Making an LVDT

[Doctorandus_P]

I've been reading a bit about LVDT's and not really convinced this is a wonderful thing. It uses a complicated way to generate

an analog voltage, and then you have to put that into an ADC.

I also bumped into the AD598. This little chip costs EUR122 from mouser, which seems quite ridiculous.

Now you've built your sensor coils and are doing some experiments, I suggest you also do some experiments with a simpler method.
Ever since the '90-ies there are a bunch of circuits floating around the 'net for an LC meter. It's build as a free running LC oscillator and a microcontroller to measure the frequency of the oscillator though some reciprocal math. Years ago I did some experiments with this, and you can do quick measurements with about 0.1Hz resolution, (and could get more resolution by integrating over more oscillation periods). The hardware is very simple, and schematics and software is floating around the 'net.

Note:
Now re-posted in the right thread. I accidentally posted this in:
https://www.eevblog.com/forum/projects/diy-variable-ac-voltage-and-frequency-source-for-calibration/msg5553581

[Kleinstein]

For the solution with µC / ADC the drive signal should ideally use the same reference as the ADC. So more like square wave from the µC with a little low pass filtering.

Something ferromagnetic near the LVDT can interfere with the readings. It gets tricky to build a shielded LVDT.

[Conrad Hoffman]

some useful stuff here- https://www.researchgate.net/publication/221918107_Linear_Variable_Differential_Transformer_Design_and_Verification_Using_MATLAB_and_Finite_Element_Analysis

The strong point of an LVDT is almost unlimited resolution. If your needs are more modest, there are simpler ways as mentioned above. I've built simple LC oscillators with a longish coil and a ferrite that moved in and out, just measuring the resulting frequency. Worked fine but today everybody wants a digital solution with as little analog stuff as possible. Things like an https://ams-osram.com/products/sensors/position-sensors/ams-as5304-linear-sensor or it's many relatives are accurate and cheap.

[Doctorandus_P]

Those HALL based sensors don't give absolute position (i.e. the pattern repeats every 4mm).

And there are more options, for example EE-SX1070 is an analog optical sensor. It costs EUR 3 and can have a resolution of 1um over a range of 1mm.
I accidentally bumped into this sensor via:
https://hackaday.com/2024/05/18/flexures-keep-this-printed-displacement-sensor-in-line/

Edit / Addition: (not worth a second full post)
I had a look at TCUT1800X01 datasheet (See post below).
The sensors in these are staggered, and have a "linear range" of around 0.2mm These seem to be optimized for digital sensing, while the EE-SX1070 has a relatigely linear range of about 1mm and is desgned for linear position sensing.

[PCB.Wiz]

And there are more options, for example EE-SX1070 is an analog optical sensor. It costs EUR 3 and can have a resolution of 1um over a range of 1mm.
I accidentally bumped into this sensor via:
https://hackaday.com/2024/05/18/flexures-keep-this-printed-displacement-sensor-in-line/

That's just a diode-transistor photo interrupter.
The narrow slots give it a quasi linear transition with % of light transfer, but it is very temperature and unit to unit dependent.
A second sensor, mounted next to the working one, (or a dual sensor like TCUT1300X01) could give some tracking compensation if you wanted to push these.

Vishay have some interesting ones with parts like TCUT1800X01 having 4 slots.

[Muffins]

Hi guys,

Just a quick update on this one. It's been slow progress since the coil was built but now I can get some things after payday. I've ordered a function generator off ebay and I'll order a couple supplies this week. Frustrating not having stock of the basic components.