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Making an LVDT

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Hello all.

I have started a new project building an LVDT, drive circuit and sensing circuit.

At the moment I'm still in the early stages, I've got some components and a spool of 0.25mm magnet wire. I'm aiming for the stroke of the LVDT to be around 40mm with the movement being slow and without vibration.

I'm trying to decide on the driving circuitry but I am a little unsure of a suitable approach as I am not that experienced with analog circuits. From what I understand I should be putting a sine wave into the main coil with a frequency in the kilohertz range.

I've looked at some sine wave generation circuits, even making a wien bridge oscillator, but I'm not sure if sine wave generators based on op-amps will be able to provide enough energy to drive the coil. As I understand the sine wave should also have no dc components to avoid saturating the coil. Surely then something like a H-bridge would work quite well to produce an alternating current through the coil but the waveform with simple switching will be a square wave. I suppose with some PWM of the input to the H-bridge a sinusoidal average voltage could be made but would that be a good solution? I've seen the components of a DC square wave be filtered out to make a sine wave but that involves using a low pass filter, which I'm not sure would be suitable in this circuit.

What should I try next?

Edit / oops. I mixed up LVDS with another inductive sensor type and that makes my post below mostly not relevant. Unless maybe, you may be interested in other inductive sensor types too...

The simplest is probably to build a free running LC oscillator, and then just measure the oscillation frequency.

For the rest it all depends on what you want to do with your sensor. Do you want low resolution movement detection (for example in a harsh / dirty environment) or do you want sub micrometer resolution. LVDT's for those cost EUR700 or more, and I guess there is a reason for that. I am curious myself what resolution you can get with DIY methods though.

LVDTs come in all shapes and sizes, I worked on a typical one that used two coils driven 180 degrees apart and a detection coil that measured how far it traveled one way or the other. I assume you are just building a single coil and will be measuring the amplitude to detect motion or will you be using a differential coil/drive? A sine wave is preferred over a square wave because of parasitic capacitances which will mess with your signals if using square. If you are worrying about drive current then use an audio power amp ic as the driver, I assume you'll want a frequency below 20kHz. A Wein bridge oscillator driving a power amp ic should work well.

Conrad Hoffman:
Look up the TI PGA970. Not sure of price or availability but the block diagram might be useful if you can't get the actual device.

There is no real need for a sine wave. A slightly smoothed / slew rate limited square would be OK too. This is usually with some phase sensitive detector and for this one need a fixed phase reference signal - this gets easier with a square wave. Also a constant amplitude is easier with a square wave.

The question is a little how one wants do do the detection.  A question is what to do with the LVDS signal: if the signal goes to a ADC / ┬ÁC anyway, one could as well directly read the AC signal and do the rectification / correlation in software.


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