Measurement system and measurement method

In the measurement system for the measurement of an object (2), the object 2) is provided with two retro-reflectors (3, 4) disposed at a known spacing. A measuring device (1) exhibits a light source (5), a video camera (7, 8) and an arrangement (9) which carries out the measurement with reference to the two image points which are formed by the light reflected by the retroreflectors (3, 4). The light source (5) is preferably a flash light source, the flashes of which are synchronized by an arrangement (6) with the imaging sequence of the video camera (7, 8).

The invention relates to a measurement system for the measurement of an 
object, and to a corresponding measurement method. 
The measurement of objects, i.e. the determination of their location, their 
distance, their velocity, their position etc., is normally very costly. It 
is necessary to determine the position of the object by means of 
appropriate theodolites or the like; the appropriate measurement values 
must then be recorded and analyzed. This is very costly, and, especially 
in the case of moving objects, it is difficult or even impossible to do 
this, if the object is moving too quickly. 
The object of the invention consists in providing a simple measurement 
system and measurement method, with which the measurement can be 
undertaken simply, rapidly and with a very high degree of automation. 
The solution according to the invention consists in that the object 
exhibits two retroreflectors disposed at a known spacing, and in that a 
measuring device is provided, which is provided with a light source, a 
video camera and an arrangement which carried out the measurement with 
reference to the two image points which are formed by the light reflected 
by the retroreflectors. 
Retroreflectors, especially triple mirrors, have the property of reflecting 
back the light, with a very low degree of scatter, substantially in the 
direction from which it comes. Accordingly, if the object is illuminated 
by the light source, then in the vicinity of the light source, with a 
video camera, even at a large distance of the object the light rays 
reflected by the retroreflectors can clearly be detected and recorded. The 
locations of these image points in the image of the video camera can then 
be determined, which can take place automatically by means of appropriate 
computers and the like. The measurement can then be undertaken on the 
basis of the known geometry of the arrangement, e.g. the focal length and 
other imaging properties of the video camera, the instantaneous 
orientation of the video camera and the known spacing of the 
retroreflectors. 
In this procedure, a white light source can be employed as the light 
source. An infrared light source can also be employed, which facilitates, 
in particular, measurement during the day. 
Expediently, the light source employed is not a continuous light source, 
but a light source which emits light flashes, e.g. a flash lamp. In this 
procedure, the light flashes are advantageously synchronized with the 
scanning frequency of the video camera. In this manner, on the one hand it 
is avoided that the image of the retroreflectors is recorded by the camera 
at unfavourable instants of the scanning cycle. Moreover, such 
synchronization can facilitate the overcoming of the problems which arise, 
in particular during the day, on account of the fact that the image of the 
light reflected by the retroreflectors is dominated by the daylight. 
By means of such light flashes and appropriately synchronized video 
cameras, it is also possible more readily to detect and to measure 
movements of the object. 
It would indeed be possible to employ a normal line camera as the video 
camera. However, a matrix CCD camera is particularly advantageous, because 
in this case direct access of the computer to the individual matrix points 
is possible. 
It is particularly advantageous if the measuring device exhibits 
arrangements for the determination of the distance of the object on the 
basis of the spacing of the two image points. In this manner, it is 
possible to carry out a particularly simple determination of the spacing, 
without complicated transit time measurements, as is the case, for 
example, with laser range finding measurements. In the course of this 
procedure, the distance of the object can be determined from the know 
spacing of the retroreflectors and the imaging geometry. 
Advantageously, the measuring device is provided with arrangements for the 
determination of the relative velocity of the object. Thus, it is possible 
not only to detect the instantaneous location of the object, but also to 
record its movement with accuracy. What is involved here is the relative 
velocity. If the measuring device itself is fixed, then this relative 
velocity is equal to the actual velocity of the object. If the measuring 
device is also moving, then the velocity of the measuring device must be 
subtracted from the relative velocity, in order to determine the absolute 
velocity of the object. 
