US20080302180A1 - Gravity Gradiometer - Google Patents
Gravity Gradiometer Download PDFInfo
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- US20080302180A1 US20080302180A1 US11/722,050 US72205006A US2008302180A1 US 20080302180 A1 US20080302180 A1 US 20080302180A1 US 72205006 A US72205006 A US 72205006A US 2008302180 A1 US2008302180 A1 US 2008302180A1
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- mount
- bar
- gravity gradiometer
- mount section
- gradiometer
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V7/00—Measuring gravitational fields or waves; Gravimetric prospecting or detecting
- G01V7/16—Measuring gravitational fields or waves; Gravimetric prospecting or detecting specially adapted for use on moving platforms, e.g. ship, aircraft
Definitions
- 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 first bar is located in a first housing which is fixed to the first mount section, the bar being connected to the first housing by a fourth flexure web for movement relative to the first housing in response to the gravitational gradient.
- actuators are provided for moving the mount about the three orthogonal axes so as to stabilise orientation of the sensor during use of the gradiometer.
- FIG. 27 is a view similar to FIG. 25 but showing the transducer of FIG. 26 in place;
- FIG. 32 is a view of part of the actuator of FIG. 31 ;
- FIG. 1 is a schematic view of a gravity gradiometer according to the preferred embodiment of the invention.
- the gradiometer shown in FIG. 1 comprises a double walled Dewar 1 which is supported in an external platform 2 .
- the external platform 2 enables adjustment of the Dewar and therefore the contents of the Dewar about three orthogonal axes.
- the external platform 2 is generally known and its adjustment by suitable motors or the like is also known. Thus, a detailed description will not be provided.
- 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).
- FIGS. 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 connecting the mount to the casing 61 .
- the top wall 24 and the peripheral wall 22 define an opening 28 .
- the peripheral wall 22 has a first part 25 , a second part 26 and a third part 27 .
- the second mount 20 is a monolithic integral structure and the first part 25 is formed by making a circumferential cut 19 through the peripheral wall except for the formation of flexure webs as will be described hereinafter.
- the third part 27 is formed by making a second circumferential cut 29 through the peripheral wall 22 except for flexure webs which will also be described hereinafter.
- the second mount 20 is mounted on the first mount 10 by locating the hub 18 into the opening 28 and the lugs 13 through respective cut-outs 16 as is shown in FIG. 7 .
- FIG. 8 shows sensor 40 mounted on the mounting.
- the sensor 40 is an Orthogonal Quadrupole 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 FIG. 8 ) orthogonal to the bar 41 and which is of the same shape as the bar 41 .
- OFR Orthogonal Quadrupole Responder
- the bar 41 and the housing 45 together with the flexure web 59 are an integral monolithic structure.
- Transducers 71 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.
- FIG. 10 is a schematic block diagram showing actuator control to stabilise the gradiometer by rotating the mounting 5 about three orthogonal axes (x, y, z).
- a controller 50 which may be a computer, microprocessor or the like outputs signals to actuators 52 , 53 , 54 and 55 .
- the actuator 52 could rotate the mounting 5 about the x axis
- the actuator 54 could rotate the mounting 5 about the y axis
- the actuator 54 could rotate the mounting 5 about the z axis.
- two of the four actuators 52 , 53 , 54 and 55 are used to rotate the mounting about each axis so that rotation about each axis is caused by a combination of two linear movements provided from two actuators.
- each actuator will be described with reference to FIGS. 31 and 32 .
- 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 as is required to stabilise the support during movement through the air either within or towed behind an aircraft.
- FIG. 12 shows the mounting 5 arranged within the casing 61 and formed by the ring 62 and the transparent hemispherical ends 63 .
- the ring 22 has connectors 69 for connecting the internal wiring from transducers 71 (see FIG. 8 ) and SQuID (Superconducting Quantum Interference Device) Electronics located in the casing 61 to the connectors 5 b ( FIG. 1 ).
- Error correction can be performed numerically based on digitised signals from the accelerometers and a temperature sensor.
- 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 FIGS. 19 to 21 .
- 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 which forms the second flexure web 33 and the third flexure web 37 .
- the cut line 88 tapers outwardly from the upper end shown in FIG. 14 to the lower end and the core 18 c tapers outwardly in corresponding shape, as best shown in FIG. 17 .
- FIG. 21 is an exploded view of the three parts 25 , 26 and 27 which make up the second mount 20 .
- an opening extends through the mount 20 which is formed by the hole 137 , hole 138 and hole 139 .
- the mount 20 shown in FIG. 21 is a monolithic structure and is merely shown in exploded view to clearly illustrate the location of the flexural webs 33 and 35 .
- the flexural web 33 shown in FIG. 21 joins with the part 26 and the flexural web 35 shown in FIG. 21 joins with the part 27 .
