Hello all,

Love this site and forum, with I could participate more but I am a newb and by all means, a hobbyist....not an EE.

Anyway, I have a theory question. I was thinking about working on a project to test this theory. Let's say I have a permanent magnet that travels along an axis in a linear motion. For example on a screw drive. Typically on a screw drive, you would put limit switches at the ends to detect end of travel. This would typically be done using microswitches or if I have a magnet traveling along, you could use a hall or reed switch. However, let's say you wanted to know the position of the magnet along the length of travel instead of just the ends. This is basically like a linear encoder.

So, the question is, could I wind up wire around a dowel rod for example to form an inductor that runs the length of the screw drive. Then, have the magnet travel parallel to and in close proximity to the inductor/coil. The theory is that the magnet would disturb the magnet field created by the inductor and cause a change of some sort. This is where I am stuck and need help. Would I generate an AC signal (maybe square wave or something using a 555 timer) and then measure current change, voltage drop or ??? Could I simply measure inductance value change? I would like to circuit to be as small and simple as possible.

Any thoughts to help me get started?

Thanks!

Mike

 

I don't see how that could give you the position of the magnet?

 

You can change the inductance due to the magnetic field and the nonlinear properties of the magnetic core. There are special ferrite materials that are used for adjustment of powerful radio transmitters. But we must take into account the strong unevenness of the magnetic field and strong nonlinearity of the core. Hardly able to create linear encoder. An alternative idea is to use two coils. Between them to make a ferromagnetic object. On one of the coils supplied alternating signal. On the other coil to remove the signal and detect. The level of the signal can be determined amount of movement.

 

You could try making a home-brew linear variable differential transformer.

 

^^^ Wot Alec_T said.

I once tried this with a capacitor, I had a set of static plates and a set of moving plates that moved in and out of the stationary plates as the mechanical part moved. It worked OK but an LVDT is better because it is linear and not affected by stray capacitance.

 

What your describing is also known as a 'magnetic scale' used in digital readouts for machines. But instead of a moving magnet they use a form of magnetic tape, like in a cassette audio tape, with bands of magnetic stripes recorded on it. And the read "head" is the moving part.