[0001] This invention relates to perimeter security systems for locating and characterising
the source of attempted and/or actual penetration of the security perimeters of relatively
large geographical areas; more particularly, the invention relates to such systems
employing structural moment detectors.
[0002] Structural moment detectors or flexural rigidity sensors are known which are constituted
by optical sensors in the form of autocollimators insensitive to linear dynamic motion
but responsive to angular deflection of one end of the sensor with respect to the
other. For example, such sensors are disclosed in the patent to Rossire, U.S. No.
3,229,511 and in the publication entitled "The Structural Rigidity Sensor: Applications
in Non-Destructive Testing", published by the Air Force Systems Command, United States
Air Force (Frank J. Seiler Research Laboratory, Publication SRL-TR-75-0017
# October,1975). See also the U.S. patents to Okubo Nos. 4,159,422 issued June 26,
1979 and 4,164,149 issued August 14, 1979.
[0003] Systems which employ structural moment detectors to measure and record certain effects
of forces acting on a structure are also disclosed in the publications described above.
For example, the
Rossire patent discloses an aircraft attitude control system.in which a structural
moment detector is used to sense wing loading and automatically adjust the attitude
of the aircraft to maintain wing loading within safe operational limits.
[0004] It is an object of the present invention to provide an improved perimeter security
system utilising structural moment detectors.
[0005] Accordingly, the present invention provides a perimeter security system for locating
and characterising the source of attempted and/or actual penetration of relatively
large geographical areas, characterised in that said system comprises,
[0006] in combination:
a) a plurality of structural moment detectors arranged at spacedpoints in ground contact
to detect intrusions and activities within and near the perimeter of said area;
b) processing means for processing said output signals to modify the information content
thereof, including rejecting components of said signals indicative of the effects
of extraneous seismic forces caused by other than said intrusions and/or activities;
and
c) signal-manipulating means for manipulating the processed signals output by the
processing means to provide secondary signals responsive to seismic forces caused
by said intrusions and/or activities.
[0007] The systems of the invention can employ substantailly conventional structural moment
detectors, the output of which is simply raw data which does not directly indicate
or give useful information concerning the effect of forces acting on a structure to
which the sensor is attached. In such cases, the raw data from the sensors is conditioned
and processed by external electronics, including microprocessors and computational
software, usually necessarily located at locations remote from the sensors themselves.
[0008] Alternatively the systems of the invention can employ structural information detectors
in which the raw signal from the optical sensor of an optical structural moment detector
is converted and processed within a unitary device (either a single-piece device or
a multi-component device in which each of the components is assembled to form an integral
unit) such that the electrical signal output of the device has a directly useful information
content which can be displayed or recorded by any conventional means or which can
be directly utilized to activate control systems.
[0009] The term "surface coordinate vector" as used herein is intended to mean both the
normal and the tangent to the surface of a structure and any angle therebetween.
[0010] The invention will now be particularly described, by way of example, with reference
to the accompanying diagrammatic drawings, in which:
Fig. 1 is a sectional view of a typical prior art structural moiment detector;
Fig. 2 is a typical schematic of the LED driver circuit of the structural moment detector
of Fig. 1;
Fig. 3 is a typical schematic of the readout electronics circuits of the structural
moment detector of Fig. 1:
Fig. 4 is a schematic drawing of a monitoring system embodying the invention;
Fig..5 is a schematic perspective view of a perimeter security system utilising the
monitoring system of Fig. 4.
Fig. 6 is a generalised block diagram illustrating the major sub-components of a structural
information detector suitable for use in the systems of Figures 4 and 5;
Fig. 7 is a circuit schematic depicting an analog circuit which functions to electrically
interface the optical detector of the structural information detector of Figure 6
with signal-processing electronics;
Fig. 8 is a circuit schematic depicting the current source circuit for light-emitting
diodes of the optical sensor portion of the Figure 6 structural information detector:
Fig. 9 is a sectional view of one particular form of the Figure 6 structural information
detector;
Fig. 10 is a perspective view of another particular form of the Figure 6 structural
information detector in which the components are carried by a semiconductor substrate;
and
Fig 11 is a perspective view of an optical system usable in the Figure 6 structural
information detector as well as in the structural moment detectors or flexural rigidity
sensors of the prior art.
