WO2012007026A1

WO2012007026A1 - Method and apparatus for automatic calibration of an electronic compass

Info

Publication number: WO2012007026A1
Application number: PCT/EP2010/059949
Authority: WO
Inventors: Dmytro Rusanovskyy
Original assignee: Nokia Corporation
Priority date: 2010-07-12
Filing date: 2010-07-12
Publication date: 2012-01-19

Abstract

In accordance with an example embodiment of the present invention, disclosed is an apparatus and a method thereof for automatic calibration of an electronic compass which comprises a housing, a magnetic field sensor mounted on a movable platform and an electromechanical device arranged to move the platform relative to the housing for determining an interference magnetic field.

Description

METHOD AND APPARATUS FOR AUTOMATIC CALIBRATION OF AN

ELECTRONIC COMPASS

TECHNICAL FIELD

[0001] The present application relates generally to electronic compasses and, more specifically, to methods and apparatuses for automatic calibration of electronic compasses.

BACKGROUND

[0002] Many navigation systems today use some type of compass to determine heading direction. A magnetic compass has been used in navigation for centuries. A recent development is an electronic compass based on magnetic field sensors. Electronic compasses offer many advantages over conventional needle-type or gimbaled compasses such as: shock and vibration resistance, and direct interface to electronic navigation systems.

[0003] The earth's magnetic field intensity is about 0.6 gauss in an open-air environment, and has a component parallel to the earth's surface pointing to the magnetic north. At the equator, the magnetic field direction is entirely a horizontal vector, but in the northern or southern hemispheres the magnetic field points partially downwards (northern hemisphere) or upwards (southern hemisphere). This angle down or up at the earth's surface is called the inclination, or dip, angle. The term magnetic north refers to the earth's magnetic pole position at which the earth's magnetic field points vertically downwards, i.e. the dip is 90°. The earth's magnetic pole position differs from true, geographic, north and it is constantly moving, currently towards true north.

[0004] The direction of the earth's magnetic field is always pointing to the magnetic north. The components of this field that are parallel to the earth's surface are used to determine compass direction. However, when a ferrous object is placed in a uniform magnetic field it will create disturbances. Magnetic distortions can be categorized as hard iron and soft iron effects. Hard iron distortions arise from permanent magnets and magnitised iron or steel, that is materials that exhibit a constant, additive field to the earth's magnetic field. A loudspeaker magnet, for example, will produce a hard iron distortion. A soft iron distortion is the result of material that influences, or distorts, a magnetic field but does not necessarily generate a magnetic field itself, and is therefore not additive. Iron and nickel, for example, will generate a soft iron distortion.

[0005] Hard and soft iron distortions vary from location to location. A particular calibration of a compass is only valid for a certain location of the compass. A new calibration is needed if the compass is re-located or re-oriented in the same location.

SUMMARY

[0006] Various aspects of examples of the invention are set out in the claims.

[0007] According to a first aspect of the present invention, an apparatus is described, which comprises a housing, a magnetic field sensor mounted on a movable platform and an electromechanical device arranged to move the platform relative to the housing for determining an interference magnetic field.

[0008] According to a second aspect of the present invention, a method is described, which comprises causing a movable platform, upon which a magnetic field sensor is mounted, to move relative to a housing for determining an interference magnetic field.

[0009] According to a third aspect of the present invention, a computer program product is provided, which comprises software program instructions where execution of the software program instructions by at least one data processor results in performance of operations that comprise execution of the method of causing a movable platform, upon which a magnetic field sensor is mounted, to move relative to a housing for determining an interference magnetic field.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] For a more complete understanding of example embodiments of the present invention, reference is now made to the following descriptions taken in connection with the accompanying drawings in which:

[0011] Figure 1 shows an example of an electronic compass system with a three-axis magnetoresistive sensor;

[0012] Figure 2a is a schematic test diagram of a magnetic field sensor output in the absence of an interference magnetic field; [0013] Figure 2b is a schematic test diagram of a magnetic field sensor output in the presence of hard and soft iron effects;

[0014] Figure 3 illustrates the principle of bidirectional calibration usable in at least some embodiments of the invention.;

[0015] Figure 4 shows an example of an apparatus according to an embodiment of the invention; and

[0016] Figure 5 is a flow diagram showing operations for perfoming a method of automatic compass calibration in accordance with an embodiment of the invention.

DETAILED DESCRIPTON OF THE DRAWINGS

[0017] An example embodiment of the present invention and its potential advantages are understood by referring to Figures 1 through 5 of the drawings.

