Arduino + HMC6343 + u-blox MAX-6

Part of my investigation into simultaneous presence in real & virtual environments involves streaming real world position, heading & orientation data into a modified Second Life viewer. After much faffing about evaluation of different accelerometers, magnetometers & GPS receivers, as well as learning about GNSS & exciting concepts like magnetic declination, hard/soft iron offsets & tilt compensation, the result is an Arduino with an HMC6343 tilt-compensated magnetometer & a u-blox MAX-6 GPS receiver.

For the interaction style that I want to investigate with the VTW project I need to know 3 things about the user;

  • position – where they are
  • heading – the direction they are looking
  • pitch – the angle they are facing (up, down, level)

Pitch is the easiest of the three – you simply use an accelerometer. I tried both the ADXL335 with signal conditioned voltage outputs & the MMA8452 which uses I2C. These are both 3-axis accelerometers so can tell you roll, pitch & yaw all at the same time. They both did what I wanted, but the MMA8452 was easier to use as it didn’t involve experimenting to find out what values represented the rest/peak positions of each axis like the ADXL335 did.

Heading (by which I mean a compass heading) also seems easy to begin with – you just use a 3-axis magnetometer (aka ‘digital compass’ or ‘e-compass’) & perform a calculation that uses the readings of the 2 axes that represent pitch & roll. I tried the HMC5883L but soon discovered that things get much more complicated if you aren’t holding the magnetometer flat – because the calculation doesn’t take the third axis into consideration, the heading becomes more useless the more you tilt it! To calculate a heading even when not held level (which is important for many applications) you need to integrate the reading from the third axis, which can be thought of as recording the magnetic field lost by the other two when tilted out of alignment, into the heading calculation. But to do this you need to know how the magnetometer is tilted – good thing we have a 3-axis accelerometer!

There are a number of good tutorials/guides for this, including these two that I followed, however if you are lazy &/or scared by maths you can also buy ICs such as the HMC6343 that combine a 3-axis accelerometer with a 3-axis magnetometer & perform the tilt compensation algorithms onboard.

Of course there must be a downside to this convenience & most obvious is the price. If you are a SparkFun sort of person the HMC6343 will set you back $149.95. To put that into perspective the ADXL335 is $24.95, the MMA8452 a paltry $9.95 & the HMC5883L just $14.95.

The HMC6343

The HMC6343 ‘3-Axis Compass with Algorithms’ is an I2C device so is easy to hook up (SDA to analog 4, SCL to analog 5 on an Arduino Uno) & interact with. The datasheet contains the protocol definition & communicating with it looks something like this (the example here changes the rate of measurements from the default of 5Hz to the fastest of 10Hz). HMC6343_ADDRESS is 0x19 & obviously you will also need to include Wire.h at the top of your sketch.

Wire.beginTransmission(HMC6343_ADDRESS);
Wire.write(0xF1);
Wire.write(0x05);
Wire.write(0x02);
Wire.endTransmission();

The first Wire.write tells the device what command we want to run, in this example 0xF1 is the ‘Write to EEPROM’ command that expects 2 binary byte arguments. The first argument is which register we want to write to, in this example 0x05 is the address of ‘Operational Mode Register 2’ which stores the measurement speed. The second argument is the value that we actually want to write to this register & in this case 0x02 means that we want measurements at 10Hz. Simple enough, no?

If you look through the sketch you’ll see this pattern repeated for setting the variation angle correcton, the Infinite Impulse Response filter & temporarily changing the orientation (so that you can mount the IC in different orientations without having to manually swap the axes, which is very handy).

The HMC6343 also allows you to access just the accelerometer readings, separately from the tilt-compensated magnetometer readings, so this single IC gives me both the (tilt compensated) heading & the pitch.

One more handy feature of the HMC6343 is the hard-iron offset calibration. Metallic stuff around the chip (wires, the GPS module, the Arduino, etc.) can affect the magnetic field detected by the magneto resistors & throw off the readings, so we can compensate for this using the built-in calibration mode. Read the datasheet to see how this works, but essentially all you have to do is send command 0x71, rotate the device in a certain manner, then send command 0x7E.

