GB2578868A

GB2578868A - A method of orientating the output of an accelerometer of a vehicle monitoring apparatus and vehicle monitoring apparatus

Info

Publication number: GB2578868A
Application number: GB1817235.3A
Authority: GB
Inventors: Wheeler Alex
Original assignee: Trak Global Solutions Ltd
Current assignee: Trak Global Solutions Ltd
Priority date: 2018-10-23
Filing date: 2018-10-23
Publication date: 2020-06-03
Also published as: GB201817235D0

Abstract

A method of orientating the output of an accelerometer Xacc, Yacc and Zacc and of a vehicle monitoring apparatus to a vehicle coordinate system Xv, Yv and Zv involves determining if the output of a gyroscope of the apparatus exceeds, or does not meet, a predetermined threshold for a predetermined period of time in order to determine if the apparatus is turning or travelling in a straight line over that period. Acceleration measured by the accelerometer over the period is then monitored to determine the rotation required to orientate the accelerometer output to the vehicle coordinate system on the assumption that the measured acceleration is all perpendicular or parallel to a forward direction of travel of the vehicle. The apparatus may be a smart phone and the gyroscope output may be a sum of an output from each axis. The accelerometer may be a three-axis accelerometer and the contribution due to gravity may be aligned with the z-axis. The gyroscope can be recalibrated when its output is substantially zero over a period of time.

Description

Intellectual Property Office Application No. GII1817235.3 RTM Date:5 April 2019 The following terms are registered trade marks and should be read as such wherever they occur in this document:

BLUETOOTH

Intellectual Property Office is an operating name of the Patent Office www.gov.uk /ipo

A METHOD OF ORIENTATING THE OUTPUT OF AN ACCELEROMETER OF A VEHICLE MONITORING APPARATUS AND VEHICLE MONITORING APPARATUS

Technical Field of the Invention

The present invention relates to a method of orientating the output of an accelerometer of vehicle monitoring apparatus and to vehicle monitoring apparatus arranged to carry out the method.

Background to the Invention

It is known to fit vehicles with apparatus to monitor vehicle use. Monitoring vehicle use can give an indication of a driver's behaviour and this may be useful in determining the insurance risk presented by a driver and thus in determining an insurance premium for the driver. Monitoring vehicle use can also indicate if a vehicle has been involved in a collision, and the nature of that collision.

Known monitoring apparatus comprises various sensors for detecting movement of a vehicle including a three axis accelerometer for detecting acceleration and a satellite navigation system receiver, such as a GPS receiver, for determining vehicle location, speed and heading. Some known monitoring apparatus is connected to vehicle outputs whereas other apparatus is entirely stand alone. In some instances a driver's smartphone is employed to monitor vehicle use.

Where vehicle monitoring apparatus is fitted to a vehicle other than by the vehicle manufacturer, or a smartphone is used to monitor use of a vehicle, it is not possible to control the orientation of the apparatus relative to the vehicle. As such, for the output of a three axis accelerometer to be useful it is necessary to orientate the output of the accelerometer so that acceleration data is provided in a standard format, ideally so that the axes along which acceleration is measured correspond to known axes of the vehicle.

One approach to this problem is to use satellite navigation data to determine the necessary orientation of accelerometer data. A problem with this, however, is that satellite navigation data is not always available and/or may be insufficiently accurate.

US9581615 discloses a method of correcting the orientation of an accelerometer of a vehicle monitoring apparatus without the need for GPS or positional sensors. The method involves first performing a vertical correction by determining the mean values of x, y, and z components of acceleration measured by the accelerometer over a whole trip and then determining the necessary rotation angles to correct the vertical orientation of the accelerometer, using the assumption that gravity imposes a constant vertical acceleration component throughout the journey.

The accelerometer data can then be corrected for the effect of gravity, the remaining correction required being for horizontal orientation. This is achieved by assuming that when the vehicle is travelling in a straight line the resultant of the gravity corrected accelerometer data arising from acceleration or deceleration of the vehicle will be constant and coincide with the direction of travel of the vehicle. The method thus involves recording the gravity corrected accelerometer data over periods where the resultant is almost constant, and then applying principal component analysis to determine the horizontal correction required.

