WO2008018826A2

WO2008018826A2 - Object detection system

Info

Publication number: WO2008018826A2
Application number: PCT/SE2007/000718
Authority: WO
Inventors: Staffan SJÖGREN
Original assignee: Scirocco Ab
Priority date: 2006-08-09
Filing date: 2007-08-09
Publication date: 2008-02-14
Also published as: WO2008018826A3; SE0701264L

Abstract

Object detection system that makes it possible to with extreme safety read a holed barcode tag in the surface treatment process of an automotive factory, and/or to reliably detect, classify and/or decode other moving objects such as a moving machine for operator safety, to classify a moving vehicle at a driving gate or truck scale for access control or debiting, and to record the direction, speed and length of a moving railcar for improved logistics control. The system comprising least one illumination unit with at least one light emitting diode with a focusing lens that transmits amplitude modulated and preferably infrared light of 700 - 1200 nm wavelength in direction to a barcode tag or other object to be detected, and, where at the other side of the barcode tag or other object a reader is located, said reader comprising a photo detector that is especially sensitive for light of the wavelength and modulation pattern that is sent out from the illumination unit.

Description

Object detection system

The present invention makes it possible to with extreme safety read a holed barcode tag in the surface treatment process of an automotive factory, but also to reliabily detect, classify and/or decode other moving objects such as a moving machine for operator safety, to classify a moving vehicle at a driving gate or truck scale for access control or debiting, and to record the direction, speed and length of a moving railcar for improved logistics control.

In the first example, a barcode tag is fastened to the car body, or to the car body's carrier structure, that shall be identified at different workcells, thereby enabling automatic identification and control of the painting process. The barcode tag has both to be compact and stand stress such as soaking in liquids, overpainting, passage through ovens, rough handling at dismounting, transportation and remounting, as well as regular cleaning from paint with solvents, high- pressure water-jet and/or freezing at temperatures. Because of the differencies between individual identification points at the workcells, the distance between barcode tag and reading equipment can vary considerably, as can passage speed and direction of passage. Because of the large number of workcells in a paint line, installation must be very easy.

Background of the invention

A number of RFID (Radio Frequency Identification) systems are since the mid '80:ies offered for automatic object identification in surface treatment applications. More or less all of these however suffer both from high tag cost and low tag reliability, mainly due to the earlier described demanding environments. The systems also fail because of metallic environments that cause undesired signal reflections that lead to false readings and errors in the process.

SUBSTITUTE SHEET (RULE 26) An alternative to RFID is described in US4401892 from 1983, where a moving tag, with a hole pattern that is unique to each tag, is illuminated so that light that pass through said holes can be detected by an array of sensors for identification of the tag. A similar solution is described in KP0032650 from 2005, in which the sensor array has been designed so that a so called bit map is created from each sensor, whereupon the output from the different sensors are compared and conclusions made about possibly wrongly decoded tags. It goes without saying that any risk for errors in the decoding will jeopartize the effectivity and cause major disorder in the manufacturing process. Typically, the read rate shall be better than 99.999%.

The mentioned barcode tags are either made from metal or from a high-grade polymer, in which a code pattern has been arranged. Commercial holed identification tag products that are offered to the market may have solved the problem with environmental resistance, but still suffer from tags that are so large and heavy that transportation, installation and use is a major problem. They are furthermore sensitive to variations in the passage speed while passing the light beam, and at what distance between light sources and sensors that the tags are passing.

Summary of the invention

The invention solves the initially described problems, including to allow a relatively small and low-cost holed barcode tag to pass at different distances and speeds between a light beam transmitter, here called illuminator, and a decoding equipment, here called reader, and still be safely read. In a special embodyment, the barcode tag is even allowed to make a full stop during passage and still be safely read.

SUBSTITUTE SHEET (RULE 26) The invention can also replace threadles, magnetic loops, mag- netoresistive detectors, radars, lasers, TV cameras or other measures that are used to detect and classify other objects that interrupt the beam between illuminator and reader, such as robotic arms, roller cages and other trolleys, trucks, trailers, railcars and cars. Thanks to its high performance, it can for instance determine the passage direction and speed of different detail objects such as the wheel of a railcar, truck or trailer, and even safely discriminate between wheels and smaller objects that may interrupt the beam such as a hanging rope, a mud flap or a towing hook.

