GB2125538A - Detecting sheet markings - Google Patents

Abstract

Apparatus for detecting a marking 2 on a portion of a sheet 1 comprises a support for the sheet, a source 4, preferably a laser, for scanning the sheet with a radiation beam 3, detector means 5 for detecting the beam 3 transmitted or reflected by such a sheet and means 9 to 13 for generating a pulse when said beam is affected by a said marking, and signaling means 14 arranged to receive any such pulse, to count the number of such pulses received, and to generate a signal in dependence upon the presence or absence of a predetermined number of said pulses in a predetermined time. A method of detecting such a marking comprises scanning such portion with a beam of radiation, generating a pulse when the beam is affected by a marking and passing that pulse to signaling means to generate a signal in dependence upon the presence or absence of a predetermined number of pulses in a predetermined time. <IMAGE>

Description

SPECIFICATION Detecting sheet markings This invention relates to an apparatus for and a method of detecting the presence or absence of a marking on a portion of a sheet.

Certain types of sheet require marking for various purposes. One example is for trade mark purposes. Another example lies in the field of safety glazings, for example glazings for vehicle windows. In many cases such glazings have to meet certain standards of safety which are laid down by authority, and it is also necessary that these glazings should be marked to indicate that they satisfy the required standard. It is in some circumstances illegal to sell such glazings without the required safety marking even though the glazing itself may be of the required standard. Furthermore, the making must often be located in a particular designated area of the sheet.

The required marking may be applied in any desired way to a sheet, for example by sand blasting, etching, serigraphy or decalcomania, but it is desirable for factory purposes to apply the marking before final treatment of the product. For example in the case of sand blasting or etching a glass sheet for a toughened windscreen, such marking must be made before the glass is tempered. In the case of laminated glazings, the markings are applied before the lamination step.

It has been common practice hitherto to mark the sheets as they progress along a production line to a further processing station, for example a tempering or laminating station, and the sheets are checked for the marking before such further processing takes place.

This checking has hitherto been done by a human operator with the inevitable result that towards the end of a working shift the operator passes as 'marked' sheets which are not marked, with the result that these sheets are made up into unsaleable products.

It is an object of the present invention to provide apparatus for monitoring such marking automatically and reliably.

According to the present invention there is provided apparatus for detecting the presence or absence of a marking on a portion of a sheet comprising: a support for a said sheet; a radiation source for delivering a beam to scan a sheet on said support; detector means for detecting the beam transmitted or reflected by such a sheet and means for generating a pulse when said beam is affected by a said marking; signalling means arranged to receive any such pulse and comprising means for counting the number of such pulses received and for generating a signal in dependence upon the pressure or absence of a predetermined number of said pulses in a predetermined time.

Such an apparatus can simply and reliably give an indication of the presence or absence of the required marking in the scanned sheet portion. Since detection depends on the presence of a predetermined number of pulses in a predetermined time, the apparatus can also be used to detect the difference between a well-formed and a partially formed marking. It is not necessary to scan the whole of a sheet since in practice, the marking would normally be located within a well defined area, for example close to one corner of the sheet.

The signal generated by the signalling means can be used to give for example a luminous signal indicating the presence or absence of a marking on the sheet examined.

Alternatively, the signal can be used to control the position of conveyor track switchgear so that the sheet follows one of two paths depending on whether it is marked or not. Or the signal can be used to sound an alarm when no marking is detected. Of course the signal generated by the signalling means can be used for any combination of these purposes and/or for any other purpose known per se.

Said radiation source is preferably a light source, and advantageously a laser light source so as to enable a narrow high energy beam to be used for scanning.

Said pulse generator means preferably includes a theshold circuit for passing pulses which exceed a predetermined threshold amplitude. This again helps to avoid spurious signals.

Such predetermined threshold amplitude is preferably adjustable so that account may be taken of the different levels of pulses which might be given off by different kinds of marking.

Advantageously, sheet detector means is provided for enabling the signalling means whenever a said sheet is in the vicinity of the path of the beam. This helps to avoid spurious signals.

In preferred embodiments of the invention the arrangement is such that said pulses are passed to a first gate arranged, when enabled by said sheet detector means, to transmit such pulses to a counter circuit which is arranged to deliver a signal when said predetermined number of pulses has been counted in said predetermined time.

Preferably, said counter circuit comprises a first monostable circuit arranged so that it can be enabled by pulses transmitted by said first gate in order to emit a signal for a predetermined time to activate the counter.

Advantageously, said counter circuit comprises a first bistable circuit arranged to control a second gate to allow passage of such counter-activating signal only after the entry of said sheet into the vicinity of the path of the beam has been signalled by said sheet detector means.

