Method and apparatus for detecting wheel slip

Abstract

A control system and method for detecting a wheel slip event SEV of a wheel of a vehicle. The method includes determining a moving average SAV(1), 70 of the wheel speed e.g. by filtering detected whee

Description

TECHNICAL FIELD The present disclosure relates to a method and apparatus for detecting wheel slip. Aspects of the invention relate to a control system, a system, a vehicle and a method. BACKGROUND It is known to detect wheel slip in a road vehicle, such as an automobile, by comparing the wheel speeds. If a variance in the wheel speed exceeds a threshold value, a controller may detect a wheel slip event at one or more of the wheels. One or more vehicle systems may be controlled in dependence on the detection of the slip event. However, this approach may not detect a wheel slip event if several of the wheels are undergoing slip at the same time. This may occur, for example, on a medium or low grip surface on which the wheel speeds may oscillate under acceleration. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a control system, a system, a vehicle and a method as claimed in the appended claims. According to an aspect of the present invention there is provided a control system for detecting a wheel slip event of a wheel of a vehicle, the control system comprising one or more processors collectively configured to: receive a wheel speed signal indicating a wheel speed of the wheel; determine a moving average of the wheel speed in dependence on the wheel speed signal; determine a first wheel speed threshold and a second wheel speed threshold in dependence on the determined moving average; detect a first cross time when the wheel speed crosses one of the first wheel speed threshold and the second wheel speed threshold; detect a second cross time when the wheel speed crosses one of the first wheel speed threshold and the second wheel speed threshold; detect a wheel slip signature pattern in dependence on the first cross time and the second cross time; and output a wheel slip detected signal indicating a wheel slip event in dependence on detection of the wheel slip signature pattern. The wheel slip signature pattern is indicative of a wheel slip event. The control system is configured to detect the wheel slip signature pattern in the wheel speed. The wheel slip signature pattern may comprise a magnitude and / or a frequency of an oscillation in the wheel speed. The wheel slip signature pattern in the present embodiment is detected in dependence on the first and second cross times when the wheel speed crosses the respective first and second wheel speed thresholds. At least in certain embodiments, the wheel slip signature pattern comprises an oscillation in the wheel speed. The oscillation in the wheel speed has a predetermined frequency or occurs within a predetermined frequency range. Detecting the wheel slip signature pattern may comprise monitoring a frequency of an oscillation in the wheel speed. The wheel slip signature pattern may be detected in dependence on a determination that the frequency of the oscillation at least substantially corresponds to the predetermined frequency or is within the predetermined frequency range. The frequency of the oscillation in the wheel speed may be determined directly or indirectly in dependence on the first cross time and the second cross time. The direct determination of the frequency may comprise calculating the frequency of the oscillation in dependence on a time interval between the first cross time and the second cross time. The indirect determination of the frequency may comprise comparing a time interval between the first cross time and the second cross time to a reference time interval of an oscillation having the predetermined frequency or a reference time interval range of an oscillation within the predetermined frequency range. The predetermined frequency range may be defined by at least one of a first frequency threshold and a second frequency threshold. The wheel slip signature pattern may comprise a magnitude of the oscillation in the wheel speed. The magnitude of the oscillation may be determined by at least one of the first wheel speed threshold and the second wheel speed threshold. The first wheel speed threshold and the second wheel speed threshold may define upper and lower thresholds. A time interval between the first cross time and the second cross time may be determined. The determined time interval is related to a time period (T) of one complete oscillation in the wheel speed. The time interval may represent a part of one or more time period of the oscillation. Alternatively, the time interval may represent one or more complete time period of the oscillation. The wheel slip signature pattern in the wheel speed may be identified in dependence on the time interval between the first cross time and the second cross time. A frequency (f) of the oscillation is the reciprocal of the time period (T) (f=1 / T). The frequency of the oscillation may be calculated directly from the time interval. The wheel slip signature pattern may be identified in dependence on a determination that the determined frequency is at least substantially equal to the predetermined frequency which is indicative of a wheel slip event, or occurs within the predetermined frequency range which is indicative of a wheel slip event. The time interval may optionally be adjusted to represent the time period of one complete oscillation. For example, if the time interval represents half of one complete oscillation, the time interval may be doubled to determine the time period of the complete oscillation. The frequency may be determined in dependence on the adjusted time interval. Alternatively, or in addition, a reference time interval and / or a reference time interval range may be defined. The reference time interval corresponds to a time interval of an oscillation occurring at the predetermined frequency which is indicative of a wheel slip event. The reference time interval range corresponds to a time interval range of an oscillation occurring within the predetermined frequency range which is indicative of a wheel slip event. The reference time interval range may be defined by at least one of a first time threshold and a second time threshold. The first time threshold may correspond to a time interval range of an oscillation occurring at the first frequency threshold; and the second time threshold may correspond to a time interval range of an oscillation occurring at the second frequency threshold. The wheel slip signature pattern may be identified by comparing the determined time interval to the reference time interval and / or the reference time interval range. A wheel slip event may be detected in dependence on a determination that the time interval is at least substantially equal to the reference time interval. Alternatively, or in addition, a wheel slip event may be detected in dependence on a determination that the time interval is within the reference time interval range. This approach may enable identification of the wheel slip signature pattern without directly calculating the frequency of the oscillation This may facilitate detection of a wheel slip signature pattern which is indicative of a wheel slip event. The control system may comprise one or more controllers collectively comprising at least one electronic processor having an electrical input for receiving an input signal; and at least one memory device electrically coupled to the at least one electronic processor and having instructions stored therein. The at least one electronic processor may be configured to access the at least one memory device and execute the instructions thereon so as to: receive a wheel speed signal indicating a wheel speed of the wheel; determine a moving average of the wheel speed in dependence on the wheel speed signal; determine a first wheel speed threshold and a second wheel speed threshold in dependence on the determined moving average; detect a first cross time when the wheel speed crosses one of the first wheel speed threshold and the second wheel speed threshold; detect a second cross time when the wheel speed crosses one of the first wheel speed threshold and the second wheel speed threshold; detect a wheel slip signature pattern in dependence on the first cross time and the second cross time; and output a wheel slip detected signal indicating a wheel slip event in dependence on detection of the wheel slip signature pattern. The detection of the wheel slip signature pattern may comprise monitoring a frequency of an oscillation in the wheel speed. The frequency of the oscillation in the wheel speed may be determined directly or indirectly in dependence on the first cross time and the second cross time. The frequency may be determined in dependence on the first cross time and the second cross time. The frequency of the oscillation in the wheel speed may be determined in dependence on the time interval between the first cross time and the second cross time. The one or more processor may be configured to determine the time