Measuring and testing – Vibration – Mechanical impedance
Reexamination Certificate
2002-03-22
2003-05-13
Chapman, John E. (Department: 2856)
Measuring and testing
Vibration
Mechanical impedance
C073S661000, C367S087000
Reexamination Certificate
active
06561031
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to prodders for probing the ground for buried explosive devices such as landmines and the like, and more particularly to a method and device for providing force feedback to the prodder and/or the user of the device.
BACKGROUND OF THE INVENTION
Despite a variety of mechanized means now available for detecting and clearing landmines, the current hand tool of choice is the hand prodder. Personnel exhibit greater confidence when traversing a minefield which has been hand-prodded by their compatriots than they do with fields cleared by other means.
The traditional hand prodder typically comprises a 30 cm long pointed rod extending from a gripping handle. The probe is generally non-magnetic to avoid setting off magnetically triggered mines. The user probes the ground ahead and excavates any hard objects which the probe contacts. As the ratio of rocks to landmines in a minefield may number 1000: 1, excavation of every contact is laborious, but very necessary.
Currently, instrumented prodders are known having ultrasonic means in the form of an ultrasonic transducer at or near the probe tip that are used for characterization of buried obstructions. These devices can be used in conjunction with a minimum metal content (MMC) detector, wherein the MMC detector first detects the ground indicating the vicinity of a land mine, and, wherein the instrumented prodder is used to probe the earth in the vicinity of the suspected land mine, the location of which may have been isolated using the MMC detector. MMC mine detectors having a search head and circuitry for detecting buried non-metallic and metallic land mines are well known. For example, U.S. Pat. No. 4,016,486 in the name of Pecori assigned to the United States of America by the Secretary of the Army, hereby incorporated by reference, discloses such circuitry.
U.S. Pat. No. 5,920,520 to Gallagher, hereby incorporated by reference, discloses an instrumented prodder having a probe in the form of an elongate, preferably non-magnetic rod including a gripping handle disposed at one end. The design of the probe is based partially upon a Split Hopkinson Pressure Bar (SHPB) apparatus. In the apparatus, a compression wave or high frequency elastic mechanical pulse is delivered via a rod to a sample, wherein a portion of the wave is reflected. The incident wave launched at the sample is reflected and/or transmitted from or through the sample, respectively, in dependence upon the characteristics of the material. The effect of mechanical impedance, which is a characteristic of a material, on a SHPB apparatus in three instances is described hereafter.
Firstly and obviously, if the mechanical impedance of a sample under test is the same as that of an incident bar in the SHPB, there will be no reflection as the sample will be displaced in a same manner as the bar itself as the compression wave is delivered. The displacement of the end of the bar is directly proportional to the strain measured (&egr;).
Secondly when the mechanical impedance of a sample is considerably greater than that of the bar, a sample's mechanical impedance tends toward being infinite and substantially the entire wave is reflected.
In a third instance when the mechanical impedance is zero, in the absence of a sample, the reflected wave is tensile but of equal magnitude to the incident wave. The phase of the wave is shifted by &pgr; and the net stress is zero; the relative displacement at the bar end equals twice that for the first instance (2&egr;).
In a SHPB device, once the relative displacement of the bars is known, the displacement of the sample is ascertained. Taking into account Young's Modulus (E) and the displacement of the bar, the imposed stress can be calculated, wherein the force applied is equal to the product of the stress and the cross-sectional area of the bar.
Since the loading on the sample becomes equal after a short time, the analysis may be somewhat simplified. Strain results may be used for only the incident bar; or alternatively, the striker bar may be directed to impact directly on the sample, and the transmitter bar alone may be used to define the sample characteristics.
It is has been found that plastics, minerals and metals may be discerned from one another by using this approach.
It has been further found that the hand held prodder disclosed by Gallagher having a rod modified to be analogous to the incident bar of a SHPB may be used to detect or discern metal, plastic and rocks.
The prodder rod is provided with one or more piezoelectric transducers capable of generating an acoustic wave into the rod and for detecting reflected waves from an object contacting the end of the rod. Conveniently, signal processing means are coupled to the transducers and are provided for analyzing the detected reflected waves for determining the characteristics of the object; more especially, for distinguishing landmines from inert rocks. The signal processor establishes measurements of the frequency-time-amplitude characteristic of the object. The reflected waves are compared with known characteristic signatures of a plurality of materials to attempt to ascertain a match within predetermined limits.
Although U.S. Pat. No. 5,920,520 describes a device that performs satisfactorily in many instances, it suffers from a problem related to the fact that acoustic coupling at the obstruction is a function of the force applied to the probe end. As a result, the results are often erroneous. This is particularly detrimental when the prodder indicates that the obstruction is a rock, when in fact it is a land mine.
Preferably, enough force will be applied to the probe end such that characterization of the obstruction can occur without causing detonation; and, preferably, a relatively consistent force will be applied to the probe end such that an accurate determination as to the character of the buried obstruction can be made. However if too little force is applied at the probe end, a poor reading may result and a mine in the vicinity of the probe may go undetected. Too much force applied at the probe end in the vicinity of a land mine may inadvertently detonate the mine.
In prior art
FIG. 1
a specimen sample is shown juxtaposed between an incident bar and a transmitter bar. A strain gauge disposed on each bar provides a signal-to-signal processor as is described heretofore.
In prior art
FIG. 2
a hand-held prodder for probing the ground for buried explosive devices such as landmines and the like is provided. The prodder comprises a rod
2
having a first end
3
flexibly supported by an annular rubber coupling
4
in a mounting nub
5
. The nub
5
is screwed into a handle
6
. The rod has a pointed second end
7
for sensing objects
8
buried in the ground
9
.
The rod
2
is 45 cm long and is formed of non-magnetic, austenitic stainless steel. Only 30 cm project from the rubber coupling
4
. The rubber coupling
4
lessens the rigidity between the rod
2
and handle
6
.
Best seen in prior art
FIG. 3
, a piezoelectric crystal
10
is glued to the first, or driver end
3
of the rod
2
. When an electric field is applied to the crystal
10
, a mechanical strain will occur and drive mechanical energy into the rod's driver end
3
. Conversely, when the crystal
10
is mechanically stressed, an electric charge is produced. A suitable crystal is a 15 mm long, 6.35 mm diameter poly-crystalline ceramic cylinder, model Sonex P-41 available from Hoechst CeramTec, Mansfield, Mass. The crystal
10
is electrically insulated from the rod
2
with a ceramic insulator
11
. Optionally, the insulator further serves to provide mechanical strength to the joint between the crystal and the rod.
Positive and negative electrical leads
12
from the crystal pass through the nub
5
for bi-directional electrical signal transmission between the crystal
10
and an electronics module
13
. Shown in
FIG. 2
, the module
13
is installed within the prodder's handle and is powered with 9 V batteries
14
.
The electronics module
13
Chapman John E.
DEW Engineering and Development Limited
Freedman & Associates
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