Description of Invention This invention relates to a vehicle safety system, and in particular to a system which analyses signals from vehicle sensors to detect pedestrian or other objects ahead of the vehicle.

Many modern vehicles includes one or more sensor which allow the vehicle to detect the presence of an object ahead of the vehicle, which is, or which may be at a future time, in the path of travel of the vehicle.

US patent publication no. US2016/0075332 discloses a system which determines the forward rate of travel of the vehicle, and also the position and lateral speed of travel of a pedestrian detected ahead of the vehicle. The system disclosed in this document aims to determine whether, based on this information, the vehicle is likely to collide with the pedestrian. Safety measures, such as early braking or activation of the vehicle's lights and/or horn to alert the pedestrian, may be engaged to help avoid a collision with the pedestrian.

It is an object of the present invention to provide an improved safety system of this type. Accordingly, one aspect of the present invention provides a safety system for a vehicle, the safety system comprising: a first sensor arrangement to determine a current velocity of the vehicle; a second sensor arrangement to detect a mobile object exterior to the vehicle, and determine a current position and velocity of the mobile object relative to the vehicle; a safety device which may be activated to protect an occupant of the vehicle and/or the mobile object; and a control unit operable to: receive and analyse signals from the first and second sensor arrangements determine whether the vehicle will collide with the object if the current velocities of the vehicle and of the object are maintained, and if so to determine the time and position of the collision; and generate an activation signal to activate the safety device based on the analysis of the signals from the first and second sensor arrangements, wherein the analysis of the signals from the first and second sensor arrangements comprises: defining a vehicle front line extending across all or part of the front of the vehicle, and calculating likely future movement of the front line through space to determine a vehicle plane, in both space and time coordinates, comprising the future path of the vehicle front line through space based on the calculated likely future movement of the front line; defining an expected future trajectory for the mobile object; and calculating the position and time of an intersection between the vehicle plane and the expected future trajectory of the mobile object, wherein the calculated position includes a position of the intersection along the vehicle front line.

Advantageously, the first sensor arrangement is operable to determine a current acceleration of the vehicle; and the calculation of the likely future movement of the front line through space assumes that the current acceleration of the vehicle will be maintained.

Preferably, the second sensor arrangement is operable to determine a current acceleration of the mobile object; and the defining of the expected future trajectory of the mobile object assumes that the current acceleration of the mobile object will be maintained.

Conveniently, the calculated time of the intersection is

-B ± S ² - 2CA Where A represents the calculated position of the vehicle relative to the object, B represents the calculated speed of the vehicle relative to the object, and C represents the calculated acceleration of the vehicle relative to the object. Advantageously,

Where: a _x and a _y represent the zero order component of a polynomial representing the difference between the position of the vehicle and the position of the mobile object along the x and y axes; b _x and b _y represent the first order component of a polynomial representing the difference between the velocity of the vehicle and the velocity of the mobile object along the x and y axes; c _x and c _y represent the second order component of a polynomial representing the difference between the acceleration of the vehicle and the acceleration of the mobile object along the x and y axes; and f _s ' _x and f _s ' _y represent the rate of change of the line parallel to the front bumper of the vehicle along the x and y axes. Preferably, the yaw rate of both the vehicle and the mobile object are assumed to be zero.

Conveniently, the position of the intersection along the vehicle front line is calculated as:

Where: g(t) describes the location of a point representing the position of the vehicle over time; h(t) describes the location of a point representing the position of the mobile object over time; and q(t) represents the vector inverse of f _s ' , which describes the orientation of a line parallel to the front bumper of the vehicle. Advantageously, the formula for calculating the position and time of the intersection is stored in a memory of the vehicle, and is not calculated in real time.

Preferably, the first sensor arrangement is operable to determine a current yaw rate of the vehicle; and the calculation of the likely future movement of the front line through space assumes that the current yaw rate of the vehicle will be maintained, and preferably also wherein: the second sensor arrangement is operable to determine a current yaw rate of the mobile object; and the defining of the expected future trajectory of the mobile object assumes that the current yaw rate of the mobile object will be maintained.

