GB2093309A

GB2093309A - Stabilization and Control of a Projectile

Info

Publication number: GB2093309A
Application number: GB8203599A
Authority: GB
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1981-02-13
Filing date: 1982-02-08
Publication date: 1982-08-25
Also published as: GB2093309B; DE3105219C2; DE3105219A1; FR2500184A1; FR2500184B1

Abstract

A projectile is stabilized by means of a beam of polarized light which is directed at the projectile, and whose direction of polarization is cyclically varied about a reference direction. The projectile carries two polarization analysers (10, 11) positioned with their preferred transmission directions mutually perpendicular, and two photo detectors (12, 13) are positioned to receive light transmitted from respective analysers (10, 11). The signals generated by the photo detectors (12, 13) are amplified and processed in a processing unit (14) to provide a flight correction signal to stabilize and/or control the movement of the projectile about its roll axis. The sign of the correction signal is determined by the signal processing unit by comparing the relative values of detected optical signals from one of the detectors (12, 13) in pairs of successive cycles of variation of the polarization direction of the incident polarized light beam, the processing unit (14) being preliminarily synchronized. <IMAGE>

Description

SPECIFICATION Method and Apparatus for Stabilisation and Control of a Projectile The present invention relates to a method for stabilising and controlling a projectile, particularly a roll-stabilised projectile utilising polarised light.

Remote control systems of this type are used to control projectiles in order to avoid the necessity of transmitting control signals by wire or by radio. Transmission by wire suffers from the disadvantage that the speed of the projectile must be limited because velocities over 300 m/sec cause problems in winding and unwinding the wire. Radio transmission of control signals, on the other hand, have the disadvantage of making it easier for an enemy to trace the operator and the firing location due to the wide dispersion of the transmission signal.

One prior art system for stabilising the position of a projectile about its roll axis (hereinafter referred to as the "roll position" of the projectile) by means of polarised laser light is described in U.S.

Patent No. 3 963 1 95. The information required for roll stabilisation of the projectile is transmitted in the form of a beam of polarised light. In this prior art system the angle between the axis of the projectile and the polarisation vector of the polarised light beam is derived indirectly from the signals produced by two detectors in the projectile. In fact if the angle between the projectile axis and the polarisation vector is 0, the detector outputs are proportional to the magnitude of sin28 and cos28, and the value of 0 is then determined from these.This known apparatus suffers from the disadvantage that if the angle 0 is small the direction of the deviation from the required direction cannot clearly be distinguished, and the detected signal which is proportional to sin20 does not give a satisfactory signal-to-noise ratio. As a result precise stabilisation of the roll position (and thus of the direction of the lift vector in the case of projectiles with a gliding effect, that is where the acceleration due to gravity is compensated by a corresponding lift) is not possible. The circuit disclosed in the above-mentioned U.S.

Patent No. 3 963 1 95 is thus unable to provide possibility of fine control of the projectile along a flight path having a slight curvature. Projectiles having such a flight path, however, are of particular interest as anti-tank weapons or as very short range weapons (that is with a range in the region of 100 m).

The present invention seeks therefore to provide a method and an apparatus for stabilising and controlling a projectile using polarised light, by which the projectile can be controlled to adopt a preferred roll position and can be automatically stabilised with a high degree of precision.

According to one aspect of the present invention there is provided a method of optically controlling and stabilising a projectile, in which a beam of polarised light is directed at the projectile in flight and the direction of polarisation of the light beam is varied cyclically about a reference position, the sign of a flight correction signal derived from a detection of the polarised light beam being determined from the relative values of detected optical signals in successive cycles.

The method of the present invention provides a correction signal for the projectile if the average value of the polarisation vector deviates from the direction of the required roll position, or from the direction of the lift vector as the case may be, for two successive cycles.

This method thus provides an advantageous means for simple control of a projectile, the direction of rotation being unambiguously associated with the course correction signal. With the method of the present invention the magnitude of the correction signal is increased considerably, and this is a factor of particular importance in the case of small deviation angles. The method of the present invention has the advantage that the required correction direction is unambiguously determined even where the angular deviation is small and, moreover, that the correction signal is linearly dependent on the rotation angle even where this latter is small. These advantages, together with the availability of a much stronger course correction signal, ensure a simple and economical regulating system.

