Detection, by an electromagnetic transponder reader, of the distance separating it from a transponder

• US 6,650,226 B1
• Filed: 04/06/2000
• Issued: 11/18/2003
• Est. Priority Date: 04/07/1999
• Status: Expired due to Term

• Current Assignee: STMicroelectronics SA (STMicroelectronics NV)
• Original Assignee: STMicroelectronics SA (STMicroelectronics NV)
• Inventors: Enguent, Jean-Pierre, Wuidart, Luc, Bardouillet, Michel
• Primary Examiner(s): Horabik, Michael
• Assistant Examiner(s): DaLencourt, Yves
• Application Number:
  US09/544,151
• Time in Patent Office: 1,321 Days
• Field of Search: 340/10.1, 340/10.34, 340/10.4, 340/572.1, 340/572.7, 340/870.31, 235/439, 235/492, 455/41
• US Class Current: 340/10.1
• 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

Abstract

A terminal for generating an electromagnetic field adapted to cooperate with at least one transponder when the transponder enters the electromagnetic field, the terminal including circuitry for determining the distance separating the at least one transponder from the terminal without requiring any transmission from the transponder to the terminal. In one example, the terminal also includes a phase regulation loop that regulates the phase of a signal in an oscillating circuit of the terminal with respect to a reference value.

Claims

1. A terminal for generating an electromagnetic field adapted to cooperate with at least one transponder when said transponder enters in the field, and comprising an oscillating circuit adapted to receive a high frequency A.C. drive voltage, the terminal including means for determining a distance separating the transponder from the terminal without requiring any transmission from the transponder to the terminal and means for regulating a phase of a signal in the oscillating circuit with respect to a reference value.

2. The terminal of claim 1, wherein said means for determining a distance measure a variable depending on a load formed by the transponder on the terminal'"'"'s oscillating circuit.

3. The terminal of claim 2, wherein said means for determining the distance measure a current in the oscillating circuit or a voltage across one or several of its elements.

4. The terminal of claim 3, wherein said means for determining the distance include an analog-to-digital converter of a measured current or voltage and a digital circuit for comparing the measured current or voltage with a correspondence table stored in the terminal.

5. A terminal that generates an electromagnetic field, the terminal being adapted to communicate with a transponder when the transponder enters the electromagnetic field, the terminal comprising:

an oscillating circuit;

a phase-regulation loop that regulates a phase of a signal in the oscillating circuit with respect to a reference value; and

a distance detector that determines a distance between the terminal and the transponder based on a value of a real part of an impedance of the oscillating circuit.

6. The terminal of claim 5, wherein the signal is a current in the oscillating circuit.

7. The terminal of claim 6, wherein the phase-regulation loop includes a phase comparator that provides an output signal based on a comparison of a measured value of the phase of the current with the reference value.

8. The terminal of claim 7, wherein the phase comparator uses the output signal to vary a capacitance of the oscillating circuit to regulate the phase of the current.

9. The terminal of claim 5, wherein the distance detector includes an envelope detector that measures a voltage at a node of the oscillating circuit to provide a measured voltage indicative of the real part of the impedance of the oscillating circuit.

10. The terminal of claim 9, wherein the envelope detector includes a capacitor for storing the measured voltage, a resistor that discharges the capacitor when the measured voltage decreases, and a diode that rectifies the voltage at the node.

11. The terminal of claim 9, wherein the distance detector includes an analog-to-digital converter that converts the measured voltage to a digital value, and provides the digital value to a processor.

12. The terminal of claim 5, wherein the distance detector includes a current measurement device that measures a current in the oscillating circuit.

13. A method for determining a distance between a terminal that generates an electromagnetic field, the terminal being adapted to communicate with a transponder when the transponder enters the electromagnetic field, and the transponder that enters the electromagnetic field, independent of the transponder transmitting any signal, the method comprising acts of:

regulating a phase of a current in an oscillating circuit of the terminal;

determining a value of a real part of an impedance of the oscillating circuit; and

determining the distance between the terminal and the transponder based on the value of the real part of the impedance of the oscillating circuit.

14. The method of claim 13, wherein the act of regulating includes:

measuring the phase of the current to provide a measured phase;

comparing the measured phase with a reference value; and

regulating the phase of the current with respect to the reference value.

15. The method of claim 14, wherein the act of regulating further includes an act of varying a capacitance of the oscillating circuit.

16. The method of claim 13, wherein the act of determining the value of the real part of the impedance includes measuring a voltage at a node of the oscillating circuit to provide a measured voltage.

17. The method of claim 16, wherein the act of determining the distance includes comparing the measured voltage with a predetermined value.

18. The method of claim 13, wherein the act of determining the distance includes converting the measured voltage to a digital value.

19. The method of claim 13, wherein the act of determining the value of the real part of the impedance includes measuring a voltage at a node of the oscillating circuit to provide a measured voltage.

20. The method of claim 13, wherein the act of determining a value of a real part of an impedance of the oscillating circuit includes measuring a current in the oscillating circuit.