Note: Descriptions are shown in the official language in which they were submitted.
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GRAVITY GRADIONIETER WITH CORRECTION OF EXTERNAL DISTURBANCE
Field of the Invention
The present invention relates to a gravity gradiometer and
to components for high precision measurement instruments.
Background of the Invention
Gravimeters are used in geological exploration to measure
the first derivatives of the earth's gravitational field.
Whilst some advances have been made in developing
gravimeters which can measure the first derivatives of the
earth's gravitational field because of the difficulty in
distinguishing spatial variations of the field from
temporal fluctuations of accelerations of a moving
vehicle, these measurements can usually be made to
sufficient precision for useful exploration only with
land-based stationary instruments.
Gravity gradiometers (as distinct from gravimeters) are
used to measure the second derivative of the gravitational
field and use a sensor which is required to measure the
differences between gravitational forces down to one part
in 1012 of normal gravity.
Typically, such devices have been used to attempt to locate
deposits such as ore deposits including iron ore and
geological structures bearing hydrocarbons.
The gravity gradiometer typically has at least one sensor
in the form of sensor mass which is pivotally mounted for
movement in response to the gravity gradient.
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International publication WO 90/07131 discloses such a
gravity gradiometer. Gravity gradiometers of that type
are typically mounted in an aircraft and carried by the
aircraft while making measurements. Consequently, the
gravity gradiometer moves with movements of the aeroplane,
which creates accelerations of the gradiometer. If the
accelerations are not compensated for, they are detected
by the gravity gradiometer and will produce noise or swamp
a.signal component that is associated with the actual the
gravity gradient which is to be detected by the gravity
gradiometer.
The gravity gradiometer disclosed in International
publication WO 90/07131 includes two sensor masses which
are orthogonally positioned and arranged to move about a
common axis. The sensor masses are suspended by pivots and
can oscillate in planes that are orthogonal to the common
axis. For measurement of the gravity gradient the
instrument is continuously rotated and a local change in
the gravity gradient results in oscillating of both sensor
masses relative to a rotated housing of the instrument.
Such arrangement has the advantage that at least some
unwanted accelerations, such as those resulting from a
sudden movement of an aircraft, are experienced by both
sensor masses in the same manner and can be eliminated.
However, the force that causes such an oscillation is very
small and results in a very small signal that is
indicative of the gravity gradient. The signal is
'30 typically accompanied by other much larger signals that
are caused by sudden movements of an aircraft in which the
gravity gradiometer may in use be positioned. Correction
of an output of the gravity gradiometer for such other
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signal components in a satisfactory manner is a
technological challenge.
Summary of the Invention
The present invention provides method of detecting a
change in gravity gradient using a gravity gradiometer,
the gravity gradiometer comprising a detector for
detecting the change in gravity gradient, the gravity
gradiometer further comprising a sensor for generating a
disturbance signal in response to an external disturbance,
the method comprising the steps of:
exposing the gravity gradiometer to the external
disturbance during operation of the gravity gradiometer;
detecting the disturbance signal;
receiving an output signal from the detector of
the gravity gradiometer; and
numerically correcting the output signal for an
impact that a component of the gravity gradiometer
experiences as a consequence of the external disturbance
using a response parameter that associates the external-
disturbance with a disturbance experienced by that
component.
The gravity gradiometer typically comprises a large number
of portions that are moveable relative to each other.
Consequently, the external disturbance, such as a
disturbance force or motion associated with a sudden
movement of an aircraft in which the gravity gradiometer
may be positioned, may be experienced differently by
different portions of the gravity gradiometer. The output
signal of the gravity gradiometer may be a signal that is
indicative if the gravity gradient. Embodiments of the
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present invention enable correction of the output of the
gravity gradiometer for the external disturbance in a
relatively accurate manner.
The response parameter typically is dependent on a
mechanical responsiveness of the component to the external
disturbance.
The gravity gradiometer may comprise a support structure
that supports the detector and that is arranged to
attenuate an impact of the external disturbance on the
detector and wherein the component is a part of the
detector.
The method may comprise correcting the output signal by
applying a correction force, the correction force being
correlated with the external disturbance, wherein the
correction force reduces an impact of the external
disturbance and the output signal is then numerically
corrected for at least a portion of a remaining impact of
the external disturbance on the output signal.
In one embodiment the method comprises the steps of:
applying an external force to the gravity
gradiometer;
detecting the external force;
analysing if the component of the gravity
gradiometer experiences a force that is correlated with
the external force; and, if the correlation has been
identified; and
calculating the response parameter.'
The component may include the detector of the gravity
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gradiometer and the step of analysing if the component of
the gravity gradiometer experiences a force that is
correlated with the external force may comprise receiving
a detector signal that includes the gravity gradient
5 signal and analysing if that signal is correlated with the
detected external force.
Alternatively or additionally, the method may comprise
calculating the response parameter numerically using a
mathematical model that represents a mechanical
responsiveness of the component.
In one embodiment the sensor for generating a disturbance
signal is one of a plurality of sensors for generating
disturbance signals and the method may comprise detecting
a plurality of the disturbance signals in response to the
external disturbance wherein each sensor is arranged to
detect at least one of: a force associated with a rotation
about one of three orthogonal axes and a force associated
with a motion along one of three orthogonal axes.
