Patent Publication Number: US-8113118-B2

Title: Spin sensor for low spin munitions

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is related to U.S. patent application Ser. No. 10/994,754, filed on Nov. 22, 2004, now U.S. Pat. No. 7,124,689, issued Oct. 24, 2006. The disclosure of the above-mentioned applications is incorporated by reference herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to fuzes for explosive devices and, more particularly, to determining an environmental condition related to when an explosive device may be safely armed. 
     2. Description of Related Art 
     Explosive projectiles must be capable of being handled safely under considerable stress and environmental conditions. In addition, explosive projectiles must be capable of detonating at the proper time. Depending on the application, this proper time may be before impact at a specific point during flight, during impact, or at some time delay after impact. As used herein the terms “warhead,” “explosive device,” and “explosive projectile” are generally used to refer to a variety of projectile type explosives, such as, for example, artillery shells, rockets, bombs, and other weapon warheads. In addition, these explosive projectiles may be launched from a variety of platforms, such as, for example, fixed wing aircraft, rotary wing aircraft (e.g., helicopters), ground vehicles, and stationary ground locations. To determine the proper detonation time, these explosive projectiles frequently employ fuzes. 
     A fuze subsystem activates the explosive projectile for detonation in the vicinity of the target. In addition, the fuze maintains the explosive projectile in a safe condition during logistical and operational phases prior to launch and during the first phase of the launch until the explosive projectile has reached a safe distance from the point of launch. Consequently, major functions that a fuze performs are: keeping the weapon safe, arming the weapon when it is a safe distance from the point of launch, detecting the target, and initiating detonation of the warhead at some definable point after target detection. 
     The first two functions are conventionally referred to as Safing and Arming (S&amp;A). Safing and Arming devices isolate a detonator from the warhead booster charge until the explosive projectile has been launched and a safe distance from the launch vehicle is achieved. At that point, the S&amp;A device removes a barrier from, or moves the detonator in line with, the warhead, which effectively arms the detonator so it can initiate detonation at the appropriate time. 
     Some S&amp;A devices function by measuring elapsed time from launch, while others determine distance traveled from the launch point by sensing acceleration experienced by the weapon. Still other devices sense air speed or projectile rotation. For maximum safety and reliability of a fuze, the sensed forces or events must be unique to the explosive projectile when deployed and launched, not during ground handling or pre-launch operations. Most fuzes must determine two independent physical parameters before determining that a launch has occurred and a safe separation distance has been reached. 
     Detecting spin of the projectile is an often-used physical parameter. However, explosive projectiles that are not shot through a rifled barrel tend to exhibit very low angular accelerations. These smaller angular accelerations and spin rates are more difficult to detect. Conventional spin sensors such as accelerometers and spin switches set to detect these low angular accelerations may be spoofed by accelerations related to platform maneuvers prior to launch. 
     Other conventional spin sensors detect the Earth&#39;s magnetic field and sense changes in position and orientation of the spinning projectile relative to the Earth&#39;s magnetic field. These devices may be quite complex and may be susceptible to electro-magnetic noise or electro-static noise. 
     There is a need for a straightforward device and robust method to sense low angular accelerations of explosive projectiles in flight while being insensitive to cross-axis accelerations from projectile launch. In addition, there is a need to discriminate between platform maneuver accelerations and spin accelerations related to projectile flight after separation from the projectile launch point. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of the present invention comprises a spin sensor, including a fuze housing, a sense weight, and a rotating induction device. The rotating induction device comprises a first element affixed to the fuze housing and a second element affixed to the sense weight. The second element is mechanically coupled to the first element such that it may rotate relative to the first element. In addition, the second element is inductively coupled to the first element such that a relative rotation between the first element and the second element generates a spin signal on an electrical connection to the rotating induction device. 
