High accuracy estimation of wheel angle using wheel mounted imu and wheel encoder for slow speed driving

Abstract

The present disclosure relates to a method for determining a wheel angle rotation of a vehicle, the method comprising: determining (S201) a threshold wheel angular speed value; determining (S202a) the wheel angle rotation of the vehicle utilizing a tick from a wheel encoder upon a determination that a wheel angular velocity value is above the threshold wheel angular speed value; and determining (S202b) the wheel angle rotation of the vehicle utilizing an Inertial Measurement Unit (IMU) MEMS sensor (102) mounted on a wheel rim (101) upon a determination that the wheel angular speed value is below the threshold wheel angular speed value.

Description

(0001) The disclosure relates generally to monitoring of vehicle speed. In particular aspects, the disclosure relates to high accuracy estimation of wheel angle using wheel mounted IMU and wheel encoder for slow speed driving. The disclosure can be applied to heavy-duty vehicles, such as trucks, buses, and construction equipment, among other vehicle types. Although the disclosure may be described with respect to a particular vehicle, the disclosure is not restricted to any particular vehicle.BACKGROUND(0002) Estimating the wheel angle and wheel rotational speed with accuracy is essential when controlling the vehicle to a desired position such as a charging station. The control needs to be with high accuracy and precision. Wheel angle estimation is closely connected to the driving speed of the vehicle. Thus, estimating the wheel angle at a multitude of driving speeds with high accuracy with current sensors is a challenge.SUMMARY(0003) According to a first aspect of the disclosure, a method for determining a wheel angle rotation of a vehicle is provided, the method comprising: determining a threshold wheel angular speed value; determining the wheel angle rotation of the vehicle utilizing a tick from a wheel encoder upon a determination that a wheel angular speed value is above the threshold wheel angular speed value; and determining the wheel angle rotation of the vehicle utilizing an Inertial Measurement Unit (IMU) MEMS sensor mounted on a wheel rim upon a determination that the wheel angular speed value is below the threshold wheel angular speed value. A technical benefit may include measuring both the wheel angle and the angular velocity of the wheel when the vehicle is driving at very low velocities or if the vehicle is at standstill. Velocity computation is based on a count time between two ticks. As the angle between two ticks in the wheel encoder is known, and the time between two ticks is measured, the average velocity from the last tick can be computed. As the update of the velocity computation is only possible when a new tick is passed, a problem at low driving speeds occurs as a tick event is very sparsely spaced in time. A challenge remains to determine if the vehicle is standing still as it would take an infinite amount of time between two ticks.(0004) Within the context of the present application, “a wheel angular value” is to be understood as the angular velocity of the wheel while driving. Within the context of the present application, “velocity” and “speed” may be used interchangeably and mean the same concept. The wheel angular velocity value may be constant. The wheel angular velocity value may not be constant. The angular velocity value of the wheel may be obtained from the determination of the wheel angle rotation.(0005) Within the context of the present application, “very low velocities” may be interpreted in terms of the wheel angular velocity lower than 1 rad / s.(0006) Within the context of the present application, “a vehicle” may be any type of vehicle configured to be operated on a road, such as an automobile or heavy-duty vehicles, such as trucks, buses, and construction equipment.(0007) The wheel encoder may determine the wheel angular velocity value by measuring the time interval between two ticks. The constant intervals mark off constant values of wheel encoder angle between two ticks. The wheel angular velocity may be obtained from the ratio of the wheel encoder angle to the time interval between two ticks.(0008) The wheel encoder may measure the wheel ticks by using a magnetic disc and a Hall sensor.(0009) According to a second aspect of the disclosure, a method for controlling the vehicle to a desired position is provided, comprising: determining a wheel angle rotation of the vehicle according to the first aspect; controlling the motion of the vehicle based on wheel angle rotation and / or wheel angular speed value at a wheel angular speed below the threshold wheel angular speed value. A technical benefit may include improved parking accuracy at a desired position.(0010) Optionally in some examples, including in at least one preferred example, the use of the method may incorporate a charging station as the desired position. A technical benefit may include improved parking accuracy to a charging station. The vehicle may possess an electric motor. The electric motor may draw power from a battery. The battery may require frequent charging. The use of the method may determine whether the vehicle is in standstill to ensure proper parking before plugging in a charging cable of the charging station to a charging port of the vehicle. The use of the method may be used for autonomous driving applications.(0011) According to a third aspect of the disclosure, a vehicle wheel angle rotation measurement system is provided, the system comprising: a wheel encoder configured to measure wheel ticks; an IMU MEMS sensor arranged at the rim of a wheel, comprising: at least one gyroscope configured to generate a wheel angular velocity data in response to a wheel rotation; and at least one two-axis accelerometer configured to generate a wheel angle data in response to the wheel rotation; and a Kalman filter configured to execute the functions of: receiving as input the data generated by the at least one gyroscope, the at least two-axis accelerometer and the wheel encoder; analyzing the received input and estimating a prediction of an angle state over time. Any technical benefit discussed in relation to the first aspect of the disclosure may be applicable to the third aspect of the disclosure.(0012) According to a fourth aspect of the disclosure, a vehicle comprising the system of the third aspect is provided. Any technical benefit discussed in relation to the first aspect of the disclosure may be applicable to the third aspect of the disclosure.(0013) The disclosed aspects, examples (including any preferred examples), and / or accompanying claims may be suitably combined with each other as would be apparent to anyone of ordinary skill in the art. Additional features and advantages are disclosed in the following description, claims, and drawings, and in part will be readily apparent therefrom to those skilled in the art or recognized by practicing the disclosure as described herein.