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 and an electrical conductor extending alongside the magnetic member, the electrical conductor being loosely mounted in the magnetic field of the magnetic member so that mechanical vibration of the cable causes the electrical conductor to move relative to the magnetic field whereby an e.m.f. and hence an electrical signal is induced in the conductor. The transducer may be used in monitoring or security apparatus.

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 meter 
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 meters tends to be considerably 
more expensive pro rata than lengths of 1000 meters or less. 
These aforementioned points highlight the major disadvantages of the 
co-axial type of sensor. 
The transducer described in the following text operates on an entirely 
different principle to those sensors 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 the transducer described, a conductor is arranged in 
such a way as to allow it to vibrate within a magnetic field in response 
to externally generated mechanical vibrations impinging on the outer 
sheath of the transducer. Since it is now possible to obtain flexible 
magnetic materials, it is possible to construct a vibration sensor in the 
form of a transducer based on this principle and offering the advantages 
of the earlier types of cable sensors. 
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 thereby to produce said signals. 
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 may be of the same 
size or preferably slightly smaller than the diameter of the conductors, 
while the slots for the active conductors are larger than the diameter of 
the conductors. 
The assembly of core and conductors is preferably sheathed with an outer 
jacket material which serves to protect the cable and which 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- or 
hazard- or other state-monitoring purposes which, in use, is coupled to 
the transducer and includes the circuitry responsive to an EMF induced in 
the transducer to produce an electrical signal indicative of said state; 
the transducer may be one per se embodying the invention.

FIGS. 1A and 1B show the general arrangement of conductors of a cable-type 
transducer in a first embodiment of the invention and their physical 
placement in relation to a core 2 of magnetic material. The magnetic core 
2, which is a flux generating mechanism and the central component of the 
transducer 1, comprises an elongate portion in the form of a flexible 
strip or tape 2s of magnetised 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 2s 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 2s is suitably only of the order of 0.05 mm thick, while 
the strip width is in the region of 6 mm. FIG. 2 shows the distribution of 
the magnetic flux as would be seen looking on the end of a section of the 
strip 2s. 
Along each edge of the core 2 is a respective active conductor wire 3a, 3b 
which is loosely fitted in a respective small bore tube or sheath 4a, 4b 
secured to, or integral with, the core 2. The conductors 3a, 3b are thus 
adjacent to the respective poles of the core 2. Any vibration transmitted 
through the sheaths 4a, 4b of the transducer 1 will cause the conductors 
3a, 3b to vibrate in sympathy and in doing so will induce an E.M.F. in the 
conductors as a result of their movement within the magnetic field. 
In FIG. 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 3a, 
3b, 5a and 5b, cross over, conductor 5a being connected to conductor 3b 
and conductor 5b to conductor 3a. All the conductors 3a, 3b, 5a and 5b; 
may be enamel coated or otherwise insulated. FIG. 1B is intended to 
facilitate an appreciation of the operation of the embodiment of FIG. 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 1 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 3a, 3b, 5a and 5b shown in FIG. 
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 3a, 3b, 5a 
and 5b 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 
active conductors 3a, 3b) secured relative to the core 2 via extruded 
outer sheathing material (not shown) run along a path which follows the 
active conductors 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 conductors adjacent to each pole of the 
magnetic material strip 2s. Normally this would mean that the desired 
signal resulting from the vibrations received would also be minimised 
except that one conductor of the pair is fixed in relation to the magnetic 
field. This is achieved by using a much thicker conductor which is 
`locked` in position by the outer abovementioned sheathing material (not 
shown). The net result therefore, is that mechanical vibrations impinging 
on the transducer 1 will induce E.M.F.s in the conductors 3a, 3b which are 
free to move in the magnetic field, thus causing a current to flow in the 
loop formed by the conductors and a 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 transducer 1, it is reasonable to assume that both 
`active` conductors 3a, 3b 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, 3a, 3b the direction of 
induced current flow in each conductor will also be the same. If the two 
conductors 3a, 3b were now connected at the same end of the transducer 1, 
the currents induced in the loop now formed with the terminating impedance 
would tend to cancel, thereby reducing the sensitivity of the transducer 
1. FIG. 3 shows this less advantageous arrangement where there are just 
two sheathed conductors 3a and 3b attached to opposite sides of a 
magnetized metallic strip 2s of a core 2. 
If however, the conductors 3a, 3b, 5a and 5b are arranged as shown in FIGS. 
1A and 1B, it can be seen that currents induced in the same direction in 
each `active` conductor 3a, 3b will be additive, thereby improving the 
overall sensitivity of the transducer 1. In practice it has been found 
that reasonable signal levels are achieved using the arrangement shown in 
FIG. 3. This can be attributed to the fact that, due to mechanical 
limitations, both active conductors 3a, 3b 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 2s, thus providing a 
degree of electrostatic shielding adjacent to the conductors 3a, 3b, 5a 
and 5b. 
FIG. 4 illustrates a variation on the arrangement shown in FIG. 3. In this 
case, a metallic magnetic strip 2s is used as the return conductor instead 
of having separate conductors such as 5a and 5b in FIGS. 1A and 1B so that 
the currents induced in the active conductors 3a and 3b are now in-phase, 
thus increasing the sensitivity of the transducer 1. This arrangement 
however, in common with that of FIG. 3, may be prone to externally induced 
electro-magnetic interference. 
