Web inspecting method

A web e.g. a fibre web as used in the manufacture of textiles is inspected e.g. for thickness uniformity or the like by transillumination, where the web is subject to ambient illumination, the transillumination being effected in modulated fashion whereby to distinguish measuring illumination from ambient illumination. Modulation may be effected by time-chopping a beam or by a narrow spectral band filter to improve signal-to-noise ratio.

This invention relates to methods and apparatus for monitoring webs of 
material and particularly travelling webs such as paper and textile webs. 
Webs are monitored for a variety of reasons, including colour, reflectance, 
transmittance and so on, which might inter alia indicate uniformity of dye 
application or thickness or homogenity of web structure. 
Among problems experienced in this field of endeavour figure 
ambient light, which may be variable, and affect readings on a temporal or 
spatial basis as the intensity of ambient light changes over time or by 
for example shadows falling across the web; 
non-uniformity of the distribution of light over the web-e.g. in some prior 
art proposals, fluorscent tubes have been used presupposing a uniform 
distribution of light emission end-to-end, now found to be inadequately 
uniform for many applications; 
changes in light emission from the light source over time, with fluctuating 
voltage supply or ageing of the source. 
The present invention provides means for solving the first of these 
problems and further inventive features deal also with the other problems. 
This invention comprises a method for inspecting a web of material by 
transillumination, where the web is subject to ambient illumination, 
comprising illuminating the web in modulated fashion whereby to 
distinguish measuring illumination from ambient illumination. 
The method may be adapted for monitoring web density by measuring the 
reduction in light intensity of light passed through the web. The light 
may be passed twice through the web before the measurement--this, provided 
the light passes twice through the same place on the web, may enhance the 
sensitivity of the measurement. 
Light from a source, which may be point-like or extended, may be traversed 
over the web, as by reflection from a moving mirror. Such a mirror may be 
rotated or oscillated so as to traverse the light from edge to edge of a 
travelling web and may be so rotated or oscillated, for example, by a 
stepper motor or by a d.c. motor whose position is monitored e.g. by a 
shaft encoder. 
Light passing through the web may be reflected back towards the source by a 
reflector behind the web. Such reflector may be a retroreflecting screen 
such as may be made by reflecting paint incorporating ballotini. 
Light from the source may be measured by a single detector. 
Variability of the intensity of the modulated illumination may be 
compensated for by detecting illumination therefrom unaffected by the web. 
Light passing through the web may be reflected back through the web by 
reflector means behind the web, the reflector means extending beyond the 
web so as to reflect to a detector modulated illumination which has not 
passed through the web. 
The modulated illumination may be with visible light or infra red light and 
may be by a laser, when modulation may be effected by switching the laser 
on and off or by using a pulsed laser or by varying a laser output, as by 
using a modulator element such as a beam chopper or by using a narrow 
spectral band filter to improve signal-to-noise ratio, this latter filter 
technique implying perhaps a wider definition of modulation than is 
customary. 
The invention also comprises apparatus for inspecting a web of material by 
transillumination, where the web is subject to ambient illumination, 
comprising modulated illumination means for transilluminating the web and 
detector means adapted to detect illumination from the web and distinguish 
the modulated illumination from ambient illumination. 
The method may be carried out by apparatus which may comprise laser means 
as said modulated illumination means directing a beam of light at a 
rotating or oscillating mirror through a beam-splitter, the mirror 
directing the beam to traverse across the width of a travelling web behind 
which is a reflector directing the beam passing through the web and 
redirecting it back towards the mirror whence to the beamsplitter which 
directs the thus reflected light to a detector, the detector being adapted 
(as by being synchronised) with respect to the laser means so as to be 
able to distinguish the modulated illumination from the ambient 
illumination.

The drawings illustrate a method and apparatus for inspecting a web 11 of 
material by transillumination, where the web 11 is subject to ambient 
illumination. 
Such inspection might be required, for example, in textile or paper making 
operations. In a textile fibre preparation plant, a card web intended for 
making yarn or a cross-folded card web intended for making non-wovens may 
be required to be monitored for regularity of area density. Such webs can 
be so monitored by transillumination, information about the area density 
being derived from the extent to which light shone through the web is 
attenuated. It is found that visible light is a good indicator of the area 
density of thin webs while for thicker webs in which most, say 80% at 
least, of the light is blocked by fibres, infra red radiation which passes 
through the fibres can give a similarly good area density indication. 
Ambient illumination, however, interferes with such measurements, and it is 
difficult to exclude it totally by shielding. 
In accordance with the invention, the web 11 is illuminated in modulated 
fashion by a light source 12 whereby to distinguish measuring 
illumination, from the source 12, from ambient illumination. 
The method is adapted for monitoring web density of a travelling web 11, by 
measuring the reduction in light intensity of light passed through the web 
11. As illustrated, the measuring light is passed twice through the web 11 
before the measurement. Whilst this is, in the illustrated arrangement, 
highly convenient, it also results in increased sensitivity. 
