GB2548597A

GB2548597A - A system for use in a vehicle

Info

Publication number: GB2548597A
Application number: GB1604918.1A
Authority: GB
Inventors: Tsampardoukas Georgios; Hampson Ric
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2016-03-23
Filing date: 2016-03-23
Publication date: 2017-09-27
Anticipated expiration: 2036-03-23
Also published as: GB2548597B; GB201604918D0

Abstract

Determining the ground speed of a vehicle 10 that has a plurality of wheels each having an associated wheel speed sensor; receiving a measured rotational wheel speed value from each of the wheel speed sensors, and processing means to determine which wheels are slipping by determining the difference between each pair of the plurality of measured rotational wheel speed values; calculating the ground speed based on the rotational speeds of the wheels that are not slipping. Determining that there is a slip event may include calculating a ground speed based on the wheel speeds of all the wheels and comparing it to an estimated ground speed. The rotational acceleration of a wheel may be used to determine a slip event if it is above a threshold. The absolute difference values between the measured wheel speeds may indicate a slip event if one wheel is offset to the others. Classification of the slip event into: lock-up or spinning may be made based on if the absolute difference is positive or negative.

Description

A SYSTEM FOR USE IN A VEHICLE

TECHNICAL FIELD

The present disclosure relates to a system for use in a vehicle and particularly, but not exclusively, to a system for determining the ground speed of a vehicle. Aspects of the invention relate to a system, to a vehicle and to a method.

BACKGROUND

The ground speed of a vehicle, also referred to as the Speed-over-Ground (SoG) or the reference speed, is an important parameter used in controlling many aspects of the vehicle and its performance. For instance, the ground speed may be used as an input to, or used in calculating parameters for input to, vehicle systems such as an anti-lock braking system (ABS), an electronic stability control system (eSCS), or a terrain identification system such as the Applicant’s Terrain Response® system.

There are various techniques currently used for estimating ground speed in real time. For instance, speed measurements may be provided by wheel speed sensors of the non-driven wheels of a two-wheel drive vehicle which are then processed using Kalman filtering. This technique, which may also be referred to as linear quadratic estimation, uses a series of measurements over time to provide an estimated value with a degree of the noise or uncertainty associated with the measurements filtered out so as to provide an estimate that is more accurate than a single measurement. For instance, the Kalman filter may use data from a GPS system, a radar sensor, an optical sensor, or some other suitable sensor. This estimation method is prone to significant error particularly during transient vehicle behaviour where the ground speed may vary significantly over a relatively short interval of time. Such behaviour commonly occurs when wheel slip also occurs: for instance, during a braking event, or during a traction event when a rotational velocity of the wheel may be too great for the terrain on which the vehicle is driving. Similarly, wheel slip may occur when a vehicle turn radius is too excessive for the current weather conditions or terrain type, particularly when ice is present. In these cases, there is reduced reliability in the wheel speed measurements and any control logic, for instance an eSCS, used to control motion of the vehicle during a high wheel slip event, for instance a braking event, may be using an estimated ground speed value that is calculated using outdated measurements.

Alternatively, direct integration of a measured longitudinal acceleration of the vehicle may be used to estimate ground speed. However, offset in the acceleration signal may cause excessive errors in estimation results. Estimation accuracy is also compromised greatly by signal noise and the offset caused by temperature drift, road ramp and vehicle pitch motion, resulting in estimation errors accumulated by the integration process.

Estimation methods such as Kalman filtering and direct integration of an acceleration signal are also more likely to estimate the ground speed more accurately because of accumulated noise. Thus, the errors in the estimation of parameters that are based on estimated ground speed, e.g. wheel slip, particularly during transient vehicle behaviour such as during a braking event, will result in errors or instabilities in the performance of control systems such as those mentioned above.

Alternative methods, which could yield reliable estimations of ground speed in all conditions, would be beneficial in the improvement and development of various stability control systems. For instance, an accurate estimation of the longitudinal wheel slip combined with an estimate of the friction coefficient, could allow for significant improvements in vehicle control in poor driving conditions. Furthermore, the measurement of the direction (and lateral speed) would allow for a better understanding of the vehicle’s trajectory and hence an improvement of the vehicle dynamic response.

It is an aim of the present invention to address one or more of the problems associated with the prior art.

SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided a system for determining the ground speed of a vehicle. The vehicle includes a plurality of wheels each having an associated wheel speed sensor for measuring rotational wheel speed.

