In Situ Wheel Position Measurement Using Inertial Measurement Units (IMUs)

Abstract

Disclosed herein are systems and methods for the in-situ determination of vehicle wheel position using an inertial measurement unit (IMU). In one aspect as the wheel is rotating, gyroscope measurements are used to find a slip angle defined between the direction of wheel travel and the direction of vehicle travel, to determine a toe alignment condition for the wheel. System and methods are also presented for using an accelerometer to measure slip angle and camber angle. Using an accelerometer or gyroscope, instantaneous wheel angle measurements can also be made to predict vehicle movement, and aid in autonomous steering and in-situ wheel alignment adjustments.

Claims

1. A method for the in-situ determination of vehicle wheel position using an inertial measurement unit (IMU), the method comprising: providing an IMU comprising an accelerometer mounted on a wheel of a vehicle; measuring acceleration on the vehicle; using the accelerometer, measuring acceleration on the wheel; comparing the acceleration on the vehicle to the accelerometer measurements on the wheel, to find a difference in acceleration; and, in response to finding the difference in acceleration, determining a wheel position with respect to the vehicle.

2. The method of claim 1 wherein finding the difference in acceleration includes finding an instantaneous difference in acceleration; and, the method further comprising: using the instantaneous difference in acceleration to predict vehicle movement.

3. The method of claim 1 wherein measuring the acceleration on the vehicle includes determining acceleration in a direction of the vehicle travel; and, wherein measuring the acceleration on the wheel includes measuring the acceleration on the wheel as the vehicle is traveling, where vehicle travel is defined by a change in vehicle geographic position.

4. A system for the in-situ determination of vehicle wheel position using an inertial measurement unit (IMU), the system comprising: an IMU comprising a gyroscope, mounted on a wheel of a vehicle, having an output to supply gyroscope measurements; a processor; a non-transitory memory; and, an application enabled as a sequence of processor executable instructions stored in the memory, the application having an interface accepting gyroscope measurements, finding an wheel angle measured with respect to a roll axis of the wheel, and in response to finding the wheel angle, determining a wheel position with respect to the vehicle.

5. The system of claim 4 wherein the application finds an instantaneous wheel angle and predicts vehicle movement.

6. The system of claim 4 wherein the application finds an average slip wheel angle and determines a wheel alignment toe condition.

7. The system of claim 6 wherein the application takes initial gyroscope measurements with the wheel in a predetermined alignment to find an initial slip wheel angle, and determines an out-of-alignment toe condition when a measured slip wheel angle differs from the initial slip wheel angle by a predetermined amount.

8. The system of claim 4 wherein the application finds a yaw rotation about a yaw axis of the wheel, where a change in yaw rotation changes a direction of wheel travel, and finds a wheel angle defined between the direction wheel travel and a direction of vehicle travel.

9. The system of claim 8 wherein the gyroscope is mounted on a non-rotating portion of the wheel, with the gyroscope yaw axis aligned in a nominal vertical direction, orthogonal to a lateral axis of the wheel and orthogonal to the roll axis of the wheel.

10. The system of claim 8 wherein the gyroscope is mounted on a rotating portion of the wheel, with a gyroscope yaw axis aligned with the radial axis of the wheel.

11. A system for the in-situ determination of vehicle wheel position using an inertial measurement unit (IMU), the system comprising: an IMU comprising an accelerometer, mounted on a wheel of a vehicle, having an output to supply accelerometer measurements; a processor; a non-transitory memory; and, an application enabled as a sequence of processor executable instructions stored in the memory, the application having an interface to accept a measurement of acceleration on the vehicle and an interface to accept accelerometer measurements on the wheel, the application comparing the acceleration on the vehicle to the accelerometer measurements on the wheel to determine a difference in acceleration and find a wheel position with respect to the vehicle.

12. The system of claim 11 wherein the application finds an instantaneous acceleration difference and predicts vehicle movement.

13. The system of claim 11 wherein the application determines an average acceleration difference and finds a wheel alignment condition.

14. The system of claim 13 wherein the application accepts an initial accelerometer measurement with the wheel in a predetermined alignment, and determines an out-of-alignment condition when the initial accelerometer measurement differs from subsequent accelerometer measurements by a predetermined amount.

15. The system of claim 11 wherein the application accepts acceleration measurements on the vehicle in the vehicle direction and accepts accelerometer measurements as the vehicle is traveling, where vehicle travel is defined by a change in vehicle geographic position, the application comparing the acceleration on the vehicle to the accelerometer measurements on the wheel to find a wheel angle between the direction of vehicle travel and a direction of wheel travel.

