Electronic compass designElectronic compass design

Principles of azimuth measurement and Principles of azimuth measurement and compass operationcompass operation

Author: Jakub Kazubek

Tutor: Lt. Col. Piotr Kaniewski, PhD

Military University of Technology

Warsaw, Poland

Faculty of ElectronicsInstitute of

Radioelectronics

Earth’s magnetic fieldEarth’s magnetic field

S

N

Rotation axis

Magnetic

North

Geographic

North

Geographic

South

Magnetic

South

11,5°

Magnetic axis Positions of the magnetic poles do not match the positions of geographic poles

The magnetic poles change their position with time

Earth’s field varies from less than 30µT to over 60µT

The The azimuth angleazimuth angle

Hz

Hy

HxHxy

Magnetic North

Heading direction

`

H

P

αThe direction parallel,

to Earth’s surface, component aligns with magnetic S-N direction

The azimuth can be calculated as:

x

y

H

Harctg 2

Magnetic field sensorsMagnetic field sensorsKMZ51 sensorKMZ51 sensor

1 2 3 4

5678

-IF +Vo -Vo -IC

+IF Vcc GND +IC

Flipping coil Compensation coil

Magnetoresistive Wheatstone

bridgemkA

VmV

/

/

KMZ51 main parameters

Parameter Typical Unit

Vcc 5 V

Sensitivity 16

Measurement range -0,2 to +0,2 kA/m

Offset voltage -1,5 to +1,5 mV/V

Temp. coefficient of the offset voltage -3 to 3

The compensation coil allows to compensate the external constant magnetic field

The flipping coil allows to flip internal magnetization vector sense

K

VV /

Sensor offset voltageSensor offset voltage

Characteristics of the sensor depend on internal magnetization sense (a and b)

The offset voltage does not depend on the internal magnetization direction

The offset voltage depends on supply voltage and can change with time

Assuming Earth’s field strength of 15 A/m, supply voltage +5V and maximum temperature

differences of 100K:

The offset voltage equals: 9mV

The voltage caused by the external field is: 1,2mV

Offset measurementOffset measurementAssume a

constant external field Hx

Measure Vn voltage

Flip internal magnetization vector

Measure Vp

voltageThe offset equals:

npoffset VVV 2

1

Tilt errorTilt error

Xk Yk Zk => Xz Yz Zz

Transformation between two coordinate systems is needed:

Tilt errorTilt errorTilt error ε depends on:

- tilt angle τ

- magnetic inclination δ sintanarctan

Tilt correctionTilt correction

cossinsinsincos1 zyx HHHH

sincos2 zy HHH

Horizontal components independent on tilt angle:

Additional elements that have to be measured:

• Vertical component Hz – third sensor

• Pitch and roll angles – two axis accelerometer

Block Block diagramdiagram

OscillogramsOscillograms

Voltage across flip coils

Trigger for flip current generator

The compass moduleThe compass module

Electronic compass Electronic compass advantagesadvantages

No mechanical parts

Digital or analogue (PWM) data output

Possibility of Hard Iron Calibration

Possibility of the True North calibration

PProject developmentroject development

Noise elimination

Implementation of the True North algorithm

Gyro correction

Software development

Thank you for your Thank you for your attentionattention