Anelok: fatigue and normalization
Ron K. Jeffries
rjeffries at gmail.com
Tue Nov 11 12:58:44 UTC 2014
Werner--as always, you share VERY useful info. It's fascinating.
I just wish this was available to a wider audience via a URL I could share.
The open and free software called "known" works well as a personal blog. I
have not (yet) installed "known" but am using a public instance e.g. mine
The "known" user interface is so-so but not terrible. It is fine for blog
entries and nice as a Tumbler-ish way to post photos (that's my initial
I apologize if there is already a way to read qi-hardware emails via a
direct URL. I am lazy.
Finally, I can NOT resist a small pun. Naming software "known" is
confusing. It reminds me of Donald Rumsfeld's memorable phrase.
Ron K Jeffries
On Nov 10, 2014 10:11 PM, "Werner Almesberger" <werner at almesberger.net>
> I'm having a bit more fun with the capacitive sensor. As mentioned,
> it didn't work very well. I first ran a series of tests to examine
> whether changing the configuration of the touch sensing module
> would improve the sensor's performance. There are about half a
> dozen parameters to play with. From what I saw, it seems that the
> parameters are about as good as they get.
> However, I noticed that one of the two channels occasionally just
> went dead for a while. Closer examination revealed that bending the
> cables between the two boards had something to do with that
> failure, but when I touched the cable at either end with tweezers,
> the sensor responded normally (any direct contact just makes it go
> off the scale).
> I then suspected the battery holder. As one can see in the middle
> image on
> one of the traces is very close to a pad. So I moved the holder a
> bit, but to no avail. I then removed it completely and began to
> probe the trace. Turns out that there was a tiny invisible fracture
> in the trace that probably responded to bending forces from the
> cable. A bit of solder bridged that gap.
> Then DFU (USB) started to fail occasionally, with an increasing
> frequency. Also here, twisting the connector one way makes things
> work while twisting it the other way makes them fail. I haven't
> looked into that any further yet, but it'll be interesting to find
> out what's happening there.
> A boring finding would be another broken trace or a bad solder
> joint. If the receptacle is damaged, that would indicate that
> either the build quality of that AB connector isn't so good, or
> that AB in general is not the best idea (in AB, the plug will not
> be stopped by the shield when reversed and the only thing stopping
> it seems to be the plastic tongue holding the contacts. Not the
> most confidence-inspiring design, but who am I to question the
> wisdom of the mighty masters of USB ...)
> But my principal suspect is the cheap plug. Maybe it's already worn
> out after a few hundred cycles. We'll see.
> Next, the daughter board failed completely. That one was easy to
> find: one of the wires resented all the bending and snapped. Maybe
> using a stranded wire (30 AWG, 7/38) wasn't such a great choice
> after all. In any case, this kind of connection will want at least
> an FPC in the end.
> With all the little nuisances fixed, I could get back to wrestling
> with the touch sensor. To get a better idea of what it really
> measured, I marked distances on the sensor, like this ...
> (The cable coming out from the back replaces the battery holder I
> had removed for better access to troubled traces.)
> ... and put my index finger at various positions. This is what the
> results looked like:
> The red line is the value of the A channel (minus the long-term
> average, i.e., the level when no finger is present), the green line
> shows the B channel. They both look nice and linear on the
> respective lower half, but each has that ugly peak about 7 mm from
> the top end. This corresponds to the finger overshooting the sensor
> area, i.e., the area of finger over the sensor plates decreases.
> But hey, this is roughly what the sum of A and B (blue) shows. So
> let's normalize the channels with the sum. This yields the magenta
> and cyan lines. They look almost too good to be true. If we add
> them together, we get an even stronger signal (black).
> When I added normalization to the algorithm, the result was quite a
> bit more useful than before. The change is small enough (I also had
> to cheat a bit, scale up and add a small offset, or it wouldn't
> feel right):
> The result was still quite noisy. I then put an averaging filter
> operating on positions on top of everything, and this now yields
> pretty acceptable performance:
> The filter is a bit awkward (I could probably have used an EWMA
> which doesn't need to record multiple values), but it's easily
> extended to chopping off samples from the beginning and the end of
> the press interval, in case we get false positions when the finger
> is still at a distance from the sensor.
> The touch sensor still isn't perfect but it's good enough for going
> back to teaching the UI a few more tricks.
> Curious what they are ? Try the simulator !
> - Werner
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