GB2494413A

GB2494413A - Vehicle speed determination

Info

Publication number: GB2494413A
Application number: GB1115398.8A
Authority: GB
Inventors: Thomas Popham; Anna Gaszczak
Original assignee: Land Rover UK Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2011-09-06
Filing date: 2011-09-06
Publication date: 2013-03-13
Also published as: WO2013034560A1; WO2013034560A9; GB201215776D0; GB2494526B; GB201115398D0; GB2494526A

Abstract

A vehicle mounted time of flight camera system 12 repeatedly captures an image of the scene ahead of the vehicle. A point of interest 14,15,16 in the scene is identified, and comparative vehicle speed determined according to the changing relative distance to the point of interest. Several points of interest, in the near and far fields, are simultaneously followed so that vehicle speed is continually updated. The scene is illuminated with a cone of infrared light 13.

Description

Improvements in Vehicle Speed Determination This invention relates to determination of vehicle speed and particularly, but not exclusively, to determination of low speed, with accuracy. The invention has specific application to off-road travel where a degree of wheel slip may be encountered, and the terrain is typically not uniform. In this specification low speed is defined as less than 20 kph. Aspects of the invention relate to an apparatus, to a calculator, to a system, to a method and to a vehicle.

Accurate determination of vehicle speed is required for many electronic vehicle systems.

One example relates to cruise control whereby a set vehicle speed is maintained regardless of gradient.

On a conventional dry road surface, such as concrete or tarmacadam, slip between the vehicle wheels and the road is negligible. Accordingly a counting technique is generally reliable, and typically consists of counting wheel revolutions to determine the distance travelled. Distance measurement can be combined with a clock signal to give an indication of vehicle speed. A counter can provide a suitable electronic input for many vehicle systems, including cruise control, distance measure, speedometer and fuel consumption per unit distance. However where traction is poor, wheels may spin on the road surface, and consequently a reliable indication of vehicle speed cannot be obtained; without such indication all systems reliant upon accurate measurement of distance travelled also become unreliable.

Another system for measuring distance travelled relies upon GAS (global positioning system) but is dependent upon an unobstructed line of sight to four or more satellites to determine absolute position. GAS works well for highway driving where speeds are high and obstructions infrequent. However at low speeds GAS resolution does not give sufficient accuracy of position to reliably determine speed of travel. Moreover, in off-road driving obstructions to the lines of sight are common so that the GPS indication may itself be frequently interrupted or unobtainable.

Yet another speed determining system relies upon inertial sensors such as low g accelerometers and gyroscopes. Such systems are expensive, and tend to suffer from drift so that the output(s) are unreliable unless frequently re-calibrated.

A fourth system relies upon colour camera recognition techniques, but fails to be effective where the scone is uniform, such as a rock field or a desert.

The present systems of determining vehicle position, and hence vehicle speed, are not sufficiently reliable when travelling off-road. Such systems cannot cope well with low vehicle speed (less than 20 kph), frequent change of vehicle attitude, and wheel slip. As a consequence the reliable application of off-road cruise control has not proved to be practicable.

Off-road cruise control is however desirable, in selected conditions, to allow the vehicle driver to concentrate on other activities, such as steering.

Aspects of the invention provide a system, a method and a vehicle as claimed in the appended claims.

According to another aspect of the invention there is provided a system for a vehicle comprising a vehicle-mounted time of flight camera arrangement, the system being arranged to illuminate and repeatedly capture an image of the scene ahead of the vehicle and to use the image data to determine the speed of movement of the vehicle with respect to said image.

The system may identify a point of interest in the repeating image, and determine speed of movement of the vehicle with respect to the point of interest. Alternatively some other data analysis technique may be used, such as using the entire image data to estimate change in relative position and orientation using an iterated closest point algorithm.

Time of flight camera systems are known. Generally speaking the camera illuminates a scene with infra-red light. An imaging chip within the camera determines the time of flight of the infra red light to the scene and back to each pixel of the chip. Thus the image on the chip gives an instantaneous representation of the distance from the camera to topographical features in the scene, rather than an image constructed from line by line scanning.

