US 20080282796 A1
A gravity gradiometer is disclosed which has a sensor in the form of bars (41 and 42) which are supported on a mounting (5) which has a first mount section (10) and a second mount section (20). A first flexure web (33) pivotally couples the first and second mount sections about first axis. The second mount has first part (25), a second part (26) and a third part (27). The parts (25 and 26) are connected by second flexure web (37) and the parts (26 and 27) are connected by third flexure web (35). The bars (41 and 42) are located in housings (45 and 47) and form a monolithic structure with the housings (45 and 47) respectively. The housings (45 and 47) are connected to opposite sides of the second mount section 20. The bars (41 and 42) are connected to their respective housings by flexure webs (59).
1. A gravity gradiometer for measuring components of the gravity gradient tensor, comprising:
a sensor for measuring the components of the gravity gradient tensor;
a mounting for providing rotation about three substantially orthogonal axes, and for supporting the sensor, the mounting comprising
(a) a first mount section;
(b) a second mount section;
(c) a first flexure web for pivotally coupling the first and second mount sections for pivotal movement about a first axis;
(d) the second mount section having a first part, a second part and a third part, the first part connecting to the second part by a second flexure web for pivotally coupling the first part to the second part for pivotal movement about a second axis orthogonal to the first axis; and
(e) the third part being coupled to the second part by a third flexure web for pivotally coupling the third part to the second part for pivotal movement about a third axis orthogonal to the first and second axes.
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This invention relates to a gravity gradiometer, and in particular, but not exclusively, to a gravity gradiometer for airborne use. The invention has particular application for measuring diagonal and off-diagonal components of the gravitational gradient tensor.
Gravimeters are widely 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.
International publication WO 90/07131 partly owned by the present applicants associated company discloses a gravity gradiometer. The gradiometer includes a gimbal bearing arrangement comprised of three concentric rings in which is mounted the sensing equipment. The sensing equipment generally comprises two spaced apart bars respectively located in shielded housings and each mounted on a web bearing. The instrument disclosed in that application is relatively complicated in that it includes a large number of parts and is relatively heavy which is a disadvantage particularly in airborne applications.
The invention provides a gravity gradiometer for measuring components of the gravity gradient tensor, comprising:
Since the mount uses the flexure webs to allow movement about the three axes, the number of components needed to make up the mounting can be reduced significantly and, indeed, minimised to two thereby reducing the complexity of the gradiometer and also the weight of the gradiometer.
In the preferred embodiment the sensor comprises a first mass and a second mass.
Preferably the first flexural web divides the first mount into a primary mount portion and a secondary mount portion, the sensor being connected to one of the primary mount portion and secondary mount portion, so that the primary mount portion can pivot relative to the secondary mount portion about the first flexural web to thereby pivotally couple the first and second mount sections for pivotal movement about the first axis.
Preferably the second mount is connected to the other of the primary mount portion and secondary mount portion.
Preferably the sensor is connected to the primary mount portion and the second mount is connected to the secondary mount portion.
In one embodiment of the invention the first mass comprises a first bar and the second mass comprises a second bar.
In the preferred embodiment of the invention the first bar is coupled to the first mount section and the second bar is coupled to the first mount section.
Preferably the first bar and second bar are arranged transverse with respect to one another. Most preferably the first bar and second bar are arranged orthogonal to one another.
Preferably the first bar is located in a first housing which is fixed to the first mount section, the first bar being connected to the first housing portion by a fourth flexure web for movement relative to the first housing in response to the gravitational field.
Preferably the second bar is located in a second housing fixed to the first mount section and connected to the second housing by a fifth flexure web so the second bar can move relative to the second housing in response to the gravitational field.
Preferably the first and second housings are coupled to the primary portion of the first mount, and the second mount is coupled to the secondary portion of the first mount.
Preferably the fourth and fifth flexure web have a common axis of rotation.
Preferably the first and second bars have associated transducers for outputting a signal indicative of movement of the bars in response to the gravitational gradient.
Preferably the first housing and first bar is a monolithic structure and the second housing and the second bar is a monolithic structure.
Preferably the second mount section is a monolithic structure.
Preferably the second flexure web is formed by making a first cut in the second mount section to separate the first and second parts, the cut extending through the second mount section except for portions which define the second flexure web.