interval. A frequency of the oscillation may be determined in dependence on the time interval. As described herein, the time interval may represent a part (i.e., a sub-section) of one or more time period (T) of one complete oscillation, or may represent the time period (T) of one or more complete oscillation. The time interval between the first and second cross times may be used to determine (or to approximate) the frequency of the or each oscillation. The frequency of the or each oscillation may be determined directly or indirectly in dependence on the first and second cross times. The wheel slip signature pattern may be detected in dependence on a determination that the frequency is within a predetermined frequency range. The control system may thereby determine if the oscillation corresponds to the wheel slip signature pattern which is indicative of the wheel slip event. A reference time interval may be defined corresponding to the time interval of an oscillation occurring at the predetermined frequency which is indicative of a wheel slip event. Alternatively, or in addition, a reference time interval range may be defined corresponding to a time interval range of an oscillation occurring within the predetermined frequency range which is indicative of a wheel slip event. The reference time interval range may be defined by at least one of a first time threshold and a second time threshold. The one or more processor may be configured to identify a wheel slip signature pattern by comparing the determined time interval to the reference time interval and / or the reference time interval range. A wheel slip event may be detected in dependence on a determination that the time interval is at least substantially equal to the reference time interval. Alternatively, or in addition, a wheel slip event may be detected in dependence on a determination that the time interval is within the reference time interval range. This approach uses the time interval to determine the frequency of the oscillation indirectly. This may enable identification of the wheel slip signature pattern without directly calculating the frequency of the oscillation. The first and second wheel speed thresholds are different from each other. At least in certain embodiments, the second wheel speed threshold is less than the first wheel speed threshold. The first wheel speed threshold may be greater than the determined moving average. The second wheel speed threshold may be less than the determined moving average. In a variant, the first wheel speed threshold may be less than the second wheel speed threshold The first wheel speed threshold and / or the second wheel speed threshold may be calculated in dependence on the determined moving average. The first wheel speed threshold may be the product of the moving average and a first factor; and / or the second wheel speed threshold may be the product of the moving average and a second factor. The first and second factors may be predefined. The first factor may be greater than or equal to one (1); and the second factor may be less than or equal to one (1). The first factor may be larger than the second factor. This may reflect a wheel slip event comprising an increase in the wheel speed followed by a decrease in the wheel speed. The first factor may be greater than one (1). The second factor may be less than one (1). The first factor may be 1.15 and the second factor may be 0.95, for example. These values are illustrative of the firstand second factors. It will be understood that the first factor and / or the second factor may be greater than or less than the stated values. The first factor and / or the second factor may be calibratable, for example for a particular drivetrain. The first and second wheel speed thresholds define an operating range (relative to the moving average of the wheel speed) in which the first wheel speed can vary without a wheel slip event being detected. The first wheel speed threshold and / or the second wheel speed threshold may be modified to adjust the operating range, for example to increase or decrease one or more of the first and second wheel speed thresholds. In a variant, at least one of the first and second wheel speed thresholds may be calculated by adding a predetermined value (positive or negative) to the moving average. For example, a first constant may be added to the moving average to calculate the first wheel speed threshold; and / or a second constant may be added to the moving average to calculate the second wheel speed threshold. By way of example, the first constant may be +1,5m / s and the second constant may be -0.5m / s. For example, the first constant may be in the range +1 m / s to +2m / s; and / or the second constant may be in the range Om / s to -1 m / s. The predetermined frequency range may be defined by at least one of a first frequency threshold and a second frequency threshold. The second frequency threshold may be lower than the first frequency threshold. The first frequency threshold may be an upper frequency threshold; and the second frequency threshold may be a lower frequency threshold. In a variant, one of the first and second wheel speed thresholds may be substantially equal to the moving average. The time interval between the first cross time and the second cross time may be compared to a reference time interval range. The wheel slip signature pattern may be detected in dependence on a determination that the time interval is within the reference time interval range. The reference time interval range may be defined by at least one of a first time interval threshold and a second time interval threshold. The first time interval threshold may be equal to the time interval of an oscillation in the wheel speed at the first frequency threshold. The second time interval threshold may be equal to the time interval of an oscillation in the wheel speed at the second frequency threshold. The first frequency threshold may be 11 Hz, 12Hz, 13Hz or MHz. The second frequency threshold may be 6Hz, 7Hz, 8Hz or 9Hz. The predetermined frequency range may be 6Hz to MHz; or 7Hz to 13Hz. At least in certain embodiments, the predetermined frequency range may be in the range 8Hzto 12Hz. The one or more processor may be collectively configured to detect an end to the wheel slip signature pattern. The one or more processor may output a wheel slip completion signal indicating the completion of the wheel slip event in dependence on the detection of the end of the wheel slip signature pattern. The detection of the end to the wheel slip signature pattern may comprise determining that the determined frequency of the change in the wheel speed is outside the predetermined frequency range. The first cross time may represent a first time when the wheel speed crosses the first wheel speed threshold; and the second cross time may represent a second time when the wheel speed crosses the first wheel speed threshold. The wheel speed may be increasing at the first cross time and decreasing at the second cross time. Conversely, the wheel speed may be decreasing at the first cross time and increasing at the second cross time. Alternatively, the first cross time may represent a first time when the wheel speed crosses the first wheel speed threshold; and the second cross time may represent a second time when the wheel speed crosses the first wheel speed threshold. The wheel speed may be increasing at the first time; and decreasing at the second time. Conversely, the wheel speed may be decreasing at the first time; and increasing at the second time. It will be understood that it is not necessary to determine the second wheel speed threshold in this implementation. As outlined above, the one or more processor may be collective configured to determine a time interval between the first cross time and the second cross time. The first cross time may be detected when the wheel speed crosses the first wheel speed threshold; and the second cross time may be detected when the wheel speed crosses the second wheel speed threshold. Conversely, the first cross time may be detected when the wheel speed crosses the second wheel speed threshold; and the second cross time may be detected when the wheel speed crosses the first wheel speed threshold. The time interval may represent a time period between an increase in the wheel speed above the first wheel speed threshold and a consecutive decrease in the wheel speed below the second wheel speed threshold; and / or a time period between a decrease in the wheel speed below the second wheel speed threshold and a consecutive increase in the wheel speed above the first wheel speed threshold According to a further aspect of the present invention there is provided a control system for detecting a wheel slip event of a wheel of a vehicle, the control system comprising one or more processors collectively configured to: receive a wheel speed signal indicating a wheel speed of the wheel; process the wheel speed signal to detect an oscillation in the wheel speed; determine a frequency of the detected oscillation in the wheel speed; detect a wheel slip signature pattern