Abstract:
There is disclosed a collar ( 100 ) which may be attached to a munition in order to control the trajectory of the munition. The collar ( 100 ) has a collar body ( 10 ); a surface ( 12 ) for capturing the projectile as it leaves the barrel; a sill ( 14 ) for supporting the surface ( 12 ) at the muzzle of the barrel; and a guidance means ( 20   a   , 20   b   , 21   a   , 21   b ) for altering the flow of air around the collar ( 100 ). The collar ( 100 ) supports itself at the muzzle and may attach to the projectile at the surface ( 12 ) to integrate with the projectile as the projectile is fired. The collar ( 100 ) is particularly suited for attachment to mortar rounds. Such a collar ( 100 ) gives a weapon operator the option of increasing the precision of a munition without having to carry a plurality of munition types.

Description:
The present invention relates to a guidance device comprising a collar for guiding a projectile, and in particular to a collar for improving the precision of a ballistic projectile. 
     BACKGROUND OF THE INVENTION 
     In the field of ballistics, the term precision describes the ability of a projectile, fired from a weapon system, to follow a predicted trajectory and hence hit an expected target; a precise projectile will, by definition, follow a predicted trajectory more closely than an imprecise projectile. Ballistic precision is commonly measured using the circular error probability (CEP). 
     In most cases, it is desirable to have a precise projectile. However, the unit cost of a projectile tends to rise with increased precision. Accordingly, it is generally understood that in designing projectiles, the benefit of providing a particularly precise projectile must be balanced against the costs of such provision. 
     It is known to have a kit, such as the XM1156 Precision Guidance Kit (as may be supplied by Alliant Techsystems to the US Army), whereby a standard (i.e. non-guided) 155 mm artillery shell may be converted into a guided munition. The kit comprises means for controlling the trajectory of the projectile. Such controlling means may include a set of control surfaces, a processor, and an actuator for moving the control surfaces in response to a correcting signal from the processor. The processor may be interfaced with Global Positioning System (GPS) and Inertial Navigation (IN) sensors to determine the correcting signal which is to be applied. 
     The kit, which further includes a fuze, can be retrofitted into a shell by detaching the fuze section of the shell from the body section of the shell and then attaching the kit to the body section. The kit may therefore give users the option of converting munitions and thus selecting the precision of each round fired. 
     However, the Precision Guidance Kit (PGK) may have a deep intrusion body that necessitates the removal of some of the shell&#39;s explosive payload in order to fit the PGK instead of the original fuze. 
     Further, the act of replacing the original fuze with the kit may be undesirably time consuming, particularly given the urgency with which a muniton may need to be fired. Indeed, it may not even be possible to replace the original fuse with the kit on the battlefield, for example if some of the payload must be removed as described above. 
     Further, the kit may only be applicable to munitions which have a detachable fuze. Where the munition does not originally have a fuze, the kit cannot readily be applied. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a collar for guiding a projectile which mitigates at least one of the disadvantages of the prior art identified above. 
     The present invention provides a guidance device for guiding the trajectory of a projectile during flight, the device comprising: a collar having a collar body configured to be located at a muzzle of a projectile barrel prior to launch and having an internal profile which cooperates with an outer surface of the projectile when the projectile is launched through the muzzle so that the collar is attached to the projectile during flight; the collar having guidance means comprising at least one adjustable control surface for controlling the trajectory of the projectile during flight, adjustment of said control surface being responsive to guidance signals received from a guidance control. 
     The internal profile of the collar may be configured to engage with a rim of the projectile barrel so that the collar is located in position to cooperate with the projectile on launch. 
     Said at least one guidance surface may be pivotally mounted to the collar to allow adjustment of said surface relative to the collar. 
     An actuator may adjust the control surface in response to said guidance signals. 