- the holes 137 , 138 and 139 define a passage through which the axle or first portion 18 a of the first mount 10 can extend when the second mount 20 is located in the first mount 10 .
- the torque produced is what constitutes the signal measured by the gradiometer.
- the first is to use internal rotational stabilization.
- Ho(s) represents the sensor assembly pivoted about the mounting 5 (as per FIG. 9 ).
- the block A(s) represents the actuator, which provides the feedback torque to effect the stabilization by canceling the applied disturbances.
- T(s) represents the sensor (or transducer) which measures the effect of the applied disturbance.
- This is the angular accelerometer.
- angular accelerometers in rotational control is unusual—usually gyros and/or highly damped tilt meters are used, but for our purpose the angular accelerometers are better, as the error is proportional to the angular acceleration disturbance.
- CMRR common mode rejection
- 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.
- 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 subtraction, 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 FIG. 10 to provide compensation signals to stabilise the mounting 5 .
- Actuator 52 shown in FIG. 31 has a hollow disc housing 310 which has a mounting bracket 311 for connecting the disc housing 310 to mounting 5 .
- the hollow disc housing 310 therefore defines an inner chamber 312 in which is located coil support plate in the form of a disc 313 .
- the disc 313 has a wide hub section 314 and two annular surfaces 315 and 316 onto which windings W 1 and W 2 of coils are wound about the hub 314 .
- the second winding W 2 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 FIG. 31 .
- the other end of the winding W 2 has a lead 338 which passes through the hole 337 into the tube 328 and to the left in FIG. 31 .
- current can circulate through the winding W 2 via the leads 333 and 338 .
- 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.
Abstract
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 a first axis. The second mount has a first part (25), a second part (26) and a third part (27). The parts (25 and 26) are connected by a second flexure web (37) and the parts (26 and 27) are connected by a 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). Transducers (71) are located in proximity to the bars for detecting movement of the bars to in turn enable the gravitational gradient tensor to be measured. The first mount section (10) has cut-outs (16) and the second mount section (20) has lugs (13) which pass through the cut-outs for connecting the first and second mount sections (10 and 20) in a Dewar (1).
Description
- 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 gravitational gradient tensor, comprising:
- a sensor for measuring the components of the gradient tensor;
- a mounting for supporting the sensor, the mounting comprising:
- a first mount section having a base and a first mount peripheral wall, the peripheral wall having a plurality of cut-outs, the first mount section being mountable for rotation about a first axis;
- a second mount section for mating with the first mount section, the second mount section having a peripheral wall; and
- connectors extending outwardly from the peripheral wall and which pass through the respective cut-outs in the first mount section so as to mount the second mount section and therefore the first mount section for rotation about a second axis and a third axis; and
- wherein the connectors are for connecting the first and second mount sections in a Dewar for cryogenic operation of the gradiometer.
- The form of the mounting according to this aspect of the invention avoids much of the weight of the gimbal rings used in prior art designs. Thus, gradiometers made in accordance with this aspect of the invention are of significantly decreased weight compared to previous designs.
- Preferably the first, second and third axes are orthogonal z, x and y axes.
- Preferably the connectors comprise radially extending lugs.
- In one embodiment the lugs are integral with the second mount section.
- In another embodiment the lugs are separate to the second mount section and are attached to the second mount section.
- Preferably the sensor is a first bar and a second bar transverse with respect to the first bar, and the second mount section has first, second and third parts.
- In the preferred embodiment of the invention the first bar is connected to the first mount section and the second bar is connected to the first mount section.
- Most preferably the first bar and second bar are arranged orthogonal to one another.
- Preferably the first mount section has a first flexural web for mounting the first mount section for rotation about the z axis.
- 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 section is cylindrical and a first cut is formed in a cylindrical wall of the section to form a second flexure web which has two web portions diagonally opposite one another, and a third flexural web is formed by a second cut in the wall and is formed by two web portions diagonally opposite one another, the first cut separating the first and second parts and the second cut separating the second and third parts.
- Preferably the first part has mounting lugs for mounting the mount within a Dewar for cryogenic operation of the gradiometer.
- Preferably the first bar is located in a first housing which is fixed to the first mount section, the bar being connected to the first housing by a fourth flexure web for movement relative to the first housing in response to the gravitational gradient.
- Preferably the second bar is located in a second housing fixed to the first mount section, and connected to the housing by a fifth flexure web so the second bar can move relative to the housing in response to the gravitational gradient.
- 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.
- In the preferred embodiment of the invention actuators are provided for moving the mount about the three orthogonal axes so as to stabilise orientation of the sensor during use of the gradiometer.
- Preferably the actuators are computer controlled.
