Method and device for measuring a dimension of a body, and use of said method

The image of a body which is located in a measuring zone is projected by a mirror into a camera having an optoelectric transducer, for instance a row of photodiodes. The stripe pattern is thrown onto the upper side of the body by a light beam which is inclined with respect to the projecting direction, said stripe pattern being imaged together with said body. The shape of the body in the vertical projection is detected by the camera and an associated electronic system, and the elevation of the body is deduced from the position of the stripe pattern thereon. Three dimensions, i.e. the actual size of the body, are thus determined. A sorting device can be controlled according to the detected size.

BACKGROUND OF THE INVENTION 
The present invention refers to a method for measuring a dimension of a 
body, the image of said body being projected onto a measuring sensor, and 
said dimension being detected by electrooptical means. Known measuring 
methods are limited to the detection of two dimensions at the most, i.e. 
the measuring sensor is capable of evaluating the two-dimensional image of 
a body at the most. In many cases, however, it is desirable to detect 
three dimensions of the body in order to evaluate its size in three 
dimensions, or its volume and weight, respectively. Such a detection of 
the actual size is e.g. desired in sorting processes, in particular for 
sorting fruit and field crops, especially potatoes. The hitherto usual 
sorting method, wherein potatoes are sized by dropping through square 
openings of graded sizes, cannot satisfy in so far as particularly slim 
potatoes are attributed to a given size although they may under given 
circumstances have the double volume of spherical potatoes in the same 
size class. However, the detection of three dimensions of bodies has been 
complicated and in certain cases impossible even under increased 
expenditure. 
SUMMARY OF THE INVENTION 
The present invention is now based the object of detecting three dimensions 
of a body without any substantial increase of expenditure, in particular 
with a single optoelectric sensor; the degree of evaluation of the 
detected data is merely a question of the amount of electronics and 
software used. This object is attained by means of a method for measuring 
the dimension of a body wherein the projection of said image is effected 
in the direction of the dimension to be measured, and said body is 
illuminated with an optical pattern in a direction which is inclined with 
respect to said projecting direction, the dimension being deduced from the 
position of said pattern on said body. The dimensions which are 
perpendicular to said projecting direction can be detected by one and the 
same sensor in a manner known per se and are evaluated by the associated 
electronics. 
The invention also refers to an application of the method wherein the 
extension of bodies is detected in three dimensions by means of a common 
image and of a common measuring sensor, and said bodies are sorted 
according to their size. Finally, the invention also refers to a device 
for implementing said method, comprising a projecting optics and an 
optoelectric measuring sensor as well as a device for illuminating a body 
with an optical pattern, the projecting direction being inclined with 
respect to the illuminating direction. 
The invention is now explained in more detail with reference to the drawing 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 schematically shows a device for sorting potatoes. On a conveyor 
belt 1, potatoes 2 are conveyed from the right to the left through a 
measuring zone 3 and beyond one end of said conveyor belt to an ejecting 
device 4. A casing 5 of relatively great length is disposed above conveyor 
belt 1. Above measuring zone 3, a first mirror 6 is provided in casing 5, 
through which the image of the measuring zone on the conveyor belt is 
projected into a camera 7. In camera 7, a row 8 of photodiodes extends in 
the horizontal direction and serves as an optoelectric sensor. The light 
values on said photodiodes may be cyclically scanned in a manner known per 
se, whereby a respective line of the projected measuring zone is surveyed 
and can be evaluated, or stored for evaluation, as the case may be. Said 
casing accommodates a second mirror 9 through which a directed light beam 
11 from a light source 10 is projected into measuring zone 3. Meanwhile, 
said light beam passes through a mask 12 which is provided with 
slit-shaped openings 13. FIG. 1 illustrates only a short section of the 
elongated mask. In fact, its length approximately corresponds to the width 
of conveyor belt 1, in such a manner that a pattern of illuminated stripes 
appears in the measuring zone over the entire conveyor belt area where 
potatoes are possibly delivered. 
A portion of said pattern is represented in more detail in FIG. 3. Each 
opening 13 of diaphragm 12, i.e. each light stripe 14 on the conveyor belt 
or on a potato 2 in the measuring zone is stepped and comprises three 
sections 14a, 14b, and 14c of equal length but increasing width. Moreover, 
the straight front edges 14d of stripes 14 are inclined by 45.degree. with 
respect to the longitudinal direction of the stripe pattern, i.e. to the 
transversal direction of conveyor belt 1. However, a different inclination 
can be chosen as well. The axis, i.e. the direction of incidence of light 
beam 11 is inclined by a certain angle of e.g. 20.degree. to 30.degree. 
with respect to the projecting direction which is identified by optical 
axis 16 in FIG. 1 and is orthogonal to the conveyor belt. The image of the 
pattern of light stripes projected onto the conveyor belt, the potatoes in 
the measuring zone, and the pattern of light stripes appearing on the 
potatoes are projected into camera 7 and onto sensor 8. Due to the 
relatively long optical path between the measuring zone on the conveyor 
belt and camera 7, the entire width of the conveyor belt, i.e. of 
measuring zone 3 can be projected onto sensor 8. 
