Interrogation method for passive sensor monitoring system

• US 20040239504A1
• Filed: 02/20/2004
• Published: 12/02/2004
• Est. Priority Date: 08/23/2001
• Status: Active Grant

• Current Assignee: Transense Technologies Plc
• Original Assignee: Transense Technologies Plc
• Inventors: Bown, George E, Beckley, John Peter, Kalinin, Victor Alexandrovich
• Granted Patent: US 7,065,459 B2
• Time in Patent Office: Days
• US Class Current: 340/572.5
• CPC Class Codes: B60C 23/0433 Radio signals
• CPC Class Codes: G06K 19/0672 with resonating marks
• CPC Class Codes: G06K 19/0675 the resonating marks being of the surface acoustic wave [SAW] kind SAW devices per se H03H9/64

Abstract

A method of determining the frequency of a plurality of resonant devices (for example three SAW devices) includes determining optimal interrogation frequencies for each of the devices, the optimal interrogation frequencies having maximum power spectral densities, accumulating a plurality of responses for each sensor, performing discreet Fourier transforms on the sampling results to estimate the three resonant frequencies, and averaging the results of the Fourier transforms to provide an indication of resonant frequencies. The averaging step may include the calculation of a standard deviation and the rejection of any results which fall more than a predetermined multiple of the standard deviation from the average frequency result. The frequency is determined by the method may be employed to calculate the pressure and temperature of the sensor devices. The sensor devices may be located in a vehicle tire.

Claims

1. A method of interrogating a plurality of resonant devices to determine the respective resonant frequencies of the devices comprising the steps of:

(1) determining, for each resonant device, an optimal interrogation frequency;

(2) repeating the interrogation of each resonant device a plurality of times at its respective optimal interrogation frequency as determined by step (1);

(3) performing spectrum estimations on the data accumulated as a result of step (2); and

(4) determining the average of the frequencies derived from step (3).

2. The method according to claim 1 wherein step (4) further includes the steps of determining the standard deviation for each of the average frequencies determined;

rejecting any frequencies which vary from the average frequency by more than a predetermined multiple of the standard deviation; and

re-calculating the average frequency after exclusion of the rejected data.

3. The method according to claim 1, wherein the optimal interrogation frequencies are determined by establishing the frequencies at which the signals from the resonant devices have a maximum power spectral density.

5. The method according to claim 3, wherein the maximum power spectral density is determined by means of a linear amplifier with automatic gain control, the optimal frequencies being chosen to maximize the ratio of peak value of the spectral density to average level of its side lobes.

6. The method according to claim 3, wherein the maximum power spectral density is determined by use of a limiting amplifier, the frequencies being chosen to maximize the length of sensor response.

7. The method according to claim 1, wherein the optimal interrogation frequencies lie within respective sub-bands of an ISM band.

8. The method according to claim 1, wherein during the repetition of step 2 of claim 1 each signal received is down-converted, sampled and accumulated to provide a coherent accumulation of optimal interrogation frequencies.

9. The method according to claim 8, wherein the coherent accumulation is achieved by using a common oscillator both in the receiver and transmitter synthesizers and a clock generator in a DPS chip.

10. The method according to claim 1, wherein the number of repetitive interrogations in step 2 and the speed at which such interrogations are conducted is such that the total interrogation period is small in comparison with the period of any cyclical movement of the sensors relative to the interrogation equipment.

11. The method according to claim 8 wherein each coherent accumulation is repeated at each interrogation frequency but with an additional phase shift of 90°

introduced into the interrogation pulse during the second cycle of accumulation of the samples are taken with the delay during the second cycle of accumulation.

13. The method according to claim 1, wherein the determined frequencies are used to calculate pressure and temperature.

14. The method according to claim 1, wherein the resonant devices are SAW devices.

15. The method according to claim 1, wherein step (3) includes performing discrete Fourier transforms of the signals obtained at step (2).

4. The method according to claim 4, wherein the maximum power spectral density is determined by frequency down-conversion, sampling the response at an intermediate frequency, and calculating discreet Fourier transforms.

12. The method according to claim 4, wherein the sampling rate is chosen in such a way that the sampling interval corresponds to a 90°

phase shift at the intermediate frequency divided by an integer.

16. A method of interrogating a plurality of resonant devices to determine the respective resonant frequencies of the devices comprising the steps of:

(1) determining, for each resonant device, a interrogation frequency;

(2) repeating the interrogation of each resonant device a plurality of times at its respective interrogation frequency as determined by step (1);

(3) performing spectrum estimations on the data accumulated as a result of step (2); and

(4) determining the average of the frequencies derived from step (3).

17. The method according to claim 16, wherein step (4) further includes the steps of determining the standard deviation for each of the average frequencies determined;

rejecting any frequencies which vary from the average frequency by more than a pre-determined multiple of the standard deviation; and

re-calculating the average frequency after exclusion of the rejected data.

18. The method according to claim 16, wherein the interrogation frequencies are determined by establishing the frequencies at which the signals from the resonant devices have a maximum power spectral density.

19. The method according to claim 18, wherein the maximum power spectral density is determined by frequency down-conversion, sampling the response at an intermediate frequency, and calculating discreet Fourier transforms.

20. The method according to claim 18, wherein the maximum power spectral density is determined by means of a linear amplifier with automatic gain control, the frequencies being chosen to maximize the ratio of peak value of the spectral density to average level of its side lobes.