VIBRATION-SENSITIVE TRANSDUCER

Abstract: A transducer in the form of a long cable for producing electrical signals in response to mechanical vibration of the cable. The cable comprises an elongate magnetic member (2) and an electrical conductor (3a, 3b, 5a, 5b) extending alongside the magnetic member (2), the electrical conduc­ tor (3a, 3b, 5a, 5b) being loosely mounted in the magnetic field of the magnetic member (2) so that mechanical vibra­ tion of the cable causes the electrical conductor (3a, 3b, 5a, 4b) to move relative to the magnetic field whereby an e.m.f. and hence an electrical signal is induced in the conductor (3a, 3b, 5a, 5b). The transducer (1) may be used in monitor­ ing or security apparatus.

Description: The present invention relates to a linear vibration-sensitive transducer designed primarily to be attached to perimeter fences or other structures although it may well find applications in other fields not directly related to the security industry. The transducer operates by producing electrical signals in response to mechanical vibrations impinging on the outer surface of the device. When the signals thus produced are electronically processed, it is possible to determine the nature of the disturbance causing the signals, and hence give warning of impending malicious acts. To date, most intruder alarm systems employing linear vibration sensors usually rely on either the natural microphony of a co-axial type cable, in which the signal generation mechanism is attributed to the tribo-electric effect, or a modification of this type in which the dielectric material of the cable is subjected to some form of processing which enhances the sensitivity of the cable. This latter family of devices include, among others, electret and piezo-electric type cables. These types of sensor, while producing adequate signal levels, are by no means ideally suited to the application outlined. The salient disadvantages of these sensors are detailed below. 1) High output impedance: If, as often is the case, the analogue signals from the sensor cable have to be returned to a central monitoring point, some form of buffer circuitry must be interposed between the sensor and the interconnecting cable so that the higher frequency components of the signal are not attenuated by the cable capacitance. Of necessity, this circuitry must be physically adjacent to the sensor and may be exposed to extremes of climatic conditions thus reducing system reliability. Also, the buffer circuitry requires a power source which, in most cases, will necessitate an additional pair of conductors to be routed to the interface box. This additional cabling involves a significant increase in the installed cost and is hence undesirable. High source impedances also pre-empt the realisation of a truly low-noise, high-gain system, since a pre-requisite of a low-noise system is a low source impedance. 2) Variations in sensor sensitivity: In the case of processed dielectric cables, which account for the largest percentage of this type of system, the degree of sensitivity is related to the level of electrical stress which can be applied to the dielectric during the sensitisation process. Since it is impossible to guarantee 100% uniformity of cable dimensions during manufacture, it follows that the maximum stress which can be applied to the cable must be modified to prevent breakdown at the weakest point in the cable. This means that all other points along the length of the cable will be less sensitive than the weakest section(s) as well as exhibiting variations of sensitivity at other manufacturing discontinuities. High cost per metre: Most of the sensors in use at the present time utilise a PTFE dielectric material which in itself is an expensive material; however a major problem in producing long lengths of PTFE cable is that it is difficult to obtain large batches of raw PTFE of sufficiently uniform characteristics to ensure close tolerances during manufacture. For this reason the cost of lengths significantly in excess of 100 metres tends to be considerably more expensive pro rata than lengths of 1000 metres or less. These aforementioned points highlight the major disadvantages of the co-axial type of sensor. The type of sensor described in the following text operates on an entirely different principle to those outlined earlier. Whereas the co-axial types of sensor involve the movement of conductors in an electric field, this sensor relies on the movement of conductors in a magnetic field. As is well known, the mechanical movement of a conductor in a magnetic field will result in the generation of an EMF across the ends of the conductor. In such sensors a conductor is arranged in such a way as to allow it to move within a magnetic field in response to externally generated mechanical vibrations impinging on the outer sheath of the sensor. Since it is possible to obtain flexible magnetic materials, it is possible to construct a vibration sensor based on this principle and offering the advantages of the earlier types of cable sensors. Such a sensor is described in US-A-3,659,257 (Witzell) which is a line-type hydrophone for use in seismographic surveys. In this hydrophone a flexible magnetic strip is housed inside a closed-cell plastic foam sheath around which is wound longitudinally an electrical conductor held firmly in contact with the sheath by adhesive tape. The sheath is to some extent flexible so that in response to certain mechanical disturbances of the cable the wire can move (flexing the sheath) relative to the magnet to have an electrical current induced in it. According to the present invention there is provided a cable type transducer for producing electrical signals in response to a mechanical vibration or impulse comprising an elongate structure comprising magnetic flux generating means and an electrical conductor movable, in response to mechanical disturbance of the cable, in the magnetic field generated by the flux generating means to produce said signals, characterized in that the electrical conductor is loosely constrained laterally of the magnetic flux generating means so as to be