Reflector for vehicle

An electromagnetic reflector which is effective for electromagnetic waves in a range from 50 to 100 GHz, radiated from an on-board radar device, is integrally incorporated with an optical reflector for effectively receiving the electromagnetic waves by the reflector and, as reflected thereby, by the radar device even though the relative angle for receiving reflected radar waves becomes larger due to a shift of lanes by the preceding vehicle or due to a curve in the road.

CROSS REFERENCE TO RELATED APPLICATIONS 
The present invention claims priority from Japanese Patent Application No. 
10-52208 filed Mar. 4, 1998, the contents of which are incorporated herein 
by reference. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a radio wave reflector adapted to be used 
for an automobile, and having a high reflection factor with respect to 
radio waves radiated from an on-board radar device or radio wave detectors 
arranged along a road. 
2. Description of Related Art 
It has been ruled that frequencies for on-board radar devices are available 
in a range of 50 to 100 GHz. Specific frequencies therefor differ among 
countries having different national characteristics. For example, the 
frequency is proposed to be 60 GHz in Japan but is 76 GHz in Europe and U. 
S. A. Testing for on-board radar devices using the above-mentioned 
frequency bands has been started. 
Meanwhile, it has been obliged for safety standards that optical reflectors 
be incorporated in a vehicle (Safety Regulations for Road Transport 
Vehicles Act. 38 (rear reflectors) and Act. 35-2 (side lamps and side 
reflectors)) in order to confirm reflected light from each of the 
reflectors when it is irradiated by a light beam emitted from a running 
front lamp during nighttime, rearward or laterally from the reflector by a 
distance of 150 m. Specifically, it is ruled that the reflecting part of 
the reflector must have an area larger than 10 cm.sup.2 and a shape other 
than a triangular shape. These reflectors are attached to the rear end and 
the sides of a vehicle in order to satisfy the safety regulations. 
The inventors have carried out tests for electromagnetic reflection 
characteristics of on-board radar devices and have found that conventional 
optical reflectors do not have satisfactory reflection characteristics in 
the available frequency range of 50 to 100 GHz. The reflection of 
electromagnetic waves on a vehicle can be obtained from a metal part of 
the vehicle, and such metal parts are almost planar so that intense 
reflection waves can be obtained only in the same direction as the 
incident direction of the electromagnetic waves. It has been found that 
this causes reflection waves from the radar to decrease at a time when the 
radar device has to exhibit its highest function, that is, at the time 
when a preceding vehicle captured and recognized by a radar device shifts 
from one into another lane or the like. 
Further, it has been found that the intensity of reflection waves is not 
satisfactory as to small sized vehicles or motorcycles. This mainly causes 
the power of output electromagnetic waves from on-board radar devices to 
be inevitably increased, and accordingly, inconvenience such as 
interference of electromagnetic waves would occur among on-board vehicles. 
SUMMARY OF THE INVENTION 
The present invention is devised in view of the above-mentioned background, 
and accordingly, one object of the present invention is to provide an 
apparatus for mounting on a vehicle an electromagnetic wave reflector 
which is effective for electromagnetic waves radiated from an on-board 
vehicle. Further, another object of the present invention is to provide an 
electromagnetic wave reflector which can be mounted on a vehicle, being 
integrally incorporated with an optical reflector whose mounting has been 
conventionally obligatory, in order to avoid significantly increasing the 
cost and the man-hours of work incurred. 
According to the present invention, there is provided a distinctive 
structure by which reflected electromagnetic waves can be effectively 
received even though the relative angle of the reflection of 
electromagnetic waves radiated from an on-board radar device varies due, 
for example, to a preceding vehicle shifting into another lane. 
That is, the present invention is characterized in that an optical 
reflector and an electromagnetic wave reflector which is effective for 
electromagnetic waves in a range from 50 to 100 GHz are formed in a one 
unit body. 
The above-mentioned reflector includes a pyramid-like part having pyramidal 
surfaces, which are arranged to be concave in a direction in which 
electromagnetic waves are incident, and having a triangular pyramid-like 
shape each ridge or edge of which is 10 to 200 mm. The reflector has an 
electromagnetic wave reflecting surface made of metal and preferably 
incorporates a protecting means made of a resin material having a high 
electromagnetic wave transmission factor in a direction in which 
electromagnetic waves are incident. 