The disturbance may also be an electromagnetic disturbance
and the sensor for generating a disturbance signal may
generate a signal in response to the electromagnetic
disturbance.
The method typically is operated by a feed-forward control
arrangement for reducing transmission of the external
disturbance to the component of the gravity gradiometer.
The gravity gradiometer typically comprises a support
structure and a pivot that couples the detector including
the component to the support structure wherein the
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response parameter is dependent on an equivalent spring
constant of the pivot.
The invention will be more fully understood from the
following description of specific embodiments of the
invention. The description is provided with reference to
the accompanying drawings.
Brief Description of the Drawings
Figure 1 is a schematic view of a gravity
gradiometer according to a specific embodiment of the
present invention.
Figure 2 is a perspective view of a first mount
forming part of a mounting of the gravity gradiometer of
according to the specific embodiment of the present
invention;
Figure 3 is a perspective view of a second mount
of the mounting according to a specific embodiment of the
present invention;
Figure 4 is a perspective view from underneath
the mount shown in Figure 3;
Figure 5 is a view of the assembled structure;
Figure 6 is a perspective view showing assembled
components of the gravity gradiometer according to another
specific embodiment of the present invention;
Figure 7 is a plan view of a bar according to a
specific embodiment of the present invention;
Figure 8 is a diagram showing actuator control
according to a specific embodiment of the present
invention;
Figure 9 is a perspective view of components of a
gravity gradiometer according to a specific embodiment of
the present invention;
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Figure 10 is a perspective view of a first mount
of a mounting according to another specific embodiment of
the present invention;
Figure 11 is a perspective view of part of the
mounting of Figure 10 to illustrate the location and
extent of the flexural web of the first mount;
Figure 12 is a perspective view of the mounting
of Figure 10 from beneath;
Figure 13 is a perspective view of the mounting
of Figure 10 including a second mount of the second
embodiment;
Figure 14 is a perspective view of a second mount
component;
Figure 15 is a perspective view of the second
mount component of Figure 14 from above;
Figure 16 is a perspective view of assembled
components of the gravity gradiometer according to a
specific embodiment of the present invention;
Figure 17 is a plan view of a housing portion for
supporting a bar according to a further embodiment of the
invention;
Figure 18 shows a component of the gravity
gradiometer according to an embodiment of the present
invention;
Figure 19 (a) - (f) is a view of transducer
elements according to a specific embodiment of the present
invention;
Figure 20 is a view similar to Figure 18 but
showing one of the transducers elements of Figure 19 in
place;
Figure 21 is a diagram to assist explanation of
the circuits of Figures 22;
Figure 22 is a circuit diagram relating to a
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specific embodiment of the invention;
Figure 23 is a frequency tuning circuit according
to an embodiment of the present invention;
Figures 24 to 26 show circuitry according to
embodiments of the present invention;
Figure 27 is a cross-sectional perspective view
through an actuator according to a specific embodiment of
the invention;
Figure 28 (a) and (b) shows components of the
.10 gravity gradiometer according to a specific embodiment of
the present invention; and
Figures 29 and 30 show block diagrams
illustrating the operation of a rotatable support system
according to a specific embodiment of the present
invention.
Detailed Description of the Specific Embodiments
Figure 1 is a schematic view of a gravity gradiometer 1
according to a specific embodiment of the present
invention. The gravity gradiometer 1 is arranged for
vertical positioning relative to a ground plane.
Throughout this specification the ground plane coincides
with an x-y plane of an x,y,z-coordination system and
consequently the gravity gradiometer is in this embodiment
arranged for orientation along the z-axis so that the r.y
and (rxx-Pyy) components of the gravity gradient tensor can.
be measured.
The function of the gravity gradiometer 1 may be briefly
summarised as follows. The gravity gradiometer has in this
embodiment two substantially identical sensor masses which
are pivotally mounted on a mounting so that they can
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oscillate relative to the mounting. The sensor masses with
mounting are rotated about the z-axis and with an angular
frequency that approximately equals half the resonance
frequency of sensor masses. A gravity gradient will result
in a force on the sensor masses which will then oscillate
relative to the mounting during that rotation. Components
of the gravity gradient tensor can be determined from the
oscillating movement of the sensor masses. For further
details on the general principal of such measurements are
described in the applicants co-pending PCT international
patent application number PCT/AU2006/001269.
The gravity gradiometer shown in Figure 1 comprises a
housing 2 which is. connected to mount'3 for connection to
an external platform (not shown). The external platform
is arranged for rotation of the housing 2 at a suitable'
angular frequency about the z-axis. Further, the external
platform is arranged for adjusting the housing 2 about
three orthogonal axes.
With reference to Figure 2 a first mount 10 is now
described. The first mount 10 forms a part of rotatable
mounting 5 which is shown in Figure 5. The mount 10
comprises a base 12 and an upstanding peripheral wall 14.
The peripheral wall 14 has a plurality of cut-outs 16.
The base 12 supports a hub 18.
Figures 3 and 4 show a second mount 20 which comprises a
peripheral wall 22 and a top wall 24. The peripheral wall
22 has four lugs 13 for supporting the mounting 5 in the
housing 2. The top wall 24 and the peripheral wall 22
define an opening 28. The second mount 20 is mounted on
the first mount 10 by locating the hub 18 into the opening
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28 and the lugs 13 through respective cut-outs 16 as is
shown in Figure 5.
