Control system for a vehicle

Abstract

Aspects of the present invention relate to a control system (10) for a vehicle (12). The control system (10) comprises one or more controller (18).The control system (10) is configured to: receive a f

Description

22 07 £5 TECHNICAL FIELD The present disclosure relates to a control system for a vehicle. Aspects of the invention relate to a control 5 system, a system, a vehicle, a method and computer readable instructions. BACKGROUND It is known to determine the speed of a vehicle using measurements of the rotational speed of one or more wheels of the vehicle combined with information regarding the size of the vehicle wheels, and in particular the 10 tyre radius. This vehicle speed may be displayed to a vehicle user on a speedometer of the vehicle, or used in other vehicle systems. Usually, the tyre size parameters used in the vehicle speed calculations relate to the tyres installed on the vehicle during initial vehicle production, and are not updated during the lifetime of the vehicle. If these parameters do not correctly reflect the true wheel size for any reason, for example due to dynamic changes in wheel size during operation of the vehicle caused by heating or cooling, or due to general 15 wear or replacement of the wheels, then the associated vehicle speed measurements may be inaccurate. 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, a method and computer readable instructions as claimed in the appended claims. According to an aspect of the present invention there is provided a control system for a vehicle. The control system comprises one or more controller. The control system is configured to: receive a 25 first signal indicative of a rotational speed of one or more wheels of the vehicle; determine a first vehicle speed value in dependence on the first signal; receive a second signal indicative of a vehicle speed from a global navigation satellite system; determine a second vehicle speed value in dependence on the second signal; perform a validation check and, if the validation check is passed, store a correction factor calculated in dependence on the first vehicle speed value and the second vehicle speed value 30 on an electronic storage device; calculate a corrected vehicle speed value using the first vehicle speed value and a stored correction factor from the electronic storage device; and output the corrected vehicle speed value, wherein the validation check comprises determining one or more of: the first vehicle speed value varies by less than a predetermined amount over a predetermined period of time; 35 the second vehicle speed value varies by less than a predetermined amount over a predetermined period of time; the first vehicle speed value is above a predetermined threshold; a ratio of the first vehicle speed value and the second vehicle speed value varies by less than a predetermined amount over a predetermined period of time; 40 a ratio of the first vehicle speed value and the second vehicle speed value falls within a predetermined range; 22 07 25 a ratio of the second vehicle speed value and the corrected vehicle speed value falls within a predetermined range; and a quality factor relating to the second vehicle speed value is above a pre-determined threshold value. The output of the corrected vehicle speed value may be to a vehicle data bus. 5 The control system advantageously outputs a corrected vehicle speed value that uses a correction factor to modify the vehicle speed as determined through rotational wheel speed measurements. The correction factor takes into account the vehicle speed as determined through use of global navigation satellite system (GNSS) measurements, and is only stored for use in the corrected vehicle speed value once it is determined that a validation check has been passed. If the validation check is passed, this indicates that the vehicle is in a 10 suitable state for a reliable correction factor based on GNSS signals to be calculated. The control system comprises 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; and wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions 15 thereon so as to undertake the steps that the control system described in the preceding paragraphs is configured to carry out. In an embodiment, the validation check comprises determining that the first vehicle speed value varies by less than a predetermined amount over a predetermined period of time. In an embodiment, the validation check comprises determining that the second vehicle speed value varies by less than a predetermined amount over a predetermined period of time. In an embodiment, the validation check comprises determining that a ratio of the first vehicle speed value and the second vehicle speed value varies by less than a predetermined amount over a predetermined period of time. In this way, the control system advantageously confirms, through monitoring of both the first vehicle speed, 25 the second vehicle speed and a ratio of the first and second vehicle speeds, that the vehicle speed is stable. This increases the likelihood that the second vehicle speed value used in the correction factor calculation and determined using the GNSS system in particular is accurate before storing a correction factor based on this speed value for use in corrected vehicle speed value calculations. 30 In an embodiment, the validation check comprises determining that the first vehicle speed value is above a predetermined threshold. A vehicle travelling below a certain speed is more likely to be travelling in a built-up area, in which buildings or other structure may block or interfere with GNSS signals and reduce the reliability and accuracy of the second vehicle speed. This sub-check of the validation check therefore reduces the likelihood of an unreliable correction factor being stored for use in corrected vehicle speed calculations. 35 In an embodiment, the validation check comprises determining that a ratio of the first vehicle speed value and the second vehicle speed value falls within a predetermined range. Through this sub-check of the validation check, the control system determines that it is appropriate to store the correction factor if the first and second vehicle speeds only differ by a relatively small amount, as defined by the predetermined range. If the first and second vehicle speed values differ by a significant amount, this may indicate that one of the first or second vehicle speed values is anomalous. Thus, this sub-check reduces the chances of a correction factor calculated using anomalous data being stored and used in future calculations. 