GB2173294A - Determining the location of defects present in flat glass - Google Patents

Abstract

The locations of defects in flat glass (1) are detected during conveyance through a detection station where the glass is scanned by a radiation beam (11). The beam sweeps transversely across the path of the glass through a distance in excess of the glass width so that the beam sweeps through both side edges. Beam deflection occurs when the beam encounters a side edge boundary of the glass (1) and beam deflection or attenuation occurs when the beam encounters a defect. Photodetectors (22,23) are located so that their irradiation or non- irradiation by the beam after it has passed through the glass depends on beam defections or attenuations by the glass, in particular detector 23 receives the beam when undeflected and detector 22 when deflected. Pulses of light are received by a detector 15 from a striped mirror 13 to indicate the position of the beam. The photodetectors are connected to a signal processor from which output signals are obtained indicative of the locations of the defects in the lengthwise direction of the glass and indicative of the transverse distances between such defects and a side edge of the glass. <IMAGE>

Description

SPECIFICATION Method of and apparatus for determining the location of defects present in flat glass This invention relates to a method of determining the location of defects present in flat glass, as it travels along a path, by scanning the glass with a beam of electromagnetic radiation and using photodetector means to detect incidence of the beam on defects in the glass. The invention also relates to apparatus for performing such a method.

The invention is particularly concerned with the location of defects in a travelling ribbon of glass, but it may also be used for the location of defects in pre-orientated glass sheets during their conveyance along a predetermined path.

Nowadays, flat glass is almost exclusively manufactured as a ribbon either in a drawing machine or a float chamber, and the ribbon is conveyed through an annealing lehr to a cutting station where it is cut into sheets for storage or further processing. Such cutting is sometimes effected automatically under computer control so that sheets of the required sizes can be cut from the ribbon with the minimum wastage of glass.

The quality rating of sheets of glass depends on the number and severity of defects in the sheets. When a ribbon or sheet of flat glass is to be cut to provide sheets of required sizes and quality ratings, it is desirable for the presence and location of significant defects to be determined beforehand so that their positions can be taken into account in determining the positions at which the glass is to be cut.

It is very desirable in industrial plant for the defect detection procedure to be automated and automatic detection methods utilising the deflecting action of defects on a scanning light beam are already known. Reference is made by way of example to BFG Glasgroup's British Patent Specification No 1 526 930 which describes a system in which a sheet or ribbon of glass travelling along a conveyor is repeatedly traversed by a scanning light beam and in which deflections of the light beam due to the presence of defects are monitored by photodetector system which passes appropriate signals to a store. The location of light-deflecting defects can be derived from the timing of those signals. The traverse number or scanning time gives an indication of the location of the defect along the glass, and its location along a line of traverse is derived by comparison with a fixed datum position.

The known detection methods using a scanning light beam are only capable of identifying the locations of defects with sufficient accuracy if it is ensured that a side edge of the glass being tested follows a constant line of motion at all times during the scanning of the glass, otherwise what is signalled is not the location of a defect across the glass, but rather its location relative to the width of the conveyer. There are circumstances in which that positioning of the work cannot conveniently be ensured. And in some plants, variation in the line of motion of the side edges of the travelling glass is unavoidable. A very important case in point is a float glass production plant.The position of the float glass relative to the width of the conveyor by which it is conveyed from the float tank and then through the annealing lehr is subject to variation in course of time in consequence of changes in the forces imposed on the glass ribbon when changing from one float glass thickness to another, and because of "snaking" of the ribbon on its conveyor. The known detection methods are therefore not satisfactory for automatically signalling the location of defects in a freshly formed ribbon of float glass and there remains a need for a method which is more suitable for that purpose.

It is an object of the present invention to fulfil that need.

According to the present invention there is provided a method of determining the location of defects present in flat glass as it travels along a path by scanning the glass with a beam of electromagnetic radiation and using photodetector means to detect incidence of the beam on defects in the glass, characterised in that:: -the glass is scanned by a beam of electromagnetic radiation which sweeps transversely across said path so that the beam traces successive transverse tracks across the glass and so that in each scanning pass the beam passes through the opposed side edge boundaries of the glass; -said photodetector means serves to detect deflections of the beam due to its incidence on one or each side edge boundary of the glass, and to detect attenuations or deflections of the beam by defects in the glass; -said photodetection means forms part of signalling means which yields output signals indicative of the positions along the scanned length of the glass of transverse tracks in which defects are detected and indicative of the distances, measured along such tracks, between such defects and a side edge boundary of the glass; and -said output signals are used to identify the signalled defect locations.

This method affords the important advantage that changes in the position which either side edge boundary of the glass occupies at the scanning station during travel of the glass through that station are rendered nugatory, in the sense that they do not affect the correctness of the signalled identifications of the defect locations on the glass. The method is therefore particularly suitable for use in automatically registering the locations of defects in a travelling ribbon of freshly formed flat glass.

But the invention can also be performed for determining the locations of defects in travelling pre-orientated sheets of flat glass whose lateral positions on a conveyor are not strictly controlled.

By suitable arrangement of the photodetection means as hereinafter more particularly described, a method according to the invention can determine the location of defects of one or more different types. These include defects which cause an incident beam of electromagnetic radiation to be deflected, by virtue of refraction. Such defects include bubbles, grains and stones which primarily determine the quality ratings of flat glass sheets. The method can also detect defects in the form of opaque zones. For example, if use is made of a photoelectric detector arranged so that it is normally irradiated by the beam but transmits a defect signal in the event of it ceasing to be irradiated because the beam is deflected to more than a predetermined extent, that detector will necessarily transmit a similar signal in the event that the beam becomes incident on an opaque zone in the glass.Additionally or alternatively, the method can detect defects which cause significant attenuation of the scanning beam. Such defects include for example certain stains, such as tin stains deriving from a float bath.

For convenience, the positions along the scanned length of the glass, of the transverse tracks in which defects are detected are hereafter referred to as the longitudinal positions of the defects whereas transverse distances between the detected defects and a side edge boundary of the glass are referred to as the ordinates of the defects.

The most important practical uses of the method according to the invention are in the automatic control of a marker or glass cutter which, during the continued travel of the glass beyond the detection station where the glass is scanned, either marks the glass at the ioca- tions of the defects or cuts the glass at positions which take account of those defect locations. Accordingly, in preferred embodiments of the invention the signalling means generates output signals which are used as a control factor in the automatic control of a glass marker or cutter for marking or cutting the travelling glass at site downstream from the station where the glass is scanned.In such embodiments of the invention the longitudinal positions of defects can be represented simply by the times of occurrence of defect detection signals because the time of arrival of a defect at a said marking or cutting station is dependent on the one hand on the moment of the initial defect detection and on the other hand on the speed of travel of the glass and the distance between the marking or cutting station and the detection station where the glass is scanned.

As an alternative to the generation of output signals which indicate longitudinal positions of defects simply by the times at which such signals occur, the signalling means can generate output signals indicative of the longitudinal distances between transverse tracks in which defects are detected and a transverse datum line. This procedure can be used for example in the detection of defects in travelling glass sheets by detecting the arrival of the leading transverse edge of each sheet at the detection station and generating output signals indicative of the longitudinal distances of the defects from that edge, which serves as the datum line. Such output signals can be fed to a recorder which reproduces the information in the form of a statistical record for glass quality classification or some other purpose.

The transverse ordinate of a defect can be determined by the signalling means directly from the scanning distance travelled by the scanning beam between the moment (indicated by the occurrence of an edge deflection signal from the photodetector means) at which the beam arrives at the location of a side edge boundary of the glass and the moment during that scanning pass at which a defect signal from the photodetection means indicates encounter of the beam with the defect.

Alternatively the said transverse ordinate can be determined by the signalling means in function of the time interval between the moments of occurrence of said edge deflection and defect signals; but for this purpose the scanning speed of the beam across the glass should be substantially constant.