[0011] As,used herein, the term "structural moment detector" means a device which measures
the integral of the structure moment between two points on the structure. Such devices
are known in the art, but, for clarity, a typical structural moment detector will
be briefly described with reference to Figs. 1-3 and the accompanying descriptive
material.
[0012] The structural moment detector shown in Figure 1 is basically an autocollimator that
is insensitive to linear dynamic motions but responds to angular deflection of one
end of the sensor with respect to the other. The structural moment detector illustrated
consists of two separate parts which are mounted at spaced locations on a beam 10.
One of the parts 11 is a support bracket 12 which carries a light-emitting diode (LED)
13, a collimating lens 14 and dual photovoltaic detectors 15. The other part 16 of
the structural moment detector consists of a support bracket
17 which carries a plane front mirror 18. The two parts 11 and 16 are suitably joined
by a bellows or other hood member (omitted for clarity of illustration) to exclude
extraneous light. The LED 13 emits an infrared light beam 19 which is collimated by
the collimating lens 14. The collimated light beam 19a impinges on the mirror 18 and,
as indicated by the dashed lines 10, is reflected back through the collimating lens
14 to the photovoltaic cells 15. Angular motions, but not linear motions, of the mirror
18 result in varying amounts of infrared radiation reaching each of the photovoltaic
cells 15. The difference in voltage output of the photovoltaic.cells 15 is then proportional
to the angular motion of the mirror 18 with respect to the cells 15.
[0013] Structural moment detectors of the Figure 1 form are capable of measurinq the deflection
of the beam with a resolution of 1 milliarc second (4.85 x 10 radians) with a range
of +6 arc seconds. Where such accuracy is not required, such devices can be fabricated
which have a resolution of at least 1 arc second with a dynamic range of +3°. Such
devices are capable of operating from DC to 50 MHz, the upper limit being established
by the frequency limitation of the photovoltaic cells.
[0014] Typical circuits which are used in conjunction with the mechanical components of
the structural moment detector of Fig. 1 are illustrated in Figs.
2 and 3. Fig. 2 is a schematic diagram of a suitable LED driver circuit which is a
simple constant current source circuit which is required to provide a light source
with constant light intensity. A typical suitable readout circuit is illustrated in
Fig. 3, which depicts an analog output circuit consisting of a first stage amplifier
with common mode rejection that permits linear operation of the photovoltaic cells.
[0015] The operation of the structural moment detector can be illustrated by reference to
a simplified example of a cantilevered beam which is loaded and the structural moment
detector is mounted at points a and b located near the supported end of the cantilevered
beam. If the deflection of the beam is measured as 6, the angle between surface tangents
at points a and b, the output voltage of the photovoltaic cells is proportional to
this angle, and according to the Area Moment Theorem

where
M is the applied moment between points a and b
E is the modulus of elasticity
I is the moment of inertia
6 is the angular difference between surface tangents at points a and b
x is the linear surface distance between points a and b.
[0016] If a load P is placed on the end of a beam of length L and is the distance between
points a and b, then

To illustrate the sensitivity of the structural moment detector, a load of 1 gram
was placed at the end of an 8" cantilevered beam. The device was mounted near the
support of the beam such that points a and b were 1.5" apart. With this load

and

Since it is impossible to load a structure without changing the total moment which
occurs between two points on the structure, it is possible to use the structural moment
detector as an extremely accurate and extremely sensitive sensor havin
q a range which far exceeds that of conventional sensors of the prior art.