A vehicle radar device transmits electromagnetic waves in front of a 
vehicle in a vehicle advancing direction and over angles around that 
direction, and receives reflected electromagnetic waves from an 
obstruction including a preceding vehicle so as to measure data relating 
to a distance to the obstruction based on the elapsed time between the 
transmission and the receiving of electromagnetic waves. For example, the 
distance to the preceding vehicle is steadily measured, and when the 
distance decreases to a certain smaller value, depending on speed, an 
alarm is automatically issued so as to allow the driver to pay more 
attention. 
An electromagnetic reflector which is effective for electromagnetic waves 
in the 50 to 100 GHz frequency range is incorporated into a single unit 
together with an optical reflector and is mounted on the rear or one side 
of a vehicle. Electromagnetic waves transmitted from a transmission 
antenna of the vehicle radar device of a trailing vehicle is reflected by 
the electromagnetic reflector of the preceding vehicle, and the thus 
reflected waves from a broad zone are received by a receiving antenna of 
the radar device. 
Electromagnetic waves are radiated from the transmission antenna over a 
predetermined angle in front of the vehicle. The electromagnetic reflector 
according to the present invention is formed in a pyramidal shape and is 
arranged so that their pyramidal surfaces form a concavity in a direction 
in which the electromagnetic waves are incident. Thus, even though the 
preceding vehicle changes its direction of travel due to a curved road or 
a shift from one lane into another, the radiated electromagnetic waves can 
be reflected by any part of the pyramidal surfaces of the electromagnetic 
reflector due to its concave shape, and accordingly, reflected 
electromagnetic waves can be effectively received. 
As to the pyramidal shape, a triangular pyramidal shape is desirable in 
order to increase the reflection characteristic value. The length of the 
edges of the triangular pyramid shape is selected in a range from 10 to 
200 mm, and the reflection characteristic value can be optionally changed 
by changing the length of those edges. 
The electromagnetic wave reflecting surface of the reflector is made of 
metal. That is, either a metal sheet may be used as the electromagnetic 
wave reflecting surface, or a metallic paint coating may be used as the 
electromagnetic wave reflection surface. Thus, the efficiency of 
reflection of electromagnetic waves can be enhanced, and a tough 
mechanical strength can be obtained. 
As to the protecting means incorporated in the electromagnetic wave 
reflector in a direction in which electromagnetic waves are incident, a 
cover made of resin materials such as acrylic resin, ABS, Teflon or 
polycarbonate is mounted to the reflector so as to protect the reflecting 
surfaces of the electromagnetic wave reflector with no hindrance to the 
transmission of electromagnetic waves and the designability thereof. This 
cover will be better understood upon considering the content of Japanese 
Patent Laid-open No. H10-79616 the contents of which are incorporated 
hereinto by reference. With this arrangement, electromagnetic waves 
radiated from an on-board radar device of a following vehicle can be 
effectively reflected from the preceding vehicle, and reflected waves can 
be surely received by the following vehicle, irrespective of variation in 
the running condition of the preceding vehicle. It is thereby possible to 
measure a vehicle-to-vehicle distance precisely. Further, the 
electromagnetic wave reflector can be integrally incorporated with an 
optical reflector which has been conventionally mounted to a vehicle, and 
accordingly, it is possible to materialize the electromagnetic wave 
reflector without the cost and working manhours from being excessively 
increased.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First, in reference to FIGS. 1 and 2 which are a perspective view and an 
exploded perspective view illustrating a reflector device in a first 
embodiment, as viewed from behind a vehicle, explanation will be made of 
the first embodiment of the present invention in which an optical 
reflector 1 and in line therewith an electromagnetic reflector 2 which is 
effective for electromagnetic waves in a range from 50 to 100 GHz are 
incorporated as a unit in frame 3. 
The electromagnetic wave reflector 2 is of the type referenced in Radar 
Cross Section Handbook, Ruck et al., Vol. 2, page 591, Plenum Press, New 
York-London, 1970, and has hollow a pyramidal shape having triangular 
surfaces. The bottom or base of the pyramid faces the inner side of the 
optical reflector 1, in a parallel relationship with the optical reflector 
1 or at an angle thereto as shown, and the bottom is open so that the 
inner sides of the pyramid are arranged in a concave shape which forms a 
cavity in a direction in which electromagnetic waves are incident. In this 
embodiment, a triangular pyramidal shape is used as the pyramidal shape of 
the part of the electromagnetic reflector, each ridge or edge of which has 
a length of from 10 to 200 mm from the common intersection of the 
triangular sides. That is, the cross-section of reflector 2 is triangular, 
and the reflector technically has a triangular hollow trihedral shape. As 
shown in the above handbook, a triangular trihedral corner reflector has 
wide angular coverage. 