The system includes receiving means configured to receive a measured rotational wheel speed value from each of the wheel speed sensors. The system also includes processing means configured to determine which of the wheels are undergoing a slip event and which of the wheels are not undergoing a slip event, said determination including determining the difference between each pair of the plurality of measured rotational wheel speed values, and the processing means (28) being configured to calculate the vehicle ground speed based on all of the measured rotational wheel speed values corresponding to the wheels that are determined not to be undergoing a slip event.

Therefore, for each of the wheels the processing means determines differences between the measured rotational wheel speed associated with that wheel and the measured rotational wheel speeds associated with each of the other wheels. For example, the processing means will determine six difference values for a vehicle with four wheels.

The invention is advantageous in that the ground speed is calculated based on current measurements of the rotational wheel speed, making the system particularly useful when the vehicle is operating in transient conditions such as undergoing a braking event, a traction event or relatively sharp cornering event. In addition, by identifying any wheels that are slipping and eliminating the relevant rotational wheel speed measurement from the ground speed determination, errors in the ground speed determination are reduced. This may be particularly useful when travelling over surfaces with a low coefficient of friction (‘low-/;’ surfaces), i.e. in conditions where wheel slip is more likely.

The receiving means and processing means may comprise an electronic processor having an electrical input for receiving the rotational wheel speed data. The system may comprise an electronic memory device electrically coupled to the electronic processor having instructions stored therein. The processor may be configured to access the memory device and execute the instructions stored therein such that it is operable to determine which of the wheels are undergoing a slip event and which of the wheels are not undergoing a slip event, and to calculate the ground speed based on the measured rotational wheel speed values corresponding to the wheels that are determined not to be undergoing a slip event.

The determination of a slip event may include calculating the ground speed based on the measured rotational wheel speed values for all of the wheels. Advantageously, this means that measurements from existing vehicle sensors may be used in the determination of wheel slip. The determination of a slip event may include determining whether the difference between the calculated ground speed and an estimation of ground speed is greater than a predetermined error difference threshold. The estimation of ground speed may be calculated using state estimation.

The determination of a slip event may include determining whether the rotational acceleration of any of the wheels is greater than a predetermined rotational acceleration threshold.

In some embodiments, if each of the absolute values of the differences between the measured rotational wheel speed of a first wheel of the plurality of wheels and the measured rotational wheel speeds of each of the other of the plurality of wheels is greater than a predetermined rotational speed difference threshold value, then it is determined that the first wheel only is undergoing a slip event.

The processing means may be configured to determine whether the first wheel is locked or spinning based on the sign of the difference between the measured rotational wheel speed of the first wheel and the measured rotational wheel speed of one of the other wheels.

In some embodiments, determining that at least two, but fewer than all, of the plurality of wheels are undergoing a slip event includes determining that the measured rotational wheel speed of at least one of the wheels is not substantially equal to zero.

The processor may be configured to calculate the ground speed by calculating an average value of the measured rotational wheel speed values corresponding to each of the wheels that are determined not to be undergoing a slip event.

The processor may be configured to determine if all of the wheels are undergoing a slip event by checking whether the measured rotational wheel speeds corresponding to all of the wheels are substantially zero. Advantageously, the system can still make a determination of the ground speed if it is determined that all of the wheels are slipping. In such a case, the receiving means may be configured to receive a tyre-road adhesion coefficient, wherein if the processor determines that all of the wheels are undergoing a slip event then the processor is configured to calculate the ground speed by estimating an acceleration or deceleration pattern of the vehicle based on the received tyre-road adhesion coefficient. The tyre-road adhesion coefficient may be received via vehicle to vehicle wireless communication.

In some embodiments, the ground speed, V, is calculated by V=Vrerinit^actmAT, where μααί is the tyre-road adhesion coefficient, m = VrefFinal^Vreflnit jS the acceleration or deceleration pattern, Vreflnit and VrefFinal are estimated ground speed values at the start and end, respectively, of a predetermined time interval, and AT is the predetermined time interval. The time interval may be greater than or equal to 100 ms.

The processor may be configured to calculate a reference wheel slip ratio in dependence on the calculated ground speed. In such an embodiment, the processor may be configured to calculate a wheel slip ratio difference between an actual wheel slip ratio for one or more of the wheels and the reference wheel ratio. The system may comprise an output configured to output the calculated wheel slip ratio difference to a stability control subsystem of the vehicle, which may improve the accuracy and reliability of such a system.

