Philips KMZ51 Installation and operating instructions

Philips Semiconductors
APPLICATION NOTE
Electronic Compass Design
using KMZ51 and KMZ52
AN00022

Electronic Compass Design using
KMZ51 and KMZ52 Application Note
AN00022
Philips Semiconductors
2
Abstract
The KMZ51 and KMZ52 from Philips Semiconductors are magnetic field sensors, dedicated to compass
applications. Both sensors rely on the magnetoresistive effect and provide the required sensitivity and linearity
to measure the weak magnetic field of the earth. While the KMZ51 is a single axis field sensor, the KMZ52
comprises a two-dimensional field sensor, as it is required for a compass, within one package. Both devices are
equipped with integrated set/reset and compensation coils. These coils allow to apply the flipping technique for
offset cancellation and the electro-magnetic feedback technique for elimination of the sensitivity drift with
temperature.
Besides the sensor elements, a signal conditioning unit and a direction determination unit are required to build
up an electronic compass. The main functions of the signal conditioning unit are amplification of the sensor
signals and offset compensation. For high resolution systems, also the temperature drift of sensitivity should be
compensated. Both compensation techniques can be implemented simply by controlling the integrated set/reset
and compensation coils of the KMZ51/KMZ52. In the direction determination unit, the azimuth is derived as the
desired compass output quantity.
The given paper shows how to realize signal conditioning and direction determination in conjunction with KMZ51
or KMZ52. Also calibration techniques with respect to interference fields, declination and tilt are pointed out.
Finally, system accuracy is covered and examples for complete compass systems are given.

Electronic Compass Design using
KMZ51 and KMZ52 Application Note
AN00022
Philips Semiconductors
3
APPLICATION NOTE
Electronic Compass Design using
KMZ51 and KMZ52
AN00022
Author:
Thomas Stork
Philips Semiconductors
Systems Laboratory Hamburg,
Germany
Ke
y
words
Earth´s magnetic field
Magnetoresistive sensors
8-segment compass
High resolution compass
Date: 00-03-30
Number of
p
a
g
es: 38

Electronic Compass Design using
KMZ51 and KMZ52 Application Note
AN00022
Philips Semiconductors
4
SUMMARY
This paper describes how to realize electronic compass systems using the magnetoresistive sensors KMZ51
and KMZ52 from Philips Semiconductors. Therefore, firstly an introduction to the characteristics of the earth´s
magnetic field is given. In the following, the main building blocks of an electronic compass are shown, which are
two sensor elements for measuring the x- and y-components of the earth field in the horizontal plane, a signal
conditioning unit and a direction determination unit.
To provide an understanding of the sensor elements, the magnetoresistive effect and the optimization of the
sensor characteristic by using barber pole structures are described briefly. Also the special features of the
KMZ5x products, such as set/reset and compensation coils are pointed out. The signal conditioning unit´s basic
function is to amplify the sensor output voltages, in order to provide reasonable input signals for the following
direction determination unit. Beyond that, offset elimination is an essential task. For high precision systems, also
sensitivity shifts due to temperature variations should be compensated. Practical methods to fulfill all these
tasks either in hardware or software are given. The direction determination unit is the building block for deriving
the azimuth as the desired compass output quantity. The azimuth is the angle between magnetic north and the
heading direction. For high resolution compass systems, this has to be done mathematically by applying the
arcus tangens function to the ratio of the two sensor signals. It is shown, how to implement this function as
software. Without this calculation, a more simple 8-segment compass can be realized, which only indicates the
nearest cardinal or intermediate point (N, NE, ...).
Besides realization of the basic compass functions, this paper also covers the calibration of electronic
compasses against external error sources like magnetic interference fields, deviation between true and
magnetic north and tilt. Finally, determination of system accuracy and examples for complete compass systems
are given.