     Another embodiment of the present invention comprises an explosive projectile including an encasement, an explosive material disposed within the encasement and configured for detonation, and a spin sensor disposed within the encasement. The spin sensor comprises a fuze housing, a sense weight, and a rotating induction device. The rotating induction device comprises a first element affixed to the fuze housing and a second element affixed to the sense weight. The second element is mechanically coupled to the first element such that it may rotate relative to the first element. In addition, the second element is inductively coupled to the first element such that a relative rotation between the first element and the second element generates a spin signal on an electrical connection to the rotating induction device. 
     Another embodiment of the present invention comprises a method of sensing fuze spin. The method comprises providing a sense weight rotationally coupled to a fuze housing, rotating the fuze housing, and detecting a relative rotation between the sense weight and the fuze housing. The method further comprises converting the detected relative rotation into a spin signal, which is sampled to develop an actual spin profile of the fuze housing. The developed actual spin profile may then be compared to an acceptable spin profile. 
     Yet another embodiment, in accordance with the present invention comprises a method of sensing fuze spin including inductively coupling a first element affixed to a fuze housing and a second element affixed to a sense weight. The inductive coupling generates a spin signal correlated to a relative rotation of the first element relative to the second element. The spin signal is sampled to develop an actual spin profile of the fuze housing. The developed actual spin profile may then be compared to an acceptable spin profile. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which illustrate what is currently considered to be the best mode for carrying out the invention: 
         FIG. 1  is a diagram of an exemplary explosive projectile incorporating the present invention; 
         FIG. 2  is a cut-away three-dimensional view of an exemplary fuze incorporating the present invention; 
         FIG. 3  is a view of an exemplary rotating induction device and sense weight in a fuze housing according to the present invention; 
         FIG. 4  is a sectional view of an exemplary rotating induction device according to the present invention; 
         FIG. 5  is another view of an exemplary rotating induction device and sense weight in a fuze housing according to the present invention; 
         FIG. 6  is a sectional view of another exemplary rotating induction device according to the present invention; 
         FIG. 7  is an exemplary electronics module for conditioning and sensing of a spin signal according to the present invention; 
         FIG. 8  is an exemplary spin signal conditioner according to the present invention; and 
         FIG. 9  is a graph illustrating a signal and spin rate of the exemplary spin signal according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, circuits and functions may be shown in block diagram form in order not to obscure the present invention in unnecessary detail. Conversely, specific circuit implementations shown and described are exemplary only and should not be construed as the only way to implement the present invention unless specified otherwise herein. Additionally, block definitions and partitioning of logic between various blocks is exemplary of a specific implementation. It will be readily apparent to one of ordinary skill in the art that the present invention may be practiced by numerous other partitioning solutions. For the most part, details concerning timing considerations and the like have been omitted where such details are not necessary to obtain a complete understanding of the present invention and are within the abilities of persons of ordinary skill in the relevant art. 
     In this description, some drawings may illustrate signals as a single signal for clarity of presentation and description. It will be understood by a person of ordinary skill in the art that the signal may represent a bus of signals, wherein the bus may have a variety of bit widths and the present invention may be implemented on any number of data signals including a single data signal. 
     In describing the present invention, the systems and elements surrounding the invention are first described to better understand the function of the invention as it may be implemented within these systems and elements. 
       FIG. 1  illustrates an exemplary embodiment of an explosive projectile  100  (also referred to as a warhead) incorporating the present invention. As illustrated in  FIG. 1 , the explosive projectile  100  includes a fuze  200  in the base of the explosive projectile  100  and an explosive material  120  encased by a body  110 . Additionally, the nose may include impact sensors  115 , such as, for example, a crush sensor, and a graze sensor. The  FIG. 1  explosive projectile  100  is exemplary only, it will be readily apparent to a person of ordinary skill in the art that the present invention may be practiced or incorporated into a variety of explosive projectiles  100  as described earlier. 