[0017] The general system of the invention is schematically illustrated in Fig. 4. As shown
in Fig. 4, the structural behaviour 41, which is effected by the forces acting on
the structure, are sensed by an array 42 of structural moment detectors (SMD's), located
on the structure. The SMD's 42 are located on the structure so as to provide primary
electronic signals 43 which are proportional to the structural behaviour parameter
of interest. The primary electronic signals 43 from the SMD array 42 are fed to signal
processing and buffering equipment 44, which includes electronic circuitry which modifies
the information content of the primary siqnals 43 (e.g., rejection of background noise,
rejection of signal components induced by other forces, etc.) and which electrically
isolate the sensors from the remainder of the system. The processed signals 45 are
then transmitted to analog- to-digital converters 46 which convert the analog information
in the processed signals 45 to a digital format compatible with various digital processors,
recorders, editors and/or display units. The digital signals 47 are then transmitted
to a data processor 48 which will usually be a single-frame computer which is capable
of accepting digital data and manipulating it in a predetermined, programmable fashion,
in order to convert the digitized measurement information into a digital representation
of the desired system data. The digital representation data 49 is optionally transmitted
to data recording/editing equipment 50 which may provide for permanent recording of
all or part of the acquired data for later use and which may, additionally, provide
manual editing capability. The recorded and/or edited data 51 may optionally be transmitted
to data display equipment 52 which provides visual display of the acquired data and,
additionally, may provide for the predetermined alteration of the means by which the
data processing equipment 48 is transforming acquired data or the manner in which.
data is digitized, recorded, edited and/or displayed. Feedback loops 53 may be optionally
provided, through which the information at one stage is fed backwardly and/or forwardly
to another stage of the system to provide improved accuracy, estimation, prediction
or other similar functions. These feedback paths may be electrical, optical, mechanical
and/or may involve human interpretations and adjustments.
[0018] According to this invention,systems are provided for detecting, warning of and characterising
the source of intrusions of the security perimeters of relatively large geographical
areas.
[0019] The extreme sensitivity of the SMD provides a seismic detector and the invention
incorporates such a detector into a system which is capable of providing information
on type, number, location, and movement of enemy forces.
[0020] The SMD is mounted on a diaphragm and used as a geophone. The sensitivity of the
SMD coupled with the variable design parameters (material, thickness, size, mounting
technique) of the diaphragm produce a geophone of remarkable performance.
[0021] When suitably deployed, any motion near the sensor produces a characteristic vibration
signature. The existence of the signal is used to indicate movement and the frequency
content is analyzed to provide information about the source. Using triangulation techniques
and/or proximity to various sensors, the location and direction of movement can be
determined.
[0022] The system is capable of detecting and monitoring movement on the ground or in below-ground
tunnels. Sensors can be hardwired to processing equipment or can be self-contained
battery powered units which communicate optically or electronically with monitoring
stations and/or aircraft.
[0023] Figure 5 depicts a perimeter security system utilising the Figure 4 monitoring system.
The perimeter security system can be utilised to protect large geographical areas
such as private estates, industrial plants, oilfields, airfields, communications installations
and, in fact, any geographical area which is required to be protected against unauthorised
entry.
[0024] The perimeter security system, according to the presently preferred embodiment of
the invention, functions to warn against unauthorised entry into the secure area 510
and, further, to give preliminary warnings of unauthorised entry or activity within
a so-called "sterile area" 511 and entry or activity in a close approach area 512.
A plurality of SMD's 401 arte buried inthe ground at spaced locations outside the
close approach area 512. Other SMD's 402 are buried in the ground within the sterile
area 511 and additional SMD's 403 are located on the posts of a fence 510A around
the sterile area. The SMD's 401, 402 and 403 are linked by appropriate cables 404
to one or more command centres 405, located within the secure area 510, which houses
the required electronics, microprocessors, software, etc.
[0025] By triangulation techniques, the unauthorised entry or activity can be identified
and located as being within any one of a plurality of perimeter zones 406 which are
segments of the perimeters of the secure area 510, sterile area 511 and close approach
area 512.
[0026] The perimeter security system of the invention provides for identification or characterisation
of the entrant or activity, location of the penetration and of approach to the security
perimeters as well as providing a system for communication among the various areas
of interest. The perimeter security system may be applied to protect areas having
widely varying geographical conditions such as deserts, swamps, urban areas and the
like, with low level maintenance and operating costs and capabilities. The components
of the systems may be easily transported to remote sites across rough terrain and
the system has very low system- originated disturbance false alarms and system failure
false alarms.
[0027] Unauthorised entry of activity within the close approach area 512 initiates a warning
signal at the control centre 405 and entry into the sterile or secure area initiates
appropriate responses such as activation of specific security systems, dispatch of
guard forces, etc.
