Electrical – How to human finger pressure be measured

I've been wanting to find out info about this subject my self and also don't have much in the way of electronics background. I believe that Raiden wants to create his own DIY fine point stylus like the Nota stylus (http://hex3.co/products/nota) which is described as electronically activated. There are also others which are coming to market in the near future.

As far as I understand they work by increasing the capacitance of the stylus point. So I've looked into what can be done to increase capacitance of the stylus.

Formulas

C = Q/V OR Capacitance = Charge / Voltage

C= εS/D OR Capacitance = (Dielectric Constance X Surface Area) / Distance between plates

If i understand correctly when you touch the screen you make a sort of capacitor due to the layer of insulating film /glass and the change in voltage at that point is the detected. (May be completely wrong about this). So in the 2nd equation if you reduce the surface area you will reduce the capacitance. So in order to increase the capacitance we need to change the dielectric constant of the material being used.

Here is an example of a fine point stylus that uses foam and water https://www.youtube.com/watch?v=alrweIVt2RQ . The Constant of water is ~80 compared to rubber of ~3 and i believe that is why this example of a fine point stylus works. Unfortunately it is inconsistent and water doesn't play nice with touch screens. I would theorise that using a material with a high constant would help reduce the size of a stylus point. For example Graphite has a constant of ~ 36.

My other thoughts are to try using an electronic circuit to add voltage to the stylus point. But thats well out of my range of understanding.

Two options:

1. Detect ground-floor and perform a reset on each arrival.

2. Install a second unit on a specific floor and update the reference pressure in the elevator micro - perhaps by wireless connection.

And ...

3. Try to compensate in software. If you can detect the difference between the rapid changes of pressure due to elevator motion and those due to atmospheric variation, you could adjust accordingly when the elevator stops (but see caution notes).

4. If there is reasonably frequent travel to upper and lower limits, you could recalibrate then. i.e.,

if (p > pmax) { // p is pressure reading. pmax = p; // Must be at top floor. pmin = p - bottom_to_top; // bottom_to_top is the pressure span } if (p < pmin) { pmin = p; // Must be at bottom floor. pmax = p + bottom_to_top; } You would have to manage power-up if the micro doesn't have non-volatile memory.

Caution

If this is an office building with air conditioning, you may have trouble with varying pressures on different floors. This may be high enough to 'swamp' the readings between floors. One way of avoiding this may be to read only when the elevator doors are closed and monitor lift-shaft pressure but this may vary also due to compression of the air during descent and vice versa.