DE2215576C3 - Device for measuring, controlling and/or displaying the movement of land vehicles - Google Patents

Description

The invention relates to a device for measuring the movement of land vehicles relative to a preferably unmarked environment with a passive mode of operation for the purpose of regulating and/or displaying.

As is known, devices for measuring movements can be implemented in different ways. On the one hand, it is possible to emit radiation with an identifier in the direction of the object to be measured using the so-called radar principle and to determine the desired measurement value from the time delay with which the signal reflected by the object reaches a corresponding receiver. On the other hand, it is known to use light emitted or reflected by the object that does not contain an identifier to determine the desired measurement values. The two measurement methods differ essentially in that one method uses a defined radiation with an identifier, while the other uses one without an identifier. For this reason, the first methods are called active measurement methods, while the second methods are passive measurement methods.

A device for measuring the speed and altitude of an aircraft is known which requires a radiating light source on the ground in order to function, i.e. in practice a marking which is projected onto a grid via an optic and which, in conjunction with this, generates optical signals which are significant for the relative speed between the aircraft and the ground. Because of the necessary marking and for the reasons set out below, this device is not suitable for solving the problem underlying this invention. To measure the speed of aircraft, it is also known to measure the speed of a moving image in a continuous optical correlation process using a parallel slit spatial filter. (Ator, Appl. Optics (Aug. 1966), Vol. 5, No. 8, pp 1325-1331) An optical and photoelectric system associated with the square wave spatial filter causes a continuous convolution with the function of a flat image, whereby the filter function is constant in its density distribution - and the image function varies in its brightness distribution in the x and y coordination with time. In this multi-slit image speed sensor, the period of the cyclic modulation of the light level is measured, which is created by image filtering for the spatial frequency of the multiple slit pattern. This measurement is done indirectly by measuring the time frequency

of the photoelectric signals. In this method, the average of the light frequencies transmitted through the equidistant filler loops is continuously calculated, i.e. a form of Fourier analysis of the image, since the amplitude of a spatial frequency component contained in the image is measured. This is therefore an optical correlation method in the sense that the filter transmission function is cross-correlated with the light flux distribution in the image.

When using a sensor arrangement with only one spatial filter, a relatively low degree of modulation of the photoelectric signals is obtained. The signal level from the ambient light is relatively high. It can be eliminated in a known manner by combining the signals from two spatial filters which are 180° out of phase with each other, whereby

When measuring in land vehicles, the problem arises in comparison to the known measuring method of the smaller distance to the ground or the measuring object. When using several spatial filters with their associated image fields to obtain push-pull signals, unacceptable parallax errors and technical functional uncertainties occur.

A stationary photoelectric arrangement for displaying moving objects and their relative speed is also known, in which the arbitrarily illuminated field of view itself or an image of the field of view is divided by a stationary grid (DE-PS 6 63 931). One or more photocells are influenced intermittently by the fact that the moving object, which is small in relation to the entire field of view, is alternately completely or partially covered or uncovered by the opaque or transparent parts of the grid or grids as it passes through the field of view. There are two complementary grids. The grids can be divided in different ways in the horizontal and vertical directions, so that a movement of the object in the horizontal or vertical direction triggers alternating currents of different frequencies.

A disadvantage of this known device is the fact that push-pull signals are generated by two separate grids or by grid-like photocells. The separate grids cause disturbing parallaxes, especially for small object distances, while the grid-like photoelectric receivers cannot be manufactured with any degree of precision and therefore the number of objects that can be measured is limited. In addition, this device does not allow the direction of movement of the measuring objects to be determined.

It is also no longer new to use an optical filter in the form of a lens drum or a slotted disk to record the movement of a land vehicle in relation to an unmarked environment, but the measurement is carried out exclusively by synchronously tracking these components to the image movement (US-PS 34 66 097). The movement is only detected by means of a frequency assessment. There is no direct measurement of the movement from the signals received. Neither direct speed measurements nor information about the direction of movement are obtained.

Another state-of-the-art arrangement that works according to the principle of time correlation is also worth mentioning, in which time-shifted radar signals are used (DE-AS 10 67 246). The time delay is a measure of the speed of the object relative to the measuring system. Since it is an active system, it has the disadvantage of being susceptible to interference in addition to being very complex.

