GB2304241A

GB2304241A - Electromagnetic interference protection

Info

Publication number: GB2304241A
Application number: GB8416472A
Authority: GB
Inventors: Ronald David Sexton
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1983-08-09
Filing date: 1984-06-28
Publication date: 1997-03-12
Anticipated expiration: 2004-06-28
Also published as: GB2304241B; GB8416472D0; FR2748889A1

Abstract

The effects of electromagnetic interference are particularly important when applied to missile sub-systems especially electronic arming 5 and guidance 4 sub-systems. Missiles may have to pass through localised zones of extreme interference (due to radars etc) and suffer the effects of interference which may result in premature arming of the warhead and/or loss of missile control. Missile sub-systems which are susceptible to interference effects may be protected by employing sensing extraneous RF electromagnetic interference incident on the missile casing, which acts like a dipole antenna. Sub-system response control means can then be gated 11, 12 or inhibited to affect the operation of the sub-system to at least reduce the susceptibility of the missile sub-system to any ill effects due to the extraneous RF electromagnetic radiation.

Description

ELECTROMAGNETIC INTERFERENCE PROTECTION This invention relates to the protection of electrical and electronic apparatus from electromagnetic interference, and is more particularly although not exclusively concerned with the protection of electronic circuits on board a missile.

The operation of the electronics on board a missile may be impaired by incident RF electromagnetic interference. Screening of the electronics helps but may not give total protection. For example, lengths of interconnecting wire within the missile or sections of the missile body may have dimensions such that they are highly resonant at particular frequencies of interference, and damaging signals may be induced despite any screening which may have been incorporated.

For safety, the screening measures taken generally have to be sufficiently extensive to allow for the possibility of quite severe interference even though such environments are only entered occasionally. The protection offered by currently employed techniques can also vary considerably according to production tolerances. This results in systems requirements which are related to the possibility of the missile system performing in conditions which it is relatively unlikely to see, thus incurring mass and cost penalties owing to the protection techniques demanded.

Additionally, since the current trend is toward electronic arming systems for missiles, which, particularly if they are vertically-launched, may travel through very localised zones of extreme field strength (due to radars etc.), and suffer interference even for a short interval, such that there results loss of the arming chain safety interlocks resulting in premature arming, or a loss of missile control, particularly where missile control is from close to the launch position.

According to one aspect of the invention, there is provided apparatus including at least one electronic sub-system, which sub-system comprises signal supply means and response-producing means for producing an output response dependent upon signals supplied by the signal supply means, the apparatus further including antenna means and detector means for sensing extraneous RF electromagnetic radiation incident on said apparatus, and sub-system response control means responsive to said detector to affect the operation of said sub-system to at least reduce the susceptibility of the apparatus to any ill effects of said extraneous electromagnetic radiation.

Preferably, said antenna means may be formed by at least one portion of the apparatus casing, and may comprise two adjacent casing portions which define a wideband dipole element.

Advantageously, said sub-system response control means is operable to inhibit the relationship between said output response and said signals supplied by said signal supply means when a level of radiation exceeding a predetermined safe-level is incident upon the apparatus. Preferably, said sub-system response control means then operates to cause said output response to be generated on the basis of signals previously received from the signal supply means.

Alternatively, said signal supply means comprises an electrical conductor member, and said sub-system response control means includes controllably-variable reactance means controlled by said detector means and connected between two spaced positions on said electrical conductor member for detuning said member with respect to said received radiation. The reactance means may comprise variable capacitance means in the form of a mechanically variable capacitor or an electronically variable capacitance diode.

As an additional protection measure, further reactance means may be connected to a metallic member forming part of the apparatus for detuning said member with respect to extraneous RF radiation incident upon the apparatus. Said metallic member may comprise a metal part of a housing or casing of the apparatus or an electrical signal supply lead within the apparatus.

Said apparatus may include at least one electronic sub-system which includes a device which will fail at a lower level of incident radiation than other components of the sub-system and cause the sub-system to fail-safe.