Typically, the camera will refresh the image of the scene repeatedly, for example at a rate of frames per second. A high refresh rate facilitates following a point of interest despite changes of viewing position due to attitude change of the vehicle, and changes in the separation distance.

Use of a time of flight camera to measure speed of a vehicle is not known.

In one embodiment the vehicle speed calculator includes a processor adapted to select a point of interest which appears to be moving in a straight line toward the vehicle. The point feature may be defined by a plurality of pixels of the imaging chip. The processor may select more than one point of interest so as to increase confidence in calculation of vehicle speed.

Relative movement of the point of interest and the vehicle need not be on the shortest line.

Triangulation techniques permit relative motion in any desired direction to be determined.

Relative speeds in relation to several points of interest provide comparison so as to give greater confidence in the calculated speed of the vehicle.

The vehicle speed calculator may select several points of interest to define a line feature, or several line features, and determine the speed of movement of the vehicle with respect to the or each line feature.

The vehicle speed calculator may select several points of interest to define a topographical array, or several such arrays, and determine the speed of movement of the vehicle with respect to the or each array. As noted above the entire image data may be used to estimate relative position, and thus speed.

Relative motion of the vehicle may not be forward with respect to the scene. For example a vehicle may shp sideways on a slope without making forward movement. The present invention allows such sideways motion to be analysed to give a relative speed across the scene. Relative motion in all directions can be computed so as to determine movement in three mutually perpendicular directions of translation and in pitch, roll and yaw.

The processor may calculate vehicle speed at the refresh rate of the camera. However a lesser rate may be selected to save processor capacity, and because speed calculation at the refresh rate is not of practicable use. Repeated speed calculation at intervals of about 1 second may be sufficient.

In an embodiment the vehicle speed calculator identifies one or more points of interest in the near field and in the far field. The near and far fields may be defined in any suitable manner, for example the near field may comprise the scene within 10 metres of the camera, and the

far field may comprise the scene beyond 10 metres.

The invention is particularly suitable as a means of providing low speed cruise control, so that the vehicle can maintain a steady speed off-road regardless of gradient.

Repeating images from a time of flight camera system can give information about the gradient ahead of a vehicle. In conjunction with information about the current attitude of the vehicle, for example from an inclinometer or like device, the system can provide speed adjustment information suitable for maintaining a pre-set cruise speed during attitude changes. Thus for example when significantly changing gradient at low speed, for example when cresting a hill, the system can prepare a vehicle engine for the new gradient in advance, thus overcoming the inherent delay between requesting a change of engine output, and delivery of the requested output. The vehicle can thereby better maintain a set speed in cruise control mode.

This technique allows handover from near field points of interest to far field points of interest as the near field points pass out of the field of view of the camera (typically beneath the vehicle).

Other suitable techniques may be used provided that a continual indication of vehicle speed is maintained by reference to points of interest within the imaged scene.

Identification of points of interest is by conventional pattern recognition techniques which form no part of the present invention. Such techniques can accommodate the relative increase in size of a point of interest in successive images, as the vehicle approaches the point of interest.

Pitching and rolling of the vehicle aids in providing three-dimensional information, so as to better allow selection and discrimination of points of interest. Such grass vehicle movement is very common in off-road driving, and the degree of such movement is generally inversely proportional to vehicle speed; thus the slower the vehicle, the more effective the system of the invention may be.

True speed of movement of the vehicle also provides data for determination of wheel traction information both on and off-road.

As noted above, counting techniques are not reliable for vehicle speed determination where wheel slip is a significant factor. Counting wheel revolutions can however be performed reliably, using for example techniques employed in anti-lock braking system.

According to a further aspect of the invention there is provided a vehicle wheel slip calculator comprising the system of the previous aspect, a wheel rotation calculator to determine the theoretical speed of rotation of a vehicle wheel commensurate with the instant speed of the vehicle, a wheel speed indicator for indicating the instant speed of rotation of said wheel, and a comparator to continually compare said theoretical speed of rotation with said instant speed of rotation in order to determine instant wheel slip.

The wheel sip calculator may determine wheel slip for all wheels of the vehicle.

Such a device can give information concerning the time level of traction at the wheels, and thus allow other vehicle systems to be more effective. For example suspension and engine adjustments may be effected in order to for example gain ground clearance or increase engine output torque; an alternative transmission ratio may be engaged and other measures appropriate to improving vehicle traction.

