Detection apparatus for structural failure in aircraft employing piezoelectric transducers

• US 3,956,731 A
• Filed: 09/27/1974
• Issued: 05/11/1976
• Est. Priority Date: 09/27/1974
• Status: Expired due to Term

• Current Assignee: Lockheed Aircraft Corporation (Martin Marietta Corporation)
• Original Assignee: Lockheed Aircraft Corporation (Martin Marietta Corporation)
• Inventors: Lewis, William H. Jr.
• Primary Examiner(s): Curtis, Marshall M.
• Application Number:
  US05/509,915
• Time in Patent Office: 592 Days
• Field of Search: 340/27, 340/421, 73/88 R, 73/88.5 SD, 324/71
• US Class Current: 340/945
• CPC Class Codes: G05D 1/0066 for limitation of acceleration or stress

Abstract

The strain energy that is released when a structural member fails is emitted, among other ways as a broad frequency acoustic emission which is detected by a piezoelectric crystal sized for a preselected natural frequency. This piezoelectric crystal converts the acoustic energy into an electronic signal which is filtered to permit passage of only predetermined frequencies. The filtered signals are then passed through a device which matches the line impedance. A differential amplifier provides signal amplification, and rejects noise, in the received signals which are then passed through a level detector, preset to only allow passage of predetermined signals which in turn trigger a latch for a light emitting diode indicating that a structural failure has occurred.

A built-in tester allows for a periodic check to be made to insure system operation. An amplified signal is oscillated causing the piezoelectric crystals to respond as if they detected an acoustic response from a structural failure which triggers the light emitting diode if the system is functioning properly.

Claims

1. An apparatus for the detection of structural failures in aircraft comprising:

at least one pair of piezoelectric transducers adapted to be secured to a component of the aircraft in a predetermined area, each of said transducers being sized for a specific resonance and adapted to convert said resonance into electrical energy of a predetermined frequency;

a first electrical circuit connected to each transducer and including a filter and a differential amplifier to pass only signals of a selected frequency, a voltage level detector to receive said selected frequency signals and preset to pass only signals of a predetermined voltage, a switch responsive to said predetermined voltage signals and an indicator actuated by the operation of said switch; and

a second electrical circuit connected to each transducer and including a power line with an on-off control, an oscillator activated by said control when operative to generate electrical signals in the associated second circuit at the predetermined frequency aforesaid and a transformer to amplify the voltage of said generated signals to a level sufficient to pass the voltage level detector aforesaid, said on-off control including a circuit maker and breaker in said first circuit and being operative to alternately connect either one of said circuits.

2. The apparatus of claim 1 wherein said piezoelectric transducers are each sized to a natural frequency of about 140 kHz.

3. The apparatus of claim 2 wherein the selected frequency level of each said filter is on the order of 32 kHz and above.

4. The apparatus of claim 3 wherein the predetermined voltage setting of each said detector is 2v.

5. The apparatus of claim 1 wherein said indicator is a light emitting diode.

6. The apparatus of claim 5 wherein said switch includes manually-operated means to reset it after operation.