Don't know exactly what I'm doing.
Following directions on one unit, don't have directions on another. Need to learn, Can't find data on web. Looked for days.
Advice, info, and opinion welcome.

I'm trying to set up (calibrate) a rotary LVDT.

We run the signal through a controller like MOOG or an A-308 signal conditioner. We are trying to get a linear output from 0 to 6 volts. Everything I find on the web gives the transformer theory, great but I understand that.

What we are trying to follow is a chart that shows 80 degrees Voltage A-B .0234 milvolts, 60 degrees voltage a-b = .0344 millvolts etc. Is this enough information? We measure A-b our values are a-lot different. And this is before the signal conditioner. Is there some standard we should be following? What are we doing wrong? I there something I need to read. Just anything to help get me going in the right direction..

Much Thanks,
David

 

David Hackenbracht,

LVDTs--and RVDTs (Rotary Variable Differential Transformers, rotary LVDTs--usually have a linear range of operation. If the LVDT/RVDT isn't adjusted such that the "stroke" of the device it's connected to is on the linear range of the LVDT/RVDT then the readings are not going to be repeatable.

For example, let's say the minimum output voltage for the linear range of the LVDT/RVDT is 0.700 VAC RMS. If the output voltage is 0.400 VAC RMS when the valve is at 0 degrees then the LVDT/RVDT output isn't going to be linear until the valve opening causes the LVDT/RVDT output voltage to be greater than 0.700 VAC RMS.

There's also usually a non-linear range at the upper end of the LVDT/RVDT output, as well. So, the initial operating position of the LVDT/RVDT has to be adjusted so that the output voltage is in the linear output range of the LVDT/RVDT and also so that the maximum travel of the valve doesn't cause the LVDT/RVDT output to exceed the upper linear output range limit.

I've seen this cause a lot of people heartache when they just bolt (or screw) a new LVDT/RVDT in place and don't adjust it to get the output to be in the linear range.

And this is probably what's needed before the signal conditioner, and then the signal conditioner needs to be adjusted to provide 0-6 volts over the range of travel of the device (say the LVDT/RVDT output is 0.234 VAC RMS to 1.45 VAC RMS; the signal conditioner would need to be adjusted so that it's output went from 0 VDC to 6 VDC for an input of 0.234 VAC RMS to 1.45 VAC RMS.

Don't know if this is your problem or not, but it's worth asking.

Also, since you're talking about such very low voltages it's usually required to use a True RMS voltmeter to measure the voltage output from the LVDT/RVDT if its operating at a high frequency (kHz). Just any old voltmeter is going to give very erratic readings; a True RMS voltmeter will give repeatable readings--if the LVDT/RVDT is adjusted such that the output is linear over the range of travel of the valve.

Please write back to let us know how you fare.