- Preferably linear and angular accelerometers are provided.
- Preferred embodiments of the invention would be described, by way of example, with reference to the accompanying drawings, in which:
-
FIG. 1 is a schematic view of a gradiometer of one embodiment of the invention. -
FIG. 2 is a perspective view of a first mount forming part of a mounting of the gradiometer of the preferred embodiment; -
FIG. 3 is a view of a second mount of the mounting; -
FIG. 4 is a view from underneath the mount ofFIG. 3 ; -
FIG. 5 is a cross-sectional view along the line IV-IV ofFIG. 3 ; -
FIG. 6 is a cross-sectional view along the line V-V ofFIG. 3 ; -
FIG. 7 is a view of the assembled structure; -
FIG. 8 is a view showing the sensor mounted on the gimbal structure; -
FIG. 9 is a plan view of a bar of the preferred embodiment; -
FIG. 10 is a diagram showing actuator control; -
FIG. 11 is a block diagram showing operation of the rotatable support system; -
FIG. 12 is a view of a gradiometer of the preferred embodiment; -
FIG. 13 is a view of a first mount of a second embodiment; -
FIG. 14 is a view of part of the mounting ofFIG. 13 to illustrate the location and extent of the flexural web of the first mount; -
FIG. 15 is a view of the mounting ofFIG. 13 from beneath; -
FIG. 16 is a view of the mounting ofFIG. 13 including a second mount of the second embodiment; -
FIG. 17 is a cross-sectional view through the assembly shown inFIG. 16 ; -
FIG. 18 is a view from beneath of the section shown inFIG. 17 ; -
FIG. 19 is a view from beneath of the second mount of the second embodiment; -
FIG. 20 is a view of the second mount ofFIG. 19 from above; -
FIG. 21 is an exploded view of the second mount of the second embodiment; -
FIG. 22 is view of the assembled mounting and sensors according to the second embodiment; -
FIG. 23 is a perspective view of the gradiometer with some of the outer vacuum container removed; -
FIG. 24 is a plan view of a housing for supporting a bar according to a further embodiment of the invention; -
FIG. 25 is a more detailed view of part of the housing ofFIG. 24 ; -
FIG. 26 is a view of a transducer used in the preferred embodiment; -
FIG. 27 is a view similar toFIG. 25 but showing the transducer ofFIG. 26 in place; -
FIG. 28 is a diagram to assist explanation of the circuits ofFIGS. 29 and 30 ; -
FIG. 29 is a circuit diagram relating to the preferred embodiment of the invention, particularly showing use of one of the sensors as an angular accelerometer; -
FIG. 30 is a frequency tuning circuit; -
FIG. 31 is a cross-sectional view through an actuator according to one embodiment of the invention; -
FIG. 32 is a view of part of the actuator ofFIG. 31 ; -
FIG. 33 is a diagram illustrating balancing of the sensors of the gradiometer of the preferred embodiment; and -
FIG. 34 is a circuit diagram of a calibration sensor used when balancing the gradiometer. -
FIG. 1 is a schematic view of a gravity gradiometer according to the preferred embodiment of the invention. - The gradiometer shown in
FIG. 1 comprises a doublewalled Dewar 1 which is supported in anexternal platform 2. Theexternal platform 2 enables adjustment of the Dewar and therefore the contents of the Dewar about three orthogonal axes. Theexternal platform 2 is generally known and its adjustment by suitable motors or the like is also known. Thus, a detailed description will not be provided. - 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. TheDewar 1 is closed by anend plate 4 which includesconnectors 5 a for connecting electrical leads (not shown) to external components (not shown). - The
canister 3 is closed by anend plate 9 which includesconnectors 5 b for connecting electric leads (not shown) to theconnectors 5 a. The gradiometer has amain casing 61 formed from a twelve-sided ring 62 and hemispherical domes 63 (seeFIG. 12 ). An internal mounting 5 is connected to thering 62. Thering 62 carries asupport 65 to which a feed throughflange 9 is coupled. Aneck plug 11 formed ofbaffles 11 a whichsandwich foam 11 b is provided above thecanister 3. Thebaffles 11 a are supported on ahollow rod 93 which extends to thecanister 3 and which is also used to evacuate thecanister 3. - With reference to
FIG. 2 afirst mount 10 of a rotatable mounting 5 (FIG. 7 ) of the gradiometer is shown which comprises abase 12 and an upstandingperipheral wall 14. Theperipheral wall 14 has a plurality of cut-outs 16. Thebase 12 supports ahub 18. -
FIGS. 3 and 4 show asecond mount 20 which comprises aperipheral wall 22 and atop wall 24. Theperipheral wall 22 has fourlugs 13 for connecting the mount to thecasing 61. Thetop wall 24 and theperipheral wall 22 define anopening 28. Theperipheral wall 22 has afirst part 25, asecond part 26 and athird part 27. Thesecond mount 20 is a monolithic integral structure and thefirst part 25 is formed by making acircumferential cut 19 through the peripheral wall except for the formation of flexure webs as will be described hereinafter. Thethird part 27 is formed by making a second circumferential cut 29 through theperipheral wall 22 except for flexure webs which will also be described hereinafter. Thesecond mount 20 is mounted on thefirst mount 10 by locating thehub 18 into theopening 28 and thelugs 13 through respective cut-outs 16 as is shown inFIG. 7 . - The