5 In an embodiment, the validation check comprises determining that a ratio of the second vehicle speed value and the corrected vehicle speed value falls within a predetermined range. Through this sub-check of the validation check, the control system determines that it is appropriate to store the correction factor if the second and corrected vehicle speeds only differ by a relatively small amount, as defined by the predetermined range. Similarly to the sub-check relating to the ratio of the first and second vehicle speed values, this sub-check 10 reduces the chances of a correction factor calculated using anomalous data being stored and used in future calculations. In an embodiment, the validation check comprises determining that a quality factor relating to the second vehicle speed value is above a pre-determined threshold value. The quality factor provides an indication of the 15 reliability of data provided by the GNSS and used to determine the second vehicle speed value. Through this sub-check of the validation check, the control system only stores a correction factor if it is determined that the quality of the data from the GNSS, which is used in the correction factor calculation, exceeds a certain threshold of reliability. In an embodiment, the control system is configured to add a predetermined offset value to the corrected vehicle speed value. This advantageously reduces the likelihood of the control system underestimating the true vehicle speed. This is advantageous especially in embodiments in which the corrected vehicle speed value outputted by the control system is displayed to a user of a vehicle, for example on a speedometer display. 25 In an embodiment, the control system is configured to calculate the correction factor before the validation check is performed. In an embodiment, the control system is configured to calculate the correction factor after the validation check is performed. In this way, in some embodiments the correction factor is calculated regardless of whether or not the validation check is passed, but in some embodiments the correction factor is only calculated if it is determined that the validation check is passed. 30 In an embodiment, the control system is configured to repeat the method periodically, at a discrete series of time intervals, and to calculate the corrected vehicle speed value using the newest stored correction factor. Thus, the corrected vehicle speed value calculations always uses the most up-to-date stored value of the correction factor. The most up-to-date stored correction factor is likely to be the most reliable for use in corrected vehicle speed calculations. 35 According to another aspect of the invention, there is provided a system comprising the control system of any preceding paragraph, a global navigation satellite system and at least one rotational speed sensor for measuring a rotational speed of at least one wheel of the vehicle. In an embodiment, the system comprises a display of a vehicle for displaying the corrected vehicle speed value to a user of the vehicle. 22 07 £5 22 07 £5 According to another aspect of the invention, there is provided a vehicle comprising the system or control system of any preceding paragraph. 5 According to yet another aspect of the invention, there is provided a method for determining a corrected vehicle speed value for a vehicle. The method comprises: receiving a first signal indicative of a rotational speed of one or more wheels of the vehicle; determining a first vehicle speed value in dependence on the first signal; receiving a second signal indicative of a vehicle speed from a global navigation satellite system; determining a second vehicle speed value in dependence on the second 10 signal; performing a validation check and, if the validation check is passed, storing a correction factor calculated in dependence on the first vehicle speed value and the second vehicle speed value on an electronic storage device; and calculating a corrected vehicle speed value using the first vehicle speed value and a stored correction factor from the electronic storage device, wherein the validation check comprises determining one or more of: 15 the first vehicle speed value varies by less than a predetermined amount over a predetermined period of time; the second vehicle speed value varies by less than a predetermined amount over a predetermined period of time; the first vehicle speed value is above a predetermined threshold; a ratio of the first vehicle speed value and the second vehicle speed value varies by less than a predetermined amount over a predetermined period of time; a ratio of the first vehicle speed value and the second vehicle speed value falls within a predetermined range; a ratio of the second vehicle speed value and the corrected vehicle speed value falls within a 25 predetermined range; and a quality factor relating to the second vehicle speed value is above a pre-determined threshold value. According to yet another aspect of the invention, there are provided computer readable instructions which, when executed by a computer, are arranged to perform a method according to the preceding paragraph. 30 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 35 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 40 the accompanying drawings, in which: Figure 1 shows a schematic representation of a control system in accordance with an embodiment of the invention; Figure 2 shows a vehicle in accordance with an embodiment of the invention, comprising the control system 5 of Figure 1; Figure 3 shows a first flow chart showing a method in accordance with an embodiment of the invention; and Figure 4 illustrates test data obtained during a test drive of a vehicle. 10 DETAILED DESCRIPTION A control system 10 in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figure 1. Referring to Figure 2, a vehicle 12 in accordance with an embodiment of the invention comprises a control system 10 as illustrated in Figure 1. 15 The control system 10 comprises a controller 18. The controller 18 comprises processing means 20 and memory means 22. The processing means 20 may be one or more electronic processing device which operably executes computer-readable instructions. The memory means 22 may be one or more memory device. The memory means 22 is electrically coupled to the processing means 20. The memory means 22 is configured to store instructions, and the processing means 20 is configured to access the memory means 22 and execute the instructions stored thereon. Although the control system 10 shown in Figure 1 comprises one controller 18, it will be appreciated that this is merely illustrative, and that additional controllers 18 may be included as part of the control system 10. 25 The controller 18 comprises an input means 24 and an output means 26. The input means 24 comprises an electrical input of the controller 18. The output means 26 comprises an electrical output of the controller 18. The electrical input is arranged to receive data contained in one or more signals, in particular one or more electronic signals, from a plurality of sources. In this embodiment, the controller 18 is configured to receive 30 wheel rotational speed data, GNSS vehicle speed data and GNSS quality factor data. The wheel rotational speed data originates from wheel rotation sensors 28 that are configured and arranged to measure the rotational speed of an associated wheel 14, and is delivered to the controller 18 via one or more electronic signals 29. The wheel rotational speed data is indicative of a rotational speed of one or more 35 wheels 14 of the vehicle 12. Specifically, the wheel rotational speed data comprises measurements of the rotational speed, co, of one or more wheel 14 of the vehicle 12. In this embodiment, each wheel 14 of the vehicle 12 is provided with a wheel rotation sensor 28, such that the vehicle 12 includes four wheel rotation sensors 28, although only two of these wheel rotation sensors 28 are 40 illustrated in Figure 2. Thus, in this embodiment, the wheel rotational speed data is indicative of the rotational 22 07.25 speed of each wheel 14 of the vehicle 12, and comprises measurements of the rotational speed, co, of each wheel 14 of the vehicle 