According to a second aspect of the invention, there is provided a missile including a sub-system which has a control input and normally responds to signals received at said input to produce an output response associated with the missile, and the missile further includes protection means for reducing the effect of electromagnetic interference upon the missile, the protection means comprising sensing means for sensing electromagnetic radiation incident upon the missile, detecting means connected to the sensing means and to said sub-system and operable for detecting a level of said radiation exceeding a predetermined safe level and for then inhibiting the relationship between said output response and said signals received at said input.

According to a third aspect of the invention, there is provided apparatus which includes electrical or electronic equipment and, so as to reduce the possibility of impairment of the operation of the equipment due to electromagnetic interference, capacitor means connected in parallel with an electrical path through, and between two spaced positions on, an electrically-conductive portion of the apparatus so as to detune said conductive portion with respect to interference components for which the portion might otherwise have a resonant relationship.

According to a fourth aspect of the invention, there is provided a safety circuit characterised by the inclusion of a device which will fail at lower levels of radiation than other components of the circuit and cause the safety circuit to fail-safe.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a diagrammatic view of one embodiment of a missile showing a simplified circuit diagram including some of the missile sub-systems and an arrangement for reducing the effect of electromagnetic interference on the missile; Figure 2 is a simplified circuit diagram of part of an electronic safety and arming unit in another missile embodiment where electrical conductors are detuned with respect to likely interference components; Figures 3 and 4 are respective diagrammatic sectional views of the bodies of two missiles where the body sections are detuned; Figure 5 shows the principle of operation of a safety circuit which may be incorporated in the embodiments mentioned above; Figure 6 is a typical circuit embodying the safety circuit; and Figure 7 shows one form of the protection device which may be used in the figure 6 circuit.

The missile 1 shown in figure 1 includes a plurality of sub-systems for performing various functions associated with the missile. The only such sub-systems shown are a gyroscopic stabilising sensor sub-system 2, a guidance information receiver 3, a missile guidance computer 4 (which receives low level analog input signals from the gyroscopic sub-system 2 and the receiver 3) and a warhead arming unit 5 (which receives an arming initiation signal at input 13 by a safety interlock system - not shown) but as will be appreciated, various other sub-systems may be present. For clarity, the sub-systems are shown separated from the missile but obviously they are in fact installed within it.

The missile casing is divided into a plurality of sections two of which, the sections 6 and 7, are dimensioned, particularly as regards the length to diameter ratio thereof, so that together they form a wideband dipole antenna element able to receive electromagnetic radiation incident on the missile. The resulting signal is fed to a threshold detector arrangement including a wideband RF receiver/detector 8 and a comparator 9 for comparing the detected signal from the receiver/detector 8 with a reference level made available from a pre-set voltage source illustrated as a potentiometer 10. The receiver/detector 8 could comprise a simple untuned diode detector.

The reference level is made such that, if it is exceeded by the signal from receiver/detector 8, the level of radiation incident upon the missile is in excess of a predetermined safe level such that it may well interfere with the operation of the various sub-systems on-board the missile. In particular, it may induce signals which appear at the control inputs to the arming unit 5 and the computer 4 and cause spurious operation, for example, the warhead might become armed prematurely and the missile guidance might become erroneous. To avoid this, when the safe radiation level is exceeded, the comparator 9 produces a signal which is applied to the arming unit 5 so as to inhibit any desired response and to respective inhibit gates 11 and 12 in the control inputs to the computer 4 from the gyroscopic sub-system 2 and the receiver 3.While these gates are inhibited, the computer may remain operable to guide the missile on the basis of stored signals previously received at the control inputs.

As will be realised, the inhibit signal from comparator 9 may also be fed to any other missile sub-systems which might be adversely affected by the interference.

Instead of being applied to the inhibit gates 11 and 12, the inhibit signal could be applied directly to the computer 4, the latter incorporating suitable software which, in response to the inhibit signals, causes the relevant input signal to be ignored.

As may be seen, the figure 1 arrangement can, by positively inhibiting missile arming and removing possibly spurious signals at the sensitive control inputs to the missile guidance computer under conditions of high electromagnetic interference, increase safety and operational availability of the missile without recourse to expensive and possibly too heavy screening arrangements. Screening can still be provided of course, so as to prevent interference by more 'normal' levels of incident radiation but does have to be so extensive as to be effective against the relatively rare severe levels. The arrangement is preferably designed in a fairly simple form such as that shown so that it is inherently reliable and does not have to be constructed or set-up with any very great precision, thereby further improving its reliability.

