Hall Encoder

Purpose

This code is designed to leverage two US1881 Hall Latch Sensors to act as an quadrature encoder for measuring the rotation of a wheel. Specifically it has been tested on the front sprocket of a bicycle.

This library includes an API which gives access to:

• Direction of rotation
• Average RPM
• Position (with resolution based on number of magnets)
• Time between rotation
• Timeout (if the rotation has stopped for a set length of time)

There is example code which showcases most of these values.

Theory

The US1881 Hall Latch Sensor can detect magnetic polarity. This is leveraged by placing magnets on a rotating surface and placing the Hall Encoder circuit in a stationary position such that the magnets rotate and their field crosses the sensor.

Originally it was assumed that the magnets would have to be arranged in sequence with opposite polarities (side by side). This is not necessary since the orientation of the sensor board causes two state (HIGH/LOW) as a single magnet passes through the magnetic field. Thus the magnets should be oriented with the polarities in one direction.

NOTE: The polarity of the magnets does matter but I have not figured out which polarity should be used.

The library is setup to allow the user to use as many magnets in the system as desired. More magnets will yield a higher resolution in terms of steps and (probably) a greater accuracy with RPM. However, if the magnets are placed too close together the system may no longer work effectively.

By having two sensors on a single board the direction of rotation can be calculated (i.e. quadrature).

Circuit

The circuit for this system is very simple with minimal requirements of:

• 2x10K resistors
• 2xUS1881 Hall Latch Sensors.

The distance between the two sensors should be roughly equal to the radius of the magnets used (or, if square magnets, half the width).

The sensors do not require a lot of current so the +5V can be powered from the Arduino.

The output of each sensor goes to separate digital input on the Arduino. The library allows you to select your desired pins.

NOTE: There is a KICAD schematic included in this repo. A circuit board could very easily be designed.

Setup

There are two ways to use this code:

1. Add the src directory to Arduino as a library, or;
2. Make a symlink of the .cpp and .h files in the directory with your code. e.g. ln -s src/* <directory with your .ino>