The first mount 10 is joined to the second mount 20. The
5 flexure web 31 is formed in the first mount 10 so that a
primary mount portion of the mount 10 can pivot about a
flexure web 31 relative to a secondary mount portion of
the mount 10. This will be described in more detail with
reference to the second embodiment shown in Figures 10 to
10 16.
The mounting 5 mounts the sensor 40 (which will be
described in more detail hereinafter and which is
typically in the form of a mass quadruple) for fine
rotational adjustment about the z-axis for stabilising the
gradiometer during the taking of measurements particularly
when the gradiometer is airborne. As described above,
rotational stabilisation about the x-and y-axis is
provided by the external platform.
Figure 6 shows sensor 40 mounted on the mounting. The
sensor 40 is an Orthogonal Quadruple Responder - OQR
sensor formed of a first mass and a second mass in the
form of a first bar 41 and a second bar 42 (not shown in
Figure 6) orthogonal to the bar 41 and which is of the
same shape as the bar 41.
The bar 41 is formed in a first housing portion 45 and the
bar 42 is formed in a second housing portion 47. The bar
41 and the second housing portion 45 is the same as bar 42
and the second housing portion 47 except that one is
rotated 90 with respect to the other so that the bars are
orthogonal. Hence only the first housing portion 45 will
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be described.
The first housing portion 45 has an end wall 51 and a
peripheral side wall 52a. The end wall 51 is connected to
rim 75 (Figures 2 and 5) of the wall 14 of the first mount
by screws or the like (not shown). The bar 41 is
formed by a cut 57 in the wall 51 except for a second
flexure web 59 which joins the bar 41 to the wall 51. The
second flexure 59 web is shown enlarged in the top view of
10 the bar 41 in Figure 7. Thus, the bar 41 is able to pivot
relative to the first housing portion 45 in response to
changes in the gravitational field. The bar 42 is mounted
in the same way as mentioned above and also can pivot
relative to the second housing portion 47 in response to
changes in the gravitational field about a third flexure
web. The second housing portion 47 is connected to base
12 (Figure 2) of the first mount 10.
The bar 41 and the first housing portion 45 together with
the second flexure web 59 are an integral monolithic
structure.
Transducers 71 (not shown in Figures 2 to 4) are provided
for measuring the movement of the bars and for producing
output signals indicative of the amount of movement and
therefore of the measurement of the differences in the
gravitational field sensed by the bars.
Figure 8 is a schematic block diagram showing actuator
control to stabilise the gradiometer by rotating the
mounting 5 about the z-axis. A controller 50 which may be
a computer, microprocessor or the like outputs signals to
actuators 53 and 54, which are arranged to rotate the
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mounting 5 about the z-axis. Each actuator is positioned
stationary relative to lugs 13 and coupled to the first
mount 10 so that the actuator can effect a rotation by a
small angle of the mount 10 with other components relative
to the lugs 13 (and other components that are stationary
relative to the lugs 13). Each actuator provides a linear
movement and is positioned so that the linear movement is
translated into a small rotation of the mount 10. The
actuators will be described in more detail with reference
to Figure 27. The position of the mounting 5 is monitored
so that appropriate feedback can be provided to the
controller 50 and the appropriate control signals provided
to the actuators to rotate the support 10 about the z-axis
as is required to stabilise the support during movement
through the air either within or towed behind an aircraft.
The specific embodiment also includes angular
accelerometers which are similar in shape to the bars 41
and 42 but the shape is adjusted for zero quadruple
moment. The linear accelerometers are simple pendulous
devices with a single micro pivot acting as the flexural
hinge.
Figure 9 is a cut away view of components of the gravity
gradiometer ready for mounting in the housing 1 which in
turn is to be mounted in the external platform 2.
The transducers 71 measure the angle of displacement of
the bars 41 and 42 and control circuitry (not shown) is
configured to measure the difference between them.
In this embodiment, the transducers 71 are constant charge
capacitors, which will be described in more detail with
reference to Figure 22.
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Figures 10 to 15 show a second embodiment in which like
parts indicate like components to those previously
described.
In the second embodiment the first mount 10 has cut-outs
80 which effectively form slots for receiving lugs (not
shown) which are connected to the mount 10 in the cut-outs
80 and also to the second mount 20 shown in Figures 15 and
16. In this embodiment the lugs are separate components
so that they can be made smaller, and more easily, made
than being cut with the second mount section 20.
In Figure 10 a cut 87 is made to define the part 18a of
the hub 18. The cut 87 then extends radially inwardly at
88 and then around central section 18c as shown by cut
101. The cut 101 then enters into the central section 18c
along cut lines 18d and 18e to define a core 18f. The
core 18f is connected to the central section 18c by the
flexural web 31 which is an uncut part between the cut
lines 18e and 18d. The part 10a therefore forms a primary
mount portion of the mount 10 which is separated from a
secondary mount portion 10a of the mount 10 except for
where the portion 18a joins the portion 10a by the
flexural web 31. The part 18a effectively forms an axle
to allow for rotation of the part 18a relative to the part
l0a in the z direction about the flexure web 31.
