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LVDT Sensitivity

I am using LabView 8.2 with an SCXI system (1001 chassis, 1315 terminal block and 1540 module).   When I set-up the DAQ assistant within my block diagram, I am having some problems knowing what I am suppose to enter for the sensititivity.  I am using a Honeywell Sensotec LVDT as the instrument which has a calibration factor of 2.138- VRMS/inch (or Voltage Root Mean Squared per Inch) at an Excitation Voltage of 3-V with a Frequency of 5-kHz.  The options for the sensitivity units within LabView are mV/V/inch.  How do I convert from VRMS/inch to mV/V/inch?  I believe that I need to divide the calibration factor by the excitation voltage (so 2.1384/3 = 0.7128), but when I use this as the input for the sensitivity (0.7128), the instrument readings are still incorrect.  The system produceds fairly accurate results with a sensitivity around 0.6421.  Can anyone help me?

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I am just guessing, but I think you also need to convert the Vrms to mV.  I believe that the mV refers to the amplitude of a sinosoidal wave.  The conversion forumula is Vrms= Vpk/sqrt(2).  Try this and see what happen.

 

Yik

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I believe you are correct that the problem has to do with the VRMS - at least that is my guess.  But the square-root of 2 doesn't work.  I also looked into using 1.11 (which is the conversion from VRMS to average Voltage).  This seems to result in more accurate results, but I am not sure if this is the correct method. 
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Hi bsmith,

 

Do you have the exact part number or a link that I can take a look at for your LVDT?

Regards,
Jim Schwartz
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Yes, I am using Honeywell-Sensotec LVDT's.  I am actually using 12 different types (which are all having the same problem) - they all are the same model type, the only difference is the sensor stroke.  Here is the information for one:

 

Model: VL7A

Part #: 060-3618-04

Sensor Stroke: +/- 3-inch

Sensitivity: 1.4190 VRMS/inch

 

Here is a link to the product data sheet:

 

http://content.honeywell.com/sensing/sensotec/pdf_catalog08/008751-1-EN_Model_VL7A_AC-AC.pdf

 

Note, within my LabView program, I am using an excitation voltage of 3.  In order to get accurate data, I have been inputting a sensitivity of 0.426 mV/V/inch - which equals 1.419 / (3 * 1.11).

 

Thanks for your help

Message Edited by bsmith24 on 03-09-2010 11:16 AM
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I can only be of limited use but I'll try...

 

The units mV/V/In describe how the ratio of measure to excitation are converted to inches. You have to divide your measured signal amplitude by the refence amplitude. THis ratio is the amount of coupling between the primary and secondary. At full displacement the mV/V should be "1".

 

The LVDT hardware does the division for you since we need the instantaneous value of excitation and measured to get a valid ratio.

 

Ben 

Retired Senior Automation Systems Architect with Data Science Automation LabVIEW Champion Knight of NI and Prepper LinkedIn Profile YouTube Channel
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