12. In other embodiments, one or more of the wheels 14 may not include a wheel rotation sensor 28. For example, 5 in some embodiments, only one wheel 14 is provided with a wheel rotation sensor 28, such that the wheel rotational speed data is indicative of the rotational speed, co, of that associated wheel 14 only. The GNSS vehicle speed data originates from the GNSS 16. As will be understood by the skilled person, the GNSS 16 is configured to use satellite signals 32 to provide positioning, navigation and timing information 10 regarding the vehicle 12. The GNSS 16 comprises a plurality of satellites 34 and an onboard navigation system 36. The onboard navigation system 36 includes an antenna 38 for receiving satellite signals 32 from the satellites 34 and processing means 40 for processing the satellite signals 32 received by the antenna 38. The processing means 40 is configured to output a signal 42 that is indicative of the speed of the vehicle to the controller 18 of the control system 10. It should be understood that although three satellites 34 are illustrated 15 in Figure 1, this is for illustrative purposes only, and other numbers of satellites 32 are possible. For the sake of clarity, the satellites 34 form no part of the present invention other than the signals they broadcast that are detected by the control system of the present invention. A GNSS quality factor data also originates from the GNSS 16, and is delivered to the controller 18 via an electronic signal 44. It should be noted that whilst signals 42 and 44 are represented in Figure 1 as two separate signals, it would be possible for a single signal to transmit the data contained in signals 42 and 44 to the controller 18. On receipt by the controller 18 of signal 44, a GNSS quality factor data is stored on the memory means 22 of the controller 18. 25 The GNSS quality factor data comprises a quality factor value that is indicative of the reliability of the GNSS vehicle speed data. The quality factor of the GNSS vehicle speed data may be affected by, for example, the surrounding environment of the vehicle, the weather and the positions of the relevant satellites. As an example, the quality factor may indicate that the GNSS vehicle speed data is less reliable if the vehicle 12 is in a built-up area compared to a more open area, because in built-up areas surrounding buildings or other structures 30 may block satellite signals from effectively reaching the antenna 38. In this embodiment the quality factor is provided as an integer between 1 to 5, inclusive. In other words, the quality factor may be 1,2, 3, 4 or 5. As explained in more detail below, a quality factor of 3 or higher, i.e. of 3, 4 or 5, indicates that the reliability of the GNSS vehicle speed data is sufficient to provide a useful correction 35 factor for use in calculating a corrected vehicle speed value, vc. A quality factor of lower than 3, i.e. 1 or 2, indicates that the reliability of the GNSS vehicle speed data is not sufficient for use in such calculations. The control system 10 is configured to calculate and output a corrected vehicle speed value, vc, via electronic signal 39 from the output means 26 of the controller 18 to a vehicle data bus 41, e.g. a Controlled Area Network 40 (CAN) bus. As explained in more detail below, the corrected vehicle speed value, vc, is calculated using data 22 07 £5 22 07 25 contained in the signals received by the controller 18, and is output for use in relevant vehicle systems, for example in a cruise control system of the vehicle 12 or in a display system of a vehicle such as a speedometer. Figure 3 illustrates a method 100 according to an embodiment of the invention. The method 100 is a method 5 of determining a corrected vehicle speed value, vc, of a vehicle 12, such as the vehicle 12 illustrated in Figure 2. The method 100 may be performed by the control system 10 illustrated in Figure 1. In particular, the memory means 22 may comprise computer-readable instructions which, when executed by the processing means 20, 10 perform the method 100 according to an embodiment of the invention. At a first step 102 of the method 100, the input means 24 of the controller 18 receives electronic signals 29, 42, 44 originating from the wheel rotation sensors 28 and the GNSS 16. In this embodiment, electronic signals 29, 42, 44 are received by the controller 20 at a frequency of 40 Hz and the method is performed by the control 15 system 10 at the same frequency of 40 Hz on receipt of electronic signals 29, 42, 44. It should be noted that in other embodiments electronic signals 29, 42, 44 may be received by the controller 20 at different time intervals, i.e. more or less frequently, and / or the control system 10 may perform the method 100 at different time intervals. As explained already, in this embodiment the signal 29, which is also referred to herein as the first signal 29, contains wheel rotational speed data indicative of the rotational speed of each wheel 14 of the vehicle 12. As explained further below, at step 104, the controller 18 determines a first vehicle speed value, v-i, in dependence on the first signal 29, and stores the first vehicle speed value, v-i, on the memory means 22. The signal 42, which is also referred to herein as the second signal 42, contains GNSS vehicle speed data 25 indicative of the speed of the vehicle 12. In this embodiment the GNSS vehicle speed data comprises a second vehicle speed value, V2, which is the speed of the vehicle 12 as determined by the GNSS 16. Thus, in this embodiment the processing means 40 of the GNSS 16 is configured to determine the speed of the vehicle using information contained in the satellite signals 32 received by the GNSS 16, and output this speed as the second vehicle speed value, V2, to the controller 18. In other embodiments the GNSS vehicle speed data may 30 include information derived from the satellite signals to be processed by the processing means 20 of the controller 18 to determine the second vehicle speed value, V2. The signal 44 contains GNSS quality factor data indicative of the reliability of the GNSS vehicle speed data. In this embodiment, for each packet of GNSS vehicle speed data delivered to the controller 18, the GNSS 16 35 provides GNSS quality factor data indicative of the reliability of the corresponding GNSS vehicle speed data. It should be noted that in other embodiments the GNSS 16 may not provide GNSS quality factor data for each and every packet of GNSS vehicle speed data provided to the controller 18. To calculate the vehicle speed, v-i, the controller 18 determines a linear speed, vl, for each wheel 14, and 40 determines the first vehicle speed value, v-i, by calculating an average of the linear speeds, vl, of the wheels 14 of the vehicle 12. As will be appreciated by a person skilled in the art, the linear speed, vl, for each wheel 14 can be calculated using the rotational speed, co, of the corresponding wheel 14, using the following equation, denoted as Equation 1: 5 vL = r x a) [Equation 1] In Equation 1, r is the tyre radius of the corresponding wheel 14, which is stored on the memory means 22 of the controller 18 for use. It should be noted that the tyre radius, r, that is stored on the memory means 22 is not updated during the lifetime of the vehicle 12, and so relates to the tyres that are installed on the vehicle 12 10 during vehicle production. It should also be noted