Abstract:
In a gun tube for firing rounds of smart ammunition, a sensor means applied to the ammunition which enables the calculation of muzzle velocity in the instant as the ammunition leaves the muzzle edge of the gun tube. The sensor relies on measurement of magnetic field induced voltages which are unique as the ammunition leaves the muzzle edge of the gun tube. Each round of ammunition has a planar transformer of wiring applied longitudinally on the side of the round, which aids in creating the necessary magnetic fields.

Description:
U.S. GOVERNMENT INTEREST 
     The inventions described herein may be made, used, or licensed by or for the U.S. Government for U.S. Government purposes. 
    
    
     BACKGROUND OF INVENTION 
     This invention relates to linear velocity measuring sensors. In particular, but not exclusively, this invention is a device that allows gun-fired, smart ammunition to precisely measure its own muzzle exit velocity for use in precision guidance. This invention allows for the precise measurement of velocity in the axial direction of motion, while cancelling the effects of motion in the radial direction. 
     Sensors are generally used in smart ammunition in order to improve precision, especially at long firing ranges. The ultimate goal is to reduce target impact dispersior of a particular round. While in flight, a smart round will use its sensors to determine if corrections are necessary to its trajectory. For instance, GPS modules, magnetometers, and gyroscopes, can be used to determine location, heading, and orientation respectively. The round&#39;s fuze will use that information to determine if maneuver needs to be performed, if the round should recalculate a timer, if the round should dud, and the like. 
     It is sometimes necessary for a smart round to know its muzzle exit velocity. In smart rounds that use a timed detonation—such as those that have an air burst mode—the time set for detonation is computed from: the range to target, the expected trajectory of the round, and an expected muzzle exit velocity. In order for the round to precisely detonate at a specific point in its trajectory, especially for air burst mode, all of these factors need to be precisely known. 
     Muzzle exit velocity is typically a value that is not measured, rather it is estimated before the round is shot based on the type of round being fired, its weight, and the type and amount of propellant being used. This is contrary to the range to target and round trajectory, both of which are already accurately determined by laser range finder and ballistic computer, respectively. 
     While certain methods have been proposed to measure muzzle velocity, each has certain shortcomings. For example, in “Apparatus for Measure the Muzzle Velocity of a Projectile” (U.S. Pat. No. 3,659,201, issued April 1972), Remo Vogelsang describes an invention where two coils sit co-axially at the end of a gun tube, each energized with a DC current source, and connected electrical in series. When a bullet is fired, it generates two voltage pulses as it passes through both coils. The time difference between each pulse is measured, and combined with a temperature compensation technique for the spacing between each coil, velocity is determined. 
     In “Inductance Type Velocity Measuring Apparatus” (U.S. Pat. No. 3,824,463, issued July 1974), Kenneth L. Oehler describes a similar invention, where again, two coils sit co-axially at the end of a gun tube. In his invention, the both coils are electrically connected as frequency determining elements of an oscillator. When a bullet is fired through the coils, the frequency of the oscillator is modulated twice. An FM discriminator circuit then detects the modulation, subsequent pulse shaping circuits produce “start” and “stop” pulses, whose time interval between along with the spacing of the coils is used to calculate exit velocity. 
     Finally, in “Device and Method for Determining the Muzzle Velocity of Projectile” (US 2004/0250615 A1, Pub Date December 2004), Aldo Alberti and Klaus Munzel describe an invention where a single coil is placed co-axially at the end of a gun tube, energized with a DC current, creating small localized magnetic field. As a bullet is fired, it changes the magnetic field within the coil, which induces a voltage pulse across the terminals of the coil. This pulse is detected, its pulse width measured, and used to determine exit velocity. 
     Nonetheless, there remains a need for a more accurate muzzle velocity measuring device as provided by this invention. 
     BRIEF SUMMARY OF INVENTION 
     This invention allows a smart round to measure its own muzzle exit velocity by using the edge of the gun tube as a reference. The round&#39;s added sensor is a form of a special planar air-core sensing transformer placed on the side of the round together with associated drive and measurement electronics also added to the round. As the sensor moves past the edge of the tube, its displacement can be determined from the sensor&#39;s voltage output. From the point of view of the round, by measuring the amount of time it takes for the edge of the tube to undergo a set displacement (relative to the sensor), the velocity of the round itself can be determined. The geometry of the air-core sensing transformer additionally cancels motion in radial directions, allowing for very precise axial displacement measurements, and hence precision velocity measurements. 
     The sensor makes use of interference between a metal surface with an abrupt ending, and an oscillating magnetic field at a fixed frequency, to determine how fast that ending is moving (relative to the sensor), past the sensor. In the case of a smart round, the metal surface would be the inner surface of the gun tube firing the round, and the edge would be the end of the tube at the muzzle. The oscillating magnetic field is created by a planar air-coil transformer attached to the side of the round, whose primary winding is energized with an ac current. As further explained herein, the secondary of the transformer has a particular geometry that will allow the position of the gun tube edge to be determined, relative to the location of the round 
     OBJECTS OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a sensor means which enables the calculation of muzzle velocity in the instant as a smart ammunition leaves the muzzle edge of a gun tube from which it is fired. 