Conveniently, the position and time of the intersection are calculated numerically. Advantageously, the mobile object is represented in the calculations by a single point in space.

Preferably, the mobile object is represented in the calculations by a plurality of points, and wherein the position and time of an intersection is calculated for each of the points.

Conveniently, the calculated position and time of the intersection between the vehicle plane and the expected future trajectory of the mobile object is based on the one of the points which is calculated to intersect with the vehicle plane at the earliest time. Another aspect of the present invention provides a vehicle incorporating a safety system according to any preceding claim.

A further aspect of the present invention provides a method of controlling one or more safety devices of a vehicle, the method comprising the steps of: determining a current velocity of the vehicle; detecting a mobile object exterior to the vehicle, and determining a current position and velocity of the mobile object relative to the vehicle; determining whether the vehicle will collide with the object if the current velocities of the vehicle and of the object are maintained, and if so determining the time and position of the collision; and generating an activation signal to activate the safety system based on the determination, wherein the determination comprises the steps of: defining a vehicle front line extending across all or part of the front of the vehicle, and calculating likely future movement of the front line through space to determine a vehicle plane, in both space and time coordinates, comprising the future path of the vehicle front line through space based on the calculated likely future movement of the front line; defining an expected future trajectory for the mobile object; and calculating the position and time of an intersection between the vehicle plane and the expected future trajectory of the mobile object, wherein the calculated position includes a position of the intersection along the vehicle front line.

In order that the invention may be more readily understood embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic view of a vehicle incorporating a system embodying the present invention; Figures 2 and 3 show the vehicle of figure 1 in motion; and Figures 4 and 5 show graphs representing the motion of the vehicle of figure 1 , along with an external mobile object.

Referring firstly to figure 1 , a vehicle 1 is shown schematically, from above. The vehicle 1 includes at least one sensor 2 which detects the presence of objects in a region of space 3 which extends forwardly in front of the vehicle 1 . The skilled person will appreciate that several different kinds of sensor are suitable for this purpose. For instance, one or more mono or stereo cameras may be used. These cameras may operate in any suitable frequencies, for instance optical or infrared. Cameras operating in infrared frequencies are typically placed at or near the front of the vehicle, forward of the vehicle's engine. This is because, during driving, the engine is likely to get hot, and if an infrared camera is positioned rearwardly of the engine then the "glow" from the heat of the engine may distort or dominate images gathered by the camera.

Alternatively, or in addition, one or more radar or lidar sensors may be used. These sensors may be placed anywhere suitable in the vehicle, but typically such sensors will be placed inside the vehicle's windscreen, near the top edge thereof, for instance in the region where the rear-view mirror is attached to the interior of the windscreen. This allows the sensors to be located relatively high up, so as to command a better field of view, and also allows the sensors to be protected behind the windscreen itself. Any other suitable sensor(s) may be used, and also any appropriate combination of sensors may be employed. As the skilled reader will be aware, in recent years many vehicle systems have combined information from different sensors to improve the accuracy of detection, and such systems are within the scope of the present invention. One or more further sensors 4 are also incorporated to determine the speed, direction of travel and acceleration of the vehicle 1 . Such sensors may include: · wheel rotation sensors, to determine the rate of rotation of the or all of the wheels of the vehicle;

• a GPS or similar receiver, to determine directly the vehicle's position on the Earth's surface;

• one or more pedal sensors, to determine the degree by which one or more of the vehicle's pedals are depressed by the driver of the vehicle;

• a steering wheel rotation sensor, to determine the angle at which the steering wheel is turned by the driver, and also the rate of change of angle of the steering wheel; and

• sensors to detect the behaviour of the driver, for instance sensors which detect the direction in which the driver is looking. This may provide information as to the likely future trajectory of the vehicle.

These sensors are known per se and will not be discussed in detail here.

