WO2008068542A1

WO2008068542A1 - Auto-calibration method for sensors and auto-calibrating sensor arrangement

Info

Publication number: WO2008068542A1
Application number: PCT/IB2006/003456
Authority: WO
Inventors: Heikki Nieminen
Original assignee: Nokia Inc
Current assignee: Nokia Inc
Priority date: 2006-12-04
Filing date: 2006-12-04
Publication date: 2008-06-12
Anticipated expiration: 2009-06-04

Abstract

A method for automatically calibrating sensors in inertial sensor systems during use is presented. Measurement values of a sensor to be calibrated are recorded in at least two different sensor orientations, and the orientations held at the time of measurement are detected simultaneously by a further sensor, which is capable of auto-calibration without any external parameters. A rotation matrix is derived from said different sensor orientations and used together with the measured sensor values for solving an equation for determining calibration coefficients of the sensor, e.g. for compensating a device-inherent offset.

Description

Auto-calibration method for sensors and auto-calibrating sensor arrangement

Field of the invention This invention is related to the field of sensor technology. In particular, the invention pertains to devices having a combination of sensors for measuring orientation and position of the device, and to an automatic calibration method for such devices.

Related Art The field of sensor technology is very wide and has great importance in any area of daily life and industry. Sensors in general are employed to measure physical characteristics, such as chemical, mechanical, thermal, electromagnetic, or optical variables. Measured signals are then transmitted and processed electronically in most cases, but other types exist as well.

For determining a position and orientation of a device, inertial measurement systems may be used. This type of sensors is usually based on a combination of sensors which measure linear and angular accelerations applied to the device in an inertial reference frame. Typical inertial measurement systems may include several accelerometers and angular rate sensors (gyroscopes) to determine the device's acceleration with respect to different, preferably orthogonal, coordinate axes. The functional principles of various types of gyroscopes and accelerometers are well known in the art. Accuracy and reliability may be further increased by combining these sensors with further elements detecting external forces, such as a magnetometer sensing the position- and orientation-dependent earth magnetic field, a barometric system for altitude measurements, or a GPS (Global Positioning System) element. The specific choice of sensors will be dependent on the intended field of application of the sensor device.

Sensor arrangements of this kind may be utilized in various applications. For example, they are commonly used within inertial navigation systems for aircrafts, spacecrafts and other vehicles. Another field of use is orientation sensing for sports technology, such as human motion sensing, pace counters, training appliances for motion analysis and similar devices. For example, sensors could be implemented within a piece of sports equipment such as a racket or golf club, and motions conducted by a user may then be analysed by a corresponding analysis device to correct and instruct the user. Alternatively, such sensors could be integrated into a wearable device that may be attached to a wrist or leg of a user to track the exact motion the user conducts with this limb. Many further applications are conceivable.

In order to provide significant and accurate measurement results, sensors need to be calibrated. This may be done directly at the production site with suitable appliances and resources. However, several characteristics of a sensor may vary and/or deteriorate during its life time due to e.g. temperature, mechanical stress, outgassing, or similar effects. This will gradually reduce accuracy of the sensor, and in some cases even render measuring results essentially useless. As an example, an accelerometer shows a certain zero-g-offset, which describes the deviation of an actual output signal from the ideal output signal if there is no acceleration (= 0 g) present. This offset needs to be compensated by calibration of the measuring values. Since the zero-g factor may drift over the device lifetime, it may be necessary to periodically calibrate the sensor in order to maintain correct measurements.

There are also chip based implementations (MEMS, microelectromechanical systems) and other less expensive sensor solutions available. With decreasing size and manufacturing cost, the range of applications and devices that may benefit from such sensors would become even wider. However, this advantage is usually accompanied with lower sensor accuracy and stability over time, as for example the zero-g offset drift described above.

Summary

A method is presented for in-field calibration of a sensor. The method may in exemplary embodiments comprise measuring first sensor values while directed in a first orientation; rotating said first sensor from said first orientation to a second orientation; measuring second sensor values while directed in said second orientation; determining orientation parameters relating to said first and second orientations; and determining at least one calibration coefficient of said first sensor based on said measured first and second sensor values and/or said orientation parameters. In this way, a calibrated orientation sensor of a combined sensor device may be used to find calibration coefficients for further sensors of the device simply by using measured sensor values recorded in different directions.