It is not necessary for the signalling means always to determine the transverse ordinate of a defect on the basis of a glass edge position signal or signals transmitted in the actual beam pass or passes in which the defect is encountered. The increments of glass length from pass to pass will be too small for there to be any significant change in the position of the glass edges at the detection station in the period between two successive passes or indeed in the period covered by many successive passes. Therefore in some methods according to the invention an average value is derived from glass edge deflection signals transmitted by the photodetection means in a multiplicity of successive passes of the beam across the glass in one direction and that average edge position value is utilised in the derivation of the output signals indicative of the transverse ordinates of defects.

Preferably said defect location output signals are generated only for defects whose transverse ordinate(s) from each side edge boundary of the glass are above a predetermined minimum value. Marginal portions of drawn or float glass are often of very inferior quality and when determining where such glass should be cut to produce required sheet sizes for marketing it is often necessary to treat such marginal portions as waste. In the case of float glass, marginal bands of the ribbon are spoiled by the action of the top rollers in the float tank and those marginal parts of the glass have to be discarded. By adopting the said preferred feature when carrying out the present invention, the presence of defects in marginal portions of the glass is ignored in the output signals which serve as defect location identification data.The width of the marginal portions to be discarded is predetermined and this dimension is therefore available as a control parameter by the signalling means. The signalling means can be arranged so that such marginal defects do not cause the generation of photoelectric detection signals or so that such signals are generated but do not influence the output signals.

Preferably the displacement of the beam is monitored by monitoring means which yields signals indicative of the beam positions relative to a fixed datum position which is outside the limits of the beam movement across the glass and such signals are used by said signalling means in the derivation of said output signals. The generation of output signals which are indicative of the transverse locations of the side edges and of defects, relative to a fixed datum position, is of advantage because the output signals are in those circumstances particularly useful in the automatic control of operations on the travelling glass by apparatus mounted at a predetermined fixed position, downstream from the region where the glass is scanned.

In the manufacture of a continuous ribbon of glass from which marginal portions have to be removed as waste, it is an advantage for the margins to be cut from the ribbon by cutters which are controlled in dependence on variations in the positions of the side edge boundaries of the ribbon so that such variations do not result in significant variations in the widths of the removed margins.For achieving this advantage, in some embodiments of the invention as applied for detecting defect locations in a continuous ribbon of glass, said datum position is fixed in relation to the frame of the glass conveyor and signals indicative of the distance of at least one side edge boundary of the ribbon from said datum position are used for automatically controlling cutting means which is mounted further downstream along the ribbon path and serves to cut a marginal portion of predetermined width from one or each side of the ribbon. Two cutters for removing the opposed margins of the ribbon can be controlled by the same signals if the width of the ribbon remains substantially unchanged. But in most cases it will be required to generate independent signals representing the transverse distances of the different side edge boundaries from said fixed datum position.

If the beam displacements are monitored relative to a fixed datum position, as is preferred, it may be necessary or desirable for the monitoring means to be periodically re-set to maintain the reliability of the monitor signals over a period of continuous performance of the method. The re-setting can be performed manually or automatically, depending on the design of the monitoring means employed. In certain preferred embodiments of the invention, signals which are transmitted from a photodetector located at said datum position or in fixed relationship thereto, responsive to irradiation of such photdetector by the scanning beam, automatically repeatedly re-set the beam displacement monitoring means so that it correctly signals the positions of the beam relative to said datum position.

Preferably the aforesaid signalling means yields output signals which represent (a) the distance travelled by the scanning beam in successive scanning cycles from said fixed datum position to the position at which the beam encounters the nearer side edge boundary of the glass and (b) the distance between said datum position and a signalled defect.

The transverse ordinate of a signailed defect is thereby indicated, it being equal to the difference between those signalled distances.

The signalling of edge boundary and defect positions in terms of their distances from the common datum position simplifies the signal processing operations.

Preferably in each scanning pass of the beam the beam is incident upon a reference photodetector, causing generation of a reference signal, just before the beam reaches the nearer side edge boundary of the glass and the first subsequent beam deflection signal is processed by said signalling means as a signal indicative of the incidence of the beam on a side edge boundary of the glass. Incidental, spurious signals which might result from beam reflections from parts of apparatus on which the beam is incident before it reaches the immediate vicinity of the glass are thereby discounted. When the beam encounters a side edge boundary of the glass the beam is deflected and this deflection can be registered by a photoelectric detector in the same way as a deflection caused by a defect. If desired one and the same detector can be used both for boundary edge and defect detection purposes.In any event the generation of a said reference signal makes it unnecessary for the boundary edge signals to be of distinctive form because they are distinguishable from defect signals by reason of their being always the first detector signals to be generated after a reference signal.

Reference photodetectors as above referred to can also serve as datum position photodetectors as above referred to. However it is better to provide said reference photodetectors and one or a pair of said datum position photodetectors, the reference photodetectors being mounted closer to the path of the glass.

Advantageously, each scanning movement of the beam is encoded as a series of signal pulses so that any given instantaneous position of the beam corresponds with a given pulse number. Such encoding is easily performed and gives signals which can readily be processed by the signalling means in conjunction with the signals indicative of glass edge or defect irradiation.

In particularly recommended methods according to the invention there are reference photodetectors as above referred to; scanning movement of the beam is encoded as a series of signal pulses which are fed- to a pulse counter which in each scanning cycle translates beam positions into numbers of pulses counting from zero or some other predetermined datum setting which corresponds with a predetermined position of the beam outside the limits of the beam movement across the glass; and in each of successive scanning passes of the beam, irradiation of the first of the two reference photodetectors encountered by the beam causes the counter to be re-set to a value representing the beam displacement distance between the said datum position and that reference photodetector.This combination of features improves the reliability of the defect location signals over periods of continued use of the method.

In certain methods according to the invention, a beam of radiation deriving from the same radiation source as the glass scanning beam is caused, synchronously with the scanning of the glass, to scan a reflector having alternating reflecting and non-reflecting bands, and the radiation quanta reflected from that banded reflector are incident on a photoelectric detector and thereby create said-signal pulses. This method of monitoring the beam displacements enables sufficiently accurate defect location signals to be generated even if there is some appreciable variation in the speed of the scanning beam during each pass across the glass. Synchronism between the scanning of the glass and the scanning of the banded reflector is preferably achieved by using a common oscillating reflector for imparting the scanning movements to the two beams.

In other embodiments of the invention the positions of the glass scanning beam in course of each scanning pass are monitored on the basis of the passage of time from the commencement of the pass. Such time can be measured by a digital clock which can deliver pulses for processing in the same way as the pulses which are delivered responsive to the radiation quanta reflected from a banded reflector in the previously described embodiment. However when monitoring the positions of an angularly oscillating beam on the basis of time intervals it is necessary for the amplitude of the beam oscillations to be sufficiently in excess of the transverse dimension of the glass to ensure that the variations in speed of the beam during its tracking across the glass are small enough to permit the required accuracy in the determination of the defect locations.Preferably the glass scanning beam is an angularly oscillating beam and the amplitude of the beam movement in the plane of the glass is at least twice the width of the glass. In these circumstances the speed of the beam during tracking across the glass will be substantially constant and the speed of such tracking can be taken as equal to the average speed of the beam over the full arc of its movement. At or near a limit of its arc of movement the scanning beam can be incident upon a photodetector from which a response signal is transmitted to the monitoring clock causing it to be re-set so as to maintain a given standard of monitoring accuracy.

The amplitude of the oscillations of an oscillating scanning beam can be automatically adjusted if the amplitude starts to depart from a predetermined range. In the case that the beam oscillations are effected by an oscillating reflector as hereinafter described, this automatic adjustment can be achieved by adjustment of energising pulses to an electromagnet responsible for applying the oscillating forces to the reflector.