first mount 10 is joined to thesecond mount 20. Thefirst flexure web 31 is formed in thefirst mount 10 so a primary mount portion of themount 10 can pivot about aweb 31 relative to a secondary mount portion of themount 10. This will be described in more detail with reference to the second embodiment shown inFIGS. 13 to 21 . - The
lugs 13 connect the mounting 5 in thecanister 3 which, in turn, locates in theDewar 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 thefirst mount 10 to move relative to thesecond mount 20 about a z axis shown inFIG. 7 . -
FIGS. 5 and 6 are views along the lines IV and V respectively which in turn are along thecuts FIG. 3 . Theperipheral wall 22 may be cut by any suitable cutting instrument such as a wire cutter or the like.FIG. 5 shows thebottom surface 19 a formed by thecut 27. As is apparent fromFIGS. 3 and 5 thecut 27 has two inverted v-shapedpeaks 34. The apex of thepeaks 34 is not cut and therefore form asecond flexure web 33 which join thefirst part 25 to thesecond part 26. Thus, thesecond part 26 is able to pivotally rotate relative to thefirst part 25 about the x axis inFIG. 7 . Thesecond cut 29 is shown inFIG. 6 and again thebottom surface 29 a formed by thecut 29 is visible. Again thesecond cut 29 forms two v-shapedpeaks 35 and the apexes of thepeaks 35 are not cut and therefore form athird flexure web 37 which connect thesecond part 26 to thethird part 27. Thus, thethird part 27 is able to pivotal rotate about the y axis shown inFIG. 7 . -
FIG. 8 showssensor 40 mounted on the mounting. Thesensor 40 is an Orthogonal Quadrupole Responder—OQR sensor formed of a first mass and a second mass in the form of afirst bar 41 and a second bar 42 (not shown inFIG. 8 ) orthogonal to thebar 41 and which is of the same shape as thebar 41. - The
bar 41 is formed in afirst housing 45 and thebar 42 is formed in asecond housing 47. Thebar 41 andhousing 45 is the same asbar 42 and thehousing 47 except that one is rotated 90° with respect to the other so that the bars are orthogonal. Hence only thehousing 45 will be described. - The
housing 45 has anend wall 51 and aperipheral side wall 52 a. Theend wall 51 is connected to rim 75 (FIGS. 2 and 7 ) of thewall 14 of thefirst mount 10 by screws or the like (not shown). Thebar 41 is formed by acut 57 in thewall 51 except for afourth flexure web 59 which joins thebar 41 to thewall 51. The flexure web is shown enlarged in the top view of thebar 41 inFIG. 9 . Thus, thebar 41 is able to pivot relative to thehousing 45 in response to changes in the gravitational field. Thebar 42 is mounted in the same way as mentioned above and also can pivot relative to itshousing 47 in response to changes in the gravitational field about afifth flexure web 59. Thehousing 47 is connected to base 12 (FIG. 2 ) of thefirst mount 10. - The
bar 41 and thehousing 45 together with theflexure web 59 are an integral monolithic structure. - Transducers 71 (not shown in
FIGS. 2 to 6 ) 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. -
FIG. 10 is a schematic block diagram showing actuator control to stabilise the gradiometer by rotating the mounting 5 about three orthogonal axes (x, y, z). Acontroller 50 which may be a computer, microprocessor or the like outputs signals to actuators 52, 53, 54 and 55. Theactuator 52 could rotate the mounting 5 about the x axis, theactuator 54 could rotate the mounting 5 about the y axis and theactuator 54 could rotate the mounting 5 about the z axis. However, in the preferred embodiment, two of the fouractuators FIGS. 31 and 32 . The position of the mounting 5 is monitored so that appropriate feedback can be provided to thecontroller 50 and the appropriate control signals provided to the actuators to rotate thesupport 10 as is required to stabilise the support during movement through the air either within or towed behind an aircraft. - The preferred embodiment also includes angular accelerometers which are similar in shape to the
bars -
FIG. 11 is a view of a feedback control used in the preferred embodiment. -
FIG. 12 is a cut away view of the gradiometer ready for mounting in theDewar 1 for cryogenic operation which in turn is to be mounted in the external platform. AlthoughFIGS. 2 to 8 show the gradiometer with thebars bars FIG. 12 . -
FIG. 12 shows the mounting 5 arranged within thecasing 61 and formed by thering 62 and the transparent hemispherical ends 63. Thering 22 hasconnectors 69 for connecting the internal wiring from transducers 71 (seeFIG. 8 ) and SQuID (Superconducting Quantum Interference Device) Electronics located in thecasing 61 to theconnectors 5 b (FIG. 1 ). - The
transducers 71 measure the angle of displacement of thebars - 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. -
FIGS. 13 to 21 show a second embodiment in which like parts indicate like components to those previously described. - In this embodiment the
