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AN #187 - kiXAHRS ONE: Atmel Xmega A1 and Sensors Xplained Print
By Natalius Kiedro

acknowledging code input from MAK3, Adrian Jansen, Per Svensson, and fruitful advice from Sergiu Sieber, Bill Premerlani, Ben Zijlstra, John Logan and last not least Mark Alberts.


Introduction

Atmel has recently released the new (blue) Xmega A1 Xplain board  and plug on sensors ATAVRSBIN1 (Inertial One)  and ATAVRSBPR1 (Pressure One). This application note shows how to use Bascom to read the sensors and to translate  the raw data into Euler angles (yaw, pitch, roll) and altitude above sea level. For Arduino users it is shown, how to make use of HEX files generated by Bascom. Due to a rather competitive pricing of Xplain boards and sensors, they are expected to find direct utilization in DIY-projects:  RC-multicopter flight controls, autopilots and flight stabilization systems in RC-planes, camera stabilization systems, 3D game controllers, and pointing devices.

 

Background

AHRS refers to an Attitude and Heading Reference System which is based on an Inertial Motion Unit (IMU). As explained in a previous application note (AN #177 - Kixrazor – Bascomer’s Electronic Flight Information System for Sparkfun’s 9DOF Razor: http://www.mcselec.com/index.php?option=com_content&task=view&id=269), sensor fusion is needed to convert the IMU’s raw data into meaningful information such as the Euler angles which describe  the orientation in 3D space. Custom designed IMU-boards combine a 3D-MEMS-gyroscope (here Invensense ITG-3200), a 3D-MEMS-accelerometer (here Bosch BMA150) and a 3D-MEMS-magnetometer (here Asahi Kasai AK8975, used in Apple’s iPhone4). More recently, such boards also contain a pressure sensor (here Bosch BMA 150), as it is common experience that a GPS derived altitude is oftenly less accurate and delayed compared to a pressure-derived altitude. Several algorithms exist for the calculation of Euler angles from IMU raw data. The simplest utilizes so called complementary filters (CF) in which accelerometer information and gyro information is “mixed” to compensate for gyro drift and to obtain the Euler angles by simple trigonometric calculations. CF (e.g. employed in the popular MultiWiiCopter) are oftenly found sufficient to stabilize hovering, they are rather fast and allow for a rapid update of brushless motor controllers. They may however cause serious problems when asking for flight stabilization from aerobatic maneuvers. The classical Kalman filters (KF) have advantages here, however the procedure itself is computationally quite demanding heavily based on matrix algebra (see AN #176 - Mini Matrix Algebra (http://www.mcselec.com/index.php?option=com_content&task=view&id=267&Itemid=57) ). Rather little matrix algebra is involved in an algorithm called Direction Cosine Matrix (DCM) which combines a good estimation of Euler angles (just 3 states of a KF) with the speed advantage of CF. More information on DCM is found in a web-document written by Bill Premerlani and Paul Bizard (Direction Cosine Matrix IMU: Theory,  http://code.google.com/p/gentlenav/downloads/detail?name=DCMDraft2.pdf&can=2&q )  

DCM is the method of choice also in this Application Note. What was possible on the SFE Razor board at 50Hz a year ago, both with Bascom and Arduino code, is now possible at almost 250 Hz and lower cost, albeit momentarily available to Arduino users only as HEX code. Note that this speed – which multicopter developers may find useful enough – is not the end of the flag. As the core DCM without sensor reads and a little printing runs at 800 Hz using Bascom (and could run even faster when making use of fixed point math instead of floating point math) the 250 Hz achieved is due to digital filtering on Invensense gyros (IMU 3000 included). Thus an Android phone app which might be viewed as the alternative to the Atmel Xplained-based approach, will also feel the 256 Hz limitation burned into the non-analog gyros of Invensense. One does not need to see this as a disadvantage, however. Digital on chip filtering has clear advantages when it comes to electric noise and noisy environments such as on a multicopter.     

 

What  one needs to do for a start?

(1) The Xmega A1 Xplained board and the Sensors Xplained Inertial ONE and Pressure ONE are available from a number of dealers, including  Atmel Store. Plug them together as shown below:   

kixahrsONE.jpg

(2) Download AVR1927: XMEGA-A1 Xplained Getting Started Guide (http://atmel.com/dyn/resources/prod_documents/doc8372.pdf )

(3) Goto http://www.atmel.com/dyn/products/tools_card.asp?tool_id=17168&category_id=163&family_id=607&subfamily_id=1965  , and find software to download as two CD icons. In one case you need to register at Atmel. Click the icons to download. I decided to place “XMEGA-A1 Xplained Example Applications and USB driver” and/or “Xplained USB CDC Driver” into subfolders of Bascom-AVR.

(4) Goto  http://atmel.com/dyn/products/tools_card.asp?tool_id=3886 and download Atmel’s FLIP programmer. Bascom and Arduino users may select FLIP 3.4.2. Click on the CD icon to start downloading. You will be asked to install the Java runtime environment if it is not installed on your Windows PC so far.

(5) Before clicking FLIP_Installer_3.4.2, Bascom users are asked to read Bascom HELP about FLIP. Have BASCOM 2.0.5.0 or later installed! Search by index for FLIP and follow instructions. Arduino users without a copy of Bascom may start installation of FLIP directly. Bascom users may do this also – but on the long run it is time-saving to use the FLIP-support build into the Bascom IDE.