The method may in exemplary embodiments comprise the calibrating of any further sensor values with said determined calibration coefficient.

For example, the orientation parameters relating to said first and second orientations may be determined using a second calibrated sensor. Optionally, these parameters may be determined using the first sensor (or both sensors combined).

In some embodiments, the first and second sensor values may be measured while the first sensor is stationary, i.e. essentially non-moving.

In exemplary embodiments, measuring of first and second sensor values may be initiated if said first sensor is stationary.

Measurements of said first and/or said second sensor may be employed in exemplary embodiments for determining whether said first sensor is stationary.

Measurements with said first sensor may, for instance, be performed continuously or at predetermined intervals.

The measured first and second sensor values may in some embodiments only then be used for calibration if said orientation parameters associated with said measured sensor values fulfil a predetermined condition.

Such a predetermined condition may for example be an angle of rotation, defined by said parameters, exceeding a predefined threshold. For example, it could be required that the angle of rotation between two calibration measurements has to exceed 20°, such that two sufficiently differing sensor results are obtained which might lead to a more precise calibration coefficient.

The orientation parameters may for example comprise a rotation matrix representing the rotational transformation between a unit vector of said first orientation and a unit vector of said second orientation. This is a convenient mathematical description for a rotational movement and can easily be implemented and applied by a processing unit such as a computer.

The determining of at least one calibration coefficient may in some embodiments comprise the solving of a predetermined set of equations for said at least one unknown calibration coefficient of said first sensor, wherein said measured first and second sensor values are variables of said set of equations. Optionally, the orientation parameters may also be variables of said set of equations.

In some embodiments, the determining of rotation parameters comprises detecting angular rate values during said measurement of first and second sensor values and during said rotational sensor motion from said first orientation into said second orientation; integrating said angular rate values over time to derive relative orientations of said first sensor at the time of said first and second measurements. This allows the use of an angular rate sensor as the second sensor, and an absolute orientation is not necessary to apply the exemplary method as described above; instead, the relative orientation between two directions derived from the integration of angular rate values is sufficient.

The first sensor to be calibrated may for example be a magnetometer or an accelerometer, or alternatively some other sensor capable of measuring parameters dependent on orientation. The second sensor may for example be a rate gyroscope. This is commonly used in combination with accelerometers and further sensors for inertial navigation/measurement systems. In exemplary embodiments, said measuring of first and second sensor values, determining of rotation parameters, and solving of said set of equations using the measured values may be repeated at least once, and the method may then further comprise averaging at least two calibration coefficients derived from said repeated measurements to obtain an averaged calibration coefficient. This may increase calibration accuracy without great expense, as only a few additional movements and calculations are necessary.

The method may in exemplary embodiments also comprise measuring at least one further sensor value in at least one further orientation; determining further rotation parameters for a rotation between said at least one further orientation and at least one of said first and second orientations; solving said predetermined set of equations also for said at least one further measured sensor value and said at least one further rotation parameter to derive at least one further calibration coefficient; averaging at least two derived calibration coefficient values to obtain an averaged calibration coefficient. This may similarly increase calibration accuracy and does not even require any further movement, but simply utilizes one or more additional measurements during said rotational movement of the device. Then, determined angles between the first and second, second and third, and first and third direction provide three calibration coefficients for averaging with only one additional measurement. With four measurements, already six calibration coefficients are available for averaging.

Said second sensor may in some embodiments be adapted to be calibrated using only its own measuring values. This enables an easy in-field auto-calibration of the combined sensor device, as the second sensor may be self-calibrated and may then be used to calibrate further sensors.

In some embodiments, the method may comprise storing at least one of the following parameters in a memory unit: a measured first or second sensor value, determined rotation parameters, a calculated calibration coefficient, a relative orientation angle, a time of a last calibration procedure.

In exemplary embodiments, the method may comprise initiating said calibration procedure by a user input signal. This allows a user to initiate a calibration when he considers it necessary.

In some embodiments, the method comprises transmitting a signal from said first and/or second sensor to an external device or vice versa.