freely movable between longitudinally extending constraints in response to said mechanical disturbance. Preferably, the flux generating means comprises an elongate portion of flexible magnetised material. This can be in the form of a tape with the electrical conductor or conductors, for example, along one edge (where there is one) or two electrical conductors (where there are two), for example one on either edge. The conductors may be made of very fine wire loosely fitting into a small bore tube attached to, or integral with, the flux generating means. To reduce the sensitivity of the transducer to external magnetic fields, the conductor or conductors may be arranged so as to be non-inductive with respect to an external applied field; this may for example be achieved by wiring them in a circuit with additional conductors which are fixed so as not to vibrate in the magnetic field and thereby not to have EMFs induced in them. Furthermore, they may be arranged so as to produce a balanced output relative to a 'centre tap'; the centre tap may be provided by one or more of the fixed conductors just mentioned, or, if the magnetised core is conductive, by the core. Wiring configurations which achieve the above functions will be described below. In an alternative arrangement, a wire loosely fitting within its sleeve may be arranged along the core in a "twisted pair" configuration with a conductor which is not movable relative to the core, e.g. a conventional insulated wire fixed to the core. The use of a twisted pair gives high inherent noise immunity, while having one wire fixed avoids signal cancellation as compared with having two loosely mounted wires. These features (the twisted pair arrangement and the use of a fixed conductor) may be used independently of one another. In an alternative embodiment the transducer may be in the form of a cable comprising a generally circular cross-section magnetized core having slots in its surface which house the electrical conductors. The core may be formed in one piece or from two or more complementary similarly-shaped parts, e.g. two parts having generally semi-circular cross-section and having recesses in its surface which, when the two halves are assembled, define the longitudinally extending slots for the conductors. In order to allow the active conductors to move with respect to the magnetic field generated by the core in response to a vibration of the cable while holding the return conductors stationary with respect to the magnetic field, the slots for the return conductors are such that the return conductors are held in a fixed position in these slots, while the slots for the active conductor are larger than the diameter of the conductor. The assembly of core and conductors is preferably sheathed with an outer jacket material serving to protect the cable and may also serve to prevent the active conductors being displaced from their slots. An electrostatic shield of foil tape may be wound around the core and conductors before sheathing with the outer jacket. In order to prevent ingress of the jacket material into the slots an extruded plastics section may be pressed into the slots before the jacket is added. The construction of the cable in an overall circular cross-section has the advantages as compared with a tape that various standard accessories and components (e.g. grommets and glands) can be used with it, that manufacture is more straightforward and that a more intense field, within which the conductor(s) move can, be generated. The invention further provides a monitoring apparatus, for security-, hazard- or other state-monitoring purposes which, in use, is coupled to the transducer and includes the circuitry responsive to an EMF induced in a the transducer to produce an electrical signal indicative of said state; the transducer may be one per se embodying the invention. The invention will be further described by way of non-limited example with reference to the accompanying drawings in which:- Figure 1a is an elevational view of one embodiment of transducer according to the present invention; Figure 1b shows, to a large scale, a schematic detail of the transducer of Figure 1a; Figure 2 illustrates the pattern of magnetic flux in the transducer core; Figures 3, 4 and 5 show further embodiments of transducer according to the present invention; Figure 6 illustrates schematically the cross-section of the embodiment of Figure 5; Figure 7a is a view, corresponding to Figure 1a, of a further embodiment of the invention; Figures 7b and 7c are details of the conductors of the transducer. Figure 8a shows the core construction of a second embodiment of the present invention; and Figure 8b is a cross-sectional view of the second embodiment. Figure 1 shows the general arrangement of conductors in a first embodiment of the invention and their physical placement in relation to core 2 of the magnetic material. The magnetic core 2, which is the central component of the assembly, comprises a flexible strip of metallic material which is permanently magnetised by subjecting it to a powerful external magnetic field; alternatively the core 2 could be made of a plastics material, preferably electrically conductive as it is advantageous for the core to be connected to the system earth of the monitor to which the transducer 1 is connected. The direction of polarisation of the field is at right-angles to the length of the strip and the sense of the field is the same throughout the length of the strip. In order that the final assembly be flexible enough, the strip is suitably only of the order of 0.05mm thick, while the strip width is in the region of 6mm. Figure 2 shows the distribution of the magnetic flux as would be seen looking on the end of a section of the strip. Along each edge of the core 1 is a respective conductive wire 3a, 3b which is loosely fitted in a respective small bore tube or sheath 4a, 4b secured to, or integral with, the core 1. The wires 3a, 3b are thus adjacent to the respective poles of the core 1. Any vibration transmitted through the sheath of the assembly will cause the wire to vibrate in sympathy and in doing so