The processor may be configured to calculate the ground speed in real time.

According to another aspect of the present invention, there is provided a method for determining the ground speed of a vehicle, the vehicle comprising a plurality of wheels each having an associated wheel speed sensor for measuring rotational wheel speed. The method comprises receiving a measured rotational wheel speed value from each of the wheel speed sensors, and determining which of the wheels are undergoing a slip event and which of the wheels are not undergoing a slip event, said determination including determining the difference between each pair of the plurality of measured rotational wheel speed values. The method also comprises calculating the ground speed based on all of the measured rotational wheel speed values corresponding to the wheels that are determined not to be undergoing a slip event.

According to a further aspect of the present invention, there is provided a vehicle comprising a system as described above.

According to a still further aspect of the present invention, there is provided a non-transitory, computer-readable storage medium storing instructions thereon that when executed by one or more processors causes the one or more processors to carry out the method described above.

For the purposes of this disclosure, it is to be understood that the control system described herein can comprise a control unit or computational device having one or more electronic processors. A vehicle and/or a system thereof may comprise a single control unit or electronic controller or alternatively different functions of the controller(s) may be embodied in, or hosted in, different control units or controllers. As used herein, the term “vehicle control system” will be understood to include both a single control unit or controller and a plurality of control units or controllers collectively operating to provide the required control functionality. A set of instructions could be provided which, when executed, cause said controller(s) or control unit(s) to implement the control techniques described herein (including the method(s) described below). The set of instructions may be embedded in one or more electronic processors, or alternatively, the set of instructions could be provided as software to be executed by one or more electronic processor(s). For example, a first controller may be implemented in software run on one or more electronic processors, and one or more other controllers may also be implemented in software run on or more electronic processors, optionally the same one or more processors as the first controller. It will be appreciated, however, that other arrangements are also useful, and therefore, the present invention is not intended to be limited to any particular arrangement. In any event, the set of instructions described above may be embedded in a computer-readable storage medium (e.g., a non-transitory storage medium) that may comprise any mechanism for storing information in a form readable by a machine or electronic processors/computational device, including, without limitation: a magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g., EPROM ad EEPROM); flash memory; or electrical or other types of medium for storing such information/instructions.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a schematic plan view of a vehicle including a vehicle control system (VCS) according to an embodiment of the invention;

Figure 2 shows a free body diagram of one of the wheels of the vehicle in Figure 1 in the case of a vehicle braking event;

Figure 3 is a flowchart showing the steps of a method for determining the vehicle ground speed as undertaken by the VCS in Figure 1;

Figure 4 shows a schematic plan view of the vehicle in Figure 1 including the notation for both the rotational speed of each wheel and the differences in rotational speed between each wheel;

Figure 5 shows the vehicle view of Figure 4 including the rotational speed differences used to determine when one or more wheels are slipping or locked for different cases: in particular, Figures 5a, 5b, 5c, 5d show the cases in which the front-left wheel, the front-right wheel, the rear-right wheel and the rear-left wheel, respectively, is locked; and Figure 5e shows the case in which both the rear-left and the rear-right wheels are locked; and

Figure 6 shows a graph illustrating how the deceleration pattern of the vehicle in Figure 1 is estimated in the case when it is determined that all of the wheels are locked.

DETAILED DESCRIPTION

In an embodiment of the present invention, a vehicle control system estimates the realtime ground speed of a vehicle by first determining which of the wheels of the vehicle are undergoing a slip event and which are not by comparing measurements of the rotational speed of the individual wheels, and second calculating the ground speed based on measurements of the rotational speed of those wheels determined not to be slipping or locked. In the described embodiment below, the ground speed is then used to determine an indication of wheel slip ratio.

Figure 1 shows a schematic plan view of a vehicle 10 that includes a vehicle body 12 and four wheels 14a, 14b, 14c, 14d. The front wheels 14a, 14b are supported by a front axle 18a and the rear wheels 14c, 14d are supported by a rear axle 18b. Each of the four wheels 14a, 14b, 14c, 14d has an associated wheel speed sensor 16a, 16b, 16c, 16d (shown collectively as block 16 in Figure 1), which measures an indication of the speed of rotation of that particular wheel. The wheel speed sensors 16 are tachometers and are located in the vicinity of the respective wheels 14a, 14b, 14c, 14d. Alternatively, the rotational wheel speed may be measured by any other suitable sensor or means, for instance an optical sensor or a shaft speed sensor.