The method according to the invention can be performed in such a way that the output signals give indication of the longitudinal dimensions of detected defects, i.e. their dimension measured in the direction of the conveyance of the glass through the scanning station. In order to achieve this advantage, in preferred embodiments of the invention the number of immediately successive beam passes in which defect-induced signals are generated when the beam is at approximately the same distance from a given boundary edge of the glass are registered and give rise to output signals indicative of defect size. In the case of ribbons of sheet or float glass, any defects usually have a major dimension in the longitudinal direction of the ribbon and that dimension of a defect is therefore normally indicative of the severity of the defect as a whole.

In preferred embodiments of the invention the beam is at all times during its irradiation of the glass reflected back through the glass before reaching the photodetector(s) from which defect signals derive. This procedure enables the method to be performed within a smaller space envelope than would otherwise be required. And the optical system can be more conveniently arranged in relation to the path of the glass. For example, when testing a ribbon of sheet or float glass travelling along a substantially horizontal path, the beam source and the defect photodetector means can be arranged at levels beneath the path of the ribbon and the reflector for reflecting the scanning beam back through the glass can be disposed above that path.Preferably the zones of the glass which are at any given time irradiated by the incident and reflected scanning beams are not separated, or are not separated by more than 2 cm. In these circumstances the occurrence of a double signal for any given defect is avoided or made easily identifiable as deriving from one defect.

According to another optional but advantageous feature, the radiation beam leaving the glass is split into a part which is transmitted to the site of a first detector and a part which is transmitted to a second detector; and one of those detectors is arranged so that it is irradiated whenever the beam is deflected and the other of such detectors is arranged so that it is irradiated unless the beam is intercepted by an opaque deflect or is deflected by a defect to more than a certain predetermined extent (as for example by a side edge of the glass). This conjunctive use of twin detectors operating in different senses enables the generation of photodetector signals which not only signal the presence of a radiation deflecting defect but also characterise it as being or not being of a particular type.In particular that use of twin detectors facilitates distinction between radiation-transmitting and opaque defects.

The invention includes a method wherein the photodetection means serves not only to detect deflections of the scanning beam such as are caused by the side edges of the glass, but also to detect defects which attenuate the beam without deflecting it. Such a method can be performed by using a single defect photodetector which is normally irradiated by the beam emerging from the glass and yields a signal only if the incident radiation falls below a predetermined minimum value either because the beam is deflected so much that the photodetector ceases to be irradiated or because the glass has a certain minimum attenuating effect on the beam. The minimum beam deflection at which the photodetector yields a response signal can be determined by the size of the photosensitive face of the detector.

It is possible to effect scanning by moving the scanning beam emitter; but preferably that emitter is stationary and the glass is scanned by a beam deflected onto the glass by an oscillating deflector, e.g. a reflector. The scanning movements can more easily be imparted in that manner.

The invention includes embodiments wherein deviations of the scanning passes of the beam to more than a given extent from a predetermined plane cause irradiation of deviation photodetectors and the scanning path is automatically corrected by response signals from such deviation detectors. This refinement affords a useful safeguard against malfunctioning due to misdirection of the scanning beam such as might result for example from the movement of apparatus components from pre-set posi tions under environmental influences.

According to another optional but rec ommended feature, the glass is scanned at a frequency of at least 20 cycles per centimetre length of the glass. This condition helps to ensure that any objectionable defect will be detected in a plurality of successive traverses, which promotes reliability.

The radiation beam used in a method ac cording to the invention is preferably a laser.

The laser beam can easily be made very nar row, which is conducive to accuracy of defect location.

The invention includes apparatus for deter mining the location of defects present in flat glass as it travels along a path, such appara tus comprising means for conveying flat glass through a testing station, means for generat ing a beam of electromagnetic radiation and causing such beam to pass repeatedly across said path and thereby trace successive transverse tracks across the glass at said testing station, and photodetector means for detecting incidences of the beam on defects in the glass.The apparatus according to the present invention is characterised in that said photodetector means is constructed to yield signals indicative of beam deflections, or of beam attenuations as well as beam deflections by the glass; and the apparatus comprises a beam monitor for generating signals which represent the beam positions, means associ ated with said beam monitor for registering, in respect of each scanning pass of the beam in one direction, or periodically in respect of a succession of such scanning passes, the posi tion or average position reached by the beam, in such pass or passes, when a first signal caused by beam deflection by the glass is received from said photodetector means; and signal processing means which is constructed so as in use to transmit output signals which vindicate the time of occurrence of a beam pass in which a later signal indicative of beam deflection or attenuation by the glass is re ceived from said photodetector means after the beam has left said position or average beam position, and which are also indicative of the distance moved by the beam in the time interval between the moment it reaches said position or average position and the mo ment of said later signal reception.

This apparatus enables not only the length wise positions of defects but also their transverse ordinates (i.e. their distances from a side edge boundary of the glass) to be automatically signalled even if the side edge boundary position varies relative to the width of the glass conveyor during the conveyance of the glass through the scanning station. In the case of continuous glass ribbons, the ac curacy of the signalled data is not affected by variations in the ribbon width passing through the glass scanning station, by undulation of either side edge boundary of the glass with respect to its line of conveyance, or by "snaking" of the glass on its conveyor. The apparatus is therefore of particular importance as applied to the identification of the sites of defects in ribbons of freshly formed float glass.

The ability of the apparatus to determine tansverse ordinates of defect positions even if the side edge boundaries of the glass do not follow a constant line of motion is attributable to the fact that the apparatus is equipped to exploit the radiation-deflecting effect which a side edge boundary of flat glass has when a radiation beam is caused to sweep towards and through that boundary. Apparatus according to the invention is intended to be used for determining the location of defects in travelling flat glass having a width (measured transversely tothe direction of its conveyance) less than the length of the path swept by the beam in the plane of the glass.Provided that condition is observed, the first signal indicative of beam deflection by the glass, which is emitted by the photodetector means in any given pass of the beam, coincides with the incidence of the beam on the nearer side edge boundary of the glass. And if in the same pass of the beam a later signal is emitted by said photodetector means, that may be attributable to the presence of a defect which deflects or attenuates the radiation beam, in which case the distance travelled by the beam between the moments of occurrence of the said first and subsequent signals is the transverse ordinate of such defect, i.e. its distance from a side edge boundary of the glass.

If a said later signal is attributable to deflection of the beam by the further side edge boundary of the glass it can be discounted by reason of the fact that the distance tracked by the beam in the time interval between the receptions of said first and that later signal substantially corresponds with the width of the glass.

The photodetection means of apparatus according to the invention can be chosen to suit the detection of different kinds of defects, for example bubbles, grains and stones which cause deflection of incident radiation, defects which are opaque to the radiation employed, and defects in the form of stains, such as tin stains deriving from a float bath, which transmit the radiation beam but in an attenuated state.

Preferably the signal processing means is connected to a control mechanism of a glass marker or cutter located along the path of conveyance of the glass, downstream from the scanning station, so that the marker or cutter marks the glass at the locations of the defects or, as the case may be, cuts the glass at positions which take account of the signalled defect locations. In this manner a cutter can be controlled so as to cut the tested glass into sheets of required sizes and quality ratings.The moments in time at which defectinduced signals are produced by the photodetection means is indicative of the longitudinal positions of the defects, i.e. the positions of the beam tracks, traced across the ribbon, in which the defects are to be found; and from the times of occurrence of those signals, the distance between the scanning station and the marking or cutting station, and the speed of the glass, the times of arrival of those defects at the marking or cutting station can be determined.

Apparatus according to the invention can alternatively be designed to yield output signals which indicate the longitudinal positions of defects in terms of their distances from a transverse datum line. For example apparatus for detecting the locations of defects in travelling glass sheets can incorporate a photodetector which is responsive to beam deflection caused by the arrival of the leading edge of a sheet between such photodetector and a radiation source. In that case that leading edge can serve as a datum line and the signal processing means can compare the times of occurrence of the datum and defect signals and tansmit output signals indicative of the longitudinal distances of defects from the leading edges of the sheets.