Furthermore, a computer program product is provided comprising computer program code adapted to carry out any of the above method steps when executed in a processing unit.

Also, a device is provided which may comprise in exemplary embodiments: at least a first and a second sensor unit arranged in a fixed spatial relationship to each other, wherein said second sensor unit is capable of determining a relative orientation of said device; a processing unit connected to said sensor units, capable of processing measured sensor values and determining orientation parameters from measured values of said sensor units; wherein said processing unit is arranged to determine at least one unknown calibration coefficient of said first sensor, based on said measured first and second sensor values and said rotation parameters. Such a device may not only be a complete sensor device ready for use, but also a module integrated in or connected to some other device; a chipset used in a device or module, or any other element providing the above features. Some elements and features may also be provided apart from each other in different modules instead of in a single entity. All device elements/parts described may be various separate or combined components of a system.

In some embodiments, the device may include a communication interface connected to said processing unit for transmitting signals to an external device. The device may also include a memory unit connected to said processing unit in some embodiments. In said memory unit, for example a predetermined set of equations may be stored, which may be solved for determining the calibration coefficient(s).

In exemplary embodiments the first sensor unit may be an accelerometer, or a magnetometer. The second sensor unit may for example be a gyroscope.

Generally, a device is provided that may comprise in exemplary embodiments: means for measuring first sensor values while directed in a first orientation; means for measuring second sensor values while directed in a second orientation; means for determining rotation parameters of a rotation between said first and second orientations using a calibrated second sensor; means for solving a predetermined set of equations for at least one unknown calibration coefficient of said first sensor, wherein said measured first and second sensor values and said rotation parameters are variables of said set of equations; and means for calibrating further sensor values with said determined calibration coefficient.

It shall be understood that the terms used here are not intended to be limiting. A second sensor may be any calibrated and/or self-calibrating sensor of a device that may be used to calibrate other sensors. There is no preference as to the order in which first and second sensor values are measured by the first sensor (which may also represent any arbitrary sensor to be calibrated). Further values may be measured by any sensor between the detection of said mentioned first and second sensor values.

Brief description of figures

In the following, exemplary embodiments of the invention will be described in more detail with reference to the appended figures, wherein

Fig. Ia shows a two-dimensional vector diagram of measured acceleration values in a sensor- fixed coordinate system;

Fig. Ib illustrates the associated rotation of a sensor device for calibration; Fig. 2 illustrates an intersection of planes representing the linear equations to solve; Fig. 3 shows example data plots from an angular rate sensor (Fig. 3 a) and an acceleration sensor (Fig. 3b) from an auto-calibration procedure;

Fig. 4 illustrates exemplary inventive method steps for automatic calibration; and Fig. 5 shows the schematic structure of an inventive device embodiment.

Detailed description of exemplary embodiments

An exemplary embodiment of an auto-calibrating sensor device may contain at least two different sensors, wherein a first one of them should be capable of being calibrated without any external measurements or values, only by means of its own sensor values. This is the case for a rate gyroscope as will be described below, for example. Variables of a second and any potential further sensor may then be calibrated using the calibrated measurements of the first sensor. The terms "first sensor" and "second sensor" are only intended to distinguish certain sensors without attributing any specific features to the respective sensors. Any of the various sensors of a device may be the self-calibrating/previously calibrated second sensor used to calibrate other sensors.

As an example, a sensing device is examined that includes at least a three-dimensional (3D) angular rate sensor or gyroscope, and a 3D acceleration sensor (accelerometer). Nonetheless, other sensors for detecting orientation dependent parameters could be used together with the gyroscope for auto-calibration as well. For example, an angular rate sensor may be used for automatic calibration of an associated magnetometer in a similar way.

In-field calibration of gyroscope elements can easily be achieved, since the effective angular rate is obviously equal to zero when the device is held in a steady position without any movement. Therefore, no external parameters are necessary for calibrating the 3D gyroscope. For the accelerometer, zero-g offset is one of the values to be calibrated. Measured acceleration values need to be corrected by a calibration coefficient compensating this offset. This is only possible if the orientation is known, since the accelerometer will always detect the resulting vector gravity acceleration value of the earth's gravity field. Thus, the effective measured acceleration when the device itself is not dynamically accelerated is the vector sum of the gravity vector and the zero-g offset vector.

