POSITION DETECTION DEVICES

Abstract: A position detection device for detecting the position of adjustable stabilizer blades (3) in a bottomhole assembly of a drill string (2) within a borehole comprises a variable reluctance magnetic circuit incorporating an inner magnet member (7) and an outer magnetisable member (4) which are movable relative to one another along a predetermined direction (5) between two states in which the reluctance of the magnetic circuit differs due to the differing relative positions of the inner and outer members (7, 4). The inner member (7) includes a detector (8) comprising a permanent magnet (9) having magnet poles oriented so as to provide a magnetic field having a portion extending transversely of the predetermined direction (5), and a magnetoresistive flux sensor (15) having a sensing axis (17) oriented relative to the magnet poles so as to provide an output indicative of the angle between the sensed flux in the vicinity of one of the magnet poles and the sensing axis (17) and dependent on the relative positions of the inner and outer members (7, 4). Such a novel position detection device provides a positive indication of the position of the stabiliser blades in such an application.

Description: "Position Dofprtion Device?;" This invention relates to position detection devices, and is more particularly, but not exclusively, concerned with detection of the position of adjustable stabiliser blades or of an adjustable bent housing in a bottomhole assembly of a drill string within a borehole. For the purpose of directional drilling within a borehole, it is known to include, within a drill string, a trajectory control device in the form of an adjustable stabiliser, such as is disclosed in US 4821817, in which equiangularly distributed stabiliser blades, which normally engage the borehole wall in order to centre the drill string within the borehole, are retractable in order to permit the drilling angle of the drill bit to be changed. Rotation of the drill bit by a downhole motor is then effected to cause the required directional drilling. Control of such a trajectory control device may be carried out by operation of a mud pump at the surface to change the flow rate of the drilling mud which is pumped down the borehole to lubricate the drill bit and bring the drilling cuttings to the surface, to thereby produce a mud actuating signal for effecting retraction or extension of the stabiliser blades. It is also known, from US 5311953, to utilise a trajectory control device for directional drilling which is in the form of a combined adjustable stabiliser and adjustable bent housing which is actuable in such a way that, in order to produce tilting of the drill bit to drill along the required direction, the stabiliser blades are first retracted, and only then is a bend induced in the housing to provide the required drill bit tilting. However both of these trajectory control devices suffer from the fact that they are difficult to control because the actual position of each stabiliser blade can only be deduced from the actuating signal which has been supplied in the mud flow and from the preceding state of the device. It is an object of the invention to provide a novel position detection device which may be used to provide a positive indication of the position of the stabiliser blades in such an application. According to the present invention there is provided a position detection device and a method of detecting the state of an adjustable downhole device, as defined by the accompanying claims. In the context of monitoring of the positions of adjustable stabiliser blades within a borehole, the inner member is preferably fixedly positioned within the part of the drill string containing the adjustable stabiliser, and the outer member is preferably a hollow end portion of a movable shaft which, in known manner, serves to effect extension and retraction of the stabiliser blades when moved axially between its two extreme positions, for example by mud pressure. In this case, the detector may be provided on an inner member within the hollow end portion of the movable shaft so that the reluctance of the magnetic circuit differs between the two extreme positions of the movable shaft. This difference, as indicated by the angle between the sensed flux and the sensing axis of the flux sensing means, provides a clear indication of the position of the movable shaft and hence provides an indication as to whether the stabiliser blades are extended or retracted. In one embodiment the magnet means is constituted by a bar magnet having opposite poles at its ends and oriented so that its magnet axis extends substantially perpendicularly to the movement between the members. Alternatively the opposite magnet poles may be aligned along the direction of movement between the members. In order that the invention may be more fully understood, reference will now be made, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic axial section through an adjustable stabiliser incorporating a linear position detection device in accordance with the invention; Figure 2 is an explanatory diagram showing the principle of flux directional control in operation of the device; Figure 3 is a diagrammatic perspective view of the detector of the device; Figure 4 is an explanatory diagram showing two