Signals from the sensors are passed to a control unit 5 of the vehicle. The control unit 5 may comprise one central processor, or may comprise two or more distributed processors throughout the vehicle. The processor is operable to receive and analyse signals from the sensors mentioned above. The vehicle also includes at least one safety device 6 which may be activated under the control of the processor. The safety devices may, upon activation, serve to alter the speed and/or trajectory of the vehicle 1 to reduce the possibility of a collision, to protect a pedestrian outside the vehicle 1 in the event of a collision with the vehicle 1 , and/or to protect the occupants of the vehicle 1 itself. These safety devices may be reversible or irreversible, and may include:

• an automated braking system, which may activate the brakes of the vehicle;

• an automated steering system, which may alter the angle of steer of the vehicle;

• one or more external air-bags;

• one or more bonnet lifters;

· one or more internal air-bags;

• one or more seat belt pretensioners;

• one or more systems which may serve to alert the driver of the vehicle to the presence of a pedestrian or other object, such as a visual, auditory or haptic alert; and

· one or more systems which may serve to alert a pedestrian to the presence of a vehicle, for instance by illuminating one or more lights of the vehicle (such as the vehicle's headlights) or producing a sound, for instance by sounding the vehicle's horn. The skilled reader will appreciate that, as a result of the information from the various sensors 2, 4 discussed above, the vehicle's processor 5 will be able to activate vehicle devices 6 which will reduce the likelihood of a crash situation occurring, and/or reduce the likely harm caused to one or more vehicle occupants, or one or more individuals with which the vehicle 1 may collide, if a crash does occur.

One example of an algorithm that may be used by the vehicle's processor 5 to analyse the signals received from the various sensors 2, 4 is described below. In this example, both spatial coordinates and time are considered together, and considered to include the following orthogonal axes:

Where x and y are orthogonal spatial axes, with x representing the forward- backward direction of the vehicle, y representing the left-right direction of the vehicle, and t representing time. These axes are collectively referred to below as "space-time".

Two functions are defined. Firstly, with reference to figure 2 (which

schematically shows the vehicle 1 in motion), a parametric function of a plane or surface in space-time is defined as:

P(s, t) = f(s) + g (t)

where s represents space (including the x and y directions), and t represents time. In this instance, this plane/surface represents the movement of the front edge of the vehicle 1 (e.g. a line extending across the front edge of the bonnet) over time, with s representing distance along a line parallel with the front edge of the bumper.

P(s, t) describes all points on all lines parallel with the front bumper at any time t. f(s) describes the orientation of the line parallel to the vehicles front bumper, and g(t) describes the location of a point (for instance a corner, or the centre of a bumper, but any other point may also be used) representing the position of the vehicle over time. Secondly, with reference to figure 3 (which again schematically shows the vehicle 1 in motion), a parametric function of a curve in space-time is defined as:

Here the curve describes how a point on a detected object external to the vehicle (e.g. a pedestrian) moves through space (x, y) over time. L(t) therefore represents the position of the external object for a time t, and h(t) describes the location of a point (for instance a corner, or the centre of a bumper, but any other point may also be used) representing the position of the external object over time.

In the above definitions of the plane and curve involve the following

constraints/assumptions:

fM = o

hi (t E ^m

g(s)

/(s) II xy— plane

Where T ⁿ and T ^m are polynomials of order n and m respectively.

The function arguments when writing functions and derivatives will be omitted and written in short notation, i.e. fx (s) = fx ds

The aim is to find s and t where P and L intersect, i.e. the predicted time and position of a collision between the vehicle and the external object. Here f _s ' _x and fsy (which appears below) represent the rate of change of the line parallel to the front bumper of the vehicle (1 ) along the x and y axes, respectively.

In this example, the solution is found by considering the point normal description of a plane.

N ^■ (p - Po) = 0

Where N describes the normal in each point of the surface. The points p ₁ and Po are points in space-time defined by the two spatial axes (x, y) and the temporal axis (t). The normal may not be constant, depending on the magnitude ofn. When these points fulfil the criteria mentioned below, these points will lie on the plane spanned by P(s, t).

dt'x

According to the above, if two points lie on a plane then the vector between them is parallel to the plane and also orthogonal to the normal vector N.

By selecting one point from the plane equation and one from the line equation, the equation given above can be fulfilled only when the line intersects the plane - in other words, where the projected trajectory of the external object (e.g. pedestrian) intersects with the projected region defined by the front edge of the vehicle. By having the plane function linear along the front of the vehicle, all vectors between two points on the plane must be parallel to the plane.