     The guidance control may comprise at least one sensor for sensing a trajectory of the projectile during flight, and a processor for comparing the sensed trajectory with a predicted trajectory and outputting guidance signals for correcting the trajectory of the projectile so that it corresponds with the predicted trajectory. The guidance control may comprise a memory for a storing the predicted trajectory of the projectile. The at least one sensor may be configured for determining the trajectory of the projectile during an initial period after launch and outputting said determined trajectory for storing by the memory as the predicted trajectory. 
     The present invention also provides a collar for guiding a projectile, the collar comprising a collar body, a surface for capturing the projectile as it leaves the barrel, a sill for supporting the surface at the muzzle of the barrel, and a guidance means for altering the flow of air around the collar, wherein the collar may attach to the projectile at the surface to integrate with the projectile as the projectile is fired. 
     Such a collar can be transported into battlefield with the munitions and the weapon system to offer a more precise firing should this be desired. The collar is simple to mount on the muzzle and does not require the detachment or reattachment of munition components prior to firing. 
     A further benefit when compared to guidance kits that require replacement of parts is that the use of the above collar will tend to minimise the number of components which must be transported after a set of precise firings. Thus this invention is in contrast to a system where fuzes may be replaced in the field because in that situation, the replacements must be brought into the field and the replaced fuzes brought back. 
     The law of the conservation of momentum dictates that as the mortar integrates with the collar, the velocity of the mortar will drop on account of the mass of the collar. It follows that the range of the integrated projectile will be less than that of an equivalent projectile. However, it is expected that in many situations, the benefits of a precise projectile compensate for the reduction in maximum range. It is nonetheless advantageous to minimise the mass of the collar wherever possible. 
     The collar may comprise a control surface, an actuator for altering the configuration of the control surface, and a guidance controller, the guidance controller comprising a navigation sensor for determining an actual trajectory the projectile is following, a memory at which data describing a predicted trajectory is stored, a processor operably connected to the actuator, the memory and the navigational sensor, wherein the processor calculates a correction signal which determines how the configuration of the control surface may be altered and transmits the correction signal to the actuator. 
     In particular the processor may calculate the correction signal by determining the difference (which may alternatively be referred to as the error or the deviation) between the actual trajectory and the predicted trajectory. 
     At the collar, the control surface may comprise a pair of canards, each canard comprising a pivot joint connecting the canard to the collar and wherein the actuator may be a ring actuator which connects to the canards so as to be able to alter the configuration of the control surface by rotating the canards about their pivots. 
     Where a pivot joint connects each of the canards to the collar body, the pivot joint is preferably connected forwards of the centre of pressure of the canard. 
     Where the ring actuator may correct the projectile course by applying a force to the control surface so as to move the control surface out of alignment with the air stream over the projectile body, such a location of the pivot leads to a stable control arrangement. This stability is conferred because as the actuator ceases to apply the force to the control surface, the air flow will return the control surface to its original configuration. 
     Such a position of the pivot should therefore also tend to simplify the control signals (i.e. the correction signal) which needs to be sent to the actuator because little consideration needs to be given to how the actuator must move in order to return the control surface to its original position; the correcting signal can consist of a set of identical signals, which rise in repetition frequency with the projectile deviation but need not be transmitted if the projectile follows the predicted trajectory. 
     By ‘forwards’, the reader will understand that this means that the pivot is mounted more towards the leading edge of the collar, i.e. further towards the oncoming air stream. 
     Further, each canard may be connected to the ring actuator at a point on the canard towards or at the trailing edge of the canard. 
     By thus positioning the interface between the actuator and the control surface, it tends to facilitate the best mechanical advantage and thus enables the weakest/lightest ring actuators to be used. 
     At the collar, the surface for capturing may be at an internal facet of the collar and may have a tapered inner diameter, operable to form an interference fit with said projectile, and thereby allow the collar to attach to the projectile. 
     Preferably, where the surface is to capture the projectile by way of an interference fit, the surface and the material providing the surface is capable of elastic deformation. Metals would be suitable materials for the material providing the surface. 