This aspect of the invention overcomes prior wheel slip measurement techniques which tend to compare instant speed of rotation of several wheels in order to identify slip. Such systems cannot give absolute information if all of the compared wheels are slipping.

According to a further aspect of the invention there is provided a method of vehicle speed calculation comprising the steps of illuminating the scene ahead of the vehicle, and repeatedly capturing an image thereof using a camera, and determining the speed of movement of the vehicle with respect to said image.

The speed of movement may be determined from the entire image data, using for example an iterated closest point algorithm, or by identifying one or more points of interest in the repeating image.

This aspect also provides a method of calculating wheel slip and comprising determining vehicle speed according to the second aspect of the invention, determining the theoretical speed of rotation of a vehicle wheel according to said determination of vehicle speed, measuring the actual speed of rotation of said vehicle wheel and determining the instant slip of said wheel.

The method is carried out continually for all wheels of the vehicle so as to obtain a substantially continuous indication of wheel slip.

Within the scope of this application it is envisaged that the various aspects, embodiments, examples, features and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings may be taken independently or in any combination thereof. For example features described in connection with one embodiment are applicable to all embodiments, except where there is incompatibility of features.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of a vehicle off-road and employing the invention.

Figs. 2-4 show schematically successive images from a moving vehicle.

With reference to the drawings, Fig. 1 represents a vehicle 10 with off-road capability travelling on an uneven rock surface 11, such as a river bed. In such conditions, as is well understood, wheel traction may be poor so that one or more driving wheels of the vehicle slips with respect to the rock surface. The vehicle driver may wish to select a low speed cruise control function, say at 5 kph, in order to maintain progressive forward travel whilst concentrating on steering. Current systems of measuring vehicle speed by counting wheel rotations are inaccurate where repeated wheel slip is encountered, and methods relying on GPS or inertial devices have the disadvantages mentioned earlier.

In the invention a forward facing time of flight camera 12 illuminates the terrain ahead of the vehicle, and generates an image thereof on an imaging chip. Illumination is indicated by the cone of infra red light 13, and it will be appreciated that any forward facing fixed location of the camera may be suitable, for example in a conventional light fitting or at the leading edge of the bonnet.

The chip image repeats at a pre-determined refresh rate, so that the speed of movement of a point of interest 14 with respect to the vehicle can be determined. The points of interest can be any topographical feature having a repeated form in successive images, but techniques for identifying and comparing successive images form no part of the invention.

The point of interest may comprise a number of pixels in an image which represent a topographical feature or may be a line feature, or a group of topographical features having an identifiable spatial relationship.

Several points of interest may be identified simultaneously, and known techniques may be used to use such points to increase confidence in the measured speed.

Time of flight camera systems have a resolution of 5 mm or better. Accordingly the point of interest may for example be a feature on a rock, such as a crack or a depression.

Furthermore several points of interest may be used successively to maintain an indication of vehicle speed as the vehicle advances over the ground. Thus a preceding point of interest 15, and a next point of interest 16 are indicated in Fig. 1. These successive points of interest may be mapped simultaneously to ensure that speed calculation is continual. Vehicle speed may for example be re-calculated at the refresh rate of the image, or may be at a slower rate.

As each image provides a distance to several points of interest within the image, and the refresh rate of the camera is known, the relative speed of each point of interest with respect to the vehicle can be calculated. In embodiments where several points of interest are identified, the mean of these points is taken to represent the speed of the vehicle, though in further embodiments other values such as the median may be used. If the number of points of interest identified in an image is above a predetermined value, such as ten, a distribution of calculated relative speeds is calculated and statistical outliers are ignored, thereby producing a more accurate result.

The selected points of interest may be allocated to distance ranges, such that at least a near and far field are identified in order to provide for new points of interest as old points of interest pass beneath the vehicle.

The invention provides a reliable indication of vehicle speed without regard to wheel spin or slip, and this facilitates cruise control at low and very low speed (e.g. less than 5 kph).