first mount 10 has cut-outs 80 which effectively form slots for receiving lugs (not shown) which are connected to themount 10 in the cut-outs 80 and also to thesecond mount 20 shown inFIGS. 19 to 21 . In this embodiment the lugs are separate components so that they can be made smaller, and more easily, made than being cut with thesecond mount section 20 which forms thesecond flexure web 33 and thethird flexure web 37. - In
FIG. 13 acut 87 is made to define thepart 18 a of thehub 18. Thecut 87 then extends radially inwardly at 88 and then aroundcentral section 18 c as shown bycut 101. Thecut 101 then enters into thecentral section 18 c alongcut lines central section 18 c by theflexural web 31 which is an uncut part between the cut lines 18 e and 18 d. Thepart 10 a therefore forms a primary mount portion of themount 10 which is separated from asecondary mount portion 10 a of themount 10 except for where theportion 18 a joins theportion 10 a by theflexural web 31. Thepart 18 a effectively forms an axle to allow for rotation of thepart 18 a relative to thepart 10 a in the z direction about theflexure web 31. - As is shown in
FIG. 14 , thecut line 88 tapers outwardly from the upper end shown inFIG. 14 to the lower end and the core 18 c tapers outwardly in corresponding shape, as best shown inFIG. 17 . - As is apparent from
FIGS. 13 to 18 , thefirst mount 10 is octagonal in shape rather than round, as in the previous embodiment. -
FIGS. 19 to 21 show thesecond mount 20.FIG. 16 shows thesecond mount 20 mounted in thefirst mount 10. As is best shown inFIGS. 19 and 20 , thesecond mount 20 has cut-outs 120 which register with the cut-outs 80 for receiving lugs (not shown). The lugs can bolt to thesecond mount 20 by bolts which pass through the lugs and into bolt holes 121. The lugs (not shown) are mounted to themount 20 before themount 20 is secured to thefirst mount 10. - In the embodiment of
FIGS. 19 and 20 , thepeaks 34 andinverted peaks 35 are flattened rather than of V-shape as in the previous embodiment. - In this embodiment,
top wall 24 is provided with acentral hole 137 and twoattachment holes 138 a. Threesmaller holes 139 a are provided to facilitate pushing of thehousing 45 off thepart 18 a if disassembly is required. When thesecond mount 20 is located within thefirst mount 10, the upper part ofcentral section 18 c projects through thehole 137, as best shown inFIG. 16 . Themount 20 can then be connected to themount 10 by fasteners which pass through theholes 138 and engage inholes 139 b (seeFIG. 13 ) in thepart 18 a. - Thus, when the
first housing 45 and its associatedbar 41 is connected to therim 75 of thehousing 10 and thesecond housing 47 is connected to thebase 12, thehousings bars flexure web 31, theflexure web 33 and theflexure web 37. - As is best seen in
FIG. 21 which is an exploded view of the threeparts second mount 20, an opening extends through themount 20 which is formed by thehole 137,hole 138 andhole 139. It should be understood that themount 20 shown inFIG. 21 is a monolithic structure and is merely shown in exploded view to clearly illustrate the location of theflexural webs flexural web 33 shown inFIG. 21 joins with thepart 26 and theflexural web 35 shown inFIG. 21 joins with thepart 27. Theholes first portion 18 a of thefirst mount 10 can extend when thesecond mount 20 is located in thefirst mount 10. - Thus, when the
second mount 20 is fixed to thepart 18 a, thesecond mount 20 can pivot with thefirst portion 10 a of thefirst mount 10 about a z axis defined by theflexure web 31 whilst the second portion formed by thepart 18 a remains stationary. Movement about the x and y axes is achieved by pivotal movement of thesecond mount 20 about theflexure webs -
FIG. 22 shows the linear andannular accelerometers 90 fixed to thehousings - 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 - 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
FIG. 10 . Here Ho(s) represents the sensor assembly pivoted about the mounting 5 (as perFIG. 9 ). The block A(s) represents the actuator, which provides the feedback torque to effect the stabilization by canceling the applied disturbances. T(s) represents the sensor (or transducer) which measures the effect of the applied disturbance. This is the angular accelerometer. Using angular accelerometers in rotational control is unusual—usually gyros and/or highly damped tilt meters are used, but for our purpose the angular accelerometers are better, as the error is proportional to the angular acceleration disturbance. - 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
FIG. 22 for ease of identification) are provided. We have two independent output signals from the pair of OQR bars 41 and 42. The first is proportional to the difference in deflection, which gives the gradient signal and the second is proportional to the sum of their deflections, which is proportional to the angular acceleration and provides the sensor for the z-axis rotational control. - 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.