Finally, a device for measuring speeds of a vehicle moving relative to the earth’s surface is known, which carries out an active measurement and which has two grids for measuring in two coordinate directions, which

lü Division directions include the desired coordinate angle (FR-PS 20 44 366).

The processing and use of the signals generated by a device of the type mentioned above can be carried out in a variety of ways.

i; distinguish between procedures relating to anti-lock protection, those which compensate for undesirable changes in direction by intervening in the control system, and those which provide information on the behaviour of other road users.

:n mer and intervene automatically in dangerous situations in the steering, braking or drive system.

If the maximum available friction force between the moving parts (e.g. wheel or chain) and the road is exceeded when a vehicle is braking, they lose their grip on the road surface. The static friction turns into the smaller sliding friction, and in the worst case the moving parts come to a standstill, so that the vehicle slides over the road surface. This not only reduces the achievable braking deceleration,

» Not only is the steering reduced, but so is the available cornering force, so that the vehicle is practically no longer steerable and begins to skid. The risk of the running links blocking is particularly high when the coefficient of friction between the running links and the road surface is low, for example on slippery roads or on black ice.

When a vehicle is braked, a certain amount of slip is always present due to the deformation of the running links, i.e. the peripheral speed of the

■ω of the running links is always smaller than the vehicle speed when braking. The greatest grip on the road is achieved, depending on the nature of the running link and the road, at slip values between 10 and 30%. In the case of complete blocking, however, the slip is 100%. The cornering force decreases steadily to zero with increasing slip, but is still sufficiently high at the slip values between 10 and 30% mentioned. This results in the task of an anti-lock device to regulate the braking forces in the vehicle, regardless of the pedal force exerted by the driver, so that the optimum slip is always set. The best results are achieved when all running links are regulated independently of one another.

According to the current state of technology, the actual driving speed and thus the slip can only be measured directly with great effort, i.e. with the help of an unbraked fifth wheel or a gyro platform. The criterion for the onset of locking is therefore the strong angular deceleration of the wheel that occurs in this case. This deceleration is due to the fact that when the vehicle switches to locking, the braking force is no longer applied to the large mass of the vehicle, but only to the large!

smaller mass of the wheel. Conversely, the wheel is subject to a very strong angular acceleration if it is suddenly released from the locked state.

In such known anti-lock devices, an inductive pulse pickup is provided on each wheel as a speed sensor.

The disadvantage of this method is that the information about the vehicle movement is only obtained indirectly through acceleration measurements. The derivation of the vehicle speed is inevitably inaccurate because disturbing parameters such as tire pressure, tire profile, vehicle load, etc., are included in the measurement as errors. This means that only an oscillation of the braking force around the desired optimal slip is achieved.

On the other hand, a very important measure for increasing road safety is measuring the distance to the vehicle in front and generating warning and control signals when a safety distance based on the speed of the vehicle is exceeded. Known systems that do not require any aids attached to the road have fundamental disadvantages: Due to the straight propagation of the rays used to measure the distance (Ik. radar, laser), it is not possible to measure the distance on curves and crests. There is also the risk that oncoming traffic, trees, roadsides, bridges, etc. will distort the measurement. The only known remedy is to equip the road and other vehicles with special, complex aids.

The object of the present invention is therefore to provide devices of the type mentioned at the outset, which avoid the disadvantageous uncertainties of known methods by means of direct, contactless speed and/or distance measurement in relation to unmarked surroundings and which can be implemented in a particularly simple and economical manner.

This object is achieved according to the invention ⁱⁿ a device of the type mentioned at the outset by the characterizing features of claims 1, 2, 4, 5. Advantageous further developments emerge from the subclaims 3, 6-15.

In order to detect the slip of the running elements of a vehicle against the ground, the optical correlator system can convert the movement over the ground into speed-proportional signals, which are then compared with signals proportional to the movement of the running elements driving and/or braking the vehicle for the purpose of slip control and/or display.

In order to detect the undesired lateral drift with the correct sign, the optical correlator system converts the movement over the ground transverse to the desired direction of travel into electrical signals proportional to the transverse movement. These signals are either obtained directly when the measuring direction of the correlator system is tracked transverse to the target value of the direction of travel or are calculated by deriving the direction difference between the measuring and target directions. These signals are fed to a servo system that influences the direction of travel.