There are, however, problems with this approach. The processing overhead in performing principal component analysis is significant. The method may incorrectly assume that a vehicle is travelling in a straight line when it is travelling around a curve of constant radius, which are commonly found on slip roads connecting to motorways or highways. The method is also not able to determine the direction of travel of a vehicle, which is addressed by determining the speed of the vehicle by using either data from the vehicle or GPS data with the attendant problems associated with having to provide vehicle data or obtain GPS data.

It is an object of embodiments of the present invention to address the problems discussed above.

Summary of the Invention

According to an aspect of the present invention there is provided a method of orientating the output of an accelerometer of a vehicle monitoring apparatus to a vehicle coordinate system comprising the steps of determining if the output of a gyroscope of the apparatus exceeds, or does not meet, a predetermined threshold for a predetermined period of time in order to determine if the apparatus is turning or travelling in a straight line over that period; and monitoring acceleration measured by the accelerometer over the period and determining the rotation required to orientate the accelerometer output to the vehicle coordinate system on the assumption that the measured acceleration is all perpendicular or parallel to a forward direction of travel of the vehicle.

According to a second aspect of the present invention there is provided apparatus for monitoring a vehicle comprising an accelerometer and a gyroscope and arranged to perform a method according to the first aspect of the invention.

Employing the output of a gyroscope to determine if the apparatus is turning or not enables the accelerometer output to be orientated to the vehicle axis without having to employ satellite navigation data and overcomes problems associated with using accelerometer data to try and determine if the apparatus is travelling in a straight line.

The gyroscope may be a multi-axis gyroscope, such as a three axis gyroscope, and the method may comprise the step of summing an output from each axis to produce the gyroscope output.

The accelerometer may be a three axis accelerometer in which case the method preferably comprises a prior step of rotating the accelerometer output to orientate the data so that one axis is substantially vertical, such that the rotation to be determined by monitoring acceleration measured by the accelerometer over the period is a horizontal rotation. This may comprise the step of calculating the mean output from each axis of the accelerometer over a period of time sufficient such that acceleration, braking and cornering forces can be assumed to average to substantially zero. The resultant mean force measured by the accelerometer can then be assumed to be due to gravity and the data rotated to that this coincides with a single axis, typically the z axis, of the accelerometer.

Acceleration measured by the accelerometer may be monitored over periods of time when the gyroscope output exceeds a predetermined threshold, indicating that the vehicle is turning. This tends to be preferred over monitoring acceleration when the vehicle is not turning, as in the latter case it is necessary to determine that the vehicle is moving rather than stationary. This can be achieved by observing noise on the accelerometer output.

The method may comprise the step of determining the rotation that maximises the accelerometer output in a chosen axis. When it is assumed that the vehicle is turning this axis should be perpendicular to the direction of straight line travel of the vehicle, typically denoted as the y axis.

This step may comprise calculating the accelerometer output in a chosen axis for a first rotation angle and repeating the calculation with first incremental increases to the angle until the angle is one increment or less away from a full rotation and selecting the angle for which the calculated value is maximum, thereby to determine the rotation angle. For example, a calculation may be made for a starting angle which is then incremented by 18 degrees so that 20 calculations are made. The size of the first increments may be chosen so that between 6 and 30 calculations are made, that is to say the increments could be between 36 degrees and 12 degrees.

The method may then comprise the further step of calculating the accelerometer output for second incremental steps around the selected angle, the second incremental steps being smaller than the first and all lying within a range of angles of the order of the first incremental step and selecting the angle for which the calculated value is maximum, thereby to determine the rotation angle. For example, calculations could be made for ten one degree increments either side of the chosen angle, resulting in a further 20 calculations being made. The size of the second increments may be chosen so that between 6 and 30 calculations are made.

The method may include a further step of calculating the accelerometer output for third incremental steps around the selected angle, the third incremental steps being smaller than the second and all lying within a range of angles of the order of the second incremental step and selecting the angle for which the calculated value is maximum.

Further such passes could be made to refine the result if desired.

This process allows a reasonably accurate value for the necessary rotation angle to be determined with a relatively small number of calculations, and thus imposes a relatively low overhead on the processing requirements of apparatus intended to perform the method.