In a further advantageous embodiment, the measuring device is provided with 
arrangements for the determination of the swing of the object relative to 
the measuring device. In this manner, it is not only possible to detect 
the swing if the object does not stand vertically, i.e. the two 
retroreflectors are not disposed precisely one above the other or beside 
one another. Indeed, with due consideration being given to this swing, it 
is then also possible to carry out a better determination of the correct 
distance. It is, of course, also possible to carry out flange finding 
without the explicit measurement of swing, by automatically taking account 
of the swing. 
If the object is stationary or moves only within a very small range, then 
the measuring device can be fixed. However, if the object moves to a 
greater extend, then a follow-up adjustment device for the measuring 
device can be provided, so that the object always remain within the image 
field, even in the case of a relatively small imaging angle of the 
measuring device. 
Advantageously, arrangements are provided for the transmission of 
information from the measuring device to the object. In this manner, the 
object can, for example, be directed from the measuring device to various 
locations, in order to carry out in this manner the measurement of a 
territory. In this procedure, the transmission of the information can take 
place by means of light, e.g. by the light of the light source. For this 
purpose, the light of the light source can be appropriately modulated, 
e.g. by variation of the flash repetition frequency. 
However, in another advantageous embodiment, the transmission of 
information is effected by a narrowly concentrated beam, in particular a 
laser beam of a microwave beam, or by a data radio transmitter. In this 
manner, less energy is required for the transmission of information. 
Moreover, laser light, microwave radiation or radio waves can be modulated 
more simply and in an improved manner, as well as with a larger bandwidth 
than conventional light. Finally, yet a further advantage of narrow 
concentration consists in that unauthorized persons cannot tap into the 
transmission of information. 
If the video camera exhibits an objective lense of variable focal length, 
as well as arrangements for the analysis of a plurality of recordings made 
with various image angles for the determination of the distance, then the 
range finding can be carried out with greater accuracy. This is based on 
the following findings. 
In the case of large distances, the following disadvantage can arise. The 
video camera, e.g.the matrix CCD camera or a line camera, has in the 
vertical direction (and also in the horizontal direction) a limited 
resolution of, for example, 500 image points or pixels. This leads also to 
a limited accuracy of range finding, as is illustrated with reference to 
the following example. 
The object is to have a distance of 1 km, and the retroreflectors at the 
object are to have a vertical distance of 1 m. Furthermore, the imaging 
geometry is to be such that the images of the two retroreflectors have a 
spacing of 25 pixels in the television image. In this, the image of one 
reflector, if the image of the other reflector is maintained stationary, 
falls only on the next image point (pixel) when the distance of the object 
becomes greater or smaller by 40 m. This inaccuracy becomes greater if the 
object distance becomes greater. At a distance of 3 km, the two images of 
the retroreflectors would only have a spacing of 8 pixels. Only if the 
distance changed by one eighth, i.e. by approximately 375 m, would the 
image of the retroreflector fall on the next image point, i.e. the 
corresponding inaccuracy would be greater, that is to say 350 m. 
These problems can now be avoided or at least very greatly diminished, if 
the video camera exhibits an objective lens of variable focal length and 
arrangements are provided for the analysis of a plurality of recordings 
made with various image angles for the determination of the distance. In 
this case, it is not so that a change in distance is not detected unless 
the image falls on the next image point. Instead of this, a plurality of 
such recordings are made, in which the image has moved already to a 
greater or lesser extent to the next image point, i.e. is detected with 
differing intensity on two adjacent image points. From this, electronic 
means can be employed to calculate a brightness distribution, which, after 
electronic determination of the mean value, leads to a substantially more 
accurate measurement of the distance. 
Advantageously, arrangements for display/printout of the results are 
provided. In particular in circumstances in which many measurement values 
are to be printed out, it is advantageous, because of the limited speed of 
the printer, to provide an intermediate store for the results. 
The measurement can be carried out in a simple manner if the measuring 
device itself is stationary or executes a known movement. If, however, the 
measuring device is not stationary and executes a specific movement which 
is not accurately known, then, advantageously, arrangements are provided 
for the illumination of a section of the direct environment of the 
measuring device, as well as arrangements for the imaging of this section 
by the camera and arrangements for the determination of the specific 
movement of the measuring device on the basis of changes of the image of 
the section. 
