Electric Trombone: Choosing a suitable slide sensor

I think your best bet is to mimic the design of electronic digital calipers  - here's a link to some pictures of the insides of one http://www.adafruit.com/blog/2010/11/08/digital-calipers-tear-down/

You might be able to modify a cheapo chinese one but you'll pay a fair bit for a long one.

There are some industrial sensors based on a tube of steel balls and using inductive sensing but these are not cheap.

MK

Ok, don't over think it.. simple, pure your picture is of the slide, but that of a "real" side, not an electric slide.

simplest an cheapest route is to use capacitance . Just like we measure fuel in a plane. But we see the wet area or lack of it, between a rod inside a pipe. what you need is a pipe and a rod and you move the rod in and out that will give you your change in capacitance. And all you need is the delta of the length of the rod. Then by picking the right ADC (ie, how many bits wide, and how fast) your about done. 

enjoy ~

I'd be concerned that the human presence might affect capacitance.

Another approach would be to use an inductive loop, like traffic signals.  Have an inner plastic tube with a loop of wire running the length of the tube, wrapped around the outside.  Then have an outer iron or other ferromagnetic tube that slides on the inner tube.  Inductance is approximately proportional to how much loop is covered with iron.

I'd be concerned that the human presence might affect capacitance.

Another approach would be to use an inductive loop, like traffic signals.  Have an inner plastic tube with a loop of wire running the length of the tube, wrapped around the outside.  Then have an outer iron or other ferromagnetic tube that slides on the inner tube.  Inductance is approximately proportional to how much loop is covered with iron.

The problem with simple capacitave sensors like Christina suggests or inductive like John suggests is that the total sensor length is at least double the working range - this may be OK because the same is true of a real trombone.

The capacitave caliper technique I suggested is widely used but perhaps a bit complicated to build.

You will find similar capacitave sensors documented in  various microprocessor manufacturers app notes about touch sensing but I'm not sure that the touch sense technolgy can be stetched to such a long device.

Annother approach you could consider is an unrolled shaft encoder, machine or paint stripes along the tube and use an optical sensor (needs two sensors 1/2 pitch apart) to detect the movement of the slider over the stripes. The resolution is only one half of the stripe pitch and since it's a relative sensor you need to count stripes passed up and down to keep track of absolute position. The slider is active.

My favorite idea so far is more complex but does allow absolute detection, a passive slider and a reasonable sensor length:

Wind several overlapping coils along the length of the tube, single layer of turns for each coil but they probably need to overlap by 50% so you end up with two layers. The slider needs to be magnetically permeable (ferrite or iron dust) and probably 1 coil length long. It will work much better inside the tube but might just about work outside. You will need to energise the coils in turn to measure the inductance and will be able to estimate the position of the slider quite accurately.

With any of the techniques suggested you will need quite  abit of signal processing and compensation to get a useable digital signal.

MK