Abstract:
An electric pulse generator includes a driver having an outer surface, a receiver, and one or more piezoelectric elements disposed between and in electrical contact with the driver and the receiver. The electric pulse generator further includes an explosive material disposed on the outer surface of the driver. A method of making an electrical pulse generator includes providing one or more piezoelectric elements, a driver, a receiver, and an explosive material and operably associating the explosive material with an outer surface of the driver. The method further includes electrically coupling the one or more piezoelectric elements between the driver and the receiver.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an electric pulse generator and a method for making the electric pulse generator. In particular, the present invention relates to an explosive-driven electric pulse generator and a method for making the explosive-driven electric pulse generator.  
         [0003]     2. Description of Related Art  
         [0004]     High-voltage, electrical pulses are employed for many different uses. For example, such pulses may be used in defense, flash X-ray, oilfield logging, and oilfield radiography applications. While electrical pulses may be generated in many different ways, one way of producing such pulses is by mechanically impacting or shocking a material that exhibits a piezoelectric effect. Generally, these materials have a crystalline structure of non-centrosymmetric unit cells. When a mechanical stress is applied to such a material, an electrical charge is produced. The voltage of the electrical charge produced by mechanically stressing a piezoelectric material is proportional to the amount of mechanical stress applied to the material. Thus, if a high-voltage electrical charge is desired, a correspondingly large mechanical stress is applied to the piezoelectric material.  
         [0005]     One way of generating a high-voltage electrical charge with a piezoelectric material is to impact the piezoelectric material with an explosive-driven member or with products (e.g., gases, particles, etc.) generated during detonation of an explosive material.  FIGS. 1 and 2  illustrate two conventional apparatuses used to generate electrical pulses. In  FIG. 1 , an electric pulse generator  101  includes a piezoelectric material  103  disposed between and in electrical contact with a housing  105  and a receiver  107 . Housing  105  defines a cavity  109  in which an explosive material  111  is disposed. Upon detonation of explosive material  111 , the products of detonation urge piezoelectric material  103  toward receiver  107 , mechanically stressing piezoelectric material  103 . The electrical charge produced by piezoelectric material  103  is electrically conducted to housing  105  and to receiver  107 , where it may be accessed via electrical leads  113 ,  115 .  
         [0006]      FIG. 2  depicts a conventional electric pulse generator  201  alternative to that shown in  FIG. 1 . Elements of electric pulse generator  201  generally correspond to those of electric pulse generator  101  (shown in  FIG. 1 ) except that a projectile  203  is disposed between an explosive material  205  and piezoelectric material  103 . Upon detonation of explosive material  205 , the products of detonation propel projectile  203  toward and into impact with piezoelectric material  103 . Projectile  203  mechanically stresses piezoelectric material  103 , producing an electrical charge. The electrical charge is conducted to housing  105  and to receiver  107 , where it may be accessed via electrical leads  113 ,  115 .  
         [0007]     Such conventional electric pulse generators, however, suffer from several problems. For example, the explosive arrangement may create a pressure pulse on detonation that is too short to sufficiently compress a thicker portion of piezoelectric material. Moreover, the explosive arrangement may produce a large peak pressure during the detonation pressure pulse, resulting in premature breakdown of the piezoelectric material. In either case, the resulting electrical pulse may exhibit a lower voltage than desired.  
         [0008]     Further, typical conventional electric pulse generators comprise a relatively large portion of explosive material. Such electric pulse generators, therefore, must be handled carefully to avoid inadvertent detonation of the explosive material.  
         [0009]     While there are many ways known in the art to produce a high-voltage electrical pulse, considerable room for improvement remains. The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.  
       SUMMARY OF THE INVENTION  
       [0010]     In one aspect of the present invention, an electric pulse generator is provided. The electric pulse generator includes a driver having an outer surface; a receiver; and one or more piezoelectric elements disposed between and in electrical contact with the driver and the receiver. The electric pulse generator further includes an explosive material disposed on the outer surface of the driver.  
         [0011]     In another aspect of the present invention, an electric pulse generator is provided. The electric pulse generator includes a driver having an outer surface, the outer surface defining a substantially helical groove and a receiver. The electric pulse generator further includes one or more ferroelectric elements disposed between and in electrical contact with the driver and the receiver and a detonation cord disposed in the groove defined by the outer surface of the driver.  
         [0012]     In yet another aspect of the present invention, a method of making an electrical pulse generator is provided. The method includes providing one or more piezoelectric elements, a driver, a receiver, and an explosive material; applying the explosive material to an outer surface of the driver; and electrically coupling the one or more piezoelectric elements between the driver and the receiver.  