-
FIG. 23 showsmain body 61 andconnector 69 with the hemispherical ends removed. -
FIG. 24 is a plan view ofhousing 45 according to a still further embodiment of the invention. As is apparent fromFIG. 24 , thehousing 45 is circular rather than octagonal, as is the case with the embodiment ofFIG. 8 . - The
housing 45 supports bar 41 in the same manner as described viaflexure web 59 which is located at the centre of mass of thebar 41. Thebar 41 is of chevron shape, although the chevron shape is slightly different to that in the earlier embodiments and has a morerounded edge 41 e oppositeflexure web 59 and a trough-shapedwall section 41 f, 41 g and 41 h adjacent theflexure web 59. The ends of thebar 41 have screw-threadedbores 300 which receive screw-threadedmembers 301 which may be in the form of plugs such as grub screws or the like. Thebores 300 register withholes 302 in theperipheral wall 52 a of thehousing 45. Theholes 302 enable access to theplugs 301 by a screwdriver or other tool so that theplugs 301 can be screwed into and out of thebore 300 to adjust their position in the bore to balance themass 41 so the centre of gravity is at theflexure web 59. - As drawn in
FIG. 24 , thebores 300 are a 45° angle to the horizontal and vertical inFIG. 24 . Thus, the twobores 302 shown inFIG. 24 are at right angles with respect to one another. -
FIG. 24 also showsopenings 305 for receiving thetransducer 71 for monitoring the movement of thebar 41 and producing signals which are conveyed to the SQUID device. Typically, the transducer is in the form of a coil and as thebar 41 moves slightly due to the gravity difference at ends of the bar, a change in capacitance occurs which alters the current in the coil to thereby provide a signal indicative of movement of thebar 41. -
FIG. 25 is a more detailed view of part of the housing ofFIG. 24 showing theopenings 305. As can be seen fromFIG. 25 , theopenings 305 haveshoulders 401 which formgrooves 402. Aspring 403 is arrangedadjacent surface 406. -
FIG. 26 shows thetransducer 71. Thetransducer 71 is formed by a generally squaremacor plate 410 which has acircular boss 407. Acoil 408 is wound about theboss 407 and may be held in place by resin or the like. Thecoil 408 may be multi-layer or a single layer coil. -
FIG. 27 shows the location of theplate 410 in theopening 305 in which the plate locates in thegrooves 402 and is biased by thespring 403 against theshoulders 401 to hold theplate 410 in place with thecoils 408 being adjacent the edge face 41 a of thebar 41. - Thus, the
coil 408 and thebar 41 form an 1 c circuit so that when thebar 41 moves, the current passing through thecoil 408 is changed. - As will be apparent from
FIG. 24 , fourtransducers 71 are arranged adjacent the ends of thebar 41. Theother housing 47 also has four transducers arranged adjacent thebar 42. Thus, eighttransducers 71 are provided in the gradiometer. -
FIG. 28 is a diagram of thebars openings 305 are shown byreference numbers 71 a to 71 e to equate to the circuit diagrams ofFIGS. 29 and 30 . - With reference to
FIGS. 29 and 30 ,transducers bar 41, andtransducers bar 42 are used to provide the gravity gradient measurements. -
Input terminals 361 provide input current to the superconducting circuits shown inFIG. 29 . Heat switches which may be in the form ofresistors 362 are provided which are used to initially set the superconducting current within the circuit. The heat switches 362 are initially turned on for a very short period of time to heat those parts of the circuit at which theresistors 362 are located to stop those parts of the circuit from superconducting. Currents can then be imposed on the superconducting circuit and when the heat switches formed by theresistors 362 are switched off, the relevant parts of the circuit again become superconducting so that the current can circulate through the circuits subject to any change caused by movement of thebars - The
transducers circuit line 365 and tocircuit line 366 which connect to aSQUID 367. - Thus, as the
bars bars transducer 71 a and therefore further away from thetransducer 71 b, and closer to thetransducer 71 h and further away from thetransducer 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
FIG. 31 ,transducers bar 41 andtransducers transducers bar 42 and thetransducers - The
transducers - To do this, the
line 366 is connected to atransformer 370. The polarity of the signals from thetransducers transducer 370 onlines outputs SQUID device 375 for providing a measure of the angular acceleration which can be used in the circuit ofFIG. 10 to provide compensation signals to stabilise the mounting 5. - 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 thebars transducers -
FIGS. 31 and 32 show an actuator for receiving the control signals to adjust the mounting in response to angular movement of the mounting 5. - The actuator shown in
FIGS. 31 and 32 are schematically shown inFIG. 10 byreference numerals FIGS. 31 and 32 will be described with reference to theactuator 52 which makes adjustment around the x axis shown inFIG. 10 . -