in dependence on a determination that the frequency of the detected oscillation is substantially equal to a predetermined frequency or is within a predetermined frequency range; and output a wheel slip detected signal indicating a wheel slip event in dependence on detection of the wheel slip signature pattern. According to an aspect of the present invention there is provided a control system for detecting a wheel slip event of a wheel of a vehicle, the control system comprising one or more processors collectively configured to: receive a wheel speed signal indicating a wheel speed of the wheel; determine a moving average of the wheel speed in dependence on the wheel speed signal; determine a first wheel speed threshold in dependence on the determined moving average; detect a first cross time when the wheel speed crosses the first wheel speed threshold; detect a second cross time when the wheel speed crosses the first wheel speed threshold; detect a wheel slip signature pattern in dependence on the first cross time and the second cross time; and output a wheel slip detected signal indicating a wheel slip event in dependence on detection of the wheel slip signature pattern. The first cross time and the second cross time may represent consecutive times when the wheel speed crosses the first wheel speed threshold. A time interval between the first cross time and the second cross time may correspond to the time period of one complete oscillation of the wheel speed. A frequency of the oscillation in the wheel speed may be determined directly or indirectly in dependence on the time interval The wheel slip signature pattern may be detected by determining that the oscillation occurs at a predetermined frequency or within a predetermined frequency range. The technique(s) described herein to detect the wheel slip signature pattern are applicable with respect to the first and second cross times associated with the first wheel speed threshold. The first wheel speed threshold may be equal to the determined moving average of the wheel speed. Alternatively, the first wheel speed threshold may be greater than or less than the determined moving average of the wheel speed. Alternatively, the first wheel speed threshold may be determined by multiplying the moving average of the wheel speed by a factor. The factor may be predefined, for example based on empirical analysis. The factor may be greater than or less than one (1). Alternatively, the first wheel speed threshold may be calculated by adding a predetermined value (positive or negative) to the moving average. For example, a first constant may be added to the moving average to calculate the first wheel speed threshold. Other techniques may be used to determine the first wheel speed threshold The control system is suitable for use in a vehicle. The vehicle may comprise a plurality of wheels. The one or more processors may be collectively configured to detect the wheel slip event on one or more of the plurality of wheels. According to a further aspect of the present invention there is provided a system comprising the control system described herein. The system may comprise at least one vehicle control unit for controlling at least one vehicle subsystem of the vehicle. The at least one vehicle control unit may be configured to modify the operation of the at least one vehicle subsystem in dependence on the wheel slip detected signal from the control system. Alternatively, or in addition, the at least one vehicle control unit may be configured to select one of a plurality of vehicle drive modes The vehicle drive modes may each define one or more operating parameter of the at least one vehicle subsystem. The at least one vehicle control unit may be configured to select one of the plurality of vehicle drive modes in dependence on the wheel slip detected signal from the control system. According to a further aspect of the present invention there is provided a vehicle comprising a control system and / or a system as described herein. According to a further aspect of the present invention there is provided a method of detecting a wheel slip event of a wheel of a vehicle, the method comprising: measure a wheel speed of the wheel; determine a moving average of the wheel speed; determine a first wheel speed threshold and a second wheel speed threshold in dependence on the determined moving average; detect a first cross time when the wheel speed crosses one of the first wheel speed threshold and the second wheel speed threshold; detect a second cross time when the wheel speed crosses one of the first wheel speed threshold and the second wheel speed threshold; detect a wheel slip signature pattern SP in dependence on the first cross time and the second cross time; and detect the wheel slip event in dependence on the detection of the wheel slip signature pattern. The first and second wheel speed thresholds are different from each other. The second wheel speed threshold may be less than the first wheel speed threshold. The detection of the wheel slip signature pattern may comprise monitoring a frequency of an oscillation in the wheel speed in dependence on the first cross time and the second cross time. The frequency of the oscillation in the wheel speed may be determined directly or indirectly in dependence on the first cross time and the second cross time. The wheel slip signature pattern may be detected in dependence on a determination that the determined frequency is at least substantially equal to a predetermined frequency or is within a predetermined frequency range. A reference time interval may be defined corresponding to a time interval of an oscillation occurring at the predetermined frequency which is indicative of a wheel slip event. A reference time interval range may be defined corresponding to a time interval range of an oscillation occurring within the predetermined frequency range which is indicative of a wheel slip event. The method may comprise identifying a wheel slip signature pattern by comparing the determined time interval to the reference time interval and / or the reference time interval range. The reference time interval range may be defined by a first time threshold and a second time threshold The first time interval threshold may be equal to the time interval of an oscillation in the wheel speed at the first frequency threshold. The second time interval threshold may be equal to the time interval of an oscillation in the wheel speed at the second frequency threshold. According to a further aspect of the present invention there is provided a control system for detecting a wheel slip event of a wheel of a vehicle, the control system comprising one or more processors collectively configured to: receive a wheel speed signal indicating a wheel speed of the wheel; process the wheel speed signal to detect an oscillation in the wheel speed; determine a frequency of the detected oscillation in the wheel speed; detect a wheel slip signature pattern in dependence on a determination that the frequency of the detected oscillation is substantially equal to a predetermined frequency or is within a predetermined frequency range; and output a wheel slip detected signal indicating a wheel slip event in dependence on detection of the wheel slip signature pattern. The oscillation may be detected in dependence on a determination that the wheel speed crosses one or more wheel speed threshold. The or each wheel speed threshold may, for example, be determined in dependence on a moving average of the wheel speed for that wheel. The frequency of the detected oscillation may be determined in dependence on a time interval between a first time and a second time that the wheel speed crosses the one or more wheel speed threshold. Alternatively, or in addition, the frequency of the detected oscillation may be determined in dependence on a time interval between a first time and a second time that the wheel speed crosses the moving average of the wheel speed. The time interval is related to a time period (T) of the oscillation. The time interval may represent a part of one or more time period or one or more complete time period. Determining the frequency of the oscillation in the wheel speed may comprise determining a time interval between the first cross time and the second cross time. The predetermined frequency range may be defined by one or more frequency threshold. At least in certain embodiments, the predetermined frequency range may be defined by at least one of a first frequency threshold and a second frequency threshold The first and second frequency thresholds may be different from each other. The second frequency threshold may be lower than the first frequency threshold. According to a further aspect of the present invention there is provided computer readable instructions which, when executed by one or more processors, cause the one or more processors to perform the method(s) described herein. 