FIGS. 5 and 6 show a variation on the arrangement of FIGS. 1A and 1B. In 
this case, active conductor/tube assemblies including active conductors 
3a, 3b, and tubes 4a, 4b, are mounted on the face of a magnetic strip 2s 
adjacent to the edges. When the extrusion process is complete and the 
conductors 3a, 3b, 5a and 5b, and magnetic strip 2s are "locked" in place 
relative to each other, it is possible to "fold" the resultant assembly in 
half as shown in FIG. 6, to form a folded transducer 1. If, as shown, the 
"active" conductors 3a,3b are "sandwiched" together between the folded 
magnetic strip 2s, 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 2s now appears to be similar to the familiar 
"horseshoe" type of magnet. The operation of folding the sensor transducer 
in half is simple to implement during the installation of such a 
transducer on say, a perimeter fence since the transducer is normally 
fixed in place with plastic ties which could also serve to maintain the 
transducer in the "folded" condition. Depending on the wall thickness of 
the sheaths or tubes 4a, 4b in which the active conductors 3a, 3b are 
received, and the finished rigidity of the assembly, it may be necessary 
to use a slightly wider magnet strip 2s 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. 
FIGS. 7A to 7C show a transducer 1 in accordance with a further embodiment 
of the invention. Here, the conductors 3a, 3b have been replaced by an 
element A comprising a conductor 3, again loosely fitting within a sheath 
4 and movable relative to a magnetic core 2. The conductor 3 is arranged 
in a "twisted pair" with, and connected at a remote end of the core 2 by a 
conductor link 8, to an element B comprising a conductor 6 which is a 
conventional wire with a conducting core 7 and a close-fitting sheath 9, 
so that the conducting core 7 is unable to move relative to the magnetic 
core 2 to the same degree as is the conductor 3. 
The twisted pair of conductors 3 and 6 are held in place, in contact with a 
magnetized surface of the metallic strip 2s by an outer jacket (not shown) 
of the resultant cable. 
The use of the conductors 3 and 6 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 conductor 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 3 and 6 in this arrangement will be identical in respect of 
factors determining resistance material, cross-sectional area, 
construction, temperature coefficient etc. 
FIGS. 8A and 8B show another embodiment of the invention where a transducer 
1 is in the form of a circular cable rather than a tape. As shown in FIG. 
8a a core 2 is constructed from two identically shaped parts 2a and 2b of 
magnetic material. Each part 2a and 2b is generally of semi-circular 
cross-section and has formed in its surface a rectangular recess 12a or 
12b which defines a longitudinally extending slot and two longitudinally 
extending, oppositely disposed radiused rebates 10a and 10b. When 
assembled as shown the two parts 2a and 2b form the core 2 having two 
pairs of longitudinally extending rectangular slots 10 and 12 (FIG. 8B) 
symmetrically disposed about a plane 11. 
The radiused rebates 10a and 10b are of such a size that the pair of 
rectangular slots 10 they define in the assembled core 2 is wider than the 
pair of rectangular slots 12. 
The two parts 2a and 2b of the core 2 are magnetized to give the pole 
configurations shown in FIG. 8B and designated by "N" and "S". Such a pole 
configuration has the advantage that the two parts 2a and 2b of the core 2 
tend to attract each other which assists manufacture. 
The transducer 1 is constructed as shown in FIG. 8B. A plurality of 
conductors 13a, 13b, 14a and 14b are all the same with return conductors 
14a, 14b in slots 12 and active conductors 13a, 13b in slots 10. 
The conductors 13a, 13b, 14a and 14b have such a diameter that the return 
conductors 14a, 14b are held firmly in fixed position in the slots 12, but 
the active conductors 13a, 13b are free to move in slots 10 in response to 
mechanical vibration of the cable and thereby generate electrical signals. 
The core 2 and conductors 13a, 13b, 14a and 14b 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 13a, 13b from coming out of the slots 10. 
A layer of foil tape 16 may be wound around the core 2 and conductors 13a, 
13b, 14a and 14b before the outer jacket 15 is put on. This provides an 
electrostatic screen and also prevents ingress of the jacket material into 
the slots 10 and 12. An alternative way of preventing this ingress is to 
press an extruded plastics section into the slots 10 and 12 before putting 
on the jacket 15. 
The conductors 13a, 13b, 14a and 14b may be connected in the similar 
configurations to those described in connection with the first embodiment 
to provide a non-inductive loop and a balanced or differential output. 
An alternative way of allowing the active conductors 13a and 13b to move 
while holding the return conductors 14a and 14b still is to make all the 
longitudinally extending slots 10 and 12 equal width and use return 
conductors of a larger diameter than the active conductors. 
The major advantages of the illustrated sensor transducer 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 magnetic 
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 aging of the cable. This contrasts with 
the electret-type of cables which show inherent charge decays over a 
period of time resulting in loss of sensitivity. 
(3) Balanced output available (in FIGS. 1 and 4) 
Since a `centre-tap` of the transducer 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 
.