Electronic Compass Design using
KMZ51 and KMZ52 Application Note
AN00022
Philips Semiconductors
5
CONTENT
1. INTRODUCTION..............................................................................................................................................7
2. EARTH´S MAGNETIC FIELD..........................................................................................................................8
3. BUILDING BLOCKS OF AN ELECTRONIC COMPASS..............................................................................10
4. MAGNETORESISTIVE (MR) SENSORS FOR COMPASS APPLICATIONS ..............................................12
4.1 The Magnetoresistive Sensor Element .........................................................................................12
4.1.1 The Magnetoresistive Effect ..............................................................................................12
4.1.2 Optimization of Sensor Characteristic using Barber Pole Structures................................13
4.1.3 Bridge Configuration ..........................................................................................................13
4.2 Set/Reset and Compensation Coils...............................................................................................14
4.3 Philips MR Sensors for Compass Systems..................................................................................15
5. SIGNAL CONDITIONING UNIT (SCU)..........................................................................................................16
5.1 Requirements...................................................................................................................................16
5.2 Offset Compensation......................................................................................................................17
5.3 Sensitivity Difference (∆S) Compensation....................................................................................19
5.4 Non-orthogonality compensation..................................................................................................20
5.5 Circuit Design ..................................................................................................................................21
5.5.1 Supply voltage considerations ...........................................................................................21
5.5.2 Flipping generator (block1)................................................................................................22
5.5.3 Pre-amplifier (block 2)........................................................................................................23
5.5.4 Offset compensation (block 3)...........................................................................................23
5.5.5 Synchronous rectifier (block 4) ..........................................................................................23
5.5.6 Integral controller (block 5) ................................................................................................23
5.5.7 Compensation coil driver (block 6).....................................................................................23
5.5.8 SCU without electro-magnetic feedback............................................................................24
5.5.9 SCU with Microcontroller ...................................................................................................24
6. DIRECTION DETERMINATION UNIT (DDU)................................................................................................25
6.1 8-Segment Compass.......................................................................................................................25
6.2 High Resolution Compass..............................................................................................................26
7. INTERFERENCE FIELD CALIBRATION......................................................................................................27
8. TRUE NORTH CALIBRATION......................................................................................................................30
9. TILT COMPENSATION .................................................................................................................................31
10. SYSTEM ACCURACY...................................................................................................................................34
11. APPLICATION EXAMPLES..........................................................................................................................35
12. REFERENCES...............................................................................................................................................37
APPENDIX 1 List of abbreviations .................................................................................................................. 38
APPENDIX 2 Unit conversions ........................................................................................................................ 38

Electronic Compass Design using
KMZ51 and KMZ52 Application Note
AN00022
Philips Semiconductors
6
LIST OF FIGURES
Figure 1 Earth´s magnetic field ........................................................................................................................... 8
Figure 2 Earth field vector................................................................................................................................... 9
Figure 3 Functional block diagram of an electronic compass........................................................................... 10
Figure 4 The magnetoresistive effect in permalloy........................................................................................... 12
Figure 5 a) R-H characteristic of a standard sensor, b) R-H characteristic of barber pole sensors............... 13
Figure 6 Barber pole sensor.............................................................................................................................. 13
Figure 7 Bridge configuration of barber pole sensors....................................................................................... 14
Figure 8 Fields generated by set/reset and compensation coil......................................................................... 14
Figure 9 Simplified circuit diagram of KMZ 51 .................................................................................................. 15
Figure 10 Effect of flipping on sensor characteristic ........................................................................................... 17
Figure 11 Block diagram of flipping circuit .......................................................................................................... 17
Figure 12 Timing diagram for flipping circuit....................................................................................................... 18
Figure 13 Typical MR sensor output characteristic (KMZ10).............................................................................. 19
Figure 14 Block diagram of flipping and electro-magnetic compensation circuit................................................ 20
Figure 15 Signal conditioning circuit for one sensor ........................................................................................... 21
Figure 16 Flip circuit with active current limiting.................................................................................................. 22
Figure 17 Signal conditioning circuit with microcontroller................................................................................... 24
Figure 18 Direction determination for 8-segment compass................................................................................ 25
Figure 19 Circuit for 8-segment compass........................................................................................................... 25
Figure 20 Typical test diagrams showing hard iron and soft iron effects............................................................ 27
Figure 21 Principle of bidirectional calibration..................................................................................................... 28
Figure 22 Tilt error............................................................................................................................................... 31
Figure 23 Tilt error magnitudes........................................................................................................................... 32
Figure 24 Definition of pitch and roll.................................................................................................................... 32
Figure 25 Electronically gimbaled compass........................................................................................................ 33
Figure 26 Analog 8-segment compass ............................................................................................................... 35
Figure 27 Analog high resolution compass......................................................................................................... 35
Figure 28 8-segment compass with microcontroller ........................................................................................... 36
Figure 29 High-end compass with microcontroller.............................................................................................. 36