[0028] The close approach area will typically be located between 5-50 feet from the perimeter
of the secure area and the sterile area will be approximately 5-10 feet wide depending
on the specific characteristics of the secure area. The zone segments may vary from
75 to 1,000 feet.
[0029] The SMD sensors used in Figures 4 and 5 are structural moment detectors (SMD's),
for example, of the prior art form shown in Figure 1. These sensors can alternatively
be implemented by means of "structural information detectors" in which raw data from
a structural moment detector is converted and processed within a unitary device (including
the SMD) such that the signal output form the device has a directly useful information
content.
[0030] Fig. 6 is a block diagram illustrating the major sub-components of a structural information
detector suitable for use in the sytems described hereinbefore. The structural information
detector, generally indicated by the reference numeral 410, is depicted for purposes
of illustration as being mounted upon a simple cantilevered beam 411 and consists
of a housing 412, an optical sensor
413, sensor power supply 414, raw signal conversion circuitry 414c and signal-processing
electronics, including a microprocessor and appropriate software 415. The optical
sensor 413, for example, a structural moment detector of the type generally disclosed
in U.S. patent 4,287,511 (sometimes also known as a "flexural rigidity sensor") measures,
as indicated by the dashed line 413a, the relative orientation of surface coordinate
vectors which are, as illustratively depicted in Fig. 6, normals 416 to the surface
411a of the beam 411. If a force F (417) is applied in the direction of the arrow 417a
to the beam 411, resultant bending of the beam 411 will cause a change in the relative
orientation of the surface coordinate vectors 416 to the positions indicated by the
dashed lines 416a, i.e., from the angular orientation
418 θ1 (shown in Fig. 6, illustratively, as 180°) to an angular orientation 418a 82 which
(as illustratively depicted in Fig. 6) is greater than θ1. Power 419 from an external
power source 419a is supplied to the circuitry 414 which, as explained below, provides
a regulated power supply 414a to the optical sensor 413. The raw data 413b from the
optical sensor 413, which could be a variable voltage or a variable current, is supplied
to the raw signal conversation portion of the circuitry 414, which converts the raw
data, as will be further explained below, to a form which is the input 414b to the
signal and data-processing electronics and software 415, which processes the converted
signal
414b and provides, as the output 415a of the structural information detector 410, a
signal which embodies useful structural information 420 which directly indicates the
effect of the force F acting on the beam 411. As indicated by the line 420a, the useful
structural information 420 can be utilised in any or all of a variety of ways, i.e.,
it can be used as the input to a direct display 520b which may be a simple galvanometric
meter, liquid crystal display, light emitting diode display, cathode ray tube or the
like. Also or alternatively,the useful structural information 420 can be used qas
the input to a semi-permanent or perment recording device 420c, such as a paper recorder,
magnetic recorder, semiconductor storage device, bubble memory storage device, or
holographic storage device. Also or alternatively, the useful sturctural information
420 can form the input to various control devices 420d such as servomotors and other
similar electromechanical devices.
[0031] As will be appreciate by those skilled in the art, it may be desirable to provide
communication paths between some or all of the individual structural information detectors
in a detector array and/or with suitable central data display, recording and/or control
components, as well as with additional hardware and software which correlate the useful
structural information outputs of the individual structural moment detectors in the
array. These details have been omitted for the purpose of clarity.
[0032] The raw data output 413b of the optical sensor 413 is (referring to Fig. 7) the input
to the signal processing electronics, including computational software therein depicted.
The function of the circuitry depicted in
Fig. 7 is to convert the raw data input 413b from the optical sensor into data suitable
for electronic data processing and then to perform the data-processing function to
yield a signal, the components of which directly provide useful structural information.