     In particular, the surface for capturing may define a generally frustoconical form. 
     As such the sill may support the frustoconical form defined by the surface at the barrel so that the axis defined by the frustoconical form is generally collinear with the axis defined by the barrel. 
     This supporting arrangement can promote an even interference fit around the projectile and so enable the collar to attach to the projectile and create a symmetrical integrated projectile. Such a symmetrical integrated projectile can be expected to have improved aerodynamic properties and tend to require less guidance. 
     Such a surface for capturing may be tapered at between 3° and 0.5°, and in particular may be tapered at approximately 1.2°. 
     The collar may comprise an air escape vent. 
     Such a provision allows the air exhausted from the barrel prior to the exit of the projectile to escape without disturbing the supported collar. 
     The collar may be formed as one or more portions operable to be fastened together. 
     A collar thus formed allows for transportation in a distributed and potentially less bulky form. 
     According to a further aspect of the invention there is provided a method of attaching a guidance collar to a projectile, the method comprising the steps of a) supporting a collar according to any one of the preceding claims at the muzzle of a barrel loaded with the projectile, b) firing the projectile from the barrel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the invention may be well understood, an exemplary embodiment of the invention will now be described with reference to the following figures of which: 
         FIG. 1   a  shows a first aspect of a collar according to the present invention; 
         FIG. 1   b  shows a cross section at X-X of the first aspect of the collar of  FIG. 1   a;    
         FIG. 2  shows a schematic diagram of the guidance controller for use in the collar of  FIG. 1   a;    
         FIGS. 3   a ,  3   b ,  3   c  and  3   d  show the sequential firing of a mortar operating in conjunction with the collar of  FIG. 1   a;    
         FIG. 4  represents the action of the collar of  FIG. 1   a  in correcting the trajectory of a projectile at a point A and a point B. 
         FIG. 5  shows an isometric aspect of the collar of  FIG. 1   a , integrated with the mortar and at Point A of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A collar  100  for guiding a mortar shell, as shown for example in  FIG. 1   a ,  FIG. 1   b  and  FIG. 5 , comprises a collar body  10 . The collar body  10  defines a generally cylindrical outer surface, which defines a collar axis  1 . The leading edge of the collar (that is the top edge in  FIG. 1   a ) is filleted so as to have appropriate aerodynamic properties. 
     A plurality of canards  20   a ,  20   b ,  21   a  and  21   b  extend from the outer surface of the collar body  10 . The plurality of canards  20   a ,  20   b ,  21   a  and  21   b  are spaced at regular intervals about the outer surface of the collar body  10 . The canards are arranged in pairs. A first canard pair, consisting of canard  20   a  and  20   b , generally occupies a first plane with canards  20   a  and  20   b  mounted on diametrically opposite sides of the collar body  10 . A second canard pair, consisting of canard  21   a  and  21   b , generally occupies a second plane with canards  21   a  and  21   b  mounted on diametrically opposite sides of the collar body  10   
     Each canard is pivotally attached to the collar body  10  by a pivot joint  30  which defines a rotational axis extending normal to the outer surface of the body  10 . The canards are arranged to be able to align with the collar axis  1  but deflect from this arrangement as they rotate about the joints  30 . Each pivot joint  30  is mounted towards the leading edge of the canard and so is forward of any component of the centre of pressure which may act laterally on the canard. 
     The collar  100  is hollow and is open towards both ends of its axis  1  to define a conduit. A first opening  16  of the conduit (alternatively referred to as the escape vent  16 ) is located at the leading edge of the collar  100  and defines a generally circular aperture, normal to the collar axis  10  and with a centre point which lies generally on the collar axis  1 . A second opening  17  is located at the trailing edge of the collar  100 . The second opening  17  defines a circular aperture normal to the collar axis  10  and with a centre point which lies generally on the collar axis  1 . 
