Inter-vehicle distance detecting device

An inter-vehicle distance detecting device comprising: a pair of image sensors on which an image of an object in front of a driving car is formed by a pair of optical systems; an image display means for displaying a first image information taken by one of the pair of image sensors; a window setting means for setting a plurality of windows partitioning a region of the image on the first image information; a distance detecting means for calculating respective distances from the driving car to the object surrounded by the respective windows by comparing a second image information in the respective windows set by the window setting means with a third image information taken by the other of the pair of image sensors corresponding to the second image information and by detecting a shift between the second image information and the third image information; a determining means for determining the object for measurement based on the distances to the respective windows provided by the distance detecting means; a gate setting means for setting a gate partitioning the region such that the gate approximately surrounds the object for measurement based on the distances corresponding to the object for measurement; a symmetry determining means for determining symmetry of a fourth image information surrounded by the gate and selected from the first image information and for obtaining a position of a symmetry line; and a predicting means for predicting the presence of a preceding car in front of the driving car by determining a first stability of a movement of the symmetry line provided by the symmetry determining means in the horizontal direction and by determining a second stability of the distances provided by a calculation result of the distance determining means over time.

This invention relates to an inter-vehicle distance detecting device 
particularly for continuously measuring an inter-vehicle distance from a 
driving car to a preceding car, in an optical distance detecting device 
employing image sensors. 
FIG. 10 is a construction diagram of a conventional optical distance 
detecting device employing image sensors, disclosed in Japanese Examined 
Patent Publication No. 38085/1988, Japanese Examined Patent Publication 
No. 46363/1988 and the like, having two optical systems on the left and on 
the right as shown in FIG. 10. 
In FIG. 10, reference numerals 1 and 2 designate left and right lenses 
disposed apart from each other by a base line length L, 3 and 4, image 
sensors disposed at positions apart from the lenses 1 and 2 by a focal 
length f, and 51, a signal treating device for calculating a distance from 
the driving car to an object 52 by employing image signals transmitted 
from the image sensors 3 and 4. 
Next, an explanation will be given of the principle of distance detection 
from the driving car to the object 52. The image signals of the object 52 
focused respectively on the image sensors 3 and 4 through the lenses 1 and 
2, are electrically superposed by successively shifting one of the image 
signals with respect to the other thereof, in the signal treating device 
51. The distance R from the driving car to the object 52 is obtained by 
the following equation (1) based on a shift quantity a whereby the above 
two image signals agree the most, by the principle of trigonometry. 
EQU R=f.times.L/a (1) 
On the other hand, a method of tracking an image of a preceding car taken 
by image sensors or the like, is disclosed by Japanese Examined Patent 
Publication No. 3352/1985. According to the publication, an operator sets 
a tracking gate (window) surrounding a target to be tracked on a display 
image plane while observing the display image plane, thereby tracking the 
image of the target. 
Since the conventional distance detecting device is constructed as above, 
wherein the distance from the driving car to the object is provided by 
comparing the image taken by the pair of left and right optical systems, 
in case of measuring the inter-vehicle distance from the driving car to a 
preceding car while running the driving car by mounting these systems on 
the driving car, when another car is running on the left hand side or the 
right hand side of the preceding car, the driver cannot recognize which 
inter-vehicle distance the device is measuring. 
Furthermore, in case of starting the image tracking of a preceding car by a 
window or the like by setting an image tracking window by a joy stick or 
by a track ball while the driver is observing a display image plane in 
driving a car, the driver is accompanied by a danger of carelessness in 
watching in front of the driving car, or the like. 
It is an object of the present invention to solve the above problem, and to 
provide an inter-vehicle distance detecting device capable of continuously 
measuring a distance from a driving car to a target, once the target is 
determined, even when the object is moving, so far as the object is in the 
scope of vision, and of finding out an object of the preceding car for 
measuring an inter-vehicle distance, even when a plurality of preceding 
cars are running. It is another object of the present invention to provide 
an inter-vehicle distance detecting device capable of automatically 
setting a tracking window for tracking the image of the preceding car, 
without observing a display image plane. 