relative positions of the inner and outer members of the device; Figure 5 is a diagram indicating a possible modified internal profile of the outer member of the device; Figure 6 is a graph showing the non-linear characteristic of the device; Figure 7 is a block diagram of the control circuitry of the device; Figure 8 is a block diagram of the control circuitry of a modified form of the device; Figure 9 is a diagrammatic axial section through the modified form of the device; and Figures 10 and 11 are explanatory diagrams showing two forms of a rotary position detection device in accordance with the invention. The functioning of the illustrated linear position detection device will be described below with reference to the monitoring of the positions of the blades of an adjustable stabiliser provided in a bottomhole assembly of a drill string within a borehole for the purpose of directional drilling, by way of example. It will be appreciated that such a position detection device may also be used to monitor the positions of adjustable parts of other trajectory control devices, as well as other downhole devices used in the directional drilling field. In addition a rotary position detection device utilizing similar principles will be described below with reference to Figures 10 and 11, for use in detecting the position of an adjustable bent housing for example. Furthermore there are a wide range of applications outside the directional drilling field in which such position detection devices may also be used. Referring to Figure 1, the adjustable stabiliser 1 is shown positioned in a drill string 2 within a borehole and includes equiangularly distributed stabiliser blades 3 which are movable between radially retracted and radially extended positions (being shown in extended positions in Figure 1) by axial movement of a tubular actuating shaft 4 in the direction of the arrows 5 within a hollow space 6 within the stabiliser 1. The manner in which the extension and retraction of the blades 3 by axial movement of the actuating shaft 4 is effected may be as described in US 5311953 referred to above. In Figure 1 the position of the actuating shaft 4 corresponding to extension of the blades 3 is shown in solid lines, whereas the position of the actuating shaft 4 corresponding to retraction of the blades 3 is shown in broken lines (slightly to the left of the position shown in solid lines). The position detection device incorporates a remote sensor module 7 (RSM) which has a casing made of non-magnetisable material, such as austenitic stainless steel, and which is fixedly positioned within the drill string 2 so that a detector 8 of the module 7 lies within the open end of the tubular actuating shaft 4 when the actuating shaft is in its lefthand position corresponding to retraction of the blades 3. The actuating shaft 4 is made of a soft magnetisable material, such as a low carbon iron. The detector 8 includes a permanent magnet 9, such as an Alnico or samarium- cobalt magnet, which is positioned so that the magnet axis joining its opposite poles 10 and 1 1 extends along a diameter at right angles to the axis of the drill string, as shown in the cross-sectional view of the actuating shaft 4 of Figure 2. On the righthand side of Figure 2 are shown the flux lines 12 extending between the poles 10 and 11 of the magnet 9 in the absence of any effect due to the magnetisable material of the tubular actuating shaft 4, that is the flux lines in free air. However, on the lefthand side of Figure 2, the flux lines 13 are shown for the case where the magnet 9 is surrounded by the magnetisable material of the actuating shaft 4, and it will be appreciated that, in the latter case, the flux lines will all be constrained by the actuating shaft 4 so that most of the flux lines pass within the material of the actuating shaft 4, and substantially all of the flux lines pass internally of the external diameter of the actuating shaft 4. Figure 2 therefore serves to show that the positions of the magnetic flux lines will differ depending on whether the actuating shaft 4 is in the lefthand position shown in Figure 1 in which it surrounds the magnet 9 or the righthand position in which it is clear of the magnet 9. Furthermore a comparison of the flux lines 12 on the righthand side of Figure 2 and the flux lines 13 on the lefthand side of Figure 2 shows that the presence of the magnetisable material of the actuating shaft 4 causes rotation of the flux lines at the pole 10 through an angle α in the direction of the magnet axis. Such a flux directional change due to the flux being squeezed into a smaller air gap between the magnet 9 and the actuating shaft 4 can be used to provide a characteristic indication of whether the actuating shaft 4 is in the position corresponding to retraction of the blades 3 or in the position corresponding to extension of the blades 3. In order to sense this flux directional change, the detector 8 includes a directional magnetoresistive flux sensor 15 which, as shown in Figure 3, is mounted on a circuit board 16 through which the magnet 9 extends transversely and on which the associated electronics of the sensor module is also mounted. The sensor 15 which is a known sensor, such as the KMZ10 magnetoresistive sensor produced by Phillips Industries, is in the form of a resistive bridge