US 20080029641 A1
An apparatus for 3-axis aerodynamic control of multi-caliber bodies comprising three shafts each having at least one canard; bevel gears attached to said shafts; and a set of three miniature stepper motors, each driving a zero backlash spur assembly attached to the drive bevel gears.
1. An apparatus for three axis aerodynamic control of a body while in flight, said apparatus comprising:
first, second and third shafts;
a canard attached to spaced ends of the first shaft;
a canard attached to one end of each of the second and third shafts,
a gear assembly operatively connected to each of the shafts;
a drive motor for driving each of the shafts through the gear assemblies; and
a controller operatively connected to each of the drive motors for integration of guidance controls to the canards to steer the body.
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14. A guided munition comprising:
a projectile body;
a stabilizing fin assembly mounted at the rear of the projectile body;
a guidance assembly mounted in a forward end of the projectile body, said guidance assembly including:
a) at least four canards movably mounted with respect to the projectile body,
b) a first shaft having first and second of said canards mounted on respective ends of said first shaft,
c) a second shaft having a third of said canards mounted thereon,
d) a third shaft having a fourth of said canards mounted thereon;
e) an optical sensor mounted in the projectile body; and
f) a controller operatively connected to each of the three shafts for independently rotating said shafts to move said canards to change the ballistic path of said projectile body in response to guidance signals received from the optical sensor.
15. The guided munition defined in
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This application claims rights under 35 USC 119(e) from U.S. application Ser. No. 60/650,705, filed Feb. 7, 2005; the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to armaments and more particularly to guided munitions. Even more particularly, the invention relates to a three axis control system and mechanism for guided mortar shells or for other small aerodynamic bodies.
2. Background Information
Mortars are one of the most commonly employed weapons in a ground combat unit. The traditional role of mortars has been to provide close and continuous fire support for maneuvering forces. Military history has repeatedly demonstrated the effectiveness of mortars. Their rapid, high-angle, plunging fires are invaluable against dug-in enemy troops and targets in defilade, which are not vulnerable to attack by direct fires. One of the major disadvantages of mortars is their comparatively low accuracy, and as a result mortars are becoming less effective in today's precision combat environment. Equipping a mortar round with a precision guidance package will increase its accuracy, enabling the mortar to be a precision munition that will be significantly more effective in wartime situations. For maximum utility, the guidance package preferably should be an inexpensive retrofit to current munitions, with a cost in production that allows its use in all situations, either as a guided or unguided weapon.
Unguided munitions are subject to aim error and wind disturbances. These factors, along with other more subtle error sources, may cause the munition to miss the target completely or require many rounds to complete the fire mission due to the resulting large CEP (Circular Error Probability). Current approaches to guided weapons are expensive and are used on larger, long range weapons. The approach of the present invention results in significantly lower cost and smaller size. This allows use with small to medium caliber weapons and significantly improves CEP which also results in a significant reduction in the quantity of rounds required to complete the fire mission which in turn results in lower overall cost and improved crew survivability. In addition, another benefit to this approach is the virtual elimination of collateral damage due to errant rounds impacting non-targeted areas. Furthermore, complete integration of a seeker/guidance error can be used in a modification to the existing fuse in order to “safe” errant rounds which are failing to meet the established CEP ground rule which further controls unwanted collateral damage by preventing detonation of off target rounds.
Mortars are typically unguided or guided by an expensive G&C (Guidance and Control) system. The cost is high for current guided mortars and unguided mortars may have poor accuracy. Also, unguided mortars may result in unacceptable collateral damage, excess cost due to the large number of rounds required to blanket the target area, and may expose the mortar crew to counterbattery fire due to the large time required to drop the necessary shells to saturate the target.
The control of a small caliber body, (or any multi-caliber munition or Micro-Air Vehicle, MAV), necessitates a space efficient, light weight, reliable, low cost hardware embodiment. Furthermore, in order to optimize performance, a full 3-axis control system is required to maximize maneuver load capability and overall control, hence minimizing CEP (or miss distance) and for flight trajectory optimization. The combination of a full 3-axis control system using, in this case canards, (although traditional aileron, rudder, and elevator control can be incorporated), in a low cost, volumetrically efficient package which does not adversely effect center of gravity has been developed. The approach of the present invention does not require shaft encoders or resolvers and integrates feedback from other onboard sensors to effect control over the body utilizing the 3-axis control scheme.
Therefore, there is a need for an accurate and cost effective means for guiding mortar munitions. There is also a need for an ultra low cost G&C approach for mortars which is compatible with a large class of rounds. Furthermore, subsequent need exists for preventing detonation of errant rounds to minimize non-target specific damage and non-combatant loss of life. Likewise, there is a need for a highly integrated control mechanism for small caliber and MAV aerodynamic bodies which provides full 3-axis control with minimal weight, cost, volume, and power impacts.