According to a first aspect of the present invention, there is provided an 
inter-vehicle distance detecting device comprising: 
a pair of image sensors on which an image of an object in front of a 
driving car is formed by a pair of optical systems; 
an image display means for displaying a first image information taken by 
one of the pair of image sensors; 
a window setting means for setting a plurality of windows partitioning a 
region of the image on the first image information; 
a distance detecting means for calculating respective distances from the 
driving car to the object surrounded by the respective windows by 
comparing a second image information in the respective windows set by the 
window setting means with a third image information taken by the other of 
the pair of image sensors corresponding to the second image information 
and by detecting a shift between the second image information and the 
third image information; 
a determining means for determining the object for measurement based on the 
distances to the respective windows provided by the distance detecting 
means; 
a gate setting means for setting a gate partitioning the region such that 
the gate approximately surrounds the object for measurement based on the 
distances corresponding to the object for measurement; 
a symmetry determining means for determining symmetry of a fourth image 
information surrounded by the gate and selected from the first image 
information and for obtaining a position of a symmetry line; and 
a predicting means for predicting the presence of a preceding car in front 
of the driving car by determining a first stability of a movement of the 
symmetry line provided by the symmetry determining means in the horizontal 
direction and by determining a second stability of the distances provided 
by a calculation result of the distance determining means over time. 
According to a second aspect of the present invention, there is provided an 
inter-vehicle distance detecting device comprising: 
a pair of image sensors on which an image of an object in front of a 
driving car is formed by a pair of optical systems; 
an image display means for displaying a first image information taken by 
one of the pair of image sensors; 
a window setting means for setting a plurality of windows partitioning a 
region of the image on the first image information; 
a distance detecting means for calculating respective distances from the 
driving car to the object surrounded by the respective windows by 
comparing a second image information in the respective windows set by the 
window setting means with a third image information taken by the other of 
the pair of image sensors corresponding to the second image information 
and by detecting a shift between the second image information and the 
third image information; 
a determining means for determining the object for measurement based on the 
distances to the respective windows provided by the distance detecting 
means; 
a gate setting means for setting a gate partitioning the region such that 
the gate approximately surrounds the object for measurement; 
a symmetry determining means for determining symmetry of a fourth image 
information surrounded by the gate and selected from the first image 
information and for obtaining a position of a symmetry line; 
a predicting means for predicting the presence of a preceding car in front 
of the driving car by determining a first stability of a movement of the 
symmetry line provided by the symmetry determining means in the horizontal 
direction and by determining a second stability of the distances provided 
by a calculation result of the distance determining means over time; and 
a tracking means for tracking an image of the preceding car predicted by 
the predicting means. 
A microcomputer in this invention measures the distance from the driving 
car to the object caught by the windows under the principle of 
trigonometry, by comparing the top and the bottom corresponding images 
taken by the pair of image sensors, setting the image signal in the 
plurality of windows as a reference signal, and by electrically detecting 
the shift between the both images, and monitors the presence or the 
absence of a preceding car or other obstacle and its approximate position. 
Furthermore, the microcomputer finds out a position of the image having a 
good left and right symmetry on the display image frame, determines the 
position as a candidate of the position of the presence of the preceding 
car and predicts the presence of the preceding car and the position of the 
image of the preceding car on the display image plane based on these 
informations. 
Furthermore, when the microcomputer predicts the position of the image of 
the preceding car on the display image plane, the microcomputer 
automatically sets a window having a size in correspondence to the 
distance from the driving car to the preceding car, and continuously 
detects the distance from the driving car to the preceding car by tracking 
the image of the preceding car by the tracking window.