In order to adapt the brakes and drive to the changing driving conditions when cornering, the lateral force is converted by means of the optical correlator system into signals with the correct sign that are proportional to the movement over the ground transverse to the desired direction of travel, and these signals are fed to a drive or braking system and/or a servo system that influences the steering in combination with signals that are preferably proportional to the vehicle speed, the slip in the direction of travel and the steering angle. These signals are also used to continuously display the difference between the maximum possible and the actual course speed.

For the purpose of measuring the distance between the vehicle and an obstacle in the direction of travel, an image structure of the obstacle is captured using at least one optical correlator system, and

&kgr;&igr; signals are obtained about distance, about the relative speed in the direction of travel and about the relative speed across the direction of travel between the vehicle and the obstacle, which, after being compared with the speed of the vehicle over the ground, are fed to a servo system that influences the speed of the vehicle and/or to a display and/or warning device. In order to detect the approach speed of following vehicles, at least one structure of the following vehicle is evaluated using a rear-facing optical correlator system, and electrical signals proportional to its approach speed and distance are generated, which, after being compared with the speed of the vehicle, are fed to a display and/or warning device.

Examples of embodiments of the devices are shown schematically in the drawings and described below. It shows

Fig. 1 a device for measuring and evaluating the slip in the direction of travel,

Fig. 2 is a functional diagram of the device according to Fig. 1,
Fig. 3 shows an example arrangement of the optical correlator system,

Fig. 4 a control ^device for eliminating unwanted changes in direction,

Fig. 5 a device for adapting the effects of brakes and drive to the changing driving conditions when cornering.

Fig. 6 Sketches to explain an anti-collision device,

Fig. 7 shows the functional diagram of a device with optical correlator systems for avoiding collisions with obstacles in the direction of travel.

Fig. 1 shows a downward-facing optical speed correlator system 1 which measures the forward driving speed above the ground without contact, regardless of its distance from the ground. A speed sensor 2, 3, 4, 5 is mounted on each of the four wheels of the vehicle shown, the output signals of which, together with the output signals of the correlator system 1, are fed to the inputs of a comparison stage 6. This in turn controls a servo system 6a located in the same housing, which operates on four brake control valves 7, 8, 9, 10 assigned to the four braked wheels. These valves are located in a known manner in the line 12 of the brake hydraulics connected to the master brake cylinder 11. The servo system 6a also operates on the differential gear 13 of the drive wheels.

The facility described so far has the following

Function. If the brake pressure exceeds a predetermined threshold value when the brake 14 is actuated, the brake servo system varies the brake pressure of each wheel via the brake control valves 7, 8, 9, 10.

until the comparison stage 6 registers a preferably adjustable target value of the slip between the wheel and the road as the difference between the output signals of the cyclists 2, 3, 4, 5 and the correlator system 1. Independently of this braking force control, the servo system controls the driving force to the slower driving wheel via the differential gear 13 when a predetermined speed difference between the two driving wheels is exceeded!

What has been described so far is shown as a functional diagram in Fig. 2.

Fig. 3 shows a special embodiment of the speed correlator system 1. At the upper end of a tubular housing 16, an optical correlator 17 and an illumination device 18 are arranged next to one another. Both are closed off from the longer lower end of the housing tube 16 by optics 15 and 19. The tube 16 is supplied with compressed air, possibly heated, via a supply line 20, so that the protective flap 22 attached at the bottom and under the influence of a counterweight 21 is opened by the air flow and clears the measuring beam path. The airstream or air from the engine fan, etc. can be used as compressed air.

A speed correlator system 1' shown in Fig. 4 is, in contrast to the above-mentioned correlator system 1, arranged and designed in such a way that it responds with the correct sign to a driving speed component directed transversely to the direction of travel. The output signals of the correlator system 1' are fed to a comparison stage 6' together with the signals of an angle sensor 24 operated by the steering wheel 23. If this comparison stage registers a lateral speed component which does not correspond to the steering wheel angle assigned to the desired direction of travel, a downstream power steering system 6a' is automatically fed in such a way that it brings the vehicle into the desired direction of travel.

If the correlator system 1' is mounted in such a way that it is controlled by the steering wheel 23 and always follows the desired direction of travel, the angle sensor 24 can be omitted.