According to the presently preferred embodiment of the structural information detector,
the conversion is performed with analog electronics, rather than digital. The analog
electronics perform two distinct functions. First, the output signal 413b from the
optical sensor is measured and converted to a proportional voltage in a voltage follower
circuit which, for clarity of illustration, are those components which are located
within the dashed line
421. The second function is to amplify the proportional voltage signal from the voltage
follower circuit
421 by feeding it through a gain control 422 into an amplifier which, for purposes of
clarity of description, are those components located within the dashed line 423. The
voltage follower circuit 421 operates as a short circuit load for the photovoltaic
cells of the optical sensor. Feedback is added in the amplifier circuit
423 to shape the upper end of the frequency response so that spurious high frequency
noiseis attenuated. This function is performed by applying the output of IC1 (pin
7) to the input (pin 6) of IC2 through a voltage divider formed by R7-R8. IC2 acts
as a low-pass filter. The output of IC2 (pin 7) is fed back through R16 to the input
3 of IC1 to act as an automatic bias adjustment. An offset voltage adjustment is provided
to remove any bias due to photocell mismatch.
[0033] The gain control 4?2 provides a means of balancing the mechanical gain of different
sensors and compensating for components variation. The nominal adjustment range is
+15%. The amplifier 423 is a high cain direct- coupled amplifier with feedback to
further attenuate high frequency noise levels. Nominal gain is 1.5 volts/ microamp
and the bandwidth is normally initially set at 50-500 Hz. In essence, A2 is a variable
cut-off low-pass filter. If the DC case is considered, its output will seek a level
such that the voltage presented to pin 3 of Al is equal to that presented to pin 2
by the voltage follower circuit. The scaling of the system is such that

where V
int is the integrator output, V
in is the first stage and U
null is the input to A1, pin 2. Thus, for low frequency, V
int represents a scaled value equal to or greater than V
in independent of the gain in the second stage of the amplifier. The implication of
this is that the gain of the second stage can be set very high for frequencies above
the autonumm roll-off without completely losing DC information.
[0034] Support and bias circuits which include the components which are, for clarity of
illustration, enclosed within the dashed line 424 are provided to provide conditioned
power and bias voltages for the components of the voltage follower 421 and amplifier
423.
[0035] The output 426 of the amplifier 423 is an analog signal which is converted to a digital
signal in the A-D converter 422a. A non-limiting, illustrative, exmaple of a suitable
analog-digital converter is manufactured by the U.S. firm Analog Devices under number
HAS1202.
[0036] The signal-processing electronics also includes a microprocessor 425 and appropriate
computational software which converts the output signal 422b of the A-D converter
422a into electrical signals 420 which directly p
lovide data related to the effect of the force acting on the structure to which the
structural information detector is attached. In the presently preferred embodiment,
this component includes a microprocessor, the necessary supporting devices and a power.
supply, details of which are omitted for purposes of clarity because they are well-known
to those skilled in the art. Suitable non-limiting examples of microprocessors which
can be employed are the TI9000 or the Intel 8086.
[0037] All of the elements described in Figs. 7 and 8 may be fabricated on a printed circuit
board using standard integrated circuits or on a single thick film substrate where
the circuits have been wire bonded to the substrate. In the embodiment of the detector
described in connection with Fig. 10, these components may be made as a single integrated
circuit on the same semiconductor substrate chip used in fabricating the optical sensor.
[0038] Another function of the signal-processing electronics is to provide a precise current
to the light emitting diode of the optical sensor.
[0039] Typical non-limiting, illustrative, values of the components of Fig. 7 are set forth
below:
Voltage Follower/Amplifier
R1 - 220K
R5 - 1K
R6 - 2.2K
R9 - 2.2M
R23 - 82
A1 - Operational Amplifier LF353/2
Second Stage Amplifier
P2 - 680K
R3 - 10K
R4 - 1M
R7 - 100K
R8 - 10K R13 - 2.2M
R15 - 220K
R16 - 10K
R17 - 1K
C2 - 47PF
C3 - 10MF
C4 - 5PF
A1 - Operational Amplifier LF 353/2
A2 - Operational Amplifier LF 353/2
Support and Bias Circuits
R10 - 100K
R11 - 1.8K
R12 - 100K
R14 - 1.8K
C5 - 10MF
C6 - 10MF
C7 - 10MF
C8 - 10MF
Z1 - LM336
Z2 - LM336
[0040] For the sake of clarity, ..the power supply circuitry has been separated from the
circuitry of Fig.7 and is shown schematically in Fig. 8. The 15 volt power supply
431 is provided by the support and bias circuitry 424 of Fig. 7.