will induce an E.M.F. in the wire as a result of its movement within the magnetic field. In Figure 1A, the signal output SO is taken from the left-hand ends of conductors 3a and 5a and is balanced with respect to a centre tap CT provided by the connection between the left-hand ends of conductors 3b and 5b. It will be seen that at the remote, righthand end, the conductors cross over, conductor 5a being connected to conductor 3b and conductor 5b to conductor 3a. All the conductors may be enamel coated or otherwise insulated. Figure 1b is intended to facilitate an appreciation of the operation of the embodiment of Figure 1A and shows the input impedance Z of the monitoring circuit. The monitoring circuit can include any suitable type of analogue and digital circuitry or both for suitably monitoring the output of the transducer and producing one or more signal outputs indicative of the state which it is desired to monitor. The circuitry may include signal processing circuitry for monitoring one or more characteristics of the transducer output e.g. spectral content, amplitude or duration, which depends on the vibration or movement which it is desired to detect. The overall arrangement of the conductors shown in Figure 1A is chosen to maximise the signal level caused by mechanical vibration while minimising the signals induced by external electro-magnetic fields. This is achieved by arranging the loop formed by the conductors to be non-inductive as far as external electro-magnetic fields are concerned. To this end, return conductors 5a, 5b (i.e.- not the vibrating conductors 3a,3b) secured relative to the core 1 via the extruded sheathing material run along a path which follows the active conductor as closely as possible so that the overall conductor assembly appears to be a bifilar-wound loop i.e. two loops of opposite senses, which will tend to cancel the effect of externally induced magnetic fields. The ultimate realisation of this goal can be achieved by utilising a twisted pair of wires adjacent to each pole of the magnetic material. Normally this would mean that the desired signal resulting from the vibrations received would also be minimised except that one wire of the pair is fixed in relation to the magnetic field. This is achieved by using a much thicker wire which is 'locked' in position by the outer sheathing material. The net result therefore, is that mechanical vibrations impinging on the assembly will induce E.M.F.s in the conductors which are free to move in the magnetic field, thus causing a current to flow in the loop formed by the conductors and the terminating impedance. Moving magnetic fields caused by external electrical disturbances however, will tend to cause currents to flow in the loop elements which cancel out as far as the resultant signal appearing across the terminating impedance is concerned. Signals resulting from mechanical vibrations are enhanced by arranging the connection of the two 'active' conductors so that the E.M.F.s induced by the vibrations are additive. Considering an impact occurring at one particular point on the sensor, it is reasonable to assume that both 'active' conductors will experience an acceleration in the same direction due to the impact. Since the direction of polarisation of the magnetic field is the same for both conductors, the direction of induced current flow in each conductor will also be the same. If the two conductors were now connected at the same end of the assembly, the currents induced in the loop now formed with the terminating impendance would tend to cancel, thereby reducing the sensitivity of the sensor. Figure 3 shows this less advantageous arrangement where there are just the two sheathed conductors 3a and 4a attached to opposite sides of the core 2. If however, the conductors are arranged as shown in Figure 1, it can be seen that currents induced in the same direction in each 'active' conductor will be additive, thereby improving the overall sensitivity of the sensor. In practice it has been found that reasonable signal levels are achieved using the arrangement shown in Figure 3. This can be attributed to the fact that, due to mechanical limitations, both conductors will never move exactly in sympathy with each other, so a resultant current will always be generated. Further improvement in rejection of unwanted interfering signals can be realised by earthing the metallic magnetic strip, thus providing a degree of electrostatic shielding adjacent to the conductors. Figure 4 illustrates a variation on the arrangement shown in Figure 3. In this case, a metallic magnetic strip 2 is used as the return conductor instead of having separate conductors such as 5a and 5b in Figures 1A and 1B so that the currents induced in the active conductors are now in-phase, thus increasing the sensitivity of the sensor. This arrangement however, in common with that of Figure 3, may be prone to externally induced electro-magnetic interference. Figure 5 shows a variation on the arrangement of Figure 1. In this case, the active conductor/tube assemblies 3a, 4a, 3b, 4b are mounted on the face of the magnetic strip 2 adjacent to the edges. When the extrusion process is complete and the conductors and magnetic strip are "locked" in place relative to each other, it is possible to "fold" the assembly in half as shown in Figure 6. If, as shown, the "active" conductors 3a,3b are "sandwiched" together between the folded magnetic strip 2, they are now subject to a more intense magnetic field as a result of the reduction in the air-gap between the poles of the magnet. The magnetic strip now appears to be similar to the familiar "horseshoe" type of magnet. The operation of folding the sensor in half is simple to implement during the installation of such a sensor on say, a perimeter fence since the sensor is normally fixed in place with plastic ties which could also serve to maintain the assembly in the "folded" condition. Depending on the thickness of the sheathing material of the tubes 4a, 4b and the finished rigidity of the assembly, it may be necessary to use a slightly wider magnet strip in order to be able to completely