As is shown in Figure 11, the cut line 88 tapers outwardly
from the upper end shown in Figure 11 to the lower end and
the core 18c tapers outwardly in corresponding shape.
As is apparent from Figures 10, 12 and 13, the first mount
10 is octagonal in shape rather than round, as in the
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previous embodiment.
Figure 14 shows a component of the second mount 20 for
mounting in the first mount 10. As is best shown in
Figures 14 and 15, the second mount 20 has cut-outs 120
which register with the cut-outs 80 for receiving lugs
(not shown). The lugs can bolt to the second mount 20 by
bolts which pass through the lugs and into bolt holes 121.
The lugs (not shown) are mounted to the mount 20 before
the mount 20 is secured to the first mount 10.
In this embodiment, top wall 24 is provided-with a central
hole 137 and two attachment holes 138a. Three smaller
holes 139a are provided to facilitate pushing of the first
housing portion 45 off the part 18a if disassembly is
required. When the second mount 20 is located within the
first mount 10, the.upper part of central section 18c
projects through the hole 137, as best shown in Figure 13.
The mount 20 can then be connected to the mount 10 by
fasteners which pass through the holes 138 and engage in
holes 139b (see Figure 10) in the part 18a.
Thus, when the first housing portion 45 and its associated
bar 41 is connected to the rim 75 of the first mount 10
and the second housing portion 47 is connected to the base
12, flexure web 31 allows movement of the housing portions
45 and 47 about the z-axis.
Thus, when the second mount 20 is fixed to the part 18a,
the second mount 20 can pivot with the first portion l0a
of the first mount 10 about a z-axis defined by the
flexure web 31 whilst the second portion formed by the
part 18a.remains stationary.
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Figure 16 shows main body 61 of the housing 1 and
connectors 69 with the hemispherical ends removed.
Figure 17.is a plan view of the first housing portion 45
5 according to a still further embodiment*of the invention.
As is apparent from Figure 17, the first housing portion
45 is circular rather than octagonal, as is the case with
the embodiment of Figure 6.
10 The first housing portion 45 supports bar 41 in the same
manner as described via flexure web 59 which is located at
the centre of mass of the bar 41. The bar 41 is of
chevron shape, although the chevron shape is slightly
different to that in the earlier embodiments and has a
15 more rounded edge 41e opposite flexure web 59 and a
trough-shaped wall section 41f, 41g and 41h adjacent the
flexure web 59. The ends of the bar 41 have screw-
threaded bores 300 which receive screw-threaded members
301 which may be in the form of plugs such as grub screws
or the like. The bores 300 register with holes 302 in the
peripheral wall 52a. of the first housing portion 45. The
holes 302 enable access to the plugs 301 by a screwdriver
or other tool so that the plugs 301 can be screwed into
and out of the bore 300 to adjust their position in the
bore to balance the mass 41 so the centre of gravity is at
the flexure web 59.
As drawn in Figure 17, the bores 300 are a 450 angle to the
horizontal and vertical. Thus, the two bores (302 shown
in Figure 17) are at right angles with respect to one
another.
Figure 17 also shows openings 305 for receiving a portion
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.of the transducers 71 for monitoring the movement of the
bar 41 and producing signals in response to the movement.
Typically, each transducer 71 is in the form of a constant
charge capacitor. One capacitor plate typically is mounted
to the bar 41 and another capacitor plate is stationary
relative to the bar 41 so that a gap is defined between
the capacitor plates. Movement of the bar changes the gap
which in turn changes a voltage across the constant charge
capacitor.
Figure 18 is a more detailed view of part of the housing
portion of Figure 17 showing the openings 305. As can be
seen from Figure 18, the openings 305 have shoulders 401
which form grooves 402.
Figure 19 (a) to (f) show portions of the constant charge
capacitor transducers 71. The transducer shown in Figure
19 (a) comprises two electrodes. A first electrode is in
this embodiment provided by a surface of the sensor bars
41 or 42, which are at ground potential, and a second
electrode is shown in Figure 19 (a) (plate 408 a).
Figure 19 (b) shows the second capacitor electrode which
comprises two separate capacitor elements 408b and 407b
which are not in electrical contact. Again, the first
electrode is provided by the sensor bars 41 or 42, which
are at ground potential. The capacitor element 408b
surrounds the capacitor element 407b. This arrangement is
used for-generating a "virtual capacitor", which will be
described below with reference to Figure 22.
Figure 19 (c) and (d) show alternatives to the embodiment
shown in Figure 19 (b) and the shown second electrodes
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comprise adjacent elements 408c, 407c and 408d and 407d
respectively.
Figures 19 (e) and (f) show capacitor elements according
to further embodiments of the present invention. The
second electrode comprises three capacitor elements 408e,
407e, 407f and 408f, 407g and 407h, respectively, and this
arrangement is also used for generating a "virtual
capacitor which will be described below.
It will be appreciated, that in variation of this
embodiment the capacitor plates may have any other
suitable cross-sectional shape.
