Measuring and testing – Vibration – Mechanical impedance
Reexamination Certificate
2000-02-14
2002-05-14
Chapman, John E. (Department: 2856)
Measuring and testing
Vibration
Mechanical impedance
C073S661000, C367S087000
Reexamination Certificate
active
06386036
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 that 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 labourious, 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. 5,754,494 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 analysing 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. 5,754,494 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.
Furthermore, it is desired that enough force will be applied to the probe end such that characterisation 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.
It is therefore an object of the invention to provide a method and device, which will overcome the aforementioned problems, related to too much force, too little force, or a varying force being applied to the probe end while in use.
It is a further object of the invention to provide an instrumented prodder for detection of land mines and the like that includes force feedback for sensing a force, such as pressure, applied to an end thereof.
It is another object of the invention to provide an instrumented prodder for detection of land mines and the like, that provides data related to characteristics of the probed object that are independent from the force of the prodder on the object.
It is a further object of the invention to provide a hand-held prodder for probing the ground for buried explosive devices such as landmines and the like, that is relatively simple, rugged, and inexpensive.
SUMMARY OF THE INVENTION
Accordingly, there is provided a detector for detecting detonatable devices or land mines, comprising:
a probe having an end for placing in contact with an object beneath the surface of the ground, the probe for providing an indication of the presence of a metal or plastic material; and;
a sensor for detecting a pressure of a fluid within the detector, the pressure related to a force at the end of the probe when the end is placed in contact with the object.
According to another embodiment, there is provided a prodder with force feedback for sensing an object buried beneath the ground, comprising:
a probe including a rod having a prodding end for prodding the object and a transducer coupled to a non-prodding end of the rod for providing an acoustic wave to the object and for receiving acoustic waves reflected from the object;
a housing including a first cavity containing a fluid and for supporting the probe such that the volume of the fluid within the first cavity is affected by pressure applied to the prodding end of th
DEW Engineering and Development Limited
Freedman & Associates
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