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 anyway 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 representation of a vehicle comprising a control system in accordance with an embodiment of the present invention; Figure 2 shows a plan view of the vehicle shown in Figure 1; Figure 3 shows a first trace representing the measured wheel speed of each of the wheels of the vehicle; Figure 4 shows a schematic representation of a control system for detecting a wheel slip event in accordance with an embodiment of the present invention; Figure 5 shows a second trace representing a measured wheel speed of one of the wheels of the vehicle and the signature pattern identified to detect a wheel slip event; and Figure 6 shows a block diagram of a method of detecting a wheel slip event in accordance with an embodiment of the present invention. DETAILED DESCRIPTION A control system 1 and a method 200 for detecting a wheel slip event SEV(n) in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figures. The vehicle 5 is described herein with reference to a reference frame comprising a longitudinal axis X, a transverse axis Y and a vertical axis Z. The reference signs herein include a suffix in the form of a whole number to differentiate between a plurality of like components on the vehicle 5. The same suffix is applied for components associated with each other, for example components forming part of the same sub-assembly of the vehicle 5. The integer n is used herein to identify a signal or event relating to a corresponding one of a plurality of features of the vehicle 5. As shown in Figure 1, the control system 1 is installed in a vehicle 5 comprising four (4) wheels W(1)-W(4). The vehicle 5 is a road vehicle, such as an automobile, a sports utility vehicle (SUV) or a utility vehicle. As shown in Figures 1 and 2, the vehicle 5 in the present embodiment is an automobile. The vehicle 5 comprises one or more torque-generating machine 11, such as an internal combustion engine (ICE) and / or an electric drive unit. The vehicle 5 may be a battery electric vehicle (BEV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV) or an internal combustion engine (ICE) vehicle. The front wheels W(1), W(2) and / or the rear wheels W(3), W(4) may be driven by the one or more torque-generating machine 11. In the present embodiment, the vehicle 5 is four-wheel drive and each of the wheels W(1)-W(4) is driven. The vehicle 5 comprises one or more torque-generating machine 11 (shown in Figure 2). In the present embodiment, the wheels W(1)-W(4) are driven by first and second torque-generating machines in the form of an internal combustion engine 11-1 and an electric drive unit 11-2. The vehicle 5 comprises a plurality of wheel speed sensors WS(1)-WS(4). Each of the wheel speed sensors WS(1)-WS(4) is associated with one of the wheels W(1)-W(4). The wheel speed sensors WS(1)-WS(4) are configured to measure a rotational speed of the wheels W(1)-W(4). The wheel speed sensors WS(1)-WS(4) in the present embodiment are the wheel speed sensors WS(1)-WS(4) used in an anti-lock brake system. The wheel speed sensors WS(1)-WS(4) measure a rotational speed of the wheels W(1)-W(4). The wheel speed sensors WS(1)-WS(4) are calibrated to output signals SLC(1)-SLC(4) indicating an instantaneous wheel speed S(1)-S(4) of each of the wheels W(1)-W(4). In the present embodiment, the output signals SLC(1)-SLC(4) indicating a raw wheel speed S(1)-S(4), i.e. an unfiltered wheel speed. The wheel speed S(1)-S(4) in the present embodiment comprises a linear (directional) velocity calculated by multiplying the wheel rotational angular speed with a nominal rolling radii of the wheels W(1)-W(4). The output signals SLC(1)-SLC(4) are in the form of linear velocity signals SLC(1)-SLC(4). Each linear velocity signal SLC(1)-SLC(4) is indicative of the linear velocity of a respective one of the wheels W(1)-4. The linear velocity signals SLC(1)-SLC(4) are output to the control system 1. In a variant, the wheel speed S(1)-S(4) may comprise the rotational speed of each of the wheels W(1)-W(4). The output signals SLC(1)-SLC(4) may comprise rotational velocity signals. It will be understood that the linear velocity and rotational speed are directly proportional to each other and may be used interchangeably. The control system 1 according to the present embodiment is configured to detect a wheel slip event SEV(n) in one or more of the wheels W(1)-W(4). The wheel slip event SEV(n) is indicative of the wheel(s) W(1)-W(4) slipping relative to the surface on which the vehicle 5 is operating. The rotational speed of the wheel W(1)-W(4) increases as the wheel W(1)-W(4) slips relative to the surface. There is a corresponding increase in the linear velocity output by the wheel speed sensors WS(1)-WS(4). The wheel slip event SEV(n) typically occurs due to a loss of traction at a contact patch between the wheel W(1)-W(4) and a surface on which the vehicle 5 is operating. The loss of traction allows the driven wheel(s) W(1)-W(4) to slip relative to the surface. The wheel slip event SEV(n) may occur when the torque applied to the wheel W(1)-W(4) exceeds the available traction. In the present embodiment, the wheel slip event SEV(n) relates to the wheel(s) W(1)-W(4) slipping relative to the surface as a result of a torque applied to the wheel W(1)-W(4) by one or more of the first and second torque-generating machines 11-1, 11-2. The available traction is dependent on the friction between the surface and each wheel W(1)-W(4) of the vehicle 5. The friction is represented by a coefficient of friction (p) which varies depending on the properties of the surface on which the vehicle 5 is operating. The coefficient of friction (p) will, for example, vary depending on surface roughness. A positive acceleration of the vehicle 5 on a surface having a low coefficient of friction (p) may cause one or more of the driven wheels W(1)-W(4) to slip. It has been recognised that, under these operating conditions, the slip of each of the driven wheels W(1)-W(4) may result in oscillations (or fluctuations) in the wheel speed S(1)-S(4) measured by one or more of the wheel speed sensors WS(1)-WS(4). The oscillations comprise one or more peaks and one or more troughs in the measured wheel speed of the or each wheel W(1)-W(4). By way of example, a first plot 50 shown in Figure 3 illustrates the wheel speed S(1)-S(4) of each of the wheels W(1)-W(4) with respect to time during a positive acceleration of the vehicle 5 on a surface having a low coefficient of friction (p). The first plot 50 comprises first-, second-, third- and fourth-wheel speed traces 65A-65D representing the wheels speeds of the first, second, third and fourth wheels W(1)-W(4) respectively. Known techniques for detecting wheel slip may comprise comparing the speed of the wheels W(1)-W(4) to each other or comparing the speed of each wheel W(1)-W(4) to an average wheel speed of two or more of the wheels W(1)-W(4). However, in scenarios comprising positive acceleration on a surface having a low coefficient of friction (p), the oscillations in the wheel speeds may mean that known techniques are not suitable for reliably detecting a wheel slip event SEV(n). The control system 1 according to the present embodiment is configured to process the linear velocity signals SLC(1 )-SLC(4) to detect a change or variation in the wheel speed S(1 )-S(4) of one or more of the wheels W(1 )-W(4) which is indicative of a wheel slip event SEV(n). In particular, the control system 1 is configured to identify a signature pattern SP in the wheel speed(s). The signature pattern SP is indicative of a wheel slip event SEV(n) and is referred to herein as a wheel slip signature pattern SP. The control system 1 is configured to detect the wheel slip event SEV(n) in dependence on the identification of the wheel slip signature pattern SP. The wheel slip signature pattern SP comprises or consists of at least one identifiable characteristic occurring in the wheel speed S(1)-S(4) indicated by the wheel speed signal SLC(1)-SLC(4). The at least one identifiable characteristic comprises or consists of a frequency (or a wavelength) of the oscillation. It has been determined that a wheel slip signature pattern SP associated with a wheel slip event SEV(n) occurs at a known frequency or within a known frequency range. The frequency or frequency range may be specific to a particular vehicle 5 ordrivetrain configuration. The frequency of the oscillation or the frequency range in which the oscillation occurs is consistent and can be used reliably to identify an oscillation in the wheel speed S(1)-S(4) which is indicative of a wheel slip event SEV(n). The control system 1 in the present embodiment is configured to detect a wheel slip event SEV(n) by identifying an oscillation in the wheel speed S(1)-S(4) which occurs at a predetermined frequency or within a predetermined frequency range. The frequency of the oscillation in the wheel speed S(1)-S(4) may be determined directly or indirectly. As described herein, the control system 1 is configured to detect only those wheel slip events SEV(n) which result in an oscillation in the wheel speed S(1 )-S(4) having a magnitude greater than or less than one or more predetermined threshold. The control system 1 is configured to output