Anelok: save the slider !

Werner Almesberger werner at almesberger.net
Wed Sep 2 16:03:21 UTC 2015


Dave Ball wrote:
> [repost - I sent the original off-list by mistake!]

Thanks ! :)

> As I understand it, the current Anelok slider is actually a linear
> slider, not a series of buttons.

Yes, it's a simple linear slider. Having multiple areas should
make it easier to associate a contact with one of the areas
(i.e., the one that produces the highest reading).

But I suspect that it could become difficult to translate this
into the linear position needed for sliding.

> Are there some alternative layouts
> of the PCB track that would make the slider less prone to
> atomspheric variations?

There are lots of layouts ;-) For basic principles, I used
Freescale's AN3863:
http://cache.freescale.com/files/sensors/doc/app_note/AN3863.pdf

There, they make sliders using multiple areas. The problem is
that this uses a lot of channels = pins. Freescale themselves
picked a more economical approach for their FRDM board:
http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=FRDM-KL26Z

This has a dual Z shape, similar to Anelok's triple Z.

One possible variation would be to remove the ground traces
between the sensor plates, and let the MCU drive one plate to
ground while measuring the other. This would leave room to
increase the plate surface and may yield a stronger signal.
Noise immunity may suffer, though, given that the GPIO acts like
a ~100 Ohm series resistor when connecting the plate to ground.

> http://www.embedded.com/design/mcus-processors-and-socs/4415133/Adaptive-Kalman-filtering-for-better-buttons-slider-capacitive-sensing

Since you mention Kalman filtering, I would indeed hope that
better signal processing could make a huge difference here.
Alas, this isn't really my field, and my contraption held
together (or not) by spit and duct tape shows it.

- Werner



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