FIELD OF THE INVENTION

The invention relates to calibration of a sensor. More particularly the invention relates, but is not limited, to in field recalibration of inertial sensors.

BACKGROUND TO THE INVENTION

Reference to background art herein is not to be construed as an admission that such art constitutes common general knowledge in Australia or elsewhere.

Inertial sensors are used in many applications to measure movement of objects. For example, vehicles, such aeroplanes and automated vehicles, and many electronic devices,. such as smart phones, have inertial sensors to determine orientation, movement, and/or other relevant variables.

Inertial sensors typically include gyroscopes, which measure the rate of change of angle, and accelerometers, which measure linear acceleration. Often such sensors are collectively packaged into an inertial measurement unit (IMU). A typical IMU will contain at least a three-axis accelerometer, and often includes one or more gyroscopes. IMUs sometimes also contain a 2 or 3 axis magnetometer for sensing the Earth's magnetic field (although not actually an inertial sensor).

Inertial sensing is often used to determine an 'attitude' of an object or a vehicle (i.e. the rotation of object or vehicle with respect to a reference frame, usually a theoretical perfectly level ground surface). In many applications, accurate inertia! sensing is critical. For example, in precision agriculture, knowledge of 'attitude' of a vehicle is required to compensate for movements of a Global Navigation Satellite Systems (GNSS) antenna through terrain level changes and undulation.

In machine control applications, such as autonomous vehicles, sensor precision is often high enough that an offset induced by the tilting of a GPS antenna mounted on a vehicle can produce a measurable positioning error (e.g. of the same order of magnitude as the GPS system itself). As a result, tilt angle is sometimes compensated with the use of angular estimates derived from sensor measurements produced by an IMU mounted in the vehicle.

For many inertial sensors, notably industrial grade inertial sensors often used in machine control applications, there are error characteristics, known as sensor bias, which change with temperature and age. These errors affect system accuracy and typically require the sensors to be sent back to the manufacturer for recalibration periodically (e.g. once per year). Such recalibration is costly and time consuming as it not only requires the device to be removed, but also requires the device to be returned to the manufacturer for a period of time, resulting in significant down-time.

Furthermore, even a yearly calibration can be insufficient in minimising bias as ambient temperature fluctuates over a year and, accordingly, temperature errors arise when the sensor is used in a different temperature range to what it was calibrated for. For example, if the sensor is calibrated in summer, the temperature errors will likely become prevalent in winter when the ambient temperature is lower.

If the user, does not send the device back to the manufacturer for factory calibration in an effort to avoid the costs and downtime then, in addition to the temperature error, age induced errors will also arise meaning that the device will lose accuracy over time.

One approach to assisting with keeping the sensors calibrated, particularly for temperature induced bias, is to add temperature sensing components and a sensor bias model to estimate the sensor bias at measured temperatures. However, this increases the cost and complexity of devices that use the sensors. Furthermore, calibration using such models often only includes temperature variation of the inertial senor over a limited temperature range. The model must also be updated as the inertial sensor ages to account for age induced bias. Updating the model is commonly done by yearly factory calibration or by calibration using additional sensors. These strategies add further cost and complexity to recalibrating the sensors.

OBJECT OF THE INVENTION

It is an aim of this invention to provide a method and system of calibrating a sensor which overcomes or ameliorates one or more of the disadvantages or problems described above, or which at least provides a useful alternative.

Other preferred objects of the present invention will become apparent from the following description. SUMMARY OF INVENTION

According to an aspect of the invention there is provided a method of determining an inertial sensor bias, the method including:

obtaining a first inertial sensor measurement at a first attitude;

obtaining a second inertial sensor measurement at a second attitude that is different to the first attitude;

obtaining a third inertial sensor measurement at a third attitude that is different to the first and second attitudes;

determining a first sphere of possible bias values for the first inertial sensor measurement, a second sphere of possible bias values for the second inertial sensor measurement, and a third sphere of possible bias values for the third inertial sensor measurement; and

determining the inertial sensor bias by calculating an intersection of the first/second, and third spheres of possible bias values.

The second attitude is preferably arbitrarily different to the first attitude and the third attitude is preferably arbitrarily different to the first and second attitudes.

Preferably the inertial sensor is an accelerometer or a gyroscope. Preferably the inertial sensor measurements are obtained from an inertial measurement unit (IMU) containing the accelerometer and/or gyroscope. Preferably the accelerometer is at least a three-axis accelerometer. Preferably the inertial sensor measurements are accelerometer measurements that consist of a measurement of gravity only. In an alternative form, the inertial sensor measurements are gyroscope measurements that consist of a measurement of the rotation rate of the Earth only.

The method may include measuring the change in inertial sensors between the measurements. A warning or prompt may be provided if the attitude calculated from the inertial sensor measurements is not changed by at least a predetermined amount.

Preferably determining an intersect of the first, second, and third spheres includes utilising linear algebra.

- Preferably the method includes moving a chassis that contains the sensors between each of the first, second, and third attitudes. Preferably the chassis is a vehicle chassis or an electronic component chassis. The sensors may be mounted on a movable component that can be moved, preferably automatically, between three different attitudes.

