Endoscopic apparatus for flaw detection on a circular knitting machine

The apparatus has particular use in the detection of flaws in articles being knitted on a circular knitting machine and aplies in particular to the manufacturing of footwear. The apparatus comprises a light source, an electro-optical sensor receiving light coming from the light source after the light has been reflected by the knitted fabric, or after it has passed through that knitted fabric. Devices are provided for processing the data produced by the electro-optical sensor. According to the invention, there is provided a linear endoscope fixedly mounted along the rotation axis of the machine, whose viewing cone is matched to a distal lens and is oriented radially towards the knitted fabric. The photosensitive zone of the electro-optical sensor is located at the image plane of a lens located at a proximal end of the endoscope.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for the optical detection of 
flaws in articles as they are being knitted in a circular knitting 
machine, and in particular to an apparatus for detecting flaws in 
footwear, irrespective of whether such flaws arise from faults in the 
knitting process or from various other origins e.g. the presence of yarns 
that are too thick or too thin, or from faults in the plaiting. 
2. Prior Art 
Flaw detection systems for fabrics based on optical inspection have already 
been proposed in the prior art. Such systems are described e.g. the 
publication "L'Industrie Textile" No. 1067 pages 277 to 279. They 
comprises a luminous source, an electro-optical sensor receiving light 
from the optical source after it has either been reflected from or passed 
through, the fabric, and means for processing data from the 
electro-optical sensor. The fabric is laid out flat and made to pass in 
front of tubular fluorescent lamps powered by a DC voltage source in order 
to illuminate the fabric throughout its width. Electro-optical cells are 
aligned along a row and extend throughout the width of the fabric. The 
lamps and the cells are either both on the same side of the fabric or on 
either side. In the first case, the cells measure the light reflected by 
the fabric. In the second case they measure the light passing through the 
fabric. Any change in the fabric's structure or surface condition 
resulting from flaws in the weaving or in the dressing brings about a 
change in the current of the photoelectric cell concerned, so triggering a 
signal which is amplified by an amplifier and sent to an analog-digital 
converter for subsequent processing. By using adjustable thresholds it is 
possible to set the level of the signal that qualifies as a flaw 
indication. 
The flaw inspection systems for fabrics described in the above article are 
not readily amenable to the detection of flaws in articles being knitted 
in a circular knitting machine and even less so in the case of knitting 
machines having a small diameter, as used in the manufacture of items of 
footwear. The reason is that in such machines, the knitted fabric is 
produced by a rotation of the knitting head and is then passed around a 
mandrel such that the part of the knitted fabric open to inspection is 
very difficult to access. 
SUMMARY OF THE INVENTION WITH OBJECTS 
The present invention has for object an optical device for flaw detection 
that overcomes the above-mentioned drawbacks and makes it possible to 
detect flaws in articles being knitted on a circular knitting loom, 
including small-diameter knitting looms as used in the manufacture of 
items of footwear. In common with other known devices, the apparatus of 
the present invention comprises a luminous source, an electro-optical 
sensor receiving light from the source after it has been reflected by the 
knitted fabric, or alternatively after it has passed through the knitted 
fabric, and means for processing data produced by the electro-optical 
sensor. 
According to the invention, the apparatus comprises a linear endoscope 
fixedly mounted along a rotation axis of the loom and having a viewing 
cone matched to a lens at a distal end of the endoscope and oriented 
radially towards the knitted fabric. Furthermore the photosensitive zone 
of the electro-optical sensor is situated substantially in the image plane 
of a lens located at a proximal end of the endoscope. 
An endoscope is a device, used e.g. in the medical field, which comprises 
an optical system having a succession of lens elements whereby an optical 
inspection can be performed at a precise zone. Endoscopes usually have 
means for bringing a luminous flux after reflection from the inspection 
zone and for retrieving the part of this flux reflected from the zone. The 
distal end corresponds to the end placed close to the inspected zone, 
while the proximal end corresponds to the end from which the incident 
luminous flux is sent and/or where the reflected flux is returned. 
Thus, the endoscope used in the context of the present invention is 
complementary to the optical flaw detection apparatus and plays an 
essential role in exploring a precise part of the knitted fabric, this 
part preferably being close to the needles associated with the machine, 
i.e. where the knitted fabric is in a uniformly stretched state. The 
endoscope also has the function of re-directing the luminous flux to a 
more accessible part of the machine after inspection of the knitted 
fabric. 
According to a first embodiment, the luminous source comprises light 
producing means and an optical fiber terminated by a totally relecting 
prism, the optical fiber being fixedly mounted within a cylinder of the 
machine and close to a wall thereof, and the prism being radially oriented 
towards the distal lens of the endoscope. In this first embodiment, the 
luminous flux received by the electro-optical sensor corresponds to light 
filtered by the corresponding zone of the knitted fabric. This 
transmissive illumination mode makes it possible to detect certain types 
of flaws--holes, thin or thick zones--by back lighting. 