Instead of using said measures, with a solution according to the herein described invention it is also possible to e.g. automatically measure the length of a passing vehicle by detecting the speed and time at each beam crossing, i.e. at the beginning and end of each passing wheel, to get the passage speed. A simple calculation of the elapsed time between passage of wheels, the distance between the wheels is automatically determined.

In this description of the invention, a limitation regarding all of its many applications has been made, this way mainly limiting the detailed descriptions to a representative embody- ment . The invention makes in this embodiment use of a holed barcode tag, the symbols of which being represented by holes in the barcode tag material and by the barcode tag material in itself, where at least one illumination unit based on a light emitting diode with a lens and light slit transmits amplitude modulated and preferably infrared light in the wavelength area 700 - 1200 nm in direction to said barcode tag, and, where at the other side of the barcode tag a reader is located so that has a photo detector that is especially sensitive for light of the wavelength and modulation pattern that is sent out from the illuminator in such a way so that the light from the illu-

SUBSTITUTE SHEET (RULE 26) minator via the barcode tag will reach the photodetector via at least one positive lens in front of the photodetector, and where a light slit is arranged at either side of the lens in front of the photodetector so that said light slit is essentially parallel with the light slit of said light emitting diode and with the symbols of the barcode tag, thereby making the sensitive area of the photo detector at each instance only being illuminated through the symbol that at that time is projected on said reader light slit of the reader when the barcode tag is passing between illuminator and reader.

In a special embodyment of the invention, the illuminator comprises at least two adjacent light emitting diodes, LEDs, each with a focussing lens and light slit arranged essentially in parallel to the light slit of the reader, or at least two groups of such LEDs with light slits and focussing lenses, where said LEDs both transmit modulated light in direction to the reader and where this transmission comprise at least two different transmission patterns that each is unique to the LED or group of LEDs in question. The special embodyment also comprises a receiver light slit and focussing lens that both are arranged in front of the photosensitive area of said receiver and where the receiver light slit is essentially narrower than the most narrow symbol of the barcode tag to be detected, or essentially narrower than the smallest resolution needed to detect and/or classify another moving object.

By using amplitude modulated light from the illuminator, the receiver in the reader can be tuned the the modulation frequency of said modulated light and therefore be given very good detection sensitivity to this light, while at the same time the resistance to interfereing light from other light sources, such as daylight from the sun, flourescent light from e.g. light tubes and incandescent light from e.g. ordinary bulbs is improved.

SUBSTITUTE SHEET (RULE 26) By said use of said modulated light and focussing lenses at both the light source and the receiver, the object detection system system can operate at such a low light intensity that even a large distance between illuminator and reader can be accepted despite the fact that the active surface of the illuminator and receiver lenses are reduced by the blocking areas of the light slits. This means that low-cost LED technology instead of a laser technology can be used in the light source or light sources. A major advantage of using a LED instead of a laser is that the relatively wide light beam of the LED allows wide tolerances in the alignment of illuminator and reader during installation.

Said measures means that the invention, compared to known holed tag solutions e.g. according to US4401892 and KR0032650 will have considerably larger signal margins and precision in the decoding safety even though a large part of the light from the LEDs is blocked out by the light slits of the LED or LEDs and receiver. Typcially, the distance between illuminator and reader can be several metres.

The light slits, with a size that is considerably smaller than the most narrow symbol of the barcode tag, or other moving object to be detected, means that the system is without focusing problems and that the barcode tag or object can pass at more or less any distance between illuminator and reader and still safely be safely detected.

The modulation frequency of the light beam shall be at least 30 kHz to obtain a good balance between high sensitivity to the illuminator signal and immunity to disturbances from other sources. In a prefered embodyment, the chosen modulation frequency is 455 kHz so that standard components from the radio receiver industry can be used, and since the higher time resolution then will allow a very fast passage even with a small barcode tag or object.

SUBSTITUTE SHEET (RULE 26) The resistance to interference from surrounding daylight and light from incandescent lamps is further improved by an infrared long-pass filter in front of the photodetector, where said filter attenuates visible light but lets through infrared of the wavelength that is sent out from the illuminator.

An especially advantagous configuration of the barcode tag is obtained if the coding is made according to interleaved 2/5, since this code type requires few symbols to represent the number of code combinations that are required by the applications, i.e. typically four decimal digits. The barcode tag can then be made very compact and lightweight.