The method may further comprise obtaining one or more further inertial sensor measurements at one or more further different attitudes.

According to another aspect of the invention there is provided a method of calibrating an inertial sensor, the method including the steps of: determining a sensor bias according to the aforementioned method; and

calibrating the inertial sensor using the determined sensor bias.

According to another aspect of the invention there is provided a method of determining a location of a chassis, the method including the steps of: determining a sensor bias according to the aforementioned method; and

determining the location of the chassis using a global navigation satellite system (GNSS) component, the inertial sensor, and the determined sensor bias.

According to another aspect of the invention, there is provided a system configured to determine an inertial sensor bias, the system including: an inertial measurement unit (IMU); and

a computing resource in communication with the IMU and including a processor and memory;

wherein the memory of the computing resource is programmed to instruct the processor to:

obtain a first inertial sensor measurement from the IMU at a first attitude;

obtain a second inertial sensor measurement from the IMU at a second attitude that is different to the first attitude;

obtain a third inertial sensor measurement from the IMU at a third attitude that is different to the first attitude and the second attitude;

determining a first sphere of possible bias values for the first inertial sensor measurement, a second sphere of possible bias values for the second inertial sensor measurement, and a third sphere of possible bias values for the third inertial sensor measurement; and

determine the sensor bias by calculating an intersection of the first, second, and third spheres of possible bias values. According to another aspect of the invention there is provided a system of calibrating an inertial measurement unit (IMU), the system including: an IMU; and

a computing resource in communication with the IMU and including a processor and memory; wherein the IMU:

obtains a first inertial sensor measurement at a first attitude; obtains a second inertial sensor measurement at a second attitude that is different to the first attitude; and

obtains a third inertial sensor measurement at a third attitude that is different to the first attitude and the second attitude;

' and wherein the processor of the computing resource:

receives the first inertial sensor measurement, the second inertial sensor measurement, and third inertial sensor measurement from the IMU;

determines a first sphere of possible bias values for the first inertial sensor measurement, a second sphere of possible bias values for the second inertial sensor measurement, and a third sphere of possible bias values for the third inertial sensor measurement;

determines a sensor bias from the intersection of the first, second, and third spheres of possible bias values; and

calibrates the IMU using the determined sensor bias.

Preferably the computing resource is an embedded system. The computing resource may automatically determine when the attitude has been changed or, alternatively, the computing resource may provide a prompt adapted to receive an input from a user to confirm when the attitude has been changed. The prompt may be graphical on a display and may assist the user in determining change in attitude.

The IMU preferably includes a three-axis accelerometer. The IMU may further include one or more angular rate sensors and/or a 2 or 3 axis magnetometer. Preferably the system includes a chassis that contains the IMU and computing resource within.

The determination of the sensor bias may be determined according to the aforementioned method. _}

Further features and advantages of the present invention will become apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, preferred embodiments of the invention will be described more fully hereinafter with reference to the accompanying figures, wherein:

Figure 1 is a flow chart illustrating steps of a method according to the invention; and

Figure 2 is a flow chart illustrating sub-steps of step 150 of the flow chart in figure 1. DETAILED DESCRIPTION OF THE DRAWINGS

The invention generally relates to determining sensor bias for an inertial sensor, particularly an accelerometer. Inertial sensors have a bias that changes with temperature and time. Such inertial sensors are used in many applications including vehicles. Although the invention is primarily described with reference to vehicles, and even more particularly with reference to land vehicles, no limitation is meant thereby and the invention could be applied to other embodiments including, for example, in electronic devices such as electronic and electromechanical tools, mobile phones, consoles, game controllers, remote controls, etc.

Figure 1 illustrates a flow chart that has steps (100 to 150) that outline a method according to an embodiment of the invention. Afirst inertial sensor measurement (f* ¹ ) is obtained (step 100) by collecting and processing data from one or more sensors, typically in an IMU, at a first attitude. In a preferred embodiment the IMU will be part of a navigation system which includes a computing resource, typically including a processor and memory. At a point when the vehicle is stationary the sensor data is received and processed by the system.

When a vehicle or device containing an accelerometer is stationary, the total force (f*) acting on the accelerometer is due to gravity and, accordingly, if scale, misalignment, and noise are known or considered to be negligible, and the only significant error in measurement is sensor bias (b _a ), then the following constraint must be satisfied: \\ f ^b - \\ = g (1)

Where g is the magnitude of acceleration due to gravity. Accordingly, for multiple arbitrary positions there are multiple equations:

II ' ^* ' - b ₀ I = g (2) II ^{*2 ~} \\ = ^g (3)

(4) i f ^» · - \\ = g (5) Expanding equation (5) for the first inertial sensor measurement results in:

(f! ^l -b +(f? -b ₊ (f! ^> -b =g ^> < ⁶ >

Each gravity measurement in equations (2)-(5) forms a sphere of possible values for the bias and hence the first inertial sensor measurement (step 100) can provide a first sphere of possible values for the bias.

