0
\$\begingroup\$

I am working with the DC-LVDT Omega 620-25

http://www.omega.com/pptst/LD620.html

Manual: http://www.omega.com/Manuals/manualpdf/M4345.pdf

Specs: http://www.omega.com/pressure/pdf/LD620.pdf

I am using the NI 9220 module on a compact DAQ for data acquisition. the module supports differential inputs and single ended mode.

NI 9220: http://www.ni.com/pdf/manuals/373920a_02.pdf

I am experience some trouble obtaining 3 digits of accuracy from the converted signal, the excitation voltage is 12.75 Volts (external adapter) and the reading is -5.9

Furthermore the second case was by using a single ended connection (as per the modules datasheet) single ended is achieved by connecting the signal - of the LVDT module to the COM input, I also placed a 100nF capacitor for noise filtering, I was able to get 2 digits of accuracy 1.12x.

Input is read with a continuous acquisition mode of 100 samples/second with a 500Hz frequency and a voltage range of -10 and 10 as device parameter options ( only differential mode is selectable)

In addition and since the LVDT is linear I should directly convert the voltage reading to distance but the output voltage is of different range the datasheet states that for an lvdt of -d/2 and d/2 it should outputs a -5 and 5 V readings.

I double checked on the sensitivity formula mv/V/mm Output Ripple: 0.02% FSO LD620-25 ±25 (±1.0)inch and with a range of -6 -> 6 the full LVDT range is (12V) therefore is its of 0.0024 mv/V * 12.75(excitation) = 0.0306mm

Example used: http://tinyurl.com/hrcshow

but i didn't use that as for calibration I am considering the initial read voltage to be the 0 reference and then subtracting it from each reading!

1) What is the difference between using a single- ended and differential modes for a DC output? and how can I achieve better accuracy

2)where do I use the calculated sensitivity and did I get the calculation correctly?

3)how can I eliminate additional noise (AC noise) that is affecting my measurement and what should I take into consideration for designing a filter and choosing it's type

when disconnecting the LVDT excitation voltage the modules outputs uV values that varies. considering the copper wire acts a receiver and generating additional noise I place the 100nF capacitor between the signal + and signal - terminals.

In addition, I am going to design a circuit to level shift the output voltage from -6 6 to 0 - 12 and make it a single ended output and connecting it to an arduino or a PIC (is it a good idea)?

Thank you in advance! I appreciate the help

\$\endgroup\$
1
\$\begingroup\$

Perhaps you have mixed some models. From your description I presume you have a +/-5V output from LVDT, the supply voltage is 10 to 30V, but this has no influence on ouptut signal.
1 Whenever possible use differential mode, but I don't see that you have this possiblity (you should point exatly what is the output/wiring of your model).
2. Probaly with calcualting 12V as reference, as said the output is +/-5V
3. Good shileded cable, sheild connected to the earth.

\$\endgroup\$
  • \$\begingroup\$ It is the same module but I am getting around -5.9 to 5.9 output the differential mode is not stable the only stable reading i got with 2 digits accuracy was the single ended mode in differential mode I tried : signal + to AI+ and signal - to AI- the LVDT white shield cable connected to COM, the second mode was single ended by connecting AI+ to signal + and AI- to COM and signal - as 1 point the datasheet of the 9220 suggest using a 1Mohm between AI- and COM but that didn't improve the reading and the LVDT suggested a 10Kohm resistor between SIG+ and SIG- but that did not improve it as well \$\endgroup\$ – chaosmind May 8 '16 at 20:03
  • \$\begingroup\$ I would change the PSU unit first, with a stable voltage LDO. The problem in differential mode, could be related to the ground connection. As for -5.9 to 5.9V output, I think you should contact Omega. Try to ground the - of PSU. \$\endgroup\$ – Marko Buršič May 8 '16 at 20:17
  • \$\begingroup\$ I have an external regulated 12 VDC adapter connected to the excitation wires of the LVDT \$\endgroup\$ – chaosmind May 8 '16 at 20:20
  • \$\begingroup\$ Then, I think you did everything as one engineer would do. Most be something wrong with the part. You can scarp a ferrite ring from some old equipment like mouse, keyboard, extern hdd,... then make few turns with both signal wires, this would work only in differential mode - it's a common mode choke, it would block noise if it's the same in both wires. \$\endgroup\$ – Marko Buršič May 8 '16 at 20:26
  • \$\begingroup\$ I tried a capacitor on the inputs! but i will try the ring wiring, i wanted to convert the differential mode to a single ended one and shift the voltage level then use a divider and connect that to a pic or arduino! In addition I have no idea on what to consider and how to design an AC filter for the input is it a low pass or high pass and how you get the RC parameters \$\endgroup\$ – chaosmind May 8 '16 at 20:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.