At least two of the wheels are driven wheels in that power is supplied to them to move the vehicle 10. The vehicle 10 includes an engine control unit having an internal combustion engine for powering the wheels via a drivetrain that includes a clutch, gearbox, differential and final drive shafts by rotating one or both of the axles 18a, 18b connected to the driven wheels. In this case, the vehicle 10 is a front wheel drive vehicle meaning only the front wheels 14a, 14b are driven wheels. The amount of power supplied to the wheels is dependent on the degree to which a driver depresses an accelerator pedal (included in block 20 in Figure 1) within a cabin of the vehicle 10. A braking system of the vehicle 10 includes two or more brakes, each brake for retarding one of the wheels. In this case, there are two brakes, each for retarding one of the front wheels 14a, 14b. The brakes are disc brakes and operate in response to the driver depressing a brake pedal (also included in block 20 in Figure 1) within the vehicle cabin. Retardation of the wheels 14a, 14b is achieved by opposing brake pads compressing against a brake disc that is fixed on the axle 18a. A brake calliper supports and moves the opposing brake pads towards the brake disc in response to receiving a braking command signal. Other types of brakes, such as drum brakes, may be used instead.

The vehicle 10 includes a state estimator (SE) block 22. This is used to provide an estimation of the ground speed using Kalman filtering for comparison with the determined ground speed, as is described below.

The vehicle 10 also includes an adhesion coefficient, μ, block 24 which determines the tire-road adhesion coefficient, μαα, and the maximum tire-road adhesion coefficient, μ-max· These values may be stored values or may be determined based on the conditions the vehicle 10 is travelling in, such as in particular weather conditions or over a particular type of terrain. In certain situations, these parameters are used to determine the ground speed, as is described below. The adhesion coefficient block 24 may be part of another Advanced Driver Assistance System (ADAS) of the vehicle 10. The tire-road adhesion characteristics of a particular surface over which the vehicle 10 is travelling may also be received wirelessly by the vehicle 10 from another vehicle (V2X communication).

The vehicle 10 includes a vehicle control system (VCS) 26 according to an embodiment the invention. The VCS 26 is configured to determine the ground speed of the vehicle 10 and to determine a reference or desired wheel slip ratio, Aref, in dependence on the ground speed determination. The VCS 26 includes a processor 28 for making the above determinations, the processor 28 having an input 30, i.e. receiving means, that is arranged to receive data from the sensors 16, pedals 20, SE block 22 and adhesion coefficient block 24.

The VCS 26 includes a data memory or memory device 32 having instructions stored therein, the processor 28 being arranged to execute said instructions in order to determine the ground speed of the vehicle 10 and to determine the reference or desired wheel slip ratio, Xref, based on the determined ground speed. The data memory 32 may be an electronic, non-transitory, computer-readable storage medium.

The processor 28 has an output 34 for providing one or more of the determinations made by the processor 28 to one or more subsystems 36 of the vehicle 10. In the described embodiment, a calculated error value between the determined (actual) and desired wheel slip values is provided to an electronic Stability Control System (eSCS) 36 of the vehicle 10, as described below. The eSCS 36 is incorporated into the traction system, which operates by controlling the engine power delivered to the wheels 14a, 14b to improve the stability of the vehicle 10 by reducing slip between the wheels 14a, 14b, 14c, 14d and the surface over which the vehicle 10 is travelling. The traction system is controlled according to a traction control protocol. The traction control protocol controls the traction system to apply varying degrees of traction torque, TLact, and/or induces operation of the eSCS 36, depending on a signal received from the output 34 from the VCS 26 of the vehicle 10.

The eSCS 36 is also incorporated into the braking system, and operates by application and release of the brake pads according to a predefined frequency. The braking system is controlled according to a braking control protocol. The braking control protocol controls the braking system to apply varying degrees of actual braking torque, Tb act, and/or induces operation of the eSCS 36, depending on a signal received from the output 34 of the VCS 26.

The operation of the VCS 26 and eSCS 36 will be described in the case of a braking event only; however, the description applies equally to a traction event.