The appropriate part of the environment, which is illuminated by a partial 
beam and is reflected into the video image, could, for example, be the 
ground in the environment. If the measuring device moves, then the image 
section of the ground also moves. From the speed of movement of the grey 
tones of this image, which speed can be determined electronically, it is 
then possible to determine the specific movement of the measuring device. 
However, if this specific movement is known, then it is possible to 
determine not only relative velocities of the object, but also the 
absolute velocities of the latter. 
The invention is also further characterized by a method for the measurement 
of an object, which method is distinguished in that the object is provided 
with two retroreflectors disposed at a known spacing from one another and 
is illuminated by a light source, in that the image of the light reflected 
by the retroreflectors is recorded by a video camera, and in that the 
measurement is carried out by analysis of the locations and movements of 
the image points of the retroreflectors. 
When reference is made here to retroreflectors, these do not in any sense 
need to be traditional triple mirrors, which have in all cases only a 
certain aperture angle of at most 60.degree.. Rather, by way of a 
retroreflector, it is also possible to provide an entire arrangement of 
such triple mirrors, which are disposed, for example, around a rod, so 
that the reflecting action occurs in all directions. In this manner, the 
measurement is also possible in circumstances in which the object, e.g. a 
rod, rotates about its own axis during the measurement.

The measurement system exhibits a measuring device (1) indicated in broken 
lines and a target (2), which has, for example, the form of a measuring 
rod. Two retroreflectors (3 and 4) or appropriate retroreflectors are 
provided at the target (2). 
The measuring device (1) exhibits a light source (5), e.g. a flash lamp in 
the white light range or infrared range, which emits a cone of light (6), 
which is directed to the target (2). In this system, the flash sequence of 
the light source (5) is set by an electronic circuit (19), which is 
synchronized with the line-scanning frequency or image-scanning frequency 
of a video camera (7), which is advantageously a matrix CCD camera. The 
light reflected by the retroreflectors (3 and 4) is recorded by means of 
this television camera (7), which, for this purpose, is provided with an 
objective lens (8), which can also have a variable focal length. 
The location of the two image points of the retroreflectors (3 and 4) 
(these image points are designated in FIG. 2 by 3a and 4a) is 
electronically analyzed in a circuit (9), by which the location of the 
object (2), the lateral movement of the object (2), a possible vertical 
movement and also a swing of the object (2) can be determined. In this 
system, it is particularly advantageous if the camera (7) is a matrix CCD 
camera, since then a direct access of the computer in the unit (9) to the 
individual image points is possible. 
The results are then placed in intermediate storage in an intermediate 
store (10) and displayed on a display (11) or printed out there. 
With the aid of the central processing unit (9), by means of an arrangement 
(12) the measuring device (1) is made to execute follow-up adjustment in 
relation to the object (2), so that the cone of light (6) continues to be 
directed onto the object (2), even in circumstances in which this object 
moves or the measuring device (1) moves. Thereby, at the same time the 
sharply concentrated beam (13) of a laser (14) is also directed to the 
object (2) or to a receiver (15) for the laser radiation at the object 
(2), so that here, by modulation of the laser beam (13), information can 
be transmitted from the measuring device to the object (2). 
By means of a small mirror (16) disposed in the lower region in front of 
the light source (5), a part of the light beam is deflected downwards, as 
is shown by broken lines. In this manner, a region of the ground (17) is 
illuminated. This region of the ground is then likewise recorded by the 
video camera (7), specifically with the aid of a further small mirror 
(18), which reflects the image of this region of the ground into the video 
camera. The result of this is the reflected-in region of the ground, shown 
at (17a) in FIG. 2) with differing grey tones. 
If now the measuring device (1) moves, then this leads to a movement of the 
grey tones, from which the actual velocity of the measuring device (1) can 
then be determined with the aid of the circuit (9). This then permits, 
from the measurement of the relative location and of the relative velocity 
of the object (2), the determination of the absolute location and the 
absolute location and the absolute velocity of the object (2).