Fig. 5 shows a speed correlator system 1" that simultaneously measures according to two coordinates in the direction of travel and transversely thereto, the output signals of which are fed to a computing unit 25 together with the signals from wheel speed sensors 2". 5" and the angle sensor 24. This computing unit 25 calculates the amount of the slippage resulting from the straight-ahead slip and the lateral downforce, taking into account the steering angle of the vehicle's running members derived from the steering servo element 26. If this amount exceeds a predetermined limit, the computer 25 sends corresponding output signals to a servo system 27. In the case shown, this servo system controls the motor, the brakes of the running members and the steering servo element 26 until the specified limit is again exceeded. A warning device 28 is activated even before the servo system starts to function. A display device fed by the computer can also be provided for continuous monitoring.

The basic aspects of distance measurement will now be explained using Fig. 6a to 6d. The problems of distance measurement of vehicles cannot be solved using distance-measuring components alone, since this would also measure the distances to obstacles that are not on a collision course (e.g. oncoming vehicles, roadsides on bends, etc.). As a result, additional information must be derived that provides information about both the approach speed and the lateral movement of the obstacle: Fig. 6a and 6c show an obstacle that is on a collision course. This is expressed in the image impression by the contours and structures of the

&iacgr;&ogr; obstacle "grows" symmetrically to the line of sight indicated by the "crosshairs".

In Fig. 6b and 6d, however, the "growth" is superimposed with a lateral image shift, the obstacle is e.g. a vehicle that is being overtaken and

is no longer on a collision course.

The growth of the image structure together with the distance gives the approach speed, the lateral displacement of the image together with the distance gives the lateral speed of the obstacle.

The ratio of both speeds to each other, along with the size and distance of the obstacle, determines whether or not there is a risk of collision.

When cornering, for example, the opposite side of the road will be present as an obstacle at a certain distance in the direction of view (and thus the direction of travel), but it will move out of the field of view at a certain lateral speed (Fig. 6e). The approach speed v" is only one component of the relative movement, and the lateral speed v, shows the observer that there is no danger of collision.

From these considerations it can be seen that in addition to the distance and its change (approaching speed), the lateral movement of the image must be recorded, namely relative to the "optical axis" corresponding to the direction of travel.

As a result, it is necessary to create an image in an image plane using a lens aligned with the direction of travel, the parts of which move across the image field according to the angular velocity. Objects that are exactly in the direction of travel when driving straight ahead and do not move at all have no angular movement. In relation to the image field, this means that a point with no lateral relative movement exists in the direction of the optical axis. The larger the angle of the image-producing rays to the direction of travel, the larger the angle of the image-producing rays becomes for a given distance and relative speed.

so the angular velocity and thus the speed at which the image points move across the image field. If, as shown in Fig. 6f, the image field of an image created by a lens is halved using a straight line that runs through the "rest point" determined by the direction of travel, the image parts at a certain distance x to the left and right of this image bisector move apart (or together) at the same and opposite speeds if and only if there is no lateral movement:

v _r (x) = I v _e (x) ..

For a given distance and approach speed , v _r or v _e increases with .r due to the dependence of the angular velocity on the angle between the direction of travel and the direction of view described above. If, for example, one were to place a

If you install an image speed meter measuring the speed of the image at a distance of .v, the approach speed can be determined from the size and sign of the pixel displacement speeds measured there by arithmetic averaging (sum signal) and the lateral speed by subtraction.

If, for energy reasons, you want to use a large-area image speed meter in each half of the image, the dependence of the values on .v is a problem, but only as long as you use linearly divided spatial frequency filters in optical correlators for this purpose. If you use a sinusoidal non-linear periodic division, for example, you can get similar signals for a certain approach speed over large value ranges of .v.

If aian halves the image into two v-halves instead of two .v-halves using a second straight line that runs horizontally through the "resting point", the same applies as already described. In fact, the image points run radially away from the "resting point".

As a result, circular arc-shaped nonlinear divisions similar to a Fresnel zone plate or sections thereof are optimally suited as spatial frequency filters f ^c ig. 6g).

In the case of land vehicles, only lateral movement, i.e. displacement in one dimension, is generally possible. Therefore, two image speed sensors in the image field are sufficient. Of course, for other applications it is also possible to insert corresponding sensors for the second coordinate and to determine the direction of a transverse movement in four quadrants from four sensor signals.