16. The system of claim 15 wherein the accelerometer is mounted on a non-rotating portion of the wheel.

17. The system of claim 16 wherein the accelerometer is aligned along an axis selected from the group consisting of a lateral axis of the wheel and a roll axis of the wheel.

18. The system of claim 16 wherein the IMU comprises a first accelerometer and a second accelerometer, with the first accelerometer aligned along a lateral axis of the wheel and the second accelerometer aligned along a roll axis of the wheel; and, wherein the application uses the first accelerometer and second accelerometer to measure the first acceleration on the wheel, and finds the wheel angle in response to the first and second accelerometer measurements.

19. The system of claim 11 wherein the accelerometer is mounted on a rotating portion of the wheel, and aligned along an axis selected from the group consisting of a lateral axis of the wheel, a radial axis of the wheel, and both the lateral and radial axes of the wheel.

20. The system of claim 11 wherein the accelerometer is aligned orthogonal to the lateral axis; and, wherein the application compares the accelerometer measurements to gravity (1G) to find a camber angle defined between vertical (1G) and the radial axis of the wheel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIGS. 1A, 1B, and 1C respectively depict the elements of caster angle, camber angle, and toe, the three primary components of tire alignment (prior art).

[0022] FIG. 2 is a schematic block diagram of a rate gyroscope (prior art).

[0023] FIG. 3 is a schematic block diagram of an exemplary single-axis micro-machined electro-mechanical systems (MEMS) vibrating structure gyroscope (prior art).

[0024] FIG. 4 is a schematic block diagram depicting an exemplary single-axis accelerometer (prior art).

[0025] FIG. 5 is a schematic block diagram depicting a system for the in-situ determination of vehicle wheel position using an inertial measurement unit (IMU).

[0026] FIG. 6A and 6B are, respectively, cross-sectional and plan views of a vehicle.

[0027] FIGS. 7A and 7B are cross-sectional views of a wheel.

[0028] FIG. 8 is a detailed plan view depicting a wheel angle measured by gyroscope.

[0029] FIGS. 9A and 9B are orthogonal cross-sectional views of the gyroscope as mounted on a non-rotating portion of the wheel.

[0030] FIGS. 10A and 10B are orthogonal cross-sectional views of a wheel with the gyroscope mounted on a rotating portion of the wheel.

[0031] FIG. 11 is a flowchart illustrating a method for the in-situ determination of vehicle wheel position using an IMU.

[0032] FIG. 12 is a flowchart illustrating a second method for the in-situ determination of vehicle wheel position using an IMU.

[0033] FIG. 13 is a diagram depicting acceleration forces on a vehicle and a wheel.

[0034] FIG. 14 is a plan view of an IMU with an accelerometer mounted on a non-rotating portion of the wheel to measure wheel angle.

[0035] FIG. 15 is a plan view depicting the IMU accelerator mounted on a rotating portion of the wheel to measure wheel angle.

[0036] FIG. 16 is a diagram depicting the accelerometer mounted on a non-rotating portion of the wheel to measure camber angle.

[0037] FIG. 17 is a diagram depicting the accelerometer mounted on a rotating portion of the wheel, aligned along a radial axis of the wheel to measure camber angle.

DETAILED DESCRIPTION

[0038] FIG. 5 is a schematic block diagram depicting a system for the in-situ determination of vehicle wheel position using an inertial measurement unit (IMU). The system comprises an IMU 502 with a wheel-mounted gyroscope 504 having an output on line 506 to supply gyroscope measurements. The system also comprises a processor 508, a non-transitory memory 510, and an application 512 enabled as a sequence of processor executable instructions stored in the memory. The application 512 has an interface on line 506 to accept gyroscope measurements via input/output (IO) 514. The IO 514 may be more than one interface. The interface may be a modem, cellular, WiFi, Bluetooth, an Ethernet card, or any other appropriate data communications interface such as USB. The physical communication links may be optical, wired, or wireless. The application 512 finds a wheel angle measured with respect to a roll axis of the wheel (see FIGS. 7A and 7B). In response to finding the average wheel angle, the application 512 determines the wheel alignment toe condition. Otherwise, the application 512 may find an instantaneous wheel angle for use in predicting subsequent wheels angles and vehicle position. The angle of the wheel with respect to the vehicle provides a useful indication of the direction of subsequent vehicle movement. The wheel angle is particularly subject to change when the vehicle is turning. When combined with other sensors, such as the steering wheel position, the heading set by an automated steering controller, or a global positioning satellite (GPS) receiver mounted on the vehicle, vehicle position can be predicted with greater accuracy. Averaged over a long period of time, or during a period of time when the vehicle is moving in a straight line, the wheel angle is a measurement of wheel slip angle. Slip angle may also be measured using a gyroscope (or accelerometer) when the steering wheel or steering controller is set to the middle (straight ahead) position. In addition, a Kalman filter might be adapted for this purpose.