It will be realised that various modifications may be made to the figure 1 arrangement. As mentioned earlier, the inhibit signal may be fed to control further sub-systems as well as the arming unit 5 and computer 4. In addition, it may be that the signal is fed to one of such other sub-systems instead of the arming unit 5 and computer 4.

The arming unit and/or computer might not even be present in the missile, or if they are, may take a different form to that described.

The same applies to the sub-systems 2 and 3, for example, a stabilising sensor may not be provided or it could be replaced by some other type of stabilising system such as an inertial guidance unit, and the receiver 3 is only needed if the missile is of the type which requires command information to be passed to it.

The electronic arming unit of figure 2 comprises an AND gate 21 which collects input signals supplied by a plurality of missile sub-systems and, provided these signals are appropriate, passes on an arming demand signal to a fuze arming unit (not shown). Only two of the possible sub-systems which might be required to contribute to the missile arming decision are shown, namely the inertial navigation sub-system 22 and the missile guidance receiver 23, and these are given by way of example (the form of the arming chain i.e. which and how many missile sub-systems are involved, obviously depends on the particular design of the missile). Each sub-system is connected to the AND gate 21 by a length of wire 24 and it may be that the dimensions of one or more of these lengths are such as to produce resonance with likely interference components.To reduce the effect of this, a fixed capacitor 25 has been connected in parallel with the path between two spaced points on the wire 24 from the inertial navigation sub-system 22, the value of the capacitor 25 being such as to detune the wire as regards the likely interference components. As will be appreciated, the prediction of resonant frequencies is sometimes difficult due to the complex nature of the impedances involved and possibly also the effects of any mutual coupling between the element for which the prediction is being made and any adjacent elements. As a result, it is generally impracticable to try to choose the dimensions of electrical connectors and such, in order to avoid resonance effects. The use of capacitors as shown however permits such avoidance, or at least reduction of the effects.

Instead of a fixed capacitor, a variable capacitor may be used and this will permit final trimming of the capacitance value during commissioning of the missile. Also, instead of a single capacitor, a network thereof including fixed and/or variable components, can be used. Different arrangements may be used on the connecting wires from different ones of the sub-systems. It may be that some of the sub-systems connecting wires will not require any detuning arrangement.

As shown for the connecting wire from the guidance receiver to the AND gate, the detuning element advantageously comprises an electronically variable capacitance diode 26 arranged to receive a control bias via resistor 27 from a receiver 28. The receiver 28 detects any incoming interference and automatically controls the capacitance of diode 26 to give optimum detuning of the associated connecting wire 24. Instead of a diode, the variable capacitance component 26 could comprise a mechanically variable capacitor (not shown), a moving vane type say, coupled to a servo-motor controller (not shown) which, in turn, is driven by the receiver 28.

As will be appreciated, for the best effect, it is preferred that the capacitors used are low inductance types and that they are connected to the respective connecting wires by wide, low inductance straps.

Figure 3 of the drawings shows a missile 30 having two metal body sections 31 and 32. Such a missile, when in flight, has an electrical response similar to that of a dipole aerial which means that it will be strongly resonant for electromagnetic interference having a frequency such that the length of the missile approaches half the wavelength of the interference. Interference at this resonant frequency may induce quite strong electrical currents in the body skin and these currents, in turn, may induce disturbing signals in the missile electronics.To avoid this, the dipole response of the missile body is adjusted, effectively altering the electrical length of the body, by connecting a capacitor 33 between two points a and b on the internal surface of the skin of body section 31, the points a and b being spaced by a distance approximately equal to one sixth of the natural resonant wavelength of the missile body. A typical value of the capacitor 33 might be around 500pF. It may be advantageous to similarly connect a second capacitor 34 but to the other body section 32.