The method described herein in fact incorporates a general regulating principle for the optical control of projectiles. Using the method of the invention it is possible, for example, to deflect the lift vector of a projectile from the vertical direction and thus control the projectile in the horizontal direction likewise. In another aspect the present invention provides apparatus for the control and stabilisation of a projectile, comprising means for directing a beam of polarised light at the projectile, including means for cyclically varying the direction of polarisation of the light about a reference position, and detector apparatus on the projectile sensitive to the direction of polarisation of the incident light beam and operative to determine the sign of a flight correction signal in dependence on the relative magnitudes of detected polarised light signals in successive cycles.

One embodiment of the invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram illustrating the manner in which the relationship between the target trajectory and projectile trajectory is used in defining the angle 0; Figure 2 is a block diagram of the control apparatus carried by the projectile; Figures 3a, 3b, 3c and 3d are diagrams iilustrating various signals and waveforms occurring within the apparatus during operation and useful in explaining the operations of the apparatus; Figure 4 is a block diagram of a part of the signal processing unit within the control apparatus carried by a projectile; and Figure 5 is a diagram illustrating the relationship between important angles related to the angle of rotation ap and the direction of the polarisation vector.

There are a number of possible reasons why the correction of the flight path of a projectile may become necessary after launch. These include: (a) lateral movement of the target between the time of firing and the time when the impact of the projectile with the target is due to occur, (b) angular deviation of the projectile from the desired direction, for example as a result of inaccuracies in launching or as a result of the effects of a side wind; (c) spontaneous rolling movement of the projectile, liable to cause horizontal drift due to the resultant lateral component of the lift.

For the case (a), if the control system has a telescopic sight for the operator the angular deviation can can be automatically determined by following the target with the telescopic sight or with the goniometer head.

In case (b), the angular deviation can be determined visually in the raster of the telescopic sight.

Alternatively, the angular deviation can be determined automatically by electronically comparing the position of the cross wire of the telescopic sight, which would be directed at the target and an illumination point of the projectila.

In case (c), the projectile's detection equipment sensitive to the optical signal transmission, which will be described below, automatically generates a correction signal which stabilises the lift vector.

Figure 1 shows the relative positions of the operator and the target, as well as the angular deviation 0(t) which has to be introduced into the flight path of the projectile to ensure that it strikes the target and which, depending on the measured distance R from the operator to the target at the time of firing, and the calculated remaining flying time At, at the time when the correction is made will result in a longer or shorter course distance St which, for small angles can be taken as St=Rpx(t). The distance Rp can be measured, before the projectile is fired, by means of a laser diode which also serves to transmit the polarised light pulses. The correction signal necessary to achieve the required transverse acceleration Bt is calculated in a microprocessor from the measured value of the angular deviation (t).

The correction signal is derived by the apparatus housed in the projectile from the rotation of the polarisation vector P0 of the received light. If this vector is detected to turn about an angle a then the magnitude of the correction signal is given by the equation bt=box sin ss.

Where ss is the angle by which the projectile has turned about its roll axis from a reference position in which the lift vector is vertical.

The angular deviation 4 and the required transverse acceleration bt is thus found to be as follows, when is a small angle: St=Rpx bt -(#t)2 2 where At is the remaining flying time, that is R,--x At= Vf Where Vf is the speed of flight of the projectile, and 2Rp.

b= (#t)2 In the case of I Vf 1 # constant, we have Rp bt=2 (RpX)2 Considering now the lift on the projectile, initially when the lift vector bo is vertical |bo|#g, whereas, after rotation of the projectile about the roll axis by an angle ss from the vertical, the transverse component bt of the lift vector is given by: bt=|bo|.sin ss The apparatus on the projectile is shown in detail in Figures 2 and 4. The projectile contains an optical window 8 with a narrow band filter 9 onto which the polarised light from the optical signal source falls. The narrow band filter 9 serves to transmit the polarised, optical signal light, which is quasi-monochromatic, while the unpolarised but wide-band background light is filtered out.These are followed by polarisation analysers 10, 11, having transmission direction E, orthogonal to each other. A polarising beam splitter may aiternatively be provided. The light then falls onto two photo-detectors 12, 13, which generate output signals Dt, Dp representing the components of incident light transverse (Dt) and parallel (Dp) to a defined vector (E) on the projectile. As can be seen in Figure 4 the electrical signals Dt, Dp are amplified in amplifiers 15, 16 to produce amplified signals Jt and Jp which are further processed in the signal processing unit 14 which is shown in greater detail in Figure 4 and of which the amplifiers 15, 16 form part.

