Remote sensing of structural integrity using a surface acoustic wave sensor

• US 5,821,425 A
• Filed: 09/30/1996
• Issued: 10/13/1998
• Est. Priority Date: 09/30/1996
• Status: Expired due to Fees

• Current Assignee: United States Of America As Represented By The Secretary Of The Army (Government of the United States of America)
• Original Assignee: The United States Of America As Represented By The Secretary Of The Army
• Inventors: Stewart, James T., Mariani, Elio A., McGowan, Raymond C.
• Primary Examiner(s): Chilcot, Richard
• Assistant Examiner(s): Noori, Max H.
• Application Number:
  US08/723,068
• Time in Patent Office: 743 Days
• Field of Search: 73/598, 73/619, 73/703, 73/768, 73/702, 73/704
• US Class Current: 73/703
• CPC Class Codes: G01N 2291/011 Velocity or travel time
• CPC Class Codes: G01N 2291/0423 Surface waves, e.g. Rayleigh waves, Love waves
• CPC Class Codes: G01N 29/022 Fluid sensors based on microsensors, e.g. quartz crystal-microbalance [QCM], surface acoustic wave [SAW] devices, tuning forks, cantilevers, flexural plate wave [FPW] devices microdevices per se B81B
• CPC Class Codes: G01N 29/041 on the surface of the material, e.g. using Lamb, Rayleigh or shear waves
• CPC Class Codes: G01N 29/14 using acoustic emission techniques echo of particles G01N29/046; measuring mechanical vibrations or acoustic waves in solids in general G01H1/00
• CPC Class Codes: G01N 29/227 related to high pressure, tension or stress conditions

Abstract

A surface acoustic wave (SAW) sensing device for remotely sensing structu integrity of a physical structure. The sensing device includes a piezoelectric substrate with a notch formed part way in the bottom of the substrate and along the width thereof. The substrate is mounted to a physical structure of interest. An antenna is coupled to the RF circuit on the substrate and is capable of receiving and transmitting a RF signal. Interdigital input and output transducers are disposed on the upper surface of the substrate. The input transducer is located adjacent one end of the substrate and the output transducer is located adjacent an opposing end of the substrate. Bus bars connect the input and output transducers. The input transducer provides a complementary first response upon receipt of an RF expanded linear/nonlinear FM signal from the antenna and transmits this compressed pulse to the output transducer. The output transducer provides a second response upon receipt of the first response and transmits the same to the antenna via the bus bar. When the substrate is strained beyond a predetermined critical level, the substrate is fractured along the notch and the first response emitted by the input transducer is prevented from being transmitted to the output transducer indicating SAW sensor failure and that structural integrity has been compromised.

Claims

1. A passive surface acoustic wave sensing device for remotely sensing structural integrity within a physical structure, comprising:

a substrate having a polished upper surface, a bottom surface, a first end and a second end, said substrate being mounted to said physical structure;

a Radio Frequency (RF) circuit formed on the upper surface of said substrate for receiving and transmitting a RF signal;

an antenna means coupled to said RF circuit;

an input interdigital transducer means and an output interdigital transducer means disposed on said upper surface of said substrate, said input transducer means being located adjacent said first end of said substrate and said output transducer means being located adjacent said second end of said substrate, said input and output transducers being interconnected through a pair of parallel bus bars which extend therebetween on said upper surface of said substrate, said input transducer means providing a complementary first response upon receipt of a RF input signal from said antenna means, said output transducer means providing a second response upon receipt of said first response from said input transducer means, said output transducer means being adapted to transmit said second response to said antenna means;

means for carrying said first response to said output transducer means when said substrate is strained below a predetermined critical level; and

means for preventing said first response from being transmitted to said output transducer means when said substrate is strained above said predetermined critical level.

2. The sensing device of claim 1, wherein said substrate is comprised of a piezoelectric material.

3. The sensing device of claim 1, wherein said carrying means extends across said upper surface of said substrate between said input and output transducer means.