The missile of figure 4 is similar to that of figure 3 except that here the fixed capacitors have been replaced by respective mechanically variable capacitors 40 and 41, for example of the moving vane type. The moving vanes of the respective capacitors are ganged together and coupled to be moved by a servo-motor 42 controlled by an interference sensing receiver 43. Together the elements form a feedback system with the capacitors being automatically adjusted to give minimum effect of the interference on the system. Instead of the mechanically variable capacitors 40 and 41 and servo-motor 42, electrically-controllable capacitors such as capacitance diodes could be used and be controlled directly by the receiver 43.

As will be appreciated, although figures 3 and 4 show missiles, the techniques described could be used to increase the interference rejection of any metallic structure, for example antennae, spacecraft and so on. Similarly, the technique described with reference to figure 2 could be applied to electronic and electrical equipment other than that used on board a missile.

In the embodiments mentioned above, extra protection may be provided by including a safety circuit which will fail-safe. Such a circuit consists of a semi-conductor diode which when subjected to radiation will fail and become a short circuit across a power supply.

The other components in the circuit are chosen so as to have a better resistance to radiation than the diode which is expected to fail.

Short circuits across the power supply will cause a large current flow and therefore a normal thermal fuze is fitted.

Figure 5 shows the principle of operation where the load 51 is in series with a switching circuit 52 and switches 54. Protection device 53 is in parallel with the load. A short circuit failure of 53 will cause failure of fuze 55.

In figure 6 the transistor switches 61 and 62 are hardened against radiation effects and in normal operation would switch the supply voltage across load 63. The protection circuit 64 is shown in more detail in figure 7 where D1 is a semi-conductor diode with little or no resistance to long or short term radiation effects. D2 is a thyristor capable of passing sufficient current to cause failure of the power supply fuze. R2 is for biasing and R1, C1 inhibit false triggering of the thyristor D2.

When the circuit is exposed to radiation D1 will fail and become short-circuited before any other component and so break the power supply circuit to the load.

Claims

1. Apparatus including at least one electronic sub-system, which sub-system comprises signal supply means and response-producing means for producing an output response dependent upon signals supplied by the signal supply means, the apparatus further including antenna means and detector means for sensing extraneous RF electromagnetic radiation incident on said apparatus, and sub-system response control means responsive to said detector to affect the operation of said sub-system to at least reduce the susceptibility of the apparatus to any ill effects of said extraneous electromagnetic radiation.

2. Apparatus according to claim 1, wherein said antenna means is formed by at least one portion of the apparatus casing.

3. Apparatus according to claim 1 or 2, wherein said antenna means comprises two adjacent apparatus casing portions defining a wideband dipole element.

4. Apparatus according to any one of claims 1 to 3, wherein said sub-system response control means is operable to inhibit the relationship between said output response and said signals supplied by said signal supply means when a level of radiation exceeding a predetermined safe-level is incident upon the apparatus.

5. Apparatus according to claim 4 wherein, when the incident radiation exceeds said safe-level, said sub-system response control means operates to cause said output response to be generated on the basis of signals previously received from the signal supply means.

6. Apparatus according to any one of claims 1 to 3, wherein signal supply means comprise an electrical conductor member, and said sub-system response control means includes controllably-variable reactance means controlled by said detector means and connected between two spaced positions on said electrical conductor member for detuning said member with respect to said received radiation.

7. Apparatus according to claim 6, wherein said controllably-variable reactance means comprises variable capacitance means.

8. Apparatus according to claim 7, wherein said variable capacitance means is an electronically variable capacitance diode.

9. Apparatus according to claim 7, wherein said variable capacitance means is a mechanically variable capacitor.

10. Apparatus according to any preceding claim including reactance means connected to a metallic member forming part of the apparatus for detuning said member with respect to extraneous RF radiation incident upon the apparatus.

11. Apparatus according to claim 10, wherein said metallic member comprises a metal part of a housing or casing of the apparatus.

12. Apparatus according to claim 10, wherein said metallic member comprises an electrical signal supply lead within the apparatus.

13. Apparatus according to claim 10, 11 or 12, wherein said reactance means is variable.

14. Apparatus according to claim 11, 12 or 13, wherein said reactance means is a capacitor.

15. Apparatus according to any preceding claim, wherein said apparatus includes at least one electronic sub-system including a device which will fail at lower levels of incident radiation than other components of the sub-system and cause the sub-system to fail-safe.