a wheel slip detected signal WSD(n) in dependence on detection of the wheel slip signature pattern SP comprising or consisting of a wheel slip signature pattern SP having these characteristics. The wheel slip detected signal indicates detection of a wheel slip event SEV(n) in one or more of the wheels W(1)-W(4). The control system 1 as illustrated in Figure 4 comprises one controller 110, although it will be appreciated that this is merely illustrative. The controller 110 comprises processing means 120 and memory means 130. The processing means 120 may be one or more electronic processing device 120 which operably executes computer-readable instructions. The memory means 130 may be one or more memory device 130. The memory means 130 is electrically coupled to the processing means 120. The memory means 130 is configured to store instructions, and the processing means 120 is configured to access the memory means 130 and execute the instructions stored thereon. The controller 110 comprises an input means 140 and an output means 150. The input means 140 may comprise an electrical input 140 ofthe controller 110. The output means 150 may comprise an electrical output 150 ofthe controller 110. The input 140 is arranged to receive the wheel speed signals SLC(1)-SLC(4) from the wheel speed sensors WS(1)-WS(4). The wheel speed signals SLC(1)-SLC(4) are electrical signals which are indicative of the wheel speed S(1)-S(4) of each of the wheels W(1)-W(4) ofthe vehicle 5 The wheel speed signals SLC(1)-SLC(4) are measured with respect to time. The output 150 is arranged to output a wheel slip detected signal WSD(1)-WSD(4) which is indicative ofthe detection of a wheel slip event SEV(n). The wheel slip detected signal WSD(1)-WSD(4) identifies the or each wheel W(1)-W(4) experiencing a wheel slip event SEV(n). As described herein, the wheel slip detected signal WSD(1)-WSD(4) is output to control operation of one or more vehicle subsystems (denoted herein generally by the reference numeral 21). The controller 110 is configured to process the wheel speed signal SLC(1)-SLC(4) received for each wheel W(1)-W(4) to detect a wheel slip event SEV(n) on that wheel W(1)-W(4). The processing is performed in respect of each ofthe wheels W(1)-W(4). The processing ofthe wheel speed S(1)-S(4) of each wheel W(1)-W(4) is independent of the wheel speed S(1)-S(4) of the other wheels W(1)-W(4). The controller 110 is configured to perform the same operation in respect of each of the wheels W(1)-W(4) to detect a wheel slip event SEV(n) at one or more ofthe wheels W(1)-W(4). The operation ofthe controller 110 to detect a wheel slip event SEV(n) at the first wheel W(1) will now be described. The controller 110 is configured to receive a first wheel speed signal SLC(1) indicating the wheel speed S(1) of the first wheel W(1). A second plot 60 shown in Figure 5 represents a first wheel speed S(1) ofthe first wheel W(1) by way of example. The first wheel speed S(1) is represented by a first wheel speed trace 65A. The controller 110 receives the first wheel speed signal SLC(1) and calculates a moving average SAV(1) ofthe wheel speed S(1) ofthe first wheel W(1). The moving average filters the wheel speed S(1) ofthe first wheel W(1) to smooth out or reduce short-term fluctuations. The moving average SAV(1) is calculated over a predetermined time period which may be calibratable, for example in dependence on empirical data. The moving average SAV(1) of the first wheel speed S(1) is illustrated in Figure 5 by a moving average wheel speed trace 70. The controller 110 calculates a first wheel speed threshold STH1 and a second wheel speed threshold STH2. The first and second wheel speed thresholds STH1, STH2 are calculated in dependence on the determined moving average SAV(1). The first wheel speed threshold STH1 is greater than the second wheel speed threshold STH2. In the present embodiment, the first wheel speed threshold STH1 represents an upper threshold; and the second wheel speed threshold STH2 represents a lower threshold The first and second wheel speed thresholds STH1, STH2 in the present embodiment are the product of the moving average SAV(1) of the wheel speed S(1) and respective first and second factors. The first and second factors are predefined. The first and second factors may be determined through empirical analysis. The first factor is greater than one (1); and the second factor is less than or equal to one (1). The first factor is larger than the second factor to reflect a wheel slip event SEV(n) comprising a relatively large increase in the wheel speed S(1) followed by a relatively small decrease in the wheel speed S(1). In the present embodiment, the first factor is 1.15 and the second factor is 0.95. These values of the first and second factors are illustrative. It will be understood that the first factor and / or the second factor may be greater than or less than the stated values. The first and second factors are calibratable, for example for a particular drivetrain. The first and second wheel speed thresholds STH1, STH2 define an operating range (relative to the moving average SAV(1) of the wheel speed S(1)) in which the first wheel speed S(1) can vary without the controller 110 detecting a wheel slip event SEV(n). The first wheel speed threshold STH1 and / or the second wheel speed threshold STH2 may be modified to adjust the operating range, for example to increase or decrease one or more of the first and second wheel speed thresholds STH1, STH2. In a variant, one or more of the first and second wheel speed thresholds STH1, STH2 may be calculated by adding a predetermined value (positive or negative) to the moving average SAV(1). For example, a first constant may be added to the moving average SAV(1) to calculate the first wheel speed threshold STH1 and a second constant may be added to the moving average SAV(1) to calculate the second wheel speed threshold STH2. By way of example, the first constant may be +1,5m / s and the second constant may be -0.5m / s. The controller 110 in the present embodiment is configured to identify one or more oscillation in the first wheel speed S(1) that exceeds the first and second wheel speed thresholds STH1, STH2. The controller 110 is configured to process the first wheel speed signal SLC(1) to identify an oscillation in the first wheel speed S(1) comprising one or more peak having a magnitude greater than the first wheel speed threshold STH1 and one or more trough having a magnitude less than the second wheel speed threshold STH2. The controller 110 is configured to monitor a frequency of the oscillation(s) in the first wheel speed S(1). The controller 110 is configured to process the first wheel speed signal SLC(1) to determine the frequency directly or indirectly. The controller 110 classifies the oscillation as corresponding to the wheel slip signature pattern SP in dependence on a determination that the determined frequency of the oscillation is within the predetermined frequency range. The controller 110 processes the first wheel speed signal SLC(1) to detect the first wheel speed S(1) crossing (or traversing) the first wheel speed threshold STH1 and / or to detect the first wheel speed S(1) crossing (or traversing) the second wheel speed threshold STH2. The first wheel speed S(1) may be increasing (positive gradient) or decreasing (negative gradient) as it crosses the first wheel speed threshold STH1 and / or the second wheel speed threshold STH2. The controller 110 is configured to detect a first cross time t1 and a second cross time t2. The first cross time t1 corresponds to a first time when the first wheel speed S(1) crosses one of the first wheel speed threshold TH1 and the second wheel speed threshold TH2. The second cross time t2 corresponds to a second time when the first wheel speed S(1) crosses one of the first wheel speed threshold TH1 and the second wheel speed threshold TH2. The controller 110 may optionally determine if the first wheel speed S(1) is increasing (positive gradient) or decreasing (negative gradient) when it crosses the first wheel speed threshold TH1 and / or the second wheel speed threshold TH2. The determination that the first wheel speed S(1) is increasing or decreasing may be used to characterise the sub-section of the oscillation represented by the first and second cross times t1, t2. In the example illustrated in Figure 5, the first cross time t1 corresponds to the time when the first wheel speed S(1) increases above the first wheel speed threshold STH1; and the second cross time 12 corresponds to the time when the first wheel speed S(1) decreases below the second wheel speed threshold STH2. The controller 110 determines a first time interval At between the first and second cross times t1, t2. The controller 110 is configured to determine if the oscillations in the first wheel speed S(1) have a frequency which is indicative of a wheel slip event SEV(n). The controller 110 is configured to detect a wheel slip event SEV(n) in dependence on a determination that the frequency of the one or more oscillation is within a predetermined frequency range. The