     An inner wall of the collar  100 , which comprises a capture surface  12 , a sill  14  and a cylindrical section  18 , extends between the first opening  16  and the second opening  17 . 
     The capture surface  12  starts at the first opening  16  and extends down into the collar body  10  up to approximately the mid point of the body length. As the capture surface  12  extends away from the leading edge of the collar it tapers out, thereby defining a generally frustoconical surface, and eventually meets the sill  14 . The sill  14  is an annular surface normal to the collar axis  1  and with its centre point generally on the collar axis  1 . The inner diameter of the annular sill  14  meets the frustoconical surface  12  and the outer diameter of the annular sill  14  meets the cylindrical surface  18 . The cylindrical section  18  extends downwards to the second opening  17 . The diameter of the second opening  17  is generally identical to the outer diameter of the annular sill  14 . 
     A set of ring actuators  40  is disposed in the collar body  10  and there are connections to each of the canards  20   a ,  20   b ,  21   a  and  21   b . Each canard is connected to the ring actuator towards the trailing edge of the canard. 
     Embedded in the collar  100  is a guidance controller  50  which, as can be seen from  FIG. 2 , comprises a navigation sensor unit  54 , a memory  52 , a processor  56  and a ring actuator I/O unit  58 . Guidance controller  50  is also provided with a power source (not shown). 
     The processor  56  is operably and independently connected to the sensor unit  54  and the memory  52  and generates as an output a correction signal  57  that is input to the I/O unit  58 . The I/O unit is operably connected to the ring actuator  40 . 
     The sensor unit  54  comprises an inertial navigation system (comprising accelerometers for sensing linear motion and gyroscopes for sensing rotational rate), a magnetometer and a Global Positioning System (GPS). 
     In operation, the collar  100  is placed loosely over the mortar shell  200  with a forked safety plate  400  slotted on to the mortar  200  to hold the mortar  200  at the collar  100 . The collar  100  may then be placed at the muzzle  310  of a barrel  300  as shown in  FIG. 3   a  to prepare the mortar  100  for firing. The collar  100  is supported at the muzzle  310  by the sill  14  which rests at the lip of the muzzle  310  and is of such a form that the collar axis is generally collinear with the barrel axis. The collar  100  is also supported by the cylindrical surface  18 , which fits around the muzzle  310 . 
     In order to fire the mortar  200  the user removes the plate  400 , which may be done remotely using a string. This stage in operation is shown at  FIG. 3   b.    
     Once the safety plate  400  is removed, the mortar  200  drops in the known manner down the barrel  300  until the pin at the base of the barrel  300  is struck and the propellant charge at the rear of the mortar is initiated. 
     The initiation of the propellant charge accelerates the mortar towards the muzzle  310  and the collar  100 . The collar  100  remains supported at the muzzle  310  until the mortar strikes and engages with the capture surface  12 . The force of the mortar striking the collar  100  at the generally frustoconical capture surface  12  sets up an interference fit between the mortar and the collar  12 . This interference fit attaches the collar  100  to the mortar  200 , thereby integrating the collar  100  with the mortar  200 . 
     Further, the frustoconical form of the capture surface  12  may cooperate with the outer surface of the mortar to tend to ensure that the collar axis and the mortar axis are collinear. Thus the integrated mortar  500  is generally symmetrical. 
     The guidance of the integrated mortar  500  is illustrated at  FIG. 4 . A ballistic trajectory can be predicted from the inclination of the barrel axis and the muzzle velocity using classical mechanics, with adjustments made for air resistance made in the known way. However, a predicted ballistic trajectory may not be followed in practice because of environmental inconsistencies (such as wind) which may cause the projectile to deviate. 
