CA1104341A - Fluxgate caliper - Google Patents

Abstract

FLUXGATE CALIPER

Abstract of the Disclosure. A borehole measuring instru-ment is disclosed which has a plurality of borehole engaging mem-bers pivotally attached by articulated mechanical arm assemblies to the instrument body. A mechanical correction linkage connects one of the arm assemblies to a signal initiator slidably located in the body, which interacts with a stationary signal generator in the body to create a signal proportional to the amount of ex-tension of the arm assemblies. In one embodiment, a magnetic fluxgate was utilized as a signal generator and a magnet was utilized as the initiator.

-1a-

Description

11~4;~1 sackground of the Inven-tion The present invention generally discloses apparatus for measuriny wellbores and more specifically involves cali-pers for measuring the diameter of a wellbore. I' is known in the art to measure a wellbore with a caliper tool, which utilizes a sensing element such as a sliding poten-tiometer. The prior art devices utilize spring loaded arms to maintain contact with the wellbore wall and have connect-ed to the sliding arms one or more elements of an electric potentiometer for determining the instantaneous borehole diameter.
The disadvantages suffered by this type of device include the exposure of the potentiometer wiring and parts to downhole conditions which lead to corrosion and failure of these parts. The prior art devices utilizing the potentio-meter also suffer from the disadvantage of having internal sliding electronic components that wear mechanically and lead to inaccuracies and failure of the electrical system.
The mechanical lead-ins to the potentiometer devices require structure for equalizing pressure between the insiae of the tool and the wellbore. Also, pressure terminals through which the electrical and mechanical lead-ins pass must be sealed off against pressure and corrosive fluids.
The present invention disposes of these disadvantages by providing a magnetic sensing system to replace the compli-cated potentiometer system of the prior art devices. The magnetic sensing system is located behind a magnetic window which seals off and protects the electronics from well fluids and pressures. The on:Ly thing exposed to welL fluids is the initiator magnet and the mechanical linkage connecting it to the caliper arms.

~"~

~, ~"

11~4~41 sroadly speaking the present inven-tion may be seen as providing a caliper for measuring borehole dia-meter while being moved through a wellbore, the cali.per comprising: elongated body means; arm linkage means pivotally connected to the body means; biasing means on the body means arranged to bias the linkage means radially outward from the body means; contact means on the linkage means arranged to contact a borehole wall; signal initiator means slidably located in the body means; initiator link-age means connecting the initiator means to the arm link-age means and adapted to slide the initiator means in linear relationship to radial movement of the contact means with respect to the body means; and, signal generator means in the body means, in close proximity to the initiator means, and adapted to generate a si.gnal.proportional to the prox-imity of the initiator means thereto; the initiator linkage .: means including a first linkage bar pivotally attached at one end thereof to the arm linkage means, a second linkage bar pivotally connecting the initiator means to the first linkage bar, and a linearizer channel in the body means;
the first linkage bar having a pin at the other end thereof continually engaged in the linearizer channel.
Brief Description of the_Drawings . - Figures lA through lC, when joined together at commonlines a-a and b-b, illustrate in cross-sectional side view the caliper apparatus of this invention.
Figure 2 represents in schematic illustration the -L - lA -.~ .