[0039] The gyroscope output may be supplied via a hardwired connection if the processor and memory are co-located in the same device. Alternatively, the processor and memory may be embedded in the console of the vehicle, in a smartphone as an application, or as an alignment function tester. In the latter cases, the IMU 502 may communicate via a wireless communication link, which may be proprietary, WiFi, or Bluetooth for example. In another aspect, the IMU 502, and optionally the monitor 516, is embedded in the valve stem of the tire or with a direct tire pressure monitoring system (TPMS). In this case, the TPMS and the wheel position system may share the same radio frequency (RF) communication link and communication protocols.

[0040] FIG. 6A and 6B are, respectively, cross-sectional and plan views of a vehicle. The vehicle 600 has a yaw axis 602, pitch (lateral) axis 604, and roll axis 606. These are the same axes typically associated with an aircraft. The vehicle 600 has a front end 608, and in the interest of simplicity it is assumed that the vehicle direction of travel 610 is forward and aligned with the vehicle roll axis 606. However, a similar understanding of the system can be obtained from a vehicle moving in the reverse direction.

[0041] FIGS. 7A and 7B are cross-sectional views of a wheel. In this example, a front wheel is shown on the right-hand side of the driver. The wheel 518 has a radial axis 700. More explicitly, the wheel many be said to have a plurality of radial axes, which may be envisioned as spokes of a wheel, or alternatively, a single radial axis (spoke) that rotates. In one particular instance the radial axis 700 is aligned in the nominal vertical direction 702, which is also the wheel yaw axis 710. In another aspect, it can be said that the rotating radial axis forms a two-dimensional plane defined by the wheel roll axis 708 and the wheel nominal vertical direction 702 (i.e., the wheel yaw axis 710). From yet another perspective, the radial axis may be said to coincide with the roll axis once per wheel revolution, and coincide with the yaw axis of the wheel once per revolution. Taking into account just the translational movement of the wheel, without consideration of wheel rotation, there exists a wheel direction of travel 704 (when the vehicle 600 is moving) which is aligned with the wheel roll axis 708. However, it should be noted that if the vehicle is making a turn, any averaged correspondence between the wheel direction and vehicle direction may change. In summary, the wheel has a lateral or pitch axis 706, a radial axis 700, a roll axis 708, and a yaw axis 710. As explained in more detail below, rotational forces 714 can be measured about the wheel yaw axis 710. These axes are intended to define the motion of a wheel as a spinning object with a transverse (e.g., forward) motion.

[0042] As is well known in the art, a gyroscope measures a rate of rotation about an axis, which when integrated, provided an angle measurement. As such, the application accepts gyroscope measurements, finds a wheel angle measured with respect to the roll axis 708 of the wheel 518, and in response to finding the wheel angle, determines the slip angle for wheel alignment analysis, or an instantaneous wheel angle for predicting vehicle movement. Although the description provided herein references just a single wheel, it should be understood that the system may simultaneously monitor several vehicle wheels. FIG. 8 is a detailed plan view depicting a wheel angle measured by gyroscope. Again, the front right wheel is used as an example shown with an exaggerated toe-in alignment, or the vehicle making a left-hand turn. In this case, the application finds the yaw rotation 714 about the yaw axis 710 (coming out of the page) of the wheel 518, where a change in yaw rotation changes the direction of wheel travel or indicates a change in the direction of wheel travel. The application then finds a wheel angle 800 (Φ.sub.2) defined between the direction of wheel travel 704 and the direction of vehicle travel 610. Note: Φ.sub.2=90° −Φ.sub.1. It should also be noted that if the slip angle is zero, the wheel direction is identical to the vehicle direction, and a gyroscope mounted to the wheel 518 should measure, on average, no rotation force about the wheel yaw axis 710.

[0043] FIGS. 9A and 9B are orthogonal cross-sectional views of the gyroscope as mounted on a non-rotating portion of the wheel. Despite the many differences between manufacturers and models, a wheel assembly typically includes a hub acting as an interface between the axle and the bearings upon which the tire rotates. In the case of a front wheel for example, at least a portion of the hub is non-rotating, while retaining a fixed relationship with the radial, roll, and lateral axes of the wheel. To simplify explanations, this portion of the hub is referred to herein as the non-rotating portion 900 of the wheel 518. Also for simplicity it is assumed that the mounting surface of the non-rotating portion 900 is formed in a plane parallel to the yaw axis (nominal vertical direction 702) of the wheel, and retains this parallel relationship despite the slip angle, camber angle, and the direction in which the wheel is steered. In the event that the plane of the non-rotating portion 900 is not parallel to the wheel yaw axis, the difference can be compensated through calibration.