As can be seen in Figure 4 the amplifier output signals Jt and Jp are both fed to an adder 18 where the sum (Jt+Jp) is formed (see also Figure 3d). The signal Jt from the amplifier 15 is also fed to a multiplier 17 which receives multiplication pulses M. The output signals from the multiplier 17 are then fed to an integrator circuit 21 which forms the average value of each pair of successive pulsed signals Jtr and Jt2 representing the transverse polarised components of the received optical signal in the first cycle and the second cycle respectively. The outputs from the integrators 20 and 21 are then fed to a divider circuit 22 which provides an output signal representing the ratio between them.

The roll position of the projectile can be defined by the transmission direction of one of the polarisation analysers on the projectile, preferably the analyser 11 having the transmission direction (see Figure 5). A spatial reference direction is provided at the firing position by the polarisation vector P0 of the incident light, which, with a reasonable degree of approximation, is not disturbed by air eddies.

Changes in the roll position (defined by Et) in relation to the polarisation vector PÒ by an angle ap, whether by control signals P0 or by spontaneous rolling movements which require to be corrected, lead to a greater relative change in the signal from the detector Dt (see Figure 2), which receives light transmitted through analyser 10 the polarisation direction of which is perpendicular to Et. A change in the magnitude of ap can thus be directly detected but not its direction of rotation +ap or -ap.

This is rendered possible by the cyclic variation of the emitted light between angles +Aa and Acg, in accordance with the principle illustrated in Figure 3. If ap is positive, for example, the signal Jtr emitted by the detector Dt in modulation cycle 1 is greater than the signal Jt2 emitted by this same detector in the subsequent modulation cycle 2. Conversely, if a, were negative, Jt1 would be smaller than the subsequent signal Jt2 In this connection it is assumed that in the initial position Po II Et. The sign of the difference (Jt1Jt2) is thus clearly correlated with the sign of ap (see Figures 3a, 3b, 3c).Whether the rotation is to the left or to the right can thus be determined from the sign of the voltage signal at the output of the integrator 21 of Figure 4. Unambiguous allocation of a certain sign to a certain direction of rotation is conditional upon having a defined phase relation of the multiplication pulses M in the projectile (see Figure 3) with the cyclic variation of the polarisation vector P0 at the light source. This synchronisation can be achieved in various ways, for example by transmitting preliminary electrical pulses from the operator's control unit to the electronic system in the projectile before the latter is fired.

The cyclic variation of the polarisation vector P0 offers the further advantage that the difference (J,,-J,,) (see Figure 3b) is a linear function of the sine of the rotation angle ap while in the absence of such variation the output signal J from a detector of the type used is proportional to the square of sin ap. This not only simplifies the dependence of the signal on the controi value but also considerably increases its magnitude, a particularly important factor in the case of small angles of rotation ap.

In order to make the control signal independent of intensity fluctuations Alo of the incident light signal, (which fluctuations may be due to variations in the light source, air turbulence, misalignment between light beam and projectile, and changing distance from light source to projectile during flight) the quotient ( < Jt1-Jt2 > )/( < Jt+Jp > ) is formed in the divider 22. The sum (Jt+Jp) is here equal to the total incident light intensity independently of the direction of the polarisation vector.