     Another object of the present invention is to provide measurement of magnetic field induced voltages which are unique as a round of smart ammunition leaves the muzzle edge of a gun tube from which it is fired. 
     It is a further object of the present invention to provide each round of smart ammunition with a planar transformer of wiring applied longitudinally on the side of such round, which aids in creating necessary magnetic fields to measure exit velocity as the smart ammunition leaves the muzzle edge of a gun tube from which it is fired. 
     These and other objects, features and advantages of the invention will become more apparent in view of the within detailed descriptions of the invention, the claims, and in light of the following drawings wherein reference numerals may be reused where appropriate to indicate a correspondence between the referenced items. It should be understood that the sizes and shapes of the different components in the figures may not be in exact proportion and are shown here just for visual clarity and for purposes of explanation. It is also to be understood that the specific embodiments of the present invention that have been described herein are merely illustrative of certain applications of the principles of the present invention. It should further be understood that the geometry, compositions, values, and dimensions of the components described herein can be modified within the scope of the invention and are not generally intended to be exclusive. Numerous other modifications can be made when implementing the invention for a particular environment, without departing from the spirit and scope of the invention. 
    
    
     
       LIST OF DRAWINGS 
       The features of the present invention and the manner in which they are attained will become apparent, and the invention itself will be better understood by reference to the accompanying drawings, wherein: 
         FIG. 1  is a view of a planar air-coil transformer according to this invention. 
         FIG. 2  is a block diagram showing pertinent major sub-systems of a system embodying this invention. 
         FIG. 3  is an illustration of a smart round  9  exiting through gun tube  10  at three select sequential moments in time. 
         FIG. 4  is a cross section of the smart round and the gun tube, at the same moments in time as depicted in  FIG. 3 . 
         FIG. 5  depicts mathematically the effects of round balloting insensitivity on the sensing transformer. 
         FIG. 6  is a diagram of voltage output of the sensing transformer&#39;s secondary, versus round displacement as the round moves past the edge of the gun tube, shown in top-down view. 
         FIG. 7  is a view of another embodiment of the present invention, with the planar air-coil transformer comprising a primary loop but in this case, having two secondary loops. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a first embodiment of the invention includes a planar loop of several turns of wire, referred to now as the “primary” of this air-coil transformer. The primary coil  1  is a planar loop of several turns, whose terminals  3  are located at the right end of the figure. The “secondary” coil  2  is a single loop (a single wire) with multiple twists. This single loop wire has its terminals ( 4 ) shown at the right side of the figure. Within single loop  2 , the twists form multiple minor loops  102 , and end, minor loops  103 ,  104 . The end loops  103 ,  104 , have half the area of a minor loop  102 . The primary is used to generate an oscillating magnetic field, while the secondary is used to measure that field, by making a voltage available at its terminals when connected to a high-impedance amplifier. This voltage is dependent on the strength of the field, the oscillating frequency, and the position of the edge of the gun tube (relative to the secondary, e.g.), as will now explained. 
     A sinusoidal current of a specific frequency is set up through the primary loop, which creates an oscillating magnetic field surrounding the loop. If the loop were located in free space, the geometry of the field could be determined by the Biot-Savart law. However, as shown in  FIG. 4 , the metal body of the round, and the metal inner surface of the gun tube, act to interfere and reshape the field. This is due to the eddy currents that are set up in the metal, that act to buck out the field from entering. The geometry of the field can be approximated using the method of image currents. 
     The presence of the inner surface of the gun tube, acts to reduce the magnitude of the magnetic field perpendicular to the surface of the round, to a value B 1 . The metal body of the round also has the same effect, but since the round and the loop move as one solid piece, this effect is immaterial to the velocity measurement process. 
       FIG. 2  is a block diagram of the present invention showing pertinent major sub-systems. The invention comprises the planar air-coil transformer as shown previously in  FIG. 1 , whose primary  1  is driven with an AC current by signal generating circuitry  5 . The voltage on the terminals  4  of the secondary of the transformer is fed into an amplifier  6 . This new amplified voltage is then synchronously demodulated (also known as lock-in amplified) with a reference signal from the signal generating circuitry. The synchronous demodulation circuitry (mixer  7 ), is shown as well. Its (new) output  211  is a voltage whose value in time is dependent on the position of the edge of the gun tube relative to the transformer, which shape is better presented at  209  by signal processor  8 . 
       FIG. 3  is an illustration of smart round  9  exiting gun tube  10  at three sequential moments in time: as the sensing transformer is still inside the tube; as the sensing transformer just exits the tube; and, as the sensing transformer is completely outside the tube. The sensing transformer  11  is located on the side of the round. 