Figure 2 shows a free body diagram of the relative directions of various parameters associated with the wheel 14a during a braking event, i.e. when the braking system applies braking torque to the wheels 14a, 14b. Details of the wheel 14a are not shown; however, the outer periphery of the wheel 14a includes a tyre 42a. The vehicle parameters include vehicle ground speed, V, rotational speed of the wheel, ω, braking torque, Tb, (that is, the applied torque of the braking system, i.e., Tb act above), downwards force on the wheel 14a, Fz (which is an estimated parameter based on a mass calculation of the vehicle 10), and the tyre braking or tractive longitudinal force, Fx. As illustrated in Figure 2, the braking torque, Tb, acts in the direction opposite to the direction of angular velocity, ω, whereas in contrast, the equivalent traction torque, Tt, would act in the same direction as the direction of rotational speed, ω.

Fx is a function of the wheel slip, which is a measure of the relationship between tyre deformation and the longitudinal force, Fx, that causes braking or acceleration. In general, the rotational speed of a wheel does not result in the expected observed longitudinal velocity (ground speed) of a vehicle because the actual movement of the tyre with respect to the surface is a mixture of rolling and slipping. In particular, the actual wheel slip, Xb act, of a wheel during a braking event is given by

where ωυί = 1,2,3,4, is the rotational speed, measured in radians per second, of that particular wheel and r is the radius of the wheel.

Note that for pure rolling motion, V = ωέΓ and so Xbact = 0, i.e. there is zero slip between the tyre of the wheel and the surface.

As is seen in the above equation, a value of the ground speed, V, is needed in order to determine the wheel slip ratio, Xb act. A method 50 as performed by the processor 28 for determining the ground speed is now described in detail. The VCS 26 receives measurements of the angular wheel speed, ω, for each of the wheels 14a, 14b, 14c, 14d from each respective wheel speed sensor 15a, 15b, 15c, 15d.

At step 52, the processor 28 first makes an estimation of the current ground speed, Vest, using Kalman filtering. This is to be used as a comparison to the ground speed as determined according to the present invention. The processor 28 retrieves the estimated ground speed, Vest, from a previous time step, along with the current wheel speed measurements from the wheels speed sensors 16, and calculates a current estimation of Vest.

At step 54, the processor 28 makes a first calculation of the ground speed, V, by combining the current rotational wheel speed measurements for all four of the wheels 14a, 14b, 14c, 14d. The rotational wheel speed measurements for each wheel 14a, 14b, 14c, 14d as measured by the sensors 16a, 16b, 16c, 16d are denoted ω1βω2,ω3,ω4, respectively. Then \/is calculated using the equation

where R(p) is the wheel rolling radius. That is, the average of all four rotational wheel speed measurements is used to make the first calculation of ground speed.

At step 56, the processor 28 compares the estimated ground speed as calculated by Kalman filtering, Vest, with the ground speed as calculated by combining the four rotational wheel speed measurements, V. The processor 28 also checks the rotational acceleration ών ώ2, ώ3, ώ4, of the wheels 14a, 14b, 14c, 14d. This is achieved by filters from the measurements of rotational wheel speed from the sensors 16. In particular, if the difference between Vest and V is less than a predetermined error threshold value and each of the rotational accelerations ώ1β ώ2, ώ3, ώ4 are less than a predetermined rotational acceleration threshold value, then V (as calculated using all four wheel speed measurements) is taken to be the ground speed and is used to then calculate the wheel speed ratio. The predetermined error and rotational acceleration threshold values are stored in the data memory 32. Expressed differently, the processor 28 checks if Vest * V to ensure the robustness of the determined ground speed value: a relatively large discrepancy between Vest and V, i.e. their difference is greater than the predetermined error threshold value, may be indicative that one or more of the measured rotational wheel speeds, ω, does not correspond closely to the actual vehicle ground speed, i.e. that particular wheel is experiencing a relatively high level of slip, and so the determined ground speed, V, based on this value of ω may not necessarily be accurate.

As such, if the difference between Fest and V \s greater than the predetermined error threshold value or any of the rotational accelerations ών ώ2, ώ3, ώ4 are greater than the predetermined rotational acceleration threshold value, then it is determined that one or more of the wheels 14a, 14b, 14c, 14d is slipping, i.e. undergoing a slip event.

In this case, the processor 28 then checks whether the brake pedal 20 is depressed at step 58. If it is, then the processor 28 starts storing speed data for a predetermined interval of time at step 60, for instance 100 ms. In particular, the estimated ground speed value, Vest, and the rotational wheel speeds ω1,ω2,ω3,ω4 are stored in the data memory 32. These stored values may be used to calculate the current ground speed, V, if it is later determined that all four wheels are slipping, as will be described below.