Electronic Compass Design using
KMZ51 and KMZ52 Application Note
AN00022
Philips Semiconductors
7
1. INTRODUCTION
The magnetic compass is a crucial navigation tool in many areas, even in times of the global positioning system
(GPS). Replacing the “old” magnetic needle compass or the gyrocompass by an electronic solution offers
advantages like having a solid-state component without moving parts and the ease of interfacing with other
electronic systems.
For the magnetic field sensors within a compass system, the magnetoresistive (MR) technology is the
preferable solution. Compared to flux-gate sensors, which could be found in most electronic compasses until
now, the MR technology offers a much more cost effective solution, as it requires no coils to be wound and can
be fabricated in an IC-like process. Due to their higher sensitivity, MR sensors are also superior to Hall elements
in this application field.
The intention of this paper is to give a general introduction of electronic compass design with MR sensors and
also to give detailed realization hints. Therefore, the basic characteristics of the earth´s magnetic field are
explained and an overview of the building blocks of an electronic compass is given. Following a description of
Philips´ magnetoresistive sensors for compass applications, the design of each building block is covered in
detail. Here, both hardware and software realisations are shown. Further sections are dedicated to special items
like interference field calibration, true north calibration, tilt compensation and system accuracy. Finally,
examples for complete compass systems are given, consisting of previously described building blocks.

Electronic Compass Design using
KMZ51 and KMZ52 Application Note
AN00022
Philips Semiconductors
8
2. EARTH´S MAGNETIC FIELD
The magnetic field of the earth is the physical quantity to be evaluated by a compass. Thus, an understanding of
its basic properties is required, when designing a compass. Figure 1 gives an illustration of the field shape.
Figure 1 Earth´s magnetic field
The magnetic field strength on the earth varies with location and covers the range from about 20 to 50 A/m. An
understanding of the earth´s field shape can be gained, if it is assumed to be generated by a bar magnet within
the earth, as pointed out in Figure 1. The magnetic field lines point from the earth´s south pole to its north pole.
Fig. 1 indicates, that this is opposite to the physical convention for the poles of a bar magnet (the background is
a historical one, in that a bar magnet´s north pole has been defined as that pole, that points towards north in the
earth´s magnetic field). The field lines are perpendicular to the earth surface at the poles and parallel at the
equator. Thus, the earth field points downwards in the northern hemisphere and upwards in the southern
hemisphere. An important fact is, that the magnetic poles do not coincide with the geographical poles, which are
defined by the earth´s axis of rotation. The angle between the magnetic and the rotation axis is about 11.5°. As
a consequence, the magnetic field lines do not exactly point to geographic or “true” north.
Figure 2 gives a 3-D representation of the earth field vector He at some point on the earth. This illustration
allows to define the quantities, which are of importance for a compass. Here, the x- and y-coordinates are
parallel to the earth´s surface, whereas the z-coordinate points vertically downwards.
•Azimuth α
The angle between magnetic north and the heading direction. Magnetic north is the direction of Heh, the
earth´s field component perpendicular to gravity. Throughout this paper, Heh will be referred to as
“horizontal” component of the earth´s field. Figure 2 shows, that:
He
x
Hey
arctan
= α (1)
Magnetic
North
Magnetic
South
True
North
True
South
EARTHFLD.GIF

