GB2087544A

GB2087544A - Coating detector

Info

Publication number: GB2087544A
Application number: GB8033471A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-10-16
Filing date: 1980-10-16
Publication date: 1982-05-26
Also published as: GB2087544B

Abstract

Apparatus for detecting uncoated sections of a continuously moving web of coated sheet material, such as coated paper, comprises means, such as an infrared LED 1, for directing onto the web 6, a beam of radiation 3 modulated at a predetermined frequency, detector means such as phototransistor 2, for generating an electrical signal dependent on the intensity of radiation 4 reflected from the web, and detection circuitry receiving the electrical signal through filter means and operable to provide an output signal when the magnitude of the filtered signal exceeds a preset value. The filter means is arranged so that the detection circuitry does not respond to short-term variations in the intensity of the reflected beam due to non-uniformity of finish of the coating on the web. Preferably the modulation frequency of the incident radiation is at least an order of magnitude greater than that of any transient changes of intensity in the region of the detector means. <IMAGE>

Description

SPECIFICATION Coating detector In the manufacture of coated papers it is important to detect skip-coating, i.e. uncoated sections.

On coating machines lamp plus photosensor devices are often used to sense and then mark the uncoated sections by actuating spray guns or tab inserters.

These sensors rely on the considerable difference between the wet coating and the dry uncoated web, when set up to differentiate between specular and diffused reflection.

Coated reels are often further processed on slitters -- rewinders at high speed. On such machinery short uncoated sections can be easily missed, even when the reel was clearly spraymarked or tabbed.

Further on some products it is difficult to see the difference between the coated and uncoated side.

Further when the coated side is shiny, the nonuniformity of small adjacent surface areas can be quite large, resulting in distinct changes in specular reflection, such changes being greater than the change due to coating.

The coating detector according to my invention can detect reliably skip-coating on all types of products -- overcoming the difficulties described above.

According to my invention a light emitting diode, preferably in the infrared radiation range, forming a narrow beam, impinges upon the surface of the coated side of a fast running web, said beam forming a shallow angle with the plane which is tangential at the point of contact between said beam and surface.

The mirror image of said beam, i.e. the specular component of the reflected beam, is sensed by a silicon phototransistor, or photo-diode.

The current through the light emitting diode (LED) is modulated by a signal S the frequency F of which is at least an order of magnitude greater than any transient light change which occurs in the environment of the detector.

For example to avoid disturbance from a 50 or 60 cycle per second mains or line operated lamp producing 100 or 120 light changes per second F is selected 100 times higher at 10 kc/s.

The current changes (at frequency F) in the photosensor, which are proportional to the specular component of the beam reflected from the coated surface, are amplified to a level of several volts, then rectified and filtered, using a time constant T in the filter, resulting in a D.C.

potential V1 of, say, positive polarity.

Said Filter, consisting conveniently of resistors and capacitors, forms one arm of a bridge. The other arm of the bridge is identical and is fed from a variable low impedance D.C. source V2 with opposite polarity to V1.

V2 is adjusted until V1 = -V2, i.e. until the bridge is balanced such that the voltage at the junction of the identical arms is O Volts, i.e.

V1-V2=0.

The bridge output V1 - V2 is then further amplified until the Gain or sensitivity control of the amplifier can sense minute or sufficient change due to the missing coating.

The out of balance bridge output due to skipcoating is conveniently sensed against a preset Gate voltage VG. When V1 - V2 amplified exceeds VG a switching amplifier is actuated, which (best pulse-stretched) operates a relay and defect indicator LED lamp to sound an alarm or stop the converting machinery.

According to the invention said time constant T is made long enough not to see variations in specular reflection for small adjacent areas due to non-uniformity of finish, and is made short enough to build up V, to its maximum value within 1 meter of web travel.

For example T is made 0.1 second. Then at a web speed of 360 meters per minute the web travels 60 centimeters in 0.1 second and skipcoating 1 meter long is safely sensed.

On the other hand non-uniformity of finish (usually with a pitch of 0.2 to 1 cm) is not sensed.

Only the mean value of the finish of the running web is recorded.

With reference to the accompanying Figure 1 infrared light emitting diode 1 emits and silicon phototransistor 2 collects narrow beams 3 and 4.

The current in the phototransistor is generated mostly by specular reflection from the coated surface area 5 of moving web 6 stretched tight over roller 7.

Beam 3 is at a shallow angle 8 to the tangential plane 9 which passes through the inspected area 5 of the coated web 6.

The angle 10 formed by the reflected beam 4 and plane 9 is identical to angle 8.

The base plate 11 and covers 12 form the essential mechanical parts of the detector head 13.

One of several practical electronic circuits, which satisfies the hereinbefore described principles of the coating detector, is described in Figure 2.

10 kc generator 1 4 modulates the light of infrared light emitting diode 1 via impedance converting operational amplifier 15, variable resistor RV1, coupling capacitor 1 6 and transistor 1 7.

Conveniently the current in transistor 1 7 and in the I.R. LED 1 is set between 20 and 80 milliamperes peak to peak. This can be checked on Test Point TP5.

The specular part 4 of the beam 3 reflected from paper web 6 modulates the current of phototransistor 2. These current variations are amplified to a level of several volts by operational amplifier 1 8.

The 10 kc output signal 1 9 is rectified by diode 20 and the 10 kc component is filtered out by resistors 21, 22 and capacitors 23, 24.

The very essential time constant of said filter, which in the example given was defined as 0.1 second, is mainly defined by the values of resistor 21 and capacitor 24.

The filter is one arm of a bridge.

The rectified mean output of amplifier 1 8 is pIusVi.

With the aid of baiancing potential divider RV2 and filter 27, 28, 29 and 30, which is identical to the already defined filter, on a second and identical bridge-arm an output potential V2 is set which is equal and has an opposite polarity to that ofV1.

Potentials V1 and V2 are connected to the high impedance input terminal 31 of amplifier 32.

The zero balance is conveniently checked on Test Point TP 1 with a centre-zero moving coil meter.

The gain of amplifier 32 is adjusted by the Sensitivity Control RV4 such that the out-ofbalance potential when the coating is not on the web is higher than the Gate Voltage set on Test Point TP4 and the Detector then actuates the Relay 33 and Skip Coat Indicator 34.

Claims

1. Apparatus for detecting uncoated sections of a continuously moving web of coated sheet material, comprising means for directing a beam of radiation onto the web, means for modulating the intensity of the incident beam at a predetermined frequency, detector means for detecting radiation reflected from the web and generating an electrical signal dependent on the intensity of the reflected radiation, and detection circuitry receiving the electrical signal through filter means and operable to provide an output signal when the magnitude of the filtered electrical signal exceeds a preset value, the filter means being arranged so that the detection circuitry does not respond to short-term variations in the intensity of the reflected beam due to nonuniformity of finish of the coating on the web.

2. Apparatus as claimed in claim 1, in which the said predetermined frequency is at least an order of magnitude greater than the frequency of transient light changes in the region of the detector means.