It is not necessary for the transverse ordinates of a defect to be determined in every instance on the basis of (a) glass edge location signal(s) transmitted by the photodetector means in the actual beam pass or passes in which such signalled defect is encountered.

The interval of time between successive passes of the beam across the glass is so small that, at least in the case that the glass being tested is a freshly formed continuous ribbon, there will be no significant alteration in the edge position of the glass at the detection station even over the period covered by many passes of the beam. That is why, in the interest of simplying the signal processing means, in some apparatus acording to the invention such processing means includes a facility which periodically determines the average of the beam positions at which photodetector signals indicative of deflection of the scanning beam by a side edge boundary of the glass are transmitted in a multiplicity of preceding passes of the beam and the transverse ordi nate output signals are derived on the basis of that average value.

Preferably the apparatus includes discounting means for preventing the output signals from the signal processing means from indicating the location of defects which are less than a certain predetermined distance from a side edge boundary of the glass. In many circum stances of potential use of the apparatus, it is of no interest to identify the location of defects near the side edge boundaries of the glass because an inferior quality of the glass in side marginal portions thereof is an inevitable result of the glass forming process itself and those marginal portions are systematically treated as waste. The width of the unusable margins depends on the type of glass forming machine by which the flat glass is produced, on the overall width of the flat glass leaving the machine and on other factors.In the case of the production of a float glass ribbon of about 3 to 4 metres in width, each of the side margins which -have to be treated as waste may for example be of the order 10-20 centimetres in width. Although the information signalled by the signal processing means can include location data even for defects which occur in those parts of the ribbon which are to be discarded, it is more satisfactory for those defects to be discounted. The signalled output data is then restricted to the more important information.

Such discounting means may form part of the signal processing means and function by blocking glass defect signals which are to be discounted. Alternatively the defect photodetector means may be associated with means which during any given pass of the scanning beam precludes the transmission of discountable signals to said signal processing means.

The invention includes apparatus as hereinbefore defined wherein the beam monitor is constructed to generate signals representing the beam positions relative to a fixed datum position which is outside the limits of the beam movement across the glass. In such apparatus, the signal processing means can deliver output signals which are indicative of side edge boundary and defect locations along the scanning path, relative to a fixed datum position. This is of advantage because such output signals are particularly useful in the automatic control of operations on the travelling glass by apparatus mounted at a predetermined fixed position, downstream from the region where the glass is scanned.

Advantageously, glass cutters are mounted at a position along the path of the glass, downstream from the testing station, and the apparatus includes means which in successive scanning cycles generates signals indicative of the distances of the beam from a datum position as aforesaid when the beam encounters the side edge boundaries of the glass, and means for controlling said cutters in dependence on those signals to cause the cutters to cut marginal portions of predetermined width from the glass during its movement away from the testing station. Such apparatus is of particular benefit in plant for producing a continuous ribbon of flat glass from which marginal portions have to be continuously removed because of the their inferior quality.By controlling the margin cutters by signals indicative of beam displacement distances from a datum position which is fixed in relation to the glass ribbon conveyor the width of the removed margins can be kept substantially constant despite changes in the lines of motion of the side boundary edges of the ribbon due to snaking of the ribbon or some other cause.

The cutters for removing the opposed margins of the ribbon can be controlled by the same signals if the width of the ribbon remains substantially unchanged. But in most cases it will be required to generate independent signals representing the transverse distances of the different side edge boundaries from said fixed datum position.

In appartus wherein the beam monitor is arranged to monitor beam displacements relative to a fixed datum position as above referred to, it may be necessary or desirable for the apparatus to include means whereby the monitoring means can be periodically re-set to maintain the reliability of the monitor signals over a period of continuous performance of the method. Such re-setting means can be designed for manual or automatic operation. In preferred embodiments of apparatus according to the invention at least one photodetector located in the scanning path of the beam, at said datum position or in fixed relationship thereto, is operatively connected to the beam monitor to cause it to be automatically repeatedly re-set by signals transmitted from such photodetector thereby to ensure that the monitor signals correctly represent the beam positions relative to said datum position.

The signal processing means of apparatus according to the invention is preferably designed to yield output signals representing (a) the distance travelled by the scanning beam in successive scanning cycles from a fixed datum position to the position at which the beam encounters the nearer side edge boundary of the glass and (b) the distance between said datum position and the position at which the beam encounters a defect in the glass. The signalling of edge boundary and defect positions in terms of their distances from a common datum position is advantageous for simplifying the signal processing operation. The signals representing distances (a) and (b) give indication of the transverse ordinate of a signalled defect, such ordinate being the difference between those signalled distances.

Advantageously the apparatus includes a pair of photodetectors (hereafter called "reference photodetectors") which are located near opposite ends of the path swept by the beam during tracking across the glass, such reference photodetectors are connected to the signal processing means which transmits output signals indicative of beam deflection by glass, and said processing means is constructed so that it does not treat a photodetector response signal as indicative of beam deflection or attenuation by a glass defect if such signal is delivered during a first part of a scanning pass of the beam, before the beam reaches a said reference photodetector. The provision of such reference photodetectors is beneficial for avoiding output signal errors because of fortuitous reflections of radiation onto photodetectors during movements of the beam beyond the ends of its tracks across the glass.

The apparatus preferably includes means which encodes each scanning movement of the radiation beam as a series of signal pulses so that any given instantaneous position of the beam corresponds with a given pulse number. Such signal pulses can be readily processed in conjunction with photodetector signals for deriving the required output signals representing defect locations.

In particularly preferred forms of apparatus according to the invention there is a pulse counter for translating beam positions in each scanning cycle into numbers of pulses counting from zero or some other predetermined datum setting corresponding with a predetermined datum position of the beam, and reference photodetectors as aforesaid are operatively connected to said counter so that in each of successive scanning passes of the beam the response signal resulting from irradiation of the first of the two reference photodetectors encountered by the beam causes resetting of the counter to a value representing the beam displacement distance between the said datum position and that reference photodetector.This combination of features is helpful for improving the reliability of the defect location signals over periods of continued use of the apparatus, particularly if the reference photodetectors are- mounted in close proximity to the positions at which the beam commences to track across the glass.

In some apparatus according to the invention, there is a reflector having alternating reflecting and non-reflecting bands, means for causing a second beam of radiation (hereafter called "monitoring beam") deriving from the scanning beam generating means to scan such reflector in synchronism with the scanning movements of the glass scanning beam, and a photodetector which is located so as to be irradiated by quanta of radiation reflected from said banded reflector and which yields said signal pulses reponsive to such irradiation.

This form of apparatus for encoding the scanning movements of the scanning beam has the advantage that the accuracy with which the pulsed signal represents the position of such beam at any given point of time is not affected by fluctuations in the speed of the beam during its tracking motion across the glass. Such speed fluctuations are inevitable in the event that the scanning beam scans the glass by to and fro angular motion and they will be of substantial magnitude unless the arc of motion of the beam is much larger than the width of the glass. The accuracy and reliability are promoted by the derivation of the monitoring beam from the same source as the scanning beam. Preferably the synchronism of the scanning and monitoring beams is ensured by using a common oscillating deflector (e.g.

a reflector) for imparting scanning oscillations to the two beams.

In other apparatus according to the invention, there is a digital clock which monitors scanning beam movement in terms of the passage of time from the commencement of each scanning pass and yields signal pulses indicative of the beam position. This form of beam monitoring apparatus avoids the need to derive a second radiation beam from the scanning beam generator and to provide the banded reflector and associated photodetector above described. By means of a monitoring clock it is possible to obtain results of similar accuracy. But if the scanning beams is angularly oscillated (as distinct from being unidirectionally rotated so as repeatedly to track across the glass in the same direction) it is necessary for the amplitude of the beam movements to be considerably greater than the width of the glass. Preferably such amplitude is at least twice the glass width.If a datum position photodetector is provided as hereinbefore referred to, as is preferred, that photodetector is preferably connected to the clock so that this is re-set to a datum setting each time the datum position photodetector is irradiated by the scanning beam.

