Distance determination between RFID tags

Patent metadata

Field	Value
Publication number	US-9767330-B2
Application number	US-201514801538-A
Country	US
Kind code	B2
Filing date	Jul 16, 2015
Priority date	Jul 16, 2015
Publication date	Sep 19, 2017
Grant date	Sep 19, 2017

Abstract

Technologies are generally described for distance determination between two or more RFID tags based on a minimum detectable signal. In some examples, a first RFID tag may establish communication with a second RFID tag, and modulate an irradiating electromagnetic field to generate backscatter signals for sequential transmission to the second RFID tag. A depth of amplitude modulation of each transmitted signal may be progressively reduced until the second RFID tag can no longer detect the modulation. The particular distance between the first and second RFID tags may then be determined based on a minimum detectable signal by the second RFID tag, which may be identified as a last transmitted signal detected by the second RFID tag. In some embodiments, distance determination between two or more RFID tags as described above may be implemented for robotic sensing, assembly systems, security, and wearable technology to track motion and/or position of objects.

First claim

What is claimed is:

1. A method to determine a distance between radio frequency identification (RFID) tags, the method comprising:
at a first RFID tag:
establishing a communication with a second RFID tag that is separated from the first RFID tag by a particular distance;
transmitting a plurality of signals sequentially to the second RFID tag, wherein each signal is transmitted at a reduced strength compared to a respective previous signal;
receiving a confirmation from the second RFID tag following the transmission of each signal, wherein the confirmation indicates that the transmitted signal is detectable by the second RFID tag;
in response to a failure to receive the confirmation from the second RFID tag:
identifying a last transmitted signal, for which the confirmation was received, as a minimum detectable signal by the second RFID tag; and
re-transmitting the last transmitted signal, identified as the minimum detectable signal, to the second RFID tag; and

determining the particular distance between the first RFID tag and the second RFID tag based on the minimum detectable signal.

2. The method of claim 1 , wherein transmitting each signal at the reduced strength compared to the respective previous signal comprises:
reducing a depth of amplitude modulation of each signal from the respective previous signal.

3. The method of claim 2 , wherein reducing the depth of the amplitude modulation of each signal from the respective previous signal comprises:
employing a pulse width modulation to vary duty cycles of a modulator of the first RFID tag.

4. The method of claim 2 , wherein the depth of the amplitude modulation is characterized as a function of field power.

5. The method of claim 1 , wherein establishing the communication with the second RFID tag at the first RFID tag comprises:
enabling the first RFID tag and the second REID tag to agree to detect the minimum detectable signal.

6. The method of claim 1 , further comprising:
communicating the last transmitted signal, for which the confirmation was received from the second RFID tag, as the minimum detectable signal to a reader.

7. The method of claim 1 , wherein the first RFID tag and the second RFID tag are passive RFID tags.

8. A radio frequency identification (RFID) tag, the RFID tag comprising:
an antenna configured to receive radio frequency (RF) field generated by an external field generator;
a demodulator configured to extract energy for tag operations from the received RF field; and
a controller configured to:
establish a communication with another RFID tag that is separated from the RFID tag by a particular distance;
transmit a plurality of signals sequentially to the other REID tag, wherein each signal is transmitted at a reduced depth of amplitude modulation compared to a respective previous signal;
receive a confirmation from the other RFID tag following the transmission of each signal, wherein the confirmation indicates that the transmitted signal is detectable by the other RFID tag;
in response to a failure to receive the confirmation from the other RFID tag:
identify a last transmitted signal, for which the confirmation was received, as a minimum detectable signal by the other RFID tag; and
re-transmit the last transmitted signal, identified as the minimum detectable signal, to the second RFID tag; and

determine the particular distance between the RFID tag and the other RFID tag based on the minimum detectable signal.

9. The RFID tag of claim 8 , wherein the controller is further configured to generate a pulse width modulation using the energy from the RF field.