       FIG. 2  illustrates an exemplary embodiment of the fuze  200  incorporating the present invention. As illustrated in  FIG. 2 , the exemplary fuze  200  includes elements forming an encasement for the fuze  200  including a base  210 , a fuze housing  220 , and an end cap  230 . The functional elements within the encasement include a safety and arming module (S&amp;A module)  250 , and a spin sensor  300 . In the exemplary embodiment of an explosive projectile  100  illustrated in  FIG. 1 , the fuze  200  is mounted in the aft end. The aft location places the fuze  200  within the “buried” warhead section adjacent to the rocket motor/guidance section, which is a relatively ineffective location for fragmentation, and is well suited for the fuze  200 . In addition, this location prevents the fuze  200  from interfering with forward fragmentation and allows an unobstructed forward target view for other sensors, such as, for example, proximity sensors. However, while the aft location is used in the exemplary embodiment of  FIG. 1 , other locations and configurations are contemplated within the scope of the invention. 
     As explained earlier, part of the S&amp;A function is to prevent premature detonation. The exemplary fuze embodiment may incorporate multiple independent environments to determine that the explosive projectile  100  may be safely armed. One environment incorporated in the exemplary embodiment of the fuze  200  is spin sensing. Spin sensing may be used to determine that the explosive projectile  100  has been launched and is following a normal trajectory wherein the spin may be caused by a rifled barrel or the aerodynamic characteristics of the explosive projectile  100 . 
       FIG. 3  illustrates an exemplary spin sensor  300  according to the present invention. The spin sensor  300  includes a sense weight  390 , a rotating induction device  310 , and a spin signal  340 . The sense weight  390  behaves as a flywheel, which creates an inertial mass that resists angular acceleration. The sense weight&#39;s mass and configuration may be modified to affect the amount of inertial force resisting angular acceleration. This modification enables adaptation of the spin sensor  300  to various spin rates and spin accelerations that may be expected of the various explosive projectiles during normal flight. 
     The rotating induction device  310  may be a device such as an alternator or an electric motor and may also be referred to herein as an alternator  310  or as an electric motor  310 . Generally, an exemplary alternator  310  includes a first element affixed to the fuze housing  220  and a second element affixed to the sense weight  390 . The first element and the second element are rotationally coupled and inductively coupled. In various embodiments, the first element may be a stator of the alternator  310  or a rotor of the alternator  310 . Similarly, the second element may be a rotor of the alternator  310  or a stator of the alternator  310 . 
     As shown in  FIG. 4 , the alternator  310  includes a rotor  320  attached to a shaft  325 , a stator  330 , and an electrical connection to a wire coil  335  within the stator  330 . The spin signal  340  may be generated in the wire coil  335  and electrical connection as the rotor  320  spins relative to the stator  330 . As depicted in  FIG. 4 , the alternator  310  may be a conventional alternating current (AC) alternator  310  or electric motor. As an AC alternator  310 , the rotor  320  comprises a permanent magnet, which, when it rotates, causes a rotating magnetic field. The stator  330  includes a wire coil  335 , which, when exposed to the rotating magnetic field, generates an AC electric signal in the wire coil  335  and spin signal  340  connected to the wire coil  335 . 
     In the exemplary embodiment of the spin sensor  300  shown in  FIG. 3 , the sense weight  390  is attached to the rotor  320 , while the fuze housing  220  is attached to the stator  330  through housing attachments  225 . This configuration allows the sense weight  390  and rotor  320  to freely rotate (or resist rotation) within the fuze housing  220 , while the stator  330 , attached to the fuze housing  220 , rotates at the same rate as the explosive projectile  100 . As the explosive projectile  100  begins to spin during flight, the stator  330  will also spin. However, the sense weight  390  and rotor  320  may resist spinning due to their inertial mass. As a result, a relative rotation develops between the rotor  320  and stator  330 , causing the coil to generate an AC signal on the spin signal  340 . Clearly, the housing attachments  225  are exemplary only. Many attachment mechanisms are possible and contemplated as within the scope of the invention. 