(0014) Examples are described in more detail below with reference to the appended drawings.(0015) FIG. 1 is an exemplary schematic view of a wheel comprising a vehicle wheel angle rotation measurement system according to an example.(0016) FIG. 2 is an exemplary flow chart illustrating a method according to the first aspect of the disclosure according to an example.(0017) FIG. 3 is a graph output from the wheel speed sensor showing the sensitivity problem close to zero angular wheel speed.DETAILED DESCRIPTION(0018) The detailed description set forth below provides information and examples of the disclosed technology with sufficient detail to enable those skilled in the art to practice the disclosure.(0019) A problem to be solved by any one aspect of the disclosure is providing a more accurate and reliable way of estimating the wheel angle and wheel angular velocity when a vehicle is driven at very low velocities or is at standstill.(0020) FIG. 1 is an exemplary schematic view of a wheel 10 comprising a vehicle wheel angle rotation measurement system 102 according to an example. The vehicle may be an automobile, a service vehicle, a truck such as a heavy-duty truck, a bus. The wheel comprises a rim 101. The vehicle wheel angle measurement system 102 may be configured to be mounted on the rim 101.The vehicle wheel angle measurement system 102 may be mounted on the periphery of the rim 101. A coordinative system is assigned to the wheel 10, where the center of rotation “o” may be chosen as the center of the coordinative system. The positive “ox” axis may be chosen in the direction the wheel rotates when steering the vehicle. The positive “oz” axis may be opposite to the ground floor. The positive “oy” axis may be along the axis of rotation “o” of the wheel 10. The coordinative system follows the ISO 8855:2011 convention to denote the axis orientations.(0021) In the exemplary schematic, the vehicle wheel angle measurement system 102 is positioned on the periphery of the rim 101. The vehicle wheel angle measurement system 102 may be mounted on the inner side or the outer side of the rim 101. The distance of the vehicle wheel angle measurement system 102 to the center “o” of the rim 101 may be n. n may correspond to the radius of the rim 101. The vehicle wheel angle measurement system 102 may be an internal measurement unit micro electronic mechanical system (IMU MEMS). For the purpose of simplicity and brevity, the IMU MEMS nomenclature will be used to signify the vehicle wheel angle measurement system 102. The IMU MEMS 102 may be assigned its own coordinate system. The coordinate system of the IMU MEMS 102 may be the same as the coordinative system of the wheel 10 displaced by n. The coordinative system may be a non-rotating coordinative system. The IMU MEMS 102 may have a secondary coordinative system, namely a rotating coordinative system at point P. The non-rotating coordinative system may have as axes the positive x axis of the IMU MEMS 102 named as “px”, the positive z axis as “pz” and the positive y axis as “py” (not shown). A radial acceleration and a tangential acceleration term denoted as pr and po, respectively are related to the rotating coordinative system. The IMU MEMS 102 may be mounted at the center of rotation of the wheel 10. In such an example the coordinative system of the wheel 10 and the IMU MEMS 102 coincide.(0022) The IMU MEMS 102 may comprise at least one accelerometer. The at least one accelerometer may be a two-axis accelerometer. The IMU MEMS may further comprise at least one gyroscope. The accelerometer may be configured to measure the radial and tangential acceleration. For the purpose of this disclosure the radial acceleration is designated as rs and the tangential acceleration is designated as ts. The gyroscope may be configured to measure the wheel 10 angular velocity around the “oy”-axis of the wheel 10.(0023) Apart from the IMU MEMS 102, the wheel 10 may comprise a wheel encoder. The wheel encoder may comprise a magnetic disc and a Hall sensor. The wheel encoder may be configured to measure the wheel 10 rotation. The wheel encoder may comprise of a magnetic disc with evenly spaced holes or slots around the circumference of the wheel encoder. The wheel encoder may comprise 100 ticks per revolution. The Hall sensor may detect the passing of the holes or slots as the magnetic disc rotates. As the magnetic disc has a limited number of holes or slots, a quantization in angle is present. In the context of this disclosure, passing of the slots may be equivalent to wheel ticks. The Hall sensor may generate an electrical signal when the magnetic disc rotates. The data generated by the Hall sensor may be processed by a processing circuit (not shown) to determine the angular velocity. The data generated may be electrical signals. The nature of the electrical signals may be in the form of a square wave. The square wave signal may correspond to a certain amount of wheel 10 rotation. A benefit of incorporating a wheel encoder with a magnetic disc may be due to a high degree of robustness. A further benefit may be operation in harsh environments without compromising on quality.(0024) FIG. 2 is an exemplary flow chart illustrating a method according to the first aspect of the disclosure according to an example. The method comprises the steps of determining S201 a threshold wheel angular speed value, determining the wheel 10 angle rotation of the vehicle utilizing a tick from a wheel encoder upon a determination S202a that a wheel angular speed value is above the threshold wheel angular speed value; and determining the wheel 10 angle rotation of the vehicle utilizing an IMU MEMS sensor mounted on a wheel rim 101 upon a determination S202b that the wheel angular speed value is below the threshold wheel angular speed value. The threshold wheel angular speed may be tied to the time between two ticks in the wheel encoder. The threshold wheel angular speed may bel rad / s.(0025) The instantaneous values obtained for the wheel angle, wheel angular velocity, and wheel angular acceleration may serve as input in a Kalman filter to set up a prediction model. The Kalman filter may be an extended Kalman filter. The Kalman filter may predict a future state. The Kalman filter use measurement data from the IMU MEMS sensor 102 and / or the wheel encoder to predict the future state. Such a future state may be: an estimate of a subsequent wheel angle, wheel angular velocity and / or wheel angular acceleration. To implement the Kalman filter, several assumptions are made: the sampling time is small, put differently the time between two measurements is small. By limiting the sampling time to a small sampling time, the measurement will be updated more frequently. An increase in the measurement update may lead to an increased accuracy of the predicted future state. The prediction model may predict the state between tick events. The prediction model may overcome the challenge of obtaining the wheel angle at very low velocities necessitating a long waiting time between ticks.