The steps may be repeated over different periods of time to make multiple determinations of the required rotation. An average, such as mean, of the determined rotations may then be calculated to yield a more reliable result.

To check that the gyroscope is functioning correctly the method may include the step of checking that the output of the gyroscope is substantially zero during a period of time which exceeds a predetermined threshold during which the output of the accelerometer s substantially zero, ignoring the effect of gravity, and indicating that the gyroscope requires recalibration, or initiating recalibration of the gyroscope, when the output of the gyroscope is not substantially zero over the period. Where there is no accelerometer output over a period of time it can be assumed that the vehicle is stationary and as such the gyroscope output should also be zero. For a three axis accelerometer, in order to ignore the effect of gravity, this step is best performed on output data which has already been flattened, by a rotation to align one axis with the vertical.

The accelerometer output may be filtered to reduce noise, for example by calculating a moving average prior to other processing of its output.

Assumed acceleration and deceleration events may be monitored over a period of time and an average magnitude for each set of events calculated. Where the magnitude of assumed acceleration events is greater than that of assumed deceleration events the accelerometer data may be re-oriented such that the presumed acceleration events are treated as braking events and vice versa. This is on the recognition that the magnitude of deceleration events, largely caused by braking, is typically higher than that of acceleration events. This step thus enables a direction of forward travel of a vehicle, or the direction of turning of the vehicle, to be estimated or determined without having to rely on any additional input other than accelerometer and gyroscope data.

The apparatus may comprise a microprocessor which may be arranged to cause the apparatus to perform the method. The microprocessor may be programmable and the invention also extends software and/or firmware which cases the apparatus to carry out the method.

The accelerometer and gyroscope may be comprised in an inertial measurement unit.

The apparatus may also comprise any one of a housing, one or more connections to an external power supply, a rechargeable battery or capacitor, a memory, a satellite navigation receiver, a wireless data connection such as a cellular modem and a wired data connection.

The apparatus may be installed in a vehicle, typically a road vehicle such as a motorcar, or commercial vehicle.

The apparatus may be comprised in a smartphone or other personal electronic device.

Detailed Description of the Invention

In order that the invention may be more clearly understood embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a schematic of vehicle monitoring apparatus; Figure 2 illustrates coordinate systems before orientation; Figure 3 shows two vehicle monitoring apparatuses mounted in a motorcar; Figure 4 is a graph of sample acceleration data before and after a cleaning step; Figure 5 is a graph of recorded gyroscope data; Figure 6 is a graph of yaw axis stabilisation; and Figure 7 is a graph of accelerometer data for the two vehicle monitoring apparatuses after performing orientation.

Vehicle monitoring apparatus 1 is shown schematically in Figure 1.

The apparatus 1 comprises a three-axis accelerometer 2, which sends its acceleration measurements to a microprocessor 4. The apparatus 1 also includes a three axis gyroscope 3 which sends rotation measurements to the microprocessor 4. The microprocessor 4 is connected with a memory 5 (or other suitable storage means) and is appropriately programmed to perform the functions described herein. The microprocessor 4 is powered by the battery 6 of a vehicle in which it is installed, and is also provided with a back-up battery 7. The microprocessor 4 is arranged to remain in a low power (sleep) mode whilst the vehicle is not in operation, and is awakened by an ignition 8 or other sense circuit when the vehicle is operated. The microprocessor 4 is operable to connect to a cellular network via a cellular modem 9 to relay data to a remote server or computer. The microprocessor 4 receives satellite navigation data (such as GPS data) from a receiver 10 provided in the apparatus 1.

The various components of the apparatus 1 are provided in a housing 11 which is, in use, fitted to a convenient position in a vehicle, without regard to the relative orientation of the housing 11 and vehicle and appropriate electrical connections made to the vehicle's electrical system to provide power to the device and a trigger to indicate to the device that the vehicle is in use. No data connection is required to the vehicle.

In an alternative embodiment the apparatus 1 could comprise a smartphone or other personal electronic device comprising at least a three axis accelerometer 2 and gyroscope 3 in which case the apparatus 1 would be self-powered and may determine that it is present in a vehicle where monitoring is required by detection of a wireless signal produced by the vehicle or a transmitter (such as a low power Bluetooth beacon) mounted in the vehicle. In yet another embodiment components of the apparatus 1 could be distributed between a smartphone and a separate housing 11 fitted to a vehicle. For example the accelerometer 2 and gyroscope 4 could be mounted to the vehicle and communicate with a smartphone which provides wireless communication and satellite navigation functions.

