Archive for March, 2011
Human–computer interaction
March 31st, 2011Tilt Compensation Azimuth ? with Pitch Ø et le Roll ?
March 19th, 2011A) Introduction
We need to calculate the Azimuth ? or its course. According to Wikipedia,the Azimuth or the course is the angle that the intended path of the boat makes with a fixed reference object (typically true north). Typically course is measured in degrees from 0° clockwise to 360° in compass convention (0° being north, 90° being east). Course is customarily expressed in three digits, using preliminary zeros if needed, e.g. 058°. This angle is measured using a compass, a magnetic compass or gyro.
The components that I have chosen to calculate my Azimuth:
HMC5843 magnetic compass is a threeaxis magnetometer. It identifies the angle of rotation relative to the magnetic north of the Earth with a resolution of 7 milligauss.
The accelerometer AXL335 delivering voltages proportional to acceleration with a sensitivity: + / – 3 g.
B) Calculate Azimuth ? without compensation:
This expression works if the HMC5843 is not tilted.
ps: Warning in Excel, you must reverse the values for atan2, it can save you time ….
You can download a sample program that demonstrates how to read a microcontroller ATMega328 HMC5843
Download: here
C) The angles Ø Pitch and Roll ? for a twoaxis accelerometer and three axis:
With the ADXL335 accelerometer, it is possible to calculate the pitch Ø and Roll ? to correct our azimuth ?. Here is a scheme to visualize the angle on the boat.
For a twoaxis accelerometer following equation:
For a threeaxis accelerometer as the HMC5843 equation here:
D) Azimuth ? tilt compensation :
The HMC5843 provides correct values if it is flat. So every time my boat will be tilted angle the rotation relative to magnetic north by the compass given no longer correct. It is possible to compensate for this error (tilt compensation) using the values of the compass and an accelerometer.
That compensation is also possible for two lines with the compass algorithm Yun Seong Cho and Chan Gook Park, which allows using the dip angle to estimate the third axis. You can download their work below
Download : TiltCompensation.pdf
For a twoaxis magnetometer, you must calculate the dip angle to approximate the third axis Z.
To calculate :
ex: ou 48.86213° coïncide avec ma latitude.
Compass for two axes, it is necessary to approximate the third axis Z by the value of the dip angle
However, to get more accurate values, it is better to use a compass as the HMC5843 three axes.
To calculate the azimuth with compensation:
Horizontal vector :
Vertical vector :
Heading :
Links : http://fr.wikipedia.org/wiki/Cap_%28navigation%29
Components selected
March 19th, 2011A) Components selected :
The main motherboard responsible for performing the calculations, the choice is the Fox board Board G20
His features
Power 
+ 5 Volts 
Consumption 
60 mA 
Processeur  ARM9™ AT91SAM9G20400 MHz d’Atmel™ 
Memory  64mbyte@ 32bit 133MHZ 
I/0  – 28 portsd’entrées/sorties
– 2 ports série (niveau 3,3 V) – 4 entrées de conversion “A/N” – bus I2C™ / SPI™ – Sortie PWM 
The GPS is the MediaTek MT3329
His features
Channels  66 channels 
Sensitivity  165dBm tracking 
Maximum update rate  up to 10Hz 
Dimension?  16mm x 16mm x 6mm 
Position Accuracy  Position Accuracy 
Cold Start  under 35 seconds (Typical) 
Warm Start  under 34 seconds (Typical) 
Hot Start  under 1 second (Typical) 
Low Power Consumption  48mA @ acquisition, 37mA @ tracking 
Low shutdown current consumption?  15uA, typical 
The magnetometer is the HMC5843 three axes:
Alimentation 
voltage of 2.53.3VDC 
Sorties 

Dimensions 
0.5×0.5″ (12.7×12.7mm) 
Alimentation 

Consommation 

Résolution 

Précision 

Filtrage 

Sorties 
I2C interface 
The accelerometer is the ADXL335
His features
Dimensions  4 mm × 4 mm × 1.45 mm LFCSP 
Sensing  3axis sensing 
Consommation  Low power – 350 ?A (typical) 
Excellent temperature stability  
Singlesupply operation1.8 V to 3.6 V 
Communication is based on XBee
Alimentation 
+ 3.3 Volts 
Consommation 
210 mA 
Puissance 
50 mW (+17 dBm) power output 
Pointtomultipointnetworking ideal for lowlatency applications
Support for large, dense networks128bit AES encryption RPSMA connector Industrial temperature rating (40° C to +85° C) Advanced mesh networking and lowpower modes supported 