In a second embodiment, the endoscope is used conventionally whereby the 
luminous flux from the light producing means is conveyed within the 
endoscope from its proximal end to its distal end. According to this 
embodiment, the endoscope serves both as a luminous source and as a means 
for conveying the light reflected from the inspected zone of the knitted 
fabric. This reflective illumination mode makes it possible to reveal 
flaws that have a good luminous contrast: spurious presence of colored 
strands, and faults in the pattern or plaiting. 
Preferably, the two above-mentioned embodiments are combined on a same 
knitting machine. In this case the inventive apparatus comprises two light 
sources, one for illumination by transmission and the other for 
illumination by relection. Switching means are provided for switching 
alternately from one source to the other. In this way, the endoscope 
alternately conveys a transmitted luminous flux and a relected luminous 
flux to the electro-optical sensor, so making it possible to detect all of 
the above-mentioned types of flaw. 
The means for processing the data from the electro-optical sensor comprise 
synchronization means capable of sampling at a frequency that is slaved to 
the rotation of the knitting machine, such that successive inspection 
zones on the knitted fabric overlap both in the direction of the columns 
and in the direction of the rows. This guarantees a total inspection of a 
tubular knitted fabric without any loss of information. 
The synchronization means can e.g. comprise a count-up encoder supplying a 
predetermined number of pulses during each rotation of the machine for 
sampling the analog signal delivered by the electro-optical sensor, a 
proximity detector supplying a fast signal at the beginning of a row, a 
detector synchronized with a knitting program for resetting the apparatus 
at the first stitch of the knitted article, and means for informing the 
apparatus of the beginning and end of the alternating movement of the 
machine, corresponding to the heel and toe sections in the case of an item 
of footwear. The latter means may be either sensors or signals produced as 
a function of the knitting programme. The electro-optical sensor may e.g. 
be a photodiode having a response curve matched to the wavelength of the 
luminous source.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A knitting machine for socks, e.g. of the double-cylinder type, is depicted 
very schematically in FIG. 1. The hatched portions correspond to a cross 
sectional view of the machine upper cylinders 1 and lower cylinders 3. 
These cylinders have longitudinal grooves in which needles are slidably 
engaged. The needles have a symmetrical configuration for a 
double-cylinder machine, can operate in the lower cylinder, as shown in 
FIG. 1, or alternatively in the upper cylinder. 
Cylinders 1 and 3 are controlled in rotation around axis 5. In the course 
of knitting, the sock having a tubular shape 6, is suspended from the 
needles 2 and drawn inside the cylinder 1 by an extraction device (not 
shown). 
The endoscope 7 is rectilinear and fixedly mounted along the rotation axis 
5 of the knitting machine. Its distal end 8 is located at the top portion 
of cylinder 1. Its proximal end 9 is outside the body of the machine and 
situated above the upper cylinder 3. 
An optical fiber 10 is fixedly mounted parallel to an interior generating 
line of the cylinder 1. It is terminated by a totally reflecting prism 11 
located at its upper end. Its lower end (not shown) is provided with a 
first luminous source. 
The exit face of the prism 11 confronts a lens 13 at the distal end of the 
endoscope 7 such that the luminous flux from the first source is guided 
along the optical fiber 10 and either reflected by the prism 11 or 
directed to the distal lens 13. 
At a portion close to its proximal end 9, the endoscope comprises a housing 
4 containing a second light source and a light guide connected to the 
rectilinear portion of the endoscope 7. The luminous flux from this second 
source is guided by the internal optical system of the endoscope 7 and 
exits from the latter through the distal lens 13 perpendicularly to the 
axis 5 of the endoscope 7, where it defines a light cone 15 having a 
30.degree. cone angle. 
The electro-optical sensor 16 is a photodiode fixed to the proximal end 9 
of the endoscope 7, as shown schematically in FIG. 2. The proximal end 9 
of the endoscope is terminated by a hollow cylindrical endpiece 17 whose 
internal surface is provided with a threading 18. The electro-optical 
sensor 16 is lodged inside a cylindrical mount 19 whose external surface 
is provided with a threading 20 made to co-operate with the threading 18 
of the end piece 17. Threadings 18 and 20 have a micrometric pitch and 
allow the position of the mount 19 to be a fine-adjusted relative to the 
end piece 17. 
The external surface 21 of the electro-optical sensor has an area of 
approximately 5 mm.sup.2, corresponding to the photosensitive zone of the 
electro-optical sensor 16, and is accurately positioned in the image plane 
of the endoscope's proximal lens 22. 
The front face of the mount 19 of the sensor 16 is closed by a window 23 
that is transparent to luminous flux. 
The electro-optical sensor 16 is connected to the processing means 
referring to FIG. 5, whose principle features shall now be explained. At 
each rotation of the knitting machine, a count-up encoder 27 delivers a 
number of pulses for sampling the analog signal delivered by the 
electro-optical sensor 16, the number of pulses being a function of the 
type of machine and corresponding e.g. to one sampling per half-column of 
stitching. A proximity sensor 28 delivers a fast signal indicating the 
beginning of a row. A sensor 29, synchronized with the knitting program, 
defines the starting point of the sock and resets the system at the first 
stitch of each sock. Another pair of sensors 30,31 indicates the beginning 
and end of the machine's alternating motions corresponding to the heel and 
toe portions of the socks. The complete processing system makes it 
possible to accurately locate the cartesian co-ordinate of each sampled 
signal. 