Figure Ia shows a vector diagram in two dimensions with measured acceleration sensor vectors ai, E₂, corresponding to the first and second sensor values mentioned above. By obtaining values from three (orthogonal) accelerometers, magnitude and orientation of the acceleration vectors are determined. The sensor values form a circle when the sensor is slowly rotated within the earth's gravity field (vectors g_l5 g₂), since the relative orientation of the gravity vector gj in relation to the apparatus-fixed coordinate system varies during the rotation. The centre of this circle is the value of the O-g-offset. ai is measured in a first orientation, while a₂ is measured in a second direction. A person skilled in the art will easily transfer these illustrations to three dimensions, where the measured sensor values form a sphere centred on the 0-g offset vector.

In Fig. Ib, a rotation conducted for the calibration procedure discussed in this example is illustrated schematically. The coordinate system is a local device-fixed coordinate system, thus rotating with the device 2 (represented by the cuboid) with respect to a horizontal earth coordinate system. For the sake of simplicity, the device rotation is shown around the z-axis of the coordinate system with a rotation angle of about 120°, but any other rotation axis (not necessarily a coordinate axis) and angle would be equally suitable for calibration. As can be derived from the figure, the initial position of the device may be arbitrary and does not need to be aligned with regard to the horizontal plane or any other reference. The gravity vector g has a constant orientation with regard to the earth horizontal coordinate system and thus changes orientation with regard to the device-fixed coordinate system.

For the calculations below, it is assumed that the zero-g offset as well as the scale factors of the accelerometer are unknown, and that the scale factors are unknown and the same for all coordinate axes.

The accelerometer zero-g offset to be determined is

V = Iv y_off ^zojf \- (X)

With the gravity vector

&i = x_g + y_g + z₈ > (2)

we can now formulate equations that describe the measured acceleration values

and ^a ₂ = [*₂ y₂ z₂] = a_0// +g₂ = a_0# +Rg₁. (4)

The rotational movement of the sensors between the measurements of a_t and a₂ is mathematically described by a rotation matrix R which transforms the vector of gi into g₂. As the zero-g offset orientation is constant in relation to the device-fixed coordinate system, offset vector a₀^ is not rotated, as can be seen from equation (4).

A person skilled in the art will be aware that equivalent calculations can be made by rotating between several coordinate systems instead of rotating gravity vectors, which would lead to a rotation matrix that is the inverse matrix of the above rotation matrix R; or alternatively e.g. by using a earth fixed coordinate system, where magnitude and direction of the gravity acceleration vector would both remain constant for each rotated position of the device, while the offset vector would be rotated by means of an appropriate rotation matrix. These transformations between coordinate systems are known in the art and not further discussed here. The calculations given in this description are intended to illustrate the principle of the invention by way of example only.

The rotation matrix of equation (4),

n _ R_2x R₂₂ i?₂₃ (5) i? ^V3,,1 i? ^■3,₀2 R ^V33

needs to be determined to obtain the correct zero-g offset for calibration. This matrix may be derived from measurement values of the gyroscope, which gives angular rate values. By integrating the angular rate over time, the orientation angle with respect to a fixed orientation or a specified initial orientation may be determined. When measuring the angular rate at two ore more different positions by slowly rotating the device, a relative angle between those positions may thus be derived. At the same time, the values (a_l5 a₂) of the sensor to be calibrated are taken at the same positions. Thus, the exact relative orientations of the device are known from the gyroscope measurement for each measuring value of the sensor to be calibrated. Then the rotation matrix for transformation between these two orientation vectors (or alternatively between two rotated and otherwise congruent coordinate systems) is unambiguously defined. Instead of rate gyroscopes as used in this example, any method of obtaining calibrated orientation information during the sensor measurements may be utilized.

If more than two positions are used for calibration measurements, even more transformations can be considered, deriving a rotation matrix for any combination or all combinations of two different orientation vectors for transformation from one vector to the other. The resultant zero-g offset values may then be averaged to determine the offset with increased accuracy.

