Alpha testing

From The Relativty Wiki
Revision as of 20:01, 25 April 2019 by Justin (talk | contribs)

Relativty has recently been working on some fairly major technical advances to our existing codebase. In addition to the hardware switch from the Arduino Due to the STM32 board for our main 6DoF tracker, we have also been working towards a full 9DoF upgrade. Positional tracking is complex field, full of obtuse terms and complicated mathematics. In a nutshell though, for those of you wondering what this upgrade will give us, it'll fix the main problem that 6DoF trackers have, which is drift.

This is best explained in the two 1 minute videos below, from Sebastian Madgwick - the guy who developed the sensor fusion algorithms that we all use as part of his Ph.D research at the University of Bristol in 2009. Basically, the direction that your 6DoF headset tracker thinks is forwards slowly drifts sideways because of sensor errors, so you have to keep on recalibrating it - but not if you have a 9DoF headset tracker. A 9DoF tracker also measures the earth's magnetic field, alongside the standard 6DoF measurements, & uses a mathematical algorithm to continually compensate for the 6DoF errors.

YouTube: 6Dof Demonstration of Basic Concepts and Drift

YouTube: 9Dof Demonstration of Zero Drift

The hardware has moved on since Sebastian Madgwick was first playing with it, as have the mathematical algorithms. An American called Kris Winer took Madgwick's algorithm and applied them to new Arduino compatible MPU-9250 sensor boards that had recently become available. He released his work to the world as "Beerware" - which means that anyone can do whatever they like with it, but if they ever get the chance, they should buy him a beer. His work is beyond epic, so if you ever meet him, please, buy him as much beer as you can!

However, to those of us who are not so mathematically adept, the algorithms are rather hard to understand. Thankfully, various people have taken them & wrapped them up in nice, approachable Arduino libraries which are pretty straightforward to use. The library that we will be using was created from Kris Winer's code by Hideaki Tai in Japan - however, as is normal in this field, there is a hardware gotcha that you have to be aware of.


The breakout boards that are widely available are supposed to be based on a specific sensor chip: The InvenSense MPU 9250 Nine-Axis (Gyro + Accelerometer + Compass) MEMS MotionTracking™ Device. Or, as you or I would call it, a 9250 chip. However, most of the boards that are now in circulation on eBay ad AliExpress are actually based on a 9255 chip instead! It's still made by InvenSense and, as far as we're concerned, it's pretty much identical to the 9250 in every respect bar one. The chips have information storage slots inside them called registers, one of which is called Who_am_I. This register lives up to its name, and just returns a simple hexadecimal code that identifies the chip - and thus the sensor. The 9250 chip Who_am_I register returns 0x71 whereas the 9255 chip Who_am_I register returns 0x73. It's a small, but rather significant difference, as it causes all the sanity checks inside the library to fail. If the chip doesn't identify itself as 0x71 (i.e. a 9250), nothing runs and everything errors.

You need to connect your sensor to your Arduino. I am working on the assumption that you have your board all prepared and ready to rock'n'roll (e.g. if you are using an STM32, you have your bootloader). I am also assuming that you have Dupont jumper wires or some way of wiring in your sensor passably. You need 4 wires. If your board is like mine, we will be using the first 4 pins. VCC goes to 3v3 on your Arduino, and GND goes to GND. Connect the SDA and SCL up as follows:

STM32 B6 B7
Nano A5 A4
Due 21 20
Uno A5 A4

As I mentioned earlier, the 9DoF sensor uses the earth's magnetic field to compensate and correct for drift. There are two important considerations when it comes to dealing with the earth's magnetic field however. The first is that it varies depending on where you physically are on the surface of the earth. The second is that it varies over time as well. This isn't quite as much of a problem as it seems though. The magic number that we need is called Magnetic Declination, and we need to work out a new value about once a year or so. That means that you should update your library & reflash your tracker sketch every now and then, to keep you accurately aligned. Thankfully, there are web pages linked to Google Maps that do the whole thing for you, and 5 seconds with a calculator gives you the number that you need to type in. The entire procedure is documented in my Magnetic_Declination.txt file. You'll need this number shortly, to update your library.

Before that though, you will need to add the library to your Arduino IDE. Go to the Sketch menu -> Include Library -> Manage Libraries. This will give you your Library Manager. In the search field enter 9250 and scroll down the list of results until you get to MPU9250 by hideakitai Version 0.1.1 and install it.

Next, we need to find out if you have a 9250 or a 9255 chip. Download the sketch called Chip_Test from here. Select your Arduino Board and Port (if required) from the menus in the IDE & flash the sketch as normal. Once it has completed, open the serial monitor. You will see something resembling either one of these two images.

Default Library settings & a 9255 chip: Looking for 0x71 but found 0x73

Updated Library settings & a 9255 chip: Looking for 0x73 and found 0x73

Basically, in the first image, I am using a completely default library, with nothing changed. I also have a 9255 chip plugged into my Arduino. It's looking for 0x71 but getting 0x73 - and it's not happy about it. Go to your Arduino libraries folder, look for MPU9250 and open MPU9250.h in a text editor. Scroll down to line 26 and look for this text:

const uint8_t MPU9250_WHOAMI_DEFAULT_VALUE {0x71};

Change 0x71 to 0x73

const uint8_t MPU9250_WHOAMI_DEFAULT_VALUE {0x73};

Whilst you are editing this file, you also need to update your Magnetic Declination, so scroll down to line 55 and look for this text:

float magnetic_declination = -7.51; // Japan, 24th June

Change it to the value for your location that you calculated earlier. It really helps if you put a date and location in the comment field next to it, so you know when & where you last updated it.

float magnetic_declination = -0.25; // London, 25th April, 2019

Save your library file, & reflash your sketch. You should now see something like the second image as it's now looking for 0x73 and getting 0x73, so it knows that it has the right chip (even if it's an updated version). If, however, you get something that looks like the second image straight away (but with 71 instead of 73), then you actually have a 9250 chip. This means that you only need to update your Magnetic Declination, so don't touch the WHOAMI_DEFAULT_VALUE.

This is a work in progress & I am actually typing it in right now. If you reload the page, I will probably have added more info...