Thanks to the dual beams in the earlier mentioned special em- bodyment, the invention makes it possible to sense not only the code or other object, but also to measure the direction and speed at each transition between code symbols or object edges. With a code carrier for instance, the reader then receives illuminator signals in eight of four states, A) light from both LEDs, B) light from a first LED while a second LED is shadowed,

C) light from none of the two LEDs since both are shadowed and

D) light from the second LED while the first LED is shadowed. By detecting the time between each state, logical decisions can be done in the reader to determine if the barcode tag is moving, stopping or retracting, to the benefit of system reliability; even if a manufacturing line would temporarily come to a stop while the code is passing by, the barcode tag is safely read. Likewise, it does not matter of the barcode tag is oriented "upside-down" thanks to that it can be read equally safe in both passage directions.

Description of drawings and preferred embodiments

The invention will now be described in detail, where

SUBSTITUTE SHEET (RULE 26) Figure 1 shows a barcode tag with holes and material representing symbols of a bar code,

Figure 2 shows a barcode tag with a reader and a single-beam illuminator unit,

Figure 3 shows a light emitting diode with lens before and after preparation with light slit,

Figure 4 shows a barcode tag reader with a photodetector and lens with light slit,

Figure 5 shows a barcode tag with a reader and a dual-beam illuminator unit,

Figure 6 shows details how two separate beams may hit a barcode tag

Figure 7 shows two light emitting diodes with lenses before and after ligh slit preparation,

Figure 8 shows a received signal if none, a first, a second or both light beams are blocked

Figure 9 shows light signal paths for the four different cases that are shown in figure 8, and,

Figure 10 shows the system applied for automatic identification of a car body or its carrier

Barcode technology is a widely used method to carry information for automatic reading, and the information carriers are then normally built up from bar-shaped black and white symbols where said symbols have either of two widths, narrow or wide.

Figure 1 shows an embodyment of a barcode tag 10 that may be manufactured from a material that is stable when exposed to heat, mechanical stress and chemicals, said material preferably comprising steel, especially cold rolled and hardened steel or stainless steel, aluminum or a high grade polymer such as PPS or PEEK. The barcode tag has been given a number of bar symbols such as 11, 12, 13 and 14 in the shape of rectangular holes such as 11 and 13, or of the material in itself such as 12 and 14. In the figure, the holed symbols are shown as white, while the material symbols are shown as dark.

In the shown example of figure 1, the barcode is of the well- known interleaved 2/5 (ITF25) type, although the invention is however not limited to this code type. ITF25 is however especially suitable for this invention since the code can be made very compact and still carry a sufficient number of unique code combinations in the applications that it is intended for, i.e. usually up to 9999. This makes very small and easily attached barcode tags possible, units that can be applied directly at the object as well as at its load carrier (skid, conveyor etc) . The ITF25 code also comprises start and stop symbols to facilitate automatic sensing of the passage direction of the code. Finally, ITF 25 offers a good redundancy check opportunity, to in a known way prevents substitution errors to cause fatal errors in the controlled process.

Figure 2 shows a system with an illuminator unit 20, a barcode tag 23 and a reader 29. The illuminator has at least one light emitting diode with a lens and light slit 21, that with preferably infrared light transmits an infrared beam through the barcode tag when it is passing between the illuminator and reader. This way, a pattern of light beams 24 will be sweeping across a lens 26 that is placed in front of the photo detector 28 in the reader.

As shown in figure 2, unless the barcode tag is very large, light from two or more holes/symbols will simultaneously hit different areas of lens 26 and via said lens, and possibly one or more lenses (not shown) between said lens and photo detector 28, reach said photo detector in such a way that the intensity of the light at the photo detector not varies in an unambiguous way according to the symbol pattern of a passing barcode tag and hence make decoding of the barcode tag data very difficult.

SUBSTITUTE SHEET (RULE 26) Said ambiguity is however prevented by a light attenuating structure on and around said lens and a light slit 25 crossing the front of said lens 26. The light slit may also be placed behind the lens or lenses, but will then have to be made with smaller dimensions and higher precision thereby possibly causing increased cost at the reader side. Thanks to said light slit, light from only one symbol at a time 27 will simultaneously illuminate the photodetector 28. Since the light slit is much narrower than the with of the most narrow symbol of the barcode tag, the system then can distinguish between wide and narrow symbols even with small barcode tags.

