Method and system for communicating with and tracking RFID transponders

• US 7,626,488 B2
• Filed: 08/31/2006
• Issued: 12/01/2009
• Est. Priority Date: 11/29/2000
• Status: Expired due to Term

• Current Assignee: Terrestrial Comms LLC (IPValuation Partners, LLC)
• Original Assignee: Mobile Technics LLC (Intellectual Ventures LLC)
• Inventors: Palmer, John P., Armstrong, John T., Richert, John D.
• Primary Examiner(s): Holloway, III; Edwin C
• Application Number:
  US11/469,477
• Publication Number:
  US 20070075834A1
• Time in Patent Office: 1,188 Days
• Field of Search: 340/10.1, 340/572.1, 340/10.2, 340/10.32, 340/10.33, 340/10.34, 340/572.4, 340/10.3
• US Class Current: 340/10.2
• CPC Class Codes: G06K 7/0008 General problems related to the reading of electronic memory record carriers, independent of its reading method, e.g. power transfer
• CPC Class Codes: G06K 7/10039 interrogator driven, i.e. synchronous
• CPC Class Codes: G06K 7/10316 using at least one antenna particularly designed for interrogating the wireless record carriers antennas in general H01Q1/22
• CPC Class Codes: G06K 7/10356 using a plurality of antennas, e.g. configurations including means to resolve interference between the plurality of antennas
• CPC Class Codes: G08B 13/2417 having a radio frequency identification chip
• CPC Class Codes: G08B 13/2488 Timing issues, e.g. synchronising measures to avoid signal collision, with multiple emitters or a single emitter and receiver

Abstract

An RFID system and method for communicating between a host computer, one or more interrogators connected to the host computer, and a large body of transponders distributed within an area covered by the interrogators. Each transponder originally has a common identification code, and upon initialization by the host computer internally generates a unique identification code based upon an internally generated random number. The host, through the interrogators, reads each of the identification codes associated with each transponder by iteratively transmitting a read identification code command along with a controlled variable. Each transponder compares the received controlled variable to an internally generated random number, and selectively transmits its identification code based upon the outcome of this comparison. After the completion of each read identification code iteration, the host adjusts the controlled variable based upon the responses received in the previous iteration. Preferably, communications between the interrogators and the transponders are DSSS signals in TDMA format, and the transponders use the random number generator to assign a time slot for transmission of their response. Each interrogator includes an antenna system utilizing a switch matrix to connect multiple antennas having different polarizations, which ensures that all transponders within the range of the interrogator receive the signals from the interrogator. In a further aspect, the interrogators are arranged in groups, each group in nearest neighbor format, to reduce the time for reading the transponders and the emissions generated when more than one interrogator is active at the same time.

Claims

1. A method of monitoring a designated security area, comprising:

transmitting a strobe command from at least one interrogator within the designated security area;

receiving signals from one or more transponders that are tagged to corresponding articles, wherein each transponder is configured to transmit an identification code in response to receiving said strobe command;

responsive to receiving an identification code from a transponder in response to the strobe command, triggering an alarm to indicate that a tagged article is located within the designated security area; and

responsive to detecting a collision of signals from two or more transponders in response to the strobe command, triggering the alarm to indicate that a tagged article is located within the designated security area, wherein the collision of signals from two or more transponders is detected by sampling a power level during a TDMA time slot and at a time outside the TDMA time slot and determining a ratio of the sampled powers.

2. The method of claim 1, further comprising transmitting multiple strobe commands over time to monitor the designated security area and determine whether a tagged article has entered the designated security area.

3. The method of claim 1, wherein the alarm is triggered when an identification code from a transponder is detected in a predetermined number of TDMA time slots after a strobe command is transmitted.

4. The method of claim 1, wherein the alarm is triggered when a collision of signals from two or more transponders is detected in a predetermined number of TDMA time slots after a strobe command is transmitted.

5. The method of claim 4, wherein a collision is detected by:

determining a background reference power level outside of a number of TDMA time slots when signals are to be transmitted from transponders;

sampling a power level during each TDMA time slot;

comparing the sample to the background reference power level;

determining a ratio of the powers; and

identifying a collision when the ratio is greater than one during a TDMA time slot and no accepted identification code is received during the TDMA time slot.

6. A system for monitoring a designated security area, comprising:

a host computer;

an interrogator in communication with the host computer to send strobe commands to transponders that are configured to return an identification code to the interrogator upon transmission of a strobe command within the security area and receive signals from the transponders, wherein the host computer is configured to activate an alarm when the interrogator receives an identification code from a transponder located in the security area after transmission of a strobe command, and wherein the host computer is configured to activate the alarm when a collision of signals from two or more transponders within the designated security area is detected after transmission of the strobe command, the collision of signals being detected by sampling a power level during a TDMA time slot and at a time outside the TDMA time slot and determining a ratio of the sampled powers.

7. The system of claim 6, wherein the host computer is configured to trigger the alarm if an identification code is received a predetermined number of times after a strobe command is transmitted.

8. The system of claim 7, wherein the host computer is configured to trigger the alarm if the identification code is received in a predetermined number of TDMA time slots after the strobe command is transmitted.

9. The system of claim 6, wherein the host computer is configured to detect a collision of signals from two or more transponders within the designated security area by:

determining a background reference power level outside of a number of TDMA time slots when signals are to be transmitted from transponders;

sampling a power level during each TDMA time slot;

comparing the sample to the background reference power level;

determining a ratio of the powers; and

identifying a collision when the ratio is greater than one during a TDMA time slot and no accepted identification code is received during the TDMA time slot.