[0044] FIG. 9B depicts the inside surface of wheel 518 and shows the non-rotating portion 900 of the wheel (hub), a rotating portion 902 of the hub, and an IMU gyroscope 504 mounted on the non-rotating portion of the wheel. The gyroscope 504 has a yaw axis 904 aligned in the nominal vertical direction 702, orthogonal to the lateral axis 706 of the wheel 518 and orthogonal to the roll axis 708 of the wheel. Note: if the wheel has a zero camber angle, the gyroscope yaw axis is aligned in the true vertical direction. Integrating the rate of rotation 906 about gyroscope yaw axis yields the wheel angle (800, see FIG. 8), with the sign of the angle indicating either toe-in (or instantaneously measured left turn) or toe-out (or instantaneously measured right turn).

[0045] FIGS. 10A and 10B are orthogonal cross-sectional views of a wheel with the gyroscope mounted on a rotating portion of the wheel. As shown, the IMU with gyroscope 504 is mounted on the rotating portion of the hub 902, but alternatively, it could be mounted on a tire surface, in the tire valve stem, or wheel rim. For simplicity of explanation it is assumed that the gyroscope yaw axis 1000 aligned with (i.e., parallel to) the radial axis 700 of the wheel 518. However, this relationship is not necessary, and any differences in axis alignment can be handled with calibration measurements. For every rotation of the wheel 518 the application finds a peak yaw rotation by measuring a maximum yaw rotation 1002 (e.g., when the IMU 504 is rotated to a position above the non-rotating portion as shown), a minimum yaw rotation 1004 (e.g., when the IMU is rotated to a position below the non-rotating portion as shown), or both minimum and maximum yaw rotations about the yaw axis of the wheel, and determines the wheel angle using the peak yaw rotation. The maximum and minimum yaw rotation measurements occur when the gyroscope yaw axis is orthogonal to the direction of wheel travel 704. Alternatively stated, the maximum and minimum yaw rotation measurements occur when the gyroscope input axis (see FIG. 2) is aligned with (and against) the direction of wheel travel 704. The terms “maximum” and “minimum” are arbitrary as they should yield the same measurement with a different sign value. Thus, the maximum yaw rotation 1002, minimum yaw rotation 1004, or average of the two may be used to determine the peak value. Integrating the peak rate of rotation (yaw rotation) about gyroscope yaw axis yields the wheel angle, with the sign of the angle indicating either toe-in (or instantaneous left turn) or toe-out (or instantaneous right turn). In some aspects, the initial position of the gyroscope with respect to the wheel must be known to correlate the measured wheel angle sign with wheel left (in) and right (out).

[0046] One relatively simple means of determining an out-of-alignment toe condition is to take initial gyroscope measurements with the wheel in a predetermined alignment to find an initial wheel angle, so that an out-of-alignment condition is determined when a measured (average) wheel angle differs from the initial wheel angle by a predetermined amount. The predetermined alignment may be the correct or desired alignment, and may include offsets or adjustment to compensate for differences between gyroscope and wheel axes.

[0047] As noted above, the slip angle measurements are made under the assumption that the relationship between the vehicle and wheel directions is relatively constant. For example, the measurements may made be made under the condition that the vehicle is moving in a straight line for an extended period of time, or that the measurements are averaged over an extended period of time. However, more instantaneous gyroscope measurements can be used to determine the instantaneous angle of the wheel with respect to the vehicle, for example, while the vehicle is making a turn. With the advent of computer aided or self-steering vehicles, knowing the instantaneous wheel angle with respect to the vehicle and vehicle direction makes prediction of vehicle movement more accurate and corrections more rapid. Further, for vehicles equipped with mechanisms for in-situ slip and camber angle adjustment, such instantaneous wheel angle measurements provide a feedback path to aid in, for example, more responsive steering or prolonging tire wear.

[0048] Returning briefly to FIG. 5, the in-situ vehicle wheel position determination system may use an IMU 502 comprising an accelerometer 1300 mounted on a wheel 518 of a vehicle, having an output on line 506 to supply accelerometer measurements. The IMU 502 may comprise only accelerometer 1300, only a gyroscope 504 (as described above), both the gyroscope and accelerometer, or multiple gyroscopes and accelerometers. The application 512 compares the acceleration on the vehicle to the accelerometer measurements to determine a wheel alignment condition or instantaneous wheel position. The axes designations used to describe the gyroscope aspects of the system are also used to describe the accelerometer aspects.