[0041] The power supply circuitry of Fig. 8 utilises two amplifiers in a high gain feedback
arrangement to provide the necessary precise current 432 to the light emitting diode.
[0042] Typical non-limiting, illustrative, values of the components of Fig. 8 are set forth
below:
Power Supply .
R18 - 3.3K
R19 - 10K
R20 - 10K
R21 - 10K
R22 - 47
C9 - 10MF
C10 - 10MF
IC3 - Operational Amplifier LF353/2
Q1 - 2N5457
Q2 - TIP21
[0043] Fig.9 is a sectional view illustrating a particular form of structural information
detector which consists of a first housing sub-assembly generally indicated by reference
character 440 containing the sensor power supply/raw signal conversion/ signal-processing
electronics 441 of Figs. 7 and 8 , a light emitting diode 442, a pair of photovoltaic
detectors 443 and a collimating lens 444 carried proximate the open end of a barrel
portion 445 formed in the housing 440. A second housing sub-assembly, generally indicated
by reference numeral 446, carries a plane surface mirror 447 on the inner end 448
of a mating barrel portion 449 formed in the housing sub-assembly 446. Although a
substantial clearance 450 is shown between the barrel portion 445 formed in the first
housing sub-assembly 40 and the barrel portion 449 formed in the second housing sub-assembly
446, it will be anderstood by those skilled in the art that this clearance is shown
only for the purpose of clarifying the mechanical relationship of the two housing
sub-assemblies 440 and 446. In actuality, the mating barrel portion 449 formed in
the second housing sub-assembly 446 is shaped and dimensioned to receive the barrel
portion 445 formed in the first housing sub-assembly 440 with an interference fit
therebetween, to form a unitary structurally integrated device which excludes ambient
light from the interior of the barrel portions 445 and 449 and which facilitates and
assists in maintaining precise optical alignment of the two sub-assemblies.
[0044] Structural information detectors of the type depicted in Fig. 9 have been successfully
manufactured and tested which are in the size range of as small as one inch in the
major dimension. Present work indicates that, eventually, this can be reduced to 1/4-1/8"
in the major dimension.
[0045] Fig. 10 illustrates another form of structural information detector in which all of
its components are carried by any suitable semiconductor substrate, such as a silicon
chip 451. The generally U-shaped chip 451 carries the light emitting diode 452 on
an inner face 453 of one of the legs of the U-shaped chip and leads 454 for providing
power to the light emitting diode. A pair of photovoltaic cells 455 are grown by known
semiconductor manufacturing techniques on the inner face 456 of the opposing leg of
the
U-shaped chip 451. Bending of the chip 451 induced by bending of a structural member
to which it is attached by any suitable technique, such as epoxy bonding, causes a
variation in the light falling on the photocells 455, depending on the relative orientation
of surface coordinate vectors (normals) 456. The circuitry of Figs. 7 and 8 is formed
by known semiconductor manufacturing techniques in the portion 457 of the silicon
chip 451. The entire chip is then received within a suitable housing indicated by
the dashed lines 458 to protect the internal components from adverse ambient environmental
effects and to prevent stray light from interfering with the operation of the optical
components 452 and 455. As in the case of the embodiment of Fi
g. 9, devices such as those depicted in
Fig. 10 can be manufactured in a size as small as 1/4-1/8" in the major dimension.
[0046] In the form of structural information detector depicted in Fig. 11, the components
of the optical sensor 410 (Fig.
6) are carried by an elongate light transmission member, generally indicated by reference
character 461, formed of a light-transmitting flexible material such as, for example,
methacrylate polymers and copolymers known in the art. A concentrating lens 462 is
formed in one end of the light-transmission member 61 and the other end 463 carries
a light source 464 such as a LED and a pair of photocells 465 which generate electrical
signals indicating the relative orientation of surface vector coordinates (normals)
466 to the surface of a structural member upon which the structural information detector
is mounted. If desired, the length of the light-transmission path 467 can be lengthened
by cutting or forming facets 468 in the external surfaces of the elongate light-transmitting
member which will reflect light beams 468 transmitted from the LED to the concentrating
lens 462 and which are then reflected 469 to the photocells 465. The entire optics
system illustrated in Fig. 11, along with the sensor power supply/raw signal conversion/signal-
processing electronics components of the structural information detector of Fig. 6,
are then enclosed in a suitable housing to protect the optics and electronics components
from adverse ambient conditions and from interference caused by stray light. The housing
is omitted in Fig. 11 for purposes of clarity. Of course, by omission of the signal
processing electronics, the Figure 11 optics system can be employed as a simple structural
moment detector.