In case you’re wondering what the difference between hard-iron offset & soft-iron offset is, the former is for things that won’t change during operation (eg the wires connecting the IC to the Arduino are always going to be there) whereas the latter refers to things that will change during operation (such as moving the IC around your lodestone collection). Hard-iron only need be calculated once, whereas soft-iron requires dynamic attention.

u-blox MAX-6

My research into GNSS led me to the u-blox MAX-6 as my best option. Impressive specification, solid performance & compatibility with SBAS including EGNOS in Europe (Wikipedia page on GNSS augmentation/SBAS here). It turns out that it’s a popular GPS receiver amongst high-altitude ballooners (the kind of people who attach Arduinos & cameras to weather balloons & let them go) as it operates correctly at high altitudes which some receivers evidently don’t. So after some discussion on the #highaltitude channel on freenode I ended up ordering a MAX-6 with the appropriate level conversion to work with an Arduino (the MAX-6 uses 3v logic, whereas the Arduino uses 5v, so conversion is required so as not to fry the module). I ordered it from here & based my sketch on the example code here.

The MAX-6 is a serial device (no convenient I2C here) so interfacing with an Arduino is slightly different. The wiki page linked above recommends not using Software Serial, however as I also want to run the HMC6343 on the same Arduino I have no choice – so far I haven’t experienced any trouble with it.

The protocol specification for the MAX-6 is substantially longer & the messages much more complex than the HMC6343. If you want to have a go at assembling the messages by hand you can do so by reading the 220+ page document & I actually had some success with this, however then discovered that the easier way is to use the u-center software which allows you to configure the receiver from Windows using tickboxes & menus then see/copy the messages from the relevant console window to paste into your sketch. The u-center software worked fine for me in a Windows 7 VM running on a Linux host, using pins 0 & 1 on an Arduino to connect the receiver directly to the USB connection via the Arduino’s UART.

Once you have your message, sending it to the receiver looks something like this (the example here sets the dynamic platform model to ‘pedestrian’);

uint8_t CFG_NAV5[] = {0xB5, 0x62, 0x06, 0x24, 0x24, 0x00, 0xFF, 0xFF,
                    0x03, 0x03, 0x00, 0x00, 0x00, 0x00, 0x10, 0x27,
                    0x00, 0x00, 0x05, 0x00, 0xFA, 0x00, 0xFA, 0x00,
                    0x64, 0x00, 0x2C, 0x01, 0x32, 0x3C, 0x00, 0x00,
                    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
                    0x00, 0x00, 0x00, 0x00};
calculateUBXChecksum(CFG_NAV5, (sizeof(CFG_NAV5)/sizeof(uint8_t)));

while (!success)
{
  sendUBX(CFG_NAV5, (sizeof(CFG_NAV5)/sizeof(uint8_t)));
  success = getUBX_ACK(CFG_NAV5);
}

The first thing we do is create an array of type unit8_t that contains the message itself. The final 2 hex values are the checksum & the call to calculateUBXChecksum on line 2 calculates these values & substitutes them into the array. To send the message we use the sendUBX method & keep on trying until the receiver sends us a confirmation.

Again you will see this pattern repeated a number of times in the sketch, to set the dynamic platform model, enable SBAS using EGNOS, disable all the messages that I don’t want & enable the ones that I do want.

Getting readings

Getting data from the HMC6343 is a simple case of sending the correct command (0x50), requesting the correct number of bytes & then reading them into sensible variables (heading, pitch, etc.).

Getting data from the MAX-6 isn’t much harder, using SoftwareSerial.available() & SoftwareSerial.Read(), however as with most GPS receivers the data that it returns is in the form of NMEA 0183 messages which look a bit like this;

$GPRMC,225446,A,4916.45,N,12311.12,W,000.5,054.7,191194,020.3,E*68

It’s not hard to parse it yourself, but why go to the effort when there are libraries like TinyGPS that can do it for you?

…using readings?

So what was the point of all this again? Well, my modified Second Life client listens on the serial port using Boost.Asio, receives these messages & then uses then to control avatar & camera – more on this soon!

2 thoughts on “Arduino + HMC6343 + u-blox MAX-6”

  1. Depending on where you buy your uBlox MAX-6Q breakout board from, you do have an I2C option. That comes in very handy for certain board layouts. If you’re interested in more info, please feel free to contact me.

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