Back to the vector equations (3) and (4) for determining the offset vector, using eq. (1) and (2) these can also be written in the form x_λ = x_off + x_g , (6)

yι = y_off +y_g , (?)

z_λ = z_off + z_g, (8)

x₂ = x_off + R_ux_g + R_ny_g + R₁₃Z _g , (9)

y₂ = y_off + R_2ϊx_g + R₂₂y_g + R₂₃z_g , (10)

and

z₂ = z_off +R₃₁x_g + R₃₂y_g +R₃₃z_g . (11)

With these 6 linear equations and 6 unknown variables, the gravity vector components can be eliminated, leading to the following equations:

(l-R_u)x_off - Rχ₂y_off -Rιs^zo_ff "fa -^Rn^xi ^{~ R}nyχ -RχιZχ) = 4*<_# +4JV +4,V -A >(¹²)

- Runoff + (l - ^22 )y₀ff - R₂S²OJT ~ b>2 ~ ^R2\^X\ ~ ^R2₂yi - ^R*Zχ ) = ^B2^Xoff + B^ off + ^BA^Zoff ~ B₁ , (13)

and

- R_3Xx_off - R₃₂y_off + (l - R₃₃)z_off - (z₂ - R₃₁X₁ - R₃₂y_λ - R₃₃Z₁)= C₂X₀₁ + C₃y_off + C,z_off - C₁ .(14) A, B and C are used here for simplifying the above expressions mathematically.

As known, each one of these three equations (12 to 14) forms a plane in graphical representation, and the intersect point of these planes corresponds to the desired solution for the zero-g offset. This plane intersection is shown in Fig. 2. The data used for the plot originates from sensor measurements gathered with a wearable sensor placed on a user's wrist. Tests have shown that e.g. golf swing motions analysed in such a way provide convenient data for calibration.

The mathematical solution for the intersection point is thus z - (AB^ - ABiIB₂C, - B₃C^₊ (B₁C₂ - B₂C₁XA₂B₃ - A₃B₂) °^ff (A₂B₄ - A₄B₂XB₂C₃ - B₃C₂)+ (B₄C₂ -B₂C₄XA₂B₃ - A₃B₂) '

and

As will be appreciated by a person skilled in the art, equations (3) and (4) above could also be solved numerically. By defining

Z₁ = E₀^ ₊ Rg₁. (18)

a solution may be found by minimizing the following expression

/_ = (^χ ₂ - Mxo_ffif + {y₂ - Λ(y_off)f + fc - f^_off)} ■ (19)

There are various procedures for numerical minimization that are well-known in the art and can easily be implemented using a processing unit or software module in the sensor device. Fig. 3 shows exemplary data plots for gyroscope (Fig. 3a) and accelerometer (Fig. 3b) measurements. Such data may for example be obtained using a wearable sensor device, including a 3D accelerometer, 3D gyroscope and 3D magnetometer. From the gathered data the zero-g offset may then be calculated as described above. Subsequently, calculation accuracy may be tested by adding the calculated offset to all acceleration sensor axes, yielding the exemplary test results as presented in Table 1 below. A zero-g offset accuracy of less than 4 mg was achieved in such calibration tests, which, basically improves stability to the resolution limit.

Table 1 - offset calculation accuracy with live data.

After the offset has been determined as described above, the unknown scale factor of the accelerometer can be calibrated as well. From equation (3), gi can be derived directly. The magnitude of gi is known to be Ig, which means that the following condition has to be satisfied scale _ factor ■ Jg₁1 = scale _ factor ^■ -Jg₁ (x)² + g_λ (y)² + g_i (zf = 9.81 W₂ 20

As mentioned before, it is assumed for this calculation that the scale factor is the same for all three coordinate axes, which holds very well in practical applications.

Method steps of the exemplary offset calibration procedure are illustrated in Fig. 4. After initiation of the calibration procedure (step 100), the orientation sensor (i.e. the gyroscope in this example) is calibrated in step 102. Preferably this sensor is capable of being calibrated without any external parameters to avoid additional errors. Then, first sensor values are measured from the sensor to be calibrated in step 104. At the same time, the orientation during this measurement is determined as an initial orientation (step 106). Using the gyroscope, the first and second orientations may be determined by recording the angular rate during a movement from the first to the second orientation, that is between step 106 and step 110, and then integrating these angular rate values over the time period of recording. However, in this example it is shown generally that any way of determining the respective (relative) orientations that were held by the device during the measurements of steps 104 and 108 is sufficient.