components of the magnetic fluxgate utilized in the invention.
Figure 3 is a schematic illustration of the circuitry of the magnetic fluxgate.
Description of the Preferred Embodiments. Referring to Figures lA through lC, there is illustrated the borehole caliper 10 having a generally cylindrical main body section 11 upon which is located a lower connection sleeve 12 and a coil expansion spring 13. A sliding collar 14 is located atop coil spring 13 and coil spring 13 rests on an annular abutment shoulder 15. Coil spring 13 preferably is in constant compression and continuously urges collar 14 upward towards abutment with shoulder 16.
Attached to collar 14 is a pivotal arm member 17 having at its outermost end a contact pad 18 pivotally connected thereto.
A secondary arm member 19 is pivotally and slidably connected at pin 20 to arm 17 and pivotally connected at pin 21 to contact pad 18.
- Also pivotally connected to pad 18 is an upper secondary arm member 22 connected thereto by pin 23, and slidably and pivot-;~ ally connected at pin 24 to the upper primary arm member 25.
Primary arm member 25 is connected at the same pin member 26 on pad 18 as arm member 17. Primary arm 25 is connected to body 11 at pivot pin 27.
It should be noted that although one arm assembly is illustrated with respect to this tool,it is possible to include one or more additional arm assemblies on the tool. When multiple arm assemblies are utilized they may or may not be equispaced around the tool body. One reason for using multiple arm assemblies in equispaced relationship is to centralize the tool body in the borehole. Conversely, other arrangements of arm assemblies may be used when it is not desirable to centralize the tool body. In one preEerred embodiment, three such arm assemblies are provided in equispaced relationship on the body l 1~434~
to maintain the instruments centralized in the wel]bore.
Operation of the arm assemblies in conjunction with the movement of the caliper through the wellbore is sub-stantially identical to the operation of the caliper dis-closed in U.S. Patent 3,977,468 assigned to Dresser Industries, Inc. assignee of the present invention.
A bow spring 28 is attached by means such as a threaded screw 29 to body 11 between arms 17 and 25 in a normally outward-flex position to initiate opening of the arm assembly and prevent lock-up of the assembly against the tool body. One primary purpose of the linkages comprising arms 17, 19, 22 and 25 and contact pad 18 is to maintain contact with the borehole wall and center the caliper instrument in the borehole. Coil spring 13 pro-vides a continuous radially outward biasing force on the arm assembly.
In addition to the arm linkage above described, a correction linkage system 30 is provided in the borehole caliper for linking the arm assembly to a sliding signal initiator 31 having an internal magnet 34 embedded therein.
The correction linkage assembly comprises a follower arm 32 having a drive arm 33 rotatably pinned thereto.
Arm 32 extends into a slot 35 machined inside arm 25 and is pinned therein in rotatable relationship by pin 36.
Drive arm 33 is rotatably attached to initiator 31 by rotatable pin 37. At the other end arm 32 has located therein a pin 38 which is continuously engaged in a follow-er channel 40 formed in the side face of follower block 39.
Pin 38 and arm 32 are attached to block 39 in a manner pre-venting pin 38 from disenyaging channel 40 at any time during operation of the instrument.
Initiator block 31 is slidably located on a slide plate 41 secured to the lower end of block 39. A guide rod 42 is } - 3 -securely embedded in follower block 39 and extends into snug fitting slidable relationship in a bore passage 43 formed in initiator block 31. The central longitudinal axis of initiator magnet 34 is maintained generally parallel to the longitudinal axis of the caliper instrument.
Also, in general coaxial alignment with magnet 34 is a magnetic window 44 comprising a non-magnetic shield sleeve 45 and a core 46 made of a magneticly permeable material such as soft iron. Directly behind the magnetic window 44 is the magnetic fluxgate 47. A biasing magnet 48 is located behind the fluxgate 47 and is secured inside an insulating plug 49. A conduit pas-sage 50 is formed through plug 49 for passage of the fluxgate wiring 51. The wiring extends into a conventional logging tool electronics section 52.
~eferring now to Figure 2, the magnetic fluxgate is illustrated in schematic view. The fluxgate generally comprises a drive mechanism consisting of a toroidal core 53 with a square loop characteristic, on which have been wound a sufficient number of turns 54 to drive the core into saturation. A sensing coil 55 is wound on an aluminum bobbin 56 with the aluminum bobbin acting as an electrostatic shield. The sensing coil and aluminum bobbin are located around the drive mechanism and this assembly is mounted inside a brass case 57 and closed with a suitable brass end plate 58 having a conduit passage 59.
Referring now to Figure 3, a schematic diagram of the circuitry of the magnetic fluxgate is disclosed. The fluxgate assembly 47 is located in a stationary position in the caliper tool. The sliding magnet 34 is located opposite the magnetic window 44 (Figure lA) which is in close proximity to fluxgate 47.
A step voltage 60 is supplied to toroidal coil 53, which voltage alternately drives the coil into saturation and then allows it to come out of saturation.