     In another embodiment of the spin sensor  300 ′ shown in  FIG. 5 , the sense weight  390  may be attached to the stator  330 , while the fuze housing  220  is attached to the rotor  320  through housing attachment  225 ′. This embodiment may enable a smaller sense weight  390  since the inertial mass of the stator  330  would be included with the inertial mass of the sense weight  390  in resisting angular acceleration. Operation of this embodiment is similar to the previous embodiment except that the stator  330  spins freely and the rotor  320  spins with the explosive projectile  100 . Clearly, the housing attachment  225 ′ of the embodiment of  FIG. 5  is exemplary only. Many attachment mechanisms are possible and contemplated as within the scope of the invention. 
     In another embodiment, rather than using a conventional AC alternator  310  or AC motor, a direct current (DC) alternator  310 ′ or DC motor may be used, as shown in  FIG. 6 . In a conventional DC alternator  310 ′, the wire coil  335  is part of the rotor  320  and connects to the spin signal  340  through a commutator  327 . The stator  330 , therefore, includes the permanent magnet. As with the AC alternator  310 , a DC alternator  310 ′ may be configured with the rotor  320  connected to the sense weight  390  and the stator  330  connected to the fuze housing  220 . Alternatively, the rotor  320  may be connected to the fuze housing  220  and the stator  330  may be connected to the sense weight  390 . 
     Conventional alternators and electric motors exhibit an attribute known as magnetic detent. This is an angular resistance to relative rotation between the rotor  320  and stator  330 . The rotor  320  and stator  330  may not rotate relative to one another until a relative angular acceleration is large enough to overcome the force of the magnetic detent. In the present invention, magnetic detent may be used to resist relative rotation of the rotor  320  and stator  330  for small angular accelerations or vibrations that may be encountered during platform maneuvers or transportation of the explosive projectile  100 . Furthermore, because the device is not sensitive to these cross-axis accelerations, precise alignment of the sensor to the longitudinal axis of the explosive projectile  100  is not needed. 
       FIG. 7  illustrates an exemplary embodiment of an electronics module for sampling and analyzing the spin signal  340 . In the  FIG. 7  embodiment, the spin signal  340  from the spin sensor  300  may be optionally connected to a spin signal conditioner  350 . If a spin signal conditioner  350  is used, the resulting conditioned spin signal  360  may be connected to a main analyzer  370  and a safety analyzer  370 ′. If a spin signal conditioner  350  is not used, the spin signal  340  may be directly connected to the main analyzer  370  and the safety analyzer  370 ′ (connection not shown). An initiation sensor  380  may be included with the electronics module or may be located in another position within the fuze  200  or explosive projectile  100  ( FIG. 1 ) and connected to the electronics module through suitable wiring and connectors. The initiation sensor  380  may be a type of sensor that detects a launch event, such as, for example, an acceleration switch or accelerometer. 
     This exemplary embodiment employs redundant, low power microcontrollers as the main analyzer  370  and the safety analyzer  370 ′. In the exemplary embodiment, the safety analyzer  370 ′ is a different part from a different vendor than the main analyzer  370 . The dual-analyzer configuration using differing parts enables a cross-checking architecture, which may eliminate both single point and common mode failures. However, other analyzer configurations are contemplated within the scope of the present invention. For example, a single analyzer may be used or more than two analyzers may be used to enable additional redundancy and safeguards against failures. 
     It may be advantageous to condition the spin signal  340  generated from the alternator  310  ( FIG. 4 ) to generate the conditioned spin signal  360 , which may then be sampled by the analyzers  370  and  370 ′. For example, the spin signal  340  may be filtered to remove unwanted noise. In addition, the spin signal  340  may be amplified or attenuated to voltage levels compatible with the analyzers  370  and  370 ′. The spin signal  340  may also be digitized, either by a circuit in the spin signal conditioner  350 , or by circuits or software in the analyzers  370  and  370 ′. 