Then in step 112, the first and second orientation value (or more generally, the determined relative orientation between these values) is used to derive a rotation matrix. The person skilled in the art will recognize that the rotation matrix is unambiguously defined by the origin and rotation angle of a movement. When unit vectors of the orientations are used, the origin of orientation is zero. Using this rotation matrix, now a predetermined equation (see calculations above) is solved in step 114, inserting the measured values of steps 104 and 108 for B₁ and a₂. The solution of this equation gives the desired sensor offset or calibration coefficient which may then be used to calibrate any further measurements of this sensor.

As mentioned above, the inventive idea as described in this example may easily be translated to other sensors, such as a magnetometer. Instead of the earth gravity vector with fixed direction and magnitude, a magnetic field vector of the earth magnetic field may be used for calibration, as long as a relative orientation between at least two measurements is known from e.g. the gyroscope. In general, any sensor responsive to a present direction dependent stationary force field (such as the earth's gravity field or magnetic field) may thus benefit from the invention.

In the example described above, both the scale factor and the offset of the accelerometer were unknown. If the scale factor of a sensor is known and also the magnitude of a constant field detected by the sensor to be calibrated, such as the magnitude of the gravity vector for an accelerometer or the magnitude of the constant magnetic field for a magnetometer, is also known, another exemplary embodiment is conceivable. In that case, values of the (first) sensor to be calibrated are measured again in two different orientations, and while the device is not moving. Such a stationary or non-moving position is determined by means of the second already calibrated sensor, for instance an angular rate sensor as above. The determination that the first and second measurements are performed in different directions may be done using the first or the second sensor, or both of them in combination. Then, by minimizing predetermined equations using these first and second sensor values from the sensor to be calibrated, the offset value may be derived. This eliminates the need for the determination of rotational parameters such as the rotation angle between measurements.

Referring again to equations (3) and (4) and Figure Ia, all possible sensor values form a sphere when assuming stationary (non-moving) measurements. When scale factor and e.g. magnitude of the gravity vector are known, the unknown offset may be solved by forming the following equations

Z₁ = (X₁ - X₀)² + (y, - j/₀)² + (z_x - zj and

/₂ = (x₂ - X₀)² + (y₂ - Jo)^{2 +} (^Z2 ^{~ z}of •

As the radius of the sphere is the known gravity vector magnitude (including the scale factor), minimizing the following equation as a function of the offset

/ = (y; - 9.8i²)² + (/₂ -9.8i²)² leads to the desired offset values (x_o,y_o,z^).

Using more than two orientations for measurements, it is easily seen that the equation to be minimized may be written in generalized form: / = (y; -9.81²)² + (/₂ -9.81²)² + - + (Λ -9.81²)².

Fig. 5 shows an example of an auto-calibrating sensor device 2. The device 2 may in exemplary embodiments comprise several sensors 10-18, whereof at least two sensors 10, 12 are of different sensor types. Three-dimensional sensor measurements may be achieved by combining three or more linear sensors in a suitable geometric arrangement, preferably in orthogonal relation to each other. An analog-digital converter 20 may be provided to convert the sensor raw signals to processible data, which may then be conveyed to a processing unit 22. The processing unit may be reserved exclusively for sensor data processing, or alternatively serve other purposes, particularly in a multifunctional device including the inertial sensor system. Memory elements 24, 26 may be provided in connection with the processing unit, such as RAM 26 and/or ROM 24 units. In these memory elements 24, 26, sensor data may be stored as well as e.g. program code that may be executed for data processing, calibration and other features. Removable memory elements or slots for such elements (such as a USB port, memory card slot, etc.) could be provided to allow a user to easily store data related to the device and measurements and to exchange such data between different devices. A power management unit 28 is included in the device to provide sufficient power to all device components. The power may be supplied via an internal source such as a battery or by a power line (not shown). Further elements may be present on such a device, as for example keypads, control wheels or buttons for user input, display elements to give system information or for displaying measured data, sound elements for any kind of audible signals to a user, and many more.