The time t of the collision may be found as follows.

By substituting p and p ₀ with P(s, t) and Lit) respectively, we can place the first point on the plane and the other one on the line

0 = iV - (P(s, t) - Lit))

fsxdt'y - fsydt'x. Evaluating the dot product

0 fsy (fx ^~ l ^~ Qx h-x) fsx ijy Qy ^y)

Separating expressions

0 fsy fx fsxfy fsy

=0

The expression f _s ' _y f _x - f _s ' _x f _y = 0 is true when considering the

constraints/assumptions mentioned above, i.e. constant acceleration and zero yaw rate in both objects.

The expressions g _t - h _t can be viewed as the difference between two polynomials resulting in a new polynomial which can be expressed as

a _i + b _i t + c _i — +

i f t ² \ , ( t ² fsy [ &x χ£ ^x ^~ ^ ^" J fsx ( Cly by t ^y ^~ 2 Where a _x and a _y represent the zero order component of a polynomial which in this case would be the difference between the position of the vehicle (1 ) and the position of the external object along the x and y axes. b _x and b _y represent the first order component of a polynomial which in this case would be the difference between the velocity of the vehicle (1 ) and the velocity of the external object along the x and y axes. Cx and c _y represent the second order component of a polynomial which in this case would be the difference between the acceleration of the vehicle (1 ) and the acceleration of the external object along the x and y axes.

This can be separated by powers of t

Recognising the 2D determinants

Here A represents the calculated position of the vehicle relative to the object, B represents the calculated speed of the vehicle relative to the object, and C represents the calculated acceleration of the vehicle relative to the object. More specifically, A is the signed unnormalised projected position distance between the line parallel to the front bumper of the vehicle 1 and the point representing the position of the external object, B is the signed unnormalised projected velocity difference between the line parallel to the front bumper of the vehicle 1 and the remote point representing the position of the external object, and C is the signed unnormalised projected acceleration difference between the line parallel to the front bumper of the vehicle 1 and the remote point representing the position of the external object. This results in a polynomial t ²

Q = A + Bt + C—+ -

With the order k = max(m, n)

And the following solutions for small k

• k = 0 = no solution

• k = l => t = - -

B

. k = 2 ^ t = - -^≡^

c

This is represented graphically in figure 4, which shows a graph of the x (direction of travel of vehicle) and y (direction transverse to the direction of travel, i.e. generally parallel with the front edge of the vehicle 1 ) directions, and also time (represented by t). A plane 7 is shown in the graph, representing the likely future trajectory of the front edge of the vehicle 1 . A line 8 is also shown, representing the calculated future path of the external object. A point 9 is calculated, with this point being the intersection between the plane 7 and the line 8. This point 9 therefore corresponds to the predicted time and location of a collision between the vehicle 1 and the external object. If the acceleration is zero for both of the vehicles, C above becomes zero and the solution becomes As noted above.

With the time variable solved, it is possible to find where on the "front" the intersection occurred, i.e. s. This is done by finding the intersection between the surface P and the curve L.

0 = P(s, t) - L(t) = /(s) + g(t) - h(t)

According to the constraints/assumptions mentioned in the beginning it is possible to rewrite the expression to the following

0 = f s + g{t) - h{t) Reordering the terms leaves a vector equation f s = -{g{t) - )

Multiplying both sides with a vector q ^T (which represents the vector inverse of / _s ') such that q ^T f _s ' = 1

The vector q ^T may be calculated, for example, using the squared L2 norm. One solution for q is The time and position of the collision can therefore, in certain cases, be calculated analytically.

It will be understood that arriving at an analytical solution confers significant benefits when compared to the calculation of individual, ad hoc numerical solutions to calculate likely time and position of impact when each potential impact event occurs. An analytical solution, which can be calculated pre- emptively and stored in a memory which is accessible for the vehicle processor, allows the calculation to be carried out very swiftly and reliably, without using valuable processing resources.

