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README.md
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README.md
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@ -12,13 +12,13 @@ Foot presses create mechanical waves. Piezoelectrics can turn these waves into v
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Method 1b: Surface acoustic waves
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---------
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Use a piezoelectric device to insert an acoustic wave onto the pad, and an array of piezos to detect its phase/amplitude at locations along the border. Pressure differences from standing feet will affect the phase/amplitude, and you can create a pressure map of the surface.
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See: https://en.wikipedia.org/wiki/Surface_acoustic_wave_sensor
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See: <https://en.wikipedia.org/wiki/Surface_acoustic_wave_sensor>
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Method 2a: Capacitive displacement sensing
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---------
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Two parallel conductive plates form a capacitor. If pressure on the top plate deforms it, that changes the capacitance. Measure this and triangulate it to find foot locations.
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This is sort of like turning the pad into a giant microphone, but with triangulation.
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See: https://en.wikipedia.org/wiki/Capacitive_displacement_sensor
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See: <https://en.wikipedia.org/wiki/Capacitive_displacement_sensor>
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WORK SO FAR
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@ -26,9 +26,9 @@ WORK SO FAR
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Method 1a is demonstrated. It works on rigid surfaces. See doc/data/itm-on-table-drop-30lb-halfway-through.wav: this was achieved using 181.5 clock single-ended ADC sampling with delay(1) between each sample with no external amplification. The piezo was held in place using an empty water glass.
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On pad: doc/data/itm-on-pad-heavy-hit-halfway-through.wav demonstrates the same setup on a Polish pad on carpet but with a full glass of water.
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On pad: <doc/data/itm-on-pad-heavy-hit-halfway-through.wav> demonstrates the same setup on a Polish pad on carpet but with a full glass of water.
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doc/data/itm-on-pad-tape-big-hit-4clock-1024buffer.{wav,png} shows what we get with a BIG stomp if the sensor is taped to a pad
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<doc/data/itm-on-pad-tape-big-hit-4clock-1024buffer.wav> and <doc/data/itm-on-pad-tape-big-hit-4clock-1024buffer.png> shows what we get with a BIG stomp if the sensor is taped to a pad
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it's really hard to get.
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Tried using flat piezos and the L-shaped ones, both behave similarly. Really difficult to get reliable waves.
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@ -40,14 +40,14 @@ Try:
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- Borrow UW's oscilloscope
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- Buy a cheapo scope?
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doc/data/itm-sandwich-51medianfilt.png shows results after taping a piezo to a flat surface, loosely setting an acryllic plate on top of it, and then tapping the plate moderately.
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<doc/data/itm-sandwich-51medianfilt.png> shows results after taping a piezo to a flat surface, loosely setting an acryllic plate on top of it, and then tapping the plate moderately.
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There is median filtering on the device side, plus additional filtering on the PC side.
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Median filter seems to be key. Even without the plate, the device can _detect_ stomps from 4-5 inches away, though not quite as cleanly.
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Placing the piezo such that half of it is in air seems to help detection. I suspect if the whole platform is oscillating, I want the other end of the piezo somewhere that's more fixed, in order to detect the oscillation of the platform itself. Alternatively, I believe a longer piezo element attached all to the surface could help. A smaller element should still receive everything, but the high frequencies are attenuated. Perhaps then, a HPF could be installed to mitigate this attenuation?
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Could also try replacing the device-side median filter with a more tuned filter (HPF)
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data/itm-pad-drop-weight-3in-away-covered-tape.png: shows clear signal dropping a 30lb weight near the sensor, even when the sensor is fully attached to the pad. Whole signal is about 5 ms.
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<doc/data/itm-pad-drop-weight-3in-away-covered-tape.png>: shows clear signal dropping a 30lb weight near the sensor, even when the sensor is fully attached to the pad. Whole signal is about 5 ms.
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51-median filt device side
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5000-sample DC removal PC side
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timescale is accurate
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@ -59,7 +59,7 @@ Resources (acoustic touch screen):
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Because of the reflections from the edges of the surface, it's able to work with just one sensor.
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- https://www.elotouch.com/touchscreen-components/catalog-product-view-id-82.html
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off-the-shelf SAW touchscreens (for sale?)
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- http://a-touch.com.tw/product-saw.htm
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- <http://a-touch.com.tw/product-saw.htm>
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shows how SAW touch screen works (it's like I thought: transmit -> interfere (from finger) -> receive
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Resources (using Piezos)
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