How to build a piston-push controller. I need some ideas

#1

Hi,

I want to build a a controller, that I can use like pushing in a button with the difference, that this "button" is more like a rod that i push into my controller. I would push against a spring, so the rod comes back on it's own. The best analogy would be a trumpet-piston. The trumpet player controls quite a lot with only three of these pistons. The exact amount of push-in is very important of course.
There are some keyboards with aftertouch, where one can push the key harder after it is already played. But afaik ine cannot push them in very much. I want this to be usable very accurately. The piston-rods should stick out about ~5cm / ~2 inches out of my so I can push them in all the way.

How can I achieve this?
I had the idea of using a fader potentiometer. But all that I found weren't sliding easily. I'd like the piston to move almost freely - only against the spring.
Another idea I had was to use a hall-sensor and move a magnet towards it. But I'm not sure if using several of these pistons near each other would be a problem (maybe I could isolate the magnetig fields sufficiently?).
The next idea would be to use an optical resistor and move an led nearer towards it (might be easier to isolate than the magnetic field).
My most complicated idea was to build a cylindrical capacitor and move the rod into a rolled alumnium foil or something thereby changing the capacitance.
The easiest way would of course be the potentiometer. I only there was a way of making it move easier. Or does anyone know of very easy-going faders with enough travel-distance?

My ultimate goal is of course to be able to controll 5 parameters accurately with one hand.

#2

Google "Linear position potentiometer" and find one the size that will work for you. You will have to add the spring setup yourself though because these are typically designed without them.

Another tack would be to make something using LEDs and photo sensors. You might actually be able to use real trumpet valves if the mechanics can be adapted. I have never taken one apart, but I assume that when the valve is pressed, a hole in it allows air to pass through it. Put a LED on one side shining through where the air pipe is, and a light sensor on the other side of the valve and there you go.

#3

Maybe look into ribon potentiometers, the rod would rest againts ribon, and as the rod moves, it initiates a different value from the ribon pot.

#4

Plus one on solving this optically. Check out the vaxmidi remake solution to continuous key positioning: A triangular window that lats through more light.