If it is determined that the brake pedal 20 is not depressed then step 60 is skipped. Irrespective of the determination at step 58, the processor 28 determines which of the wheels 14a, 14b, 14c, 14d is slipping or locked at step 62. This is achieved by comparing each of the rotational wheels speeds ωνω2,ω3,ω4 against one another. With reference to Figure 4, the processor 28 calculates the following values:

The one or more wheels that are slipping may be identified by analysing the six calculated values above. That is, if the modulus of any of the differences in wheel speed above is relatively large, then that is an indication that one of the relevant wheels is in a state of high slip, i.e. the wheel is locked or spinning. Specifically, if the modulus of any of the differences is above a predetermined rotational speed difference threshold value, then that is indicative that a wheel is slipping.

Figures 5a-e illustrate respectively the differences needed to show: the front left wheel 14a is locked or spinning; the front right wheel 14b is locked or spinning; the rear left wheel 14c is locked or spinning; the rear right wheel 14d is locked or spinning; and both the rear left and rear right wheels are spinning.

Referring to Figure 5a, if the absolute values of Δω1,Δω4,Δω6 (indicated by the arrows, and as defined above) are all above the predetermined rotational speed difference threshold value, then it is determined that the front left wheel 14a only is either locked or spinning. This is because these three values being greater than the threshold means that the rotational speed of the front left wheel 14a is (significantly) different than the rotational speed of each of the front right wheel 14b, rear left wheel 14c and the rear right wheel 14d, and this taken to be indicative that the front left wheel 14a is locked or spinning. The sign of one of Δω!,Δω4,Δω6 can then be used to determine which of these is the case, if that information is needed, for instance if Δω4 (= ω4 -ω2) is positive then the front left wheel 14a is rotating at a speed greater than the front right wheel 14b (i.e. the front left wheel 14a is spinning), and conversely if Δω! is negative then the front left wheel 14a is taken to be locked.

The processor 28 applies a similar analysis to determine that one of the other three wheels 14b, 14c, 14d is locked or spinning. Referring to Figure 5b, if the absolute values of Δω!,Δω2,Δω5 are all above the predetermined rotational speed difference threshold value, then it is determined that the front right wheel 14b only is either locked or spinning. Referring to Figure 5c, if the absolute values of Δω2,Δω3,Δω5 are all above the predetermined rotational speed difference threshold value, then it is determined that the rear right wheel 14d only is either locked or spinning. Referring to Figure 5d, if the absolute values of Δω3,Δω4,Δω5 are all above the predetermined rotational speed difference threshold value, then it is determined that the rear left wheel 14c only is either locked or spinning.

The processor 28 can also determine if more than one wheel is locked or spinning. Referring to Figure 5e, if Δωχ is below the predetermined rotational speed difference threshold value (i.e. the rotational speeds of the front left wheel 14a and the front right wheel 14b are substantially equal or similar), both Δω2,Δω4 are above the predetermined rotational speed difference threshold value (i.e. the rotational speeds of the front right wheel 14b and the rear right wheel 14d are different), and ω1ψΟ (i.e. the front left wheel 14a is not locked) then it is determined that both the rear left and rear right wheels 14c, 14d are locked.

Once the processor 28 analyses the wheel speed differences using a relation matrix to determine which wheels are slipping or locked, at step 64 the processor 28 makes a further check to determine if all four of the wheels 14a, 14b, 14c, 14d are locked. For instance, this may be achieved by checking whether the wheel speeds a)t, ω2, ω3,ω4 are all equal to, or substantially equal to, zero.

If it is determined that not all of the wheels 14a, 14b, 14c, 14d are slipping, then at step 66 the processor 28 determines the ground speed, V, based on the rotational wheels speeds of those wheels determined not to be slipping. For instance, referring back to Figure 5a, in the case in which the processor 28 determines that the front left wheel 14a is slipping, then the ground speed is calculated to be

That is, the average of the measured rotational wheel speeds corresponding to those wheels determined not to be slipping is used to calculate the ground speed. Similarly, referring back to Figure 5e, in the case in which the processor 28 determines that the rear left and rear right wheels 14c, 14d are slipping, then the ground speed is calculated to be

This calculated value of V may then be used to calculate Xb act in the manner described above.