Preferably the third flexure web is formed by a second cut in the second mount section which separates the second and third parts, the second cut extending all the way through the second mount section except for portions which define the third flexure web.
Preferably the second mount section is cylindrical and the first cut is formed in a cylindrical wall of the section to form the first flexure web which has two web portions diagonally opposite one another, and the second flexural web is formed by the second cut in the wall and is formed by two web portions diagonally opposite one another.
Preferably the first part has mounting lugs for mounting the mount within a Dewar for cryogenic operation of the gradiometer.
Preferred embodiments of the invention would be described, by way of example, with reference to the accompanying drawings, in which:
The gradiometer shown in
A vacuum canister 3 is provided in the Dewar and the Dewar is supplied with liquid gas such as liquid helium He so that the gradiometer can operate at cryogenic temperature. The Dewar 1 is closed by an end plate 4 which includes connectors 5 a for connecting electrical leads (not shown) to external components (not shown).
The canister 3 is closed by an end plate 9 which includes connectors 5 b for connecting electric leads (not shown) to the connectors 5 a. The gradiometer has a main casing 61 formed from a twelve-sided ring 62 and hemispherical domes 63 (see
With reference to
The first mount 10 is joined to the second mount 20. The first flexure web 31 is formed in the first mount 10 so a primary mount portion of the mount 10 can pivot about a 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
The lugs 13 connect the mounting 5 in the canister 3 which, in turn, locates in the Dewar 1 for cryogenic operation of the gradiometer.
The Dewar is in turn mounted in a first external platform for course rotational control of the gradiometer about three orthogonal x, y, x axes. The mounting 5 mounts the sensor 40 (which will be described in more detail hereinafter and which is preferably in the form of a mass quadrupole) for much finer rotational adjustment about the x, y and z axes for stabilising the gradiometer during the taking of measurements particularly when the gradiometer is airborne.
The first flexure web 31 allows the first mount 10 to move relative to the second mount 20 about a z axis shown in
The bar 41 is formed in a first housing 45 and the bar 42 is formed in a second housing 47. The bar 41 and housing 45 is the same as bar 42 and the housing 47 except that one is rotated 90° with respect to the other so that the bars are orthogonal. Hence only the housing 45 will be described.
The housing 45 has an end wall 51 and a peripheral side wall 52 a. The end wall 51 is connected to rim 75 (
The bar 41 and the housing 45 together with the flexure web 59 are an integral monolithic structure.
Transducers 71 (not shown in
The preferred embodiment also includes angular accelerometers which are similar in shape to the bars 41 and 42 but the shape is adjusted for zero quadrupole moment. The linear accelerometers are simple pendulous devices with a single micro pivot acting as the flexural hinge.
The transducers 71 measure the angle of displacement of the bars 41 and 42 and the control circuitry (not shown) is configured to measure the difference between them.
Error correction can be performed numerically based on digitised signals from the accelerometers and a temperature sensor.
The transducers 71 are SQUID based transducers and the error correction is made possibly by the large dynamic range and linearity of the SQUID based transducers.
In this 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
As is shown in
As is apparent from
In the embodiment of
In this embodiment, top wall 24 is provided with a central hole 137 and two attachment holes 138 a. Three smaller holes 139 a are provided to facilitate pushing of the housing 45 off the part 18 a if disassembly is required. When the second mount 20 is located within the first mount 10, the upper part of central section 18 c projects through the hole 137, as best shown in
Thus, when the first housing 45 and its associated bar 41 is connected to the rim 75 of the housing 10 and the second housing 47 is connected to the base 12, the housings 45 and 47 and their associated bars 41 and 42 are therefore able to move about three orthogonal axes defined by the flexure web 31, the flexure web 33 and the flexure web 37.
As is best seen in
Thus, when the second mount 20 is fixed to the part 18 a, the second mount 20 can pivot with the first portion 10 a of the first mount 10 about a z axis defined by the flexure web 31 whilst the second portion formed by the part 18 a remains stationary. Movement about the x and y axes is achieved by pivotal movement of the second mount 20 about the flexure webs 33 and 35 as previously described.
The gravity gradient exerts a torque on a rigid body with any mass distribution provided it has a non-zero quadrupole moment. For a planar body, in the x-y plane and pivoted about the z-axis, the quadrupole is the difference between moments of inertia in the x and y directions. Thus a square or circle has zero quadrupole moment, while a rectangle has a non-zero value.