In apparatus of preferred construction, the signal processing means is designed so as in operation to yield output signals indicative of the number of immediately successive beam passes in which a beam deflection or attenuation signal is transmitted when the beam is at approximately the same distance from a side edge boundary of the glass. That design of the signal processing means has the advantage that the output signals of the apparatus give not only information about the locations of radiation deflecting defects, but also about the sizes of the defects. The signals indicative of defect sizes can be further processed by a selector mechanism which determines the quality of different areas of the flat glass on the basis of the number, distribution and sizes of defects therein.

According to another optional but advantageous feature, the apparatus comprises a reflector which is arranged so that it reflects the scanning beam back through the glass and the photodetector means for detecting defectinduced beam deflections or attenuations is arranged so as to be responsive to deflections or attenuations of that reflected beam after its second emergence from the glass. This design feature is of appreciable benefit in reducing the space requirements of the apparatus. According to one possible arrangement, the radiation beam source and the photodetector means for detecting defect-induced beam deflections or attenuations can be installed beneath the path of conveyance of the glass and the scanning beam reflector can be arranged above that path.The beam source can be arranged to emit the beam directly upwardly towards the path of the glass or the beam can be emitted in a horizontal or nearly horizontal direction onto a reflector which reflects the beam upwardly towards the said path.

The latter arrangement enables the apparatus to function in circumstances in which the optical length of the scanning system is greater than can be accommodated in the available vertical space. The beam source can of course alternatively be located for directing the scanning beam downwardly onto the glass but that arrangement entails more difficulty and expense in mounting the appartus in position for use.

Another optional but nevertheless very advantageous feature, resides in the provision of a beam splitter for splitting the scanning beam, after it leaves the path of travel of the glass, into two derivative- beams, and of separate photodetectors for said derivative beams, such photodetectors being arranged so that when the apparatus is in use one of them will be irradiated only when the scanning beam is deflected by the glass whereas the other of them will be irradiated unless the scanning beam is prevented from being transmitted by the glass or is deflected to more than a certain predetermined extent. It is an advantage of this twin-detector arrangement that it enables information signals to be generated which indicate not only the location of a defect but also whether the defect falls into or does not fall into a particular type category.For example, if the said one detector detects a beam deflection but the said other detector remains irradiated, this is indicative of a type of defect which has only a small refracting effect on the incident radiation; and if at any given time neither detector is irradiated that is indicative of interception of the scanning beam by an opaque zone in the glass.

The invention includes apparatus wherein the photodetection means comprises a photodetector which is arranged so that when the apparatus is in use it is normally irradiated by the beam emerging from the glass and which yields a signal if the incident radiation falls below a predetermined minimum threshold value. Such a photodetector means is able to detect and signal defects which attenuate the scanning beam without deflecting it. The photodetection means of such apparatus has of course also to detect beam deflections such as are caused by the edges of the glass. Such large beam deflections as well as said beam attenuations can be detected by one and the same photodetector having a sensitive face of such size that it remains irradiated unless the scanning beam is deflected to more than a predetermined minimum extent.

Preferably the beam generating means is held so that it remains stationary when the apparatus is in use and the apparatus comprises a deflector, e.g. a reflector, coupled to means for oscillating such deflector to produce the scanning movements of the beam. A particularly recommended oscillating mechanism for oscillating the deflector comprises a torsion element attached to the deflector and means, e.g. electromagnetic means, for oscillating such element at its natural frequency.

Such an oscillating system has been found to be capable of functioning accurately and reliably. The oscillating mass can be very small.

The electromagnet can be energised by voltage pulses controlled by the deflector movement itself so that the pulse frequency coincides with the fundamental resonant frequency of the torsion element.

In some apparatus according to to the invention there are deviation photodetectors located for yielding a response signal if the path of the scanning beam deviates to more than a certain extent from a predetermined plane, and such deviation photodetectors are operatively connected to adjustment means to cause automatic correcting adjustments of said path.

This additional feature affords a useful safeguard against malfunctioning due to misdirection of the scanning beam such as might result for example from the movement of apparatus components from pre-set positions under environmental influences.

According to another recommended feature, the means for generating the scanning beam, the defect photodetectors, and means for imparting scanning movements to the beam, are integrated into a unitary assembly. The installation of the apparatus in appropriate relationship to a given glass conveyor is thereby facilitated.

Preferably the radiation beam emitter is a laser gun. The use of a laser is advantageous for making the instantaneously irradiated zone of the glass as small as possible and thereby giving the apparatus a very high resolution capability.

An embodiment of the process and of the apparatus according to the invention, selected by way of example, will now be described with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a scanning and detection system in course of use; and Figure 2 is a block diagram of a signalling means used in this system.

Fig. 1 represents the scanning and detection system as viewed in the direction of conveyance of a glass ribbon 1. The beam emitter 2 and a number of photodetectors for monitor ing the scanning movements of the beam and detecting beam deflections are mounted at a site below the path of the glass ribbon and above that path there is a concave reflector 3 for reflecting radiation back to such detectors.

The beam emitter 2 is preferably a laser gun, and the radiation beam will in the following description be refered to as a laser beam.

The emitter 2 is mounted in a fixed position.

The laser beam is split by a semi-mirror-cumreflector 4 into a transmitted part 5 and a reflected part 6. This reflected part is further reflected by reflectors 7 and 8. The transmitted beam 5 and the beam 9 reflected by mirror 8 are both incident on a reflector 10. The centre of reflector 10 is in the plane bisecting and containing the axis of curvature of the reflector 3.

The reflector 10 is mounted on a torsion rod and is. oscillated by an electromagnet having in its energisation circuit a switch which is operated by the reflector movements so that the reflector is oscillated at the natural frequency of the rod. The oscillation frequency of the reflector 10 is 800 cs/sec. The oscillation of reflector 10 causes the scanning beam 11 reflected therefrom to scan the glass ribbon 1 at that frequency. Assuming, for example, that the speed of the glass ribbon is 20 cm per second, the result is that the ribbon is exposed to 40 scanning cycles of the beam per centimetre length of the ribbon.

The beam 9 is transformed by the oscillating reflector 10 into an oscillating reflected beam 12 which sweeps to and fro in precise synchronism with the scanning beam 11. The beam 12 scans a concave striped reflector 13 comprising reflecting and non-reflecting stripes arranged alternately along the path swept by the beam. The reflector 13 therefore intermittently reflects incident light, the light being reflected in a succession of discrete quanta or pulses as represented by broken line 14.

These light pulses are directed towards the site of a photodetector 15. The centre of that photodetector and the centre of the oscillating reflector 10 are symmetrically disposed with respect to the plane bisecting and containing the axis of curvature of the reflector 13 so all of the light pulses are received by the photodetector 15 notwithstanding the oscillating movements of the beam 12 from which the pulses are generated. The synchronous scanning beam 12, the striped reflector 13 and the detector 15 together serve for encoding the scanning movements of the main scanning beam 11 as a series of electrical signal pulses which are processed by a signal processing means as hereafter described.

The amplitude of oscillation of the oscillating reflector 10 is such that the path swept by the beam in the plane of the glass ribbon extends appreciably outside the limits of the ribbon path. In fact the amplitude is such that the beam sweeps from a datum position denoted by line X at one side of the ribbon path to a position represented by line Y on the other side of such path. In consequence, the beam sweeps through both side edge boundaries of the ribbon during each transverse scanning movement of the beam. In the illustrated embodiment of the invention reference photodetectors 16 and 17 are installed adjacent the opposed ends of the reflector 3 for a purpose to be described, and the scanning beam 11 also sweeps across those photodetectors in each of its scanning movements.

While sweeping the glass ribbon, the scanning beam 11 is reflected back through the ribbon by the reflector 3 as a reflected scanning beam 18. The optical arrangement of the system is that the locations where the glass is instantaneously irradiated by the incident and reflected scanning beams are very close together (preferably within 2 cm of each other) and preferably they overlap or are contiguous.

