Magnetic Spheres Link Up to Form Rotary Encoder Ring

If you need to measure how far a shaft has rotated, an encoder is the way to go. But what if you actually wanted to build one yourself? As hacker “ao2” puts it, this solution involving a chain of magnetic spheres is likely an example of constraints driving its creativity.

While an optical encoder could normally be used for angular sensing, this was ruled out because of a possible noise issue in the final project. Instead, he used 21 small magnetic spheres to form a circle that wraps around a pen that acts as a spindle in his demo. When these magnetic spheres pass in front of a pair of Hall effect sensors, this creates the effect of a quadrature signal, meaning that not only can it measure angular velocity, but the setup can also tell in which direction the shaft is turning.

Because two sensors are used, the circle of 21 magnets, or 42 total poles, produce a total of 82 pulses per revolution, giving a resolution of 360 / 84, or 4.29 degrees per revolution. This is apparently good enough to suit ao2’s needs for the mystery project that he’s cooking up.

For now, you can see the device demonstrated with a small LCD display in the video below.