In a schematical manner, FIG. 1 shows the connection between sensor 8 and 
the electronic system 15 of the device. Outputs 16 of said electronics are 
connected to the ejecting device 4 where they act upon valves which 
control compressed air for ejecting cylinders 17 whose piston rods 18 act 
upon a respective movably mounted finger 19 each. Ejecting device 4, whose 
length approximately corresponds to the width of the conveyor belt, is 
provided with a series of ejecting fingers 19. According to FIG. 1, one of 
said ejecting fingers has just been lifted up by the associated piston rod 
18 in order to eject a potato 2 sliding down over said finger. 
As already mentioned, the device represented in FIG. 1 allows a detection 
of three dimensions and thus of the actual shape and size of the potatoes. 
The portions of the potatoes which are projected into camera 7 allow a 
detection of the contours in the vertical projection. The elevation or 
thickness of the potatoes is detected by means of the described pattern of 
light stripes. With reference to FIG. 2, the principle of the detection of 
the elevation is explained by means of a very simple body 20. Due to the 
inclined incidence of illuminating beam 11, the higher the surface on 
which the stripe pattern falls, the more said stripe pattern, as seen from 
above, is displaced towards the right in FIG. 1 or in FIG. 2. On the 
conveyor belt, i.e. at the elevation 0, said stripes are located so far on 
the left that they lie completely outside observing plane 16'. This 
situation corresponds to a level 0. At a next higher lever 21, light 
stripes 14 are located further to the right, and their narrow sections 14a 
lie within the area of observing plane 16'. In FIG. 2, arrows 11' 
indicate the direction of incidence of light beam 11, which is responsible 
for the transversal displacement of the stripe pattern. At the next higher 
level 22 of body 20, stripes 14 appear still further displaced to the 
right, so that now their middle sections 14b lie in the area of observing 
plane 16'. At the highest level 23, stripes 14 appear still further 
displaced to the right, so that now their largest sections 14c are located 
in the area of observing plane 16'. From the position of light stripes 14, 
i.e. their displacement with respect to the initial position on conveyor 
belt 1, the elevation of determined portions of a body can be deduced, and 
there are different possibilities for detecting the amount of said 
displacement by means of a computer and thus the elevation 44 of a 
determined point. 
Concerning the precise detection of the elevation, FIG. 3 shows that light 
stripes 14 overlap in the longitudinal direction of the stripe pattern. By 
scanning the luminous values in the observing plane 16, 16', traversely to 
conveyor belt 1 by means of sensor 8, the periodical sequence of light and 
shadow of the bar pattern is detected. Yet, when the bar pattern is 
laterally displaced on the visible upper side of a potato with respect to 
the bar pattern on the conveyor belt, as shown in FIG. 2, then the 
mentioned periodical signal will be phase-shifted by a certain amount 
corresponding to this elevation as well, and, as shown in FIG. 3, said 
phase difference can amount to several periods. By means of sections 14a, 
14b, and 14c of light stripes 14, the phase difference in integral periods 
is determined, and by means of the position of front edge 14d, the phase 
difference within a period. Whether the detection is effected in the area 
of the narrow sections 14a, of the middle sections 14b, or of the large 
sections 14c is easily determined by determining either the width of the 
scanned stripe or the pulse ratio between light and shadow. This 
arrangement already allows a coarse measure, in the present example in 
three ranges, or in a greater or smaller number of graduated ranges. 
Additionally, the precise phase, i.e. the precise time of entry into the 
illuminated zone during detection can be determined, whence the precise 
elevation of the body can be determined according to the resolution which 
is given by the sensor. The precise resolution in this case comprises e.g. 
eight respective graduations, which is to say that the measuring range is 
divided into "octaves" which are defined by the stripe sections 14a, 14b, 
and 14c, and within which again eight values can be detected. 
Likewise it is possible to position the narrow ends 14a of stripes 14 in 
the area of the observing plane 16' at an elevation of 0 already if it is 
intended to detect even small elevations safely and precisely. 