If, however, it is determined that all of the wheels 14a, 14b, 14c, 14d are slipping, then the processor 28 determines Vby a different process. The processor 28 estimates the vehicle deceleration pattern based on the tyre-road adhesion value, μαοί, as received from the adhesion coefficient block 24, and the speed data stored at step 60. Specifically, with reference to Figure 6, at step 68 the processor 28 calculates the gradient, m, of the change in estimated ground speed, Vest, over the predetermined interval of time that the speed data is stored using

where VrefInit and VrefFinal are the estimated ground speed values, Vest (as estimated by Kalman filtering, for instance) at the start and end of the predetermined interval, respectively, and AT is the time interval (e.g. 100 ms).

The ground speed, V, is then determined at step 70 using

This is the value of l/that can then be used to calculate Ab act. This determined value of the ground speed value, V, improves the equivalent estimation by Kalman filtering, i.e. VrerFinal, by taking into account the adhesion coefficient, which may be particularly significant during transient behaviour such as a braking event.

Once the wheel slip ratio, Ab_act, has been calculated, the processor 28 compares this value against a reference wheel slip, Ab_ref, and the difference between the two is output by the VCS 26 to the eSCS 36. The eSCS 36 then controls the amount of braking torque delivered to the wheels 14a, 14b, 14c, 14d via the brakes based on the above-mentioned difference and the brake torque requested by the driver as measured by the amount the brake pedal 20 is depressed. The reference or target wheel slip is the desired value of slip to give optimum vehicle performance and is dependent on many factors including type of surface a vehicle is travelling over. The reference wheel slip may be the value of slip that gives the maximum coefficient of friction of a tyre. A starting value, or ‘initial guess’, of the reference wheel slip, Abref, may be determined by the processor 28 by solving the differential equation

for Abref, where J is the moment of wheel inertia and M is the mass of the vehicle 10. In practice, the optimal wheel slip ratio is dependent on various parameters, in particular the weather conditions and surface type. Therefore, subsequently, the processor 28 determines the reference slip ratio, Ab ref, based on the sources of information that are most appropriate in the present conditions. In particular, the processor 28 receives the wheel speed measurements, ω, the maximum adhesion coefficient (from V2X, ADAS or the data memory 32), the determined actual slip ratio, T<j_acti and the requested braking torque, Tb req, and uses predefined rules to classify and select the appropriate source of information for the estimation of the reference wheel speed ratio. For instance, this may be based on inputs relating to the ambient temperature and/or the windshield wipers.

Many modifications may be made to the above examples without departing from the scope of the present invention as defined in the accompanying claims.

The above-described embodiment describes the case where a wheel slip event occurs in a so-called braking mode, i.e. during a braking event; however, a wheel slip event can occur in other vehicle modes. For instance, a wheel slip event can occur in a so-called traction mode, i.e. when there is a relatively large amount of requested torque via the accelerator pedal. While wheel lock may be more common in a braking mode, wheel spin may be more common in a traction mode. In addition, a wheel slip event can occur with no requested braking or traction torque but with a large steering angle request, i.e. when a steering wheel of the vehicle 10 is turned through a relatively large angle. The broadest scope of the invention covers any vehicle mode in which wheel lock or spin occurs.

As the above-described embodiment does not take into account the steering wheel angle, it is assumed that the vehicle 10 is travelling in a straight line and therefore that there is zero transverse slip, i.e. that the determined ground speed is in fact the longitudinal speed of the vehicle and the determined wheel slip ratio is the longitudinal wheel slip. In different embodiments, this may readily be extended so as to include the transverse components of the ground speed and wheel slip ratio.

Although shown separately in Figure 1, the state estimator block 22 and/or the adhesion coefficient block 24 may be integral with the VCS 26.

Claims

1. A system for determining the ground speed of a vehicle, the vehicle comprising a plurality of wheels each having an associated wheel speed sensor for measuring rotational wheel speed, and the system comprising: receiving means configured to receive a measured rotational wheel speed value from each of the wheel speed sensors; and processing means configured to determine which of the wheels are undergoing a slip event and which of the wheels are not undergoing a slip event, said determination including determining the difference between each pair of the plurality of measured rotational wheel speed values, and the processing means being configured to calculate the ground speed based on all of the measured rotational wheel speed values corresponding to the wheels that are determined not to be undergoing a slip event.