Fig. 7 shows a schematic representation of a structure for a passive anti-collision device designed on the basis of the above considerations. As can be seen, a speed correlation system Γ" measuring in four quadrants is provided, the output signals of which, assigned to a coordinate direction, are fed to a summing stage 34 or 34' and a difference former 35 or 35'. The output signals of these stages are fed to a ratio former 36, which relates the sum and the difference of the signals associated with a coordinate direction to one another. A computing unit 37 is connected downstream of this. The correlator system is assigned a distance measuring system 38, the output signals of which are also fed into the computing unit 37. The output signals of the computing unit can, as indicated, be evaluated in various ways. Firstly, it is possible to use these output signals to feed a servo system 39, which reduces the speed of the vehicle until the speed of the vehicle is less than or equal to that of the other vehicle. If the If, however, the appropriate object is a stationary object, the vehicle is braked. In addition to such a servo system, a display device 40 and/or a warning device 41 will be provided, by which the impending danger is clearly shown.

It is advantageous to install the two measuring systems &Ggr;" and 38 behind the windshield of the vehicle, where they are protected against the effects of the weather.

The advantage of the new passive anti-collision system compared to the known system is that obstacles on a collision course can be clearly distinguished from other obstacles (e.g. roadsides) because they do not generate an asymmetrical signal component in the measuring system.

By using photoelectric receivers that respond to IR rays, it is possible to receive signals from a vehicle ahead even when there is no daylight and the reverse lights of the vehicle ahead have failed.

It is advantageous to provide two optical correlator systems on the vehicle, one of which measures in two coordinate directions and is assigned to the front end of the vehicle, while the other correlator system, which measures transversely to the direction of travel, is assigned to the rear end of the vehicle. This arrangement is necessary for the early detection of the vehicle's rear swerving.

5 sheets of drawings

Claims (15)

