Antenna system and method for reading low frequency tags

• US 6,750,771 B1
• Filed: 07/24/2001
• Issued: 06/15/2004
• Est. Priority Date: 08/10/2000
• Status: Active Grant

• Current Assignee: Savi Technology Incorporated
• Original Assignee: Savi Technology Incorporated
• Inventors: Brand, Timothy K.
• Primary Examiner(s): Hofsass, Jeffery
• Assistant Examiner(s): NGUYEN, HUNG T
• Application Number:
  US09/912,076
• Time in Patent Office: 1,057 Days
• Field of Search: 340/572.7, 340/572.1, 340/572.4, 340/572.5, 340/10.1, 340/10.2, 340/10.3, 343/756, 343/748, 343/742
• US Class Current: 340/572.7
• CPC Class Codes: G06K 7/10336 the antenna being of the near field type, inductive coil
• CPC Class Codes: G06K 7/10346 the antenna being of the far field type, e.g. HF types or dipoles
• CPC Class Codes: G06K 7/10435 the interrogation device being positioned close to a conveyor belt or the like on which moving record carriers are passing conveying in accordance with bodily destination marks, see B65G47/46, sorting of objects carrying identity markings, see B07C5/34

Abstract

A reader reads identifiers from tags on pallets conveyed past the reader. The reader includes two interleaved linear arrays of antennas with circularly polarized fields. Each antenna is composed of a pair of crossed rods phased to have adjacent antennas of an array generate circularly polarized fields of opposite rotation. The vector components of the polarization in the direction across the width of the conveyor have peaks and nulls, and the interleaved arrays are arranged such that the nulls of one array'"'"'s fields are covered with the peaks of the other array'"'"'s fields. This arrangement allows the reader to the identifier from the tag when the tag is at any orientation. A tag at the side of the reader is aligned in the direction of travel by rails on the conveyor. The reader has antennas aligned in the direction of travel to read such tags.

Claims

1. A method of reading an identifier of a tag being conveyed by a conveyor, the tag having an antenna having any orientation in the plane of the conveyor, comprising the steps of:

generating a first circularly polarized field having a first rotation and a null;

generating a second circularly polarized field having a second rotation opposite the first rotation and a peak, the peak of the second field covering the null of the first field;

powering the tag with the first or second field responsive to the first or second field aligning with the orientation of the tag'"'"'s antenna, wherein the powered tag changes a load on the tag'"'"'s antenna responsive to data stored in the tag;

detecting impedance changes caused responsive to the load on the tag'"'"'s antenna;

determining the tag'"'"'s identifier responsive to the detected impedance changes; and

wherein there are a plurality of first fields and a plurality of second fields, the first fields are generated by a first linear array of antennas, the second fields are generated by a second linear array of antennas, adjacent antennas in the first and second arrays generate fields having opposite rotation, and the first and second linear arrays are linearly interleaved such that the peaks of the second fields cover the nulls of the first fields.

2. The method of claim 1, wherein the antennas of the second array are each physically rotated on an axis to reduce coupling to adjacent antennas of the first array.

3. The method of claim 2, wherein the antennas of the second array are each rotated on their axis by 45 degrees relative to the antennas of the first array.