TECHNICAL FIELD

[0001] The present invention generally relates to a vehicle license plate (also colloquially called blank plate or blank) fitted with a radio frequency identification (RFID) tag. The present invention has particular, although not exclusive application for e-tolling and enforcement purposes.

BACKGROUND

[0002] The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge.

[0003] Vehicle plates are manufactured using either plastic sheeting (typical thickness > 3 mm) or metal sheeting (typical thickness > 0.8 mm). Most plates use a retro-reflective sheeting as background with opaque letters on it. In some cases, the characters are reflective with the background opaque. In some countries, like Australia, the background is overlaid with an image and/or pattern. This image/pattern is largely for aesthetic reasons and attracts a premium price. The retro-reflective sheeting and printing are typically applied to the back of a plastic plate and to the front of a metal plate. Metal plates are typically embossed with a border to improve stiffness. Characters on metal plates are also typically embossed. The embossed areas are covered with ink using rollers (known in the industry as inking) or epoxy film (known in the industry as foiling) to create accurate character images. This embossment may be reversed to raise the background resulting in reflecting characters.

[0004] A metal plate can be fitted with an on-metal RFID tag. In this case the plate acts as the conducting ground plane of the tag antenna. As such, the plate size and the metal parts of the vehicle behind the plate impacts the RF performance of which a typical impact is to reduce the forward aperture of the tag antenna radiation pattern.

This in turn limits the ability to read the tag from overhead, side and in-road readers.

The tag size (footprint and thickness) is limited since the tag shall not obscure the ability to read the characters on the plate. The RF performance is also impacted by the placement of the tag on the plate since the metal surface on which the tag is placed should overlap the tag by at least a factor (based on the tag antenna design) of the frequency; which is ~20 mm at the UHF frequencies. Embossment, aesthetic and security elements of the plate may also impact the RF performance (read range and aperture). On-metal tags, as a result, are marginally effective for the identification of vehicles on open roads.

[0005] A metal plate can also be fitted with an in-metal embedded tag. These tags typically use the plate as the tag antenna. Slot tag antennas are a preferred

embodiment. In-metal embedded tags suffers the same limitations than on-metal tags, especially from the impact of the vehicle parts and shape, since the plate, used as an antenna, will radiate forwards along the road, but also backwards to the vehicle. The backwards signal is reflected by the metal parts of the vehicle which then interfere with the forwards radiation. These reflections are difficult to control and often require a specific design for a specific vehicle and plate placement. In-metal embedded can also not be used directly on a metal surface. Many vehicle's plate attachment is directly on metal.

[0006] A plastic plate can also be fitted with a RFID tag. In this case, both the vehicle body metal and the dielectric effect of the plastic base used pose a challenge. The most common retro-reflective sheeting used in the construction of vehicle license plates are metalized. A plastic plate with metalized retro-reflective sheeting poses a bigger RFID challenge. Modern non-metalized retro-reflective sheeting (called prismatic retro-reflective sheeting) is now available, and is durable enough to be used on vehicle license plates. This, however, does not resolve the RFID challenges faced by dense plastic plates, since the highly variable vehicle metal body still poses a big RF challenge.

[0007] Prior art, similar in design of this invention, uses a cavity slot antenna in a hollow plastic plate. In effect the plate is constructed to limit the backwards radiation, to eliminate the impact of the vehicle's metal body. The direct placement of the plate on metal issue is resolved, though the aperture reduction effect is not. Undesirably, the body of the cavity slot plate needs to be enclosed by a conducting layer to form a Faraday cage. The slot is formed in the front conducting layer of the plate wherein a RFID tag is placed to allow the slot to become a resonating slave antenna of the tag. The slotted cavity poses challenges for mass manufacturing, especially when considering that a vehicle license plate is a commodity under severe cost pressure.

[0008] The preferred embodiment provides for improved plate manufacturing and/or reduces substantially the impact of the plate placement and vehicle on the RF performance.

