Problem with 3 wire LVDT

3 wire lvdt

Hi everybody,
I am working on a project where i'm using an LVDT for precise measurments of a linear valve displacement. The problem is that i have no documentation related to the sensor, and i have no idea about the correct wiring.

I have three terminals coming out of the valve, two of which are shorted and seem to be the input coil. By trial and error, i found that by applying the AC signal across these terminals, a DC output is generated on the third terminal with respect to ground (which is one of the two terminals of the input coil). This DC voltage has a range of 250mV, and has a very low signal to noise ratio.

This makes me unsure if i'm going in the right way, it seems there is an integrated signal conditioner that gives me this DC output, but i'm not yet sure, and i have no idea about LVDT's with three wires (noting that a DC input voltage produces no response either).

The LVDT is Czech made, and no documentation is available anywhere, could anyone please help me with my problem, I have tried everything and it seems i am almost totally lost.

Thankyou very much for your help...

Re: 3 wire lvdt

hey friend
what type of signal conditoning your using?
are you using any analog technique using AD598/AD698 ? or controlling Digitally....
Tell me so that we can think of possible problems that could occur

hey i am working on something like same project now. so lets we do some knowledge sharing this will help you and me.

thank u

Re: 3 wire lvdt

I didnt hear about a LVDT having 3 wires
there are three veriaties of LVDTS are available
4-wire LVDT - secondaries connected in series opposed condition
5-wire LVDT - having secondary at cetre tap
6 wire LVDT - two isolated secondaries

make sure, your LVDT is right one. refer datasheet of it, it will be helpfull to you

Search for '3 wire LVDT' on Google. There are plenty of results.

Keith

ok keith
But seems, 5 wire LVDT they are calling as 3-wire LVDT.
wire 1, 2 : primary excitation
wire 3, 4 : secondary outputs
Wire 5: centre tap wire

am i right?

i think few manufacturers calling 5-wire LVDT as 3-wire LVDT

---------- Post added at 07:47 ---------- Previous post was at 07:43 ----------

keith1200rs said:

Search for '3 wire LVDT' on Google. There are plenty of results.

Keith

Click to expand...

Hello Mr. keith

can you please help on synchronous phase demodulation in AD698 signal conditioning unit?
I could'nt able to understand, where the phase information is decoding. It used the transfer function as A/B.
could you please tell me how the phase demodulation is taking place ?

thank u

I think your LVDT may have the signal processing inside like this www.sst-shinkawa.co.jp/pdf/manual/e/vm21p_me.pdf

Without any sort of identification for the device it is impossible to tell.

Keith.

But seems, 5 wire LVDT they are calling as 3-wire LVDT

Click to expand...

There are also sensors that have three terminals, without a separate exitation port. Review the respective manufacturer manuals. Universal signal conditioners should be able to deal with all kinds of sensors, 3 - 5 wires connection scheme.

P.S.: It seem like you didn't read the AD698 datasheet thorougly. Figure 7 is e.g. showing a three wire LVDT connection scheme.

---------- Post added at 09:41 ---------- Previous post was at 09:23 ----------

I think your LVDT may have the signal processing inside

Click to expand...

This is apparently the case with the transducer describer in the original 2007 post.

hi fvM
can you give me a brief explanation on How synchronous phase sensitive demodulation is taking place in the universal signal conditioner AD698 ?
i got confused on the internal IC operation.

You can see the operation principle in the block diagram Figure 6. It's basically multiplying the sign of the reference input with the A and B signal input and low-pass filtering the product.

Imagine a sine input signal. If it's in phase with the reference (0 degree phase shift), the product is a full-wave rectified sine, and the filter output a maximum positive DC voltage. A 180 degree signal will give maximum negative demodulated signal. At 90 degree input phase, the sign is switched at the sine peaks, the filtered mean value ist zero.

I dont Know much about LVDT's but found this article must be of some help

LVDT

The link says about LVDT :smile:

It can operate over a temperature range from -650C° to 600C°

Click to expand...

The whole of that site seems rather dubious. It just goes to show you shouldn't believe everything you see on the internet!

Keith.