Need help with LVDT setup on USB-9215

[Bocchini]

I'm having trouble setting up and LVDT to my USB-9215. The voltage that my daq bored is reading goes from a little over 5 V to 3.3ish V  at regular intervals (see attached screenshot). I've measure the output of the LVDT with a voltmeter and it stays constant at the 5 V reading. I don't understand why it keeps dropping to 3.3 V. If anyone has any insight it would be appreciated.

[Meadow]

Hi Bocchini,

Can you give description of the LVDT sensor that you are using?

Do you have an oscilloscope to see, measure and capture the output from the LVDT?

[Bocchini]

I've attached the data sheets. The LVDT goes to a Signal conditioner, and I'm connecting the outputs from the Signal conditioner to the DAQ board.

[Bocchini]

After checking both the DAQ and the LVDT signal conditioner, they both seem to work propertly on there own. I called someone from honeywell and he said the two might be slightly incompatible due to impedence differences (or something, I'm not an Electrical Engineer). I was told that it might be as simple as adding a resistor, but he hasn't gotten back to me yet. If anyone has experience doing something like this let me know.

[Meadow]

Hi Bochini,

LVDT signal conditioning module will provide the excitation voltage and reading of the LVDT signal. The result of the reading is displayed on the signal conditioning module as well as available for DAQ as +-5V from the output channel of the signal conditioning.

I would check the output reading in the signal conditioning display to check on possible issue of the installation and to ensure that the LVDT signal conditioning is calibrated for operation as mentioned in the section 4.0 of the DM Manual.

Should there be no issue with the signal conditioning module (LVDT value displayed correctly), then we check on the output to ensure that the output represents the displayed LVDT value.

Let me know if these helps.

[Bocchini]

I have fixed my issue. To get rid of the intermittent voltage drop, I added a 10 megaohm resitor from the negative important to the COM channel. This is actually mentioned on pg 13 of the USB-9215 manual though it didn't say what happened if this was not done. I believe the next best version USB-9215A has internal circuitry to avoid this problem. Just thought I'd relay this information if anyone else gets stuck on it.

[KyleTTU]

Hi Bocchini, 

Thats great to hear that the addition of a pull down resistor solved your problem.  It seems your signal was floating outside the range of the device and causing some pretty bad measurements.  This resistor only really needs to be used with floating differential signals which is why it isn't a required peice of hardware. 

Regards, 

Kyle S