that in some embodiments a tyre size correction factor may be applied to the tyre radius, r, at this stage in order to account for differences between the tyre radius, r, stored on the memory means 22 and the actual trye radius. At step 106, the controller 18 determines the second vehicle speed value, V2, in dependence on the second 15 signal 42, and stores the second vehicle speed value, V2, on the memory means 22. In this embodiment, in which the GNSS vehicle speed data contains the second vehicle speed value, V2, step 106 simply comprises extracting the second vehicle speed value, V2, from the GNSS vehicle speed data and storing this second vehicle speed value, V2, on the memory means 22. In embodiments in which the GNSS vehicle speed data does not include the second vehicle speed value, V2, itself, but rather includes information received from the satellites 32 that enable calculation of the second vehicle speed value, V2, step 106 comprises calculating the second vehicle speed value, V2, using the GNSS vehicle speed data and storing this calculated second vehicle speed value, V2, on the memory means 22. 25 Following completion of steps 102, 104 and 106, the controller 18 carries out a validation check 107 at steps 108 to 118. The validation check 107 determines if a correction factor for use in a corrected vehicle speed, vc, calculation, should be stored. As explained in more detail below, the correction factor re-calibrates the vehicle speed as determined through wheel rotational speed measurements, i.e. the first vehicle speed value, vi, using the vehicle speed as determined using GNSS data, i.e. the second vehicle speed value, V2, to provide a more 30 reliable corrected vehicle speed, vc, for e.g. display to a driver of the vehicle on a speedometer. In this embodiment, the validation check 107 comprises seven sub-checks that must all be passed in order for the validation check 107 to be passed, but in other examples it would be possible for the validation check 107 to include more or fewer sub-checks. In this embodiment the validation check 107 is carried out at a frequency 35 of 40 Hz, on receipt of the electronic signals 29, 42, 44 by the controller 20, but it should be noted that the validation check 107 may be carried out more or less frequently in other embodiments. At step 108, the controller 18 compares the first vehicle speed value, v-i, with a predetermined threshold value stored on the memory means 22, and determines if the first vehicle speed value, v-i, is above the predetermined 40 threshold value. If the first vehicle speed value, v-i, is above the predetermined threshold value, the controller 18 determines that sub-check of step 108 has been passed. 22 07 £5 22 07 £5 Thus, the predetermined threshold value of step 108 provides a minimum speed at which the vehicle 12 must be travelling in order for the validation check 107 to be passed. This ensures that the correction factor is only stored if the vehicle 12 is travelling above a minimum speed, below which it is deemed likely that the vehicle 5 12 is travelling in a built-up area, where surrounding buildings and / or other structures may interfere with the satellite signals 32 using to determine the second vehicle speed value, V2 In this example, the predetermined threshold is 30 kph, but in other examples this threshold value may differ. At step 110, at a time tx, the controller 18 determines if the first vehicle speed value, v-i, has remained 10 substantially constant over a predetermined period of time directly preceding time tx. For this, the controller 18 determines if the first vehicle speed value, v-i, varies by less than a predetermined amount over the predetermined period of time, so as to allow for minor fluctuations in vehicle speed that may be expected when the vehicle 12 is travelling at a substantially constant speed. 15 The controller 18 identifies a set of stored first vehicle speed values, v-i, determined during the predetermined period of time, and calculates the standard deviation of this set of stored first vehicle speed values, v-i. The controller 18 then compares the calculated standard deviation with a threshold stored on the memory means 22, and determines if the calculated standard deviation is smaller than the stored threshold. If the calculated standard deviation is smaller than the stored threshold, then the controller 18 determines that the sub-check of step 110 has been passed. In this example, the stored threshold is 0.1 and the predetermined period of time is 5 seconds, such that step 110 determines if the first vehicle speed value, v-i, has remained substantially constant over the preceding 5 25 seconds. In other embodiments, the threshold and predetermined period of time may vary. It should be understood that each time the standard deviation of the first vehicle speed values, v-i, is calculated in accordance with step 110 of the method 100, the controller 18 does so on the basis of the first vehicle speed values, v-i, stored most recently on the memory means 22, and falling within the relevant time window. As such, 30 repetition of the method 100, e.g. at a frequency of 40 Hz, results in the system performing a moving standard deviation calculation in respect of the first vehicle speed values, v-i. At step 112, the controller 18 carries out a similar procedure as that undertaken at step 110, but this time for the second vehicle speed value, V2. That is, at step 112, at a time tx, the controller 18 determines if the second 35 vehicle speed value, V2, has remained substantially constant over a predetermined period of time directly preceding time tx, by determining if the second vehicle speed value, V2, varies by less than a predetermined amount over the predetermined period of time. For this, the controller 18 identifies a set of stored second vehicle speed values, V2, determined during the 40 predetermined period of time, and calculates the standard deviation of this set of stored second vehicle speed values, V2. 22 07 £5 The controller 18 then compares the calculated standard deviation with a threshold stored in the memory means 22, and determines if the calculated standard deviation is smaller than the stored threshold. If the calculated standard deviation is smaller than the stored threshold, then the controller 18 determines that the 5 sub-check of step 112 has been passed. In this example, the stored threshold for step 112 is 0.1 and the predetermined period of time is 5 seconds, such that step 112 determines if the second vehicle speed value, V2, has remained substantially constant over the preceding 5 seconds. In other embodiments the threshold and predetermined period of time may vary. It 10 should also be noted that whilst the stored threshold and predetermined period of time used in steps 110 and 112 are the same in this embodiment, it would be possible for these values to vary across these steps in other embodiments. At step 113, the controller 18 calculates a ratio, Rvi.v2, of the first and second vehicle speed values, vi and V2, 15 and stores the ratio, Rvi,v2 on the memory means 22. At step 114, the controller 18 determines if the ratio, Rvi.v2, falls within a predetermined range. For this, the controller 18 compares the ratio, Rvi.v2, with upper and lower limits of the predetermined range that are stored on the memory means 22. If the ratio, Rvi.v2, falls within the predetermined range, then the controller 18 determines that the sub-check of step 114 has been passed. In other words, if the ratio, Rvi.v2, is smaller than the upper limit or greater than the lower limit then the controller 18 determines that the sub-check of step 114 has been passed. In this example, the predetermined range for step 114 is between 0.95 and 1.05, such that the upper limit is 25 1.05 and the lower limit is 0.95, but in other examples these values may vary. At step 115, the controller 18 determines if the ratio, Rvi.v2, has remained substantially constant over a predetermined period of time directly preceding time tx, by determining if the ratio, Rvi.v2, varies by less than a predetermined amount over the predetermined period of time. 30 Forthis, the controller 18 identifies a set of stored ratios, Rvi.v2, calculated using first and second vehicle speed values, vi and V2, received by the controller 18 within the predetermined period of time, and calculates the standard deviation of this set of stored ratios, Rvi,v2. 