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    Solution: Capacitive Encoder

    Here, I'll try PCB-fabbing a reference circle with triangular teeth, and two pads to read through.

    As the ring moves, capacitance should vary between the pads - I can read this as a value on an ADC pin and track that to figure position.

    Commercial Sensors

    I'm dubious about Neils TX / RX settle time - 100us (yikes!). I'm looking into using some other IC's - drivers that are made for this - to do the conversion.

    TI has two - the FDC1004 and the FDC2112 - both are I2C devices. The FDC2112, rather than doing rise / delay, watches deviation in resonant frequency at the sensor location. Oddly, none that I can find use Neils TX / RX Scheme for environmental cancelling, all use TX / GND.

    In short, these solutions seem cool, but complicated, and I need to take some more time with the bare-bones of this problem to determine if I can make something work. Like,

    • does rise-time drop with smaller pads? less charge to distribute...
    • how many up/down sets do I need for a reasonable measurement?
    • adc resolution -> bar width / spacing
    • grounding, stack setup, tx pads, rx pad(s)?

    Understanding the Phenomenon

    It's good to try to get a grounding on what's really going on. To get the basics in order, I go here.

    Capacitance is C = \frac{k * e * A}{d} where 'k' is the relative permittivity of the material between the plates, 'e' is a constant making 'k * e = 1' in air, 'A' is the area of the plates and 'd' is the separation of those plates.

    Some quick thoughts

    • do static tx / rx pads. reference is metallic thing that moves between them, changing the permeattivity of space between them
    • bc capacitance is C = \frac{k * e * A}{d} where E is constant, A is area, and K is relative permeattivity (k ~= 1 for air)
    • 'metallic thing' can be double-sided pcb w/ sawtooth or sine pattern to read
    • can do spreadsheet maths for tx / rx areas, see what total capacitance range will be (1pf -> 15pf?)
    • can figure apparent rise-time calculator, probably?

    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/pplate.html

    http://www.ti.com/lit/an/snoa927/snoa927.pdf

    To quickly lay this out, I'll take the linear case... and a triangular wave to read.

    • notebook pic

    • or whiteboard, do pitch, do 1/4 -> 1/2 -> 3/4

    So I can build a quick spreadsheet for this. Using basic areas and http://hyperphysics.phy-astr.gsu.edu/hbase/electric/pplate.html

    C = ke_0A/d

    What changes as the imaged plate moves is the permittivity of the space between the tx and rx pads. Copper has a technically infinite permittivity, so in some sense what we're doing is decreasing the space between the pads, when the copper passes through. shrugman