The electromagnetic wave reflector 2 is molded from a resin material, and 
the inside surfaces thereof are subjected to surface treatment with metal. 
The three tabs 2' shown in FIG. 2 as projecting outwardly from the edges of 
the bottom of the electromagnetic wave reflector 2 may be used as stops 
for assembly purposes. 
FIG. 3 is a view for explaining the condition of reflection of light and 
electromagnetic waves by the optical reflector 1 and the electromagnetic 
wave reflector 2 in the first embodiment. FIG. 3 shows that when light and 
electromagnetic waves are irradiated to the surface of the optical 
reflector 1 in a direction perpendicular to the lens surface of the 
optical reflector, the light irradiated to the optical reflector 1 is 
directly reflected by the surface thereof, and the incident light is 
refracted by a prism formed at the rear surface thereof and is radiated 
therefrom as reflected light. Meanwhile, electromagnetic waves received 
from a radar device (not shown) transmit once through the optical 
reflector 1 and are repeatedly reflected by the electromagnetic reflector 
2, and then, the electromagnetic waves are finally reflected toward the 
radiation side after they transmit again through the optical reflector 1 
toward the source of the electromagnetic waves. 
In FIG. 3, the illustrated sides of the electromagnetic wave reflector 2 
have a length L which is in the range 10 mm to 200 mm. 
FIG. 4 is a graph showing an example of actual measured reflection 
characteristic values which were measured with the use of a triangular 
pyramidal shaped reflector in the first embodiment of the present 
invention. In this figure, the broken line indicates a reflection 
characteristic curve which was obtained by radiating electromagnetic waves 
to a vehicle which does not incorporate the electromagnetic wave reflector 
2. The reflection characteristic values RCS (Radar Cross Section) (dB) are 
taken along the ordinate, and receiving angles (.theta.) of 
electromagnetic waves are taken along the abscissa It is noted that, as 
shown in FIG. 5, the receiving angles on the left side are considered 
negative (-.theta.), and those on the right side are considered positive 
(+.theta.). 
As understood from FIG. 4, reflected waves from a vehicle which 
incorporates the electromagnetic wave reflector 2 of this invention are 
reflected by larger values over a wide range in comparison with reflected 
waves from a vehicle which does not incorporate the electromagnetic wave 
reflector 2. 
It is noted that the reflection characteristic values (RCS) of a triangular 
pyramidal shaped reflector using a right-angled isosceles triangle can be 
obtained from the following formula: 
EQU RSC=(4.pi.L.sup.4)/3.lambda. 
where L is the length of the three isosceles edges which meet at a common 
point (the apex) and .lambda. is a wavelength. It has been known that, the 
reflection characteristic value varies as listed in the following table if 
the isosceles edge length is changed: 
TABLE 
______________________________________ 
L (mm) 
RCS (dB) 
______________________________________ 
66 5 
88 10 
156 20 
______________________________________ 
Accordingly, an electromagnetic wave reflector for any of various kinds of 
vehicles can be designed by changing the length of the isosceles sides of 
the triangular pyramidal shape. 
FIGS. 6A and 6B show examples of the reflector device including the optical 
reflector and electromagnetic reflector of the first embodiment of the 
present invention. That is, FIG. 6A shows the left side one of a pair of 
arrangements which are respectively disposed on the left and right sides 
of the rear part of a vehicle. In this example of the reflector device, 
the optical reflector 1 and the electromagnetic wave reflector 2 are 
integrally incorporated with each other together with a tail lamp 4 and a 
turn indicator 5 in a frame 3. Further, FIG. 6B shows an example of the 
reflector device in which the optical reflector 1 and the electromagnetic 
wave reflector 2 are incorporated in a single unit. 
