UniPOV Kickoff#

A while back, I discovered MonkeyLectric, a persistence-of-vision (POV) display mounted to the spokes of a bicycle wheel and stabilized using a magnet on the fork.

Generic POV display

There are some interested challenges obviously to making this work on a bike wheel, such as resolution, mapping and timing requirements when converting a Cartesian image to polar coordinates, synchronization between multiple boards, etc.

A unicycle display presents another unique challenge: stabilization. Bikes remain (relatively) flat in motion, unless traveling over a speed bump or pothole. That means the fork is a suitable alignment feature for the display. Unicycles frames, on the other hand, are constantly pitching forward and backward - an image aligned to the frame would be a jarring experience for any viewer.

How can we get around this? We need another way to accurately track the wheel angle relative to the Earth. A few ideas come to mind:

  1. Measure deflection of the tube as it is sandwiched between the rim and ground,
  2. Measure eccentricity of the rim with a strain gauge,
  3. Measure inertial reference frame of the wheel as it rotates (accelerometer/gyro),
  4. Measure wheel position relative to the frame, and measure frame inertial reference relative to the Earth,
  5. Measure distance to the ground.

The first two options are interesting and somewhat practical, but would not provide a consistent estimate of angle without many/continuous sensors along the rim. Option 5 is interesting as well, and could be done with a strip of range sensors along the rim, but it aligns the image to the road instead of the Earth and is not immune to discontinuities in the surface. Options 3 and 4 are likely the most feasible, and simplistic from a hardware perspective, but require a fair bit of modeling to get right.

Ultimately, I decided option 3 would be best, as it greatly simplifies the hardware and installation.

Aside from the technical challenge of image alignment, there are a number of other facets to tackle:

  • Power: desire 4 hours on a charge
  • LED brightness
  • Resolution: number of pixels, refresh rate
  • User interface: SD card? Wifi/BLE + app?
  • Preprogrammed patterns? Images? Videos?
  • Safety: with appropriately angled lights, headlights and tailights could be achieved
  • Water resistant (I do live in Portland, after all)

For this to come to fruition, here are the phases I envision:

  • Phase 1: Test board to verify MCU, LED driver, power stage, IMU driver, and memory interface.
  • Phase 2: Desktop mockup to verify application logic, spoke synchronization, static stabilization, and wireless interface.
  • Phase 3: Full-scale functional prototype for 36" Coker.

Stay tuned for Phase 1 updates..


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