fold the assembly into a "U" shape. The fold could, of course, be imparted during manufacture, if desired. Figures 7a to 7c show a further embodiment of the invention which is intended to reduce or eliminate these disadvantages. Here, the conductors 3a, 3b have been replaced by an element A comprising a conductor 3, again loosely fitting within its sheath 4 and so movable relative to the magnetic core 2, arranged in a "twisted pair" with, and connected at the remote end by a conductor link 8 to an element B comprising a wire 6 which is a conventional wire with a conducting core 7 and a close-fitting sheath 9 and so the conducting core 7 is unable to move relative to the magnetic core 2 to the same degree as is the wire 3. The twisted pair of conductors are held in place, in contact with the surface of the metallic strip by the outer jacket of the cable. The use of the wires in a twisted pair configuration overcomes the first problem of induced interference as it has inherent immunity to external electromagnetic interference. The use of the wire 6 is to overcome the problem of signal cancellation, since it will not vibrate within the magnetic field of the core 2 and will therefore prevent signal cancellation occurring. In order to maintain a balanced system, both conductors in this arrangement will be identical in respect of factors determining resistance material, cross-sectional area, construction, temperature coefficient etc. Figures 8a and 8b show a second embodiment of the invention where the transducer is in the form of a cable rather than a tape. As shown in Figure 8a the core 2 is constructed from two identically shaped parts 2 a and 2 b of magnetic material. Each part is generally of semi-circular cross-section and has formed in its surface a rectangular recess 12 a or 12 b which defines a longitudinally extending slot and two longitudinally extending, oppositely disposed radiused rebates 10 a and 10 b . When assembled as shown the two parts form a core having two pairs of longitudinally extending slots 10 and 12 symmetrically disposed about the plane 11. The radiused rebates are of such a size that the pair of slots 10 they define in the assembled core is wider than the pair of rectangular slots 12. The two parts 2 a and 2 b of the core are magnetized to give the pole configurations shown in Figure 8B and designated by "N" and "S". Such a pole configuration has the advantage that the two parts of the core tend to attract each other which assists manufacture. The transducer is constructed as shown in Figure 8b. The conductors 13 a , 13 b , 14 a and 14 b are all the same with return conductors 14 a , 14 b in slots 12 and active conductors 13 a , 13 b in slots 10. The conductors have such a diameter that the return conductors 14 a , 14 b are held firmly in fixed position in the slots 12, but the active conductors 13 a , 13 b are free to move in slots 10 in response to mechanical vibration of the cable and thereby generate the electrical signals. The core and conductors are sheathed in an outer jacket 15 which is of plastics material extruded onto the cable. The outer jacket 15 serves the dual function of protecting the cable and preventing the active conductors 13 a , 13 b from coming out of the slots 10. A layer of foil tape 16 may be wound around the core and conductors before the outer jacket 15 is put on. This provides an electrostatic screen and also prevents ingress of the jacket material into the slot. An alternative way of preventing this ingress is to press an extruded plastics section into the slots before putting on the jacket. The conductors may be connected in the similar configurations to those described in connection with the first embodiment to provide a non-inductive loop on a balanced or differential output. An alternative way of allowing the active conductors to move while holding the return conductors still is to make all the longitudinally extending slots equal width and use return conductors of a larger diameter than the active conductors. The major advantages of the illustrated sensor constructions include:- 1) Low output impedance: Since the signal produced is developed across a copper conductor or other good conductor, the output impedance is very low. This enables the signal to be transmitted along much longer lines without degradation than is possible with the co-axial type of sensors. Also, a much higher signal to noise ratio can be realised for the reasons outlined above. 2) Uniform signal response: It is comparatively easy to ensure that the magnetic material is subjected to exactly the same magnetising force along the length of the strip. Consequently the signals induced will be less likely to vary in amplitude for a given force than is the case with the co-axial sensors. Also, since the magnetic strip can be metallic in nature, it is fairly simple to ensure its dimensional accuracy during manufacture. This should result in a more uniform sensor construction than has been possible previously. Another feature of this type of sensor is that since the magnetic effect is, to all intents and purposes, permanent, there should be no loss of sensitivity due to ageing of the cable. This contrasts with the electret-type of cables show inherent charge decays over a period of time resulting in loss of sensitivity. 3) Balanced output available (in Figures 1 and 4): Since a 'centre-tap' of the sensor is available at each end of the assembly, it is inherently a balanced type of line which enables a high degree of common-mode rejection to be realised. This reduces the complexity of the processing circuitry necessary to otherwise achieve this. The above described embodiments of the invention have both conductors as being able to move within their sheath. A problem with this arrangement is that, being a simple loop, it is susceptible to induced interference from external electromagnetic fields. Also, since each conductor tends to accelerate in the same direction under the influence of an external mechanical distrubance, the currents induced in each conductor will be in the same direction and thus will tend to cancel when connected as shown.