Actuator 52 shown inFIG. 31 has ahollow disc housing 310 which has a mountingbracket 311 for connecting thedisc housing 310 to mounting 5. Thehollow disc housing 310 therefore defines aninner chamber 312 in which is located coil support plate in the form of adisc 313. Thedisc 313 has awide hub section 314 and twoannular surfaces hub 314. - The
disc 313 is also provided with aradial bore 319 and ahole 320 at the periphery of thedisc 313 which communicates with thebore 319. Ahole 321 is provided at thehub 314 and communicates with thebore 319 and extends to ahollow rod 328 which locates in atube 330. Therod 330 is fixed to thedisc 313 and also to supportframe 340 which is fixed to main body 61 (not shown inFIG. 31 ). Thetube 330 is connected to thedisc housing 310 for movement with thedisc housing 310 relative todisc 313,rod 328 andframe 340. - The winding W1 provided on the
face 315 has a lead 331 which passes through thehole 320 and then through thebore 319 to thehole 321 and then through thetube 328 to the right, as shown inFIG. 31 . A lead 332 from the other end of the winding W1 passes through thehole 321 and through thehollow rod 328 also to the right so that current can be supplied to the winding W1 through theleads 331 and 332. - The second winding W2 provided on the
face 316 has a lead 333 which passes through aradial hole 334 and bore 345 in thedisc 313 and then throughhole 337 totube 328 and to the left inFIG. 31 . The other end of the winding W2 has a lead 338 which passes through thehole 337 into thetube 328 and to the left inFIG. 31 . Thus, current can circulate through the winding W2 via theleads - When the windings W1 and W2 are energised or the current passing through the windings changes, the
disc housing 310 is moved relative to thedisc 313 andframe 340 and because thedisc housing 310 is connected to the mounting 5 by thebracket 311, the mounting 5, in the case of theactuator 52, is adjusted. The movement of thedisc housing 310 is generally a longitudinal movement (i.e. linear movement) in the direction of the axis of thetube 330 androd 328. To facilitate such movement, clearance is provided between the ends of therod 330 and theframe 340 and about thedisc 313. Thebracket 311 is offset relative to the flexure web (such as the flexure web 37) so that movement of thehousing 310 applies a torque to thefirst part 25 of the mounting 5 to cause rotation of thepart 25 about theflexure 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 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″ (FIG. 22 ) could also be provided. Thelinear accelerometers 90″ do not apply active compensation but may apply corrections to the final measured gradient data. Thus, data relating to linear acceleration can be recorded and possibly used in later processing. - One or both of the
bars - In the preferred embodiment, four angular accelerometers are provided with two of the accelerometers being formed by the
bars - The
disc 310 prevents flux from the windings W1 and W2 from leaving the actuator and because theleads 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 thehollow disc housing 310 is formed in twoparts part 310 b forming a closure panel which enables thedisc 313 to be located in thechamber 312 and then thedisc housing 310 closed by locating theplate 310 b in place. - With reference to
FIGS. 33 and 34 , the manner in which the balance of thebars capacitors - 1. To measure the residual linear acceleration sensitivity of each bar 41 (and 42) to enable the bars to be mechanically balanced using the
grub screws 301 described with reference toFIG. 24 , before operation at low temperatures; and - 2. To measure the induced linear acceleration sensitivity of each
bar - The
bars capacitors capacitors bar 41 will cause onecapacitor 400 to increase and theother capacitor 401 to decrease by the same amount, as is shown inFIG. 33 , whereas thermal expansion will cause both outputs of thecapacitors capacitors -
FIG. 33 shows that as thebar 41 pivots, the gap applicable to thecapacitor 400 decreases and the gap of thecapacitor 401 increases. - The
capacitors face 41 a of the bar 41 (and the corresponding face on the other bar 42) andsecond plates 405 which are spaced from theface 41 a. The gap between the plates of therespective capacitors -
FIG. 34 shows the calibration circuit applicable to thecapacitor 400. A circuit for theother capacitor 401 is identical. - The
capacitor 400 forms a high Q-factor resonant circuit withinductor 410. Theinductor 410 andcapacitor 400 are provided parallel tocapacitors capacitor 413 to anamplifier 414. The output of theamplifier 414 is provided to afrequency counter 415 and also fed back between thecapacitors line 416. Thecapacitor 400 therefore determines the operating frequency of theamplifier 414 which can be read to a high precision. - If the
bar 41 is out of balance, thefrequency counter 45 will tend to drift because of the imbalance of the bar. This can be adjusted by moving thegrub screws 301 into and out of the masses as previously described until balance takes place. Theamplifier 414 can then be disconnected from thefrequency counter 415 so that the gradiometer can be arranged within theDewar 1 with the other parts of the circuits shown inFIG. 34 in place. - 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.