(0026) To implement the model a state-space representation of a system is introduced. The state-space representation of the system consists of a set of input, output, and variables related by first-order differential equations. These state variables may be represented as a vector. The state variables may change over time in a way that depends on the values they have at previous instance and on the values of the input variables. Output variables may depend on the state vector and may also depend on the input variables.(0027) An example of the state-space may be represented as below in discrete time:x(k 1) = A x(k) B u(k)y(k) = C x(k) D u(k) (1)where x(k) may be the state vector at time instance k. A, B, C and D may be a state matrix, an input matrix, an output matrix and a feedforward matrix, respectively. Further, y (k) may be an output vector and it(k) may be an input variable. The state vector may be an unknown quantity. The state vector may be estimated using known measurements y (k) and known inputs it(k). Equation (1) may be reformulated using predicted state as x(k):x(k 1) = A x(k) B u(k ) (2) y(k) = C x(k) D u(k ) (3)Equation 2 is called the process model. Equation 3 is called the measurement model.To minimize the error between y(k) and y(k), the equation may then be reformulated as:x(k\k — 1) = A x(k — 1|k — 1) Bu(k — 1) (4)x(k\k) = A x(k\k — 1) K (y(k) — Cx(k\k — 1)) (5)where K is the Kalman gain used to minimize the error of y (k) - y(k), x(k|k — 1) is the predicted state estimate at time k before incorporating the measurement, x(k — l|k — 1) is the previous state estimate after the measurement update, and x(k\k) is the updated state estimate after incorporating the measurement.To expand on the previous equations, the process and measurement model for thesystem in the context of this disclosure may be given by the following matrices:Docket No.: P20024-0790 - 211511398where T is the sampling time, qk-i and rk are additive white Gaussian noise terms for the process and measurement model, respectively, 6k is the wheel angle of state k, ωk is the wheel angular velocity of state k, ak is the wheel angular acceleration of state k, ωgyro,k is the wheel angular velocity measured by the gyroscope in the IMU, and θwss,k is the wheel rotation angle measured by the wheel encoder.The state equation 7 may further be reduced by making several approximations. If the pitch of the vehicle is neglected and assuming no longitudinal slip and vertical acceleration in slow speed conditions, the parameters aox and aoz can be approximated as:aox = vx = rω = raαoz = gwhere r is the radius of the wheel and g is the gravitational acceleration. Thus, the measurement model may be given as:The state update with the above measurement model requires all measurements to be available at the same time instant. A challenge posed by this requirement is the nonutilization of all the IMU MEMS measurement data for the measurement update. The challenge arises from the IMU MEMS’ higher data output rate compared to the wheel encoder. The challenge may be overcome by using a high sampling wheel encoder measurement. Alternatively, the challenge may be overcome by implementing a filter that processes each available measurement instantaneously, without waiting for other measurements to become available. Such a filter may have multiple measurement model definitions. The filter may choose the relevant measurement model depending on the available set of observations.(0028) FIG. 3 is a graph output from the wheel speed sensor showing the sensitivity problem close to zero angular wheel speed. The x-axis shows the number of samples, the yaxis shows the amplitude of wheel angular speed in rad / s.(0029) Examples of the disclosure follows.(0030) Example 1: A method for determining a wheel 10 angle rotation of a vehicle, the method comprising: determining S201 a threshold wheel angular speed value; determining S202a the wheel angle rotation of the vehicle utilizing a tick from a wheel encoder upon a determination that a wheel angular speed value is above the threshold wheel angular speed value; and determining S202b the wheel angle rotation of the vehicle utilizing an Inertial Measurement Unit (IMU) MEMS sensor mounted on a wheel rim upon a determination that the wheel angular speed value is below the threshold wheel angular speed value.(0031) Example 2: A method for controlling a vehicle to a desired position, comprising: determining a wheel angle rotation of the vehicle according to Example 1 ; controlling S203 the motion of the vehicle based on wheel angle rotation and / or wheel angular speed value at a velocity below the threshold wheel angular speed.(0032) Example 3 : The method according to Example 2, wherein the desired position is a charging station.(0033) Example 4: A vehicle wheel angle rotation measurement system, the system comprising: a wheel encoder configured to measure wheel ticks; an IMU MEMS sensor 102 arranged at the rim 101 of a wheel 10, comprising: at least one gyroscope configured to generate a wheel angular velocity data in response to a wheel rotation; and at least one twoaxis accelerometer configured to generate a wheel angle data in response to the wheel rotation; and a Kalman filter configured to execute the functions of: receiving as input the data generated by the at least one gyroscope, the at least two-axis accelerometer and the wheel encoder; analyzing the received input and estimating a prediction of an angle state over time.(0034) Example 5: A vehicle comprising the system according to Example 4.(0035) The terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting of the disclosure. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. It will be further understood that the terms "comprises," "comprising," "includes," and / or "including" when used herein specify the presence of stated features, integers, actions, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, actions, steps, operations, elements, components, and / or groups thereof.(0036) It will be understood that, although the terms first, second, etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element without departing from the scope of the present disclosure.(0037) Relative terms such as "below" or "above" or "upper" or "lower" or "horizontal" or "vertical" may be used herein to describe a relationship of one element to another element as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. It will be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element, or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present.(0038) Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.(0039) It is to be understood that the present disclosure is not limited to the aspects described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the present disclosure and appended claims. In the drawings and specification, there have been disclosed aspects for purposes of illustration only and not for purposes of limitation, the scope of the disclosure being set forth in the following claims.

