Tuna Gül

Systems

Hall-sensor absolute encoder Archive

An attempt at a cheap absolute encoder for our FRC robot. A ring of Hall sensors around a magnet, and the circle geometry to find the magnet, worked out by hand in C.

Role
Algorithm and C code
Period
Jul–Oct 2022
Team
With a teammate · Team NF
Stack
C · Python · Arduino

Code ↗

Illustration of the method with simulated readings: six Hall sensors on a ring, a distance circle around each, the pairwise intersections nearest the magnet's path, and their average marking the magnet and the shaft angle.

Why

In 2022 a teammate and I tried to build a cheap absolute encoder for our FRC robot. A magnet turns with the shaft, and a ring of analog Hall sensors sits around it. If each reading can be turned into a distance, every sensor gives a circle the magnet must lie on, and where the circles meet is the magnet, so the shaft angle.

What I did

I wrote the algorithm and the C code by hand: first a prototype in Python, then plain C for an Arduino, with my own small geometry library for points, lines and circles.

How it works

  • Two circles: the intersection of two circles is reduced to their radical line, then a line–circle quadratic gives the two points.
  • The shortcut: the code first intersects the first two circles. If one of the two points lies on every other circle too, that is the answer and it returns right away. Three circles whose centres are not on one line meet in at most one point, so the match cannot be a coincidence.
  • The usual case: real readings never meet perfectly, so every pair of circles is intersected, the point closer to the magnet's path is kept from each pair, and the kept points are averaged. The search runs in buffers set up in advance, with no malloc inside it.
  • Calibration is stored in EEPROM behind a versioned header.

Where it stopped

It never got past simulated readings. Turning a raw Hall reading into a distance was never written, so every test ran on distances computed from a known magnet position. It is still one of my favourite bits of math.

Related

Building cross-section with four elevator shafts, the cars at their floors and passengers queued at the landings, fed by a small floor-versus-time trace of the last minute, from a small dispatch simulation.

Elevator dispatch simulator Won

A building and elevator simulator in Rust, written in a hackathon week with one teammate, with cascaded PID control for every car. Our team took 1st place at Boğaziçi AI UpForAChallenge.

Two-camera distance to targets Archive

Distance to the reflective FRC targets from two cheap cameras, without a full stereo calibration. Blobs are paired across the two frames, and distance is fitted against 1/disparity from real measurements.
Illustration of a streaming JSON decoder: a small nested list drawn as an indented tree on a white card, its deeper part boxed in amber as a lazy generator below the max_depth cut, leaving the card through a port beside the root bracket as a stream of token chips on a winding track, with chevrons showing the direction, that runs off to the right.

DBEX Archive

A JSON-compatible encoder and decoder in Python built on generators, so it can stop at a chosen depth and hand back the rest lazily. On PyPI since 11th grade.