A limit to how close to the illuminator 20 that the barcode tag can pass is given by the with of the light source 21, as counted along the passage direction of the barcode. If the light source is too wide, the projected symbol pattern 24 at the reader's light slit will be blurred. The invention solves this problem in an elegant way by using a light slit also in front of a LED 21. Using a laser as light source is not recommended for several reasons, since this means almost impossible alignment restrictions when the system is to be installed and operated. Eye safety is another problem with such a laser solution, that also is comparatively expensive. The invention therefore makes use of a LED with a focusing lens as its light source 21, where the emission area of said LED for said reasons is partly blocked with a light slit at the lens to form a flattened light beam towards the reader.

In a preferred embodyment of the light source 21, as shown in figure 3, a standard infrared LED 31 with an integrated focusing lens 32 is used. The radiating area of LED 21 has however been given a rectangular shape 33 that is close to parallel to the light slit in the reader and to the symbols of a barcode tag that is to be read. The in this way shaped rectangular radiating lens are 33 is equivalent to a separate light slit in front of the lens. Integrating the light slit with the LED lens has the advantage that the slit position is precise in relation to the position of the radiating chip 34 inside the LED and therefore provides high performance and reproducable performance of the light beam. For barcode tag identification, the with of the LED light slit may typically be about 10 % of the LED' s diameter so that the length to width ration of radiating area of the LED is about 10:1. For further improved performance, leakage light through surfaces 35 may be blocked, e.g. by paint, metallisation or non-transparent labels.

Figure 4 shows a reader where a flattened light beam 41 hits a receiver lens 42 and where a part of the light beam is passing through a light slit 43 and furthermore is focussed by said lens to concentrate its energy at photo detector 44 with high irradiance.

Using the here described light slit configurations at both illuminator and reader makes compact barcode tags possible and furthermore offers excellent tolerance to variations in distance between reader and illuminator, as well as large tolerance to at which positions between reader and illuminator that the barcode tag can pass and still be safely read. At the same time, the system is easy to set up and use thanks to the relatively wide beam of the LED light as compared to e.g. a laser. The orientation tolerance will typically be in the order of 5°.

The performance is further improved by means of high-frequency amplitude modulation of the LED light, this way giving the reader both excellent sensitivity and high time resolution for accurate decoding of barcode tags that cross the beam. Using an ordinary 5mm diameter infrared LED for example, said LED having 5 mm diameter and 20° beamidth, modulating the LED with pulse bursts at a frequency of 455 kHz, and arranging the light slits so that they are about 0.5 mm wide, a 4-digit bar-

SUBSTITUTE SHEET (RULE 26) code tag that is less than 30 cm in size can be safely read at any passage position between reader and illuminator and at passage speeds up 2 m/s or more. This performance is far superior to all other known holed barcode tag identification systems .

An even more sophisticated functionality of the invention is illustrated in Figure 5, showing how two separate flattened light beams 50, 51 from two LEDs 52, 53 in illuminator 54 are directed towards a reader 55 that has a light slit 56 in front of a lens 57 and a photodetector 58. Said light beams 50 and 51 are blocked or passed on to the reader depending on the instant position of the symbols 59 in a barcode tag that moves in the beams between illuminator 54 and reader 55.

Figure 6 is an extract from figure 5 and shows in detail how the beams are blocked or passed on. The beams 50 and 51 are also differently modulated, so that reader 55 can detect from which one of, or both of, LEDs 52 or 53 light originates that reaches photodetector 58.

As with the single-beam solution, in order to achieve sufficient precision in the beam blockage, the light sources 53 and 54 hase an extension in direction of the barcode passage that is substantially smaller than the width of the narrow bars, otherwise a part of the light from the beam that is intended to be blocked would pass through an adjacent symbol in the barcode tag and thereby make decoding of the barcode tag information impossible.

Now, in a preferred embodyment of the invention as shown in figure 7, the light sources 52 and 53 are built up of two closely spaced LEDs 71, 72. The transmitting areas 73, 74 of the LEDs' combine lens and light slit functionality, and are preferably spaced closer to each other than half the width of the most narrow symbol of the barcode tag. Said configuration

SUBSTITUTE SHEET (RULE 26) can for instance be achieved by machining two standard LEDs as shown in figure 7, mounting these in close proximity to each other and covering the light-emitting machined surfaces 75 and 76 with a light-blocking paint, a metal layer or with non- transparent labels. A high-performance dual flattened-beam light source has this way been constructed, that furthermore is cheap, accurate, powerful, safe to the eye, at that offers two individually coded beams that are wide enough to make orientation of the illuminator towards the reader at installation uncritical.