1. A perimeter security system for locating and characterising the source of attemped
and/or actual penetration of the security perimeter of relatively large geographical
areas, characterised in that the said system comprises, in combination;
a) a plurality of structural moment detectors (401, 403), arranged at spaced points in ground contact to detect intrusions and activities
within and near the perimeter of said area;
b) processing means (405) for processing said output signals to modify the information
content thereof, including rejecting components of said signals indicative of the
effects of extraneous seismic forces caused by other than said intrusions and/or activities;
and
c) signal-manipulating means (405) for manipulating the processed signals output by the processing means to provide
secondary siganls responsive to seismic forces caused by said intrusions and/or activities.
2. A system according to Claim 1, characterised in that each said structural moment
detector (401,402 403) and its associated processing and signal-manipulating means
(405) is constituted by a respective structural information detector (410), said detector
(410) comprising, in combination:
a) a housing (412) mounted on a ground structure,
b) optical means (413) within said housing (412) for detecting the relative orientation
(413a) of spaced surface coordinate vectors (416a) of said structure , and for generating
primary signals (413b) in response to changes in said orientation;
c) circuit means (414c) within said housing (412) for converting said primary signals (413b) to a form usable by signal-processing
electronics; and
d) signal-processing electronics means (415) within said housing (412), including
computational software, for processing the converted primary signals (414b) to said
secondary signals (415a).
3. A system according to Claim 2, characterised in that the optical means (413), circuit
means (414c), and signal-processing electronics means (415) of the structural infoimation
detector (410) are carried by a single-piece semiconductor substrate (451) received
within said housing (412).
4. A system according to Claim 2, characterised in that the housing (412) of the structural
information detector (410) comprises first and second housing sub-assemblies (440,446)
and in that:
a) said first housing sub-assembly (440) contains said circuit means (441), said signal-processing
electronics means (441), and those components of said optical means (413) which include
a light source (442), a plurality of photovoltaic detectors (443) and a collimating
lens (444); the light source (442) and photovoltaic detectors (443) being carried
on the inner face of a hollow barrel portion (445) of said, first housing sub-assembly
(440) and said collimating lens (444) being carried proximate the open end of said barrel portion (445);
b) said second housing sub-assembly (446) contains, as another component of said optical
means (413), a plane surface mirror (447) carried on the inner end (448) of a barrel portion (449)of the second housing sub-assembly (446), the two said
housing barrel portions (445, 449) being shaped and dimensioned to mate one within
the other with an interference fit therebetween to form a unitary structurally-integrated
device; and
c) said first and second housing sub-assemblies (440,446) are mountable at spaced
points.
5. A system according to Claim 2, characterised in that the said optical means (410)
of the structural information detector comprises:
a) an elongate light-transmission member (461) formed of a light-transmitting flexible
material and having a light-transmitting/ receiving end (463) and a reflecting end;
b) a concentrating lens (462) formed in the reflecting end of the member (461);
c) a light source (464) carried by said light-transmitting/receiving end (463) of the
member, the light source (464) being positioned such as to direct light through the
member (461) toward said reflecting end;
d) light-receiving means (465) carried by the light-transmitting/receiving end (463)
of the member, the light-receiving means (465) being arranged to receive light transmitted
through said member from the light source (464) to the concentrating lens (462) and
reflected thereby back through said member to the light-receiving means (465), and
to generate electrical signals in response to the received light; and
e) means for connecting the said ends of the light-transmission member (461) to spaced
points on the ground such that ground movement causes corresponding bending of the
light-transmission member (461) and, in turn, causes a variation in the amount of
light received by the light-receiving means (465).