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DAQmx AI Bridge Measurements - First and Second Electrical and Physical Values

Hello,

Can someone explain the meaning behind the parameters "first electrical value", "first physical value", "second electrical value", and "second physical value", when configuring the scaling information for an AI Bridge measurement (pressure or force)?

If I have a load cell rated 0 to 200 lbs with a sensitivity of 2mv/V and an excitation voltage of 2.5V ...

I thought the parameters should be:

electrical units = mVolts/Volt

1st electrical value = 0

1st physical value = 0

2nd electrical value = 5 (2mv * 2.5V = 5mv)

2nd physical value = 200

But it seems the 2nd electrical value is actually supposed to be the sensitivity of the sensor (2mv in this case).

Am I missing something here?

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Re: DAQmx AI Bridge Measurements - First and Second Electrical and Physical Values

Hi ebloohm,

These values correspond to the minimum and maximum values that you want DAQmx to scale. I found some documentation on how this function works:

When you use this scaling type, NI-DAQmx performs linear scaling between each pair of electrical and physical values. The input limits specified with minimum value and maximum value must fall within the smallest and largest physical values. For any data outside those endpoints, NI-DAQmx coerces that data to the endpoints.

If your table is something like:

Electrical | Physical

-----------+---------

0 mV       | 0 lb

2 mV       | 50 lb

and the input signal is negative, the table scale will return 0 lb.

Thomas N.
Applications Engineer
National Instruments
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Re: DAQmx AI Bridge Measurements - First and Second Electrical and Physical Values

Maybe I'm misunderstanding something...

If I have a 2mV/V, 50lb sensor.  With a 2.5V excitation:

Isn't the voltage at full scale 2mV * 2.5V = 5mV?

Is that not right?

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Re: DAQmx AI Bridge Measurements - First and Second Electrical and Physical Values

Hi ebloohm,

That's correct. For your load cell, an excitation of 2.5v will give you a rangle of 0-5mv for the output. At 0lbs you'll get 0mV, and at 200lbs you'll get 5mV. The values in the table are used to scale the mV readings to lbs. The sensitivity of the load cell is already taken into consideration with the mV range values. If the loadcell had a sensitivity of 3mV/V you would instead have 0 and 7.5 in your table. The values in your original post are the correct ones.

Thomas N.
Applications Engineer
National Instruments
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Re: DAQmx AI Bridge Measurements - First and Second Electrical and Physical Values

Sorry to add on to this thread, but I'm a bit confused on the second electrical value. Easy enough that we want sensitivty*excitation, but that would have units mV (mV/V*V). The options with the AI Force Bridge are in the form of sensitivity units, e.g., mV/V or V/V. What obvious thing am I missing here?

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Re: DAQmx AI Bridge Measurements - First and Second Electrical and Physical Values

The values that you would pick for first and second electrical values seem to be values that are defined by the specs of the sensor.

"Specifies from which electrical unit to scale data. Select the same unit that the sensor data sheet or calibration certificate uses for electrical values."

http://zone.ni.com/reference/en-XX/help/370471W-01/daqmxcfunc/daqmxcreateaiforcebridgetwopointlincha...

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