The invention includes a method of detect ing the presence or absence of a marking on a portion of a sheet comprising scanning such portion with a beam of radiation, monitoring the transmitted or reflected beam to generate a pulse when said beam is affected by a said marking and passing said pulse to signalling means to generate a signal in dependence upon the presence or absence of a predetermined number of said pulses in a predetermined time.

Such method preferably includes one or more of the following optional features: said sheet is scanned with a laser light beam; the amplitudes of said pulses are compared with a threshold value and the pulses are only passed when their amplitudes exceed such threshold; such threshold value is adjustable; said signalling means is blocked unless the presence of a sheet in the vicinity of the path of the beam is detected; unless blocked, pulses are passed to a counter circuit which delivers a signal when said predetermined number of pulses has been counted in a predetermined time; unless blocked, said pulses enable a first monostable circuit to cause it to emit a counter-activating signal for a predetermined time; and said counter-activating signal is blocked until after the entry of a said sheet into the path of the beam.

An example of the present invention will now be described in greater detail with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a functional diagram of an embodiment of apparatus according to the invention, and Figure 2 is a graph indicating signals transmitted by various elements of the diagram of Fig. 1.

In Fig. 1, a transparent sheet 1 bearing a marking 2 is conveyed through the optical path of a laser beam 3 emitted by a laser light source 4. Photoreceptor means 5 comprising a photo-cell 6 and a focussing lens 7 is arranged to intercept the laser beam 3. The signal emitted by the photo-cell 6 is passed to a DC amplifier 8 arranged to deliver a substantially constant voltage to a capacitor 9 while the laser beam is incident on the photocell. When the laser beam 3 is interrupted or attenuated, the capacitor 9 delivers a pulse to an AC amplifier 10 which in turn passes a pulse to a noise eliminating circuit 11. The pulse is then passed to a rectifier circuit 1 2 and thence to an adjustable threshold circuit 13.

When the pulse is of sufficient amplitude, the adjustable threshold circuit 1 3 then passes pulse signals indicative of the presence of a marking 2 on the sheet 1 to a gate ET1 of signalling means 14.

A detector P for example a photo-electric detector or a microswitch is located close to the path of the laser beam 3 to indicate the presence or absence of a transparent sheet 1 and and is arranged to cause or allow delivery of a signal (compare Fig. 2) when such a sheet is present to the gate ET1 of the signalling means to enable it to pass pulse signals from the comparator 1 3. Such signals are passed to a counter 1 5 and to a monostable circuit Or1, the latter being arranged to emit a signal (compare Fig. 2) for a predetermined time from its receipt of the pulse signal from the gate ET1. This predetermined time is eg adjusted to a value corresponding to the time it would take the whole of a said marking 2 to be scanned by the laser beam 3.The timer signal from oscillator circuit OS1 is passed to a second gate ET2 which also receives input from a first flip-flop FF1. This flip-flop is enabled by the sheet presence indicating signal from the photo-electric detector P and is actuated by the end of a first timer signal to allow passage of a second timer signal through the second gate ET2 to the counter 1 5 when a second pulse or pulse group is delivered from the comparator 1 3 to the monostable circuit OS1 via the first gate ET1. By this arrangement, the pulse signal which will be generated as the edge of the sheet crosses the path of the laser beam will not register at the counter. Of course other ways of achieving this result are possible.The counter 1 5 then commences to count these pulses for as long as the timer signal runs. When the required predetermined number of pulses has been reached, the counter passes a signal to a second flip-flop FF2 which is set to high level when no signal reaches it from the sheet presence detector P. A signal indicative of the required number of pulses causes the output of the second flip-flop FF2 to drop. The output from the second flip-flop FF2 is passed to a third gate ET3. A second monostable circuit OS2 is arranged to deliver a pulse when the sheet presence indicating signal fed to it ceases, and this pulse also goes to the third gate ET3, where it is blocked by the absence of signal from the second flip-flop FF2.

The second flip-flop FF2 is a memory flipflop which is arranged to revert to high level with a short delay after the signal from the sheet presence detector P stops, this reversion taking place after the sheet has left the path of the beam and so after the pulse from the second monostable circuit OS2.

If on the other hand, an insufficient number of pulses is counted in the duration of the timer signal then the second flip-flop is not disabled and its output continues at high level. Thus when the second monostable circuit OS2 emits its pulse on removal of the transparent sheet 1, the third gate ET3 is able to pass a signal to an alarm 1 6.