Patent claims: 1. Device for measuring the movement of land vehicles in relation to a preferably unmarked environment with a passive mode of operation for the purpose of regulating and/or displaying, characterized in that an optical correlator system (1, 1', 1", &Ggr;") is mounted on the vehicle and has imaging optics, at least one grid structure and associated therewith at least two photoelectric receivers, on whose grid structure the environment to be recorded according to its relative movement to the land vehicle is imaged, so that electrical push-pull signals proportional to this movement in the image structure are generated optically, from which at least one output signal proportional to the speed over ground in the direction of travel can be derived, that means (2, 3, 4, 5) are provided for generating electrical signals proportional to the movement of the running members driving and/or braking the vehicle, and that a comparison stage (6) fed with both the output signals of the means mentioned and with the output signals of the correlator system is provided, whose output signals are used for slip control and/or display (Fig. 1, 2). 2. Device for measuring the movement of a land vehicle, preferably in relation to an unmarked environment with a passive mode of operation for the purpose of control and/or display, characterized in that an optical correlator system (1, 1", 1", 1") is mounted on the vehicle, which has an imaging optic, at least one locking circuit and associated therewith at least two photoelectric receivers, on whose grid structure the environment to be recorded according to its relative movement to the land vehicle is imaged, so that electrical push-pull signals proportional to this movement of the image structure are generated optically, from which at least one output signal proportional to the movement over the ground transversely to the desired direction of travel can be derived, these signals either being obtained when the measuring direction of the correlator system (1') is tracked transversely to the desired value of the direction of travel or being determined by deriving them from the difference in direction between the measuring and desired direction by means of a comparison stage (6'), and that these signals are fed to a steering system (6a') which influences the direction of travel (Fig. 4). 3. Device according to claims 1 and 2, characterized in that the signals with the correct sign proportional to the movement over the ground transverse to the desired direction of travel in combination with the signals which are preferably proportional to the vehicle speed, the slip in the direction of travel and the steering angle are fed to a computing unit (25), the output signals of which control the servo system (27) influencing the drive or braking system and/or the steering and/or are used for the continuous display (28) of the difference between the maximum possible and the actual cornering speed (Fig. 5). 4. Device for measuring the movement of land vehicles in relation to a preferably unmarked environment with a passive mode of operation for the purpose of regulating and/or displaying, characterized in that an optical correlator system (1, 1', 1", &Ggr;") is mounted on the vehicle, having an imaging optic, at least one grid structure and associated therewith at least two photoelectric receivers, on whose grid structure the environment to be recorded according to its relative movement to the land vehicle is imaged, so that electrical push-pull signals proportional to this movement of the image structure are generated optically, from which output signals proportional to the speed over ground can be derived, and that for the purpose of measuring the distance between the vehicle and an obstacle in the direction of travel, at least one forward-facing optical correlator system (&Ggr;") is provided, with which an image structure of the obstacle is recorded and signals about the distance, about the relative speed in the direction of travel and about the relative speed transverse to the direction of travel between the vehicle and obstacle are obtained, which, after a comparison with the speed of the own vehicle over the ground, are fed to a servo system that influences the speed of the own vehicle and/or a display and/or warning device (Fig. 7). 5. Device for measuring the movement of land vehicles in relation to a preferably unmarked environment with passive operation for the purpose of control and/or display, characterized in that an optical correlator system (1, 1', 1", &Ggr;") is mounted on the vehicle, having an imaging optic, at least one grid structure and associated therewith at least two photoelectric receivers, on whose grid structure the environment to be recorded according to its relative movement to the land vehicle is imaged, so that electrical push-pull signals proportional to this movement of the image structure are generated optically, from which output signals proportional to the speed over ground can be derived, and that a rear-facing optical correlator system is provided, which records at least one image structure of a following vehicle and generates electrical signals proportional to its approach speed and the distance of the following vehicle, which are fed to a display and/or warning device after a comparison with the speed of the vehicle itself. 6. Device according to claim 1, characterized in that a) preferably each running member is assigned a rotation speed sensor (2—5), that b) an optical correlator system (1) is assigned to the chassis, measuring the relative speed over the ground without contact and independently of the distance between it and the ground, and that c) the output signals of the rotation speed sensor and the output signals of the correlator system are fed to a comparison stage (6) which is tuned to a preferably adjustable target value of the slip, the output signals of which - if necessary after running of an amplifier - are fed as control signals to a servo system (6a) which influences the rotational speed of the respective rotating element. 7. Device according to one of the preceding claims, characterized in that the optical corrector (17) and optionally an illumination device (18) are mounted in a tubular housing (16) and that this housing has a closure (22) at the bottom which is effective when the vehicle is stationary and can be opened when the vehicle is moving. 8. Device according to claim 7, characterized in that means are provided which generate an overpressure in the tubular housing (16) when the vehicle is moving. 9. Device according to claim 2, characterized in that at least one optical correlator system (I ¹ ) measuring transversely to the direction of travel is provided, the output signals of which are of the correct sign are fed to a comparison stage (6') to which a target value is optionally fed as a comparison value, and that a power steering system (6a') is connected downstream of this comparison stage. 10. Device according to claim 3, characterized in that preferably each of the running members is assigned a rotary encoder (2", 5") and that an optical correlator system (1") is provided which measures both transversely and in the direction of travel with the correct sign, the output signals of which are fed together with the signals from the rotary encoders (2", 5") to a computing unit (25) in which the results of the lateral drive and straight-ahead slip are determined and compared with a predetermined limit value, taking into account the turning angle of the running members of the vehicle, and that a servo system (27) is provided which acts on the friction or the brakes of the running members and/or the steering of the vehicle when this limit value is exceeded. 11. Device according to claim 10, characterized in that two correlator systems are present, one of which measures in two coordinate directions and is assigned to the front end of the vehicle and the other, which measures transversely to the direction of travel, is assigned to the rear end of the vehicle. 12. Device according to claim 10 or 11, characterized in that the arithmetic unit (25) is assigned an alarm display and/or warning device (28). 13. Device according to claim 4, characterized in that an optical correlator system (&Ggr;") is provided, which is aligned in the direction of travel and is preferably suitable for measuring in two coordinate directions, the electrical output signals of which, using the object perspective and its change, are fed to a comparison stage for each coordinate direction running perpendicular to the direction of view, that this comparison stage comprises a summing stage (34, 34'), a difference stage (35, 35') and/or ratio-forming stage (36) and that this comparison stage is followed by a computing unit (37) to which signals originating from a distance measuring system (38) are additionally fed, and that a servo system (39) and/or display system (40) and/or warning system (41) influencing the driving speed of the vehicle is connected to the output of the computing unit (37). 14. Device according to claim 5, characterized in that an optical correlator system is provided with its viewing direction directed backwards, the electrical output signals of which together with electrical signals proportional to the speed of the vehicle itself are fed to a comparison device, which is followed by a warning device. 15. Device according to claim 4, 5, 9, 13 or 14, characterized in that the correlator system is arranged in the vehicle behind the windshield.