The torque produced is what constitutes the signal measured by the gradiometer.
There are two dynamical disturbances which can also produce torques and consequently are sources of error.
The first is linear acceleration.
This produces a torque if the centre of mass is not exactly at the centre of rotation—i.e. the bar is “unbalanced”. The bars 41 and 42 are balanced as well as possible (using grub screws to adjust the position of the centre of mass) but this is not quite good enough, so there is a residual error. This error can be corrected by measuring the linear acceleration and using this to numerically subtract away the erroneous part of the signal.
The second is angular motion.
There are two aspects to angular motion, each of which produces a different error.
The first is aspect angular acceleration.
Angular acceleration produces a torque on the mass distribution through its moment of inertia (even if the quadrupole moment is zero). This is an enormous error and two preferred techniques are used to counteract it.
The first is to use internal rotational stabilization. This is depicted in the block diagram of
The second is to use common mode rejection CMRR—that is why 2 orthogonal bars are needed. For the two bars, the error torque produced by the angular acceleration is in the same direction, but the signal torque produced by the gravity gradient is in opposite direction.
Therefore, by measuring the difference in deflection between the two bars, the gradient is sensed but not the angular acceleration.
Therefore, two separate angular accelerometers 90 (labeled 90′ in
The x and y axes require separate angular accelerometers. Rotational stabilization about these axes is required because the pivot axes of the two bars are not exactly parallel and also to counteract the second form of error produced by angular disturbance, discussed below.
The second aspect is angular velocity.
Angular velocity produces centrifugal forces, which are also a source of error. The internal rotational stabilization provided by the actuators reduces the angular motion so that the error is below 1 Eotvos.
The housing 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 more rounded edge 41 e opposite flexure web 59 and a trough-shaped wall section 41 f, 41 g and 41 h 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 52 a of the housing 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
Thus, the coil 408 and the bar 41 form an 1 c circuit so that when the bar 41 moves, the current passing through the coil 408 is changed.
As will be apparent from
With reference to
Input terminals 361 provide input current to the superconducting circuits shown in
The transducers 71 a, 71 b, 71 g and 71 e are connected in parallel to circuit line 365 and to circuit line 366 which connect to a SQUID 367.
Thus, as the bars 41 and 42 rotate about their respective flexure web, the bars 41 and 42, for example, come closer to the transducer 71 a and therefore further away from the transducer 71 b, and closer to the transducer 71 h and further away from the transducer 71 g respectively. This therefore changes the current flowing through the transducers and those currents are effectively subtracted to provide signals for providing a measure of the gravity gradient.
As is shown in
The transducers 71 a, 71 b, 71 g and 71 h 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.
To do this, the line 366 is connected to a transformer 370. The polarity of the signals from the transducers 71 a and 71 b and 71 g and 71 h are reversed so that the output of the transducer 370 on lines 371 and 372 is an addition of the signals rather than a substraction, as is the case when the gradient is measured so the addition of the signals gives a measure of the angular movement of the bars. The outputs 371 and 372 are connected to SQUID device 375 for providing a measure of the angular acceleration which can be used in the circuit of
Thus, according to the preferred embodiment of the invention, the angular accelerometers 90′ provide a measurement of angular acceleration, for example, around the x and y axes, and the angular accelerometer formed by the bars 41 and 42 and the transducers 71 a, 71 b, 71 g and 71 h provide a measure of the angular accelerometer around the, for example, z axis.
