Subject:
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Re: Multiplexing 4 touch sensors on an RCX input port
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Newsgroups:
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lugnet.robotics.rcx
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Date:
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Tue, 9 Sep 2003 22:11:39 GMT
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Viewed:
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3593 times
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In lugnet.robotics.rcx, Steve Hassenplug wrote:
> In lugnet.robotics.rcx, Mark Bellis wrote:
> > You can use four touch sensors per input port on the RCX.
>
> While in theory, this sounds very cool, in pratice, I doubt it will work quite
> that well.
>
> The biggest problem will be that the standard touch sensors may not be 100% on
> when you press it. This means the sensor itself can provide some resistance,
> depending on how hard you press it.
>
> You can test this by hooking up a touch sensor (configured to display raw
> values) and softly press it. You'll notice, the harder you press it (to a
> point) the more the number changes. (I forget if they go up or down, using the
> standard software/firmware). Then, as you release the button (VERY slowly) you
> see the numbers change, again.
>
> So, the sensors won't give you a solid on/off reading. I believe the reading
> can easily vary 5 or 10 percent.
>
> That means, if you have 4 touch sensors, there are 16 possible combinations, so
> each can vary by at most 6.25% (100%/16readings=6.25%) or there is 6% between 1
> and 2 or 2 and 3
>
> So, given that each sensor could vary as little as 5%, then at BEST you have
> 1.25% difference. At worst, the reading differ by -20% (readings overlap 20%)
>
> It doesn't exactly come out like that, but let's just say it won't work very
> well.
Good analysis.
Series can work quite well as long as you know that the touch sensors are
touched mutually exclusively, but you are right parallel doesn't work well. I
remember trying to come up with good resistance values to do parallel, and
didn't have much luck.
You might be able to do it with two touch sensors, right? Two sensors can have
four possible combinations, so you get 25% per combination. This is higher than
the 20% worst case, so it might work.
With three touch sensors, you get 8 combinations, at 12.5% per combination. I
don't think that would work worst case.
Kevin
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