         [0013]     The present invention provides significant advantages, including: (1) the ability to apply pressure to the piezoelectric element or elements for a longer period of time, thus increasing the voltage outputted from the piezoelectric element or elements; (2) the ability to apply more consistent pressure to the piezoelectric element or elements, thus decreasing the likelihood of damage to the element or elements; and (3) the ability to tailor the electric pulse waveform depending upon the implementation.  
         [0014]     Additional objectives, features and advantages will be apparent in the written description which follows.  
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0015]     The novel features believed characteristic of the invention are set forth in the appended claims. However, the invention itself, as well as, a preferred mode of use, and further objectives and advantages thereof, will best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings, in which the leftmost significant digit(s) in the reference numerals denote(s) the first figure in which the respective reference numerals appear, wherein:  
         [0016]      FIG. 1  is a stylized, cross-sectional view of a first conventional electric pulse generator;  
         [0017]      FIG. 2  is a stylized, cross-sectional view of a second conventional electric pulse generator;  
         [0018]      FIG. 3  is a side, elevational view of an illustrative embodiment of an electric pulse generator according to the present invention;  
         [0019]      FIG. 4  is a cross-sectional view of the electric pulse generator of  FIG. 3  taken along the line  4 - 4  of  FIG. 3 ;  
         [0020]      FIG. 5  is graphical representation of illustrative waveforms for embodiments of the electric pulse generator of the present invention having varying numbers of piezoelectric elements;  
         [0021]      FIG. 6  is a graphical representation of illustrative output voltages for embodiments of the electric pulse generator of the present invention having varying numbers of piezoelectric elements;  
         [0022]      FIG. 7  is a graphical representation of illustrative waveforms for embodiments of the electric pulse generator of the present invention having explosive materials with varying helical pitches; and  
         [0023]      FIG. 8  is a graphical representation of illustrative waveforms for substantially equivalent electric pulse generators according to the present invention. 
     
    
       [0024]     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will, of course, be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.  
         [0026]     The present invention represents an explosive-driven apparatus for generating an electrical pulse. In various implementations, the apparatus includes an explosive material disposed on an outer surface of a driver. When the explosive material is detonated, products resulting from the detonation urge the driver into increasing contact with a piezoelectric material. The piezoelectric material is compressed between the driver and a receiver, thus generating an electrical pulse.  
         [0027]      FIGS. 3 and 4  depict one particular illustrative embodiment of an explosive-driven electric pulse generator  301  according to the present invention.  FIG. 3  presents a side view of generator  301 , while  FIG. 4  provides a cross-sectional view of generator  301  taken along the line  4 - 4  of  FIG. 3 . In the illustrated embodiment, generator  301  includes one or more piezoelectric elements  303  disposed between a driver  305  and a receiver  307 . An outer surface  309  of driver  305  defines a groove  401 , which is shown more clearly in  FIG. 4  and extends helically along outer surface  309 . An explosive material  311 , which is only shown in  FIG. 3 , is disposed in helical groove  401 . Note that explosive material  311  is not shown in  FIG. 4  to better illustrate groove  401 . A dielectric portion  313  is disposed around piezoelectric elements  303 , between driver  305  and receiver  307 . Electrical leads  315 ,  317  are electrically coupled with driver  305  and receiver  307 , respectively, for accessing the electrical pulse generated by electric pulse generator  301 .  
         [0028]     When explosive material  311  is detonated, piezoelectric elements  303  are compressed by a resulting pressure wave traveling along the length of driver  305 , as indicated by arrows  321  (only shown in  FIG. 3 ). Piezoelectric elements  303  are, therefore, compressed between driver  305  and receiver  307 . Piezoelectric elements  303  produce an electrical pulse as a result of being compressed, which can be accessed via leads  315 ,  317 .  
         [0029]     Still referring to  FIGS. 3 and 4 , features of various particular embodiments of electric pulse generator  301  will now be discussed. As indicated above, one or more piezoelectric elements  303  are disposed between driver  305  and receiver  307 . It should be noted that any suitable number of piezoelectric elements  303  may be employed in the present invention. For example, only one piezoelectric element  303  may be included or a plurality of piezoelectric elements  303  may be utilized. It is generally desirable, although not required, for a plurality of piezoelectric elements  303  to be bonded along facing surfaces. In one particular embodiment, piezoelectric elements  303  are bonded along facing surfaces with a conductive epoxy, such as a conductive silver epoxy.  