[0009] The slot antennas (be it in the plate or in a cavity), by its nature, has a narrow aperture. The desired aperture should ideally be wide both in the vertical and horizontal planes when considering a vehicle mounted plate so that roadside, in-road and overhead antennas will detect all vehicle plates. Often plates are tilted by users to avoid detection by cameras. The downward facing plate, due to directionality of the cavity slot antenna, will most likely not be read by an overhead reader. Similarly, a side reader will have read challenges. Although an enforcement officer with a handheld reader must be a safe distance away from the traveling path of the vehicle, the face-on angle from which the plate can be read is increased which reduces safety.

[00010] An additional issue with slot antennas lies in their inherent linear polarity. The best-read performance with RFID is achieved when both the reader and the tag uses the same linear polarisation. The common practice for windscreen RFID labels is to place them horizontally. Services, like parking and electronic tolling, therefore use horizontally polarised antennas. The slot antenna for various RF reasons, but mostly because a horizontal slot fits the structure of a plate, has a vertical polarisation (a slot antenna is an inverse of a di-pole antenna). Accordingly, reading services are forced to undesirably use cross or circular polarised reader antennas with an unavoidable performance penalty of at least 3 dB.

[00011] The preferred embodiment provides for improved RF reading of the license plate tag when compared with a slot antenna.

[00012] Other prior art involves the use of on-metal RFID tags. On-metal tags are designed to be placed directly on vehicle metal, and by their nature have a directional radiation pattern, typically with an overly narrow aperture for the vehicle identification.

[00013] On-metal tags and in-metal embedded tags need to be placed on or in the face of the plate which restricts the placement and the size of the on-metal tag. Owing to radio signal propagation, an on-metal and in-metal tag needs to be placed so that there is ample metal on all sides of the tag in the mounting plane. The ideal position is in the center of the plate, however, doing so would undesirably interfere with the image readability of the plate, reduce the on-metal tag's size and also reduce its efficiency.

[00014] The preferred embodiment avoids obscuring the image readability of the plate.

SUMMARY OF THE INVENTION

[00015] According to one aspect of the present invention, there is provided a vehicle license plate including:

a radio-frequency (RF) shield for locating adjacent a vehicle;

a spacer for locating adjacent the shield; and

a radio frequency identification (RFID) tag spaced from the shield by the spacer.

[00016] Advantageously, the tag is spaced from the shield by the spacer which may provide a wider beam to improve reading of the tag when compared with a cavity slot antenna tag. The shield may be expansive compared with the compact tag to mitigate the effects of variances of the metal body of the vehicle and thereby improve tag reading.

[00017] The tag may be embedded within the spacer to avoid obscuring during image readability of the plate. The spacer may be hollow. The spacer may include internal ribs. The ribs may be located between a planar base and a planar lid. The ribs may define recesses in which the tag is located. The tag may be flush with the ribs when located in the recesses.

[00018] The spacer may be non-conductive. The spacer may be non-metallic. The spacer may be plastic. The spacer may have a low relative permittivity (er) and a similar dielectric value to air. The spacer may be about 8 to 10mm thick, greater than 100mm high and/or greater than 150 mm wide.

[00019] The shield may include metal. The shield may be metalised on only one side, with that side facing the vehicle. [00020] The license plate may further include another shield (or matching layer) between the tag and the shield. The other shield may be located between the shield and an antenna of the tag. The tag may include a cross-polarised antenna. The other shield may be conductive or metal, acting as a DC ground for electrostatic discharge and shielding the tag from reflected RF waves from the shield. The other shield may be integrally formed with the tag.

[00021] The license plate may further include retro-reflective sheeting located adjacent the tag. The license plate may further include an image layer located adjacent the retro-reflective sheeting. The image layer may include a background image and/or characters, without the need for fasteners which could otherwise degrade the image readability of the plate. The license plate may further include an endmost transparent layer.

[00022] According to another aspect of the present invention, there is provided a method for manufacturing a vehicle license plate, the method involving:

providing a radio-frequency (RF) shield which is to be located adjacent a vehicle; locating spacer adjacent the shield; and

spacing a radio frequency identification (RFID) tag from the shield using the spacer.