As an example, Figure 20 shows the location of the
capacitor elements 407b and 408b in the opening 305 and
opposite a corresponding second capacitor plate 411. In
this embodiment the capacitor elements 407b and 408b are
provided in the form of metallic foils that are positioned
on insulating body 409. The plate 411 is metallic and
positioned on the bar 41. In this embodiment plate 411
provides one capacitor element that opposes capacitor
elements 407 b and 408 b. In this case the bar 41 may be
of relatively low electrical conductivity or may be
electrically insulating.
If bar 41 is provided in the form of a metallic material
of sufficiently high electrical conductivity, the bar 41
itself may also provide a capacitor element and a portion
of the bar 41 may directly oppose the capacitor elements
407b and 408b without the plate 411, as discussed above in
the context of Figure 17.
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Figure 21 is a diagram of the bars 41 and 42 showing them
in their "in use" configuration. The transducers which
are located in the openings 305 are shown by reference
numbers 71a to 71e.
As will be apparent from Figure 21, four transducers 71
are arranged adjacent the ends of the bar 41. The second
housing portion 47 also has four transducers arranged
adjacent the bar 42. Thus, eight transducers 71 are
provided in the gradiometer.
Referring now to Figures 22 and 23 transducer circuitry
360 is now described. Each. of the transducers 71a to 71e
is a constant charge capacitor and comprises a first
capacitor electrode. Each of the transducers 71a to 71e
has a second capacitor electrode that is positioned
opposite a respective first capacitor electrode and fixed
in position relative to the housing portions. The first
capacitor electrode is in this embodiment provided by a
surface the sensor bars'41 or 42. For example, each
transducer 71a - 71e may have a second electrode of the
type as shown in Figure 19.
Oscillating movement of the sensor masses 41 and 42
results in a movement of the first capacitor electrodes
(surfaces of the sensor bars 41 or 42) relative to the
second capacitor electrodes. That movement changes the
gaps between respective first and second capacitor
electrodes and results in a voltage change across the
constant charge capacitor transducers 71a to 71e.
If the transducers are of the type as shown in Figure 19
(b) to 20 (d), then separate component transducers are
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formed between the first electrode and each capacitor
element of the second electrode, such as 407b and 408b. In
this case Figure 22 shows the transducer circuitry for the
component transducers formed between the first plate and
one of the two elements and an analogous circuitry
(labeled accordingly) is used for the component
transducers formed between the first electrode and the
other capacitor elements.
If the transducers are of the type as shown in Figure 19
(e) and 19 (f), then separate component transducers are
formed between the first electrode and each of the three
capacitor elements, such as 408e, 408e and 407f. In this
case, Figure 22 shows the transducer circuitry for the
component transducers formed between the first electrode
and one of the three elements and two analogous
circuitries (labeled accordingly) are used for the
component transducers formed between the first plate and
the other capacitor elements.
Each constant charge capacitor component transducer 71a to
71e has a separate bias voltage by a respective bias
voltage source Vgapy applied to it. Figure 22 shows
component transducer 71a to 71e with one of the capacitor
elements being connected to ground potential. As discussed
above, these capacitor elements are surfaces of the sensor
bars 41 and 42, which are in this embodiment electrically
conductive and connected to ground potential. The
polarities of the voltages provided by the bias voltage
sources 361a to 361e and the electrical interconnections
between the constant charge capacitor component
transducers 71a to 71e are chosen so that the electrical
signals generated by all transducers are combined with the
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same polarity if the sensor masses 41 and 42 oscillate in
opposite directions. Such oscillation in opposite
directions typically is generated by a gravity gradient.
If the sensor masses 41 and 42 move in the same direction,
5 one half of the electrical signals generated by the
constant charge capacitors component transducers 71a to
71e has one polarity and the other half has an opposite
polarity. Consequently, in this case, the electrical
signals typically cancel each other. Such movement in the
10 same direction may for example be generated by a sudden
movement of the aircraft in which the gravity gradiometer
is positioned and consequently the transducer circuitry
360 illustrated in Figure 22 reduces the effect of such
sudden movements and the effect of a number of other
15 external forces or external angular accelerations that are
not related to the gravity gradient.
The combined electrical signal is directed to a low noise
amplifier which will be described in the context of Figure
20 23.
The transducer circuitry 360 shown in Figure 22 also
comprises locking capacitors C$ajy which are arranged so
that the applied bias voltages VBOY cannot reach the lower
noise amplifier. The locking capacitors 362a to 362e
typically have a capacitance that is larger. than 10 times,
or even larger than 100 times that of the respective
constant charge capacitor component transducers 71a to
71e.
Further, the transducer circuitry 360 comprises resistors
RB.pY 363a to 363e. These resistors typically have a very
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high resistance, such as 1GC2 or more, and are arranged for
substantially preventing flow of charges and thereby
providing for the component transducers 71a to 71e to
operate as constant charge capacitors.
The bias voltages applied to the constant charge
capacitors generate electrostatic. forces. Consequently,
each transducer 71a to 71e can also function as an.
actuator.
If the transducers 71 are of the type as shown in Figure
19(a), then the circuitry 360 shown in Figure 22 is
sufficient. However in a specific embodiment of the
present invention the transducers are of the type as shown
in Figures 19 (b) to 19(d) and comprise two component
transducers. In this case two circuitries 360 are used,
one for the component transducers formed between the first
electrodes and one of the.capacitor elements, and the
other for the component transducers formed between the
first electrodes and the other capacitor elements. This is
schematically indicated in Figure 25. A first circuitry
360 is used for measurement purposes (differential mode,
"DM") and a second circuitry 360 is used to provide
feedback for external rotational motion correction (common
mode, "CM"), which will be described below with reference
to Figures 28 and 29.