16. Apparatus according to any preceding claim and substantially as hereinbefore described with reference to the accompanying drawings.

17. Apparatus according to any preceding claim and substantially as hereinbefore described with reference to the accompanying drawings.

17. A missile including a sub-system which has a control input and normally responds to signals received at said input to produce an output response associated with the missile, and the missile further includes protection means for reducing the effect of electromagnetic interference upon the missile, the protection means comprising sensing means for sensing electromagnetic radiation incident upon the missile, detecting means connected to the sensing means and to said sub-system and operable for detecting a level of said radiation exceeding a predetermined safe level and for then inhibiting the relationship between said output response and said signals received at said input.

18. Apparatus which includes electrical or electronic equipment and, so as to reduce the possibility of impairment of the operation of the equipment due to electromagnetic interference, capacitor means connected in parallel with an electrical path through, and between two spaced positions on, an electrically-conductive portion of the apparatus so as to detune said conductive portion with respect to interference components for which the portion might otherwise have a resonant relationship.

19. A safety circuit characterised by the inclusion of a device which will fail at lower levels of radiation than other components of the circuit and cause the safety circuit to fail-safe.

Amendments to the claims have been filed as follows 1. Apparatus including a casing containing at least one electronic sub-system, which sub-system comprises signal supply means and response-producing means for producing an output response dependent upon signals supplied by the signal supply means, the apparatus further including detector means coupled to said casing for sensing extraneous RF electranagnetic radiation incident on said apparatus, and sub-system response control means responsive to said detector means to affect the operation of said sub-system to at least reduce the susceptibility of the apparatus to any ill effects of said extraneous electromagnetic radiation.

2. Apparatus according to claim 1, wherein said casing comprises two adjacent portions defining a wideband dipole element.

3. Apparatus according to claim I or 2, wherein said sub-system response control means is operable to inhibit the relationship between said output response and said signals supplied by said signal supply means when a level of radiation exceeding a predetermined safe-level is incident upon the apparatus.

4. Apparatus according to claim 3, wherein when the incident radiation exceeds said safe-level, said sub-system response control means operates to cause said output response to be generated on the basis of signals previously received from the signal supply means.

5. Apparatus according to claim 1 or 2, wherein signal supply means comprise an electrical conductor member, and said sub-system response control means includes controllably-variable reactance means controlled by said detector means and connected between two spaced positions on said electrical conductor member for detuning said member with respect to said received radiation.

6. Apparatus according to claim 5, wherein said controllably-variable reactance means comprises variable capacitance nedllS to c1aimk wherein said variable L wfierein said variable capacitance means is an electronically variable capacitance diode.

8. Apparatus according to claim 6, wherein said variable capacitance means is a mechanically variable capacitor.

9. Apparatus according to claim 1 including reactance means connected to a metallic member forming part of the apparatus for detuning said member with respect to extraneous RF radiation incident upon the apparatus.

10. Apparatus according to claim 9, wherein said metallic member comprises part of the casing.

11. Apparatus according to claim 9, wherein said metallic member comprises an electrical signal supply lead within the apparatus.

12. Apparatus according to claim 9, 10 or 11, wherein said reactance means is variable.

13. Apparatus according to claim 10, 11 or 12, wherein said reactance means is a capacitor.

14. Apparatus according to any preceding claim, wherein said apparatus includes at least one electronic sub-system including a device which will fail at lower levels of incident radiation than other components of the sub-system and cause the sub-system to fail-safe.

15. Apparatus according to any one of the preceding claims, wherein a safety circuit is provided which will fail at lower levels of radiation than other components of the circuit and cause the safety circuit to fail-safe.

16. A missile including a casing containing a sub-system which has a control input and normally responds to signals received at said input to produce an output response associated with the missile, and the missile further includes protection means for reducing the effect of electromagnetic interference upon the missile, the protection means comprising sensing means for sensing electromagnetic radiation incident upon the casing, detecting means connected to the sensing means and to said sub-system and operable for detecting a level of said radiation exceeding a predetermined safe level and for then inhibiting the relationship between said output response and said signals received at said input.