EXAMPLE 1 
An explanation will be given of an Example of an inter-vehicle distance 
detecting device of this invention in reference to the drawings, as 
follows. FIG. 1 is a block diagram showing the construction of a first 
Example of this invention. In FIG. 1, a part the same with that in the 
conventional distance detecting device shown in FIG. 10, is attached with 
the same notation. In FIG. 1, reference numerals 1 and 2 designate the 
lenses 3 and 4 composing the top and the bottom optical systems, 3 and 4, 
the two-dimensional image sensors provided respectively corresponding to 
the lenses 1 and 2 and 5, a preceding car to be tracked. Numerals 6 and 7 
designate analog-to-digital (hereinafter, A/D) converter, 8 and 9, 
memories, 10, a microcomputer and 11, a display image plane for displaying 
an image taken by the top image sensor 4, which is controlled by the 
microcomputer 10. A numeral 12 designates a window setting device for 
setting windows for designating an object for measuring a distance from a 
driving car to the object, on the image, and 13, an image tracking 
instruction switch operated by a driver. 
Next, an explanation will be given of the operation. When the driver 
operates the image tracking instruction switch 13, as shown in FIG. 2, the 
window setting device 12 sets a plurality of windows for the distance 
measurement 21, 22, 23, 24, 25 and 26 at predetermined positions of the 
display image plane 11 wherein an image in front of the driving car taken 
by the top image sensor 4, is displayed. As shown in FIG. 3, when an image 
of a preceding car 5a enters the image, the image 5a can be caught by the 
plurality of windows 21 to 26 for the distance measurement. 
By the way, when the object for the distance measurement is an automobile, 
the image thereof viewed from behind is often provided with a left and 
right symmetry. Therefore, employing such a characteristic, the presence 
of the preceding car and the position of the image of the preceding car on 
the displayed image plane, are specified by performing a determination of 
symmetry thereof in a region wherein the plurality of windows for the 
distance measurement are set and by having the distance information. 
First, as shown in FIG. 5, based on the distance information provided from 
the windows 23, 24 and 25 catching the preceding car, a gate 33 having a 
size in correspondence to the distance, is prepared. An evaluation of the 
symmetry is carried out by performing the calculation by the equation (3) 
shown below with respect to the image signal in the gate by shifting the 
gate 33 in the region 34 wherein the plurality of windows 21 through 26 
for distance measurement are set, on the memory. S (i,j) designates the 
image signal in the symmetry determining region 34, which shows the grey 
level, for instance, in 256 grey scales. Notations m and n are 
respectively the width and the height of the gate 33, and w is the width 
of the symmetry determining region 34. 
##EQU1## 
The best symmetry is provided at the position wherein the value of A.sub.k 
is minimized by changing k in the above equation. However, actually there 
are several objects having good symmetry on the load, other than the 
preceding car. As shown in FIG. 11, according to the result of calculation 
by the above equation (3), there are a number of minimum values. It is 
determined that a candidate of the preceding car is present at the 
position of k (horizontal coordinate in the region 34) giving these 
minimum values. An absolute coordinate X.sub.n of a candidate point of the 
preceding car in the horizontal direction on the image plane 36, is given 
by the following equation (4) by determining the left end horizontal 
coordinate of the symmetry determining region 34 as p.sub.O. 
EQU X.sub.n =k+p.sub.O +m/2 (4) 
The microcomputer 10 selects a region corresponding to the above window 24 
in the memory 8 wherein the image signal of the bottom image sensor 3 is 
memorized, and calculates a summation of absolute values of differences of 
signals at every top and bottom pixel by successively shifting the image 
signal of the memory 8 with respect to the above reference image signal, 
pixel by pixel. Accordingly, the position of the image which fits the 
image in the window 24 the most, is provided by successively shifting the 
image pixel by pixel. 
At this occasion, as shown in FIG. 4, the region of the memory 8 which is 
related to the calculation, is a region 30 corresponding to the position 
of the window 24. A shift quantity of the pixel which minimizes the 
summation of the absolute values of the differences of signals, which is 
obtained by comparing the top and the bottom pixels, is determined to be n 
pixels, the pitch of the pixel, P, the base line length of the optical 
system, L, and the focal length of the lenses 1 and 2, f. Then, the 
distance R from the driving car to the preceding car is provided by the 
following equation (2). 
EQU R=f.times.L/n.times.P (2) 
In this way, the distance from the driving car to the object in the window 
24 can be measured. 