In one embodiment of the present invention, a set of three miniature stepper motors, each driving a zero backlash spur gear assembly, are used to drive bevel gears attached to a set of three shafts. In another embodiment the stepper motors each drive a planetary gear assembly which drives a worm gear and a pitch gear connected to each of the three shafts. Attached to each shaft are the canards. A common shaft has a canard attached at opposite ends of the shaft. Two individual shafts each have a single canard affixed at an end thereof. In this manner, roll, pitch, and yaw steering commands can be executed through the canard frame to maneuver the munition in order to increase system accuracy. This technique is robust and also can be adapted to conventional MAV airframes to improve aerodynamic maneuvering performance.
The present invention is further described with reference to the accompanying drawings wherein:
Similar numbers refer to similar parts throughout the drawings.
A preferred embodiment of the three axis aerodynamic controlled munition of the present invention is indicated generally at 1, and is shown in assembled position in
Munition 1 can be an optically guided mortar shell such as shown and described in detail in a pending patent application entitled, Optically Guided Munition, being filed concurrently herewith, the contents of which are incorporated herein by reference. At the forwardmost end of nose 11 is located an optical seeker subsystem 15 for detecting an optical illuminator or other type of radiator located at a target. The details and manner of operation of one type of optical seeker which can be used in munition 1 is shown and described in detail in a pending patent application entitled Optically Guided Munition Control System And Method being filed concurrently herewith, the contents of which are incorporated herein by reference. Operationally connected with optical seeker assembly 15 is the three axis aerodynamic control system of the present invention indicated generally at 17.
Nose 11 is formed with a Ram Air Turbine (RAT) 14 which includes a plurality of air ducts 19 which supply air through openings 21 formed in taper nose portion 12 for controlling an alternator and switch plate assembly 23 and a safe/arm rotor assembly 25 (
A first embodiment of the three axis control system 17 is shown in detail in
Although canards 33 are shown mounted on tapered nose portion 12, they can be mid-body wings or similar aerodynamic control surfaces without departing from the concept of the invention.
In accordance with one of the features of the invention, common shaft 45 and individual canard shafts 53 and 55 each are driven through a gear assembly 51 by three drive motors, each of which is indicated generally at 57. Each drive motor 57 preferably is a high torque two-phase stepper motor, and in the preferred embodiment has an 18° step size. Each motor 57 is connected to a planetary gear 59, which in turn is connected to a worm gear 61 which drives a pitch gear 63 that is connected either directly to a respective canard or to the pivot lock and fold pivot therefore. In this preferred embodiment, planetary gear 59 has a gear ratio of approximately 256/1 and an output torque of approximately 1.77 in/lb.
In a second embodiment as shown in
Thus, drive motors 57, which preferably are stepper motors, can be connected by various gear assemblies, such as gear assemblies 51 and 52 to the canards, which if desired will be through a pivot lock 47 and fold pivot 49, and in another embodiment directly to the canards. In either construction the gear assemblies and drive motors will rotate the canards about their pivots 43 to control the pitch, roll and yaw movement of the munition body after passing through apogee and in its descent toward a target along a ballistic path. The particular canard shown in
Although the above-described three-shaft and associated gear assembly and drive motor is the preferred construction, it is understood that a four-drive motor configuration could be utilized wherein each supports roll and pitch, as well as a single or two-motor drive motor configuration which requires higher complexity transmission with independent shaft shifting functions and various gear arrangements, which would provide the desired results but in a more complicated and complex manner.
Common shaft 45 is driven through gear assemblies 51 or 52 by a stepper motor which is attached to one of the three zero backlash reduction spur gear heads or to the planetary gear, worm gear, pitch gear and spur gear arrangement. This common shaft is capable of generating a force normal to the plane of the canards in both positive and negative senses and is utilized in a left/right or up/down mode only. The two independently driven canards 33B and 33C, each of which is attached to a separate shaft by a gear assembly are capable of bi-directional control, and thus when used in a common mode are capable of generating the same normal force as described for the common shaft assembly in either a positive or negative direction, and is capable of supporting both the left/right or up/down maneuver directions. However, when driven in a differential mode, the pair of canards are capable of generating a roll torque, thus positioning the total canard frame, (in this case the mortar roll orientation), in the angular orientation required to support maximum maneuver capability and for removing spin generated due to any tail asymmetries or manufacturing defects.
The stepper motors also provide the angular state of the canards based on the magnet pole energized. Minor corrections can be applied in a single step or multiple step modes and active feedback from a guidance algorithm used to null out oscillations about the intended course line, thus implicitly correcting any potential angular misorientation of the canards caused by launch conditions. In this way complicated shaft encoders or resolvers are eliminated further reducing cost.
One manner in which the three axis control system and mechanism of the present invention is used to control an aerodynamic body, such as a munition, is shown in diagrammatic block form in
One particular manner in which the drive and control mechanism is used to control the flight of munition 1 is set forth below.