predetermined frequency range is defined by at least one of a first frequency threshold FTH1 and a second frequency threshold FTH2. The first frequency threshold FTH1 is larger than the second frequency threshold FTH2 The controller 110 is configured to detect a wheel slip event SEV(n) in dependence on the identification of one or more oscillation having a frequency within the predetermined frequency range The first frequency threshold is defined as 12Hz; and the second frequency threshold is defined as 8Hz. The first frequency threshold may be defined as a value less than or greater than 12Hz, for example 11 Hz or 13Hz. The second frequency threshold may be defined as a value less than or greater than 8Hz, for example 7Hz or 9Hz. It will be understood that one or more of the first and second frequency thresholds may have a different value. In the present embodiment, the controller 110 is configured indirectly to determine if the oscillation in the wheel speed S(1) has a frequency within the predetermined frequency range. In particular, the controller 110 is configured to determine if the first time interval At is indicative of an oscillation having a time period (T) within a reference time interval range. The reference time interval range corresponds to a time interval of an oscillation having a frequency which is indicative of a wheel slip event. The reference time interval range in the present embodiment is defined by a first time threshold and a second time threshold The controller 110 is configured to determine if the first time interval At is indicative of a time period (T) of an oscillation within the reference time interval range. The controller 110 is configured to detect a wheel slip event in dependence on a determination that the first time interval At is indicative of an oscillation having a time period for one complete cycle which is between the first and second time thresholds. The first time threshold corresponds to the time interval of an oscillation having a frequency equal to the first frequency threshold; and the second time threshold corresponds to the time interval of an oscillation having the second frequency threshold. In the present embodiment the first frequency threshold (12Hz) corresponds to a period of 83.33ms fora complete cycle; and the second frequency threshold (8Hz) corresponds to a period 125ms fora complete cycle. It will be appreciated that the first time interval At represents only a portion of the complete cycle. In the present example, the first time interval At represents half of the period of one complete cycle. A halftime period of the oscillation is in the range 41,66ms (83.33*0.5) to 63ms (125*0.5). The reference time interval range is defined by a first time threshold of 40ms and a second time threshold of 70ms in this example. Other values may be defined for the first time threshold and / or the second time threshold. The controller 110 in the present embodiment is configured to detect an oscillation in the wheel speed S(1) in dependence on a determination that the first time interval At is in the reference time-interval range of 40ms to 70ms (i.e., 40ms<At<70ms). The controller 110 starts a timer once the first wheel speed S(1) crosses the first threshold STH1. If the timer indicates that the first wheel speed S(1) crosses the second threshold STH2 at a time greater than or equal to 40ms and less than or equal to 70ms (i.e., 40ms<At<70ms), the controller 110 identifies an oscillation indicative of a wheel slip event SEV(n). If the timer indicates that the first wheel speed S(1) crosses the second threshold STH2 at a time less than 40ms (i.e., At<40ms) or at a time greater than 70ms (i.e., At>70ms), the controller 110 determines that the oscillation is not indicate of a wheel slip event SEV(n). While the first wheel speed S(1) continues to alternate between crossing the first and second thresholds STH1, STH2 within the predefined time interval, the controller 110 detects a wheel slip event. Different relationships between the first time interval At and the period of one complete cycle may be defined. The relationship may, for example, be determined in dependence on empirical data. The first time interval At may be compared to first and second threshold time periods modified accordingly. In a variant, the controller 110 may calculate the frequency of the oscillation in the wheel speed S(1) in dependence on the first time interval At. By determining the frequency of an oscillation, the controller 110 may more reliably detect a wheel slip event SEV(n). The controller 110 can compare the measured frequency of the oscillation to a predetermined frequency or a predetermined frequency range which is associated with (i.e., indicative of) a wheel slip event SEV(n). The frequency range may, for example, be defined as between 8Hz and 12Hz. The upper limit and / or the lower limit of the frequency range may be increased or decreased, as described herein. By identifying an oscillation occurring at the predetermined frequency or in the predetermined frequency range, the controller 110 can detect a wheel slip event SEV(n) at the first wheel W(1). The controller 110 is configured to characterise the oscillation in the wheel speed S(1) as indicative of a wheel slip event SEV(n) in dependence on a determination that the frequency of the oscillation is at least substantially equal to the predetermined frequency or is within the predetermined frequency range. In a variant, the first time interval At may represent a time period between two or more peaks and / or two or more troughs in the wheel speed S(1). Alternatively, the first and second times t1, t2 may correspond to a peak and a trough in the wheel speed S(1) such that the first time interval At is approximately equal to half the period of one cycle. Alternatively, the first time interval At may correspond directly to the period of the oscillation in the wheel speed S(1). The first and second times t1, t2 may represent the same point on consecutive oscillations in the wheel speed S(1). For example, the first and second times t1, t2 may represent consecutive peaks or troughs in the wheel speed signal SLC(1). Alternatively, the first and second times t1, t2 may correspond to the times on consecutive oscillations when the wheel speed S(1) crosses one of the following: the first wheel speed threshold STH1, the second wheel speed threshold STH2 or the moving average SAV(1) of the wheel speed S(1). The control system 1 is configured to output a wheel slip detected signal WSD(n) in dependence on detection of the wheel slip signature pattern SP In particular, the control system 1 is configured to output the wheel slip detected signal in dependence a determination that the wheel speed signal SLC(1) comprises an oscillation having a frequency within the predetermined frequency range. The control system 1 is configured to output a wheel slip detected signal in respect of one or more of the wheels W(1)-W(4). The wheel slip detected signal WSD(n) indicates that a wheel slip event SEV(n) has been detected at one or more of the wheels W(1)-W(4). The wheel slip detected signal WSD(n) may be output repeatedly for the duration of the wheel slip event SEV. Alternatively, or in addition, a signal may be output to indicate that the wheel slip event SEV(n) is no longer detected. As outlined above, the wheel speeds S(1)-S(4) are processed independently of each other to detect a wheel slip event SEV(n) on each wheel W(1)-W(4). The moving average SAV(1)-SAV(4) is calculated independently for each of the wheel speeds S(1)-S(4) to detect the wheel slip event SEV(1)-SEV(4) at each of the wheels W(1)-W(4). It will be understood that first and second wheel speed thresholds STH1-STH2 are calculated for each of the wheel speeds S(1)-S(4) in dependence on the respective moving averages SAV(1)-SAV(4) to detect the wheel slip event SEV(1)-SEV(4) at each of the wheels W(1)-W(4). The wheel speed S(1)-S(4) of each wheel W(1)-W(4) is compared to the corresponding first and second wheel speed thresholds STH1-STH2 calculated forthat wheel W(1)-W(4) to detect a wheel slip signature pattern SP. The wheel slip event SEV(1)-SEV(4) may thereby be detected at each wheel W(1)-W(4). The control system 1 is incorporated into a system 15 provided on the vehicle 5. The system 15 comprises the control system 1 and at least one vehicle control unit VCU The vehicle control unit(s) VCU is configured to control operation of one or more vehicle subsystems 21. As described herein, the control system 1 is in communication with the at least one vehicle control unit VCU. In particular, the control system 1 is configured to output the wheel slip detected signal to the at least one vehicle control unit(s) VCU. The at least one vehicle control unit(s) VCU is configured to control operation of the one or more vehicle subsystems 21 in dependence on the wheel slip detected signal. The or each vehicle control unit VCU comprises an electrical processor configured to receive the wheel slip detected signal(s) WSD(n) from the control system 1. The one or more vehicle control unit VCU controls operation