Alternatively, the circuitries 360 may also be connected
so that "virtual capacitors" are formed. This will be
described below in more detail and is schematically
indicated in Figure 24.
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In another specific embodiment of the present invention
the transducers are of the type as shown in Figures 19 (e)
or 19 (f) and comprise three component transducers. In
this case three circuitries 360 are used. This is
schematically indicated in Figure 26. In this embodiment
two circuitries 360 are used for measurement purposes and
arranged so that "virtual capacitors" are formed. A third
circuitry 360 is used to provide feedback for external
rotational motion correction.
The following will describe how relative mechanical
properties of the sensor masses 41 and 42 can be tuned.
The resonance frequencies of the sensor masses 41 and 42
depend on the square of the electrostatic forces and
therefore the square of the applied bias voltage. For
example, the resonance frequencies may be tuned using a
mechanical test set up in which external forces are
applied to the sensor masses 41 and 42. if the resonance
frequencies are not identical, the bias voltages can be
adjusted until the resonance frequencies are identical.
The sensitivities of the transducer capacitors for sensing
the movement of the sensor masses is linearly dependent on
the electrostatic forces and thereby linearly dependent on
the applied bias voltages. Consequently, it is possible
to tune both the resonance frequencies and the
sensitivities of the transducers
Figure 23 shows a schematic circuit diagram of a low noise
.30 amplifier according to a specific embodiment of the
present invention. The low noise amplifier circuitry 366
is used to amplify the electrical signal generated by the
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transducer circuit 360 and to provide active feedback to
control properties of the transducers and sensor masses 41
and 42.
The amplifier circuit 366 simulates an impedance ZL and an
ohmic component of ZL provides active damping of resonant
electrical signals generated by the constant charge
capacitor component transducers 71a to 71e described
above. The active damping reduces the Q-factor of the
resonance and thereby increases the bandwidth within which
the resonance can be generated. That electrical damping
results in mechanical damping by generating electrostatic
damping forces at the constant. charge capacitor component
transducers 71a - 71e. Typically, the active damping is
adjusted so that the gravity gradiometer has a bandwidth
of the order of 1 Hz and the Q-factor of the active
damping is close to 0.5.
The impedance ZL-also has an imaginary component, which is
dependent on a simulated capacitance CL in parallel with
the simulated resistor RL. The imaginary component actively
controls the resonance frequency of the sensor masses 41
and 42 via the constant charge capacitor transducers 71a -
71e by simulating a change of the "stiffness" of the
pivotal coupling of the sensor masses 41 and 42 and
thereby fine-tunes the resonance frequency of the sensor
masses 41 and 42. As described above, the transducer
circuit 360 is arranged so that resonant oscillations in
which the sensor masses 41 and 42 oscillate in opposite
directions result in an additive electrical signal. The
simulated capacitance CL of the simulated impedance ZL
allows fine tuning of the resonance and thereby further
helps distinguishing that resonance oscillation from other
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common mode oscillations in which the sensor masses 41 and
42 oscillate in the same direction.
In this embodiment the amplifier circuit 366 provides
"cold damping", which introduces very little thermal
noise. Passive damping, such as damping using a
conventional resistor, is avoided as this would result in
thermal noise.
As described above, the constant charge component
capacitors 71a - 71e may combine sensing and actuator
functions. The amplifier circuit 366 provides an active
feedback loop between sensing and actuator functions and
provides electronic feedback control of mechanical
properties of the sensor masses 41 and 42.
The amplifier circuit 366 comprises an input 368 and an
output 369'. Further, the amplifier circuit 366 comprises a
low-noise j-FET differential amplifier 370 and impedances
Zi, Z2 and Z3. The low noise amplifier 370 has two input
terminals 371 and 372 and the impedance Z1 is connected
between the output terminal 369 and the low noise
amplifier input 371. The impedance Z2 is connected between
the output terminal 369 and the low noise amplifier input
372. The impedance Z3 is connected between the terminal 372
and a common ground terminal 373.
The amplifier circuit 366 simulates the impedance ZL with
ZLZ3 (eql)
Zz
Sv
The amplifier 370 has noise matched resistance RoPt=
S;'
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The term Sv is the spectral density of amplifier's voltage
noise and the term Si is the spectral density of
amplifier's current noise. In this embodiment the.
amplifiers noise matched resistance is a few 1MQ.
5
Further, the amplifier 370 has a noise temperature
Topt 2ks' (kB: Bolzman constant) of less than 1K.
e
The noise density Sr of the gradient error produced by
10 thermal noise near resonance is given by
U T
2i
Sr = a 2 o (eg. 2)
mA~Q""
15 where A. is the radius of the gyration of the sensor masses
41 and 42 and Qact the effective Q-factor associated with
the.active damping, M is the mass of the senor masses 41
and 42 and fo is the resonance frequency. The noise density
Sr is dependent on the noise of the amplifier and not on
20 the physical temperature of the amplifier circuit,.which
allows "cold damping" and control of other mechanical
properties without introducing significant thermal noise
at normal operation temperatures such as at room
temperature.
