Some XLoBorg code to get you started

Here’s code to create a graphical compass for your XLoBorg from PiBorg. It’s a work in progress, working fine for 2-dimensional headings (ie: keep the Pi on a flat, level surface). It’s got the code in for 3-dimensional headings using the accelerometer as well as the magnetometer to enable the Pi to be tilted but this is *ahem* not quite working yet (my maths is a bit screwy it seems). I’ll post an update when it’s working good ’n proper.

I’ve included support for Declination which is pretty important to get an accurate compass heading. Just define your location(s) as constants near the top and edit the declinationAngle = BRISTOL / 1000 to equal your location and hey presto.

I also found a very interesting PDF from Honeywell all about how these combined accelerometer + magnetometer compasses work. Definitely worth a read.

# Calculate and generate a graphical display of the compass heading
# calculated from the output of XLoBorg from

# Author: Colin Deady, 2012.
# Released under Creative Commons Attribution Share Alike,

# Credit to Bryan Oakley for the MyApp class code that generates the clock face:
# Released under Creative Commons Attribution Share Alike (,

# NO liability is accepted WHATSOEVER for inaccurate data generated by this program.
# If you use it to navigate the Pacific / Sahara / other place of your choice and
# end up falling over the edge of the world and the last thing you see are some giant
# elephants then that is your get, not mine.

import Tkinter as tk; import time
from math import cos,sin,pi, atan, atan2, floor, asin
import sys

# Load the XLoBorg library
import XLoBorg

# Tell the library to disable diagnostic printouts
#XLoBorg.printFunction = XLoBorg.NoPrint

# Start the XLoBorg module (sets up devices)

# define a target direction that we can use to rotate the Pi towards
# this could be built upon to be the target angle that a robot steers towards
targetDirection = 120

# Declination - very important!
BRISTOL = -2.2
declinationAngle = BRISTOL/1000

# create a "clock face" upon which we can render the compass heading of XLoBorg
class MyApp(tk.Tk):
def __init__(self, *args, **kwargs):
tk.Tk.__init__(self, *args, **kwargs)


self.w = tk.Canvas(self, width=320, height=320, bg="#111", relief= "sunken", border=10)

self.w.create_oval(10,10,330,330, fill="cyan", tags="compassbg1")
self.w.create_oval(70,70,270,270, fill="blue", tags="compassbg2")
self.w.create_line(0,0,0,0, fill="red", width="3", tags="compass")
self.w.create_line(0,0,0,0, fill="yellow", width="3", tag="target")

legendbg = "#fff"
uzr1 = tk.Label(self, text="N", bg=legendbg ), y=12)
uzr2 = tk.Label(self, text="S", bg=legendbg ), y=311)
uzr3 = tk.Label(self, text="E", bg=legendbg ), y=160)
uzr4 = tk.Label(self, text="W", bg=legendbg ), y=162)

e = tk.Button(self,text="Quit", command=self.Quit)


def update_compass(self):

# Read and render the raw compass readings, correcting by our manual calibration values
compass = XLoBorg.ReadCompassRaw()
print str(compass[0])+","+str(compass[1])+","+str(compass[2])
mX = compass[0] - -248
mY = compass[1] - 370
mZ = compass[2] - 1384
# print "mX:"+str(mX)+",mY:"+str(mY)

# Read the raw accelerometer readings
accel = XLoBorg.ReadAccelerometer()
print str(accel[0])+","+str(accel[1])+","+str(accel[2])
aX = accel[0]
aY = accel[1]
aZ = accel[2]

# some notes on tilt compensation

# =======================================================================================
# The following code does NOT yet work... it should but my maths is a bit screwy
# When I manage to fix it then we'll have a compass that copes with being tilted, huzzah!
rollRadians = asin(aY)
pitchRadians = asin(aX)
# up to 40 degree tilt max:
cosRoll = cos(rollRadians)
sinRoll = sin(rollRadians)
cosPitch = cos(pitchRadians);
sinPitch = sin(pitchRadians);

vX = mX * cosPitch + mZ * sinPitch;
vY = mX * sinRoll * sinPitch + mY * cosRoll - mZ * sinRoll * cosPitch;
# =======================================================================================

# get the heading in radians
heading = atan2(vY,vX)

# correct for declination
heading +=declinationAngle

# Correct negative values
if (heading < 0):
heading = heading + (2 * pi)

# Check for wrap due to declination and compensate
if(heading > 2*pi):
heading -= 2*pi

# convert to degrees
heading = heading * 180/pi;

# get the base degrees
heading = floor(heading)
print heading

angle = heading*pi*2/360
ox = 165
oy = 165
x = ox + self.size*sin(angle)*0.45
y = oy - self.size*cos(angle)*0.45
self.w.coords("compass", (ox,oy,x,y))

angleTarget = targetDirection*pi*2/360
xt = ox + self.size*sin(angleTarget)*0.45
yt = oy - self.size*cos(angleTarget)*0.45
self.w.coords("target", (ox,oy, xt, yt))

gapAngle = abs(targetDirection - heading) # we want the angle without the sign
# if the gap angle is more than 180 degrees away then it is closer than we think
if gapAngle > 180 :
gapAngle = 360-gapAngle
if gapAngle < 0 :
print "Target is " + str(gapAngle) + " degrees anticlockwise"
elif gapAngle > 0 :
print "Target is " + str(gapAngle) + " degrees clockwise"
print "Target acquired!"

self.after(500, self.update_compass)

def Quit(self):

app = MyApp()


RoverBot1 (aka: "George") runs amok on BBC Points West

I feel so honoured. My cunningly crafted (read: “a bit of a bodge, but hey it works”) RoverBot1 who gained the name George at the November DigiMakers event in Bristol has been featured on BBC Points West. Scroll forward to ~53.5 seconds in (or better still, watch the entire article). Yes there he is, scuttling across a table.

Behind the scenes of course it’s probably worth mentioning that George had actually started to run amok and my hand was there trying to coral him away from Certain Doom should he decide to leap from the table top. There he was: Python program running nicely, moving forwards, turning this way and turning that way on command when suddenly I lost control via the keyboard and he refused to respond, with his wheels left permanently turning. I’ve managed to recreate this mishap and it seems that when the battery voltage drops enough the Raspberry Pi freezes and George carries on repeating the last command he was given, which in this case was clearly “run amok”.

Still, it looks good on TV :)

Here’s George before he started misbehaving on The MagPi stand that I jointly ran with Meltwater of

A mechanical arm that can shift gears in less than a twentieth of a second

Look what Father Christmas brought me for Christmas:


Build time was about half of Christmas Day. It is a pretty good design and seems fairly sturdy. The weight of the 4 D battery cells keeps it nicely planted on the table and the inclusion of an on/off switch on the base keeps battery drain to a minimum. My only gripe would be that it does not use stepper motors and instead relies on a worrying “click, click, I’m failing” noise when any gear reaches the end of its travel.

So far it has been connected up to my Linux Mint netbook and runs a treat after following the instructions by maxinbjohn. Where John writes “Select the 'USB ROBOTIC ARM' and build the kernel” you will want to:

sudo make -f Makefile
sudo make install

from within the directory you downloaded the files to (or use git to clone the files). That seemed to work fine for me.

Now to connect it to one of my Raspberry Pi’s for a bit of Python programming.