Software and/or firmware running on the microprocessor 4 cause the microprocessor 4 to implement processes which orientate the output of the accelerometer 2 and gyroscope 3 to a vehicle coordinate system, so that different apparatus 1 mounted in different ways on the same or different vehicles will provide vehicle use data in substantially the same form. Operation of the processes is now described in more detail.

Coordinates and rotations The three axis accelerometer 2 measures proper acceleration so the acceleration due to gravity is always present in its output. The direction of the gravity vector is indicated in the accelerometer 2 output as an equivalent acceleration in the opposite direction along the same axis.

Typically, in use, the three axis accelerometer 2 is in some arbitrary orientation relative to the body of the vehicle to which the apparatus 1 is fitted.

There are two relevant coordinate systems, as shown in Figure 2, the accelerometer coordinate system and the vehicle coordinate system. The three accelerometer axes of the accelerometer coordinate system are denoted in the figure as x,,, and za." The three vehicle axes of the vehicle coordinate system are denoted as.x.", yr and z", xi, is parallel to the direction of travel of the vehicle in a straight line, yr is perpendicular to xi, and zr, and zi, is vertical.

It is advantageous to know acceleration information in terms of the vehicle coordinate system and to achieve this both accelerometer 2 and gyroscope 3 output data is passed through an algorithm which rotates collected data so that it is orientated in the vehicle coordinate system, irrespective of the relative orientation of the apparatus 1 and the vehicle.

A rotation matrix R for rotating a vector by a yaw, pitch and roll angle of a, /3 and y respectively is given by the product R = Rz(a)Ry(fl)Rx(Y) where 1 0 0 Rx(y) FO cos( y) -sin( y) 0 sin(y) cos( y) cos(fl) 0 sin(fl) Ry(13) = 0 1 0 -sin( ig) 0 cos(p) -sin( a) cos( a) 01 0 1 cos(a) Rz(a) = [sin(a) Output data in the accelerometer coordinate system can thus be orientated from the accelerometer axis to the vehicle axis by obtaining sufficiently accurate values of a, 13 and y, whereby applying the rotation matrix R to collected accelerometer 2 and gyroscope 3 data will rotate said data such that it can be interpreted as though the accelerometer 2 were aligned with the vehicle so that the accelerometer and vehicle coordinate systems are the same.

The apparatus 1 is arranged to automatically determine the values of a, 13 and y, and to apply the rotation matrix to accelerometer 2 and gyroscope 3 data, as well to check that the rotation remains correct.

Filtering Data In use, the accelerometer 2 and gyroscope 3 send measured acceleration and rotation data about the three accelerometer axes to the microprocessor 4 at 100Hz. This data is sampled at the rate of 10Hz, as such a sample rate provides sufficient data.

Alternatively the accelerometer 2 and gyroscope 3 could be arranged to provide data at the rate of 10Hz in which case no sampling step would be required.

The accelerometer data is relatively noisy. To reduce this noise the microprocessor 4 is arranged to filter the accelerometer data by a noise filtering step, 5 performed by applying an exponential moving average to the raw data according to x[i] =(T x x[i -1]) + (1-T x x[i]) where x[i] is the current reading, x[i -1] is the previous reading and T is a cleaning threshold.

Determining pitch and roll angles -vertical orientation compensation The filtered accelerometer data is sampled at a rate of 10Hz and processed by the microprocessor 4 to determine values for the pitch and roll angles /3 and y required to rotate the data so that the contribution due to gravity is aligned with the z axis of the accelerometer coordinate system, i.e. the z axes of the accelerometer and vehicle coordinate systems are aligned. This is done as follows: I5 1 Over a time period, for example of the order of a minute, sufficient for acceleration, braking and cornering to average to zero, the microprocessor 4 records filtered accelerometer data received from the accelerometer 2 in each of the X ace, Yacc and za" axes and calculates the mean of the recorded values in each respective axis to obtain < xa" >, < > and < za, >.

- -Yacc 2. It is assumed that in any single time period gravity is a constant downwards force relative to the vehicle.

