McLVDT: a straw-based sensor

We saw [Kevin’s] home-built Linear Variable Differential Transformer in a YouTube video last week and wanted to know more. We’re in luck, he agreed to share all the details as well as a bunch of information on these sensors. An LVDT is used to measure distance along a straight path. Unlike a linear optical encoder, this method uses measurements of inductance between two electrical coils to judge the distance.

[Kevin] used some magnet wire wrapped around two straws of different diameter to fabricate his sensor. A signal generator is connected to the primary coil and the resulting signal induced in the secondary coil is measured to reveal the change in physical position. Check out the video after the break to see the results.

It’s not hard to get your hands on a McDonald’s straw (hence the name ‘Mc’LVDT), a smaller inner straw, and a few feet of magnet wire. This will be a fun one to try when those dark winter days start to get to you.

13 thoughts on “McLVDT: a straw-based sensor

  1. The problem with this approach is that the circuit is very sensitive to metallic masses or magnetic fields nearby and it needs a fair amount of power to work reliably on linear distances higher than a few centimeters. If you search the net you will find that the same principle is used to build simple metal detectors. Here’s a much simpler approach I saw implemented some decades ago in a volume pedal for electronic instruments: a soft plastic strip is printed in a way it goes linearly from totally transparent to totally black. The strip is then put in between a bright led and a phototransistor or photodiode (the original used a lamp+photoresistor couple) and enclosed in a small box to screen it from external light sources. By moving the strip one effectively controls the amount of light the sensor receives, hence the output voltage. It’s very cheap and reliable, albeit not as a digitally encoded linear sensor, still it’s an excellent way to build simple linear sensors.

  2. So I dug out my bag of LDVT’s I rescued from being thrown out at work and snipped off the mil spec connector. I have 5 wires. Checked the resistances and it looks like my 5th wire is a center tap in between blue and yellow:

    blue/green = 168 Ohm
    yellow/green = 148 Ohm
    blue/yellow = 315 Ohm
    red/black = 162 Ohm

    I am a bit perplexed how you could utilize a center tap. I think this is all the motivation I needed to finally break down and buy a signal generator.

    Great project; it would also make a great science fair project!

  3. @pookeye

    Yup, that was helpful. Mainly the (A-B)/(A+B) bit. That is what I was assuming but, was having a hard time visualizing it in my head at the time.

    Now, what to do with ten 1″ travel LVDT’s…

    An analog joystick would only need three…

    Or just an AVR based LVDT tester.

    Ok, off the the drawing board…

  4. If you put “Mc” in front of anything McDonalds will sue the pants off you. They learned that behavior from Apple who sues the pants off you if you put “i” in front of anything. If you’re a glutton for punishment call the thing “iMcLVDT” he he.

  5. We have these all over the ship I work on, using them for differential pressure and pressure detectors. There is a bellows connected to a high pressure side and outside is a low pressure. The magnetically permeable core then moves up and down based on the pressure, giving us an indication of steam pressure and flow. I can tell you how much of pain it is to calibrate them, especially when you remove the tool, complete changing the coupling of the coils.

  6. It does not “measure the inductance” (nor does OP claim to).

    This is simply a transformer with variable coupling (mutual inductance) between the single primary and the two (balanced) secondary windings.

    The signals at the secondary ends are always in-phase and 180 deg out of phase with the energisation, but when they differ in *amplitude* the vector *resultant* swings in phase.

    Generally speaking such *linear* optical transducers aren’t as accurate or linear as LDVT’s.

    Go4it. The single winding is energised from an oscillator, the secodary centre tap can be grounded, and the two end signals taken to a difference amplifier/rectifier. You can just use opposing rectifiers off each side to resolve position as a voltage (a rather more robust homebrew LDVT is here;)

    A better method is to use a *Synchronous Detector* aka gated rectifier (the re-injection “carrier” is already available from the drive oscillator).

    See also NE5521

  7. I’ve got a few of these from a semiconductor fab mask aligner that I ripped apart a couple of years ago. Wasn’t sure what they were, but I saved them anyway. They’re set up with a plunger that moves through (I’m guessing) a coil. I’ll have to dig them out and take a look again..

  8. still useful, perhaps it could be repurposed for the RepRap project.

    i used a surplus optical strip from a demised desktop 6*4″ photo printer (the led was visible rather than infrared) and that worked somewhat.

  9. I’ve just designed a system at work that uses this kinda thing, except the LVDT itself it about 10 feet long and superconducting :p

    If you can afford the price tag, then it’s well worth getting a decent signal conditioner, something like the AD698. Also, if your really keen on knowing the exact “zero” point, then you need decent phase detection, as the amplitude will just become noise by itself. Most LVDT’s have a fairly rapid phase reversal, but they can be designed to have a much slower reversal, which allows for even more accurate positioning.

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