         [0030]     Generally, piezoelectric elements  303 , or a single piezoelectric element  303  if only one is present, may comprise any material that exhibits a piezoelectric effect. In one particular embodiment, one or more of piezoelectric elements  303  comprise a ferroelectric material. Ferroelectric materials are a sub-class of piezoelectric materials that contain natural dipoles that can be reversed in the presence of a strong, external electric field. Ferroelectric materials tend to display a very strong piezoelectric effect but can be de-poled and lose their piezoelectric properties when subjected to high electric fields, high temperatures, or excessive pressures.  
         [0031]     While many different ferroelectric materials may be utilized in the present invention, one particular class of ferroelectric materials conform to the formula ABO 3 , wherein A is a large, divalent, metal ion and B is a tetravalent, metal ion. Examples of materials exhibiting large, divalent, metal ions are lead, strontium, and barium. Examples of materials exhibiting tetravalent, metal ions include titanium and zirconium. One particular ferroelectric material suitable for use as one or more of the piezoelectric elements  303  is PbZrO 3 —PbTiO 3  solid solution, known as PZT. PZT is a polycrystalline ceramic comprising two ferroelectric materials, lead zirconate and lead titanate. PZT is a hard, dense material exhibiting a relatively strong piezoelectric effect and an extremely high electrical permittivity, in the range of about 1000ε 0  to about 3000ε 0 . In one particular embodiment, piezoelectric elements  303  comprise the material EC-64 PZT from EDO Electro-Ceramic Products of Salt Lake City, Utah.  
         [0032]     Still referring to  FIGS. 3 and 4 , driver  305  may comprise any suitable, conductive, solid material (i.e., not a gas or a fluid) and the selection of the particular material for driver  305  may be implementation specific. For example, the material comprising driver  305  may be selected depending upon the material&#39;s density, weight, electrical conductivity, acoustic properties, or the like, as one of ordinary skill in the art would appreciate having the benefit of the present disclosure. Driver  305  may, for example, comprise aluminum, an aluminum alloy, steel, or the like.  
         [0033]     While driver  305  is depicted in  FIGS. 3 and 4  as being substantially right cylindrical, the scope of the present invention is not so limited. Rather, driver  305  may take on any suitable shape, such as a frustum of a cone, a prism, or the like.  
         [0034]     Outer surface  309  of driver  305 , as depicted in  FIGS. 3 and 4 , defines groove  401  (shown only in  FIG. 4 ) that is generally helical in form and semi-circular in cross-section. The scope of the present invention, however, is not so limited. Rather, groove  401 , however, may take on other forms or cross-sectional shapes depending upon the characteristics of the electrical pulse generated by electric pulse generator  301 . For example, in certain embodiments, outer surface  309  of driver  305  may define a groove  401  that is generally linear in form, extending generally along a length of driver  305 . Moreover, groove  401  may exhibit a cross-sectional shape that is, for example, rectangular or angular, irrespective of the form of groove  401 . It should be noted, however, that some embodiments of electric pulse generator  301  may omit groove  401 , such that explosive material  311  is applied to outer surface  309  of driver  305 .  
         [0035]     Still referring to  FIGS. 3 and 4 , piezoelectric elements  303  are compressed between driver  305  and receiver  307  upon detonation of explosive material  311 . While receiver  307  is depicted in  FIGS. 3 and 4  as being generally right cylindrical, the scope of the present invention is not so limited. Rather, receiver  307  may comprise any shape suitable for receiver  307 . For example, a portion of a structure housing electric pulse generator  301  may serve as receiver  307 . Moreover, receiver  301  may comprise any of a wide variety of materials, particularly any conductive, solid material. Receiver  307  may, for example, comprise aluminum, an aluminum alloy, steel, or the like.  
         [0036]     As discussed above, explosive material  311  is applied to outer surface  309  of driver  305 . Explosive material  311  may comprise, for example, cast, putty, and extruded forms of materials containing cyclotrimethylene trinitramine (RDX), cyclotetramethylene tetranitramine (HMX), pentaerythritoltetranitrate (PETN), trinitrotoluene (TNT), or the like. Note that this particular list of explosive materials  311  is neither exhaustive nor exclusive. Moreover, explosive material  311  may take on the form of a detonating cord, such as “A-Cord” from Austin Powder of Cleveland, Ohio. In one particular embodiment, explosive material  311  is detonating cord comprising a nylon housing containing about five grams of PETN per meter of length and having a detonation velocity of about 6900 meters per second. Note that explosive material  311  may be detonated by any suitable means.  