Electronic Compass Design using
KMZ51 and KMZ52 Application Note
AN00022
Philips Semiconductors
9
The azimuth is the reading quantity of a compass. Throughout this paper, αis counted clockwise from
magnetic north, i.e. north is 360° or 0°, east is 90°, south is 180°, west is 270°.
•Inclination or dip δ
The angle between the earth´s field vector and the horizontal plane. As already pointed out, the inclination
varies with the actual location on earth, being zero at the equator and approaching ±90° near the poles. If a
compass is tilt, then inclination has to be considered, as explained in section 9.
•Declination λ
The angle between geographic or true north and magnetic north. Declination is dependent on the actual
position on earth. It also has a long term drift. Declination can be to the east or to the west and can reach
values of about ±25°. The azimuth measured by a compass has to be corrected by the declination in order
to find the heading direction with respect to geographic north. This is pointed out in section 8.
x
(heading
direction)
y
(right)
y
(right)
z
(down)
He
Hez
Hex
Hey
αλ
δ
magnetic
north
true
north
azimuth
inclination
or dip
declination
HEVECTOR.AI
Heh
Figure 2 Earth field vector

Electronic Compass Design using
KMZ51 and KMZ52 Application Note
AN00022
Philips Semiconductors
10
3. BUILDING BLOCKS OF AN ELECTRONIC COMPASS
Figure 3 shows a functional block diagram of an electronic compass. This is a minimum set-up, i.e. these
elements are principally required for any electronic compass. The functional blocks – except the field sensor –
can be realized either as hardware or as software. In the following, the functions of each block are summarized.
A detailed description of each block together with examples for realization are given in the following sections.
2-dimensional
field sensor
KMZ52 Signal Conditioning Unit
(SCU)
Direction Determination
Unit (DDU)
Interface to
other system,
e.g. navigation
Display
BLOCKDIA.AI
Vx
Vy
- Amplification
- Offset elimination
(- Temperature
compensation of
sensitivity)
- Amplification
- Offset elimination
(- Temperature
compensation of
sensitivity)
azimuth derivation
x-channel
y-channel
Iflip, Icomp
S·Hex
Iflip, Icomp
S·Hey
flip
coil
compensation
coil
Hex-Sensor
flip
coil
compensation
coil
Hey-Sensor
Figure 3 Functional block diagram of an electronic compass
•Magnetic field sensors
As already pointed out, the task of a compass is to measure the azimuth α, i.e. the angle between north and
heading direction, as defined by equation (1). Therefore, the strengths of two horizontal earth field components
have to be measured: one in heading direction (Hex) and one sidewards (Hey). This requires two magnetic field
sensors, both aligned parallel to the earth´s surface, but rotated by 90 degrees with respect to each other.
Philips´ magnetoresistive sensor technology is an optimum choice for measuring weak magnetic fields like the
earth´s field. The KMZ52 is a sensor device, which is perfectly matched to this application, as it comprises two
extremely sensitive field sensors in the required configuration in one SO16 package. Furthermore, the KMZ52
comprises for each of its sensors a set/reset coil needed for offset elimination and a coil for the compensation of
sensitivity temperature drift. Thus, the KMZ52 makes any external coils obsolete, which are required when using
other sensor products. Section 4 describes Philips´ magnetoresistive sensor technology and introduces those
products, which are dedicated to compass applications.
•Signal conditioning unit (SCU)
The purpose of this block is to deliver output voltages proportional to the field strengths Hex and Hey
respectively. Therefore, the signals delivered by the magnetic field sensors have to be amplified. Also offsets
have to be eliminated. Beyond that, the amplification of one SCU channel should be trimable in order to allow