       FIG. 4  is a cross section of the smart round and the gun tube, at the same moments in time as depicted in  FIG. 3 . The regions containing AC currents  11 , and eddy currents induced on the inner surface of the gun tube  12 , are also shown. The strength of the magnetic field  13  generated by the primary, is depicted by arrow lengths between the round and the inner surface of the gun tube. (The longer the lengths of the arrows, the stronger is the field). 
       FIG. 5  illustrates mathematically the effects of round balloting insensitivity on the sensing transformer. A small section of the secondary  14  is shown, comprised of four loop areas a 1 , a 2 , a 3 , and as. While a 1  and a 2  are subject to oscillating magnetic field strength B 1 , a 3  and a 4  are subject to oscillating magnetic field strength b(y)B 2 , where the magnetic field strength of B 2  is modulated by the amount of balloting (balloting here is accounted for by a ratio ‘b’ which indicates some diminished field strength, in the ‘y’ direction, where ‘y’ is perpendicular to the longitudinal axis of the gun tube). Equations 15 show calculations for the Electromotive Forces (EMFs) around each of the areas, and also the voltage at the terminals of the entire section. The magnetic fields are assumed to vary in time as e jwt . When the transition region between B 1  and b(y)B 2  is coincident with the boundary between a 2  and a 3 , all of the EMFs around each of the loops cancel, and hence the voltage output of the section is zero. This effect is always the same, regardless of the value of b(y). Therefore it can be seen that the zero crossing of the output voltage will be insensitive to balloting. 
       FIG. 6  is a diagram of voltage output  16  of the sensing transformer&#39;s secondary ( 17 ) versus its displacement as the round moves past the edge of the gun tube  18 , shown in top-down view. 
       FIG. 7  is a view of another embodiment of the present invention, with the planar air-coil transformer comprising a primary loop (as in  FIG. 1 ) but in this case, two subsection secondary loops  19  and  20 , instead of just the one full secondary loop  2  (as in  FIG. 1 ), are used. It is also possible to have multiple subsectional stages like  19  and  20  (instead of just the two as shown here in  FIG. 7 ), all if within the primary loop. 
     As the round moves toward the edge of the tube, the edge of the gun tube starts to move past the loop (relatively speaking). As a result, two regions of the magnetic field are created: one of a large perpendicular magnitude B 2 , and one of the said smaller magnitude B 1 . Once the round is fully out of the tube, the magnitude is once again entirely B 2  over the surface of the sensor. 
     This entire effect, as mentioned, is diagrammed in  FIG. 4 , with magnetic field intensities B 1  and B 2  depicted as arrows in region  13 . Rectangle  11  depicts the current carrying region of the primary loop in side view, while rectangle  13  depicts the region of the inner surface of the gun tube that carries image currents. 
     As mentioned, the secondary sits entirely within the primary, and is exposed to both oscillating field intensities B 1  and B 2 . The turns of the secondary are arranged such that when it is exposed to uniform field intensity throughout its entire length, the sensor is nulled, and ideally no voltage appears at the secondary terminals. This is because all of the EMFs around each of the turns of the secondary cancel. The turns can be viewed as loops of equal area, exposed to equal magnetic field intensities, but wired in criss-cross. 
     As mentioned, as the sensing transformer is swept past the edge of the gun tube, this null condition is upset. The output voltage of the secondary becomes amplitude modulated, with an amplitude that swings from positive to negative in an oscillating manner. The zero crossings of the amplitude of this AM signal are illustrated in  FIG. 5 . As the transition region from B 1  to B 2  passes between areas a 2  and a 3 , the EMFs around the loops cancel. Even with balloting (modeled as modulating B 1  to b(y) B 1 , with y being the amount of balloting off axis), the EMFs around the loops still cancel. 
     The voltage signal from the secondary is then synchronously demodulated, using the original AC drive signal as a reference. This recovers the amplitude modulation, as shown in  FIG. 6 . This signal will go through a single temporal period, as the edge of the gun tube passes through a single spatial period over the loops. The times between the zero crossings represent the flight time over fixed distances, from which velocity can be calculated. 
     Other possible embodiments of the invention include the case where the planar air-coil transformer comprises one primary and two or more secondary coils. As was mentioned with respect to  FIG. 7 , the geometry of the planar air-coil transformer with two secondary coils  19  and  20  are two subsections of the original secondary coil in the preferred embodiment. Each subsection consists of one zero-crossing zone, creating two displacement references. The electronic measurement circuitry can look for the zero-crossings, and determine the time between when the edge of the gun tube has crossed them (relatively speaking), yielding a velocity measurement. 
     While the invention may have been described with reference to certain embodiments, numerous changes, alterations and modifications to the described embodiments are possible without departing from the spirit and scope of the invention as defined in the appended claims, and equivalents thereof.