10. The RFID tag of claim 9 , further comprising a modulator configured to:
receive the pulse width modulation; and
vary a duty cycle based on the received pulse width modulation such that the depth of the amplitude modulation of each signals transmitted to the other RFID tag is varied.

11. The RFID tag of claim 10 , wherein the modulator includes an antenna shunting transistor.

12. A system to determine a distance between radio frequency identification (RFID) tags, the system comprising:
a reader;
a first RFID tag comprising:
a first antenna configured to receive a radio frequency (RF) field;
a demodulator configured to extract energy for tag operations from the received RF field; and
a first controller configured to:
establish a communication with a second RFID tag that is separated from the first RFID tag by a particular distance;
transmit a plurality of signals sequentially to the second RFID tag, wherein each signal is transmitted at a reduced depth of a first amplitude modulation compared to a respective previous signal;
receive a first confirmation from the second RFID tag following the transmission of each signal, wherein the first confirmation indicates that the transmitted signal is detectable by the second RFID tag;
in response to a failure to receive the first confirmation from the second REID tag, identify a last transmitted signal for which the first confirmation was received as a first minimum detectable signal by the second RFID tag;
communicate the last transmitted signal, identified as the first minimum detectable signal, to the reader; and
determine the particular distance between the first RFID tag and the second RFID tag based on the first minimum detectable signal; and

the second RFID tag comprising:
a second antenna configured to receive the RF field and the plurality of signals transmitted from the first RFID tag at varying depths of the first amplitude modulation; and
a second controller configured to send the first confirmation to the first RFID tag in response to a determination that the transmitted signal is detectable by the second RFID tag.

13. The system of claim 12 , wherein the system is integrated within a robotic arm.

14. The system of claim 12 , further comprising:
an external field generator configured to generate the RF field.

15. The system of claim 14 , wherein the first RFID tag and the second RFID tag are bathed in the RF field generated by one of the external field generator or the reader.

16. The system of claim 12 , wherein the second controller is further configured to:
transmit another plurality of signals sequentially to the first RFID tag, wherein each signal, of the another plurality of signals, is transmitted at a reduced depth of a second amplitude modulation compared to a respective previous signal of the another plurality of signals;
receive a second confirmation from the first RFID tag following the transmission of each signal of the another plurality of signals, wherein the second confirmation indicates that the transmitted signal, of the another plurality of signals, is detectable by the first RFID tag;
in response to a failure to receive the second confirmation from the first RFID tag, identify a last transmitted signal, of the another plurality of signals, for which the second confirmation was received as a second minimum detectable signal by the first RFID tag; and
determine the particular distance between the first RFID tag and the second RFID tag based on the second minimum detectable signal.

17. The system of claim 16 , wherein the second RFID tag is configured to communicate the second minimum detectable signal by the first RFID tag to the reader.

18. The system of claim 17 , wherein the reader is configured to:
determine a strength of each of the first minimum detectable signal and the second minimum detectable signal.

19. The system of claim 18 , wherein the reader is furt

Assignees

• Empire Technology Dev Llc

Inventors

• Roberts Michael John

Classifications

• G06K7/10128 Primary

  the step consisting of detection of the presence of one or more record carriers in the vicinity of the interrogation device · CPC title

• G06K7/10188

  the repeating consisting of intelligently propagating data from record carriers via intermediate stations to the interrogation device, e.g. a distant RFID or RFID falling in a "shadow" region sending its identification data to an interrogation device using at least the help of one further RFID that is positioned in a region "visible" to the interrogation device, the further RFID therefore functioning as a relay station · CPC title

• G06K7/10475

  arrangements to facilitate interaction with further interrogation devices, e.g. such that at least two interrogation devices may function and cooperate in a network of such devices · CPC title

• B25J13/088

  with position, velocity or acceleration sensors · CPC title

• G01S13/75

  using transponders powered from received waves, e.g. using passive transponders {, or using passive reflectors} · CPC title