That's a rather ingenious way to measure displacement.

Why not use a linear resistance coil or track or even a 10-turn pot with a rack and pinion or a flex band. A voltage divider circuit is a lot simpler, more linear, accurate and stable.

You could even use the same coil as variable linear resistor by scraping off the insulation and putting a wiper on it.

Thank you for your reply.
In fact, the environment that the device will work is a harsh environment because maybe oil penetrates inside the device, and mechanisms base on electrical connection may be failed.

 

hi M,
Do you have a Inductance measuring unit in your workshop.?
If you measure the Inductance change over the range of travel if the inserted rod, it should be possible to create a LTS simulation.
This would help with project development.
Is the frequency still 20kHz sine and at what voltage level?
E

 

Your Wien network is wrong. R6 and C9 should be in series.

 

Your Wien network is wrong. R6 and C9 should be in series.

True.
But as TS reports it is running at the correct frequency then it is most probably just on the schematic not on the real world circuit.

 

In the original post, you asked about alternatives.

I'd be looking at a LVDT

https://www.omega.co.uk/technical-learning/linear-variable-displacement-transducers.html#

https://www.te.com/usa-en/industries/sensor-solutions/insights/lvdt-tutorial.html

https://en.wikipedia.org/wiki/Linear_variable_differential_transformer

Its a transformer, with a sliding core inside,
you have two, counter wound primary and a single secondary, with a sliding core in the middle.
you put a sine wave in the primary, and synchronous receive at the secondary,

As its an AC signal, and synchronous rectified, the amplification is AC coupled, so no DC drift.

used a lot in situations where rugged, sensitive position measurement is needed.

 

On second thoughts, would a linear variable capacitor work better? It would be easier to construct.
I would use the capacitor in an RC oscillator circuit and measure frequency instead of measuring voltage.

 

@MrChips,
an interesting thought use capacitance,
one could actualy do a LVDT , but using capacitance plates,
I'd think it would be much more variable with ambient conditions,
A transformer LVDT is very easy and rugged,
I once "repaired" one using card board rolled up and a large bolt from the boat as the slider.

 

hi M,
Do you have a Inductance measuring unit in your workshop.?
If you measure the Inductance change over the range of travel if the inserted rod, it should be possible to create a LTS simulation.
This would help with project development.
Is the frequency still 20kHz sine and at what voltage level?
E

Hi Eric,
No, I have not LCR meter, but I calculated it by voltage divider with the C series, and the inductance obtained about 10mH when the rod was out of the coil.
I found when the rod was quite in the coil, the frequency increased from 3400 Hz to 50 kHz, I have not an oscilloscope and I measured the frequency by frequency meter on my multimeter.

I replaced the C4 with 470 ohms resistor and I saw that the frequency stabled for different positions of the rod.

 

Your Wien network is wrong. R6 and C9 should be in series.

You are right, I made a mistake when drawing the circuit.
In practice, I connected R6 and C9 in series.

 

In the original post, you asked about alternatives.

I'd be looking at a LVDT

https://www.omega.co.uk/technical-learning/linear-variable-displacement-transducers.html#

https://www.te.com/usa-en/industries/sensor-solutions/insights/lvdt-tutorial.html

https://en.wikipedia.org/wiki/Linear_variable_differential_transformer

Its a transformer, with a sliding core inside,
you have two, counter wound primary and a single secondary, with a sliding core in the middle.
you put a sine wave in the primary, and synchronous receive at the secondary,

As its an AC signal, and synchronous rectified, the amplification is AC coupled, so no DC drift.

used a lot in situations where rugged, sensitive position measurement is needed.

At first, I tried to test a LVDT but I have not a good result, in the future, I will work on LVDT.
if you agree, let's work on this work.
My issue with lvdt was a limited range of sensitivity and in some positions, the output did not change by changing the rod.
Definitely, LVDT is a good way for displacement measuring in rugged environments.

 

LVDT has a dead spot in the centre,
where the two signals cancel out,
this can lead to dithering at centre,
this is either "cured" by software, or not stopping in the centre but to one side,
lots of techniques around on web for ways to improve things,
synchronous detectors are a big step,
detecting both the "left" and "right" coil separately and doing the maths in S/w allows the variable amplitude to be minimised..

If you go for the Ardino route,
then there are off the shelf libraries for it, just have a web search

e.g. https://github.com/sebfio/lvdt


I'd suggest a fairly fast Arduino , with good ADC / DAC's

e.g. https://www.pjrc.com/teensy/


good luck,

start a new thread with any specific questions.