[0049] FIG. 13 is a diagram depicting acceleration forces on a vehicle and a wheel. The application accepts acceleration measurements on the vehicle in the vehicle direction, expressed as vector 1400, and may accept accelerometer measurements, expressed as vectors 1402a and 1402b, as the vehicle is traveling, where vehicle travel is defined by a change in vehicle geographic position. It could also be said that acceleration forces on the vehicle (and wheel) oppose the vehicle direction. The acceleration on the wheel 518 can be broken into a component 1402a aligned with the wheel roll axis 708 and a component 1402b aligned with the wheel lateral axis 706. The application compares the acceleration on the vehicle to the wheel accelerometer measurements to find a wheel angle 1404 between the direction of vehicle travel 610 and the direction of wheel travel 704, to determine a wheel toe alignment or instantaneous wheel angle useful in the prediction of vehicle movement.

[0050] FIG. 14 is a plan view of an IMU 502 with an accelerometer mounted on a non-rotating portion 900 of the wheel 518 to measure wheel angle. The IMU accelerometer may be aligned along either the lateral axis 706 of the wheel or the roll axis 708 of the wheel. As used herein, the terms “aligned along” or “aligned with” a particular axis or direction signifies that the accelerometer maximally measures acceleration in that axis or direction. Assuming a small non-zero slip angle, a majority of the acceleration on the wheel should be along the wheel roll axis 708. Knowing the acceleration along the wheel roll axis 708 for example (1402a) and the acceleration on the vehicle (1400), the third side of the vector triangle, acceleration along the wheel lateral axis 706 (1402b), and wheel angle 1404 can be calculated using trigonometry.

[0051] Returning briefly to FIG. 13, the acceleration on the vehicle represented by vector 1400 can be measured using an auxiliary sensor 1406 mounted on vehicle 600. The auxiliary sensor 1406 may be a vehicle-mounted accelerometer aligned along the direction of vehicle travel or more than one vehicle-mounted accelerometer, a vehicle speedometer configured to calculate acceleration, the combination of a first accelerometer mounted on the wheel aligned along the lateral axis with a second accelerometer mounted on the wheel aligned along the roll axis of the wheel (as explained below), a GPS receiver mounted to the vehicle, or combinations of these devices.

[0052] In one aspect, the auxiliary sensor is not required. Returning to FIG. 14, the IMU 502 may comprise a first accelerometer and a second accelerometer, with the first accelerometer aligned along the lateral axis 706 of the wheel and the second accelerometer aligned along the roll axis 708 of the wheel. The application uses the first accelerometer and second accelerometer to measure the acceleration on the wheel, and using trigonometry, finds the acceleration on the vehicle by summing the first and second accelerometer measurements of the first acceleration, and thus finds the wheel angle 1404.

[0053] FIG. 15 is a plan view depicting the IMU 502 accelerator 1300 mounted on a rotating portion 902 of the wheel 518 to measure wheel angle. In this aspect, the IMU 502 with accelerator is aligned along the lateral axis 706. If mounted along the lateral axis, the measurement is described as in the explanation of FIG. 14, where vector 1402b is compared to vector 1400 to find vector 1402a and the wheel angle 1404. Alternatively, if the accelerometer is aligned with the radial axis 700 of the wheel, the measurement is more complicated, as the accelerometer may also measure forces associated with gravity and angular acceleration. In one aspect, these undesired measurements are subtracted-out or canceled using a peak accelerometer measurement by measuring a maximum accelerometer measurement (when the wheel radial axis 700 is aligned with the wheel roll axis 708) and a minimum accelerometer measurement (a half wheel rotation from the maximum second accelerometer measurement). Otherwise, assuming the forces due to gravity on a rotating wheel are self-canceling, measured forces due to angular acceleration or angular velocity can be calculated and removed from the accelerometer measurements knowing the vehicle acceleration or velocity, and the distance (radius) from the accelerometer to the wheel center. In one aspect, two accelerometers can be used to measure forces along the wheel lateral and radial (peak measurements) axes and summed to supply a measurement of acceleration on the vehicle.

[0054] To measure the camber angle, the accelerometer is aligned orthogonal to the wheel lateral axis, and the application compares the accelerometer measurements to gravity (1G), as measured along a true vertical axis, to find a camber angle defined between true vertical (1G) and the radial axis of the wheel, when the radial axis is aligned in a nominal vertical direction.