(6) FLIP 3.4.2 misses the configuration file for the Xmega A1 Xplained board. Follow AVR1927 (Chapter 5) where to find it. Briefly: The file ATxmega128A1.xml needs to be copied into ..\Atmel\Flip 3.4.2\bin\PartDescriptionFiles.

(7) Follow AVR1927, chapter 5, how to set up a virtual comm port for your Xmega A1 Xplained board.

(8) AVR1927, Chapter 5, recommends to use Batchisp for the bootloading of Hex-Files. Bascom and FLIP are easier to use, however. Bascom users simply select FLIP as the programmer, load the kiXAHRS_ONE.BAS file, and compile. Before “Send to chip” from Bascom, the SW0-switch of the Xplained A1 board needs to be pressed while plugging in the USB cable.

(9) Arduino users may start FLIP instead. Press SW0 on the Xplained board and hold it while connecting board and PC using USB cable. From the menu: File->Load Hex file->kiXAHRS_ONE.HEX. Click the button showing a USB cable. Select RS232 for programming via USB virtual comm port. Connect the port from the menu which pops up, and press RUN. The standard baud rate is 115200, 8N1. Don’t forget to close FLIP after bootloading completion.

 

 

Testing kiXAHRS_ONE using a terminal program

Bascom users have a terminal emulator integrated in the IDE. This is also available in the free demo copy of Bascom 2.0.5.0. Plug in the kiXAHRS_ONE combo. LED1 flashes rapidly while the combo measures the magnitude of gravity in accelerometer units. Right after, a stream of data starts to scroll:

The first three numbers are the Euler angles roll (X), pitch (Y) and yaw (Z). The roll axis is the one which connects both boards (pointing to the pressure sensor), the pitch axis seperates them while pointing to the top side (USB) and the yaw axis is perpendicular to the board pointing downwards. The last three numbers indicate altitude above sea level in meter, gyro temperature (under a lamp) in degree Celsius and looptime in microseconds. There are two phases of data streaming.  In the first 30 seconds the data show larger variations (phase I) reflecting an alternative to "analog offsetting" previously discussed on the Diydrones forum.  (http://diydrones.com/forum/topics/why-analog-offsetting-in-dcm?xg_source=activity ).  After omega_I filling the combo switches to phase II automatically - phase II has smaller Kp's and Ki's than phase I - the data are smoother here.

Pressing SW7 on the board will show:

on a Bascom terminal (hoping that Bascom may also pop up on a Atmel page;) and a second later there is a main menu:

From here you can change baudrates, looptimes, type of outstream,do magnetometer calibration, read I2C sensor registers, and store your setting in EEPROM. Baudrates may range from 9600 bps to 256000 bps with 115200 bps as the default.  Selectable loop times range from 15000 microseconds
(default) to 4000 mics (250 Hz). Actual loop times are always a bit longer due to I2C “quanta”. For example, the selection of 250 Hz will result in an average of about 240 Hz, if one selects Just Hz as the outstream option. Data streaming is possible in a number of formats including kixlines (
http://www.mcselec.com/index.php?option=com_content&task=view&id=265&Itemid=57 ), SF9DOMAHRS based on Arduino code, the format shown a few screenshots before, raw data display and just Hz. There is an option for compass calibration in which you are asked to rotate the combo around all three axes and bring it into as many different orientations as possible, and a further option to read the I2C-registers of each of the 4 sensors in binary, hex, and decimal formats. The following screenshot shows the registers of the ITG-3200:

From the main menu, it is possible to store all settings in EEPROM (just by pressing the Enter key (CR). This way it is rather simple to fix the data stream to suit the need of a given application. Planes, for example, need another loop timing than multicopters with high speed brushless controllers.

Visualisation using AeroSimRC

AeroSimRC is a commercial (but affordable) RC flight simulator of which a fully functional demo having a runtime limitation of 2 min is downloadable from: http://www.aerosimrc.com/j/index.php/en/downloads . The simulator is a general purpose RC simulator, especially suited for the training of FPV flight however. This is because AeroSimRC allows to utilize rather easily satellite photographs from the flight area of your choice. These data (e.g. from Google Maps) can be combined with digital elevation data from NASA’s SRTM mission which are available on public FTP servers accessed by AeroSimRC.

I have written a plugin for AeroSimRC (plugin_AeroSimRC.DLL) which allows to expand the capabilities of AeroSimRC for three things:

(1) The “safe” testing of autopilot hardware on ground. This is done by letting AeroSimRC stream GPS, altitude, attitude, heading, battery level to the autopilot which then streams radio stick settings back to AeroSimRC. The approach is called “hardware in the loop simulation” (HILS) and as all streaming is in the kixline format,  it is called kixHILS.

(2) The usage of AeroSimRC as a ground station during real flight. In this mode the autopilot streams GPS, altitude, attitude, battery level, stick settings to the ground via a telemetry connection. AeroSimRC then visualizes these data “as if” they would result from a flight simulation. One may select a view from the pilot, from the air, from a follower, or from above (map view)

(3) Visualization of kixAHRS and kixRazor data streams using a vehicle of your choice. The following screenshot shows a typical view – I have selected a quadcopter here. Moving the kiXAHRS around its axes leads to corresponding moves of the quad on the screen:

 

 

To enable the visualization of kiXAHRS_ONE within AerosimRC, select the kixline format, 115200 bps, 15000 mics, and store these settings in EEPROM. The plugin comes with the plugin_kixAHRS_AerosimRC.ZIP  folder. Find instructions how to install in the kixPLUGIN_README2.txt

Please note that updates of kixAHRS_ONE will be in the AR7212 folder of the Forum at MCS electronics.

Have fun

Natalius