Monitoring a deposit on a travelling web

Method of monitoring a line of adhesive deposited on a travelling paper web to check for accidental omission of adhesive wherein the adhesive has a distinguishing relative permittivity and is sensed by a capacitive proximity sensor. The web may be plugwrap material for cigarette filter rod, the adhesive being provided to anchor filter tow in position when the plugwrap is wrapped around it. The method avoids various disadvantages associated with other automatic monitoring devices such as optical devices and infra-red devices.

TECHNICAL FIELD 
The invention was initially designed to monitor continuously a thin line of 
adhesive deposited on a travelling web of paper which is subsequently 
wrapped around a tow of filter material to form a filter rod for 
cigarettes. Such paper is traditionally white and typical adhesives have 
the same colour which makes it difficult to detect any accidental failure 
by the adhesive applicator. The deposited adhesive is used to anchor the 
filter material in position when the paper web is wrapped around it to 
form the continuous filter rod. Should the adhesive fail to be deposited 
on the paper web a considerable length of faulty filter rod may be 
produced by a high speed rod-making machine before the fault is detected 
and remedial action can be taken. 
There are various kinds of sensors available but for one reason or another 
these are not suitable for monitoring the deposited adhesive. One kind of 
sensor uses reflected light, another uses light transmitted through the 
paper, but their respective operations suffer from the disadvantages that 
it is difficult to distinguish the white adhesive on its white carrier web 
and furthermore the lighting in the operating region of the sensor may 
vary, e.g. by reflection from adjacent machine surfaces or from the 
presence of personnel. Another form of sensor uses infra-red light but 
this too can be influenced by any adjacent heat source. 
The present invention arose from the realisation that if the adhesive 
deposited on the paper web is selected to have a relative permittivity 
(formerly called dielectric constant) which is sufficiently high to 
distinguish it from the carrier web a capacitive sensor known per se can 
be used to detect any omission of the adhesive without incurring any of 
the disadvantages referred to above in connection with other kinds of 
sensor. 
The present invention is not however limited in its application to adhesive 
applied to a plugwrap material. 
INVENTION 
Method of monitoring a liquid substance applied to a travelling web of 
material to check for accidental omission of the substance, which method 
comprises: 
(a) applying to a travelling web of material a liquid substance which has a 
relative permittivity sufficiently high to distinguish it from the web 
material, 
(b) passing the web carrying the substance through the field of a 
capacitive proximity sensor forming part of an electric circuit which is 
adapted to initiate a desired response where an omission of the adhesive 
is sensed by the sensor.