[0018] The magnetic field sensors, or magnetometers, used in electronic compasses measure at least one of the strength and direction of the earth's magnetic field to determine the heading relative to the magnetic north. There are several types of magnetometers. A fluxgate sensor consists of a set of coils around a core comprising excitation circuitry, that is capable of measuring magnetic fields with a resolution of less than 1 milligauss. Solid-state Hall effect sensors produce a voltage proportional to the applied magnetic field and also sense polarity. Magneto-inductive sensors incorporate a coil surrounding a ferromagnetic core whose permeability changes within the earth's magnetic field. The coil serves as an inductive element in an oscillator, whose frequency is proportional to the magnetic field being measured. Magnetoresistive sensors may be made of thin strips of nickel-iron permalloy magnetic film whose electrical resistance varies with a change in applied magnetic field. A magnetoresistive sensor will be used to illustrate various embodiments of the present invention but should not be viewed as limiting in any manner the various forms in which the exemplary embodiments may be realized.

[0019] Magnetoresistive sensors are well suited to measuring both linear and angular position and displacement in the earth's magnetic field. They are popular in magnetic field sensing applications as they have a well defined axis of sensitivity and may be mass produced as an intergrated circuit. The reaction of the magnetoresistive effect is very fast and not limited by coils or oscillating frequencies. Recent magnetoresistive sensors show sensitivities below 0.1 milligauss and allow reliable magnetic readings in moving vehicles at rates up to 1000 times a second. In a typical configuration, four resistors are connected in a Wheatstone bridge to permit measurement of both field magnitude and direction along a single axis.

[0020] Figure 1 shows an example of an electronic compass system with a three-axis magnetoresistive sensor. A three-axis magnetoresistive sensor utilizes three channels of wheatstone bridges to convert a magnetic field into a differential output voltages. An electronic compass system 18 with a three-axis magnetoresistive sensor 10 is shown in Fig. 1. Three magnetoresistive sensor outputs are coupled to analog-to-digital converters (ADCs) 14 via amplifier circuits 12. The ADCs 14 are coupled to a microprocessor 16. Note that the compass system 18 may comprise only a microprocessor interface and the microprocessor 16 may be placed outside the compass system 18. The compass system 18 may further comprise a tilt- sensing element, not illustrated in Fig. 1 , to measure the gravitational direction.

[0021] Compasses, especially hand held compasses, are not always horizontal to the earth's surface. This makes it more difficult to determine the azimuth, or heading direction, since errors introduced by tilt angles can be quite large depending on the the dip angle. A tilt sensor provides two-axis measurement of compass assembly tilt, known as picth and roll axis. Roll refers to the rotation around the X axis, or forward direction, and pitch refers to the rotation around the Y axis, or left-right direction. XYZ output from the three-axis magnetoresistive sensor 10 can be combined with pitch and roll outputs from a two-axis tilt sensor to compute a tilt-compesated heading.

[0022] After the azimuth is determined, a declination correction can be applied to find true north. However, compensating the tilt and declination is not necessarily enough to elicit an accurate heading. Another consideration for heading accuracy are the effects of nearby ferrous materials and equipment on the earth's magnetic field. The effects of ferrous materials including iron, nickel, steel and cobalt will distort, or bend, the earth's magnetic field which in turn will alter the compass heading.

[0023] Examples of equipment that produce local magnetic fields and thus could interfere with the earth's magnetic field are magnets in loudspeakers, generators, electrical motors and other electric and electronic systems.

[0024] The output of the magnetic field sensor corresponds to the three axes of the resultant magnetic field comprising the geomagnetic field, distortions and any local magnetic fields relative to the orientation of the magnetic field sensor. The x, y and z components of the magnetic sensor orientation is a set of unique values in the numerical coordinate frame of the sensor. If the magnetic field sensor is rotated in the 3D space, the sensor's magnetic field outputs in the numerical coordinate frame of the sensor can provide information of sensor's orientation variation.

[0025] When a compass is operating in an open area in the absence of any ferrous objects, there are no distortion effects affecting the earth's magnetic field. The azimuth would be calculated using the relationship:

azimuth = acrtan (y/x)

where x and y represent the earth's horizontal magnetic field components, that is the magnetic fields in the X and Y plane. However, in a realistic environment, the output at each sensor's axis will vary due to the magnetic distortions and interferences around the magnetic field sensor. The earth's field at the compass may be superimposed by other magnetic fields or distorted by nearby ferrous materials.