The two light sources are connected to a switch alternating the power 
supply to each source. 
The operation of the apparatus shall now be explained. For an easier 
understanding, the present description shall cover the case where only the 
luminous flux coming from the first source is used, this flux being 
transmitted through the knitted fabric. However, the following can also 
apply to a flux coming from the second source and reflected by the knitted 
fabric, as well as to an alternation of these two luminous fluxes. 
During knitting, the knitted fabric 6 descending from the needles 2 is 
uniformly stretched to enter the space comprised between the prism 11 and 
the distal end 8 of the endoscope 7. The luminous flux 24, produced by the 
first luminous source and guided by the optical fiber 10, is reflected by 
the prism 11, whereupon it reaches the distal lens 13 of the endoscope 
after having passed through a given zone of the knitted fabric 6. 
Accordingly, the flux received by the distal lens corresponds to the image 
of the inspected zone of the knitted fabric 6. 
The dimension of the inspected zone, i.e. the illuminated area, is adapted 
to the fabric's gauge to obtain an optimum level of filtering and 
contrast. As a typical value, an area of 6 mm.times.6 mm would correspond 
to a gauge of 14. Preferably, the area corresponds to a square whose sides 
contain 4 stitches. 
The luminous flux received by the distal lens 13 is conveyed along the 
endoscope 7 up to the image plane of the proximal lens 22, corresponding 
to the plane where the external surface 21 of the electro-optical sensor 
has been positioned. The electro-optical sensor 16 thus receives the real 
image of the inspected zone. 
Because of the rotation of the cylinder 1--and thus the knitted fabric 
6--as well as the gradual lowering of the knitted fabric 6 towards the 
interior surface of the cylinder 1 during the knitting process, the 
luminous flux continuously illuminates the knitted fabric in a helical 
fashion. The count-up encoder causes the continual flow of data produced 
by the electro-optical sensor 16 to be cut up into a succession of 
instantaneous portions each corresponding to the real image of an 
inspected zone. The number of sampling pulses per rotation of the machine 
is a function of the loom dimension, being e.g. 400 for a machine having 
200 needles. 
The number of sampling pulses and the dimensions of the inspected zone are 
determined such that there is a partial overlap of the successively 
inspected zones, both in the row and column directions. 
Accordingly, the processing means connected to the electro-optical sensor 
16 are active during the knitting of a sock for receiving and storing 
successive items of data each corresponding to a specific area of the 
sock. These data collectively represent the mapping of the light 
transmitted through the sock. Each instantaneous data, termed pixel, is 
indexed by the row number and the half-column number. The intensity of 
each pixel, i.e. the grey level, is proportional to the quantity of light 
passing through the inspected zone. The origin of the pixel coordinates is 
defined by means of different sensors providing the synchronization 
function. Each identified pixel is encoded by a grey level and is stored 
as image data by the processing means. 
The grey level is encoded e.g. using a fault-free reference. FIG. 3 is a 
curve depicting the histogram of the grey level for the real image of a 
pixel for a fault-free reference sock. Using the histogram of the 
reference image, there is determined the minimum and maximum grey level 
values (25, 26) which define the characteristic limits of the image for a 
given probability, these values lying between a grey level corresponding 
to black (equal to zero) and a grey level corresponding to white (equal to 
255). An encoding matrix is shown in FIG. 4 in the form of a diagram 
having the input grey level, corresponding to the data that is delivered 
by the electro-optical sensor 16, indicated along the X-axis, and having 
the output grey level, corresponding to the data that is reproduced and 
stored, indicated along the Y-axis. The grey level of the pixel, comprised 
between the minimum value (25) and maximum value (26) are materialised in 
the form of a white spot. The grey levels outside that interval are 
considered as flaws and are materialised in black. This encoding is 
effected in real time. After the knitting of the reference sock there is 
obtained a totally white image except for those points corresponding to 
the image levels that are not considered in establishing thresholds (25) 
and (26), and which appear in black. 
When knitting all other socks, the measurements are encoded in real time in 
accordance with that same matrix. When the knitting of each sock under 
inspection is completed, there is generated an image that is filtered 
relative to the reference image. After this filtering, the black spots 
appearing on image make it possible to accurately determine the existence, 
nature and position of possible faults. With the use of suitable means, 
the identification of these faults enables a subsequent decision to be 
taken, e.g. the immediate stoppage of the machine in case of a detection 
of a continuous vertical flaw, or a repeated flaw, or a conditional 
stoppage upon detection of a horizontal flaw. 
This control phase can be implemented in real time with decisions taken 
immediately during the knitting process for a sock. In that last case, the 
data processing system has a dual role--it simultaneously acquires a given 
image and processes the preceding image. 
The present invention is not limited to the embodiments described in the 
foregoing, which were given purely as examples, but covers all other 
variants. In particular, the image acquisition at the distal end 9 of the 
endoscope 7 can be obtained by electro-optical sensors other than the 
photodiode e.g. by an area-array CCD or linear-array CCD.