The relative levels and durations of pulses emitted by the sheet presence detector P, the comparator 13, the first gate ET1, the first monostable circuit OS1, the first flip-fop FF1, the second gate ET2, the second flip-flop FF2, the second oscillator OS2 and the third gate ET3 are indicated in Fig. 2.

In operation, the leading edge of a sheet 1 passes the sheet presence detector P at time T1, this sends P high. At time T2 the edge of the sheet intersects the laser beam causing one or more pulses to be emitted by the comparator and passes through gate ET1 to the counter 1 5 and to the first monostable circuit OS1 so that that in turn emits a high timer signal for a predetermined time. The counter is not at this stage counting since no signal is reaching it through the second gate ET2. Decay of the timer signal at time T3 from OS1 while the sheet detector signal from P is high sends the first flip-flop high (FF1).

Thus when the next timer signal is delivered at time T4 from the first monostable circuit the gate ET2 is open and the counter can count the number of pulses delivered. When at time T5, sufficient pulses have been counted, the second flip-flo.p FF2 is deactivated to prevent third gate ET3 from passing any signal, as shown in solid lines.

If however time T6 passes without a sufficient number of pulses being counted, the counter-activating timer signal from the first monostable circuit OS1 being then arranged to stop, the second flip-flop FF2 remains high (as shown in dotted lines) and when the sheet leaves the presence detector P and P signal drops thus deactivating the first flip-flop and also causing the second monostable circuit to emit a pulse to the third gate ET3 which in turn feeds a signal to the alarm circuit '(in dotted lines), at time T7.

Claims (18)

1. Apparatus for detecting the presence or absence of a marking on a portion of a sheet comprising: a support for a said sheet; a source for delivering a beam of radiation to scan a sheet on said support; detector means for detecting the beam transmitted or reflected by such a sheet and means for generating a pulse when said beam is affected by a said marking; signalling means arranged to receive any such pulse and comprising means for counting the number of such pulses received and for generating a signal in dependence upon the presence or absence of a predetermined number of said pulses in a predetermined time.

2. Apparatus according to Claim 1, wherein said radiation source is a laser light source.

3. Apparatus according to Claims 1 or 2, wherein said pulse generator means includes a threshold circuit for passing pulses which exceed a predetermined threshold amplitude.

4. Apparatus according to Claim 3, wherein said threshold amplitude is adjustable.

5. Apparatus according to any of Claims 1 to 4, wherein sheet detector means is provided for enabling the signalling means whenever a said sheet is in the vicinity of the path of the beam.

6. Apparatus according to Claim 5, wherein the arrangement is such that said pulses are passed to a first gate arranged, when enabled by said sheet detector means, to transmit such pulses to a counter circuit which is arranged to deliver a signal when said predetermined number of pulses has been counted in said predetermined time.

7. Apparatus according to Claim 6, wherein said counter circuit comprises a first monostable circuit arranged so that it can be enabled by pulses transmitted by said first gate in order to emit a signal for a predetermined time to activate the counter.

8. Apparatus according to Claim 7, wherein said counter circuit comprises a first bistable circuit arranged to control a second gate to allow passage of such counter-activating signal only after the entry of a said sheet into the vicinity of the path of the beam has been signalled by said sheet detector means.

9. A method of detecting the presence or absence of a marking on a portion of a sheet comprising scanning such portion with a.

beam of radiation, monitoring the transmitted or reflected beam to generate a pulse when said beam is affected by a said marking and passing said pulse to signalling means to generate a signal in dependence upon the presence or absence of a predetermined number of said pulses in a predetermined time.

10. A method according to Claim 9, wherein said sheet is scanned with a laser light beam.

11. A method according to Claim 9 or 10, wherein the amplitudes of said pulses are compared with a threshold value and the pulses are only passed when their amplitudes exceed such threshold.

1 2. A method according to Claim 11, wherein such threshold value is adjustable.

1 3. A method according to any of Claims 9 to 12, wherein said signalling means is blocked unless the presence of a sheet in the vicinity of the path of the beam is detected.

1 4. A method according to Claim 13, wherein, unless blocked, pulses are passed to a counter circuit which delivers a signal when said predetermined number of pulses has been counted in a predetermined time.

15. A method according to Clam 14, wherein, unless blocked, said pulses enable a first monostable circuit to cause it to emit a counter-activating signal for a predetermined time.

1 6. A method according to Claim 15, wherein said counter-activating signal is blocked until after the entry of a said sheet into the path of the beam.

1 7. A method according to Claim 9 and substantially as herein described.

18. Apparatus according to Claim 1 and substantially as herein defined.