[0055] FIG. 16 is a diagram depicting the accelerometer mounted on a non-rotating portion of the wheel to measure camber angle. The accelerometer is aligned in a nominal vertical direction 1600, orthogonal to the lateral axis 706 of the wheel 518 and orthogonal to a roll axis 708 of the wheel (coming out of the page). The difference between the nominal vertical direction 1600 and true vertical (1G) direction 1606 defines the camber angle 1602. The force of gravity is known, and is represented by vector 1604. The force association with gravity when the accelerometer 1300 is aligned in the nominal vertical direction is measured and is represented by vector 1604a. The third side of the vector triangle represented by vector 1604b is calculated using trigonometry, as is the camber angle 1602. Note: this measurement can be made with the vehicle stationary or in motion. Further, the measurement may be averaged or instantaneous.

[0056] FIG. 17 is a diagram depicting the accelerometer 1300 mounted on a rotating portion 902 of the wheel, aligned along (parallel to) the radial axis 700 of the wheel 518 to measure camber angle. Ideally, the accelerometer should align with the nominal vertical direction 1600 to measure the camber angle 1602. However, since the accelerometer 1300 is rotating with the wheel 518, it only aligns with the nominal vertical direction once per revolution. Thus, the application accepts accelerometer measurements, in a plurality of instances subsequent to a corresponding plurality of vehicle movements, when the wheel is stationary, and determines a maximum accelerometer measurement, represented by vector 1604a. It is assumed that with enough measurements the accelerometer will eventually align with the nominal vertical direction 1600 on at least one occasion. The application compares this maximum accelerometer measurement to gravity (1 G, represented by vector 1604), to find camber angle 1602. Note: equivalent results can be found using the minimum accelerometer measurement and comparing it to zero. Again, if the accelerometer (or gyroscope) is not aligned with the radial, roll, or lateral axes of the wheel, which may also be referred to herein as “true” axes, the differences between accelerometer (or gyroscope) axes and wheel axes can be accounted for through calibration or then use of multiple orthogonally aligned accelerometers (or gyroscopes).

[0057] In one aspect, the application accepts an initial accelerometer measurement with the wheel in a predetermined alignment (slip or camber angle), and determines an out-of-alignment condition when the initial accelerometer measurement differs from subsequent accelerometer measurements by a predetermined amount. It may not always be possible to perfectly align the accelerometer (or gyroscope) in the optimal axis, orientation, or direction. One solution is the find the calibration (misalignment) angle, and factor this calibration angle into the measurements or calculations. In a related variation, the application may accept initial accelerometer (or gyroscope) measurements with the wheel having a known zero slip angle or camber angle. Then, the accelerometer or gyroscope misalignment can be mechanically adjusted, or the measured misalignment factored into the measurements and calculations.

[0058] In another aspect, a single acceleration measurement may be made using two or three orthogonally aligned accelerometers if one accelerometer cannot be perfectly aligned in an intended axis or direction, with the assumption that the intended acceleration measurement can be determined by summing the multiple orthogonal accelerometer measurements. This same principle of, using two or three orthogonally aligned gyroscopes, can also be applied to the measurement of a rate of rotation about an intended axis.

[0059] In one aspect, the IMU may comprise one or more accelerometers and one or more gyroscopes, combined for the purposes of calibration, refinement of data, and ease of calculation. A conventional gyroscope measures angular rotation speed, and has non-zero bias offset that jitters and varies with sensor temperature. Over time, the bias offset creates integration errors. A conventional accelerometer measures the direction of sensor acceleration, but it cannot distinguish between gravity and inertial acceleration (i.e., an axiom of General Relativity). Therefore, the measurement of slip angle, for example, using both an accelerometer and gyroscope may be useful in providing calibration and reference measurements. In one aspect, the gyroscope and/or accelerometer can be calibrated to remove jitter and bias errors when measurements are able to determine that the IMU is not in motion (e.g., when the vehicle is parked).

[0060] The IMU may potentially comprise a magnetometer. A conventional magnetometer measures the direction of a local magnetic field, and it cannot distinguish between Earth's field and any other nearby fields. Unfortunately, magnetometer readings also typically include significant noise, and they may be influenced by metal in the wheel and vehicle.

[0061] As noted above, the slip angle measurements made using the accelerometer are made under the assumption that the relationship between the vehicle and wheel directions is relatively constant. For example, the measurements may be made under the condition that the vehicle is moving in a straight line for an extended period of time, or the measurements may be averaged over an extended period of time. However, more instantaneous accelerometer measurements can be used to determine the instantaneous angle of the wheel with respect to the vehicle, for example, while the vehicle is making a turn. With the advent of computer aided or self-steering vehicles, knowing the instantaneous wheel angle with respect to the vehicle and vehicle direction makes prediction of vehicle movement more accurate than simply measuring vehicle movement, so that corrections can be made more rapidly. Further, vehicles equipped with mechanisms for in-situ slip and camber angle adjustment may use wheel angle measurements as feedback data to aid in more responsive steering or prolonging tire wear.

