getting that this value is strain or voltage.

i have a strain gage of 120 ohm with gage factor two. please clear my doubt which i have with this as early as possible.

[Broken_Arrow]

From the sound of it, what you are probably seeing is voltage. Here's an example:

Your strain gauge has a Full Scale Output Voltage of 1mV/V.

Your strain gauge has a Full Scale value of 100 inch pounds.

Your excitation voltage is 1V.

You are reading 500 micro volts. That's 50 inch pounds.

Get it?

[M.P.Singh]

ok

if we are seeing voltage then why strain is taken as output, we can take voltage as output directly.

[johnsold · Accepted solution]

Think of the data being represented in several different ways.  Your input is strain.  The strain gauge converts that to resistance.  The bridge changes the resistance to a voltage.  An amplifier multiplies the voltage by a constant and perhaps subtracts an offset.  The analog to digital converter changes the continuous analog voltage to a sequence of voltages with two possible values (0 or 5 V) at each time point in the sequence (digital).  The computer processes these digital voltages as numbers which have a close correlation with the analog voltage magnitude.

But you are trying to measure strain, so you do not really care about analog or digital voltages, except needing to know what the relationship is between them and the original strain.

Your data acquisition system measures voltages. You must provide the conversion to units of strain via calibration.

Lynn 

[Broken_Arrow]

Well said Lynn. 

One way to get actual strain units into your indicator is to use the “From Custom Scale” type input rather than “Strain”, and in MAX, make a custom scale which correlates the output to strain (micro-strain most likely). Then tell the DAQmx Analog Input VI what custom scale you are using. From that point forward, the value populated into the indicator will be Strain.  

The other way is to simply scale the voltage, using math functions, just before you populate the indicator, as Lynn said.