Claims (20)
1. A gravity gradiometer for measuring components of the gravitational gradient tensor, comprising:
a sensor for measuring the components of the gradient tensor;
a mounting for supporting the sensor, the mounting comprising:
a first mount section having a base and a first mount peripheral wall, the peripheral wall having a plurality of cut-outs, the first mount section being mountable for rotation about a first axis;
a second mount section for mating with the first mount section, the second mount section having a peripheral wall; and
connectors extending outwardly from the peripheral wall and which pass through the respective cut-outs in the first mount section so as to mount the second mount section and therefore the first mount section for rotation about a second axis and a third axis; and
wherein the connectors are for connecting the first and second mount sections in a Dewar for cryogenic operation of the gradiometer.
2. The gravity gradiometer of claim 1 wherein the first, second and third axes are orthogonal z, x and y axes.
3. The gravity gradiometer of claim 1 wherein the connectors comprise radially extending lugs.
4. The gravity gradiometer of claim 3 wherein the lugs are integral with the second mount section.
5. The gravity gradiometer of claim 3 wherein the lugs are separate to the second mount section and are attached to the second mount section.
6. The gravity gradiometer of claim 1 wherein the sensor is a first bar and a second bar transverse with respect to the first bar, and the second mount section has first, second and third parts.
7. The gravity gradiometer of claim 6 wherein the first bar is connected to the first mount section and the second bar is connected to the first mount section.
8. The gravity gradiometer of claim 6 wherein the first bar and second bar are arranged orthogonal to one another.
9. The gravity gradiometer of claim 1 wherein the first mount section has a first flexural web for mounting the first mount section for rotation about the z axis.
10. The gravity gradiometer of claim 9 wherein 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.
11. The gravity gradiometer of claim 10 wherein the second mount section is cylindrical and a first cut is formed in a cylindrical wall of the section to form a second flexure web which has two web portions diagonally opposite one another, and a third flexural web is formed by a second cut in the wall and is formed by two web portions diagonally opposite one another, the first cut separating the first and second parts and the second cut separating the second and third parts.
12. The gravity gradiometer of claim 11 wherein the first part has mounting lugs for mounting the mount within a Dewar for cryogenic operation of the gradiometer.
13. The gravity gradiometer of claim 6 wherein the first bar is located in a first housing which is fixed to the first mount section, the bar being connected to the first housing by a fourth flexure web for movement relative to the first housing in response to the gravitational gradient.
14. The gravity gradiometer of claim 13 wherein the second bar is located in a second housing fixed to the first mount section, and connected to the housing by a fifth flexure web so the second bar can move relative to the housing in response to the gravitational gradient.
15. The gravity gradiometer of claim 6 wherein the first and second bars have associated transducers for outputting a signal indicative of movement of the bars in response to the gravitational gradient.
16. The gravity gradiometer of claim 13 wherein the first housing and first bar is a monolithic structure and the second housing and the second bar is a monolithic structure.
17. The gravity gradiometer of claim 1 wherein the second mount section is a monolithic structure.
18. The gravity gradiometer of claim 1 wherein actuators are provided for moving the mount about the three orthogonal axes so as to stabilise orientation of the sensor during use of the gradiometer.
19. The gravity gradiometer of claim 18 wherein the actuators are computer controlled.
20. The gravity gradiometer of claim 1 wherein linear and angular accelerometers are provided.
Applications Claiming Priority (7)
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AU2005905524 | 2005-10-06 | ||
AU2005906669A AU2005906669A0 (en) | 2005-11-29 | Gravity Gradiometer | |
AU2005906669 | 2005-11-29 | ||
AU2006900193A AU2006900193A0 (en) | 2006-01-13 | Gravity gradiometer | |
AU2006900193 | 2006-01-13 | ||
PCT/AU2006/001271 WO2007038820A1 (en) | 2005-10-06 | 2006-08-31 | Gravity gradiometer |
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US11/722,058 Active 2027-06-15 US7823448B2 (en) | 2005-10-06 | 2006-08-31 | Actuatory and gravity gradiometer |
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