         [0037]     If groove  401  exhibits a helical form, a pitch P and the number of turns or revolutions of the helix can be varied to change certain electric pulse characteristics, such as the waveform shape of the electric pulse. Generally, a smaller pitch P results in a longer rise time to peak voltage, a higher peak voltage, and an overall longer pulse width. Generally, it is desirable that pitch P be tailored so that the longitudinal velocity of detonation along the length of driver  305  is proportional to the wave velocity (i.e., approximately the speed of sound) in driver  305 . This proportion affects the amount of reinforcement and the length of the detonation wave, determining the shape and magnitude of the wave incident upon the piezoelectric elements  303 . For example, this relationship may be expressed as:  
             VOD   z     ⁢     :       =       VOD           (   P   )     2     +       (   C   )     2           ·   P       ,       
 
 wherein P represents the pitch of groove  401  (and explosive material  311 ), C represents the circumference of driver  305 , and VOD represents the velocity of detonation of explosive material  311 . When VOD z  is substantially equal to the wave velocity in driver  305 , the explosive wave-fronts impact piezoelectric elements  303  at approximately the same time, creating a short but powerful pressure pulse. If, however, VOD z  is slower than the wave velocity in driver  305 , a longer, weaker pulse may be produced. 
 
         [0038]     Moreover, it is generally desirable, that the time of detonation is longer than the time required for the detonation wave to propagate through the one or more piezoelectric elements  303 . For example, this relationship can be expressed as:  
             t   pulse       t   piezo       &gt;   1     ,       
 
 wherein t pulse  represents the time of detonation and t piezo  represents the time required for the detonation wave to propagate through the one or more piezoelectric elements  303 . 
 
         [0039]     The time of detonation (t pulse ) may be represented by:  
           t   pulse     =               (   P   )     2     +       (   C   )     2         VOD     ·     N   turns         ,       
 
 wherein P represents the pitch, C represents the circumference of driver  305 , VOD represents the velocity of detonation of explosive material  311 , as discussed above, and N turns  represents the number of turns of explosive material  311 . 
 
         [0040]     The time required for the detonation wave to propagate through the one or more piezoelectric elements (t piezo ) may be represented by:  
           t   piezo     =         N   piezo     ·     T   piezo         V     sound   ⁢           ⁢   in   ⁢           ⁢   piezo           ,       
 
         [0041]     wherein N piezo  represents the number of piezoelectric elements  303 , T piezo  represents the thickness of each piezoelectric element  303 , and V sound in piezo  represents the velocity of sound in the material of the piezoelectric elements  303 .  
         [0042]     Dielectric portion  313  is provided between driver  305  and receiver  307 , about piezoelectric elements  303 , to inhibit surface flashover between driver  305  and receiver  307  along piezoelectric elements  303 . The occurrence of surface flashover generally inhibits the peak voltage produced by piezoelectric elements  303  and, thus, is typically undesirable. In one embodiment, materials suitable for use as dielectric portion  313  are those that are capable of holding off a voltage corresponding to about the breakdown voltage of the piezoelectric elements  303 . Moreover, suitable dielectric materials include materials that are capable of curing in deep crevices to completely encapsulate piezoelectric elements  303 , exhibit adequate surface adhesion, and can be prepared with a minimal amount of air bubbles or other features that can cause electric field enhancements. It is also desirable to employ a dielectric material that cures at near room-temperature, since some piezoelectric materials may become de-poled when subjected to elevated temperatures. Examples of such dielectric materials include polyurethanes, polystyrenes, epoxies, transformer oils, silicone rubbers, and the like.  
         [0043]     For example, dielectric portion  313  may comprise RTV11 two-part silicone rubber from GE Silicones of Wilton, Conn. Primers may be applied to the piezoelectric elements  303 , driver  305 , and/or receiver  307  prior to applying the dielectric material to aid in adhesion of the dielectric material. For example, S4155 primer from GE Silicones may be used prior to applying the RTV11 silicone rubber as the dielectric material. Other materials that may be suitable as dielectric portion  313 , depending upon the particular implementation, include Hysol® E40FL two-part epoxy from Loctite Corporation of Rocky Hill, Conn. and Univolt N61B transformer oil from Exxon Mobil Corporation of Fairfax, Va. Other suitable materials include 3145-RTV and IS808 silicone rubbers from GE Silicones.  