Furthermore, a communication interface 30 may be included, connected to the processing unit, which allows for transmission of the measured sensor values or any further values derived from those measurements to an external device. The interface 30 could for example be a wireless interface, such as Bluetooth, WiFi, radio, or infrared interface; or also a wired interface using an optical or electrical data line. In this way, sensor data may be transmitted to a desktop computer or handheld device for further evaluation and processing, or for storing data values. Also, some of the device units described above may be located at a remote device, such as a handheld device, to facilitate a lightweight and small sensor device. Any signal transfer between the units as described may then alternatively be performed via the communication interface.

The calibration sequences may be started automatically by the device when suitable movements are detected, or could also be initiated by a user. In some exemplary embodiments, measurements by the sensor to be calibrated may be performed in two different directions if data from the second (orientation) sensor shows that the directional/angular difference between those two directions is large enough to ensure accurate calibration measurements. Alternatively, the sensor to be calibrated may be able to detect such a directional difference itself. In other cases, a measurement for calibration may be initiated when it is detected that the device is essentially motionless. This may be determined using the first or second sensor or a combination of both (optionally also any other sensors included in the device, such as a magnetometer).

Instead of only initiating calibration measurements in response to certain conditions as mentioned above, some or all sensors of the device may take continuous measurements or measurements at certain predefined intervals. Then, it could subsequently be determined whether some of the recorded measurements are suitable for calibration. Criteria for suitable measurements may vary.

Commands for initiating a calibration sequence may be given by some user input directly at the device. Alternatively, a calibration may be initiated via the communication interface described above. The device could be adapted to prompt the user to initiate a calibration at predetermined intervals, such as every other week. When a calibration sequence is started, instructions may optionally be given to a user to ensure a correct execution of the calibration procedure. Instructions may for example include a text displayed on a screen, telling the user to keep the device still for a certain time and then to perform a slow rotation around a device axis, as described in the vector example above of Fig. 1.

Although exemplary embodiments of the present invention have been described, these should not be construed to limit the scope of the appended claims. Those skilled in the art will understand that various modifications may be made to the described embodiments and that numerous other configurations or combinations of any of the embodiments are capable of achieving this same result. Moreover, to those skilled in the various arts, the invention itself will suggest solutions to other tasks and adaptations for other applications. It is the applicant's intention to cover by claims all such uses of the invention and those changes and modifications which could be made to the embodiments of the invention herein chosen for the purpose of disclosure without departing from the spirit and scope of the invention.

Claims

1. A method for calibration of a first sensor, comprising measuring first sensor values while directed in a first orientation; rotating said first sensor from said first orientation to a second orientation; measuring second sensor values while directed in said second orientation; determining orientation parameters relating to said first and second orientations; and determining at least one calibration coefficient of said first sensor based on said measured first and second sensor values and said orientation parameters.

2. The method of claim 1 , further comprising calibrating any further sensor values with said determined calibration coefficient.

3. The method of any previous claim, wherein said orientation parameters relating to said first and second orientations are determined using a second calibrated sensor.

4. The method of any previous claim, wherein said orientation parameters relating to said first and second orientations are determined using said first sensor.

5. The method of any previous claim, wherein said first and second sensor values are measured while said first sensor is stationary.

6. The method of any previous claim, wherein measuring of said first and second sensor values is initiated if said first sensor is stationary.

7. The method of claim 5 or 6, wherein measurements of said first and/or said second sensor are employed for determining whether said first sensor is stationary.

8. The method of any previous claim, wherein measurements with said first sensor are performed continuously.

9. The method of any of claims 1 to 7, wherein measurements with said first sensor are performed at predetermined intervals.

10. The method of any previous claim, wherein said measured first and second sensor values are only used for calibration if said orientation parameters associated with said measured sensor values fulfil a predetermined condition.

11. The method of claim 9, wherein said predetermined condition comprises an angle of rotation that is defined by said parameters exceeding a predefined threshold.

12. The method of any previous claim, wherein said orientation parameters comprise a rotation matrix representing the rotational transformation between a unit vector of said first orientation and a unit vector of said second orientation.

13. The method of any previous claim, wherein said determining of at least one calibration coefficient comprises solving a predetermined set of equations for said at least one unknown calibration coefficient of said first sensor, wherein said measured first and second sensor values are variables of said set of equations.