The connection between the signal at the output of the signal processing unit 14 and the angle αp between the instantaneous direction of the polarisation vector P0 and the reference direction defined by Et, can be shown to be as follows:: a1=a+Aa a2=a-Aa Et=Eo x.cos a EEo x sin a Intensity laE2 It=Io x sin2α Ip=Io x cos2α It1=Io sin2(α+#α) It2=Io sin2(α-#α) The currents J and the outputs of the amplifiers 15 and 16 are given by Jt1=Vt x It1 and also Jp=Vp x Jt2=Vt x 1t2 where Vt, Vp are the effective amplification factors, which are also dependent on the area and efficiency of the photo-detectors 12, 1 3.From the above it can be seen that since: sin (αp+#α)=sin αp cos#α+cosαp sin #α and sin (αp-#α)=sin αp cos Aa-cos ap sin Aa (Jt1-Jt2)/(IoVt)=sin2(αp+#α)-sin2(αp-#α) =(sin ap cos Aa2+2 sin αp cos αp sin Aa cos Aa+(cos ap sin #α)2 -(sin αp cos #α)2+2 sin αp cos αp sin #α cos #α;-(cos αp sin #α)2 =4 sin αp cos αp sin Aa cos Aa =sin 2ap sin (2Aa); moreover since from the above it can be seen that Jt+Jp=Vt It+Vp Ip =Io(Vtsin2α+Vp cos2α) =Io. V for Vt=Vp=V JtrJt2 =sin 2ap sin (2#α) Jt+Jp for cases in which amplification factors Vt=Vp=VI Approximation for small angles of rotation αp: Jt1-Jt2 #2αp.sin(2#α); Jt+Jp that is the signal at the output of the divider 22 is a linear function of the rotation angle αp for polarisation vectors < Po, its magnitude being optimised for #α=45 .

Claims

1. A method of optically controlling and stabilising a projectile, in which a beam of polarised light is directed at the projectile in flight and the direction of polarisation of the light beam is varied cyclically about a reference position, the sign of a flight correction signal derived from a detection of the polarised light beam being determined from the relative values of detected optical signals in successive cycles.

2. A method of optically controlling and stabilising a projectile as claimed in Claim 1, in which the direction of polarisation of the transmitted light beam is cyclically varied by +450 from the mean position.

3. A method of optically controlling and stabilising a projectile as claimed in Claim 1 or Claim 2, in which the incident light beam on the projectile is separated into two mutually perpendicular components and a roll correction signal is generated as the quotient of signals representing the mean value of the difference between signals of successive pairs of signals representing one of the two said perpendicular components of the incident light and the mean value of the sum of the signals representing the two mutually perpendicular components of the incident light.

4. A method as claimed in Claim 3, in which the said difference signal is formed by multiplying the pulses representing the said one component of the incident light with a square wave multiplication pulse train having a defined phase relation with respect to the cyclic variation of the polarisation of the transmitted polarised light beam.

5. Apparatus for the control and stabilisation of a projectile, comprising means for directing a beam of polarised light at the projectile including means for cyclically varying the direction of polarisation of the light about a reference position, and detector apparatus on the projectile sensitive to the direction of polarisation of the incident light beam and operative to determine the sign of a flight correction signal generated thereby in dependence on the relative magnitude of detected polarised light signals in successive cycles.

6. Apparatus as claimed in Claim 5, in which the apparatus carried by the projectile includes two polarisation analysers having mutually perpendicular transmission directions, and two photodetectors each positioned to receive light transmitted through a respective analyser whereby to produce electrical signals representing mutually perpendicular components of the incoming optical signal.

7. Apparatus as claimed in Claim 6, in which the output signals from the two photodetectors are fed to a signal processing unit incorporating means operative to generate first signals as the mean value of the difference between successive signals representing one of the two mutually perpendicular components of the incident light, and a summing circuit and integrator operative to generate second signals as the mean value of the sum of the signals from the two detectors representing the two mutually perpendicular components of the incident light, the output signal from the signal processing unit representing the quotient of the said first and second signals.

8. Apparatus as claimed in Claim 7, in which the means for generating the said first signals include a multiplier which is fed with amplified output signals from the said one detector and with a multiplication pulse train having a defined relation with the cyclic variation of the transmitted polarised light beam whereby to produce an output signal representing the difference between and successive signals in each pair of successive signals from the said one detector, and an averaging circuit producing an output signal representing the average value of the said difference between successive signals from the said one detector constituting the values of the said one component of the polarised incident light in successive cycles.

9. Apparatus as claimed in any of Claims 5 to 8, in which there is further provided a spectral filter in advance of the or each said polarisation analyser operative to filter out the background light.

10. A method of controlling and stabilising a projectile substantially as hereinbefore described with reference to the accompanying drawings.

11. Apparatus for controlling and stabilising a projectile substantially as hereinbefore described with reference to the accompanying drawings.