If the most narrow symbol of the barcode tags is in the order of 4 mm, the width of the combined lens and light slit should preferrably be in the order of 0.5 mm and the preferred distance between the two LED light slits about 2 mm. A bar width of 4 mm is very reasonable since it can then be covered with at least up to 1 + 1 mm of paint from the automotive paint line process and still provide a 2 mm light gap for satisfactory function of the system.

Figures 8 and 9 show a sequence with four different states that occur while a barcode symbol is passing through the beam. The light beams from LEDs 91 and 92 are individually modulated with single or paired pulse bursts as shown in B and D respectively of figure 8.

In order to achieve sufficient sensitivity of the receiver by bandpass filtering, as well as to achieve sufficient capacity to handle fast-passing barcode tags, the pulse frequency in said bursts 81 is higher than 30 kHz, and in a preferred high- performance embodument 455 kHz where standard radio receiver components can be used for filtering and amplification of the signals from the reader's photodetector. A burst shall have at least two, but preferably more than 8 pulses in a row to obtain a good integration effect in said receiver circuits (not shown) .

SUBSTITUTE SHEET (RULE 26) In state A, light beams from both LED 91 and LED 92 are passing through hole symbol 93 in a barcode tag. Both beams therefore reach photodetector 95 through light slit 98 of light blocking structure 95, said light slit in this case being adapted in directly in front of lens 98. Since both beams are received by the photodetector 95, the output signal of the detector will be the sum of the two beams modulations as shown with A in figure 8.

In state B, light beam from LED 91 is blocked by symbol 95 of the barcode tag 94, while light beam from LED 92 still reaches the photo detector. The output signal from the photo detector at this position of the barcode tag will consequently be as shown with B in figure 8.

In state C, light from both LEDs 91 and 92 are blocked by barcode symbol 95, and no signal will be output from the photodetector as shown with C in figure 8.

In state D, light from LED 91 can pass through symbol 94, while light from LED 92 is blocked by symbol 95, and signals as shown with state D in figure 8 will be output from the photodetector .

By suitable logical algorithms in the reader, it is now possible, from the order and time of events A, B, C and D, to accurately determine the time and state change when each and every edge of a barcode symbol is crossing the combined beam from LEDs 91 and 92. The barcode information can this way be decoded regardless of speed variaions during passage, even if the barcode tag comes to a full stop or reverses.

Figure 10 shows a complete installation with a car body 100, to which a code carrier 101 has been attached e.g. inside the wheelhouse so that it can be penetrated by light from illuminator 102 and provide a decodable light intensity variation in

SUBSTITUTE SHEET (RULE 26) the photo detector of reader 103. As an alternative, a barcode tag 104 is attached to the load carrier 105. The most suitable installation alternative varies from case to case and car model to car model, and is of no significance to the invention in itself.

The invention can also be used to identify other objects than the described car bodies and where electronic tags are difficult to use, such as to identify containers with melted material in the steel and aluminum industry.

The barcode tag system can furthermore detect and classify other objects that interrupt the beam between illuminator and reader, such as robotic arms, roller cages and other trolleys, trucks, trailers, railcars and cars. The invention can then be used to determine the passage direction and speed of different detail objects such as the wheel of a railcar, truck or trailer.

If the invention is used for detection of larger objects, such as the wheel of a vehicle, the light slit width may be larger - than with a barcode tag, as long as it is considerably smaller than the smallest object or object movement that is to be detected. If the resolution in detection of an object for instance shall be ten times that of a 4 mm narrow barcode symbol, i.e. 40 mm, the width can be about ten times as wide as the 0.5 mm width typically used for a barcode tag. Hence, the slit can be as wide as the diameter of a 5 mm LED, comprising the LED in itself.