[0026] If no distorting effects are present, rotating a magnetic field sensor through a minimum of 360° and plotting the resulting data as y axis vs. x axis will result in a circle centered around the origin and radius equal to the magnitude He of the earth's magnetic field, as shown in Figure 2a. However, the presence of hard and/or soft iron effects may produce a perturbation of the circle as a simple offset from the origin in the case of a hard-iron effect, or deform the circle to produce an ellipse in the case of a soft iron effect. It is also possible that both effects will be exhibited simultaneously, as shown in Figure 2b. Also interfering fields may be present. The sum of distortion and interfering fields may be called an interference magnetic field.

[0027] Interference field compensation ideally means converting the shifted and/or deformed circle into a perfect circle around the origin. To accomplish this, the interference magnetic field has to be determined in order to correct the compass readings. Generally, this calibration procedure should be carried out individually for each compass and location. As an example, each individual car or user location in an office among computers has its own interference magnetic field. Even when mounting the same compass into the same environment again, for example after maintenance, it is recommended to carry out the calibration again. Thus, the calibration procedure should be simple and ideally automatic. [0028] Calibration methods become straightforward if soft iron effects are negligible compared to hard iron effects. In this case only the components of a constant interference magnetic field have to be measured and compensated. In practice, soft iron effects are usually much weaker, provided there are no ferrous materials at or near the compass. In this case, satisfactory results may be achieved by using a method which may be called a bidirectional calibration method.

[0029] Figure 3 illustrates the principle of bidirectional calibration usable in at least some embodiments of the invention. A bidirectional calibration method comprises that two measurements are carried out with the compass at the same location, but at a heading difference of 180°. For both measurements, respective field components Hx and Hy are stored. Generally, the field at the compass is equal to the sum of earth's magnetic field vector He and interference magnetic field vector Hi. After a compass rotation of 180°, He appears with equal magnitude but opposite sign, whereas Hi appears unchanged as its source is fixed with respect to the compass.

Thus, the vector sum of both measurements HI + H2 is: H1 + H2 -

Rearranging this vector equation, the interference magnetic field components as a function of the measured field components are yielded as:

1

Hix = - (Hlx + H2x)

Hiy = ^ (Hly + H2y)

[0030] Once the interference field components, Hix and Hiy, have been measured, their effect can be compensated by generating opposite field components -Hix and -Hiy at the respective sensors, or by subtraction of the interference field components from the respective sensor output signals. Subtraction may be carried out at a microprocessor, for example.

[0031] If tilt compensation is done in bidirectional calibration, it may be applied prior to determining hard iron corrections.

[0032] Figure 4 shows an example of an apparatus according to an example embodiment of the invention. In this example, a magnetoresistive sensor 44 is mounted on a movable platform 42. The platform 42 has one or more steps of freedom. In the example of Fig. 4 the platform 42 is configured to rotate around the z-axis as shown by a vector of rotation 48. The platform 42 is mounted on a chassis 40 so that it can be moved or rotated. The chassis 40 may be part of a housing of a host device or a separate chassis inside the housing of the host device. A housing of a host device may substantially enclose the host device and provide structural support for the host device. A housing may be constructed of plastic, aluminum, glass or other durable and rigid materials. In some embodiments, the housing may be made of non- ferromagnetic materials. The platform may be constructed of materials similar to those used to construct the housing. A electromechanical device 46 is connected to the platform 42 for causing the platform 42 to move relative to the housing of the host device.

[0033] The electromechanical device 46 may be for example a piezoelectric crystal. The piezoelectric crystal produces a mechanical strain and/or stress when an electric field is applied to it. That is, the application of the electric field creates mechanical deformation in the crystal. When a piezoelectric crystal is placed in an electric field, or when charges are applied by external means to its faces, the crystal exhibits strain , that is, the dimensions of the crystal change, and if the direction of the applied electric field is reversed, the direction of the resulting strain is also reversed.

[0034] When a first electric field is applied to the piezoelectric crystal in Figure 4, the piezoelectric crystal stretches in the direction of the arrow 45. The stretching may be configured to cause the platform 42 to rotate counterclockwise. The rotation may correspond to 180°.

[0035] Automatic calibration using the bidirectional calibration method is explained in more detail with reference to the flow diagrams of Figures 5a - 5c.