FIGS. 7A to 7D show examples of the mounting thereof on the rear end of 
different kinds of vehicles. FIG. 7A shows the mounting on a large-sized 
truck. On the left side is the arrangement shown in FIG. 6A in which the 
optical reflector 1 and the electromagnetic wave reflector 2 of the 
reflector device are located in one and the same frame together with the 
tail lamp 4 and the turn indicator 5. Symmetrically, on the right side is 
a mirror image of the arrangement in FIG. 6A. Further, the arrangement 
shown in FIG. 6B in which the optical reflector 1 and the electromagnetic 
wave reflector 2 of the reflector device are incorporated in a single unit 
is located in the center part of the vehicle. 
FIG. 7B shows the mounting on the rear end of a passenger vehicle of a FIG. 
6A arrangement on the left side and a mirror image of FIG. 6A on the right 
side. FIG. 7C shows the mounting at the center of the rear end of a middle 
size bus of the arrangement shown in FIG. 6B. FIG. 7D shows the FIG. 6B 
arrangement mounted on the rear end of a motorcycle. 
One or a plurality of the reflector devices according to the present 
invention may be arranged in accordance with the kind of vehicle. For 
example, reflector devices may be arranged on opposite sides of a vehicle. 
In this case, electromagnetic waves received from a radar device and 
reflected by the reflector devices can be received multidirectionally, and 
accordingly, the receiving zone can be enlarged. This arrangement is 
effective for a radar device which monitors a side of a vehicle or for a 
traffic survey in which radar devices are arranged along a road. 
Reference is now made to a second embodiment as shown in FIGS. 8 and 9. 
FIG. 8 is a perspective view illustrating a reflector device in a second 
embodiment of the present invention, as viewed from behind a vehicle, and 
FIG. 9 is an exploded perspective view of the FIG. 8 arrangement. An 
optical reflector 11 and a triangular pyramidal shaped electromagnetic 
reflector 2 which is effective for electromagnetic waves in a range of 50 
to 100 GHz are incorporated in a single unit as in the first embodiment. 
The optical reflector 11, however, is formed with a triangular cut-out 
opening 6 having a shape corresponding to the shape of an opening surface 
of the electromagnetic wave reflector 2, upon which electromagnetic waves 
are incident. A protecting cover 7 serving as a protecting means is fitted 
in the cut-out opening 6. This protecting cover 7 is made of a resin 
material having a high transmission factor with respect to electromagnetic 
waves, such as acrylic resin, polycarbonate resin, ABS (acrylonitrile 
butadiene styrene) resin and Teflon resin. 
FIG. 10 shows the relationship between the thickness t and the transmission 
factor T with respect to electromagnetic waves at a frequency of 60 GHz 
for four dielectric materials: acrylic resin, polycarbonate resin, ABS 
resin and Teflon resin. FIG. 10 was originally shown in Japanese Patent 
Application Laid-Open No. H10-79616, the contents of which are 
incorporated by reference. The relationship between the thickness t of the 
dielectric material and the transmission factor T is such that the 
transmission factor does not proportionally decrease as the thickness t 
increases. However, for example, in the case of the acrylic resin, the 
transmission factor T is 87% when the thickness t is 2.3 mm, but the 
transmission factor T is 95% when the thickness t is 3.1 mm. Further, it 
decreases to 85.5% when the thickness t is 3.9 mm while it increases to 
93% when the thickness t is 4.6 mm. 
With the use of the characteristic, it is possible to select a thickness t 
which can satisfy the mechanical strength and also exhibit a high 
transmission factor T. For example, in the case of using the acrylic 
resin, when the thickness is set to t=4.6 mm, a high transmission factor 
of 93% can still be obtained. 
In the case of the second embodiment, since the surface upon which 
electromagnetic waves are incident is made of a resin material having a 
high transmission rate for electromagnetic waves, the reflectivity can be 
further enhanced in such a condition that the reflecting surface is 
sufficiently protected. 
As stated above, according to the present invention, electromagnetic waves 
radiated from an on-board radar device can be effectively reflected in a 
broad zone, the vehicle-to-vehicle distance can be precisely measured even 
though the preceding vehicle runs on a curved road or shifts from one to 
another zone, thereby ensuring a safe distance. Further, since an 
electromagnetic reflector can be integrally incorporated with a 
conventionally incorporated optical reflector, it is possible to 
materialize the on-board electromagnetic wave reflector without incurring 
a significant increase in the cost and the man-hours of work. 
While the foregoing description has detailed the invention, the invention 
is not to be limited by such details but only by the scope of the appended 
claims.