Claims: A cable type transducer (1) for producing electrical signals in response to a mechanical vibration or impulse comprising an elongate structure comprising magnetic flux generating means (2) and an electrical conductor (3a, 13a, 13b) movable, in response to mechanical disturbance of the cable, in the magnetic field generated by the flux generating means (2) to produce said signals, characterized in that the electrical conductor (3a) is loosely constrained laterally of the magnetic flux generating means (2) so as to be freely movable between longitudinally extending constraints (4a, 10, 12) in response to said mechanical disturbance. A cable type transducer according to claim 1 characterised in that the magnetic flux generating means (2) is formed from a permanently magnetised metallic or plastic material. A cable type transducer according to claim 1 or 2, characterised in that the electrical conductor (3a, 13a, 13b) is disposed so as to be non-inductive with respect to an externally applied electro-magnetic field. A cable type transducer according to any one of the preceding claims, characterised in that the magnetic flux generating means (2) is in the form of a tape. A cable type transducer according to any one of the preceding claims, characterised in that said electrical conductor (3a) comprises at least one run, lengthwise of the cable, of a wire loosely fitting into a sheath (4a) whose walls form said constraints. A cable type transducer according to claim 5, characterised in that the sheath (4a) is integral with the magnetic flux generating means (2). A cable type transducer according to claim 6 characterised in that said conductor (3a, 13a, 13b) comprises two runs of conductor arranged in a twisted pair configuration. A cable type transducer according to claim 1, 2 or 3, characterised in that the electrical conductor (13a, 13b) is located in slots (10, 12) in the periphery of a cylindrical core of the cable, the walls of the slots forming the said constraints. A cable type transducer according to claim 8, characterised in that the core (2) constitutes the magnetic flux generating means. A cable type transducer according to claim 8 or 9, characterised in that the core (2) comprises two parts (2a, 2b) meeting on a diameter of the cross-section of the core (2). A cable type transducer according to claim 10, characterised in that at least one slot (12) is wholly defined in one of the parts of the core (2a, 2b) and at least one slot (10) is defined by both parts of the core (2a, 2b) and straddles their common diameter plane. A cable type transducer according to claim 10 or 11 characterised in that said electrical conductor comprises two runs, one (13a, 13b) housed in a first slot (10) whose width is larger than the diameter of the electrical conductor (13a, 13b) it houses, and a second run (14a, 14b) housed in a further slot or slots (12), the width of the further slot (12) being such that the second run (14a, 14b) of electrical conductor is held in a fixed position in the further slot (12). A cable type transducer according to claim 12 wherein the first slot (10) straddles the common diameter plane of two parts of the core (2). A cable type transducer according to any one of the preceding claims wherein the movable electric conductor (3a, 3b, 13a, 13b) consists in a movable run extending along the structure, is connected with a fixed run of electrical conductor (7, 14a, 14b) so as to provide a balanced output. A cable type transducer according to any one of the preceding claims wherein the movable electrical conductor or run of electrical conductor (3b, 14a, 14b) is connected with an electric conductor (6) fixed in position relative to the magnetic flux generating means so as to provide a differential output. A cable type transducer according to claim 16 characterised in that the magnetic flux generating means (2) constitutes the fixed position electrical conductor (6). Monitoring apparatus including a cable type transducer according to any one of the preceding claims.

Inventors: MACALINDIN, IAN
Applicants: GEOQUIP SECURITY SYSTEMS LIMITED
Application year: 1986
Main classification: G01D5
IPC codes: G01H11, G01V1