[00023] Advantageously, the plate may be readily assembled in layers, with minimum parts and processing, to provide for improved plate manufacturing.

[00024] The method may further involve locating the tag within the spacer. The method may involve locating the tag in one or more recesses of the spacer, and attaching a plate to embed the tag within the spacer. The method may involve affixing another plate to the spacer.

[00025] The method may involve affixing a retro-reflective sheeting adjacent the spacer. The method may involve affixing an image layer adjacent the retro-reflective sheeting.

[00026] Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[00027] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows:

[00028] Figure 1 is a cross sectional view of a partial vehicle license plate in accordance with an embodiment of the present invention;

[00029] Figure 2a is a radiation pattern of a known vehicle license plate with a cavity slot antenna;

[00030] Figure 2b is a radiation pattern of the vehicle license plate of Figure 1 ; and

[00031] Figure 3 is an exploded perspective view of the complete vehicle license plate of Figure 1 .

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[00032] According to an embodiment of the present invention, there is provided a multilayered vehicle license plate 100 as shown in Figure 1 . The license plate 100 includes a radio-frequency (RF) base shield 102 for locating adjacent a vehicle 104. A hollow plastic spacer 106 is provided for locating adjacent the base shield 102.

[00033] The license plate 100 further includes a UFIF RFID tag 108 (i.e. passive transponder) spaced from the shield 102 by the spacer 106. The tag 108 includes an upper antenna 1 10, and a lower circuitry active component 1 12. An antenna matched shield 1 14 is provided at the base of the tag 108 to shield it from reflected RF signals from the base shield 102.

[00034] Figure 2a shows a narrow radiation pattern 200 for a vehicle license plate with a cavity slot antenna, whereas Figure 2b shows a wide radiation pattern 202 for the vehicle license plate 100. Advantageously, the tag 108 is spaced from the base shield 102 by the spacer 106 which provides a wider beam envelope 204 to improve reading of the tag 108 when compared with the narrower beam envelope 206 of the cavity slot antenna tag. The aperture of the tag 108 in both the horizontal and vertical planes is increased

[00035] RFID tag reading involves a technique of modulating radar where the reader emits a strong signal to the tag 108. The strong signal powers the passive tag 108. The reader modulates the signal to communicate a message to the tag 108, emitting a constant wave (CW) when listening for a response message from the tag 108. The tag 108 modulates the reflected signal to communicate the message to the reader. The base shield 102 is expansive compared with the compact tag 108, to mitigate the ground effects or variances of the metal body of the vehicle 104, thereby improving tag reading.

[00036] Figure 3 shows the complete vehicle license plate100 in detail. The tag 108 is embedded within the spacer 106 (and plate 100) which advantageously avoids obscuring of the characters during image reading of the plate 100. The spacer 106 includes a middle layer 200. The middle layer 200 defines internal longitudinal ribs 202 defining separated parallel hollow channels. The spacer 106 also includes a planar base 204 and a planar lid 206 between which the middle layer 200 is sandwiched. The ribs 202 define upper recesses 208 in which the tag 108 is located. The tag 108 sits flush with the top of the ribs 202 when located in the recesses 208.

[00037] The light-weight spacer 106 is non-conductive and non-metallic. The spacer components 200-206 are typically formed from plastic, having a low relative permittivity (er) and a similar dielectric value to air yielding low RF interference. The spacer 106 is about 8 to 10mm thick, greater than 100mm high and greater than 150 mm wide.

[00038] The base shield 102 is metalised. The tag antenna 1 10 is located on the opposite non-metalised side of the base shield 102. The base shield 102 forms a RF shield, and results in strong reflection when compared with the reflection from the tag antenna 1 10.

