Thermal deformation compensating range finder

Details Thermal deformation compensating range finder Thermal deformation compensating range finder

2000-06-20

2002-01-08

Tarcza, Thomas H. (Department: 3662)

Optics: measuring and testing

Range or remote distance finding

Triangulation ranging with photodetection, but with no...

C396S097000

Reexamination Certificate

active

06337736

ABSTRACT:

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to a range finder or distance measuring apparatus, which is used in, for example, an apparatus for detecting the distance to a vehicle running in front. More specifically, the present invention relates to a range finder that improves an measurement error due to unequal thermal deformation distribution of the plastic material forming the finder by a characteristic of low thermal conductivity of the plastic material based on heat generated by the current flowing through a CCD sensor chip.
In the following, the same reference numerals and symbols in the drawings are used for designating the same or corresponding constituent elements.
A range finder or distance measuring apparatus, which measures a distance to an object by electrically comparing the images focused by two adjacent optical systems based on the principle of triangulation, has been used widely. At first, the principle of triangulation will be explained below.
FIG. 3
is a schematic drawing for explaining the principle of triangulation. Referring now to
FIG. 3
, object images
23
,
24
are formed on optical sensor arrays
25
,
26
by focusing lens
1
a,
1
b.
Since triangles
27
and
28
are similar to triangles
27
′ and
28
′ respectively, the distance L to an object is expressed by the following formula (1).
L=Bf
/(
x
1
+
x
2
)=
Bf/x
  (1)
Here, B is a distance between optical axes of the focusing lenses
1
a
and
b
(hereinafter referred to as a “base line length”) and f is a focal length. Since B and f are constants, the distance L to the object is determined by detecting the shift length x of the object images.
FIG. 4
shows a cross section of a conventional range finder constructed on the basis of the principle described with reference to FIG.
3
. Referring now to
FIG. 4
, the conventional range finder includes lenses
1
a
and
1
b
spaced apart from each other for a base line length B, lens supporting means
2
for supporting the lenses
1
a
and
1
b,
CCD packages
3
a
and
3
b
, and CCD supporting means
4
for supporting the CCD packages
3
a
and
3
b.
The CCD packages
3
a
and
3
b
include CCD chips
25
′ and
26
′, respectively. The CCD packages
3
a
and
3
b
are arranged such that the optical sensor arrays on the respective CCD chips
25
′ and
26
′ are spaced apart from each other for the base line length in the focal plane of the lenses
1
a
and
1
b.
Each of the CCD packages
3
a
and
3
b
includes a plastic casing
6
and a transparent plastic plate
8
fixed to the plastic casing
6
. The CCD chips
25
′ and
26
′ are bonded to the respective plastic casings
6
with a thermosetting adhesive AH.
FIGS.
5
(
a
) and
5
(
b
) show bonding portions between the CCD supporting means
4
and the CCD packages
3
a
,
3
b
shown in FIG.
4
. FIG.
5
(
a
) is a vertical cross sectional view of the bonding portions between the CCD supporting means
4
and the CCD packages
3
a
,
3
b
. FIG.
5
(
b
) is a cross sectional view taken along line
5
b —
5
b
in FIG.
5
(
a
).
Referring now to FIG.
5
(
b
), bonding ribs
5
A, each shaped with a rectangular ring and protruding around an image ray hole HL through which an image ray from the lens
1
a
or
1
b
passes, are formed on the bottom surface of the CCD supporting means
4
. The CCD packages
3
a
and
3
b
are fixed to the CCD supporting means
4
such that the transparent plastictes
8
for the CCD packages
3
a
and
3
b
are bonded to bonding planes
5
, which are lower end faces of the bonding ribs
5
A.
A plastic, such as cycloolefin polymer, which exhibits a low water absorbing capacity and excellent optical characteristics is used for the lenses
1
a
and
1
b,
lens supporting means
2
, CCD supporting means
4
, plastic casings
6
and transparent plates
8
.
Thus, by making all the constituent elements except for the CCD chips
25
′ and
26
′ by the same material, any relative positional shift does not occur between the lenses nor between the CCDs, since all the constituent elements except for the CCD chips
25
′ and
26
′ expand or contract thermally at the same rate in response to the environmental temperature changes.
FIG.
8
(
a
) is a cross sectional view of an entire range finder structure (hereinafter referred to as a “range finder module” for the sake of convenience) at a certain temperature corresponding to the range finder designated in
FIG. 3
for explaining the principle of triangulation. FIG.
8
(
b
) is another cross sectional view of the range finder module thermally expanded due to an environmental temperature rise while maintaining the relative positional relations of the constituent elements in the range finder module including the lenses
1
a
,
1
b
and the CCD chips
25
′,
26
′.
In FIG.
8
(
a
), the distance L to the object is expressed by the foregoing formula (1) based on the principle of triangulation. When the range finder module has thermally expanded as shown in FIG.
8
(
b
), the product (B+&Dgr;B)×(f+&Dgr;f) of the base line length (B+&Dgr;B)and the focal length (f+&Dgr;f) after the thermal expansion is proportional to the shift length (x
1
′+x
2
′)=(x+&Dgr;x) after the thermal expansion. Therefore, the distance L to the object detected after the thermal expansion is the same as the distance L detected before the thermal expansion.
Since the range finder module is made of the same material, all the constituent elements thermally expand uniformly in all the directions and the similarity relations between the foregoing triangles are maintained.
As far as the range finder module is made of the same material and the temperature is uniform throughout the module in the conduction state of the CCD chips in the same manner as in the environmental temperature rise, no problem occurs on the accuracy of the distance measurement.
However, even if the entire range finder module is made of the plastic material, the CCD chips
25
′ and
26
′, which are semiconductor optical sensors, generate heat when a current is supplied to the CCD chips
25
′ and
26
′, and the generated heat causes thermal expansions of the CCD packages
3
a
,
3
b
sealing the CCD chips therein and the CCD supporting means
4
, to which the CCD packages are fixed.
Moreover, due to the low thermal conductivity, which is a characteristic of the plastic material, the influences of the CCD supporting means
4
and the lens supporting means
2
to the thermal expansion are different and an error occurs in the measured distance by nonuniform thermal conductivity.
As described with reference to
FIGS. 4 and 5
, the CCD packages
3
a
and
3
b
are connected to the CCD supporting means
4
such that the transparent plates
8
are fixed to the respective lower end faces (bonding planes
5
) of the bonding ribs
5
A formed on the bottom surface of the CCD supporting means
4
.
Due to this configuration, when the CCD chips generate heat, thermal conduction occurs from the heat sources, i.e. CCD chips, to the lens supporting means
2
via the plastic casing
6
, the transparent plates
8
and the CCD supporting means
4
in order. Therefore, even if all the constituent elements have the same temperature when a current starts to flow through the CCD chips, the CCD supporting means
4
will start expanding earlier and the lens supporting means
2
will start expanding at a certain period later.
Even when the temperatures of all the constituent elements become sufficiently stable, a certain temperature difference occurs between the CCD supporting means
4
and the lens supporting means
2
. Due to the temperature difference, the optical axes connecting the CCD supporting means
4
and the lens supporting means
2
after the current conduction shift with respect the corresponding optical axes before the current conduction, in such a direction that the base line length on the side of th

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