The component transducers 71a, 71b, 71g and'71h are also
used to form angular accelerometers for measuring the
angular movement of the mounting 5 so that feedback
signals can be provided to compensate for that angular
movement.
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Figure 27 shows an actuator for receiving the control
signals to adjust the mounting in response to angular
movement of the mounting 5.
The actuator shown in Figures 27 is also schematically
shown in Figure 8 by reference to numerals 53 and 54. The
actuators are the same and Figure 28 will be described
with reference to the actuator 54.
The actuator 54 comprises in this embodiment a permanent
NdFeB.magnet 410, a soft iron core 411, a non-magnetic
spacer 412 (aluminium, delrin), mumetal or permalloy
housing 413, a voice coil assembly 414, a hollow rod 428
and a tube 430 that forms part of the housing 413 and in
which the hollow rod 428 is rotatably mounted.
The voice coil assembly 414 is mounted onto rod 430 and
the permanent magnet 410 and the soft iron core 411 are
provided with internal bores through which the rod 430
penetrates so that the rod 430 with voice coil assembly
414 can move axially relative to the iron.core 311 and the
magnet 410. Electrical connections for the voice coil
assembly 414 are fed through the hollow rod 430.
As described above, one or both of the bars 41 and 42 can
also be used as an angular accelerometer to provide a
measure of angular movement of the mounting 5 so that
appropriate feedback signals can be generated to
compensation for that movement by control of the actuators
previously described.
Figures 28 (a) and (b) show schematic plan and cross-
sectional view of the gravity gradiometer 1. As indicated
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previously, the gravity gradiometer 1 comprises a housing
2 that is rotated by.an external mounting about a z-axis.
The external mounting comprises an inner stage 500 and an
intermediate stage 502 and an outer stage 504. The housing
2 is mounted so that it is rotated with the inner stage
500 by z-drive 508 with bearings. The z-drive provides
continuous rotation at a very stable speed. The rotational
frequency is in this embodiment'selectable between 0 and
20 Hz. The' intermediate stage 502 including the inner
10' stage 500 is rotable about the x-axis by x-drive 510,
which includes bearings and the outer stage 504 is rotable
with the intermediate stage 502 about the y-axis by y-axis
drive 512 which also include suitable bearings. The outer
stage with y-axis drive is mounted on springs 516 in a
support frame 518.
The external mount 3 includes an IMU (inertial measurement
unit), which contains gyroscopes, accelerometers, GPS
receivers and a computer. The IMU is not shown in Figure
28 (a) or (b). The IMU measures rotation about the x-, y-
and z- axis and is coupled to drives in a feedback loop.
This will be described below in more detail with reference
to Figure 29.
The external mounting is arranged to gyro-stabilize the
housing 2 about the x-,y- and z-axis with a. gain factor of
approximately 100 DC and a bandwidth of 20 Hz. This is
achieved using .the above-described 3-axis "gimbal" bearing
arrangement with direct drive torque motors (508, 510 and
512). In this embodiment, fine-tuning of the motor drive
for correction of rotation about the z-axis is achieved
using the "common mode" signal provided by respective
transducer components positioned within the housing 2.
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Figure 29 shows a block diagram 600 that illustrates how
the common mode signal, generated within the housing 2
("internal platform"), is used for rotational z-axis
correction of the external support structure ("external,
platform").
Blocks 602 and 604, labelled "response to motion" and
"response to force" respectively, both represent the
gimbal structure of the support structure 3. Each gimbal
consists of three main components, namely a frame, a part
supported by the frame via a bearing and an actuator which
applies a torque (force) to this part. Each gimbal has two
independent inputs, namely motion applied to the frame and
a force applied directly to the part suspended by the
frame. It has only one output, namely the angular position
of the supported part and this responds differently to the
two inputs.
Feedback force Fe counteracts an external disturbance Z.
This may be expressed by the following equation
Xe = Hf Fe + HZ Z (eq. 3)
where Hf and HZ are constants.
Equation 3 may be written as
Xe = Hf ( Fe + Ke Z } (eq. 4)
where Ke = HZ/Hf.
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An external motion, such as a motion of an aircraft in
which the gravity gradiometer 1 is positioned, produces an
equivalent force Ke Z, which is counteracted by Fe
generated by the actuator 610. In Figure 29 block 602
"Response to motion" represents Ke and block 604 "Response
to force" represents He. The sensor 606 for the external
platform is the IMU, which contains gyroscopes,
accelerometers, GPS receivers and a computer. This
provides a signal (usually digital) which measures the
angular position and angular rate of the supported part of
the innermost gimbal. This signal is used in the controller
608 (also usually digital) to implement the feedback.
The internal platform may be represented in an analogous
manner where blocks 612 and 614 labelled "response to
motion" and "response to force" respectively, both
represent the z-axis gimbal structure within the housing
2. The transducer sensors 71 and the actuator 54 have been
described above.