The reflected beam 18 after leaving the glass ribbon is incident upon a semi-transparent reflector 19 and is thereby split into a transmitted part 20 and a reflected part 21.

The drawing inevitably makes it appear that the incident and reflected scanning beams 11 and 18 are both in the same plane, but in fact they are not, otherwise the oscillating reflector 10 and the curved reflector 13 would obstruct the reflected scanning beam. The transmitted part 20 travels towards a photodetector 22 whereas the reflected part 21 travels towards a photodetector 23. An intercepting disc 24 is located centrally in front of the sensitive surface of the photodetector 24 and intercepts the transmitted part 20 of the reflected scanning beam 18 whenever and for as long as the scanning beam 11 and therefore also the reflected scanning beam 18 is not deflected by the glass ribbon. A deflection of those beams occurs whenever the scanning beam encounters a side edge boundary of the ribbon, and is liable also to occur when those beams encounter a defect in the glass.Any significant deflection of the scanning beam results in irradiation of the photodetector 22 by the transmitted part 20 of the reflected scanning beam. The reflected part 21 of the reflected scanning beam 18 is at all times incident on the photosensitive surface of the photodetector 23 except at times when the reflected scanning beam is deflected by the glass. If desired the photodetector 23 can be of a sensitivity such that it is responsive to falls in the incident radiation flux to below a certain threshold value so that the photodetector can signal the incidence of the scanning beam on glass defects which attenuate the beam without deflecting it.

The successive distance increments which in any given scanning pass are swept by the beam along the reflector 3 and the striped reflector 3 are not in constant relationship to the distance increments simultaneously swept across the glass. In order to keep signalling error due to this disparity as small as possible it is desirable for there to be a long optical path between the oscillating reflector 10 on the one hand and the glass and reflector 3 on the other hand. In an actual apparatus giving good results, the distance between the oscillating reflector and the reflector 3 is 7-8 metres.

Reference is now made to the block diagram of the signalling means (Fig. 2). At the left-hand side of this diagram are shown the photodetectors 15, 16, 17, 22 and 23 shown in Fig. 1. The beam-monitoring pulses delivered from the photodetector 15, are fed after amplification to a pulse counter 26, as also are the signals which are delivered by the photodetectors 16, 17 which are mounted close to the scanning beam reflector 3.

Monitoring pulses delivered from the photodetector 15 during displacements of the beam to the right in the aspect of Fig. 1 are registered as successive increases of the value registered by the counter; whereas monitoring pulses delivered during leftward scanning passes are registered as successive diminutions of such valve. The counter can be re-set periodically so that its pulse count continues to represent the beam position, relative to the fixed datum position X, with a required degree of accuracy. Such setting can be performed manually at any required intervals. The counter performance can for example be monitored by a visual display which reveals when resetting is required. However the counter is preferably set and re-set automatically.In preferred embodiments of the apparatus the re-setting occurs automatically, in each cycle of the scanning beam or at intervals of two or more cycles, responsive to incidence of the beam on one or more photodetectors. For example, if a datum position photodetector is located at X, the counter can be automatically re-set to a datum, e.g. zero, setting by a signal transmitted from such photodetector responsive to its irradiation.In that case it is preferable for the counter to be re-set responsive to irradiation of the reference photodetector 16 during a rightward pass of the beam and again re-set responsive to irradiation of the reference photodetector 1 7 during a leftward pass of the beam so that the pulse counts registered when the beam is at the positions of those reference photodetectors correctly represent the respective distances of those photodetectors from the datum position X. As an alternative, said datum-position photodetec; tor can be omitted and the re-setting of the counter can be effected soley by signals from the reference photodetectors 16, 17, such signals serving to set the counter to pulse count numbers which correctly represent the said respective distances.

In consequence of the counter re-setting procedure, the value registered by the counter 26 is at all times a correct representation of the position of the beam relative to the fixed datum position.

Monitoring pulses indicative of the varying scanning beam position are fed from the counter 26 to an edge position computer 27 to which the photodetectors 22, 23 are connected via an OR circuit 32. The computer comprises a gate circuit 28, a tally device 29, an accumulator/calculator 30 and a register 31.

During each scanning pass of the scanning beam 11, the first deflection of the beam by the glass occurs when the beam first encounters a side edge boundary of the glass. The photodetector signal responsive to that beam deflection is fed to the corresponding gate circuit 28, as are the beam position monitoring pulses from counter 26. The gate circuit transmits from the counter 26 to the accumulator/calculator 30 a counter output signal which is indicative of the beam position at the instant that such beam deflection signal is delivered to that gate circuit via the OR circuit 32. Such signal transmission via the gate circuit 28 occurs in each of a plurality of scanning cycles of the beam, the number of such plurality being determined by the setting of the tally device 29.The beam position signals transmitted from the gate circuit in the preset number of scanning cycles are accumulated in the accumulator/calculator 30 and this device then calculates the averages of the accumulated beam positions and transmits signals representing such averages to the edge position register 31. In a particular embodiment the tally setting is 128 cycles and the accumulation of the 128 monitoring pulse numbers representing the location of a ribbon edge, and the computation of the average value, takes 8 seconds.

The signals representing the mean positions of the respective glass edges over the time intervals determined by the setting of the tally device 29 are fed to a comparator circuit 33.

They are also fed to mechanism for controlling the positioning of glass cutters for cutting the glass ribbon at a site downstream from the defect detection station, including cutters for cutting the glass ribbon into sheets of required sizes and quality ratings and cutters for continuously removing side marginal portions of the ribbon. By. using the said signals to control movements of margin cutters in a direction transverse to the path of the ribbon, the width of the margins cut from the glass can be kept constant even over periods during which there is a variation in the positions of the ribbon edges where they pass through the cutting station. Said glass edge position signals can also optionally be supplied to an indicator or recorder 34 which can be inspected by an operator.

The beam positions which coincide with beam deflections by defects in the glass are registered by a defect register 35. To this end, signals from the photodetectors 22 and 23 are fed to such defect register 35 via an inhibitor circuit 36 one purpose of which is to filter off noise, including spurious signals occa sioned by incidental light reflections from parts of the glass conveying and testing installation.

The register 35 receives beam position moni -toring signals from the counter 26 and transmits to the comparator circuit 33 signals indicative of positions of the beam at the in stants at which beam deflection signals are received from -the photodetectors 22 and 23.

The inhibitor circuit 36 also receives signals from the photodetectors 16 and 17 which are located close to the ends of the scanning beam reflector 3 and only transmits to the defect register 35 deflection signals which oc cur during movement of the beam between those detectors.

Signals indicative of beam positions which are coincident with defects signalled by the detectors 22 and 23, like the ribbon edge location signals from the edge position regis ter 31, are fed to the comparator circuit 33 and to a gate circuit 37. In the comparator circuit the- defect position signals from defect register 35 are compared with edge position signals from register 31 to determine which defects are more than a certain predetermined distance from each edge of the ribbon. In re sponse to defect position signals which are in that category the comparator circuit transmits an enable signal to the gate circuit 37. This signal opens the gate circuit and allows the corresponding defect location signals received from defect register 35 to pass to a micropro cessor 38.This microprocessor is also con nected, via an OR circuit 39, with reference detectors 16, 17 so as to receive signals in dicative of the moments when the beam is commencing a new pass across the glass.

Such signals are of course important in deter mining the sizes of individual defects.

The microprocessor has three output lines 40-42. Line 40 transmits signals representing the transverse distance of the defects from datum position X. Line 41 transmits signals indicative of the nature of a signalled defect.

Defects are signalled in different categories depending on whether or not they cause beam deflection sufficient to interrupt irradia tion of detector 23. Line 42 transmits signals indicative of the length of a signalled defect, a factor which is derivable from the number of successive scanning passes of the beam in which beam deflection is signalled at approxi mately the same beam position. In this con nection, it is often convenient to register small defects in very close proximity to each other as if they were a single defect. For example, if defect signals occur at the same beam position more than once within an interval of up to forty scanning cycles such signals can be treated as representing a single defect.

