Rollino sensor

Rollino

An attempt to make a cheap, solid and low profile DIY finger trackball.

The ball floats just 1 mm above the table on three ceramic support balls. Two PMW3610 sensors look up at the ball from below its equator, placed so their lines of sight are orthogonal (X, Y and twist are all observable). A Seeed XIAO nRF52840 running ZMK and a 602020 LiPo lie flat in a squashed dome in front of the ball. The XIAO is soldered upside down onto a base PCB that sits in a groove in the underside and reaches round under both sensors, each wired up with a short FPC ribbon; a cover over the whole underside hides it, held by the same screws.

• case/rollino.scad - parametric OpenSCAD model (ball size, sensor/MCU placement, battery, ...). Set part to assembly, exploded, hardware, section, or body / base_cover for printable parts.
• case/renders/ - preview renders: case/render_previews.sh (OpenSCAD) and case/render_exploded.sh (Blender, with KiCad's 3D models of the boards).
• pcb/ - KiCad projects for the sensor boards and the base board: schematics and routed boards, generated from the case model (see pcb/readme.md).

• parts.md - everything to order for one Rollino.
• datasheets/ - component datasheets. The PMW3610 datasheet is available here.

Firmware

ZMK, built by GitHub Actions on every push (download firmware.zip from the run, then double-tap the XIAO's reset and copy the .uf2 onto it). It's a standalone Bluetooth/USB mouse.

• zmk/boards/shields/rollino/ - the shield: pins, the two PMW3610 sensors (Zephyr's driver), three optional buttons.
• zmk/src/ball_fusion.c - combines the two sensors: each one only sees the ball's surface sliding past in its own tilted plane; together they give the ball's whole rotation. Rolling moves the pointer (as if the finger on top dragged it), twisting about the vertical axis scrolls.
• config/rollino.keymap - the buttons: left, right, middle click.

Calibrating the sensors' orientation. The fusion needs to know which way each sensor's X axis points (rotation in the shield's ball_fusion node, plus the sensor's invert-x / invert-y), which depends on how the chip sits on the sensor PCB. Roll the ball right: the pointer should go right; towards you: down; twist it: scroll, with no drift. If one direction is off, try one sensor at a time (cover the other's lens) and fix its rotation / inversion until rolling right moves the pointer mostly right.