[0036] Figure 5a shows an example procedure, where a first measurement of the magnetic field is performed prior to rotation of the platform 42 and a second measurement is performed when the platform 42 has been rotated by substantially 180°. At phase 510 outputs of the three-axis magnetoresistive sensor 10 are read by the microprocessor 16 and may be stored into a memory. After the first measurement has been performed, the platform 42 is rotated, for example by 180°, by applying an electric field to the piezoelectric crystal as shown in block 512. At phase 514 outputs of the three-axis magnetoresistive sensor 10 are again read by microprocessor 16 and may be stored into a memory. The interference magnetic field, or the interference field components Hix and Hiy, is then determined at phase 516 based on the first and second measurements as described above in connection with Fig. 3. In the description of Fig. 5, the electromechanical device 46 is discussed as a piezoelectric crystal but the invention is not limited thereto.

[0037] It is also possible to perform the first measurement when the platform 42 has been rotated and the second measurement after the platform 42 has returned to its initial position where no electric field is applied to the piezoelectric crystal, as illustrated in the process of Figure 5b. In this case, the electric field is first applied to the piezoelectric crystal to cause the platform 42 to rotate, for exampleby 180°, phase 520. The outputs of the three-axis magneto resistive sensor 10 are read at phase 522. Then, at phase 524 the electric field is removed from the piezoelectric crystal and the platform 42 returns to its initial position, for example inertially or guided by a suitable spring mechanism. The initial position may correspond to a rest position. At phase 526 outputs of the three-axis magnetoresistive sensor 10 are again read by the microprocessor 16. Finally, the interference magnetic field, or the interference field components Hix and Hiy, is determined at block 528 based on the first and second measurements. As can be seen, the difference between the process of Fig. 5a and the process of Fig. 5b is substantially the order or measurement, whereas the mathematic process underlying the interference magnetic field estimation is the same.

[0038] Alternatively, as shown in Figure 5c, the application of the first electric field may result a first movement, for example 90°, phase 530. When a second electric field, opposite to the first electric field, is applied to the piezoelectric crystal it causes the piezoelectric crystal to deform, thus resulting a second movement of the platform 42, phase 536. This second movement may also be 90°, and since the direction is substantially opposite to the first movement the net result is a movement of 180°. In this case, the first measurement on phase 532 is performed when the platform 42 has been rotated in a first direction in block 530 and the second measurement at phase 538 is performed when the platform 42 has been rotated in a second, opposing, direction in phase 536. In between the first measurement in phase 532 and rotation of the platform 42 to the second direction at phase 536, the electric field may be removed from the piezoelectric crystal and the platform 42 may momentarily return to its initial position, phase 534. As in previous examples, the interference magnetic field is determined in phase 539 based on the first and second measurements.

[0039] The implementations of the electromechanical device 46 are not limited to the piezoelectric crystal. It may be any suitable device that is capable to cause a movement or rotation of the platform 42. It may be for example a minituarized motor or engine. For example, many mobile phones have a vibrator comprised of a flywheel motor with a weight, causing the mobile phone to vibrate when the motor turns and the weight throws if off balance and creates a wobble. The vibrator's motor may be configured to rotate also the platform 42 when the vibrator is activated. This configuration has an advantage that the in embodiments where the compass is comprised in such a mobile phone, the compass may be calibrated every time the vibrator is activated, for example every time the phone receives a call. It is also possible that a mobile phone is configured to briefiy acticate the vibrator to cause movement and/or rotation of platform 42 responsive to a user of the mobile phone invoking an application that makes use of the compass.

[0040] A microprocessor 16 may be configured to control the calibration process by applying the electric field to the piezoelectric crystal, or other electromechanical device 46, for obtaining the required measurements. The instructions and required algorithms may be stored into a memory to which the microprocessor 16 has access. The microprocessor 16 receives measurements from the three-axis magnetoresistive sensor 10, in some embodiments via amplifiers 12 and ADCs 14, and optionally also from a tilt-sensor, and determines the interference magnetic field based on the received measurements. The calculated interference field components may be stored in the memory and subtracted from every reading. The instructions required to perform the described calibration process may be computer instructions and they may collectively form a computer program configured to cause a calibration process of the described type to be performed.

[0041] The calibration process may be performed for example every time navigation software in a device is started, or at a device start-up. After that, the calibration may be perfomed at some predetermined time intervals, for example hourly, daily or once every five minutes. It is also possible that a user commands, via a user interface, the microprocessor 16 to perform the calibration. Measurements from other sensors may also be exploited to decide when to perform the calibration. For example, if the host device has a GPS (General Positioning Sytem) or any other navigation satellite system receiver, it may be decided that the compass is calibrated every time the device has moved some pre-determined distance from the previous calibration point. Also an orientation sensor may be used to decide that the calibration is to be performed if the orientation of the device has been changed by pre-determined amount. [0042] In one example embodiment, a gyroscope output may be used to ensure that platform 42 rotates in relation to the housing. That is, if the host device is rotating with the same direction and with the same speed as platform 42, the calibration may fail because the platform 42 does not move sufficiently relative to the housing of the host device.