As stated above, when a window 35 for tracking surrounding the image 5a of 
the preceding car is set, the image tracking operation of the image 5a of 
the preceding car by the window 35 for tracking, is initiated (FIG. 7). 
The image tracking operation is similar to that in conventional devices 
disclosed in Japanese Examined Patent Publication No. 33352/1985, or 
Japanese Examined Patent Publication No. 35305/1989 and a detail 
explanation will be omitted. Furthermore, normally, as a window for 
catching an image of a preceding car which is an object for tracking and 
for distance measurement, a standard window corresponding to an ordinary 
passenger car is set. However, in the actual running, a large-sized 
vehicle or a small-sized vehicle such as a motor cycle may be the 
preceding car. Therefore, a large window, a medium (standard) window and a 
small window are prepared, and a driver can select a suitable window by 
manual operation in accordance with the kind of the preceding car after 
the tracking is started, as shown in FIGS. 12(a) and 12(b). 
As stated above, as shown in FIG. 4, with respect to the object caught by 
the windows for distance measurement 21 through 26 which are set in the 
display image plane 11, the image comparison is performed in calculation 
regions 27, 28, 29, 30, 31 and 32 respectively corresponding to these 
windows, and distances from the driving car to the object can be measured 
respectively for the windows for distance measurement 21, 22, 23, 24, 25 
and 26. 
The measurement values of the distances with respect to the windows for 
distance measurement 21 through 26 measured as above, are compared by the 
microcomputer 10, and windows showing approximately the same measurement 
values having comparatively short distances are selected among the 
contiguous windows. 
That is to say, the windows for distance measurement 23, 24 and 25 showing 
the distances shorter than that of the background, are selected. When the 
distance measurement values of the windows for distance measurement 23, 24 
and 25 do not change rapidly over time, it is predicted that a preceding 
car is present at the position wherein these windows are set. 
When the object for distance measurement is an automobile, the image 
thereof viewed from behind the automobile are often symmetric in the left 
and right direction, generally. Accordingly, employing such a 
characteristic, the symmetry is determined in a region wherein the 
plurality of the windows for distance measurement, are set, and the 
presence of the preceding car and the position of the image of the 
preceding car on the display image plane, are specified by the symmetry 
determination along with the distance information. 
First, as shown in FIG. 5, in accordance with the distance information 
provided by the windows 23, 24 and 25 catching the preceding car, a gate 
33 having the size corresponding with the distance, is prepared. The 
evaluation of the symmetry is carried out by performing the calculation by 
the following equation (5) with respect to the image signal in the gate 
33, by shifting the gate 33 in a region 34 wherein the plurality of 
windows for distance measurement 21 through 26, are set in the memory. S 
(i,j) in equation (5) is the image signals in the region 34 for 
determining the symmetry, which, for instance, represents a grey value in 
256 grey scales. Furthermore, notations m and n are respectively the width 
and the height of the gate 33, and W, the width of the region 34 for 
determining the symmetry. 
##EQU2## 
In equation (5), the symmetry has the best value at a position wherein the 
value of A.sub.k is minimized. However, actually, there are a number of 
objects having good symmetries on the road other than the preceding car. 
The calculation result of equation (5) has a number of minimum values, and 
the positions having these minimum values are candidates of the position 
for the presence of the preceding car. 
Although a number of candidates of the preceding car may be pointed out as 
above, the preceding car to be tracked on the image, does not abruptly 
move in the horizontal direction, and the inter-vehicle distance from the 
driving car to the preceding car, does not change abruptly over time. 
Therefore, by determining a stability of movement of these candidates in 
the horizontal direction over time, and a stability of the distance 
information provided by the windows for distance measurement, the 
preceding car can be selected among the candidates. In this way, the 
position of the image of the preceding car on the display image plane 11, 
can be specified. 