2. A system according to Claim 1, the receiving means and processing means comprising an electronic processor having an electrical input for receiving the rotational wheel speed data, and the system comprising an electronic memory device electrically coupled to the electronic processor and having instructions stored therein, wherein the processor is configured to access the memory device and execute the instructions stored therein such that it is operable to determine which of the wheels are undergoing a slip event and which of the wheels are not undergoing a slip event, and to calculate the ground speed based on the measured rotational wheel speed values corresponding to the wheels that are determined not to be undergoing a slip event.

3. A system according to Claim 1 or Claim 2, wherein determination of a slip event includes calculating the ground speed based on the measured rotational wheel speed values for all of the wheels.

4. A system according to Claim 3, wherein determination of a slip event includes determining whether the difference between the calculated ground speed and an estimation of ground speed is greater than a predetermined error difference threshold.

5. A system according to Claim 4, wherein the estimation of ground speed is calculated using state estimation.

6. A system according to any previous claim, wherein determination of a slip event includes determining whether the rotational acceleration of any of the wheels is greater than a predetermined rotational acceleration threshold.

7. A system according to any previous claim, wherein if each of the absolute values of the differences between the measured rotational wheel speed of a first wheel of the plurality of wheels and the measured rotational wheel speeds of each of the other of the plurality of wheels is greater than a predetermined rotational speed difference threshold value, then it is determined that the first wheel only is undergoing a slip event.

8. A system according to Claim 7, the processing means being configured to determine whether the first wheel is locked or spinning based on the sign of the difference between the measured rotational wheel speed of the first wheel and the measured rotational wheel speed of one of the other wheels.

9. A system according to any of Claims 1 to 6, wherein determining that at least two, but fewer than all, of the plurality of wheels are undergoing a slip event includes determining that the measured rotational wheel speed of at least one of the wheels is not substantially equal to zero.

10. A system according to any previous claim, the processor being configured to calculate the ground speed by calculating an average value of the measured rotational wheel speed values corresponding to each of the wheels that are determined not to be undergoing a slip event.

11. A system according to any previous claim, wherein the processor is configured to determine if all of the wheels are undergoing a slip event by checking whether the measured rotational wheel speeds corresponding to all of the wheels are substantially zero.

12. A system according to Claim 11, the receiving means being configured to receive a tyre-road adhesion coefficient, wherein if the processor determines that all of the wheels are undergoing a slip event then the processor is configured to calculate the ground speed by estimating an acceleration or deceleration pattern of the vehicle based on the received tyre-road adhesion coefficient.

13. A system according to Claim 12, the tyre-road adhesion coefficient being received via vehicle to vehicle wireless communication.

14. A system according to Claim 12 or Claim 13, the ground speed, V, being calculated by V=Vreflnit^actwAT, where μαα is the tyre-road adhesion coefficient, m = Vrefp^ai^Jrefmit jS t^e acceieration or deceleration pattern, vrefInit ancl ^refFinal are estimated ground speed values at the start and end, respectively, of a predetermined time interval, and AT is the predetermined time interval.

15. A system according to Claim 14, wherein the time interval is greater than or equal to 100ms.

16. A system according to any previous claim, wherein the processor is configured to calculate a reference wheel slip ratio in dependence on the calculated ground speed.

17. A system according to Claim 16, the processor being configured to calculate a wheel slip ratio difference between an actual wheel slip ratio for one or more of the wheels and the reference wheel ratio.

18. A system according to Claim 17, comprising an output configured to output the calculated wheel slip ratio difference to a stability control subsystem of the vehicle.

19. A system accords to any previous claim, the processor being configured to calculate the ground speed in real time.

20. A method for determining the ground speed of a vehicle, the vehicle comprising a plurality of wheels each having an associated wheel speed sensor for measuring rotational wheel speed, and the method comprising: receiving a measured rotational wheel speed value from each of the wheel speed sensors; determining which of the wheels are undergoing a slip event and which of the wheels are not undergoing a slip event, said determination including determining the difference between each pair of the plurality of measured rotational wheel speed values; and calculating the ground speed based on all of the measured rotational wheel speed values corresponding to the wheels that are determined not to be undergoing a slip event.

21. A vehicle comprising a system according to any of Claims 1 to 19.

22. A non-transitory, computer-readable storage medium storing instructions thereon that when executed by one or more processors causes the one or more processors to carry out the method of Claim 20.

23. A system, a method or a vehicle substantially as hereinbefore described with reference to the accompanying figures.