[0043] In another example embodiment, further piezoelectric crystals may be connected to the platform 42. With these additional piezoelectric crystals the platform 42 can be moved relative to x and y axes to compensate for tilt of the host device. The calculations for calibrating the compass may be simplified if the tilt is compensated by stabilizing the platform 42 to the horizontal position, that is tilt compensation may be omitted when calculating the heading. In this embodiment, the tilt sensor is mounted to the platform 42 in addition to the magnetic field sensor.

[0044] The embodiments of the invention are not limited to the bidirectional calibration with sensor movement of 180° but the platform 42 may be configured to rotate a whole circle or 360°. In this case a plurality of measurements may be performed.

[0045] Without in any way limiting the scope, interpretation, or application of the claims appearing below, a technical effect of one or more of the example embodiments disclosed herein is that the electronic compass can be automatically calibrated thus eliminating a need for manual operations by the user.

[0046] Embodiments of the present invention may be implemented in software, hardware, application logic or a combination of software, hardware and application logic. The software and/or application logic may reside on a memory, a microprocessor or a central processing unit, CPU. If desired, part of the software, application logic and/or hardware may reside on a memory, and part of the software, application logic and/or hardware may reside on a microprocessor or on a CPU of the host device. In an example embodiment, the application logic, software or an instruction set is maintained on any one of various conventional computer- readable media. In the context of this document, a "computer-readable medium" may be any media or means that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer, with one example of a computer described and depicted in Figure 1. A computer- readable medium may comprise a computer-readable storage medium that may be any media or means that can contain or store the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer.

[0047] If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above-described functions may be optional or may be combined.

[0048] Although various aspects of the invention are set out in the independent claims, other aspects of the invention comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims.

[0049] It is also noted herein that while the above describes example embodiments of the invention, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications which may be made without departing from the scope of the present invention as defined in the appended claims.

Claims

WHAT IS CLAIMED IS

1. An apparatus, comprising:

a housing;

a magnetic field sensor mounted on a movable platform; and

an electromechanical device arranged to move the platform relative to the housing for determining an interference magnetic field.

2. The apparatus as claimed in claim 1, wherein the electromechanical device is arranged to move the platform by rotating it within the housing.

3. The apparatus as claimed in claim 1 or 2, wherein the platform is arranged to be rotated around the vertical axis.

4. The apparatus as claimed in any preceding claim, wherein the platform is arranged to be rotated 180 degrees.

5. The apparatus as claimed in any preceding claim, wherein the magnetic field sensor is a magnetoresistive sensor.

6. The apparatus as claimed in any preceding claim, wherein the electromechanical device is a piezoelectric crystal.

7. The apparatus as claimed in any of claims 1 to 6, wherein the apparatus is an electronic compass.

8. A mobile communication device comprising the apparatus of any of claims 1 to 7.

9. A method, comprising:

causing a movable platform, upon which a magnetic field sensor is mounted, to move relative to a housing for determining an interference magnetic field.

10. The method as claimed in claim 9, wherein the platform is arranged to be rotated around a vertical axis.

1 1. The method as claimed in claim 9 or 10, wherein the platform is arranged to be rotated 180°.

12. The method as claimed in any of claims 9 to 1 1, wherein the platform is arranged to be rotated by applying an electric field to a piezoelectric crystal.

13. The method as claimed in any of claims 9 to 12, wherein a first measurement is performed when the platform is in a first position relative to the housing, and a second measurement is performed when the platform has been moved to a second position relative to the housing.

14. The method as claimed in any of claims 9 to 12, wherein a first measurement is performed when the platform has been moved to a second position relative to the housing, and a second measurement is performed when the platform has returned to the first position relative to the housing.

15. The method as claimed in any of claims 9 to 12, wherein a first measurement is performed when the platform has been moved to a first position, and a second measurement is performed when the platform has been moved to a second position, wherein neither of the first and the second position correspond to a rest position.

16. The method as claimed in claim 15, wherein the platform is moved to the first position by rotating it to a first direction, and the platform is moved to the second position by rotating it to a second direction. The method as claimed in claim 15 or 16, wherein the first position corresponds to a movement of 90° from an initial position and the second position corresponds to a movement of -90° from the initial position.

A non-transitory computer-readable medium that contains software program instructions, where execution of the software program instructions by at least one data processor results in performance of operations that comprise execution of the method of any one of claims 9 through 17.