STRUCTURAL HEALTH MONITORING CIRCUIT

• US 20080231294A1
• Filed: 03/19/2008
• Published: 09/25/2008
• Est. Priority Date: 03/19/2007
• Status: Abandoned Application

• Current Assignee: Ndsu-Research Foundation
• Original Assignee: Ndsu-Research Foundation
• Inventors: You, Chao, Wang, Shirui
• Application Number:
  US12/051,682
• Publication Number:
  US 20080231294A1
• Time in Patent Office: Days
• US Class Current: 324/725
• CPC Class Codes: G01N 2203/0244 Tests performed "in situ" or after "in situ" use
• CPC Class Codes: G01N 27/02 by investigating impedance

Abstract

A structural health monitoring circuit apparatus and method are based on electrical impedance variations of a piezoelectric patch, which is attached to a structure to be monitored. The circuit compares a known good sweep of frequency-impedance pairs with a contemporaneous sweep to generate an alarm when an error bound is exceeded. The impedance of the piezoelectric patch is determined though adjustment of a variable reactance in a bridge configuration. By suitable design of the bridge elements, the electrical impedance of the piezoelectric patch may be directly measured. A microprocessor controlled version of this device consumes less than 2 W of power, which may be further reduced by further large scale integration or reduction to a state machine on a programmable gate array. Ultimately, this device may give personnel warnings to aircraft, automobiles, bridges, elevated roads, buildings, or home structural failures.

Claims

1. An apparatus, comprising:

a piezoelectric patch attached to a structure;

means for measuring electrical impedance of the piezoelectric patch; and

means for outputting the measured the electrical impedance of said piezoelectric patch at an input frequency to a computer readable medium.

2. The apparatus of claim 1, wherein the means for measuring comprises:

a resonant bridge, comprising;

a variable reactance as an element in a first leg in the resonant bridge, and

the piezoelectric patch as an element in a second leg in the resonant bridge;

a clock generator that drives a frequency input to the resonant bridge;

a first peak detector electrically connected to the first leg of the resonant bridge;

a second peak detector electrically connected to the second leg of the resonant bridge;

a differential amplifier comprising inputs from the two peak detector outputs;

a window comparator having an input coupled to an output of the differential amplifier; and

a control circuit having an input coupled to an output of the window comparator;

wherein an output of the control circuit independently controls the variable reactance and the clock generator.

3. The apparatus of claim 2, wherein the variable reactance comprises one or more elements selected from a group consisting of:

a digitally controlled resistor, a digitally controlled capacitor, and a digitally controlled inductor.

4. The apparatus of claim 2, comprising:

means for monitoring a state of health the structure.

5. The apparatus of claim 4, wherein the means for monitoring comprises:

a comparison between an initial known good state of the structure; and

a subsequent unknown state of the structure.

6. The apparatus of claim 5, wherein the known good state and the subsequent unknown state are determined by a sweep of frequencies and their corresponding variable reactance set points to achieve balance of the resonant bridge.

7. An apparatus, comprising:

a piezoelectric patch attached to a structure;

a clock generator;

a bridge circuit comprising an input coupled to an output of said clock generator, said bridge circuit configured to monitor variations in electrical impedance of said piezoelectric patch;

a set of two peak detectors, each with an input coupled to an output of said bridge circuit;

a differential amplifier with inputs coupled to an output of the two peak detectors;

a comparator with an input coupled to an output of the differential amplifier;

a control circuit with an input coupled to an output of said comparator, wherein;

the control circuit controls an output frequency of the clock generator, and

the control circuit controls a variable reactance within the bridge circuit; and

a data output to a computer readable medium, comprising a set point of the clock generator and a set point of the variable reactance within the bridge circuit.

8. An apparatus as recited in claim 7, wherein the variable reactance comprises a digital controlled component, such as a digital resistor or digital capacitor.

9. A structural heath monitoring apparatus, comprising:

a clock generator;

a bridge circuit having an input coupled to an output of said clock generator, said bridge circuit configured for monitoring variations in electrical impedance of said piezoelectric patch;

a set of two peak detectors, each with an input coupled to an output of said bridge circuit;

a differential amplifier with inputs coupled to an output of the two peak detectors;

a comparator with an input coupled to an output of the differential amplifier;

a control circuit with an input coupled to an output of said comparator, wherein;

the control circuit controls an output frequency of the clock generator, and

the control circuit controls a variable reactance within the bridge circuit;

wherein said apparatus is configured to electrically couple the piezoelectric patch to a structure and to monitor variations in electrical impedance in the piezoelectric patch that are indicative of structural heath of said structure; and

a data output to a computer readable medium, comprising a set point of the clock generator and a set point of the variable reactance within the bridge circuit.

10. An apparatus as recited in claim 9, wherein the bridge circuit variable reactance comprises a digital resistor or a digital capacitor.

11. A method of structural health monitoring, comprising:

providing a structural health monitoring circuit attached to a structure;

providing an initial known good frequency sweep of the structural health monitoring circuit attached to the structure;

subsequently sweeping the structural health monitoring circuit attached to the structure to generate a contemporaneous frequency sweep; and

comparing the initial known good frequency sweep with the contemporaneous frequency sweep to generate a differential error.

12. The method of claim 11, comprising:

outputting to a computer readable medium the differential error.

13. The method of claim 12, wherein the comparing step is a digital comparing step.

14. The method of claim 12, wherein the comparing step is an analog comparing step.

15. The method of claim 12, wherein the initial known good frequency sweep comprises one or more frequencies.

16. The method of claim 15, wherein the initial known good frequency sweep is performed in-situ after the structure has been completed.

17. The method of claim 15, wherein the initial known good frequency sweep is performed prior to installation of the structure.

18. The method of claim 15, wherein the initial known good frequency sweep is generated off-line through numerical modeling of the structure.

19. The method of claim 15, wherein the initial known good frequency sweep spans a frequency range from about 53 kHz to about 164 kHz.

20. The method of claim 11, comprising:

generating an alarm when the differential error exceeds an error limit.

21. The method of claim 20, comprising:

transmitting the alarm a computer readable medium.

22. The method of claim 20, wherein the alarm is an audible and/or visual alarm for personnel that may be injured by damage to the structure.

23. The method of claim 20, wherein the error limit is based on an average calculation.

24. The method of claim 20, wherein the error limit is based on a root mean square (RMS) calculation.

25. A computer readable medium comprising a programming executable capable of performing on a computer the method of claim 11.