3. For the time period the microprocessor 4 then inputs the averaged recorded values into z' = -< xci" > sin A' +< Z > cos [3 g =< ya" > sin y + z' cos y and solves to find the two pairs of values of)3 and y which maximise the value of gin the positive direction.

4. Due to the nature of the equations at 3 there are two theoretical answers that maximise the value of g in each instance. To account for this the calculations are repeated over multiple time periods to produce multiple pairs of angles of)3 and y. For each pair the microprocessor 4 calculates a variable s according to s = (fl x c) + g where c is a constant, and sorts the results by s to obtain the peak value, which corresponds to the correct true pitch and roll angles. Including the constant c introduces a bias towards one of the two solutions to the equations at step 3.

5. The microprocessor 4 then repeats steps 1 to 4 to produce multiple pitch and roll values. A mean of these vales is calculated, each successively calculated value being included in the calculation of the mean provided that it within a tolerance of the current mean value (or the initial value), such as say within 50 degrees of the initial or current mean value. This way, if the calculated values have tended towards the wrong solution to the equations at 3 they will; be excluded from the mean. In the unlikely event that the first pair of values are incorrect the mean will not converge to a result.

The thus determined mean values offl and y are then used to rotate the averaged accelerometer data using matrices Ry(P) and Rx(y), so that zo" corresponds to zo. For convenience the resulting data is referred to as flattened accelerometer data.

Checking gyroscope stability It is important to ensure that the gyroscope 3 is properly calibrated in order to determine an accurate value of the yaw angle a. This is achieved by the microprocessor 4 determining a time period, typically of the order of 10 seconds or so, in which no acceleration is detected by the accelerometer 3 when the effect of gravity is ignored, i.e. that xo" and ya" in the flattened accelerometer data are substantially zero, implying that the apparatus 1 and thus the vehicle are stationary or moving at a constant speed. The microprocessor 4 then checks that the average gyroscope output during this period is close Lo zero. If it is not, the gyroscope 3 is recalibrated before orientation can proceed.

Determining yaw angle -horizontal orientation compensation The yaw angle a is that by which the flattened accelerometer data must be rotated to align xo" and yo" with x, and yz, . The microprocessor 4 is arranged to determine a by using a combination of accelerometer and gyroscope data as follows: I. The microprocessor 4 calculates a rotational movement field Ao by summing each gyroscope axis reading.

2 It is assumed that significant rotation only occurs when the vehicle is cornering. The microprocessor 4 thus identifies periods when the magnitude of Ao is above a threshold, and it is assumed that these periods correspond to the vehicle turning left or right.

3. For each such period when the vehicle is turning the microprocessor 4 finds the value of a which maximises the magnitude of the flattened accelerometer output in the accelerometer y axis. This is achieved by calculating the cornering force a using a = xa" sin a + ya" cos a and varying a to find the value which maximises the magnitude of a.

To do this, a is initially calculated for a = 0 degrees and then recalculated with a ncreas ng in 18 degree increments until a = 342 degrees. a is then recalculated in one degree increments for ten degrees either side of the value of a yielding the highest value for a and the value of a selected which leads to the highest magnitude of a of all the values calculated. This yields a value for a accurate to one degree by calculating 40 different values of a.

4 The microprocessor 4 repeats steps 1 to 3 over multiple rotation events, and calculates a mean value for the yaw angle a.

The determined yaw angle is then used to rotate the flattened accelerometer and gyroscope data using the Rz(a) rotation matrix to produce re-oriented accelerometer and gyroscope data, based on the vehicle coordinate system.