In the above-described embodiment the gravity gradiometer
1 is arranged so that rotation about the z-axis is
controlled to a fixed uniform rotation speed. The input
signal for controlling the motion is provided by the IMU
606 and directed to the controller 608. However, the IMU
606 may only have limited accuracy at the higher
frequencies and to improve the z-axis rotational
correction further, an angular acceleration derived from
the above-described "Common Mode" signal from the internal
transducers 71 is used for fine-tuning. This same signal
is also used inside the internal platform in a feedback
loop to stabilise the instrument against applied angular
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acceleration (via actuator 54). The specification for this
internal feedback system is stringent and to ease this
requirement, some of the burden is transferred to the
external platform in that manner.
5
In a variation of the above-described embodiment the IMU
may also be used in a feed-forward configuration.
Figure 30 shows a block diagram 650 that illustrates
10 stabilisation (no rotation) about the x-and y- axis, which
is performed exclusively by the external platform. All
elements of Figure 30 were already described above and
function in an analogous manner to inhibit rotation about
the x- and y- axes.
Referring now to Figure 29, a method of correcting an
output from the gravity gradiometer in accordance with a
specific embodiment of the present invention is now
described. As described above, external disturbance
forces, such as forces that are associated with sudden
impacts on an aircraft in which the gravity gradiometer 1
is positioned, cause equivalent forces Ke Z on components
of the gravity gradiometer. For example, the equivalent
force Ke Z may be measured by an IMU. The output signal of
the gravity gradiometer 1, in this embodiment the output
signal of the transducers sensors 71, is corrected for an
impact by such external forces.
The external disturbance forces and the equivalent forces
as experienced by a component if the gravity gradiometer
(the component may for example be the internal platform
including the gravity gradient detector or the IMU on the
external platform) are related to each other by the above
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mentioned respective constants Ke, which in this embodiment
are determined as follows. The gravity gradiometer 1 is
initially exposed to an external force and an equivalent
disturbance force is then measured-for.example by one-of
the.IMUs. The constant Ke is determined from the ratio of
the external force and the measured equivalent force.
Alternatively or additionally, the constants Ke are
determined using a mathematical model that models a
mechanical responsiveness of the component to the external
force.
In this embodiment the motion of the gravity gradiometer 1
is corrected for components of an external disturbance
force in three orthogonal directions and for rotation
about three orthogonal axes. Consequently, the motion of
the gravity gradiometer 1 is corrected for 6 degrees of
freedom. The gravity gradiometer 1 comprises a plurality
of sensors (such as the IMUs and the transducer sensors
71) and is arranged so that motions for each one of the 6
degrees of freedom are detected. For each degree of
freedom it is initially investigated if the respective
component of the applied external force causes an
equivalent component force that is correlated with that
component of the external force. Once a correlation has
been identified, for example between the external force
and an output signal of the gravity gradient detector as
detected by the transducer sensors 71, respective
constants Kei (i= 1,2,3,4,5 or 6) are then obtained
mathematically and/or from the ratio of the measured
components of the equivalent disturbance force and the
respective components of the external disturbance force.
In this example the external disturbance is a mechanical
disturbance. However, it will be appreciated that in an
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alternative example the external disturbance may be an
electrodynamic disturbance and the gravity gradiometer may
comprise a disturbance sensor that generates an electrical
signal in response to a detected electromagnetic
disturbance.
In use of the gravity gradiometer 1 the output can be
corrected for influences of external disturbance forces in
the following manner. As described above, components of
equivalent external disturbance forces are measured by
sensors of the gravity gradiometer. Actuators such as
actuator 610 or the transducer sensors 71, may be used to
counteract the detected components of the equivalent
external disturbance force. Such correction forces may
only partially correct for the equivalent external forces
or correction forces may not be applied. In this case the
components of the external disturbance forces are
calculated from the measured equivalent external
disturbance forces. The output of the gravity gradiometer
is then numerically corrected for the external disturbance
force using the previously determined respective constants
Kei.
In one specific variation the gravity gradiometer may also
be arranged so that the motion of the gravity gradiometer
1 is corrected for 6 degrees of freedom (rotation about
three orthogonal axes and translation along three
orthogonal axes).
Although the invention has been described with reference
to particular examples, it will be appreciated by those
skilled in the art that the invention may be embodied in
many other forms. For example, the transducers may not
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necessarily be provided in the form of constant charge
capacitors, but may be provided in the form of any other
suitable type of capacitor including those that do not
allow simulation of a virtual capacitor. Further, it is
to be appreciated that the amplifier circuitry 366 shown
in Figure 24 is only one embodiment and a variety of
variations from the described embodiment are possible.
In addition, the gravity gradiometer may be arranged for
measuring other components of the gravity gradient, in
which case the gravity gradiometer would not be arranged
for operation in the described orientation. For example,
the gravity gradiometer may be arranged to measure the r.,
and (F -ry) or rXZ and (r,,,-ryy) of the gravity gradient.
The reference that is being made to documents WO 90/07131
and PCT/AU2006/001269 does not constitute an admission
that these documents form a part of the common general
knowledge in Australia or in any other country.
In the claims which follow and in the preceding
description of the invention, except where the context
requires otherwise due to express language or necessary
implication, the word "comprise" or variations such as
"comprises" or "comprising" is used in an inclusive sense,
i.e. to specify the presence of the stated features but
not to preclude the presence or addition of further
features in various embodiments of the invention.