Figs. 2-4 indicate schematically how large discontinuities may appear in frames spaced by several seconds. For ease of illustration three discontinuities in the near/far continuum are shown. The near and far criteria may be set or adjusted according to circumstances of vehicle use, and may for example have a transition at 5 metres. More than two distance bands may be defined. Also, for ease of illustration a generally flat surface is depicted from which discontinuities project upwardly -depressions may also be used as discontinuities, and the ground surface may not be flat.

The illustrated frames of Figs. 2-4 are several seconds apart. It will be appreciated that the refresh rate is tens of frames per second, so that much information is available for processing, and image discrimination.

Fig. 2 shows discontinuities 14-16 in the middle, near and far field at time t = 0. Fig. 3 shows a representation at time t = 5 seconds; the discontinuities are closest to the vehicle, and accordingly larger. The speed of the vehicle can be calculated since it can be assumed that the discontinuities are in a fixed location.

Fig. 4 shows a representation at time t = 10 seconds. The largest discontinuity has disappeared under the vehicle, and has been replaced by a new discontinuity 17 in the far

field.

The repeated images generated on the imaging chip may be used to generate a topographical plan view of the terrain ahead of the vehicle -a so-called birds-eye view. The passage of the vehicle over such a view can be tracked as the images repeat, and the relative speed with respect to a reference location, or in a reference direction, may be determined. Thus the changing attitude of the vehicle can be accommodated within the invention provided that the point or points of interest remain within the field of view of the camera. Conventional techniques can be used for identification and discrimination of topographical features which change relative position due to change of vehicle attitude. The rapid refresh rate of the system of the invention ameliorates this task.

Claims

<claim-text>Claims 1. A speed calculator for a vehicle comprising a vehicle mounted time of flight camera system having a forward facing camera, the system being configured to illuminate and repeatedly capture an image of the scene ahead of the vehicle, wherein the system is further configured to identify a point of interest in the repeating image and determine the speed of movement of the vehicle with respect of said point of interest.</claim-text> <claim-text>2. A calculator according to claim 1, wherein said point of interest is defined by a plurality of pixels of an imaging chip.</claim-text> <claim-text>3. A calculator according to claim 1 or claim 2, and adapted to select a point of interest which appears to be moving in a substantially straight line toward the vehicle.</claim-text> <claim-text>4. A calculator according to any preceding claim, wherein said system is adapted to identify simultaneously several points of interest in the repeating image.</claim-text> <claim-text>5. A calculator according to claim 4, wherein the system identifies a point of interest in the near field, and a point of interest in the far field.</claim-text> <claim-text>6. A calculator according to any preceding claim, wherein a point of interest is at a topographical discontinuity.</claim-text> <claim-text>7. A calculator according to claim 6, wherein said discontinuity projects from a notional ground surface plane.</claim-text> <claim-text>8. A calculator according to claim 6 or claim 7, wherein said discontinuity is a topographical line feature.</claim-text> <claim-text>9. A calculator according to any of claims 6-8, wherein said discontinuity is greater than a predetermined size.</claim-text> <claim-text>10. A method of vehicle speed calculation comprising: providing a time of flight camera system on a vehicle; facing the time of flight camera in the direction of vehicle travel; illuminating the scene ahead of the vehicle, and repeatedly capturing an image thereof; identifying a point of interest in the repeating image; and determining the speed of movement of the vehicle with respect to said point of interest.</claim-text> <claim-text>11. A method according to claim 10, wherein said identifying step comprises identifying a point of interest that appears to be moving in a substantially straight line toward the vehicle.</claim-text> <claim-text>12. A method according to claim 10 or claim 11, comprising identifying several points of interest simultaneously; and for each point of interest determining the speed of movement of the vehicle.</claim-text> <claim-text>13. A method according to any of claims 10-12, comprising identifying a point of interest in the near field and a point of interest in the far field.</claim-text> <claim-text>14. A method according to any of claims 10-13, wherein a point of interest is selected according to minimum size criteria.</claim-text> <claim-text>15. A method according to claim 14, wherein said minimum size criteria are variable.</claim-text> <claim-text>16. A vehicle having a calculator as claimed in any of claims 1 -9.</claim-text> <claim-text>17. A system, a calculator, a vehicle or a method constructed and/or arranged substantially as described herein with reference to the accompanying drawings.</claim-text>