having two opposing terminals to which an alternating current is applied so as to produce a sense output at the other two terminals. The sensor 15 is mounted so as to lie close to the pole 10 with its sensing axis 17 parallel to the magnet axis, and is angled about its sensing axis so that the plane of the sensor 15 is at a slight angle relative to the corresponding tangent to a circle centred on the magnet axis and passing through the sensing axis, in order to cause a magnetic bias to be applied in the plane of the sensor 15 perpendicularly to the sensing axis. The sensor 15 will thereby provide an output which is directionally proportional to the cosine of the angle α between its sensing axis and the flux direction. Other types of directional flux sensor may be used in place of the sensor 15, such as a Hall effect sensor which would have no magnetic bias requirement. Although, in the arrangement shown in Figure 3, the detector comprises only a single flux sensor 15, it is advantageous for the detector to include more than one flux sensor in order to compensate for any positional errors due to lack of concentricity of the detector, for example. In one possible arrangement the detector includes a respective flux sensor adjacent each of the magnet poles, whereas, in another possible arrangement, three flux sensors are positioned so as to compensate for positional errors along each of the three dimensions. Although the detection of the position of the actuating shaft 4 has been described above as though the detector 8 is wholly surrounded by the actuating shaft 4 in one position of the actuating shaft and wholly outside the actuating shaft in the other position of the actuating shaft, it is alternatively possible to arrange for the actuating shaft to surround the detector 8 at all times, but for the actuating shaft to be provided with an internal taper 20 as shown in Figure 4 such that, when the actuating shaft is in one extreme position, the detector is located relative to the taper 20 as shown at 21 in Figure 4, whereas, when the actuating shaft is in the other extreme position, the detector is located in the position 22 relative to the taper 20. It will be appreciated that, because the magnetisable material of the actuating shaft surrounds the detector more closely in the position 21 than it does in the position 22, the flux directional change between these two positions can again be used as an indication of the position of the actuating shaft, even though the detector remains within the actuating shaft in both positions. Although Figure 4 shows the detector in two positions 21 and 22 for the purposes of explanation, it will be appreciated that, in practice, the detector will remain stationary and the surrounding actuating shaft will be movable between the two positions. The directional flux sensor is incorporated in an oscillator circuit which provides a variable frequency output signal having a frequency dependent on the position of the actuating shaft. Figure 6 is a graph of the frequency of such an output signal plotted against the displacement of the actuating shaft, and it will be appreciated from this graph that the output characteristic is non-linear with respect to displacement. If the associated electronics incorporates a microprocessor, such non- linearity can be compensated for by the provision of suitable processing software. However, an alternative arrangement would be to profile the internal diameter of the actuating shaft so as to provide an internally curved taper 24 as shown in Figure 5, so as to cause the output characteristic to be linearised without the need for special processing software. Figure 7 is a block diagram of the oscillator circuit of the detector showing that the output of the direcuonal flux sensor 15 is supplied by way of an amplifier 26 to an oscillator element 27, feedback control of the current supply to the sensor 15 being effected by a temperature compensation control element 28 The output signal from the oscillator circuit of the detector may be transmitted to the surface in any known manner, for example by means of mud pulses, electromagnetically or inductively or directly along a wireline to the surface. In one embodiment the output signal is supplied to an MWD tool which modulates the mud flow along the dπll string in known manner so as to transmit mud pulses which are capable of being picked up at the surface and decoded to provide a surface reading of the output of the detector. A data signalling system which may be used in this application is disclosed in US 5163521. In a modification of the above described position detection device, material property changes affecting calibration, such as an increase in temperature, are compensated for by using a reference directional flux sensor 30, as shown in the circuit of Figure 8. The reference sensor 30 is included in a control loop which further comprises an amplifier 31 and a reference control element 32, and supplies a reference control output to a measurement control element 33 which in turn controls the current supply to the sensor 15. I I As shown in Figure 9 the reference sensor 30 is associated with a separate reference magnet 35 which has the same orientation as the magnet 9 but is disposed within a section 36 of the actuating shaft 4 having an internal profile of constant