The light source 12 is a conveniently point-like source, though an external 
source could also be used, and light from it is traversed over the web 11 
by reflection from a moving mirror 13. The direction of web travel is 
indicated by the arrow 14--the light beam 15 is traversed from edge to 
edge of the web 11 by the mirror 13 oscillating about an axis parallel to 
the arrow 14. The mirror, which could then be multi-faceted, could equally 
well rotate and in fact this may be a simpler mechanical arrangement. 
The mirror 13 is a lightweight but rigid mirror mounted on the shaft of a 
stepper motor 16. Such a motor desirably has 26,000 steps per revolution, 
more desirably twice that number, and is capable of some 7,000 revolutions 
per minute. The motor 16 is controlled so as to oscillate the mirror 13 
over an angle such as oscillates the beam 15 right across the travelling 
web 11 and to some small extent either side thereof. Whilst a stepper 
motor is convenient from a control point of view, if high traverse speeds 
are required a d.c. motor or galvanometer-type drive, perhaps with a shaft 
encoder for position determination, could be substituted. 
The beam 15 is reflected back towards the point-like source 12 by a 
reflector 17 behind the web 11. The reflector 17 is a retroreflecting 
screen utilising microbeads so that incident light is reflected back in 
the direction whence it came. Thus light from the oscillating mirror 13 is 
reflected straight back thereto no matter at what angle it strikes the 
screen 17. Light striking the oscillating mirror 13 from the screen 17 is 
reflected back towards the source 11, the oscillating mirror not having 
moved perceptibly in the short time it takes for the light to leave it and 
return to it. In fact, it may be desirable in many instances if the motor 
16 moves the mirror 13 stepwise so that the beam dwells a short period of 
time for a measurement to be made. 
A beamsplitter 18 is situated in the path between the source 12 and the 
mirror 13. This allows light from the source 12 to reach the mirror 13 and 
directs light returned from the mirror 13 to a silicon photodiode detector 
19. A polarising beamsplitter and a quarter wave plate can be used to 
improve the amount of light reflected to the detector. 
The light source 12 is a helium-neon laser, which gives a narrow coherent 
beam. A laser of 0.5 mW power will be adaequate for most fibre webs; 
higher powered lasers, e.g. 1.5 mW, will be more suitable for heavier 
textiles. Infra red lasers can be used instead of visible light lasers for 
dense webs where transmission is then predominantly through rather than 
between fibres. 
Some lasers are pulsed lasers, i.e. inherently delivering light 
intermittently. Such are of course inherently modulated. Where a 
continuous laser is used, it can be modulated by switching it on and off 
or by varying its intensity in some way, or by a beam chopper. The rate of 
pulsing in whatever fashion should be such as, where individual 
measurements are being made at "dwell" points in the traverse across the 
web, pulsing or modulation must be apparent during such measuremnts. This, 
of course, means that switching the laser on and off may best be effected 
electronically. 
After attenuation by its passage through the web 11, the modulated beam 15 
is sensed by the photodiode 19 and an electrical output therefrom is 
analysed by a microprocessor or computer 21, FIG. 5. For a regular 
mark/space modulation, the electrical signal will contain a set of low 
"space" levels corresponding to ambient illumination alone. It is a simple 
matter for the computer to separate the two levels and average each of 
them, subtract the low level average from the high level average and 
output the result in numerical form. Such should then be independent of 
the level of ambient illumination. 
More sophisticated modulation may be employed where ambient illumination 
may itself contain oscillatory components, as for example from fluorescent 
lighting. Here a pattern of on/off periods can be imposed on the source 11 
and electronically synchronised in the microprocessor or computer 21--the 
latter, in fact, may generate signals controlling the light source 11. 
The microprocessor or computer 21 may also provide the driving pulses for 
the stepper motor 16, so that the latter can readily be adjusted as to 
traverse width, speed and frequency by keyboard input and system software, 
there being if required a motor drive encoder 22 for such purpose (FIG. 
5). 
The microprocessor or computer 21 may well then "know" where on the web the 
mirror 13 is directing the beam 15, or this information may be derived 
from or verified by a motor shaft encoder 23. 
The beam location information is then combined with the photodiode 
information to give an obscuration distribution across the web 11 and such 
can be represented graphically on a VDU 41 (FIG. 5) as in top trace 42a 
which is essentially an area density distribution transversely of the web 
11. 
Coupling this information with web travel speed information derived e.g. 
from a component of the web producing or processing machinery allows a 
second graphical representation to be made on the VDU this time of area 
density distribution lengthwise of the web-trace 42. 
All of this information can, of course, be differently displayed if 
desired, for example, numerically, and may be output in hard copy on a 
printer 42. 