The signals transmitted via lines 40-42 are transmitted more or less simultaneously with the incidence of the scanning beam on the defects giving rise to those signals. Therefore the times at which those signals are transmitted are indicative of the longitudinal positions of the defects. Taken in conjunction with the speed of travel of the glass, such times of defect signal transmission enable the times of arrival of the defects at any given position downstream from the scanning station to be determined.

For the size classification function the microprocessor incorporates a number of multi-cell registers. Defect location signals are transmitted to one or another cell of each register, depending on the transverse ordinate of the signalled location. Defect location signals representing corresponding transverse ordinates are transmitted to the same register cells. The cells of each register have associated exit gates on which a threshold charge value is conferred which is appropriate to a particular defect size category. There are different threshold charge values, one for each register. The number of registers corresponds with the number of different size categories into which defects are to be classified.Location signals representing the same transverse ordinate and transmitted in immediately successive passes of the scanning beam have a cumulative charging effect on the cells to which they are transmitted. Therefore the number of charge increments accumulated in any given cell is indicative of the lengthwise dimension of the defect causing the cell load -ing. The addition of a charge increment to any given cell is accompanied by a decrease in the threshold charge value conferred on the associated gate. If the threshold charge value on any gate falls to zero a signal is transmitted from the associated cell thereby indicating that the lengthwise dimension of the defect having the transverse ordinate which pertains to that cell is at least equal to the threshold value represented by the pre-set gate charge.If an increment of charge is not added to a cell in any given pass of the beam then any charge previously accumulated by the cell and the residual charge increment on the associated cell are automatically drained.

A refinement of the apparatus described and illustrated, which has been found to be beneficial, involves the installation of photodetectors located abreast of the correct tracking path of the scanning beam across the reflector 3 and the connection of those photodetectors to an adjustment device for the oscillating reflector 10 so that that reflector is automatically adjusted in the event that vibration or some other disturbing influence causes the beam to move lateraily in one direction or the other away from its correct track along the reflector 3. Such additional photodetectors may for example be located near one end of the reflector 3, for example in a plane which is normal to the plane of Fig. 1 and betw.een reference photodetector 16 and the corre sponding end of the reflector 3.

In a modification of the described apparatus, the photodetector 22 is dispensed with. The photodetector 23 alone is used for detecting the glass edge locations and the locations of defects in the glass. The photodetector 23 can in that case perform as previously described or it can be designed so that it also yields a signal in the event that the incident radiation flux falls below a certain value, thereby indicating attenuation of the beam, caused for example by a stain on the glass. In those circumstances the apparatus will not signal the location of defects which have only a slight deflecting effect on the scanning beam. In the event that a signle glass edge and defect location photodetector is used there is of course no need for the semi-transparent mirror 19. The photodetector can be at the location of photodetector 22 in the drawing. Of course various other photodetector arrangements are possible within the scope of the invention. For example photodetector 23 can be retained in its illustrated position for the purpose of detecting large beam deflections and photodetector 22 can be designed and used without the mask 24 for detecting beam attenuations.

Claims (43)

1. A method of determining the location of defects present in flat glass as it travels along a path by scanning the glass with a beam of electromagnetic radiation and using photodetector means to detect incidence of the beam on defects in the glass, characterised in that - the glass is scanned by a beam of electromagnetic radiation which sweeps transversely across said path so that the beam traces successive transverse tracks across the glass and so that in each scanning pass the beam passes through the opposed side edge boundaries of the glass; -said photodetector means serves to -detect deflections of the beam due to its incidence on one or each side edge boundary of the glass, and to detect attenuations or deflections of the beam by defects in the glass;; -said photodetection means forms part of signalling means which yields output signals indicative of the positions along the scanned length of the glass of transverse tracks in which defects are detected and indicative of the distances, measured along such tracks, between such defects and a side edge boundary of the glass, and -said output signals are used to identify the signalled defect locations.

2. A method according to Claim 1, wherein the signalling means generates output signals which are used as a control factor in the automatic control of a glass marker or cutter for marking or cutting the travelling glass at a site downstream from the station where the glass is scanned.

3. A method according to Claim 1 or 2, wherein an average value is derived from glass edge deflection signals transmitted by the photodetection means in a multiplicity of successive passes of the beam across the glass in one direction and that average value is utilised in the derivation of the output signals indicative of the defect locations.

4. A method according to any preceding claim, wherein said output signals are generated only for defects which are more than a certain predetermined distance from each side edge boundary of the glass.

5. A method according to any preceding claim, wherein the displacement of the beam is monitored by monitoring means which yields signals indicative of the beam positions relative to a fixed datum position which is outside the limits of the beam movement across the glass and such signals are used by said signalling means in the derivation of said output signals.

6. A method according to claim 5, applied for detecting defect locations in a continuous ribbon of glass, wherein said datum position is fixed in relation to the frame of the glass conveyor and signals indicative of the distance of at least one side edge boundary of the ribbon from said datum position are used for automatically controlling cutting means which is mounted further downstream along the ribbon path and serves to cut a marginal portion of predetermined width from one or each side of the ribbon.

7. A method according to claim 5 or 6, wherein signals which are transmitted from a photodetector located at said datum position or in fixed relationship to that datum position, responsive to irradiation of such photodetector by the scanning beam, automatically repeatedly re-set said beam displacement monitoring means to ensure that it continues correctly to represent the positions of the beam relative to said datum position.

8. A method according to any of claims 5 to 7, wherein said signalling means yields output signals which represent (a) the distance travelled by the scanning beam in successive scanning cycles from said fixed datum position to the position at which the beam encounters the nearer side edge boundary of the glass and (b) the distance between said datum position and a signalled defect.

9. A method according to any preceding claim wherein, in each scanning pass of the beam, the beam is incident upon a reference photodetector, causing generation of a reference signal, just before the beam reaches the nearer side edge boundary of the glass and the first subsequent beam deflection signal is processed by said signalling means as a signal indicative of the incidence of the beam on a side edge boundary of the glass.

10. A method according to any preceding claim, wherein each scanning movement of the beam is encoded as a series of signal pulses so that any given instantaneous position of the beam corresponds with a given pulse number.

11. A method according to claims 9 and 10, wherein said signal pulses are fed to a pulse counter which in each scanning cycle translates beam positions into numbers of pulses counting from zero or some other predetermined datum setting which correspoonds with a predetermined position of the beam outside the limits of the beam movement across the glass, and wherein in each of successive passes of the beam, irradiation of the first of the two reference photodetectors encountered by the beam causes the counter to be re-set to a value representing the beam displacement distance between the said datum position and that reference photodetector.

12. A method according to any Claim 10 or 11, wherein a beam of radiation deriving from the same radiation source as the glass scanning beam is caused, synchronously with the scanning of the glass, to scan a reflector having alternating reflecting and non-reflecting bands, and the radiation quanta reflected from that banded reflector are incident on a photoelectric detector and thereby create said signal pulses.

13. A method according to Claim 10 or 11, wherein the positions of the scanning beam in course of each scanning pass are monitored on the basis of the passage of time from the commencement of the pass.

14. A method according to Claim 13, wherein the scanning beam is an angularly oscillating beam and the amplitude of the beam movement in the plane of the glass is at least twice the width of the glass.

15. A method according to any preceding claim, wherein the number of immediately successive beam passes in which defect-induced signals are generated when the beam is at approximately the same distance from a given boundary edge of the glass are registered and give rise to output signals indicative of defect size.

16. A method according to any preceding claim, wherein the beam is at all times during its irradiation of the glass reflected back through the glass before reaching the photodetector(s) from which defect signals derive.