14. The method of any previous claim, wherein said determining of at least one calibration coefficient comprises solving a predetermined set of equations for said at least one unknown calibration coefficient of said first sensor, wherein said measured first and second sensor values and said orientation parameters are variables of said set of equations.

15. The method of any previous claim, wherein said determining of orientation parameters comprises detecting angular rate values during said measurement of first and second sensor values and during said rotational sensor motion from said first orientation into said second orientation; integrating said angular rate values over time to derive relative orientations of said first sensor at the time of said first and second measurements.

16. The method of any previous claim, wherein said first sensor is an accelerometer.

17. The method of any previous claim, wherein said first sensor is a magnetometer.

18. The method of any previous claim, wherein said calibrated second sensor is a rate gyroscope.

19. The method of any previous claim, wherein said measuring of first and second sensor values, determining of orientation parameters, and determining of said at least one calibration coefficient using the measured values is repeated at least once, the method further comprising averaging at least two calibration coefficients derived from said repeated measurements to obtain an averaged calibration coefficient.

20. The method of any previous claim, further comprising measuring at least one further sensor value in at least one further orientation; determining further orientation parameters relating to said at least one further orientation and at least one of said first and second orientations; determining at least one further calibration coefficient in dependence of said at least one further measured sensor value and said at least one further orientation parameter to derive at least one further calibration coefficient; averaging at least two derived calibration coefficient values to obtain an averaged calibration coefficient.

21. The method of any previous claim, wherein said second sensor is adapted to be calibrated using only its own measuring values.

22. The method of any previous claim, further comprising storing at least one of the following parameters in a memory unit: a measured first or second sensor value, determined orientation parameters, a calculated calibration coefficient, a relative orientation angle, a time of a last calibration procedure.

23. The method of any previous claim, further comprising initiating said calibration procedure by a user input signal.

24. The method of any previous claim, further comprising transmitting a signal from said first and/or second sensor to an external device.

25. A computer program product comprising computer program code adapted to carry out any of the method steps of any previous claim when executed in a processing unit.

26. A device comprising at least a first and a second sensor unit arranged in a fixed spatial relationship to each other, wherein said second sensor unit is capable of determining a relative orientation of said device; a processing unit connected to said sensor units, said processing unit being capable of processing measured sensor values and determining orientation parameters relating to the orientation of said device from measured values of said sensor units; wherein said processing unit is arranged to determine at least one calibration coefficient of said first sensor based on said measured first and second sensor values.

27. The device of claim 26, wherein said processing unit is further arranged to calibrate any further measured values of said first sensor with said determined calibration coefficient.

28. The device of claim 26 or 27, further comprising a communication interface connected to said processing unit for transmitting signals to an external device.

29. The device of any of claims 26 to 28, further comprising a memory unit connected to said processing unit.

30. The device of claim 29, wherein said predetermined set of equations is stored in said memory unit.

31. The device of any of claims 26 to 30, wherein said first sensor unit is an accelerometer.

32. The device of any of claims 26 to 31 , wherein said first sensor unit is a magnetometer.

33. The device of any of claims 26 to 32, wherein said second sensor unit is a gyroscope.

34. A chipset comprising at least a first and a second sensor unit arranged in a fixed spatial relationship to each other, wherein said second sensor unit is capable of determining a relative orientation of said chipset; a processing unit connected to said sensor units, said processing unit being capable of processing measured sensor values and determining rotation parameters from measured values of said sensor units; wherein said processing unit is arranged to determine at least one calibration coefficient of said first sensor based on said measured first and second sensor values; and wherein said processing unit is further arranged to calibrate any further measured values of said first sensor with said determined calibration coefficient.

35. A device comprising means for measuring first sensor values while directed in a first orientation; means for measuring second sensor values while directed in a second orientation; means for determining orientation parameters relating to said first and second orientations; and means for determining at least one calibration coefficient of said first sensor, in dependence of said measured first and second sensor values and said orientation parameters.

36. The device of claim 35, further comprising means for calibrating further sensor values with said determined calibration coefficient.

37. The device of claim 35 or 36, further comprising means for determining whether said first and second sensor values are suitable for calibration.