INDUCTIVE EVALUATION OF THE COUPLING FACTOR OF AN ELECTROMAGNETIC TRANSPONDER

• US 20100321164A1
• Filed: 06/15/2010
• Published: 12/23/2010
• Est. Priority Date: 06/19/2009
• Status: Active Grant

• Current Assignee: STMicroelectronics SAS (STMicroelectronics NV)
• Original Assignee: STMicroelectronics SAS (STMicroelectronics NV)
• Inventors: Wuidart, Luc
• Granted Patent: US 8,395,485 B2
• Time in Patent Office: Days
• US Class Current: 340/10.4
• CPC Class Codes: G06K 7/10158 methods and means used by the interrogation device for reliably powering the wireless record carriers using an electromagnetic interrogation field

Abstract

A method for evaluating the current coupling factor between an electromagnetic transponder and a terminal, wherein a ratio between data representative of a voltage across an oscillating circuit of the transponder and obtained for two pairs of inductive and capacitive values of this oscillating circuit is compared with one or several thresholds, the two pairs of values preserving a tuning of the oscillating circuit to a same frequency.

Claims

1. A method for evaluating the current coupling factor between an electromagnetic transponder and a terminal, wherein a ratio between data representative of a voltage across an oscillating circuit of the transponder and obtained for two pairs of inductive and capacitive values of this oscillating circuit, is compared with one or several thresholds, the two pairs of values preserving a tuning of the oscillating circuit to a same frequency.

2. The method of claim 1, wherein:

first data relative to the level of a D.C. voltage provided by a rectifier across the oscillating circuit are measured and stored for a first pair of inductive and capacitive values of the oscillating circuit; and

second data relative to the level of said D.C. voltage are measured and stored for a second pair of inductive and capacitive values of the oscillating circuit.

3. The method of claim 1, wherein the evaluation provides a position of the current coupling factor with respect to an optimum coupling position with one of the two pairs of values.

4. The method of claim 1, wherein said thresholds are a function of the inductive values of the respective pairs of values.

5. The method of claim 1, wherein the coupling factor is evaluated against an optimum coupling position with the first pair of values.

6. A method for protecting an electromagnetic transponder against a possible overheating based on the evaluation of the coupling of claim 1, wherein a detuning of the oscillating circuit is caused if the ratio between the current coupling and the optimum coupling ranges between two thresholds.

11. The transponder of claim 7, further comprising a processing unit programmed to implement the method of claim 1.

7. An electromagnetic transponder comprising:

an oscillating circuit upstream of a rectifying circuit capable of providing a D.C. voltage when the transponder is in the magnetic field of a terminal; and

at least one switchable inductive element and at least one switchable capacitive element capable of being functionally connected in parallel with the oscillating circuit.

8. The transponder of claim 7, wherein said switchable inductive and capacitive elements are connected to the output terminals of the rectifying circuit.

9. The transponder of claim 7, wherein:

first switchable inductive and capacitive elements are connected between a first terminal of the oscillating circuit and the ground;

second switchable inductive and capacitive elements are connected between a second terminal of the oscillating circuit and the ground.

10. The transponder of claim 7, wherein the inductive elements are formed by active circuits.