The light source 11 may be variable in intensity or other factors may 
change with time such for example as the response of the photodiode 19 or 
the performance of other optical components of the system. It is arranged, 
to compensate for such variability, that the system is calibrated from 
time to time, and this can for greatest assurance of accuracy be done 
before and after each traverse of the beam 15. As noted above, the screen 
17 extends beyond either side of the web 11 so that the beam 15 can "see" 
the screen 17 without the intervention of the web 11. An intensity 
measurement at these extreme traverse positions sets the base level of 
illumination for the traverse against which the observation caused by the 
web can be evaluated. The calibration could, of course, be carried out on 
light captured more directly from the source without having to provide the 
extension of the screen 17 beyond the web--the latter, however, has the 
advantage that the light has travelled more or less the same distance 
through the same atmosphere for both actual and calibration intensity 
measurements. 
A problem to be dealt with in the illustrated arrangement is best seen in 
FIG. 2 where three beam 15 positions are shown. The left-hand beam 15 is 
the calibrating beam at extreme traverse. The right-hand beam is at the 
mid-line of the web directly beneath the oscillating mirror 13. The middle 
beam is seen to be angled with respect to the right-hand beam and the path 
length D.sub.1 through the web 11 is greater than the corresponding length 
D.sub.0 of the right-hand beam. The relationship would appear to be a 
simple trigonometrical one, but this must be approached with caution as 
the nature of the web might complicate matters-because of fibre 
orientation, light might penetrate preferentially at some angles. So 
experience with different types of web might suggest that calibration in 
this regard also be effected. 
The calibration may take the form of preliminary measurements on particular 
web types which give rise to a correction function which can be applied to 
the photodiode output dependent on the instantaneous beam angle or 
position. 
The problem posed by the angular dependence of the beam path length through 
the web 11 can of course be avoided by ensuring that the beam always 
passes through at the same angle, preferably perpendicularly. This can be 
done by an arrangement as illustrated in FIG. 6 where the beam 15 from the 
light source 11 is deflected by the mirror 13 upwardly to a parabolic 
mirror 51 whence it is reflected vertically downwardly no matter what the 
position of the mirror 13. The reflector 17 behind the web 11 can then be 
a simple specular reflector. Such an arrangement might be cumbersome in 
practice. However, the parabolic mirror 51 might be replaced by a Fresnel 
mirror which will not occupy the same vertical extent as a truly parabolic 
mirror, or a Fresnel lens could be interposed between the mirror 13 and 
the web 11, to refract the beam so as to pass perpendicularly through the 
web 11. 
These arrangements would appear cumbersome in practice by comparison with 
the retro-reflecting screen arrangement principally described. 
FIG. 3 illustrates a clearing arrangement for the screen 17 in which an 
air-blowing nozzle 31 is traversed from end to end thereof by a rodless 
air cylinder 32. In a fibre plant, fibre and fly will tend to drop from 
the web 11 which would, were it not cleaned away regularly, impair the 
performance of the system, particularly if the accumulation were different 
at the extreme mirror positions so that the traverse-on-traverse 
calibration procedure became unreliable. 
FIG. 4 illustrates another configuration in which the screen 17 is above 
the web 11 and therefore in a position in which fly will not tend to 
accumulate on it. The laser 12, mirror 13 and associated equipment is 
beneath the web 11, if necessary in a pit, if there is insufficient space 
between web and floor to accommodate it. A simple blower 33 can keep fly 
away from this equipment, or the pit may be held at a slight over-pressure 
as by a fan 34 so that the net airflow through its aperture is outwards. 
With a 7,000 rpm stepper motor and a traverse angle of 120.degree., which 
means the mirror 13 turns through an angle of 60.degree. for each beam 
traverse, traverse frequencies of some hundreds of traverses per second 
can be contemplated. With a one meter web width a traverse angle of 
60.degree. (30.degree. of the mirror 13), and a 26,000 step-per-revolution 
motor 16, there can be up to about 850 separate dwell points between steps 
made. In practice it is probable that one measurement per centimeter would 
be quite sufficient for most purposes, but clearly with a 52,000 step 
motor, resolution can easily be brought down to millimeter levels. 
If a web is traversed in say 0.01 seconds and, say 100 measurements made, 
each measurement must take place in somewhat less than 0.1 milliseconds. 
The modulation rate must be faster than this, of course, and the light 
source must pulse, therefore, at a rate in the order of 10.sup.5 Hz at 
least. 
It is not necessary to use a laser as the light source 12, of course. A 
strobing discharge lamp with suitable optics could be used, and also a 
quartz-halogen filament lamp with mechanical strobing as by a beam chopper 
which could, also, of course, be used with a laser. But a laser clearly 
has advantages in terms of beam fineness and intensity. 
For a web travelling at one meter per second, traversing the beam every 
0.01 second results in measurements being made at centimeter intervals 
along the web. 
Instead of modulating the measuring illumination by varying its intensity 
with time, modulation could also be effected in the sense of using 
illumination with a special spectral signature, which may be produced by a 
narrow spectral bandwidth filter. Applying such a filter to the detector 
so as to match the characteristics of the source (whether those 
characteristics are inherent or produced by a like filter) will improved 
the signal-to-noise ratio facilitating detection even in ambient light 
conditions which would otherwise interfere.