Patent Application
20080048834
A preferred embodiment of the present invention will now be described with reference to the accompanying drawings wherein:
An example of an antenna loop 10 is shown in
Hence a single loop antenna 10 having its axis oriented with an oblique angle x relative to a direction of movement 12 of a tag bearing object 11 or translation of the antenna in conjunction with rotation of either the tag bearing object or the antenna should eliminate the effect of null coupling.
One problem with a rotating reading antenna is how to form a connection to cables feeding signals to the rotating antenna to and from transmitting and receiving circuits associated with the interrogator. The present invention addresses this problem by introducing a second antenna loop 22. The second antenna loop 22 may be fixed relative to the reading tunnel in closely spaced relationship to rotatable cylinder 21 and loop antenna 20.
Fixed antenna loop 22 may be actively coupled to the transmitting and receiving circuits associated with the interrogator. Fixed antenna loop 22 may include a construction that includes a self-balun method which entails cable entry at opposite ends of a break in a single turn loop in which tuning elements (not shown) may be located. Placing cable entry opposite the tuning elements may serve to electrically balance the loop with respect to ground for a loop which otherwise would be physically balanced with respect to ground. This approach may reduce far field radiation resulting from stray electric fields.
Rotatable antenna loop 20 forms a passive antenna that is electromagnetically coupled to fixed antenna loop 22. Because antenna loop 20 has a main axis that is oriented obliquely relative to a direction of movement of tag bearing objects, its axis changes orientation relative to the tag bearing objects as loop antenna 20 and cylinder 21 rotate. The electromagnetic field associated with antenna loop 20 also rotates and changes its orientation relative to the tag bearing objects as they pass through the reading tunnel. This facilitates reading of the tags and addresses coupling nulls between the reader antenna and the tags which are randomly oriented relative to the objects.
The main axis of loop conductor 20 may be oriented at an acute angle relative to the direction of movement of the tags. Preferably the main axis of loop conductor 20 is oriented at approximately 45 degrees relative to the direction of movement of tag bearing objects.
Antenna loop 20 may be rotated at any suitable rate to maintain an optimum reading rate. The speed of rotation may be controlled automatically depending upon reading requirements. In one form a relatively high rotating speed, for example 300 rpm, may be adapted to effectively create a 3D interrogation field. The 3D field is a real rotating field (not simulated by several antennas) that may allow a longer interrogation time of the tags and may provide a more robust reading operation without field interruptions. A tunnel reader incorporating a rotating antenna structure according to the present invention may be installed horizontally or vertically.
In the case of a magnetically coupled system in which tagged objects are passed through or in the vicinity by an aperture of a loop antenna or the antenna structure is translated across the objects, an electrical shield in the form of a tube may be placed around the loop antenna. The axes of the shield may be parallel to the direction of movement of the objects.
To electrically shield a circular loop with a conducting cylinder of diameter D1 with minimal detuning, the area in the plane of the loop between the loop and the shield can be thought of as requiring the same reluctance R presented to the flux as the cross-sectional area of the loop. It turns out that in this case where D2=√2D1 (and shield length>D1+loop height), the ratio of inductance with shield to inductance without shield is 0.84 (for a loop height to diameter ratio<0.1). For a ratio of inductance with shield to inductance without a shield of 0.95, the diameter of the shield is required to be twice that of the loop (D2=2D1). This latter amount of detuning is practically acceptable. The method described can also be used for a loop and shield cross-section of a regular polygon by considering the diameter of a circle circumscribed by the loop. Other more general shapes require calculation of flux paths.
The reason that a shield reduces inductance arises from a condition of shielding wherein the magnetic field outside the shield is zero (or very small). This being the case, a tangential magnetic field inside the shield material must likewise be zero. In order to maintain boundary conditions between the tangential magnetic field at the surface inside of the shield and the tangential magnetic field inside the shield material, a surface current on the inside edge of the shield must flow in order to produce a magnetic field inside the shield material which cancels the field that would have been in that region had the shield not existed. This current, however flows in anti-phase with that of the loop, so a subtracting field is present at the centre of the loop. As the definition of inductance is L=NΨ/l, then a reduction in Ψ causes a reduction in L (for constant l).
Likewise, L=N2/R, where N is the number of turns of the loop, so a reduced flux path (as the shield closes in on the loop) has an increased reluctance R which is also consistent with reduced L.