The attitude of the sensors is then changed (step 110). For a land vehicle, this may be achieved by driving to a different location on non-level ground such as a hill. Some vehicles may have equipment that can change the attitude such as, for example, an excavator standing up on its excavator bucket. For electronic devices, or the like, the attitude may be changed by resting the device on an angle different to the first measurement.

A second inertial sensor measurement is obtained at the second attitude (step 120): {f ^~b j Atf ^~b J ⁺ (f! ^{2 ~b} J (7)

Much like for the first inertial sensor measurement the second inertial sensor measurement provides a second sphere of possible values for the bias. The second sphere is different to the first sphere but overlaps. The intersection between the two spheres forms a circle of possible values for the bias.

The attitude of the sensors is then changed a second time (step 130) and a third inertial sensor measurement is obtained at the third attitude (step 140):

The third inertial measurement provides a third sphere of possible values for the bias. The third sphere is different to the first and second spheres but still overlaps and the intersection between the three spheres converges to a single physically possible point that represents the bias (b ₀ ). The point of convergence must therefore be determined to determine the sensor bias (step 150).

In determining the intersection of the three spheres, the three inertial sensor measurements are considered (step 152 of figure 2). Using equations (6), (7), and (8) a sphere of possible bias values for each measurement is determined (step 154) and all that is required is to determine the intersection of the spheres.

Subtracting equation (7) from equation (6) results in: ((Λ* ^ι ) ² -( ; ² ) ² )+2^^ -/;> )+

(( ^¾ ) ² -( ² ) ² )+ -/ *)+

((^) ² -(^ ² ) ² )+2^^ - ^ )= 0 (9) which is linear in

Subtracting equation (8) from (6) and (8) from (7) and rearranging into matrix form results in:

Equation (10) is solvable using standard linear algebra techniques and the bias is therefore determined. Equation (10) is preferably solved directly; for example, by performing a single matrix inversion and a single matrix multiplication (i.e. determining b=A ^"1 f where equation (10) is in the form of Ab=f)- While other methods may also be utilised they require more computations and are therefore less efficient.

More measurements can also be used in equation (10) by subtracting the relevant sensor measurement equations in the same manner as constructing equation (9) and solving using standard estimation techniques.

The sensor data is typically processed using signal processing to determine an estimate of the specific force at the relevant attitude. The estimate of the specific force includes signal processing to account for other factors such as, for example, removal of engine vibration (if the engine is running) or other disturbances. Although the described embodiment primarily relates to accelerometers and a measurement of gravity at three different attitudes, it will be appreciated that the same method could be applied to gyroscopes and a measurement of the rotation rate of the Earth at three different attitudes. Advantageously the method and system according to the present invention allows a sensor to be easily calibrated without the need to send the sensor, or equipment containing the sensor, to a third party or back to the manufacturer. The invention can easily be carried out in a vehicle by driving the vehicle to three different attitudes. This allows the sensors to be recalibrated at minimal cost and with minimal downtime to an operator. Additionally, the relative ease of recalibration means that the sensors can be recalibrated frequently ensuring that any sensor bias due to age or temperature is kept to a minimum, even due to seasonal changes, and the like, if desired. A further advantage of the present invention is that no temperature sensors, or other additional components, are required in order to try to estimate the sensor bias. This reduces costs and complexity of devices utilising the invention compared to those that use bias models, and the like, to estimate the bias. Furthermore, the present invention is typically more accurate than devices that use a bias model as the bias is actually measured and not merely assumed to match the bias model.

The method and system of the present invention are also independent of mounting orientation of the sensors, ensuring that the sensors can be mounted regardless of how they are located within a chassis (e.g. vehicle chassis). In the case of a vehicle application, the invention can also be carried out separately from the vehicle by removing the device with the sensors and arranging the device at different attitudes. Although this requires removal of the device which may be more time consuming than in-field calibration in the vehicle, this allows the sensors to be calibrated in situations where it may not be practical for the vehicle to move to three different attitudes (e.g. where the vehicle is on flat land). Furthermore, the cost and time savings relative to sending the device back to a third party or the manufacturer for calibration are still realised.

In this specification, adjectives such as first and second, left and right, top and bottom, and the like may be used solely to distinguish one element or action from another element or action without necessarily requiring or implying any actual such relationship or order. Where the context permits, reference to an integer or a component or step (or the like) is not to be interpreted as being limited to only one of that integer, component, or step, but rather could be one or more of that integer, component, or step etc.

The above description of various embodiments of the present invention is provided for purposes of description to one of ordinary skill in the related art. It is not intended to be exhaustive or to limit the invention to a single disclosed embodiment. As mentioned above, numerous alternatives and variations to the present invention will be apparent to those skilled in the art of the above teaching. Accordingly, while some alternative embodiments have been discussed specifically, other embodiments will be apparent or relatively easily developed by those of ordinary skill in the art. The invention is intended to embrace all alternatives, modifications, and variations of the present invention that have been discussed herein, and other embodiments that fall within the spirit and scope of the above described invention.

In this specification, the terms 'comprises', 'comprising', 'includes', 'including', or similar terms are intended to mean a non-exclusive inclusion, such that a method, system or apparatus that comprises a list of elements does not include those elements solely, but may well include other elements not listed.