         [0044]     One particular preferred embodiment of electric pulse generator  301  is described below and in reference to  FIGS. 5-8 . It should be noted that the scope of the present invention is not limited to the particular characteristics of this embodiment. In this embodiment, electric pulse generator  301  includes one or more piezoelectric elements  303  disposed between and in electrical contact with a solid, aluminum, right cylindrical driver  305  and a solid, stainless steel, right cylindrical receiver  307 . Facing surfaces of piezoelectric elements  303 , driver  305 , and receiver  307  are adhesively bonded by a conductive silver epoxy. In this embodiment, driver  305  has an outside diameter of about 2.5 centimeters and receiver  307  has an outer diameter of about seven centimeters, although these dimensions can vary depending upon the implementation. Driver  305  has a length of about 15.2 centimeters and receiver  307  has a length of about 15.2 centimeters. Groove  401 , defined by outer surface  309  of driver  305 , has a helical form and exhibits a width and depth of about 4.8 millimeters. Explosive material  311  comprises A-Cord detonating cord having an about 4.2 millimeter nylon housing containing about five grams of PETN per meter of length. Dielectric portion  313  comprises S4155 primer and RTV11 silicone rubber from GE Silicones. Possible outputs of this particular embodiment of electric pulse generator  301  are provided below.  
         [0045]      FIG. 5  illustrates possible outputs for electric pulse generator  301  described above when varying the number of piezoelectric elements  303 .  FIG. 5  presents voltage-time graphs representing electrical pulses generated by electric pulse generator  301  having one, six, ten, and 20 substantially round piezoelectric elements  303 . In each case, each piezoelectric element  303  is about five millimeters thick and about 25 millimeters in diameter. In the embodiment comprising a single piezoelectric element  303 , driver  305  defines helical groove  401  having a pitch of about 25 millimeters and including three revolutions about driver  305 , beginning at the top revolution (i.e., distal to piezoelectric elements  303 ). The six and ten piezoelectric element  303  embodiments include driver  305  defining helical groove  401  having a pitch of about 16.9 millimeters and six revolutions about driver  305 , beginning at the top revolution. The 20 piezoelectric element  303  embodiment includes driver  305  defining helical groove  401  having a pitch of about 12.7 millimeters and 12 revolutions beginning at the top revolution.  
         [0046]     In each of these embodiments, piezoelectric element  303  or piezoelectric elements  303  are compressed or excited using A-cord detonating cord as explosive material  311  disposed in helical groove  401 .  FIG. 5  shows the output voltage and the duration of the pulse increases as the number of piezoelectric elements  303  is increased. Note that more explosive material  311  is used for greater numbers of piezoelectric elements  303  to provide adequate compression of piezoelectric elements  303 .  
         [0047]      FIG. 6  illustrates a comparison of average voltages produced by embodiments of electric pulse generator  301  having varying numbers of piezoelectric elements  303 , as described above in relation to  FIG. 5 . Each data point represents an embodiment having a particular number of piezoelectric elements  303  having thicknesses of about five millimeters and diameters of about 25 millimeters. As the number of piezoelectric elements  303  is increased, the output voltage per piezoelectric element  303  is reduced, while the overall output voltage increases.  
         [0048]     As illustrated in  FIG. 7 , varying the pitch of the helical driver results in varying rise times as well as changes in peak voltage. In particular,  FIG. 7  depicts a comparison of waveforms generated by embodiments of electric pulse generator  301  having six piezoelectric elements  303 , each of about 5 millimeters in thickness and about 25 millimeters in diameter. As can be seen in  FIG. 7 , increased pitch of helical groove  401  and explosive material  311  results in faster rise time but lower output voltage.  
         [0049]     It should be noted that output voltages of substantially equivalent electric pulse generators  301  are substantially equivalent. In other words, the output voltage of a particular embodiment of electric pulse generator  301  is reproducible.  FIG. 8  illustrates waveforms for three substantially equivalent electric pulse generators  301 . Each electric pulse generator  301  includes six piezoelectric elements  303  and a driver  305  defining a helical groove  401  with a pitch of about 16.9 millimeters. In this embodiment, three revolutions of A-cord detonating cord are disposed in helical groove  401 , beginning at the top revolution (i.e., distal to piezoelectric elements  303 ). In  FIG. 8 , the waveforms are offset in time to avoid overlap and to better illustrate similar rising edges and peak voltages.  
         [0050]     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below. It is apparent that an invention with significant advantages has been described and illustrated. Although the present invention is shown in a limited number of forms, it is not limited to just these forms, but is amenable to various changes and modifications without departing from the spirit thereof.