35 The controller 18 then compares the calculated standard deviation with a threshold stored in the memory means 22, and determines if the calculated standard deviation is smaller than the stored threshold. If the calculated standard deviation is less than the stored threshold, then the controller 18 determines that the subcheck of step 115 has been passed. 22 07 £5 In this example, the stored threshold for step 115 is 0.1 and the predetermined period of time is 5 seconds, such that step 115 determines if the ratio, Rvi.v2, has remained substantially constant over the preceding 5 seconds. In other embodiments the threshold and predetermined period of time may vary. 5 At step 116, the controller 18 calculates a ratio, RV2,vc, of the second vehicle speed value, V2, and the corrected vehicle speed value, vc, using the last corrected vehicle speed value, vc, stored on the memory means 22. As explained below, in this embodiment a corrected vehicle speed value is calculated at step 122, after the validation check has been passed. If a corrected vehicle speed value, vc, has not yet been calculated and stored through step 122 of the method 100, i.e. if the method 100 is being carried out for the first time, then 10 the controller 18 may calculate a corrected vehicle speed value, vc, at step 116 for use in the ratio, RV2,vc, calculation. It should be noted that a pre-calculated correction factor is stored on the memory means 22 even before the method 100 is carried out for the first time, such that all relevant calculations of the method 100 that require use of a correction factor (such as the corrected vehicle speed value, vc, calculation) can be performed on the first run of the method 100. 15 The controller 18 determines if the ratio, RV2,vc, falls within a predetermined range. For this, the controller 18 compares the ratio, RV2,vc, with upper and lower limits of the predetermined range that are stored in the memory means 22. If the ratio, RV2,vc, falls within the predetermined range, then the controller 18 determines that the sub-check of step 116 has been passed. In other words, if the ratio, RV2,vc, is smallerthan the upper limit or greater than the lower limit then the controller 18 determines that the sub-check of step 116 has been passed. In this example, the predetermined range for step 116 is between 0.95 and 1.05, such that the upper limit is 25 0.95 and the lower limit is 1.05, but in other examples these values may vary. At step 118, the controller 18 determines if the quality factor is above a predetermined threshold stored on the memory means 22. For this, the controller 18 compares the latest quality factor received in the GNSS quality factor data with the predetermined threshold. If the quality factor is above the predetermined threshold, the 30 controller 18 determines that the sub-check of step 118 has been passed. As discussed already, the quality factor is provided as an integer between 1 to 5, inclusive. The predetermined threshold in this embodiment is 2, such that the sub-check of step 118 is passed if the quality factor is 3, 4 or 5, but the sub-check of step 118 fails if the quality factor is 1 or 2. In other embodiments the predetermined 35 threshold may vary. If all of the sub-checks of steps 108 to 118 are passed, the controller 18 determines that the validation check 107 has been passed and continues to step 120. 40 At step 120, the controller 18 calculates and stores the correction factor on the memory means 22. 22 Q7 25 The correction factor is calculated in dependence on the first vehicle speed value, v-i, and the second vehicle speed value, V2, and in this embodiment the correction factor is the ratio of the second vehicle speed value, V2, and the first vehicle speed value, v-i: correction factor = — [Equation 2] V1 5 At step 122, the controller 18 calculates a corrected vehicle speed value, vc, and stores the corrected vehicle speed value, vc, on the memory means 22. The corrected vehicle speed value, vc, is calculated using the first vehicle speed value, v-i, and the latest stored correction factor, i.e. the most recent correction factor to have been stored on the memory means 22. Specifically, the corrected vehicle speed value, vc, is calculated by 10 multiplying the last stored correction factor by the first vehicle speed value, v-i: vc = latest stored correction factor x v, [Equations] If one or more of the sub-checks of steps 108 to 118 are not passed, the controller 18 determines that the validation check 107 has been failed, and the method proceeds directly from step 118 to step 122, missing out 15 step 120. In other words, if the validation check 107 is failed, the controller 18 does not calculate orstore a new correction factor, but instead proceeds to calculate a corrected vehicle speed value, vc, using the last correction factor that was stored on the memory means 22. In such case the last stored correction factor will be the correction factor that was calculated and stored during the last run of the method in which the validation check 107 was passed, or the pre-calculated correction factor stored on the memory means 22 before the method 100 is carried out for the first time (in the case where the validation check 107 has not been passed on any previous run of the method). 25 Thus, it will be appreciated that a corrected vehicle speed value, vc, is calculated each time the method is carried out (i.e. at a frequency of 40Hz in this example), regardless of the outcome of the validation check 107. If the validation check 107 is passed, the controller 18 calculates and stores on the memory means 22 a correction factor at step 120, and uses this latest stored correction factor in the subsequent calculation of the 30 corrected vehicle speed value, vc, at step 122. If the validation check 107 is failed, the controller 18 does not calculate orstore a new correction factor, but instead moves directly to step 122 where the corrected vehicle speed value, vc, is calculated using a correction factor already stored on the memory means 22. 