Instead of using threadles, magnetic loops, magnetoresistive detectors, radars, lasers, TV cameras or other measures for vehicle detection, with a solution according to the herein described invention it is possible to e.g. automatically measure the length of a passing vehicle by detecting the speed and time at each beam crossing, i.e. at the beginning and end of

SUBSTITUTE SHEET (RULE 26) each passing wheel, to get the passage speed. A simple calculation of the elapsed time between passage of wheels, the distance between the wheels is automatically determined.

Furthermore, it is possible to count how many axles that a passing train may have, a function that is highly valuable to discover failed readings of electronic tags on passing rail- cars.

The extreme precision of the invented system can even detect narrow objects such as the presence, direction and speed of a sliding current connector on top of locomotives, where almost no other technique is available in the market. Using low-cost standard components, suitable modulation frequencies and other here described measures, such a system will work accurately at speeds up to 300 km/h or more.

Relevant details of the invention can now be summarised as follows .

Δn object detection system comprising least one illumination unit 20 based on at least one light emitting diode 21, 31 with a focusing lens 32 that transmits amplitude modulated and preferably infrared light of 700 - 1200 nm vawelength in direction to a barcode tag 10 or other object to be detected, and, where at the other side of the barcode tag or other object a reader 29, 55 is located, said reader comprising a photo detector 44, 58 that is especially sensitive for light of the wavelength and modulation pattern 81 that is sent out from the illumination unit and where said light furthermore will reach said photodetector via at least one positive lens 26, 42 and furthermore has a light slit 25, 43 is arranged at either side of said lens in front of the photodetector, said light slit being significantly narrower than the most narrow symbol 13, 14 of the barcode tag to be detected, or significantly narrower than the smallest resolution needed to detect

SUBSTITUTE SHEET (RULE 26) onother object, and that said at least one light emitting diode has an effective radiating area 33 that is restricted either by a separate light slit in front of its lens or by the shape of said lens in itself has been restricted in size so that in case of barcode tag detection a rectangular radiating area 33 is formed that is essentially parallel to the light slit 43 in the reader and to the symbols 11, 12, 13 and 14 of the barcode tag that are to be detected, or, in case of detection of other objects than a barcode tag, such as the wheel of a vehicle, the light slit in front of the photodetector is considerably narrower than the smallest object or object movement that is to be detected, and the radiating area of the light emitting diode has a width not larger than or equal to the diameter of the light emitting diode and in this latter case may comprise the light emitting diode in itself.

In one embodyment, the illuminator 54 if arranged for reading of barcode tags it comprises at least two adjacent light emitting diodes 52, 53 and 71,72, LEDs, each with a focussing lens and light slit 73, 74 arranged essentially in parallel to the light slit 56 of the reader 55, or if arranged for detection of other objects than barcode tags, such as the wheel of a vehicle, the light slits in front of the photodetector is considerably narrower than the smallest object or object movement that is to be detected, and the radiating area of the light emitting diodes have a width not larger than or equal to the diameter of the light emitting diodes and in this latter case may comprise the light emitting diodes in themselves.

The system is then characterised by that said LEDs 52, 53 and 71, 72 transmit bursts 81 of modulated light in direction to the reader 55, where the modulation frequency in said bursts is least 30 kHz, and where said transmissions comprise at least two different transmission patterns that each is unique to the individual LED. The system may furthermore adapted to distin-

SUBSTITUTE SHEET (RULE 26) guish between four states of signal reception to the reader, i.e. A) light from both LEDs, B) light from the first LED while the second LED is shadowed, C) light from none of the two LEDs while both are shadowed and D) light from the second LED while the first LED is shadowed, thereby through detecting the time between each state change, logical decisions are done in the reader to determine if a passing barcode tag is moving, stopping or retracting. It may also be adapted to measure the length between wheels of a passing vehicle by detecting the speed and time at each beam crossing, i.e. at the beginning and end of each passing wheel, to get the passage speed and possible acceleration or retardation, and by calculation of the elapsed time between passage of wheels, the distance between the wheels is determined.

In the invention, the light slits 73, 74 at the LEDs in the illuminator 54 have a width that is typically 10 % if the diameter of the LED and that said light slits 73, 74) may form an integral part of the illuminator LEDs 71, 72.

The illuminator LEDs 71, 72 may furthermore have material being removed along their sides in such a way that the light emitting surfaces 73, 74 are located closer to each other otherwise possible. Also, the surfaces 35, 75, 76 that are not intended for radiation can be covered by an opaque material such as paint, metal or a label.

