pxt-heading

Introduction

Microbit makes a great controller for a robot buggy. But every vehicle ideally needs to know which way it's pointing.

If you know exactly how far each wheel has rotated, then from their diameter you can compute the difference in path-lengths. Knowing the axle-length then lets you compute changes of direction. However, only a few buggies have rotation sensors.

A rather simpler method is just to use a compass! The problem is that the built-in microbit compass only works when it is horizontal, yet most robot buggies mount it vertically.

This pxt-heading extension is designed to meet the need for an orientation-independent compass, based on the magnetometer readings.

Magnetometer

The earth's magnetic field has a horizontal component at every location (apart from the magnetic poles). Whatever the microbit's mounting orientation, as the buggy turns around, this North-South component sinusoidally changes the flux readings in at least two of the magnetometer channels. These sinusoidal curves will be 90 degrees out-of-phase, but need to be corrected in various ways.

Calibration

The first task is to determine which two of the three axes to use. Then we'll need to compensate for field-distortions due to the buggy itself (i.e. fixed metalwork and motor magnets close to the microbit mounting-point).

Finally we'll need to balance up the detected field-strengths (depending where you are located on the globe) so that we can apply simple trigonometry to compute the angular bearing with respect to North.

heading.scan()

It is obviously not feasible for this extension to know how to turn your buggy in any particular direction, so you must first set it spinning on-the-spot (by running its motors in opposite directions) and then call heading.scan(ms) function to perform a magnetic scan of 3-D readings. The scanning time (ms) should be sufficient to complete more than one full rotations.

heading.prepare()

This function analyses the data collected during the scan to choose the appropriate axes and how to balance them. As an interesting by-product, it returns the spin-rate of the buggy during the previous scan (in RPM), letting you compare the effects of setting different motor speeds.

heading.degrees()

Having performed the scan, and prepared the measuring set-up, this is the function that returns the current compass heading in degrees (0 to 360)

rpm2Speed(diameter, axle)

A utility function to help with motor calibration. This function converts the spin-rate achieved with wheels turning in contrary directions, into the equivalent linear speed when both wheels are going forwards. Calculations are based on the wheel-diameter and axle-length.

It should be noted that for a given power setting, inertial and frictional effects may mean that the actual wheel-rotation speeds achieved will differ between the two situations, so motor calibration can only ever be approximate.

Open this page at https://grandpabond.github.io/pxt-heading/

Use as Extension

This repository can be added as an extension in MakeCode.

• open https://makecode.microbit.org/
• click on New Project
• click on Extensions under the gearwheel menu
• search for https://github.com/grandpabond/pxt-heading and import

Edit this project

To edit this repository in MakeCode.

• open https://makecode.microbit.org/
• click on Import then click on Import URL
• paste https://github.com/grandpabond/pxt-heading and click import

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• for PXT/microbit

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