Max + Phidgets interface questions: sensitivity, hysteresis and delay ?

    Jul 30 2011 | 9:47 pm
    Folks, I could use your help !. I'm trying to use a bunch of 1129 touch sensors with Max to build a sort of musical instrument.. I have no trouble creating a simple demo where the 1129's can trigger simple midi notes in Max.. The issues I'm having are with sensitivity, hysteresis and delay
    Sensitivity- I've attached the 1129 touch point to a piece of fine copper mesh behind a thin lexan sheet . I'm easily able to sense my hand on the other side of the lexan.. what's bugging me a bit is that the plate is quite sensitive to surroundings.. eg. metal beneath a table, etc. I was trying to figure out a how to calculate the right size for my sensing pad.. and/or.. a way that I could discriminate between static coupling and the presence of the hand. I believe the sensor is ratiometric.. but the phidget interface in max seems to be showing only two values.. 0 or 999, Is there a way to change the scaling on these things so I can some interpretation about how near and/or big something is ?
    hysteresis- I'm also noticing that there is some sort of built in hysteresis in the 1129. I.e the Phidgetinterface registers presence of a hand at (say) 2cm distance.. but then does not register that the hand is not present until it's raised to (say) 5 cm. Is this hysteresis somehow in the phidget interface.. or is it set in the 1129. If it's the latter, is it possible to control ? It's creating some difficulty in 'playing' the instrument
    delay- similar question.. I've created a simple feedback mechanism using leds to indicate when the 1129 sensor is active. There appears to be a surprisingly long (many mS) long delay between the placing of your hand and the sound of the midi note. I've tried changing the sample rate (setSampleRate) message on the phidgetinterface.. I've tried -1 (trigger on change) and 1 (1 ms poling i think. or is it 100mS ? ) and 0.. (no clue).. -1 seems to have the least delay... Any ideas on how to improve this ?
    Is it possible that the whole 1129-> phidget 8/8/8 controller ->Max interface is going to be too slow to use to trigger a musical instrument ? Have others tried to do similar things and either succeeded or failed ?
    Any help/comment on any of the above would be most appreciated ! -jc
    ps. On a similar bent.. does anyone have a contact touch sens design that would work with phidgets ? Perhaps one that works with coupled noise (ie. you need to physically touch the antenna).. that would be a nice compliment to the existing sensors...
    pss. . one suggestion.. there should be some sort of of better solder connection for the sensing tab on the 1129..
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    • Jul 30 2011 | 10:26 pm
      Looking at the datasheet for that 1129 sensor it states that touch value goes from 0>1000 only. It is a switch. Although capacitive sensors can be progressive it would seem this one is not. Plus the whole calibration/recalibration would seem to lead to potential latency as you have identified
      Maybe you would be better off with FSRs
    • Jul 31 2011 | 1:16 am
      initially I intended to resist responding to your post, as I'm not familiar with the Phidget platform - I favour DIY sensors and Arduino; also, it would seem (from reading a bit about the platform and sensor in question) that you're pretty much tied to that binary-state 0 or 999 response.
      I also don't like referring to one platform/environment, where someone has invested time/money/effort in using a different one.
      Then I read your last question about similar designs. I recently abandoned a resistive touchscreen DMI interface, due to unreliable and jittery touch data and I'm currently implementing a panel consisting of a DIY FSRs matrix. With limited electronics/programming chops, I've had some success (with very early pre-alpha prototypes). You can see the results and journal here:
      Any support from the phidget community?
      Best regards Brendan