Acoustic emission contact fuze with signal processing capability

An acoustic emission contact fuze that is responsive to high-frequency actic emission stress waves and distinguishes between such waves caused by impacts and signals caused by the operating environment.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present application relates to acoustic emission contact fuzes for 
missiles and, more particularly, to an acoustic emission contact fuze with 
signal processing capabilities. 
2. Description of the Prior Art 
Piezoelectric contact fuzes are well known in the art. The fuzes presently 
in use are sensitive only to direct impacts and are not sensitive to 
glancing blows or contact where only a portion of the missile is involved. 
The conventional methods used to increase the sensitivity to glancing blows 
are such means as wires on the outside surfaces of the fins, or 
accelerometers of increased sensitivity. The wires are separated from fin 
structure surface by a thin layer of a crushable insulator. Upon an impact 
with a target a short circuit is created which triggers a detonator. The 
fragility of this system requires extra precautions in the shipping and 
handling of the missiles such as the use of fin guards. In addition, a 
check must be made of damage to the structure immediately prior to firing. 
Accelerometers consisting of a piezoelectric material with a mass attached 
have also been used. Increasing the sensitivity of the accelerometer can 
provide some reaction to glancing blows. As the sensitivity is increased 
however, environmental factors can lead to false contact signals. 
It has been proposed that a fuze could be constructed by placing a 
piezoelectric transducer on the structural elements of the missile linked 
to the fin structure. When an impact occurs with the fin structure the 
supporting structure would be deformed. Deformation of the structure would 
bend the piezoelectric crystal causing an electrical signal which could be 
detected. The signals then could be used to trigger the warhead. 
Previously such attempts have been unsuccessful due to sensitivity to 
normal vibration or motor-chuffing in flight. These prior configurations 
have been limited to stress waves of a frequency below that utilized by 
the claimed invention. 
Major disadvantages of present techniques are insensitivity to glancing 
blows and/or sensitivity to flight environmental conditions and the 
requirement of extra handling precautions. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention provides a piezoelectric transducer 
located so as to be sensitive to high-frequency acoustic emission stress 
waves. Signal processing circuitry is connected to the transducer to 
distinguish waves caused by the plastic deformation of missile structures 
from those caused by normal vibrations in flight or motor chuffing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
When a material is deformed to failure by loading acoustic emission stress 
waves are generated due to crystal slippage. Acoustic emission stress 
waves were not researched fully until the late 1960's. The waves generated 
are of frequencies from 100 KHz to 1 MHz. These waves were not utilized 
until that date due to the insensitivity of prior art piezoelectric 
devices to frequencies above a few tens of KHz. The claimed invention 
comprises a method for detection of these waves, and separation of the 
signals generated by these waves from noise and other aoustic signals, and 
the use of the processed signal to trigger a fuze device. 
Any plastic deformation of a metal causes the generation of acoustic 
emission stress waves. These waves are much higher in frequency than 
acoustic waves generated by wind buffeting or motor-chuffing. The 
invention distinguishes acoustic emission stress waves by their higher 
frequency and amplitude from signals caused by the operating environment. 
In FIG. 1, a missile 11 collides with a target 12. A fin 13 of the missile 
is damaged in the collision. The deformation of the fin's metal generates 
acoustic emission stress waves 14. The waves are detected by a 
piezoelectric transducer 16. The transducer's signal is then fed to the 
signal processor 17 which on receipt of acoustic emission stress waves 
fires a squib 18 which detonates the warhead 19 of the missile. 
A block diagram of the signal processor 17 is shown in FIG. 2. The signal 
from the transducer first goes to an amplifier 21 which raises it to a 
level suitable for processing. The signal then enters a bandpass filter 22 
and those portions within the range of the filter, here 100 KHz to 1 MHz, 
are amplified in a post amplifier 23. The signal is now measured against a 
preset reference by the window comparator 24 and 26. Those signals above 
the threshold go on to the timer 27. The timer 27 is set to pass a signal 
to the firing circuit 28 if the timer receives two pulses in 12.5 
microseconds. The firing circuit 28 functions as a switch to fire an 
explosive squib 18 on receiving a signal from the timer. 
The schematic diagram of the comparator circuit is shown in FIG. 3. The 
voltage level of the signal required to generate output pulses from window 
comparator 24, 26 is preset by adjustment of a potentiometer 29. Detection 
of both the positive and negative parts of a sinusoidal input signal can 
be accomplished by placing the comparator "in/out" switch 31 in the "in" 
position. When the switch is in the "out" position, the comparator will 
detect only positive going signals. When the input at pin 4 of U4 or pin 3 
of U5 exceeds the preset threshold the output voltage at pin 9 of U4 and 
U5 switches from 2.0-4.0 VDC to 0-0.8 VDC. The width of the output pulse 
will remain at this voltage level for the length of time that the input 
voltage exceeds the preset threshold. 
FIG. 4 is a circuit diagram of the timing gate. 
If two pulses, either positive or negative are fed to the input of the 
comparator (comparator in/out switch in the "in" position), a "fire" pulse 
will be generated at pin 10 of U7B. This is accomplished when a negative 
going signal into pin 4 of U7A produces a positive going signal at pin 6 
of U7A. This positive going signal is fed to pin 13 of U7B. The charging 
time of the capacitor 32 holds pin 13 at a "O" logic level and as a 
consequence the output at pin 10 is held to a logic "O". Where the second 
negative going pulse is fed to pin 4 of U7A and pin 11 of U7B, capacitor 
32 is charged thereby driving the input of pin 13 with a "I" logic level 
and the output at pin 10 is switched to a "I" logic level. 
The most recent embodiment of the invention consists of a piezoelectric 
transducer 16 mounted securely and acoustically coupled to a portion of a 
guided missile airframe 11. The location is generally within the missile 
structure at a point close to the portion of the missile which is most 
often expected to impact with the target, such as missile wings or nose. 
Transducer 16 has no external mass connected to the crystal to allow 
function as an accelerometer but functioning rather as an acoustic 
emission stress wave transducer. The output of transducer 16 is processed 
by a solid-state circuitry 17, previously described in reference to FIG. 
2. The bandpass filter 22 is designed to pass signals of extremely high 
frequency and prevent the transmission of lower frequency signals. In the 
disclosed embodiment of the invention the bandpass filter is an active 
filter. Bandpass filter 22 thus ensures that only signals in the frequency 
range of acoustic emission stress waves will be processed. The output of 
bandpass filter 22 is amplified and transmitted for further signal 
processing. The signal then enters window comparator 24, 26 and only 
pulses that are above a set magnitude pass. These pulses are analyzed in 
timer 27 and if two pulses are received in 12.5 microseconds, indicating 
the high frequency of acoustic emission stress waves, firing circuit 28 is 
activated causing detonation of squib 18 and the warhead. 
The signal processing means herein described are the most recent method of 
detecting the acoustic emission stress waves and distinguishing them from 
environmental signals. Alternative signal processing means are possible 
and within the skill of the proficient engineer recognizing 
well-understood trade-off's. It is likely that future development of the 
invention may utilize other signal processing means, within such scope. 
The circuit configuration given is a preferred means of accomplishing this 
end and constitutes the best mode of practicing the invention presently 
contemplated.