Correction for left/right confusion As the rotational movement field Ao is determined using un-oriented gyroscope data the relationship between positive and negative movement (i.e. turning left or right) can be reversed in some orientations of the vehicle monitoring apparatus 1. To detect and correct this, the microprocessor 4 monitors braking and accelerating forces along the vehicle x axis throughout a journey, or combination of journeys, until sufficient acceleration and/or deceleration events have been recorded, in one example 20 acceleration and 20 deceleration events. The mean positive and negative accelerations are determined and an assumption is made that in a typical journey mean braking force will greater than mean accelerating force. This enables the direction of forward vehicle travel to be estimated and to enable the microprocessor 4 to re-orient the accelerometer data, if necessary, so that positive accelerations in the flattened and reoriented accelerometer x axis represents acceleration of the vehicle in a forward direction. Following completion of orientation of the accelerometer and gyroscope data the microprocessor 4 may store and/or transmit to a remote server or computer orientated accelerometer and/or gyroscope data or data derived from this data indicative of vehicle usage and this may, for example, be used to record driver behaviour and/or calculate insurance risk. Where the accelerometer 2 detects vehicle acceleration which exceeds predetermined thresholds the microprocessor 4 may generate a driver behaviour warning. This may be stored and/or transmitted to a remote server or computer and/or cause a warning message to be transmitted or displayed to a driver of the vehicle.

Diagnostics During determination of the transformation necessary to orientate the accelerometer and gyroscope data the microprocessor 4 sends a message to the server as each stage of the algorithm is completed, in particular determining pitch and roll angles, determining yaw angle and correcting for left/right confusion, so that the progress of the algorithm can be monitored. Diagnostic information such as accelerometer data and pitch, roll and yaw angles (where available) are also transmitted.

The microprocessor 4 also will notify the server if the algorithm fails at any stage, allowing for timely diagnosis of problems with hardware or software.

Maintenance Three maintenance stages are performed once orientation is complete.

The microprocessor 4 is arranged to periodically recalculate the pitch and roll angles necessary to flatten the data. If the recalculated values depart from the currently used values by more than a predetermined threshold the microprocessor will notify the server and restart the orientation process. The periods over which the pitch and roll angles are re-calculated in practice will depend on how accurate and consistent chosen apparatus tends to be. In an example pitch and roll angles are recalculated every 30 minutes when the vehicle is in use, and the newly calculated values are adopted if they depart from the currently used values by more than 10 degrees.

Likewise, the microprocessor 4 is also arranged to periodically recalculate the yaw angle necessary to align the accelerometer 2 and vehicle x axes. If the recalculated values depart from the currently used value by more than a predetermined threshold the microprocessor 4 will notify the server and restart the reorientation process. Again, the period over which the yaw angle is re-calculated in practice will depend on how accurate and consistent chosen apparatus tends to be. In an example the yaw angle is recalculated every 20th detected rotational event.

When the orientated accelerometer data is such as to trigger a driver behaviour warning the microprocessor 4 is arranged to compare the data with GPS (or other) data relating to the same event, to ensure its accuracy. If the two data sets are not sufficiently consistent the microprocessor 4 will notify the server and reorientation will be triggered.

Examples

The orientation algorithm was tested by mounting two separate vehicle monitoring apparatuses in different orientations inside a motorcar. The locations of the vehicle monitoring apparatuses are illustrated in Figure 3. A first vehicle monitoring apparatus 15 was mounted horizontally to a central console 17 and a second apparatus 16 mounted vertically to a headrest 18.

The accelerometer and gyroscope data for each apparatus 15, 16 were filtered using the filtering step described above. The results of this step are illustrated in Figure 4 which shows a single axis output from a single accelerometer on the vertical axis against time on the horizontal axis both before 41 and after 42 the filtering step. As can be seen from the figure, noise on the data is greatly reduced by the filtering step, and the resulting average acceleration line remains an accurate representation of variations in acceleration during a time period.

The pitch and roll angles were determined by each apparatus 15, 16, and the data was rotated to align with the vertical axis of the motorcar.

The gyroscope data was used to identify cornering events by looking for periods in which the gyroscope 3 reading exceeded a threshold value. The gyroscope data is shown in Figure 5 which shows the calculated rotation field measured by a single gyroscope 3 on the vertical axis against time on the horizontal axis. For each detected period of rotation (the period where the rotation field exceeds the positive 20 or negative 21 threshold for at least a predetermined time period), the yaw angle was determined as described above. A mean yaw angle was updated following each determination. Figure 6 shows the initially volatile yaw angle converging close to the true value after approximately five detected cornering events. In this test, a greater amount of braking force was detected than accelerating force, and so the left/right correction was not applied.