     During its flight the collar  100  monitors its trajectory  120  using the navigational sensors in unit  54  to feed data into the processor  56 . Before applying any correcting signal, the processor  56  compares the monitored trajectory  120  to a set of predicted trajectories stored in the memory  52 . The processor thus determines that, of the possible predicted trajectories which the projectile  500  may follow, projectile  500  is intended to follow a particular predicted trajectory  110 . By making this determination in the early part of its flight, which is the part of its flight where the weather may have least effect on the trajectory, the selection of the predicted trajectory should tend to be correct. 
     Once the integrated mortar  500  has determined the predicted trajectory  110 , the controller  50  may regulate the actual trajectory  120  of the integrated mortar  500 , attempting to conform the actual trajectory  120  to the predicted trajectory  110 . 
     In the present embodiment, where the projectile is a free falling mortar which is not spinning in flight, the processor will rely on signals from magnetometer sensors and GPS sensors to determine the position of the projectile  500 . 
     Inertial Navigation sensors (in particular the accelerometers) at the projectile  500  will tend to give null readings for most of the flight because, in a projectile describing pure ballistic flight, there is a net zero acceleration at a strapdown accelerometer sensing the lateral axes within the projectile (a small deceleration followed by small acceleration will be sensed in the longitudinal axis). However, in other embodiments of the collar  100 , especially those where the projectile spins in flight, the IN sensors may include solid-state rate gyros and their output may be considered in determining the actual position of the projectile. 
     The processor  56  may, by frequently sampling the position of the projectile  500  from the signals from the sensors  54 , determine the actual trajectory  120  of the projectile  500 . 
     Once the actual trajectory  120  is determined, the processor  56  can compare the actual trajectory  120  to the predicted trajectory  110 . At the point A of  FIGS. 4 and 5 , the processor  56  determines that the actual trajectory  120  differs from the predicted trajectory  110 . In order to conform the actual trajectory  120  to the predicted  119 , the processor  56  sends a correcting signal  57  to I/O unit  58 . I/O unit  58  then outputs a more powerful signal to the ring actuator  40 , which signal momentarily energises the ring actuator  40  so that the ring actuator  40  momentarily deflects the canard pair  20   a ,  20   b  to apply lift to the integrated mortar  500 . The course of the integrated mortar should then alter and once the ring actuator is de-energised, the air flow will return the canard pair  20   a ,  20   b  to their original configuration. 
     In a similar manner, at the point B, the processor  56  determines that the integrated mortar  500  is now above the predicted trajectory  110  and so the correcting signal  57  is generated to energise the ring actuator  40  so that the canards deflect in the opposite direction to that at point A. 
     These exemplary corrective actions having been taken at the points A and B, the projectile  500  proceeds to land at the target Y, which is the predicted target for the predicted trajectory  110  and so avoids potentially sensitive targets X and Z. 
     For each canard pair, there are two types of corrective action which can be taken. The first type is for both canards to be deflected a specific amount in a first (glide) direction. The second type is for both canards to be deflected by the same specific amount but in a second (brake) direction. A simple control algorithm may be employed whereby the frequency of repetition of this corrective action is proportional to the deviation of the actual trajectory  120  from the predicted trajectory  110 . However, the invention alternatively contemplates the use of more sophisticated control methods which employ for example PID control algorithms. 
     The collar body  10  may be made from milled aluminium or an alloy of aluminium. Where the collar is for attaching to an 81 mm mortar, the first opening has a diameter of approximately 78mm and tapers at approximately 1.6° to a diameter of approximately 80 mm at the inner diameter of the annular sill  14 . The outer collar body diameter is 108 mm. With such a fabrication, the capture surfaces are the surfaces of the milled aluminium form. 
     The remaining components would be well known to skilled men in this field. Such skilled men would for example be aware of the need to use components in the guidance controller  50  which were sufficiently robust to function under the high accelerations encountered upon firing. 
     In the above described embodiment, the collar  100  is for attaching to and guiding a mortar round and in particular an 81 mm mortar round. However, the skilled man would realise that the invention could be applied to other calibres of mortar and indeed, other types of projectile.