Next, as shown in FIG. 7, a tracking window 35 is set by the window setting 
device 12 at the position specified on the display image plane 11. The 
size of the tracking window 35 is the same with that of the gate 33 which 
has been set in accordance with the measurement values of the windows for 
distance measurement 23, 24 and 25. That is to say, since the size of the 
image of the preceding car 5a varies in accordance with the distance from 
the driving car to the preceding car, when the distance measurement value 
is comparatively large, the tracking window 35 having small dimension is 
set (FIG. 7). When the distance measurement value is comparatively small, 
a large gate 33a for determining the symmetry, is prepared as shown in 
FIG. 6. At this occasion, the region for determining the symmetry is 
designated by 34a in FIG. 6. Accordingly, a large tracking window 35a is 
set as shown in FIG. 8. 
When the tracking window 35 surrounding the image of the preceding the 
image of the preceding car 5a is set as above, the image tracking action 
of the image of the preceding car 5a by the tracking window 35, is 
started. This image tracking action is the same with that in a 
conventional device disclosed in Japanese Examined Patent Publication No. 
33352/1985, or Japanese Examined Patent Publication No. 35305/1989, and a 
detailed explanation will be omitted. 
Next, the inter-vehicle distance from the driving car to the preceding car 
5 caught by the tracking window 35, is detected. This inter-vehicle 
distance is detected by the principle of trigonometry wherein the top and 
bottom image signals are compared as in the method explained in the above 
equation (2), determining the image signal in the tracking window 35 
tracking the image of the preceding car 5a as the reference image signal 
for calculating the inter-vehicle distance. Wherever the image of the 
preceding car 5a moves in the display image plane 11, the tracking window 
35 tracks the image of the preceding car 5a, and continuously detects the 
inter-vehicle distance from the driving car to the preceding car 5. 
FIG. 9 is a flowchart showing the above operational procedure. First, the 
driver operates the image tracking instruction switch 13 in a state 
wherein the preceding car is present in front of the driving car (Step 
S101). The microcomputer 10 inputted with an operation signal, controls 
the window setting device 12 and sets a plurality of windows for distance 
measurement 21 through 26 at predetermined positions on the display image 
plane 11 (Step S102). 
Next, the operation measures the distances from the driving car to the 
object caught by the respective windows (Step S103). The operation reads 
the measured distance information and the positions of the windows in the 
memory (Step S104). The operation selects contiguous windows showing the 
same measured value having comparative short distances, and predicts the 
presence of the preceding car (Step S105). 
Next, the operation prepares the gate for determining symmetry having the 
size in accordance with the measured distance, and samples out the 
candidates of the preceding car (Step S106). The operation specifies the 
position of the image of the preceding car by determining the stability of 
movement of the candidates over time and the stability of the measured 
distance from the driving car to the candidates over time (Step S107). 
Furthermore, the operation deletes the windows for distance measurement 
which have been displayed until that time (Step S108). The operation sets 
the tracking window having the size in accordance with the inter-vehicle 
distance at the position selected as above (Step S109). Thereafter, the 
operation continuously detects the inter-vehicle distance from the driving 
car to the preceding car caught by the tracking window (Step S110). 
In the explanation of the above embodiments, the two optical systems are 
disposed on the top and on the bottom. However, the systems can be 
arranged on the left and on the right or on a slanted plane with the same 
effect. 
As stated above, according to the first aspect of the present invention, 
the image of the preceding car in front of the driving car is taken. The 
plurality of windows are set at the predetermined positions on the display 
image plane displaying the image taken by these image sensors. The 
distances from the driving car to the object caught by these windows, are 
detected. The symmetry in the left and right direction is determined. The 
stability of movement of the determined symmetry point in the horizontal 
direction, and the stability of the detected distance information, are 
determined. Accordingly, the invention has an effect wherein the presence 
of the preceding car and the position thereof on the display image plane 
can be predicted. 
Furthermore, according to the second aspect of the present invention, the 
tracking window surrounding the image of the preceding car is newly set at 
the position of the presence of the image of the specified preceding car. 
The distance from the driving car to the preceding car caught by the 
tracking window, is detected. Therefore, the tracking window for tracking 
the image of the preceding car, is automatically set. Furthermore, since 
the image of the preceding car is displayed on the display image plane, 
this invention has an effect wherein the driver can be informed of the 
preceding car for detecting the inter-vehicle distance.