35 It should be noted that in some embodiments the correction factor is calculated prior to step 122, for example before carrying out the validation check 107. In that case, the correction factor is calculated even if the validation check 107 is failed, but is only stored if the validation check 107 is passed. At step 124, the controller 18 calculates an adjusted corrected vehicle speed value, vcadJ, by adding a predetermined offset value or parameter stored in the memory means 22 to the corrected vehicle speed value, vc. In this way, step 124 assists in ensuring that the vehicle speed value output by the method 100 is not an underestimate of the true speed of the vehicle 12. This guards against, for example, display of a speed that is lower than the true vehicle speed that could result in the driver of the vehicle inadvertently exceeding a speed limit, for example. In this embodiment, the offset value utilised in step 124 is 2kph for all corrected vehicle speed values, vc. The offset value may vary in other embodiments, and furthermore it would be possible for different offset values to be applied depending on the corrected vehicle speed value, vc. At step 126, the controller 18 outputs the adjusted corrected vehicle speed value, vcadj, to the vehicle data bus 41 via the output means 26 of the controller 18. 22 07b 25 Once output by the control system 10, the adjusted corrected vehicle speed value, vcadj, may be utilised by other systems of the vehicle 12 as desired. For example, the adjusted corrected vehicle speed value, vc adj, may be displayed to a vehicle user, in particular a driver of the vehicle 12, on a speedometer display of the vehicle. Additionally or alternatively, the adjusted corrected vehicle speed value, vcadj, may be used in other relevant systems of the vehicle 12 that utilise vehicle speed measurements, such as cruise control systems. It should be noted that in some embodiments the controller 18 may process the adjusted corrected vehicle speed value, vcadj, further before outputting this value to the vehicle data bus 41. For example, in some embodiments the controller 18 may undertake smoothing and rounding operations to prevent flickering of the adjusted corrected vehicle speed value, vc adj, across outputted values. In some embodiments, the controller 18 may convert the adjusted corrected vehicle speed value, vc adj, to different units, if required or desired. For example, if the adjusted corrected vehicle speed value, vcadj, is calculated in kph, the controller 18 may convert the adjusted corrected vehicle speed value, vc adj, to mph. The described method advantageously allows for a more accurate vehicle speed in the form of the adjusted corrected vehicle speed value, vc adj, to be generated and, for example, presented to a user of the vehicle via the vehicle speedometer. The adjusted corrected vehicle speed value, vcadj, is determined using a correction factor that takes into account the vehicle speed as determined from data from the GNSS 16. Thus, the adjusted corrected vehicle speed value, vcadj, is advantageously calculated in dependence on both the vehicle speed as determined from the rotational speed of the vehicle wheels (i.e. the first vehicle speed value) and the vehicle speed as determined from GNSS 16 data (i.e. the second vehicle speed value). Actually, if the validation check is passed, the corrected and adjusted vehicle speed, vcadj, is dependent on the vehicle speed, v-i, as determined from the rotational speed of the vehicle wheels only to the extent that this speed is employed in the validation check. That is, if the validation check passes, the corrected vehicle speed, vc = is simply equal to V2, because in this case the value of vi used to calculate the latest correction factor is the same as the first vehicle speed value vi to which the latest correction factor is applied in the corrected vehicle speed calculation. If the validation check fails, then the vehicle speed reported by the controller 18 to the bus 41 is calculated by multiplying vi by the latest correction factor stored on the memory means 22. 22 07.25 Furthermore, the correction factor is stored for use in adjusted corrected vehicle speed value, vcadJ, calculations only when a validation check 107 is passed. If the validation check 107 is failed, then a new correction factor is not stored, and subsequent adjusted corrected vehicle speed values, vc adj, use a correction factor already stored on the memory means 22, in particular the latest correction factor stored on the memory 5 means 22 e.g. the last time the validation check 107 was passed. This ensures that the correction factor is only stored for use in subsequent corrected speed value calculations when it is determined that the reliability of the correction factor is deemed sufficient. As an example, the validation check 107 may be failed if it is determined that the vehicle 12 is travelling below a certain speed, as this indicates that the second vehicle speed value, V2, may be less accurate due to blocking of GNSS signals by buildings associated with low-speed 10 areas. In this way, the correction factor is not stored for use in such a situation in which the second vehicle speed value V2, which is used in the correction factor calculation, may be less accurate or reliable. Turning now to Figure 4, test data obtained during a test drive of a vehicle 12 is illustrated in a graph of vehicle speed on the y-axis against time on the x-axis. The curve 130 illustrated on the graph of Figure 4 represents 15 the vehicle speed as determined using wheel rotational speed measurements, i.e. first vehicle speed values, vi. The curve 132 illustrated on the graph of Figure 4 represents the true vehicle speed, as determined using GPS apparatus that mimics GNSS input to the vehicle 10. The curve 134 on the graph of Figure 4 does not represent vehicle speed, but rather illustrates whether or not a validation check to determine if a correction factor should be stored for use has been passed. When the curve 134 is shown extending along the x-axis at y = 0, this indicates that the validation check has been failed at the associated time windows of the test drive. When the curve 134 is at y = 125 kph, this indicates that the validation check has been passed at the associated time windows of the test drive. Specifically, in Figure 4, the curve 134 illustrates that the validation check is only passed between t=513.5s and t=527s of the test drive. 25 It will be appreciated that the y value chosen for the curve 134 to indicate that the validation check has been passed is arbitrary - the curve 134 does not represent a speed value, but rather indicates whether or not the validation check has been passed or failed (i.e. a true or false value). The curve 136 illustrated on the graph of Figure 4 represents corrected vehicle speed values, vc, calculated 30 by applying a correction factor to the first vehicle speed values, v-i. Before t=513.5s, the corrected vehicle speed values, vc, are identical to the first vehicle speed values, v-i, i.e. no correction is applied to the first vehicle speed values, v-i, such that the correction factor can be considered to be 1. However, after t=513.5s, once the validation check has been passed for the first time, an appropriate correction factor is calculated and stored, and this stored correction factor is used to calculate corrected vehicle speed values, vc, after t=513.5s. 35 As will be appreciated from Figure 4, after t=513.5s, when an appropriate correction factor is being applied after the validation check has been passed, the corrected vehicle speed values, vc, advantageously more closely represent the true vehicle speed than the first vehicle speed values, v-i. 40 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.