The sensing coil 55 is connected by leads 61 and 62 to an amplifier 63 which feeds into the electronics section 52 of the caliper. A biasing magnét 64 is located ~ehind the fluxgate to shift the zero point upward into the region of higher voltage where a better signal-to-noise ratio can be obtained.
During operation of the fluxgate assembly, the toroidal coil 53 is alternately driven into and out of saturation by appli-cation of step voltage 60. During saturation, the flux from magnet 34 entering magnet window 44 is driven out of the sensing coil 55. At the end of each saturation stage, the toroidal coil moves into a non-saturated condition and flux from magnet 34 ~oves into sensing coil 55 inducing a voltage therein which is amplified at 63. This results in a stepped signal 65 representing a voltage proportional to the intensity of the flux from magnet 15- 34.
This output of the fluxgate from sensing coil 55 is an amplitude-modulated carrier of the same frequency as the input voltage 60. The output 65 is further processed in the electronics section 52 and may be transmitted as a DC voltage via a logging cable to electronic instruments at the surface. This DC voltage is directly proportional to the flux density of magnet 34 which in turn varies with the distance of the magnet from fluxgate 47.
In typical operation, the apparatus described with reference to Figures 1-3 serves to provide a signal directly proportional to the diameter of the wellbore. This is accom-plished by the expansion of the linkage arms 17, 19, 22 and 25 radially outward, forcing the contact pad 18 into continuous contact with the borehole wall as the logging instrument moves upward in the borehole.
Variations in the borehole will cause radial movement of pad 18 with respect to tool body 11 as the tool moves along the borehole. In this preferred embodiment, the tool is provided ~ lQ~41 with three sets of linkage arms and contact pads, although only a single set is illustrated. ~his illustrated assembly is typical of the othe~ two arm mechanisms and all three mech~nisms are gener-ally located in equispaced relationship around the circumference of the tool body. m e op~ration of the linkage arms and contact pads with movement of the instrument along the wellbore is more particularly described in the afQrementioned U.S. Patent 3,977,468.
One set of linkage arms is provided with the mechanical correction linkage assembly 30 converting the non-linear radial move-ment of pad 18 into linear movement of the sliding magnet 34. m us, with the provision of linkage arm 32 having a follcwer pin 38 engaged in a follower channel 40, the movement o~ magnet 34 along the longitu~inal axis of body 11 is directly and linearly proportional to the movement of pad 18 radially inward and outward from bady 11. This removes the non-linear cc~ponent arising from the polæ action of the linkage arm 25 about pin 27.
t This conversion of radial movement of pad 18 into directly proportionate coa~ial movement of magnet 34 results in a magnetic flux entering magnetic windcw 44, having a flux density proportionate to the radial extension of pad 18 from body 11. Since the flux density of magnet 34 sensed in fluxgate 47 is not linearly proportional to the dis-tance of magnet 34 from the fluxgate, this non-linear magnetic oomponent may be converted into a linear signal either in electronics section 52 or in the control panel instruments located on the ground.
The advantages of the present invention are obvious frow the description given above and include the fact that the entire electro-nics syste~ may be secluded in the body section and sealed off by the magnetic window 44, thus eliminating the aforementioned problems of ex-posed wiring and press~re seals needed in the potenticmeter type de-vices. The present invention !? - 6 -3~1 also eliminates the internal sliding electrical parts that suffer wear from friction encountered in the potentiometer device.
There also is no requirement for a pressure equalization system using bellows and hydraulics such as utilized in the potentiometer calipers.
The only exposure to the well fluid in this invention is the initiator magnet and the mechanical linkage connecting it - to the caliper arms. The mechanical linkage assembly 30 is reliable and uncomplicated and serves to remove a complex non-linearity component of movement in the caliper system. The seIection of the proper curve for channel 40 depends upon physical characteristics of the caliper arm assemblies such as the various lengths of the various arms, the physical dimensions of the con-tact pads and the locations of the pivot pins connecting the arms and pads to each other and to the body.
Although a specific preferred embodiment of the present invention has been described in the detailed description above, the description is not intended to limit the invention to the particular forms or embodiments disclosed therein since they are to be recognized as illustrative rather than restrictive and it will be obvious to those skilled in the art that the invention is not so limited. For example, whereas certain materials are cited for the construction of components within the fluxgate, it is obvious that other suitable materials could be substituted therefor. Also, whereas a toroidal core having a square loop configuration is utilized as the driving coil, it is obvious other configurations of driving coils could be utilized. Addi-tionally, it is possible to substitute an eddy current sensor or radioactive sensing system for the magnetic fluxgate system and still retain many of the advantages o~ the magnetic system.
These and all other modifications and changes of this type are deemed to be embraced by the spirit and scope of the invention except as the same may be necessarily limited by the appended claims or reasonable equivalents thereof.

Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A caliper for measuring borehole diameter while being moved through a wellbore, said caliper comprising:
elongated body means;
arm linkage means pivotally connected to said body means;
biasing means on said body means arranged to bias said linkage means radially outward from said body means;
contact means on said linkage means arranged to contact a borehole wall;
signal initiator means slidably located in said body means;
initiator linkage means connecting said initiator means to said arm linkage means and adapted to slide said initiator means in linear relationship to radial movement of said contact means with res-pect to said body means; and, signal generator means in said body means, in close proximity to said initiator means, and adapted to generate a signal proportional to the proximity of said initiator means thereto;
said initiator linkage means including a first link-age bar pivotally attached at one end thereof to said arm linkage means, a second linkage bar pivot-ally connecting said initiator means to said first linkage bar, and a linearizer channel in said body means, said first linkage bar having a pin at the other end thereof continually engaged in said linear-izer channel.

2. The caliper of claim 1 wherein said arm linkage means comprises an assembly having a double pair of articulated arms pivotally connected at three points to said contact means.

3. The caliper of claim 1 wherein said signal initia-tor means comprises a magnetic element and said signal generator means comprises a magnetic flux sensor adapted to sense magnetic flux magnitude and generate an electronic signal proportionate to said flux magnitude.

4. The caliper of claim 3 wherein said flux sensor comprises a magnetic flux window arranged in close proxi-mity to said magnetic element, with a sensing coil and a driving coil behind said window, said driving coil arrang-ed to alternately drive said sensing coil into and out of saturation in response to a step voltage applied to said driving coil.

5. The caliper of claim 4 wherein said signal generator means is located in fluid tight relationship in said body means and said caliper further comprises electronic linear-izing means for linearizing a magnetic flux signal, said electronic linearizing means being connected to said gener-ator means.

6. The caliper of claim 4 wherein said flux sensor fur-ther comprises a non-magnetic case, said sensing coil is wound on a non-magnetic sleeve, and said driving coil fur-ther comprises a toroidal coil with a square loop character-istic; said toroidal coil being located inside said sleeve which is located in said case.

7. The caliper of claim 4 further comprising a biasing magnet behind said sensing coil and said driving coil, in close proximity thereto.

8. A caliper for measuring a borehole diameter while moving through the borehole, said caliper comprising:
an elongated body member adapted to be moved longi-tudinally along a borehole;
one or more arm assemblies attached pivotally to said body member and each having a well-bore con-tact pad thereon;
spring means on said body member for biasing said arm assemblies radially outwardly from said body member;
a signal initiating member slidably mounted in said body member;
a mechanically linearizing connection linkage linking said signal initiating member to one of said arm assemblies said linkage including a first linkage bar pivotally attached at one end thereof to said one arm assembly, a second linkage bar pivotally connect-ing said signal initiating member to said first link-age bar, and a linearizer channel in said body member, said first linkage bar having a pin at the other end thereof continually engaged in said linearizer channel whereby said signal initiating member is slidable in linear relationship to radial movement of said contact pad with respect to said body member;

9. The borehole caliper of claim 8 wherein said signal initiating member comprises a magnet and said signal generator comprises a magnetic fluxgate adapted to generate a signal having an amplitude proportional to the density of flux from said magnet passing into said fluxgate.

10. The borehole caliper of claim 9 further comprising a biasing magnet behind said magnetic fluxgate arranged to in-crease the signal-to-noise ratio of said fluxgate.

11. The borehole caliper of claim 9 wherein said magnetic fluxgate comprises a non-magnetic housing, a sensing coil wound on a non-magnetic bobbin in said housing, and a drive coil inside said bobbin adapted to selectively drive said sensing coil into saturation.