In more general terms than the specific example described above, in embodiments of the invention the vehicle's processor employs an algorithm in which a plane is defined, with the plane representing the space in which the vehicle may move. If the vehicle is assumed to be travelling over a generally flat surface, the plane will be parallel with this flat surface. The height of the plane above the flat surface may be defined to be of the same, or substantially the same, height as a part of the vehicle which is most likely to make first contact with an object with which the vehicle collides, for instance the front bumper of the vehicle.

A region of the plane is defined, which comprises the part of the plane corresponding to the projected future trajectory of the front edge of the vehicle as the vehicle moves forwardly through space-time. An object external to the vehicle, which is detected by the vehicle sensors or is otherwise communicated to the vehicle, for instance through vehicle-to-vehicle communication ("V2V communication") is preferably represented by a point. In other embodiments, an object external to the vehicle may be represented by a plurality of points, and each individual point will be analysed in the same way as the single point included in the discussion below. In the case of another vehicle, one point may be defined for each corner of the vehicle. For other objects which are detected, such as a pedestrian, two or more points may be defined at positions which appear to correspond to points on the external surface of the object, and the skilled person will realise how this may be achieved. In these examples, if one or more of the points are calculated to intersect with the predicted future path of the vehicle, it is preferred that a future impact with the external object is based upon the point which is calculated to intersect with the predicted future path of the vehicle at the earliest time.

To return to the example in which the external object is represented by a single point, the projected trajectory of the point through space time is also modelled, based on the current detected position and velocity of the object.

An intersection between the region of the plane and the point representing the external object is then calculated. If the point of intersection appears in the past (i.e. at a time before the present time), it may be calculated that the projected trajectory of the vehicle and the object are diverging, and that no future collision is likely.

However, if the intersection occurs in the future, the algorithm may then calculate the time at which the collision will take place. In addition, the position of the collision along the length of the front edge of the vehicle may be determined.

With reference to figure 5, a further graph is shown representing analysis of the situation where the yaw rate of the vehicle 1 and of the external object are not assumed to be zero. The yaw rate of the vehicle 1 may be determined through any suitable means (e.g. through the use of on-board accelerometers, a positioning system such as GPS, through detection of the angle of steering wheel, or any combination of these). The yaw rate of the external object may be detected by the vehicle's sensors 2, as described above, through tracking of the motion of the external object over time.

Where the yaw rate of the vehicle 1 and of the external object are not assumed to be zero, the analytical solution presented above may not apply, and in this case a numerical solution may be employed, i.e. a solution that uses one or more numerical approximations to arrive at solutions to the available equations, rather than one that relies on a pre-defined or "closed" solution. The skilled reader will appreciate how the analysis set out above may be adapted to generate a suitable numerical solution when the yaw rate of the vehicle 1 and/or of the external object are not assumed to be zero.

Returning to figure 5, this graph once again shows the x, y and t axes. In common with figure 4, a plane 10 is defined, representing the calculated likely future trajectory of the front edge of the vehicle 1 . A line 1 1 is also defined, representing the calculated future trajectory of the external object, and a point 12 of intersection of this plane 10 and line 1 1 is also calculated. Once again, this point 12 represents the calculated time and position of an impact between the vehicle 1 and the external object. To calculate a numerical solution, one route would be to use the bisection method to get a starting point, and then use a faster method such as the Newton-Raphson method or the Gauss-Newton method. The skilled reader will understand how these approaches may be used,

As discussed above, if a future collision appears to be imminent or likely, then one or more vehicle safety systems may be activated. Which system is activated may depend upon, for instance, the projected speed of the impact, and the projected time until the impact. If the speed of the vehicle and the object is relatively low, and/or the collision is projected to be a relatively long time into the future, the system may be activated to try and warn the driver of the vehicle and/or the object (if the object is a vehicle with its own driver, or a pedestrian), to try and cause the driver and/or the external object to take action to avoid the collision.

If the speed of the vehicle and the object is relatively high, and/or the collision is projected to occur within a relatively short space of time, active safety systems such as internal or external airbags may be deployed, as discussed above.

It will be understood that the use of safety systems embodying the invention may result in collision detection which, compared to existing techniques, is fast, consumes fewer valuable processing resources, and is less prone to errors.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.