[00039] Turning briefly again to Figure 1 , the matched shield 1 14 (or matching layer) forms the base of the tag 108, and is located between the tag antenna 1 10 and the base shield 102. It is matched with the tag antenna 1 10. The matched shield 1 14 is also conductive or metal, acting as a DC ground for electrostatic discharge and shielding the tag 108 from reflected RF waves from the base shield 102. The floating tag antenna 1 10 has a high level of immunity from the variable size base shield 102 and the

unpredictable vehicle metal body 104. This foregoing construction provides the freedom of size to use a cross polarised antenna 1 10.

[00040] Returning to Figure 3, the license plate 100 further includes non-metalised retro-reflective sheeting 210 located adjacent the spacer lid 206 and the tag 108. The license plate 100 further includes an image layer 212 located adjacent the retro- reflective sheeting 210. The image layer 212 includes a background image and characters (e.g. ABC-234) in the foreground. The image layer 212 can applied through any combination of printing, lasering, etching, micro embossing, sand basting, foiling (hot and cold methods) or other method on either or both the retro-reflective sheeting 210 and the top lid 206. There is no need for fasteners which could otherwise degrade the image readability of the plate 100 or interfere with the RF.

[00041] The plate 100 improves both the image readability and the RFID readability when compared to current standard and RFID enabled vehicle license plates. The plate 100 also reduces the manufacturing complexity and associated costs, and increases the freedom of adding personalised features to image layer 212 of the plate 100. The tag 108 is embedded to protect the tag-antenna radiation pattern from the effects of the highly variable metal bodies of vehicles 104.

[00042] A method for manufacturing a vehicle license plate is now briefly described.

[00043] Initially, the tag 108 with matching shield 1 14 is located within the spacer 106. In particular, the rib recesses 208 receive the tag 108, and the spacer middle layer 200 is securely sandwiched between the affixed planar base 204 and planar lid 206.

[00044] Next, the assembled spacer 106 is affixed adjacent the base shield 102, with the tag 108 spaced from the base shield 102.

[00045] Next, the retro-reflective sheeting 210 is affixed adjacent the assembled spacer 106. [00046] Next, the image layer 212 is affixed adjacent the retro-reflective sheeting 210.

[00047] Advantageously, the plate 100 may be readily assembled in layers, with minimum parts and processing, to provide for improved plate manufacturing.

[00048] A person skilled in the art will appreciate that many embodiments and variations can be made without departing from the ambit of the present invention.

[00049] In the preferred embodiment, the RFID tag 108 has a single unitary construction which incorporates the RFID tag chip 1 12, antenna radiator 1 10 and matching shield 1 14. Other embodiments may separate these parts for RF performance and manufacturing reasons. In one embodiment, the spacer middle layer 200 may independently receive the matching shield 1 14, which is separate from the antenna 1 10 and circuitry 1 12 of the tag 108.

[00050] In one embodiment, the top lid 206 of the spacer 106 includes a recess for receiving the tag 108.

[00051] In one embodiment, the spacer middle layer 200 is integrally formed with one or both of the planar base 204 and planar lid 206.

[00052] In one embodiment, the base shield 102 include sheeting, metallisation or conducting inks and epoxies.

[00053] In one embodiment, the bottom lid 204 may be made from a conducting material, including for example conducting plastics, in which case the base shield 102 is absent.

[00054] The top three plate layers 206, 210, 212 are interchangeable, with the retro- reflective sheeting 210 always lower in the stack than the image layer 212. A

transparent top lid may be placed on top of the stack to provide the following

advantages:

• It protects the image layer 212.

• It may contain security features such as transparent holographic images and water marks. • It may be textured for both aesthetic and security reasons.

• It may allow for the independent forming and image application in a reel-to-reel (continuous flow) manner. In this case the strip of top layer 206 can be used in the manufacturing process as a conveyer belt upon which the other components are added. The layer 206 is wider then the spacer 202 to which it is adhered. The last step will be to cut (mechanical or by laser) the plate 100 from the "conveyer belt".

[00055] In one embodiment, the retro-reflective sheeting 210 and the spacer lid 206 can be reversed.

[00056] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect.

[00057] Reference throughout this specification to‘one embodiment’ or‘an embodiment’ means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases‘in one embodiment’ or‘in an embodiment’ in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.