FIG. 4 shows an example of a possible occurrence of certain measuring 
signals. If the detection is effected in the area of a narrow stripe 
section 14a, an impulse 14a' will appear whose duration corresponds to a 
unit. The rising edge of said impulse designates the precise position of 
the stripe. The position of the stripe pattern, i.e. the elevation of the 
detected portion of a body can be deduced from the impulse duration and 
from the precise time of the impulse beginning. Another impulse 14c' has a 
duration of three units and therefore corresponds to a detection in 
section 14c of the stripe pattern, and again the precise position of the 
bar pattern or the elevation of the detected spot can be deduced from the 
time or the phase of the impulse rise. Another impulse 14b' shows a 
duration of two units and thus corresponds to a middle section 14b of the 
stripe pattern. 
The scanning of the luminous values in diode row 8, i.e. along measuring 
zone 3 on conveyor belt 1 is effected with a high frequency, so that e.g. 
at intervals of the order of 1 mm, all potatoes in the measuring zone are 
monitored. All values detected in the process are stored and afterwards 
used for evaluation, i.e. in order to determine the shape of each potato 
in the plan view and the elevation of each detected point of the potato. 
The greatest detected elevation can then be singled out and used for the 
classification according to the size of the potato. It is also possible, 
however, to effect further calculations by means of the microprocessor and 
to determine the volume and especially the center of gravity of the 
potato. The determination of the approximate center of gravity (the 
underside of the potato cannot be monitored) is significant for setting 
the precise time of ejection by ejecting device 4. According to the 
classification of each potato, a stronger or a weaker ejecting impulse is 
produced, or none at all, so that the potatoes are delivered to different 
locations and are thus sorted. The microprocessor here determines the time 
and the intensity of the ejecting movement, as well as the ejecting finger 
or fingers 19 to be actuated, of course. It has been found that the 
correct detection and storage of the values associated to a determined 
potato entail no particular problems even if a plurality of potatos are in 
measuring zone 3 simultaneously. A certain difficulty could arise if two 
potatoes are directly adjoining in measuring zone 3. In this case, two 
potatos might be taken for one. This difficulty can be encountered due to 
the inclination of stripes 14. It is understood that the above-mentioned, 
considerable phase shifting of the periodical detecting signal occurs when 
the stripe pattern is traversely displaced, which means that during the 
displacement, i.e. during the rise to a greater elevation, a frequency 
variation will occur, namely an increase of the frequency in the case of 
rising flanks of a body and a decrease of the frequency in the case of 
falling flanks of a body. Arrows 24 and 25 in FIG. 3 indicate that in the 
case of a downward displacement of the pattern, i.e. when the elevation is 
increasing, the temporal distance between the detection of the front edges 
of the stripes is greater (arrow 24) than in the case of a contrary 
displacement on the falling flank of the monitored body. It is thus 
possible in this manner to determine the value and the sense of the 
inclination of a flank of a body. It is further possible, for example for 
the monitoring of potatoes, to detect only elevation values occurring in 
flat places, i.e. where the variation of the frequency resulting from the 
detection of the stripes is low or zero. Now, if this frequency suddenly 
jumps from a reduced to an increased value when adjoining potatoes are 
monitored, it can be deduced that two adjacent potatoes are concerned, and 
the measuring signals can be stored and processed accordingly. 
FIG. 5 shows an alternative embodiment of the mask 12, i.e. of the stripe 
pattern. In this case, stripes 14" are no longer stepped but have a 
continuously varying width. Here the elevation might be deduced directly 
from the stripe width, i.e. the pulse ratio of light and shadow at the 
examined spot. However, it may be advantageous in this case as well to 
effect first a coarse division into phase differences of integral periods, 
and within the latter, a fine division, the detected stripe width or the 
detected pulse ratio of light and shadow determining the phase difference 
by integral periods, i.e. the "octave", while the fine division is 
effected by means of the time of entry into a stripe during scanning. 
In the case of the stripe pattern according to FIG. 5, an inclination of 
said stripes is unnecessary if a detection of frequency variations, i.e. 
of flank inclinations of the body and/or a high resolution is not 
required. 
Although the invention has been explained above with reference to a sorting 
device for potatoes, it is understood that any kind of bodies can be 
tested in a corresponding manner, said bodies passing under the measuring 
device on a conveyor belt or being held in a certain position. 
It has been found that it is advantageous to provide a constant background 
lighting in addition to the stripe illumination described above. Thus it 
is avoided that the contours of potatoes which are presently in the full 
shadow of the stripe pattern will be unprecisely monitored. 
The described embodiment with immobile optical elements of the measuring 
device is particularly simple and reliable. However, it would be possible 
as well to direct the image of the measuring zone to a suitable 
transducer, e.g. a photodiode, periodically by means of a rotating or 
oscillating mirror and to evaluate the data of said transducer accordingly 
.