of one or more of the vehicle subsystems 21 in dependence on the wheel slip detected signal generated by the control system 1. It will be understood that the vehicle control unit VCU and the control system 1 may be combined into the same control unit. The vehicle 5 comprises a plurality of the vehicle subsystems (denoted generally by the reference numeral 21 and shown schematically in Figures 2 and 4). The vehicle subsystems 21 in the present embodiment include, but are not limited to, a traction control system 21 A, a propulsion (or engine) management system 21B, a transmission system 21C, a steering system 21D, an anti-lock braking system (ABS) 21E, a suspension system 21F and a differential system 21G. The vehicle 5 may comprise less than or more than six (6) vehicle subsystems 21. The vehicle control unit VCU is provided to control operation of the vehicle subsystems 21). In particular, the vehicle control unit VCU is configured to control or to modify the operation of the at least one vehicle subsystem in dependence on the wheel slip detected signal from the control system 1. The vehicle control unit VCU may be configured to implement one or more vehicle drive mode DM(n). The or each vehicle drive mode DM(n) defines one or more operating parameter of at least one of the vehicle subsystems 21. Each vehicle drive mode configures the at least one of the vehicle subsystems 21 in dependence on the current or prevailing operating conditions. For example, the vehicle drive mode may configure the vehicle subsystems 21 in dependence on a determined (or estimated) surface friction. A first drive mode for controlling the vehicle subsystems 21 when the vehicle 5 is operating on a surface having a high coefficient of friction (such as a road); and a second drive mode for controlling the vehicle subsystems 21 when the vehicle 5 is operating on a surface having a low coefficient of friction (such as a wet grass or ice). At least in certain embodiments, the vehicle control unit VCU may select one of the vehicle drive modes in dependence on the wheel slip detected signal received from the control system 1. For example, the vehicle control unit VCU may increase a weighting for selecting the second drive mode in dependence on receipt of the wheel slip detected signal. The weighting may represent a probability that the second drive mode is appropriate for the prevailing conditions. Figure 6 illustrates a method 200 according to an embodiment of the invention. The method 200 is a method of detecting a wheel slip event SEV(n) in one or more of the wheels W(1)-W(4) of a vehicle 5, such as the vehicle 5 illustrated in Figure 1. The method 200 may be performed by the control system 1 illustrated in Figure 1. In particular, the memory device 130 may comprise computer-readable instructions which, when executed by the processor 120, perform the method 200 according to an embodiment of the invention. The method 200 will now be described with reference to Figure 6. The method 200 is described with reference to a first wheel W(1). It will be understood that the method 200 is performed in respect of each of the wheels W(1)-W(4). The method 200 comprises measuring a wheel speed S(1) of a first wheel W(1) (BLOCK 205). The wheel speed S(1) may, for example, be measured by a wheel speed sensor WS(1) associated with the first wheel W(1). The wheel speed S(1) may comprise a linear wheel speed or a rotational wheel speed. A moving average SAV(1) of the wheel speed S(1) is calculated (BLOCK 210). At least one of a first wheel speed threshold STH1 and a second wheel speed threshold STH2 are calculated in dependence on the determined moving average SAV(1) (BLOCK 215). The second wheel speed threshold STH2 is less than the first wheel speed threshold STH1. The first wheel speed threshold STH1 may be greaterthan the determined moving average SAV(1) of the wheel speed S(1). Alternatively, or in addition, the second wheel speed threshold STH2 may be less than the determined moving average SAV(1) of the wheel speed S(1). A first cross time t1 is detected when the wheel speed S(1) crosses one of the first wheel speed threshold STH1 and the second wheel speed threshold STH2 (BLOCK 220). A second cross time t2 is detected when the wheel speed S(1) crosses one of the first wheel speed threshold STH1 and the second wheel speed threshold STH2 (BLOCK 225). A wheel slip signature pattern SP is detected in dependence on the first cross time t1 and the second cross time t2. The wheel slip signature pattern SP may be detected in dependence on a frequency of an oscillation in the measured wheel speed S(1). The method comprises monitoring the frequency (f) of an oscillation in the wheel speed S(1). The frequency of the oscillation may be determined directly or indirectly. The frequency of the oscillation may, for example, be determined in dependence on the first cross time t1 and the second cross time t2. In the present embodiment, a time interval At is determined between the first cross time t1 and the second cross time t2 (BLOCK 230). The frequency of the oscillation is determined in dependence on the time interval At (BLOCK 235). The wheel slip signature pattern SP may be detected in dependence on a determination that the determined frequency is at least substantially equally to a predetermined frequency or is within a predetermined frequency range (BLOCK 240). The determined frequency of the oscillation may, for example, be compared to first and second frequency thresholds FTH1, FTH2. A first wheel slip event SEV(1) is detected in dependence on the detection of the wheel slip signature pattern SP in the wheel speed S(1) of the first wheel W(1) (BLOCK 245). A wheel slip detected signal WSD(1) is output to indicate detection of the wheel slip event SEV(1) at the first wheel W(1) (BLOCK 250). The wheel slip detected signal WSD(1) may be output repeatedly for the duration of the wheel slip event SEV. Alternatively, or in addition, a signal may be output to indicate that the wheel slip event SEV(n) is no longer detected. The signal may comprise a wheel slip completion signal WSC(n) to indicate the completion of the wheel slip event SEV(n). The wheel slip completion signal WSC(n) may be generated in dependence on a determination that the wheel slip signature pattern SP is absent. The wheel slip completion signal WSC(n) may be generated in respect of each wheel W(1)-W(4). The detection of the end to the wheel slip signature pattern may comprise determining that the determined frequency of the change in the wheel speed S(1) is outside the predetermined frequency range. As outlined herein, the oscillations in the wheel speed S(1) typically occur as the vehicle 5 accelerates. The control system 1 may optionally be configured to determine that the vehicle 5 is undergoing (positive) acceleration, for example greater than or equal to a predetermined threshold. The control system 1 may be configured to detect a wheel slip event SEV(n) in dependence on a determination that the vehicle 5 is accelerating. The detection of the wheel slip event SEV(n) may be conditional on the vehicle accelerating. This may provide an additional entry condition for detection of the wheel slip event SEV(n). Similarly, the method 200 may comprise determining that the vehicle 5 is undergoing (positive) acceleration. Detection of the wheel slip event SEV(n) may comprise determining that the vehicle 5 is accelerating. Alternatively, or in addition, the control system 1 may be configured to detect a wheel slip event SEV(n) in dependence on a determination that a reference speed of the vehicle 5 is greater than or less than a predetermined threshold speed. The oscillations in the wheel speed S(1) are more likely to occur at a low ambient temperature, for example below a threshold temperature. The threshold temperature may be predefined, for example as 0°C, 5°C or 10°C. The control system 1 may optionally be configured to determine an ambient temperature. The detection of the wheel slip event SEV(n) may be conditional on the ambient temperature being less than the threshold temperature. The control system 1 may be configured to detect a wheel slip event SEV(n) in dependence on a determination that the ambient temperature is less than the threshold temperature. This may provide an additional entry condition for detection of the wheel slip event SEV(n). Similarly, the method 200 may comprise determining that an ambient temperature is less than the threshold temperature. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. 5 LABELS FOR METHOD 200 BLOCK LABEL 205 MEASURE WHEEL SPEED 210 DETERMINE MOVING AVERAGE OF WHEEL SPEED 215 CALCULATE AT LEAST ONE WHEEL SPEED THRESHOLD 220 MONITOR WHEEL SPEED TO IDENTIFY FIRST CROSS TIME 225 MONITOR WHEEL SPEED TO IDENTIFY SECOND CROSS TIME 230 DETERMINE TIME INTERVAL (At) BETWEEN FIRST AND SECOND CROSS TIMES 235 DIRECTLY OR INDIRECTLY DETERMINE FREQUENCY OF OSCILLATION 240 IDENTIFY WHEEL SLIP SIGNATURE PATTERN IN DEPENDENCE ON OSCILLATION FREQUENCY 245 DETECT WHEEL SLIP EVENT IN DEPENDENCE ON WHEEL SLIP SIGNATURE PATTERN 250 OUTPUT WHEEL SLIP DETECTED SIGNAL