The actuator shown in
Actuator 52 shown in
The disc 313 is also provided with a radial bore 319 and a hole 320 at the periphery of the disc 313 which communicates with the bore 319. A hole 321 is provided at the hub 314 and communicates with the bore 319 and extends to a hollow rod 328 which locates in a tube 330. The rod 330 is fixed to the disc 313 and also to support frame 340 which is fixed to main body 61 (not shown in
The winding W1 provided on the face 315 has a lead 331 which passes through the hole 320 and then through the bore 319 to the hole 321 and then through the tube 328 to the right, as shown in
The second winding W2 provided on the face 316 has a lead 333 which passes through a radial-hole 334 and bore 345 in the disc 313 and then through hole 337 to tube 328 and to the left in
When the windings W1 and W2 are energised or the current passing through the windings changes, the disc housing 310 is moved relative to the disc 313 and frame 340 and because the disc housing 310 is connected to the mounting 5 by the bracket 311, the mounting 5, in the case of the actuator 52, is adjusted. The movement of the disc housing 310 is generally a longitudinal movement (i.e. linear movement) in the direction of the axis of the tube 330 and rod 328. To facilitate such movement, clearance is provided between the ends of the rod 330 and the frame 340 and about the disc 313. The bracket 311 is offset relative to the flexure web (such as the flexure web 37) so that movement of the housing 310 applies a torque to the first part 25 of the mounting 5 to cause rotation of the part 25 about the flexure web 37.
In the preferred embodiment of the invention, four actuators are provided for providing actual adjustment about the various axes and flexure webs and the actuators operate in combination in response to signals received from the angular accelerometers to maintain stability of the mounting 5 when the gradiometer is in use.
For cryogenic operation of the gradiometer, the mounting 5, housings 45 and 47, bars 41 and 42, the hollow disc housing 310, coils, and electrical leads referred to previously, are all made from superconducting material such as niobium.
In embodiments of the invention where the gradiometer is not cryogenically operated, the components can be formed from other materials such as aluminium.
The angular accelerometers 90′ have zero quadrupole moment which means that the centre of mass coincides with the flexure web and that consequentially they are insensitive to both gravity gradient and centrifugal force. Linear accelerometers 90″ (
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.
In the preferred embodiment, four angular accelerometers are provided with two of the accelerometers being formed by the bars 41 and 42. The use of four accelerometers arranged at 45° angles with respect to one another enables adjustment about the x, y and z axes by torque supplied from two or more of the actuators at any one time.
The disc 310 prevents flux from the windings W1 and W2 from leaving the actuator and because the leads 331 and 332 and 333 and 338 leave the actuator through the elongate tube 330, the ability of flux to pass out of the actuator is substantially prevented.
Thus, spurious magnetic fields which may detrimentally effect operation of the instrument are not generated by the actuator and therefore do not influence the sensitivity or operation of the instrument.
The tube 330 preferably has a length to diameter ratio of 10:1 at the least.
The disc plate 316 is preferably formed from macor and the hollow disc housing 310 is formed in two parts 310 a and 310 b. The part 310 b forming a closure panel which enables the disc 313 to be located in the chamber 312 and then the disc housing 310 closed by locating the plate 310 b in place.
With reference to
The bars 41 and 42, in their respective housings, are rotated in a jig (not shown) through 360°. This provides an acceleration range of 2 gE, which is typically 100 times greater than the accelerations which may be conveniently applied at low temperature. A typically requirement is for the capacitors 400 and 401 to be able to detect 0.1 nm over a period of 1 to 20 minutes. A pair of capacitors 400 and 401 is required for each bar to provide some discrimination against sensor drift, since rotation of the bar 41 will cause one capacitor 400 to increase and the other capacitor 401 to decrease by the same amount, as is shown in
The capacitors 400 and 401 are formed by the face 41 a of the bar 41 (and the corresponding face on the other bar 42) and second plates 405 which are spaced from the face 41 a. The gap between the plates of the respective capacitors 400 and 401 must typically be resolved to about 1 ppm.
The capacitor 400 forms a high Q-factor resonant circuit with inductor 410. The inductor 410 and capacitor 400 are provided parallel to capacitors 411 and 412 and connect via capacitor 413 to an amplifier 414. The output of the amplifier 414 is provided to a frequency counter 415 and also fed back between the capacitors 412 and 411 by line 416. The capacitor 400 therefore determines the operating frequency of the amplifier 414 which can be read to a high precision.
If the bar 41 is out of balance, the frequency counter 45 will tend to drift because of the imbalance of the bar. This can be adjusted by moving the grub screws 301 into and out of the masses as previously described until balance takes place. The amplifier 414 can then be disconnected from the frequency counter 415 so that the gradiometer can be arranged within the Dewar 1 with the other parts of the circuits shown in
Since modifications within the spirit and scope of the invention may readily be effected by persons skilled within the art, it is to be understood that this invention is not limited to the particular embodiment described by way of example hereinabove.
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.