pengtoh's blog

I got my hand on a few micro:bits and a Kitronic :MOVE mini buggy. They were meant for my kids. But I had always wanted to build a remote controlled self-balancing robot. What's the easiest way to get something working? Repurpose parts from the buggy, add Lego and some code!

Here's version 1. It is fragile and wobbly. With a simple PID controller written in Javascript using makecode/PXT, it self-balances on carpet for over 20 minutes but doesn't do so well on hard floors.

I use a second micro:bit to instruct the robot to turn using the micro:bit's radio service. Turn too much and it falls over!

The bot:

Stripped of the Lego frame:

As you can see, it isn't exactly as "kid-friendly" as just adding Legos.

I had to add a digital gyroscope (MPU-9255), an edge connector and solder some connectors to get access to the I2C pins. See parts list below.

The hardware turns out to be too flimsy. The motors do not stay in position. The wheels have limited friction on hard floors. Tuning the PID parameters was hard and time consuming with so many sources of errors.

On the software side, I had to read the MPU-9255 specification and write code to talk to it using I2C. There is no existing makecode module for that 9-axis gyroscope/accelerometer/compass chip. The version of Javascript in makecode does not support floating point or trigonometric functions. Certainly, this can be made more "kid-friendly".

In the end, I was just amazed that it somewhat self-balanced on carpet for more than 20 minutes. It is not anywhere near Segway stable, which is not surprising given the rather primitive control algorithm.

Parts:

BBC micro:bit
2x Feetech FS90R 360 degrees servo motors from the :MOVE mini buggy.
Kitronic servo:lite board from the :MOVE mini buggy.
InvenSense MPU-9255 9-axis gyroscope, accelerometer, compass.
Pololu Wheel for FEETECH FS90R/FT90R Micro Servo 60x8mm
Wires, bolts
Lego pieces

The servo:lite board isn't necessary. It holds 3xAAA batteries to generate regulated 3.3V volts for both the micro:bit and the servos. The micro:bit can actually run off 2xAAA batteries directly and the servos can be powered unregulated with up to 6V (4xAA batteries).

Version 3 is the updated version 1 without the servo:lite board and Legos.

Links to all the micro:bit robot posts:

July 6th to July 16th

11 days

900km

15 mountain passes

18,000m of climb

To remind myself to do something crazy once in a while.

Many thanks to my wonderful wife for taking care of the kids while I was away.

Check out Piaw's detailed trip report for the whole tour.

Day 1, July 6th

83km, 1240m climb.
Luzern, Sarnen, Melchtal, Stockalp, half way up Melchee-Frutt, Sarnen, train to Meiringen, bus to Rosenlaui

Day 2, July 7th

86km, 1640m climb.
Rosenlaui, Grosse Scheidegg, Grindelwald, Interlaken, Giessbach, Meiringen, Rosenlaui

Day 3, July 8th

60km, 2090m climb.
Rosenlaui, Innertkirchen, Grimsel Pass, Furka Pass, Tiefenbach

Day 4, July 9th

116km, 840m climb.
Tiefenbach, St Gotthard Pass, Bellinzona, Grono

Day 5, July 10th

86km, 2350m climb.
Grono, San Bernardino Pass, Passo dello Spluga, Campodolcino

Day 6, July 11th

78km, 1780m climb.
Campodolcino, Maloja Pass, St Moritz, Pontresina, Berninahaus

Day 7, July 12th

87km, 1020m climb.
Berninahaus, Bernina Pass, Livigno Pass (Forcola di Livigno), Livigno, Munt la Schera Tunnel, Ofen Pass (Passo del Forno), Santa Maria Val Müstair, Schluderns