The barcode tag is preferably manufactured with interleaved 2/5 coding and that the light beams (50, 51) are individually modulated with single or paired pulse bursts.

The invention has now been described with a number preferred embodyments, but can of course be further varied within the limits of the patent claims.

SUBSTITUTE SHEET (RULE 26)

Claims

1. An object detection system comprising least one illumination unit (20) based on at least one light emitting diode (21, 31) with a focusing lens (32) that transmits amplitude modulated and preferably infrared light of 700 - 1200 nm vawelength in direction to a barcode tag (10) or other object to be detected, and, where at the other side of the barcode tag or other object a reader (29, 55) is located, said reader comprising a photo detector (44, 58) that is especially sensitive for light of the wavelength and modulation pattern (81) that is sent out from the illumination unit and where said light furthermore will reach said photodetector via at least one positive lens (26, 42) , characterised by that a light slit (25, 43) is arranged at either side of said lens in front of the photodetector, said light slit being significantly narrower than the most narrow symbol (13, 14) of the barcode tag to be detected, or significantly narrower than the smallest resolution needed to detect onother object, and that said at least one light emitting diode has an effective radiating area (33) that is restricted either by a separate light slit in front of its lens or by the shape of said lens in itself has been restricted in size so that in case of barcode tag detection a rectangular radiating area (33) is formed that is essentially parallel to the light slit (43) in the reader and to the symbols (11, 12, 13, 14) of the barcode tag that are to be detected, or, in case of detection of other objects than a barcode tag, such as the wheel of a vehicle, the light slit in front of the photodetector is considerably narrower than the smallest object or object movement that is to be detected, and the radiating area of the light emitting diode has a width not larger than or equal to the diameter of the light emitting diode and in this latter case may comprise the light emitting diode in itself. to the diameter of the light emitting diode and in this latter case may comprise the light emitting diode in itself.

2. A system according to claim 1, characterised by that the illuminator (54) if arranged for reading of barcode tags it comprises at least two adjacent light emitting diodes (52, 53 and 71,72), LEDs, each with a focussing lens and light slit (73, 74) arranged essentially in parallel to the light slit (56) of the reader (55), or if arranged for detection of other objects than barcode tags, such as the wheel of a vehicle, the light slits in front of the photodetector is considerably narrower than the smallest object or object movement that is to be detected, and the radiating area of the light emitting diodes have a width not larger than or equal to the diameter of the light emitting diodes and in this latter case may comprise the light emitting diodes in themselves.

3. A system according to claim 2, characterised by that said LEDs (52, 53 and 71, 72) transmit bursts (81) of modulated light in direction to the reader (55), where the modulation frequency in said bursts is least 30 kHz, and where said transmissions comprise at least two different transmission patterns that each is unique to the individual LED.

4. A system according any claim 2 or 3, characterised by that the system is adapted to distinguish between four states of signal reception to the reader, i.e. A) light from both LEDs,

B) light from the first LED while the second LED is shadowed,

C) light from none of the two LEDs while both are shadowed and D) light from the second LED while the first LED is shadowed, thereby through detecting the time between each state change, logical decisions are done in the reader to determine if a passing barcode tag is moving, stopping or retracting.

5. A system according any of claims 2 to 4, characterised by that it is adapted to measure the length between wheels of a passing vehicle by detecting the speed and time at each beam crossing, i.e. at the beginning and end of each passing wheel, to get the passage speed and possible acceleration or retardation, and by calculation of the elapsed time between passage of wheels, the distance between the wheels is determined.

6. A system according to any of the previous claims, characterised by that said light slits (73, 74) at the LEDs in the illuminator (54) have a width that is typically 10 % if the diameter of the LED.

7. A system according to any of the previous claims, characterised by that said light slits (73, 74) form an integral part of the illuminator LEDs (71, 72) .

8. A system according to any of the previous claims, charac- terised by that the illuminator LEDs (71, 72) have material being removed along their sides in such a way that the light emitting surfaces (73, 74) are located closer to each other otherwise possible.

9. A system according to any of the previous claims, characterised by that the surfaces (35, 75, 76) that are not intended for radiation are covered by an opaque material such as paint, metal or a label.

10. A system according to any of claims 2 - 9, characterised by that the barcode tag is manufactured with interleaved 2/5 coding and that the light beams (50, 51) are individually modulated with single or paired pulse bursts.