17. A method according to any preceding claim, wherein the radiation beam leaving the glass is split into a part which is transmitted to the site of a first detector and a part which is transmitted to a second detector; and one of those detectors is arranged so that it is irradiated whenever the beam is deflected arid the other of such detectors is arranged so that it is irradiated unless the beam is intercepted by an opaque defect or is deflected by a defect to more than a certain predetermined extent.

18. A method according to any preceding claim, wherein the photodetection means not only detects deflections of the scanning beam such as are caused by the side edges of the glass but also detects defects which attenuate the beam without deflecting it.

19. A method according to any preceding claim, wherein the scanning beam is emitted by a stationary emitter and the scanning beam is deflected on the glass by an oscillating deflector.

20. A method according to any preceding claim wherein deviations of the scanning passes of the beam to more than a given extent from a predetermined plane cause irradiation of deviation photodetectors and the scanning path is automatically correct by response signals from such deviation detectors.

21. A method according to any preceding claim, wherein the glass is scanned at a frequnecy of at least 20 cycles per centimetre length of the glass.

22. A method according to any preceding claim, wherein the scanning beam is a laser.

23. Apparatus for determining the location of defects present in flat glass as it travels along a path, such apparatus comprising means for conveying flat glass through a testing station, means for generating a beam of electromagnetic radiation and causing such beam to pass repeatedly across said path and thereby trace successive transverse tracks across the glass at said testing station, and photodetector means for detecting incidences of the beam on defects in the glass, characterised in that said photodetector means is constructed to yield signals indicative of beam deflections, or of beam attenuations as well as beam deflections by the glass; and the apparatus comprises a beam monitor for generating signals which represent the beam positions, means associated with said beam monitor for registering, in respect of each scanning pass of the beam in one direction, or periodically in respect of a succession of such scanning passes, the position or average position reached by the beam, in such pass or passes, when a first signal caused by beam deflection by the glass is received from said photodetector means; and signal processing means which is constructed so as in use to transmit output signals which indicate the time of occurrence of a beam pass in which a later signal indicative of beam deflection or attenuation by the glass is received from said photodetector means after the beam has left said position or average beam position, and which are also indicative of the distance moved by the beam in the time interval between the moment it reaches said position or average position and the moment of said later signal reception.

24. Apparatus according to Claim 23, wherein the signal processing means is connected to a control mechanism of a glass marker or cutter located along the path of conveyance of the glass, downstream from the scanning station, so that the marker or cutter marks the glass at the locations of the defects or, as the case may be, cuts the glass at positions which take account of the signalled defect locations.

25. Apparatus according to Claim 23 or 24, wherein there is discounting means for preventing the output signals from the signal processing means from indicating the location of defects which are less than a certain predetermined distance from a side edge boundary of the glass.

26. Apparatus acording to any of claims 23 to 25, wherein the beam monitor is constructed to generate signals representing the beam positions relative to a fixed datum position which is outside the limits of the beam movement across the glass.

27. Apparatus according to claim 26, wherein glass cutters are mounted at a position along the path of the glass, downstream from the testing station, and wherein there is means which in successive scanning cycles generates signals indicative of the distances of the beam from said datum position when the beam encounters the side edge boundaries of the glass, and wherein there is means for controlling said cutters in dependence on those signals to cause the cutters to cut marginal portions of predetermined width from the glass during its movement away from the testing station.

28. Apparatus according to claim 26 or 27, wherein at least one photodetector located in the scanning path of the beam, - at or in fixed relation to said datum position, is operatively connected to said beam monitor to cause it to be automatically repeatedly re-set by signals transmitted from such photodetector thereby to ensure that the monitor signals correctly represent the beam positions relative to said datum position.

29. Apparatus according to any of claims 26 to 28, wherein said signal processing means is designed to yield output signals representing (a) the distance travelled by the scanning beam in successive scanning cycles from said fixed datum position to the position at which the beam encounters the nearer side edge boundary of the glass and (b) the distance between said datum position and the position at which the beam encounters a defect in the glass.

30. Apparatus according to any of claims 23 to 29, wherein there is a pair of photodetectors (in subsequent claims called "reference photodetectors") which are located near opposite ends of the path swept by the beam during tracking across the glass, such reference photodetectors are connected to the signal processing means which transmits output signals indicative of beam deflection by the glass, and said processing means is con structed so that it does not treat a photodetector response signal as indicative of beam deflection or attenuation by a glass defect if such signal is delivered during a first part of a scanning pass of the beam, before the beam reaches a said reference photodetector.

31. Apparatus according to any of claims 23 to 30, wherein there is means which encodes each scanning movement of the radiation beam as a series of signal pulses so that any given instantaneous position of the beam corresponds with a given pulse number.

32. Apparatus according to claims 30 and 31, wherein there is a pulse counter for tran slating beam positions in each scanning cycle into numbers of pulses counting from zero or some other predetermined datum setting cor responding with a predetermined datum position of the beam, and wherein said reference photodetectors are operatively connected to said counter so that in each of successive scanning passes of the beam the response signal resulting from irradiation of the first of the two reference photodetectors encountered by the beam causes re-setting of the counter to a value representing the beam displacement distance between the said datum position and that reference photodetector.

33. Apparatus according to Claim 31 or 32, wherein there is a reflector having alter nating reflecting and non-reflecting bands, means for causing a second beam of radiation (hereafter called "monitoring beam") deriving from the scanning beam generating means to scan such reflector in synchronism with the scanning movements of the glass scanning beam, and a photodetector which is located to be irradiated by quanta of radiation reflected from said banded reflector and which yields said signal pulses reponsive to such ir radiation.

34. Apparatus acording to Claim 31 or 32, wherein there is a digital clock which monitors scanning beam movement in terms of the pas sage of time from the commencement of each scanning pass and yields signal pulses indica tive of the beam position.

35. Apparatus according to any of claims 23 to 34, wherein the signal processing means is designed so as in operation to yield output signals indicative of the number of im mediately successive beam passes in which a beam deflection or attenuation signal is transmitted when the beam is at approxi mately the same distance from a side edge boundary of the glass.

36. Apparatus according to any of claims 23 to 35, wherein there is a reflector which is arranged so that it reflects the scanning beam back through the glass and the photodetector means for detecting defect-induced beam deflections or attenuations is arranged so as to be responsive to deflections or attentua tions of that reflected beam after its second emergence from the glass.

37. Apparatus according to any of claims 23 to 36, wherein there is a beam splitter for splitting the scanning beam, after it leaves the path of travel of the glass, into two derivative beams, and there are separate photodetectors for said derivative-beams, such photodetectors being arranged so that when the apparatus is in use one of them will be irradiated only when the scanning beam is deflected by the glass whereas the other of them will be irradiated unless the scanning beam is prevented from being transmitted by the glass or is deflected to more than a certain predetermined extent.

38. Apparatus according to any of claims 23 to 37, wherein the photodetection means comprises a photodetector which is arranged so that when the apparatus is in use it is normally irradiated by the beam emerging from the glass and which yields a signal if the incident radiation falls below a predetermined minimum threshold value.

39. Apparatus according to any of claims 23 to 38, wherein the beam generating means is held so that it remains stationary when the apparatus is in use and the apparatus comprises a deflector coupled to means for oscillating such deflector thereby to produce the scanning movements of the beam.

40. Apparatus according to claim 39, wherein the means for oscillating said -deflec- tor comprises a torsion element attached to the deflector and electromagnetic or other means for oscillating such element at its natural frequency.

41. Apparatus according to any of- claims 23 to 40, wherein there are deviation photodetectors located for yielding a response signal if the path of the scanning beam deviates to more than a certain extent from a predetermined plane, and such deviation photodetectors are operatively connected to adjustment means to cause automatic correcting adjustments of said path.

42. Apparatus according to any of claims 23 to 41, wherein the means for generating the scanning beam, the defect photodetectors, and the means for imparting scanning movements to the beam, are integrated into a unitary assembly.

43. Apparatus according to any of claims 23 to 42, wherein the means for generating said radiation beam is a laser gun.