       FIG. 8  illustrates an exemplary spin signal conditioner  350 . In this spin signal conditioner  350 , resistor R 1  and capacitor C 1  form a simple low pass filtering function to eliminate potential high frequency noise. Resistor R 2  and Resistor R 3  form a voltage divider, which acts in conjunction with the operational amplifier A 1  to form a simple two-state digitizer. The voltage divider defines a voltage threshold for the digitizer. The digitizer acts to drive the conditioned spin signal  360  high any time the spin signal  340  exceeds the voltage threshold and to drive the conditioned spin signal  360  low any time the spin signal  340  goes below the voltage threshold. Of course, if the analyzers  370  and  370 ′ ( FIG. 7 ) are configured to evaluate a multi-state-digitized signal, a more complex analog-to-digital converter may be implemented in the spin signal conditioner  350 , or within the analyzers  370  and  370 ′. A person of ordinary skill in the art will recognize that many other implementations and modifications of the spin signal conditioner  350  are possible and contemplated as within the scope of the present invention. 
       FIG. 9  includes waveforms to illustrate an exemplary spin signal  340  and a rotation rate waveform  345 . Initially, the spin signal  340  is shown as beginning at zero volts. Then, as the alternator  310  begins relative rotation, the spin signal  340  begins to oscillate. It can be seen from the spin signal  340  that the spin signal  340  increases in amplitude during the time period shown on the waveform. Also, the rotation rate waveform  345  illustrates the increasing frequency of the spin signal  340  during the same time period. The analyzers  370  and  370 ′ ( FIG. 7 ) may use the characteristics of the spin signal  340  to develop a spin profile for the explosive projectile  100  ( FIG. 1 ). 
     In operation of the exemplary embodiment of the spin sensor  300  shown in  FIG. 3 , the stator  330  portion of the alternator  310  is affixed to the fuze  200  substantially along a longitudinal axis of the explosive projectile  100  ( FIG. 1 ). As a result, as the explosive projectile  100  spins after launch the stator  330  spins. Due to the magnetic detent of the alternator  310 , spin will not result in relative rotation between the rotor  320  and the stator  330  until an angular acceleration threshold greater than the magnetic detent is exceeded. When the magnetic detent is overcome, the inertial mass of the sense weight  390  combined with the rotor  320  resists spinning, causing relative rotation between the rotor  320  and stator  330  of the alternator  310 . The relative rotation generates an AC signal on the spin signal  340 , which may be sensed by the main analyzer  370  and safety analyzer  370 ′ ( FIG. 7 ). The spin signal  340  may be processed to develop an actual spin profile, which may be compared to an acceptable spin profile to determine if the spin signal  340  conforms to expectations of normal flight of the explosive projectile  100 . Acceptable spin profiles may be developed from modeling or empirical testing and analysis of the explosive projectile  100 . In addition, the analyzers  370  and  370 ′ may include multiple acceptable spin profiles stored within them, enabling the proper acceptable spin profile to be selected at an appropriate time, such as, for example, a user selection prior to launch. A variety of parameters may be included in the actual spin profile and the acceptable spin profile, such as, for example, revolution count, spin rate, increase in spin rate and spin signal amplitude. 
     By way of one, nonlimiting example, an acceptable spin profile may be defined as at least four transitions from the spin sensor  300 , with each transition occurring at an increasing rate. The system may be configured such that the main analyzer  370  and the safety analyzer  370 ′ wait for a signal from the initiation sensor  380  indicating a valid launch event. After a valid launch event, the analyzers  370  and  370 ′ may sample the spin signal  340  to develop the actual spin profile. If the actual spin profile conforms to the acceptable spin profile, the analyzers  370  and  370 ′ may signal that a valid spin environment has been achieved. If the actual spin profile does not conform to the acceptable spin profile within an expected time window, a valid spin environment may have not been achieved and the fuze  200  may be shut down. 
     In addition, if multiple analyzers are used, a valid spin environment may require all analyzers to reach a same conclusion on a comparison of the actual spin profile to the acceptable spin profile. Of course, a person of ordinary skill in the art will recognize that many other spin profiles are contemplated within the scope of the present invention. 
     Although this invention has been described with reference to particular embodiments, the invention is not limited to these described embodiments. Rather, the invention is limited only by the appended claims, which include within their scope all equivalent devices or methods that operate according to the principles of the invention as described.