Referring to the drawings a web of permeable paper 10 for use as plugwrap 
in a machine for making cigarette filter rods is drawn past the nozzle 11 
of an adhesive applicator which deposits on the web a continuous thin line 
of liquid adhesive 12 which extends parallel to the opposing sides edges 
of the web. The adhesive 12 is provided to anchor cigarette filter 
material to the plugwrap when the latter is subsequently wrapped around 
the filter material to form continuous filter rod. The web carrying the 
adhesive 12 is drawn past the sensing head of a capacitive proximity 
sensor 13 but without touching it. A liquid adhesive 14 for the lap or 
seam seal of the wrapped plugwrap is deposited on the web 10 in a 
continuous thin line parallel to the line of anchorage adhesive 12 and 
then the web carrying the two lines of adhesive is guided to enter a 
garniture on a known rod-making machine (not shown) which wraps the web 
around a tow of filter material to form a continuous filter rod having a 
longitudinal lap seal. The adhesive 12 which is now on the inner surface 
of the tubular plugwrap serves to anchor the two in position. The 
continuous filter rod is then cut transversely into lengths of multiple 
rod sections which pass to a fluted transfer drum. For the purpose of this 
invention the drum is provided with an air jet ejection system for faulty 
rods, i.e. those whose plugwrap lacks the required anchorage adhesive. The 
acceptable rods are then discharged onto conveyor trays which carry them 
to a cigarette making machine. An example of such a filter rod-making 
machine which may be adapted to carry out the present invention is the 
HAUNI KDF2 machine. The paper web and the adhesive 12 have the same colour 
to avoid staining the web. Conventionally the colour is white. The 
adhesive 12 is preferably polyvinyl acetate in aqueous emulsion, its 
composition being PVA solids 40-70% by weight with the remainder water. An 
alternative adhesive is a starch adhesive made with water. Whatever the 
adhesive chosen it must have a relative permittivity which is sufficiently 
high to distinguish it from the web material and thus facilitate the 
operation of the sensor. 
The sensor 13 for sensing an absence or omission of adhesive 12 which may 
be total or partial, e.g. an interruption in a line of such adhesive, has 
a sensing head located close to but spaced from a guided straight path of 
the web 10 and directed at where the line of adhesive 12 should appear. 
The sensor which forms part of electrical circuitry is used to generate an 
electro-static field in the target area which crosses the path of the web. 
When a substance solid or liquid enters this field the capacitance is 
changed and a switch is operated. Consequently the sensor 13 can be set to 
sense the accidental absence of the water-containing adhesive. 
A suitable capacitive proximity sensor is marketed by Hymatic Industrial 
Controls Limited of Worcestershire under the registered trade mark 
PROXISTOR. With the particular sensor the capacitive figures listed below 
illustrate how the water-containing adhesive can affect the field of the 
sensor 13. 
______________________________________ 
Material Capacitive 
______________________________________ 
Mild Steel 1.0 
Water 0.65 
PVA 0.5 
Plugwrap (paper) 
&lt;0.1 
______________________________________ 
Referring to FIG. 2 the sensor 13 which is connected to power lines 20, 21 
includes a proximity switch 22 shown in its normal open position before 
operation. The sensor is connected to a first relay 23 for opening a first 
pair of contacts 24 which are biassed to a closed position and is also 
connected via power line 20 to one of the contacts 24. A second relay 26 
for closing a second pair of contacts 25 which are separate from the first 
pair and are biassed to an open position is connected to a solenoid for a 
supply valve in the adhesive applicator 11 via lines 27, 28. Finally, two 
lines 29, 30 one connected to a contact 25 the other to the power line 21 
lead to a relay (not shown) for actuating the air jet system for ejecting 
faulty filter rod from the rod-making machine which receives the web 10 
after deposit of the lap seal adhesive 14. The sensor 13 is mounted on a 
bracket secured to the rod-making machine with its sensing face parallel 
to the web path. The sensing face is spaced from the web path, e.g. by 2 
mm, to avoid physical contact with the adhesive. 
The above-described monitoring system may be operated as follows. A motor 
for driving the web 10 is started up and then the supply valve in the 
applicator is opened to allow a pumped supply of liquid adhesive 12 to be 
deposited by the nozzle 11 on the web travelling past it. This operation 
of the solenoid to open the supply valve causes the second relay 26 to be 
energised and close the contacts 25. Thus both sets of contacts are now 
closed. As soon as the sensor 13 which is scanning the web 10 senses the 
deposit of adhesive 12 its proximity switch 22 is automatically closed, 
the first relay 23 becomes energised to open the contacts 24 and thereby 
break the circuit for the rod ejector mechanism. If and when the sensor 13 
senses an interruption in the line of deposited adhesive 12, i.e. a lack 
of future internal anchorage for the filter tow, its proximity switch 22 
is automatically opened, whereupon the relay 23 becomes inoperative, the 
contacts 24 are freed to close and the relevant circuit is energised via 
line 29 to cause the now faulty filter rod to be ejected from the 
rod-making machine at a point upstream of its normal discharge. Thus the 
ejector system only operates when both the proximity switch 22 and the 
adhesive supply valve are open. 
The invention has proved so reliable that it is sufficient to have only a 
single line of adhesive 12 for anchorage of the filter tow to its 
plugwrap. 
The capacitive proximity sensor has the advantages that it can be located 
out of contact with the travelling web, thereby increasing the scanned 
area and avoiding contact with the adhesive, it can be located on either 
side of the web, its operation is not affected by proximity of a metallic 
and possibly vibrating rod-making machine and it is not affected by 
ambient light or by an adjacent heat source.