Flight Control Subassembly—The overall projectile center of gravity is controlled in order to maintain static and dynamic stability and to optimize maneuverability. The aerodynamic control surfaces are canards 33 with a trapezoidal planform, double wedge (symmetrical) surfaces, which are deflected to generate normal force, which in turn generates a pitching moment on the mortar shell causing the new trim attitude to be achieved and accelerating toward the target. The canard panels are located relative to the drive shafts at the longitudinal center of pressure location providing a torque balanced aerodynamic surface. This minimizes the torque required of stepper motors 57 to generate the requisite normal force from the canard surface enabling low cost miniature motors to be incorporated into the design.
The choice of a double wedge planform, while not normally preferred for subsonic rounds, is a cost effective planform from the productibility perspective with the thick portion 41 of the airfoil ideally suited to incorporation of the canard hinge or pivot (
The actual drive motors and gear assembly options for the canards are miniature, high torque, 2 phase stepper motors with an 18 degree step size. The motors are driven at a rate of between 0 and 15,000 rpm which translates, through the gear assemblies to a rate of up to 3000 degrees/second well in excess of the required Bandwidth of the control system. The motors are stepped at a high rate then paused to allow the effect of each command to settle (aerodynamically) through the system. Each motor is capable of 0.5 A total current draw at stall. The motors are attached via the gear mechanisms to the canard shafts in one embodiment using right angle bevel gears at a 1/1 ratio (
Roll Control—Zero roll is maintained by using a Proportional-Integral-Derivative (PID) control loop 70 shown diagrammatically in
Steering Control—Steering control has two separate components: YAW (left/right) control, in which the canards, acting in pairs, provide horizontal displacement, and Elevation (up/down) in which the canards, again operating in pairs, provide an increment or decrement to the projectile range. Two of the diagonally opposed canards also provide the roll control discussed above.
Left/Right Steering Correction—The horizontal steering correction term is determined from the left/right centering error determined from the sensor array. This error is used to determine the necessary correction to drive the canards to correct any lateral aiming error through a L/R steering loop 75 (
Up/Down Steering Correction—Vertical steering correction is done in a similar manner to the horizontal steering correction. However, unlike the left/right correction where the desired horizontal angle of attack is known and equals zero (at the detector array centerline as shown in
The trajectory correction approach involves estimation of the trajectory by a trajectory estimator 78 and a determination of the impact point relative to the target. If the mortar is on course, the target will be centered with respect to the left/right center line. It will also be aimed at the proper elevation angle vs. time. Thus, the downward look angle will follow a specific time history. See body axis graph 80 in
For a cross track error the location of the target with respect to the left/right of the bore sight center line is the “horizontal” error signal. This error is used to deflect the canards to correct the cross track error. In this case a trajectory estimate is not needed. Only an estimate of down is required to roll the mortar to zero degree roll angle.
To correct along-track errors, the “vertical” error signal is computed from the difference between the nominal bore sight look down angle and the bore sight look down angle measured by the seeker. To implement this approach, the trajectory is estimated (using time to apogee and launch speed). This trajectory estimate is then used to provide the nominal look down angle to the impact point. This nominal angle is time dependent and decreases a few degrees per second. This nominal value is compared to the seeker value. If the nominal value exceeds the seeker value then the current trajectory will pass over the target. In this case a downward correction is applied. If the nominal value is less than the measured value, an upward correction is applied.
In the absence of gravity, the nominal bore sight look down angle would be zero. In this case we have the “direct fly in” approach. The effect of gravity diminishes as the mortar closes on the target for short range shots using high quadrant elevation (>45 degrees) because the approach angle is closer to vertical. Thus in the end the ballistic correction approach morphs into a direct fly in approach.
When a maneuver command is applied, the mortar develops an angle of attack (AOA). This AOA shifts the look angle to the target. For a 0.2 g maneuver a 6-DOF (degrees of freedom) model shows that the AOA will be about 1.9 degrees. Thus, if the projectile is initially aimed 1 degree to the right of the target in the horizontal direction and a 0.2 g left maneuver is commanded, the target look angle will be 0.9 degrees to the right. As the mortar velocity vector turns left towards the target, the look angle will move further to the right. This does not indicate an over shoot. In fact the turn must be continued until the look angle is 1.9 degrees to the right. At this point the canards are zeroed and the AOA trims back to zero. With zero AOA and the velocity vector pointing to the target, the look angle will be zero. It is critical to account for this AOA effect when steering because the effect the expected AOA will be of comparable magnitude to the aim angle error.
Further details of one type of guidance and control system for use with the three axis control apparatus of the present invention is shown and described in pending patent application entitled, Ballistic Guidance Control For Munitions, being filed concurrently herewith, the contents of which are incorporated herein by reference.
While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.