[0062] FIG. 11 is a flowchart illustrating a method for the in-situ determination of vehicle wheel position using an IMU. Although the method is depicted as a sequence of numbered steps for clarity, the numbering does not necessarily dictate the order of the steps. It should be understood that some of these steps may be skipped, performed in parallel, or performed without the requirement of maintaining a strict order of sequence. Generally however, the method follows the numeric order of the depicted steps. The method starts at Step 1800.

[0063] Step 1802 provides an IMU comprising a gyroscope, mounted on a wheel of a vehicle. Step 1804 takes gyroscope measurements. Using the gyroscope measurements, Step 1806 finds a wheel angle measured with respect to a roll axis of the wheel (the direction of wheel travel). In response to finding the wheel angle, Step 1808 determines the wheel position with respect to the vehicle. In one aspect, Step 1806 finds an average wheel angle and Step 1808 determines a wheel alignment toe condition. In another aspect, Step 1806 finds an instantaneous wheel angle and Step 1810 predicts subsequent wheel positions and vehicle movement in response to determining the wheel position in Step 1808.

[0064] In one aspect, Step 1803 takes initial gyroscope measurements with the wheel in a predetermined alignment to find an initial slip wheel angle. Then, Step 1808 determines an out-of-alignment condition when the determined slip wheel angle differs from the initial slip wheel angle by a predetermined amount.

[0065] In one aspect, taking gyroscope measurements in Step 1804 includes finding a yaw rotation about a yaw axis of the wheel, where a change in yaw rotation changes (indicates a change in) the direction of wheel travel. Step 1806 finds a wheel angle defined between the direction of wheel travel and the direction of vehicle travel. Again, Step 1806 may determine an average wheel angle so that Step 1808 determines a toe alignment condition for the wheel. Otherwise, Step 1806 determines the instantaneous wheel angle.

[0066] If Step 1802 mounts the gyroscope on a non-rotating portion of the wheel, the gyroscope may have a yaw axis aligned in a nominal vertical direction, orthogonal to the roll axis of the wheel and orthogonal to a lateral (pitch) axis of the wheel. In one aspect, Step 1803 calibrates the gyroscope to account for differences between the gyroscope yaw axis alignment in the nominal vertical direction and a true vertical direction (1 G). See the explanation of FIGS. 9A and 9B for details.

[0067] If Step 1802 mounts the gyroscope on a rotating portion of the wheel, the gyroscope has a yaw axis aligned with the radial axis of the wheel. For every rotation of the wheel, Step 1804 finds a peak yaw rotation in response to measuring a maximum yaw rotation and/or a minimum yaw rotation about the yaw axis of the wheel, and Step 1806 finds the wheel angle using the peak yaw rotation. See the explanation of FIGS. 10A and 10B for details.

[0068] In another aspect, Step 1803 takes gyroscope calibration measurements while the wheel is rotating, with the vehicle stationary (e.g., mounted on a lift or conveyor belt track). For every rotation of the wheel, Step 1804 finds a peak yaw calibration rotation by measuring a maximum yaw rotation calibration and/or a minimum yaw rotation calibration about the yaw axis of the wheel, and Step 1806 modifies a measured the slip angle using the peak yaw calibration rotation. Generally, Step 1803 calibrates the gyroscope to account for differences between the gyroscope yaw axis alignment in a nominal radial axis of the wheel and a true radial axis of the wheel. The calibration may be a mechanical adjustment to move the gyroscope axis, or a computational offset used in measurements and calculations. FIG. 12 is a flowchart illustrating a second method for the in-situ determination of vehicle wheel position using an IMU. The method begins at Step 1900. Step 1902 provides an IMU comprising an accelerometer mounted on the wheel of a vehicle. Step 1904 measures acceleration on the vehicle. Using the accelerometer, Step 1906 measures acceleration on the wheel. Step 1908 compares the acceleration on the vehicle to the accelerometer measurements on the wheel. In response to finding a difference between the acceleration measurements, Step 1910 determines a wheel position with respect to the vehicle. In one aspect, Steps 1904 and 1906 respectively measure average accelerations on the vehicle and wheel, and Step 1910 determines a wheel alignment condition. In another aspect, Steps 1904 and 1906 measure instantaneous accelerations, and Step 1912 predicts subsequent wheel positions and vehicle movement based upon the determined wheel position.