Claims

22 07 £51. A control system for a vehicle, the control system comprising one or more controller, the control system configured to:5 receive a first signal indicative of a rotational speed of one or more wheels of the vehicle;determine a first vehicle speed value in dependence on the first signal;receive a second signal indicative of a vehicle speed from a global navigation satellite system; determine a second vehicle speed value in dependence on the second signal;perform a validation check and, if the validation check is passed, store a correction factor calculated10 in dependence on the first vehicle speed value and the second vehicle speed value on an electronicstorage device;calculate a corrected vehicle speed value using the first vehicle speed value and a stored correction factor from the electronic storage device; andoutput the corrected vehicle speed value,15 wherein the validation check comprises determining one or more of:the first vehicle speed value varies by less than a predetermined amount over a predetermined period of time;the second vehicle speed value varies by less than a predetermined amount over a predetermined period of time;the first vehicle speed value is above a predetermined threshold;a ratio of the first vehicle speed value and the second vehicle speed value varies by less than a predetermined amount over a predetermined period of time;a ratio of the first vehicle speed value and the second vehicle speed value falls within a predetermined range;25 a ratio of the second vehicle speed value and the corrected vehicle speed value falls within apredetermined range; anda quality factor relating to the second vehicle speed value is above a pre-determined threshold value.

2. The control system of claim 1, configured to add a predetermined offset value to the corrected vehicle speed 30 value.

3. The control system of claim 1 or 2, wherein calculating the correction factor comprises calculating a ratio of the second vehicle speed value and the first vehicle speed value.