Claims

1. A control system for detecting a wheel slip event of a wheel of a vehicle, the control system comprising one or more processors collectively configured to:receive a wheel speed signal indicating a wheel speed of the wheel;determine a moving average of the wheel speed in dependence on the wheel speed signal;determine a first wheel speed threshold and a second wheel speed threshold in dependence on the determined moving average;detect a first cross time when the wheel speed crosses one of the first wheel speed threshold and the second wheel speed threshold;detect a second cross time when the wheel speed crosses one of the first wheel speed threshold and the second wheel speed threshold;detect a wheel slip signature pattern in dependence on the first cross time and the second cross time; andoutput a wheel slip detected signal indicating a wheel slip event in dependence on detection of the wheel slip signature pattern.

2. A control system as claimed in claim 1, wherein detecting the wheel slip signature pattern comprises: monitoring a frequency of an oscillation in the wheel speed in dependence on the first cross time and the second cross time;the wheel slip signature pattern being detected in dependence on a determination that the frequency of the oscillation corresponds to a predetermined frequency or is within a predetermined frequency range.

3. A control system as claimed in claim 2, wherein monitoring the frequency of the oscillation in the wheel speed comprises:determining a time interval between the first cross time and the second cross time;determining the frequency of the oscillation in dependence on the time interval between the first cross time and the second time; anddetecting the wheel slip signature pattern in dependence on a determination that the determined frequency of the oscillation corresponds to the predetermined frequency or is within the predetermined frequency range.

4. A control system as claimed in claim 2, wherein monitoring the frequency of the oscillation in the wheel speed comprises:determining a time interval between the first cross time and the second cross time;comparing the time interval to a reference time interval or a reference time interval range, wherein the reference time interval corresponds to the time interval of an oscillation occurring at the predetermined frequency and the reference time interval range corresponds to the time interval range of an oscillation occurring within the predetermined frequency range; anddetecting the wheel slip signature pattern in dependence on a determination that the time interval corresponds to the reference time interval or is within the reference time interval range.

5. A control system as claimed in any one of claims 2, 3 or 4, wherein the predetermined frequency range is defined by at least one of a first frequency threshold and a second frequency threshold.

6. A control system as claimed in claim 5, wherein the first frequency threshold of 12Hz and the second frequency threshold is 8Hz.

7. A control system as claimed in any one of the preceding claims, wherein the one or more processors is collectively configured to:detect an end to the wheel slip signature pattern; andoutput a wheel slip completion signal indicating the completion of the wheel slip event in dependence on the detection of the end of the wheel slip signature pattern.8 A control system as claimed in claim 7 when dependent directly or indirectly on claim 2, wherein detecting the end to the wheel slip signature pattern comprises determining that the frequency of the oscillation in the wheel speed does not correspond to the predetermined frequency or is outside the predetermined frequency range.

9. A control system as claimed in any one of the preceding claims, wherein:the first cross time is detected when the wheel speed crosses the first wheel speed threshold; and the second cross time is detected when the wheel speed crosses the second wheel speed threshold.

10. A control system as claimed in any one of the preceding claims, wherein the vehicle comprises a plurality of wheels, the one or more processors being collectively configured to:detect the wheel slip event on one or more of the plurality of wheels.

11. A system comprising the control system of any one of the preceding claims and at least one vehicle control unit for controlling at least one vehicle subsystem of the vehicle, the at least one vehicle control unit being configured to modify the operation of the at least one vehicle subsystem in dependence on the wheel slip detected signal from the control system.

12. A system comprising the control system of any one of claims 1 to 10 and at least one vehicle control unit for selecting one of a plurality of vehicle drive modes, the vehicle drive modes each defining one or more operating parameter of at least one vehicle subsystem, wherein the at least one vehicle control unit is configured to select one of the plurality of vehicle drive modes in dependence on the wheel slip detected signal from the control system.

13. A vehicle comprising a control system as claimed in anyone of claims 1 to 10; ora system as claimed in claim 11 or claim 12.

14. A method of detecting a wheel slip event) of a wheel of a vehicle, the method comprising:measure a wheel speed of the wheel;determine a moving average of the wheel speed;determine a first wheel speed threshold and a second wheel speed threshold in dependence on the determined moving average;detect a first cross time when the wheel speed crosses one of the first wheel speed threshold and the second wheel speed threshold;detect a second cross time when the wheel speed crosses one of the first wheel speed threshold and the second wheel speed threshold;detect a wheel slip signature pattern in dependence on the first cross time and the second cross time; anddetect the wheel slip event in dependence on the detection of the wheel slip signature pattern.

15. A method as claimed in claim 14, wherein detecting the wheel slip signature pattern comprises: monitoring a frequency of an oscillation in the wheel speed in dependence on the first cross time and the second cross time; anddetected the wheel slip signature pattern in dependence on a determination that the frequency is within a predetermined frequency range.

16. Computer readable instructions which, when executed by one or more processors, cause the one or more processors to perform the method as claimed in claim 14 or claim 15.

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