Claims

1. A method for determining a wheel angle rotation of a vehicle, the method comprising: determining (S201) a threshold wheel angular speed value;determining (S202a) the wheel angle rotation of the vehicle utilizing a tick from a wheel encoder upon a determination that a wheel angular speed value is above the threshold wheel angular speed value; anddetermining (S202b) the wheel angle rotation of the vehicle utilizing an Inertial Measurement Unit (IMU) MEMS sensor (102) mounted on a wheel rim (101) upon a determination that the wheel angular speed value is below the threshold wheel angular speed value.

2. A method for controlling a vehicle to a desired position, comprising:determining a wheel angle rotation of the vehicle according to claim 1; controlling (S203) the motion of the vehicle based on wheel angle rotation and / or wheel angular speed value at a wheel angular speed below the threshold wheel angular speed value.

3. The method of claim 2, wherein the desired position is a charging station.

4. A vehicle wheel angle rotation measurement system, the system comprising:a wheel encoder configured to measure wheel ticks;an IMU MEMS sensor (102) arranged at the rim (101) of a wheel (10), comprising:at least one gyroscope configured to generate a wheel angular velocity data in response to a wheel rotation; andat least one two-axis accelerometer configured to generate a wheel angle data in response to the wheel rotation; anda Kalman filter configured to execute the functions of:receiving as input the data generated by the at least one gyroscope, the at least two-axis accelerometer and the wheel encoder;analyzing the received input and estimating a prediction of an angle state over time.

5. A vehicle comprising the system according to claim 3.