4. The sensing device of claim 3, wherein said preventing means includes a notch formed across the width of said substrate in said bottom surface thereof, said substrate and said carrying means being adapted to fracture along said notch when said substrate is strained above said predetermined critical level.

6. The sensing device of claim 1, wherein said output transducer means includes a plurality of output taps spaced at time-ordered intervals.

7. The sensing device of claim 1, wherein said output transducer means includes a plurality of output taps phase-coded at 0°

and 180°

.

8. The sensing device of claim 1, wherein said output transducer means includes a plurality of output taps spaced at time-ordered intervals and phase coded at 0°

and 180°

.

9. The sensing device of claim 1, wherein said input RF signal has a predetermined bandwidth and said first response is in the form of a pulse substantially equivalent to the reciprocal of said bandwidth of said RF signal.

5. A method of remotely sensing structural integrity within a physical structure utilizing a passive surface acoustic wave sensing device of the type which includes a piezoelectric substrate having an input transducer means and an output transducer means disposed thereon in a spaced apart relation, said method comprising the steps of:

rigidly mounting said surface acoustic wave sensing device to said physical structure so that said sensing device will be strained when said physical structure is strained;

transmitting a RF signal to said input transducer means;

transforming said RF signal into an acoustic wave having a distinct compressed pulse from said input transducer means;

transmitting said acoustic wave signal to said output transducer means along said substrate when said physical structure is strained below a predetermined level;

providing means for detecting receipt of said acoustic wave by said output transducer means; and

preventing said acoustic wave signal from being transmitted to said output transducer means when said physical structure is strained above a predetermined level.

13. The method of claim 5, wherein said method for preventing said acoustic wave from being transmitted to said output transducer means includes forming a physical break across said substrate between said input transducer means and said output transducer means.

10. A passive surface acoustic wave sensing device for remotely sensing structural integrity within a physical structure, comprising:

a substrate having an upper surface, a bottom surface, a first end and a second end, said substrate mounted to said physical structure so that when said structure is strained, said substrate is also strained;

an antenna means coupled to said substrate for receiving and transmitting a RF signal;

an interdigital input transducer and two spaced apart output sensing taps disposed on said upper surface of said substrate, said input transducer being located adjacent said first end of said substrate and said output sensing taps being located adjacent said second end of said substrate with said input transducer interconnected to said sensing taps through a pair of parallel bus bars which extend therebetween on said upper surface of said substrate;

said input transducer providing a complementary first response upon receipt of an input RF signal from said antenna means; and

means for carrying said first response from said input transducer to said output sensing taps, each one of said output sensing taps providing an output response upon receipt of said first response emitted from said input transducer, each one of said output responses being transmitted to said antenna means via said carrying means, and each one of said output responses being separated in time by an amount corresponding to the separation between said output sensing taps so that any strain in said substrate will create a change in the phase velocity, time delay and time separation between said output sensing taps.

11. The sensing device of claim 10, wherein said substrate is comprised of a piezoelectric material.

12. The sensing device of claim 10, wherein said carrying means extends across said upper surface of said substrate between said input transducer and said output sensing taps.

14. A method of remotely sensing structural integrity within a physical structure utilizing a passive surface acoustic wave sensing device of the type which includes a piezoelectric substrate having an input transducer means and a pair of output transducer means disposed thereon in a spaced apart relation, said method comprising the steps of:

mounting said surface acoustic wave sensing device to said physical structure so that said sensing device will be strained when said physical structure is strained;

transmitting a RF signal to said input transducer means;

transforming said RF signal into an acoustic wave having a distinct compressed pulse from said input transducer means;

transmitting said acoustic wave to said output transducer means along said substrate when said physical structure is strained below a predetermined level;

modifying said delay time separation between output taps of said acoustic signals as it is transmitted along said substrate when said physical structure is strained above a predetermined level; and

providing means for detecting the receipt of said acoustic signals by said output transducer means.