[0069] In one aspect, Step 1908 compares acceleration measurements when the wheel is known to have a predetermined alignment condition, and Step 1910 determines an out-of-alignment condition when the comparison of the acceleration on the vehicle differs from the accelerometer measurement by a predetermined amount. In a different aspect, Step 1903 calibrates the accelerometer to account for a difference between a nominal axis of the wheel and a true axis of the wheel. The calibration may involve a physical manipulation of the accelerometer axis or a measurement used to offset or cancel subsequent measurements and calculations.

[0070] In one aspect, Step 1904 determines acceleration in a direction of the vehicle travel. Step 1906 measures the acceleration on the wheel as the vehicle is traveling, where vehicle travel is defined by a change in vehicle geographic position.

[0071] If Step 1902 mounts the accelerometer on a non-rotating portion of the wheel, the accelerometer may be aligned along either the lateral (pitch) axis of the wheel or the roll axis of the wheel. Either alignment provides an acceleration vector along one of the wheel axes, permitting the wheel angle to be calculated using trigonometry. See the explanation of FIG. 14 for details.

[0072] In one aspect, Step 1902 mounts a first accelerometer aligned along a lateral axis of a non-rotating portion of the wheel and a second accelerometer aligned along a roll axis of a non-rotating portion of the wheel. Step 1906 uses the first accelerometer and second accelerometer to measure the acceleration on the wheel, and Step 1904 measures the first acceleration on the vehicle by summing the first and second accelerometer measurements of the first acceleration.

[0073] Otherwise, Step 1904 measures the acceleration on the vehicle using an auxiliary sensor such as a vehicle-mounted accelerometer aligned along the direction of vehicle travel. Optionally, two or three accelerometers may be orthogonally mounted on the vehicle. The auxiliary sensor may also be a vehicle speedometer (configured to calculate acceleration), a GPS receiver mounted on the vehicle, or a combination of sensors. As explained above, vehicle acceleration can also be calculated by measuring acceleration along both the wheel roll and lateral axes.

[0074] If Step 1902 mounts the accelerometer on a rotating portion of the wheel, it may be aligned with either the lateral or radial axis of the wheel. See the explanation of FIG. 15 for details In another aspect, Step 1902 mounts a first accelerometer aligned along a lateral axis of the wheel and a second accelerometer aligned along a radial axis of the wheel. Step 1906 measures the first acceleration on the wheel using the first and second accelerometers, and Step 1904 measures the acceleration on the vehicle by summing the first and second accelerometer measurements. More explicitly, Step 1906 makes second accelerometer measurements by finding a peak accelerometer measurement by measuring a maximum second accelerometer measurement and/or a minimum second accelerometer measurement. Then, Step 1908 compares acceleration on the vehicle to the accelerometer measurements by comparing the acceleration on the vehicle to the peak accelerometer measurements on the wheel.

[0075] In a different aspect, Step 1902 aligns the accelerometer orthogonal to a lateral axis of the wheel. Step 1908 compares the accelerometer measurements to gravity (1G), and Step 1910 finds a camber angle defined between vertical direction (1G) and a radial axis of the wheel. If Step 1902 mounts the accelerometer on a non-rotating portion of the wheel, it is aligned in a nominally vertical direction, orthogonal to the lateral axis of the wheel and orthogonal to the roll axis of the wheel. See the explanation of FIG. 16 for details.

[0076] If Step 1902 mounts the accelerometer on a rotating portion of the wheel, it is aligned along the radial axis of the wheel. Step 1906 measures the acceleration on the wheel by accepting accelerometer measurements in a plurality of instances subsequent to a corresponding plurality of vehicle movements, when the wheel is stationary, and determines a maximum accelerometer measurement. Step 1910 determines the camber angle as Step 1908 compares the maximum accelerometer measurement to gravity (1 G). Alternatively, a minimum accelerometer measurement can be compared to zero. See the explanation of FIG. 17 for details.

[0077] In one aspect, with the accelerometer mounted on a rotating portion of the wheel, Step 1903 rotates the wheel in a condition where the wheel is known to have a zero camber angle, and calibrates the accelerometer alignment to read a zero peak accelerometer measurement. If the accelerometer is mounted on a non-rotating portion of the wheel, the same calibration can be made without rotating the wheel.

[0078] An IMU system and method have been provided for measuring wheel alignment and instantaneous wheel position with respect to a vehicle. Examples of particular process steps and devices been presented to illustrate the invention. However, the invention is not limited to merely these examples. Other variations and embodiments of the invention will occur to those skilled in the art.