Looking at why shielding is required in the first place, if a large loop is required for clearance of an object passing through the loop, two problem factors enter into the RFID system. One factor is that in order to maintain an acceptable field at the centre of the loop sufficient current must be provided from the interrogator. As a loop's perimeter becomes larger, the radiation properties diverge from that of an electrically small loop due to non-uniform current distribution around the loop, resulting in increased radiation. The loop can be constructed by segmenting the periphery into segments joined by series capacitors of low enough reactance to not affect the matching of the loop or with a judicious choice of reactance to facilitate the matching. An alternative segmentation in the form of “pie slice” sections whose effect from the radial currents cancel is not practical for an object passing through and a further implementation where the feed is external to the loop and (possibly the shield) is unwieldy in complexity. Once the loop behaves as an electrically small loop, shielding becomes one solution to further reduce radiation to acceptable EMC limits.
A second factor is that a larger loop picks up more external noise through reciprocal reasoning of why it radiates more.
With a shield causing a reduction in inductance, a direct reduction in flux (and hence H) for the same current occurs, therefore increased current is required from the interrogator leading to increased power output and internal interrogator noise.
Other multiple antenna configurations are possible to create a field and such structures may require shielding from external noise or attenuation of propagating field in one direction for which a technique as described below may be equally suitable. Nevertheless, a single loop is desired in most applications due to its simplicity.
To reduce the diameter of the shield, a material with a higher permeability than that of air may be used between the loop and the shield to provide a lower reluctance path. To calculate a required amount of magnetic material to be placed between the loop and the shield, a value of reluctance may be provided that would result in the value of the loop's initial inductance in the absence of the shield. A material such as ferrite is desirable due to its low conductivity, which prevents (or at least keeps to a minimum) surface currents on the magnetic material which may act in the same way as currents on the inside of the shield. For the case of conducting material, it may be laminated in planes perpendicular to a line around the perimeter and may require more material (increase the inductance to a value greater than the loop) to counteract inductance reducing effect of the surface currents.
Large toroids or flat disks with holes in the centre are not commonly available so practically, the magnetic material may be in the form of rods or slabs placed in a picket fence or polygon fashion respectively. For the latter structures, a demagnetising factor associated with the material may be estimated by the following formulas.
For a rod of diameter d and length L,
N
d=(1−w2)/w2*(1/(2w)*In((1+w)/(1−w))−1), where w=√(1−(d/L)2).
The effective permeability is then calculated by
μeff=μr/(1+(μr−1)Nd).
The reluctance of a magnetic pathway is R=l/(μS) where l is the centre-line length and S is the area of cross section. For the case of using rods, reluctance of a single rod may be calculated and the reluctance of each rod is one of n in parallel in the magnetic circuit, so
L
loop
=N
2/(Rrod/n)
is used to find the number of rods required.
This method may get close to a final requirement of magnetic material, but the volume of magnetic material may require adjustment for the following reasons. Firstly the formula for reluctance assumes uniform magnetic field at the air magnetic material interface, which is approximately true for narrow rods or slabs. Secondly, the rods need to be long enough to maintain enough radius of curvature of the flux lines at the centre of the loop in order for a randomly oriented tag to dwell long enough to couple to the field while it passes through the loop. This second case relates to two inductors having the same value of inductance, but with differing distributions of field within their turns. Using a thin wall cylinder as the loop (a loop with some height) may assist in keeping the radius of curvature of the field at the centre from becoming too small for good tag coupling when a single turn loop is used.
To complete the shielding, a shield length>D1+loop height may be required to allow enough flux return area for a cylinder with closed ends. In order to pass objects through the loop, the ends may be required to be opened, thus relaxing this requirement, but in order to prevent too much field escaping the cylinder ends, the tube's length preferably is made such that it acts as a waveguide beyond cut-off, which may apply attenuation to the wave present at the operating frequency. For a magnetic loop case, the arrangement may launch a TE22 wave mode, although a conservative approach may be to make the shield long enough to give a required attenuation for the dominant mode. The attenuation required comes from the amount that the unshielded loop was over the EMC limit. The length, l, with the source at the centre of the waveguide, is related to attenuation by the formula:
[attenuation dB]=20* log 10*exp(−jβ*l/2)
where β will be complex when operating below the cut-off frequency.
Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.
| Number | Date | Country | |
|---|---|---|---|
| 60839483 | Aug 2006 | US |