4. The control system of any preceding claim, wherein calculating the corrected vehicle speed value comprises multiplying the stored correction factor and the first vehicle speed value.35 5. The control system of any preceding claim, configured to calculate the correction factor before the validationcheck is performed.

6. The control system of any of claims 1 to 4, configured to calculate the correction factor after the validation check is performed.22 07.

257. The control system of any preceding claim, configured to repeat the steps that the control system is configured to carry out periodically, at a discrete series of time intervals, and to calculate the corrected vehicle speed value using the newest stored correction factor.

8. A system comprising the control system of any preceding claim, a global navigation satellite system and at 5 least one rotational speed sensor for measuring a rotational speed of at least one wheel of the vehicle.

9. The system of claim 8, comprising a display of a vehicle for displaying the corrected vehicle speed value to a user of the vehicle.

10. A vehicle comprising the system of claim 8 or 9, or the control system of any of claims 1 to 7.

11. A method for determining a corrected vehicle speed value for a vehicle, the method comprising:10 receiving a first signal indicative of a rotational speed of one or more wheels of the vehicle;determining a first vehicle speed value in dependence on the first signal;receiving a second signal indicative of a vehicle speed from a global navigation satellite system;determining a second vehicle speed value in dependence on the second signal;performing a validation check and, if the validation check is passed, storing a correction factor calculated in dependence on the first vehicle speed value and the second vehicle speed value on an electronic storage device; andcalculating a corrected vehicle speed value using the first vehicle speed value and a stored correction factor from the electronic storage device,wherein the validation check comprises determining one or more of:20 the first vehicle speed value varies by less than a predetermined amount over a predeterminedperiod of time;the second vehicle speed value varies by less than a predetermined amount over a predetermined period of time;the first vehicle speed value is above a predetermined threshold;25 a ratio of the first vehicle speed value and the second vehicle speed value varies by less than apredetermined amount over a predetermined period of time;a ratio of the first vehicle speed value and the second vehicle speed value falls within a predetermined range;a ratio of the second vehicle speed value and the corrected vehicle speed value falls within a 30 predetermined range; anda quality factor relating to the second vehicle speed value is above a pre-determined threshold value.

12. Computer readable instructions which, when executed by a computer, are arranged to perform a method according to claim 11.CLAIMS1. A control system for a vehicle, the control system comprising one or more controller, the control system configured to:receive a first signal indicative of a rotational speed of one or more wheels of the vehicle;determine a first vehicle speed value in dependence on the first signal;receive a second signal indicative of a vehicle speed from a global navigation satellite system;determine a second vehicle speed value in dependence on the second signal;perform a validation check and, if the validation check is passed, store a correction factor calculated in dependence on the first vehicle speed value and the second vehicle speed value on an electronic storage device;calculate a corrected vehicle speed value using the first vehicle speed value and a stored correction factor from the electronic storage device; andoutput the corrected vehicle speed value.

2. The control system of claim 1, wherein the validation check comprises determining one or more of:the first vehicle speed value varies by less than a predetermined amount over a predetermined period of time;the second vehicle speed value varies by less than a predetermined amount over a predetermined period of time;the first vehicle speed value is above a predetermined threshold;a ratio of the first vehicle speed value and the second vehicle speed value varies by less than a predetermined amount over a predetermined period of time;a ratio of the first vehicle speed value and the second vehicle speed value falls within a predetermined range;a ratio of the second vehicle speed value and the corrected vehicle speed value falls within a predetermined range; anda quality factor relating to the second vehicle speed value is above a pre-determined threshold value.

3. The control system of claim 1 or 2, configured to add a predetermined offset value to the corrected vehicle speed value.

4. The control system of claim 1,2 or 3, wherein calculating the correction factor comprises calculating a ratio of the second vehicle speed value and the first vehicle speed value.

5. The control system of any preceding claim, wherein calculating the corrected vehicle speed value comprises multiplying the stored correction factor and the first vehicle speed value.

6. The control system of any preceding claim, configured to calculate the correction factor before the validation check is performed.

7. The control system of any of claims 1 to 5, configured to calculate the correction factor after the validation check is performed.

8. The control system of any preceding claim, configured to repeat the steps that the control system is configured to carry out periodically, at a discrete series of time intervals, and to calculate the corrected vehicle speed value using the newest stored correction factor.

9. A system comprising the control system of any preceding claim, a global navigation satellite system and at least one rotational speed sensor for measuring a rotational speed of at least one wheel of the vehicle.

10. The system of claim 9, comprising a display of a vehicle for displaying the corrected vehicle speed value to a user of the vehicle.

11. A vehicle comprising the system of claim 9 or 10, or the control system of any of claims 1 to 8.

12. A method for determining a corrected vehicle speed value for a vehicle, the method comprising:receiving a first signal indicative of a rotational speed of one or more wheels of the vehicle;determining a first vehicle speed value in dependence on the first signal;receiving a second signal indicative of a vehicle speed from a global navigation satellite system;determining a second vehicle speed value in dependence on the second signal;performing a validation check and, if the validation check is passed, storing a correction factor calculated in dependence on the first vehicle speed value and the second vehicle speed value on an electronic storage device; andcalculating a corrected vehicle speed value using the first vehicle speed value and a stored correction factor from the electronic storage device.

13. Computer readable instructions which, when executed by a computer, are arranged to perform a method according to claim 12.

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