After the orientation algorithm had been performed, the data from the two vehicle monitoring apparatuses 15, 16 was compared. This is shown in Figure 7 which plots 71) reoriented cornering (i.e. accelerometer x-axis) data and 72) acceleration/deceleration data (i.e. accelerometer y-axis) data for each apparatus against time. As can be seen from the figure the data for the two apparatuses were consistent. The increased noise on the accelerometer data for the apparatus mounted to the motorcar headrest 16 is thought to be due to increased vibration of the mounting surface.

The described apparatus enables the efficient orientation of accelerometer data with a reduced processing overhead and/or greater accuracy than prior art arrangements. Use of a combination of accelerometer and gyroscope data enables an accurate determination of any yaw correction required. As described above this is conveniently achieved by determining when a vehicle is turning and then determining a yaw correction which maximises force in a y axis direction. Yaw correction could alternatively be achieved by determining when a vehicle is travelling in a straight line by looking for periods when the gyroscope output is zero, and then assuming that the all accelerative forces arise from acceleration and braking and determining a yaw correction which maximises force in an x axis direction.

The above embodiment embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMSA method of orientating the output of an accelerometer of a vehicle monitoring apparatus to a vehicle coordinate system comprising the steps of: determining if the output of a gyroscope of the apparatus exceeds, or does not meet, a predetermined threshold for a predetermined period of time in order to determine if the apparatus is turning or travelling in a straight line over that period; and monitoring acceleration measured by the accelerometer over the period and determining the rotation required to orientate the accelerometer output to the vehicle coordinate system on the assumption that the measured acceleration is all perpendicular or parallel to a forward direction of travel of the vehicle.
2. A method as claimed in any preceding claim wherein the gyroscope is a multi-axis gyroscope and the method comprises the step of summing an output from each axis to produce the gyroscope output.
3. A method as claimed in either claim 1 or 2 wherein the accelerometer is a three axis accelerometer and the method comprises a prior step of rotating the accelerometer output to orientate the data so that one axis is substantially vertical, such that the rotation determined by monitoring acceleration measured by the accelerometer over the period is a horizontal rotation.
A method as claimed in any preceding claim wherein acceleration measured by the accelerometer is monitored over periods of time when the gyroscope output exceeds a predetermined threshold, indicating that the vehicle is turning.
A method as claimed in any preceding claim comprising the step of determining the rotation that maximises the accelerometer output in a chosen axis.
6. A method as claimed in any preceding claim comprising calculating the accelerometer output in a chosen axis for a first rotation angle and repeating the calculation with first incremental increases to the angle until the angle is one increment or less away from a full rotation and selecting the angle for which the calculated value is maximum, thereby to determine the rotation angle.
7. A method as claimed in claim 6 comprising the step of calculating the accelerometer output for second incremental steps around the selected angle, the second incremental steps being smaller than the first and all lying within a range of angles of the order of the first incremental step and selecting the angle for which the calculated value is maximum, thereby to determine the rotation angle.
8. A method as claimed in any preceding claim comprising repeating the steps over different periods of time to make multiple determinations of the required rotation and calculating an average of the determined rotations.
9. A method as claimed in any preceding claim comprising the step of checking that the output of the gyroscope is substantially zero during a period of time which exceeds a predetermined threshold during which the output of the accelerometer is substantially zero, ignoring the effect of gravity, and indicating that the gyroscope requires recalibration or initiating recalibration of the gyroscope when the output of the gyroscope is not substantially zero over the period.
10. A method as claimed in any preceding claim comprising the step of calculating a moving average of the accelerometer output prior to other processing of the output.
11. A method as darned in any preceding claim comprising the step of monitoring assumed acceleration and deceleration events over a period of time, calculating an average magnitude for each set of events and where the magnitude of assumed acceleration event is greater than that of assumed deceleration events re-orienting the accelerometer data such that the presumed acceleration events are treated as braking events and vice versa.
12. Apparatus for monitoring a vehicle comprising an accelerometer and a gyroscope and arranged to perform a method according to any preceding claim.
13. Apparatus as claimed in claim 12 further comprising a microprocessor wherein the microprocessor s programmed to cause the apparatus to perform the method.
14. Apparatus as claimed in either claim 12 or 13 installed in a vehicle.
15. Apparatus as claimed in any of claims 12 to 15 wherein the apparatus comprises a smartphone or other personal electronic device.