cross-section, so that the sensor 30 provides an output which is unaffected by the position of the actuating shaft 4. Figure 9 also shows the two possible extreme positions of the actuating shaft 4 with respect to the magnet 9, and indicates the manner in which the output frequency of the device varies with position. Since the output of the reference sensor 30 will change only in response to noise sources, such as temperature and magnetic variations, which will also affect the output of the measurement sensor 15, the use of such a reference sensor can serve to minimise any calibration shift. Figures 10 and 11 are diagrammatic representations of two possible rotary position detection devices in accordance with the invention utilising similar operational principles to the linear position detection device already described. Such rotary position detector devices may be applied, for example, to an indexed barrel cam mechanism used for activating an adjustable bent sub downhole. In the embodiment of Figure 10 the detector incorporating the magnet 9 is fixed in position and is surrounded by a rotatable sleeve member 40 made of magnetisable material and having an annular wall 41 having a cross-section incorporating two relatively thin portions 42 and two relatively thick portions 43. As the sleeve 40 is rotated in the direction indicated by the arrow 44, the variation in the air gaps in the vicinity of the two poles of the magnet 9 will result in modulation of the output of a directional flux sensor (not shown) so that the output is indicative of rotation of the sleeve 40 relative to the fixed detector in a similar manner to the detection of linear movement already described. Where applied to an adjustable bent sub, the sensor may provide an indication of the precise position of the adjustment mechanism of the bent sub and hence of the angle to which the bent sub has been set. The arrangement of Figure 11 is similar that of Figure 10 except that the sleeve member 40 has an annular wall 45 having a relatively thin portion 46 and a relatively thick portion 47 disposed diametrically opposite one another (rather than two relatively thin portions and two relatively thick portions as in Figure 10). In this case the sensing of the rotational movement of the sleeve member 40 will take place in a similar manner to that already described, except that full 180° angular movement will be required to change from a state of minimum reluctance of the magnetic circuit to a state of maximum reluctance. In both the arrangements of Figures 10 and 1 1, the inner profile of the sleeve member may be stepped, rather than being continuously variable. This would enable switching of the device between two or more well-defined states corresponding to the positions of the steps. Such stepped inner profiles may also be provided in the linear position detection devices already described. Finally, although, in each of the position detection devices described above, the detection of relative movement between the two members is effected by means of an arrangement in which an inner detector member which includes a permanent magnet is positioned within an outer member made of magnetisable material, it is also possible in further non-illustrated embodiments of the invention for the positions of the two members to be reversed so that the inner of the two members is made of magnetisable material, and the outer member incorporates the detector including a permanent magnet and a flux sensor for sensing the flux directional change due to relative movement between the two members.

Claims: CLAIMS 1. A position detection device having a variable reluctance magnetic circuit incorporating a magnet member and a magnetisable member which are movable relative to one another between two states in which the reluctance of the magnetic circuit differs due to the differing relative positions of said members, wherein the magnet member includes a detector comprising magnet means having magnet poles oriented so as to generate a magnetic field having a portion extending transversely of the relative movement between said members, and flux sensing means having a sensing axis oriented relative to the magnet poles so as to provide an output indicative of the angle between the sensing axis and the sensed flux in the vicinity of one of the magnet poles, said angle being dependent on the relative positions of said members. 2. A device according to claim 1, wherein the magnetisable member is a hollow outer member having a central axis, and the magnet member is an inner member positioned on said central axis, said relative movement between said members extending linearly along said central axis. 3. A device according to claim 2, for monitoring of the positions of adjustable stabiliser blades of an adjustable stabilizer provided in a drill string within a borehole, wherein the magnet member is fixedly positioned within part of the drill string containing the adjustable stabiliser, and the magnetisable member is a hollow end portion of a movable shaft which serves to effect extension and retraction of the stabiliser blades when moved axially between its two extreme positions, the detector on the magnet member being positioned within the hollow end portion of the movable shaft so that the reluctance of the magnetic circuit differs between the two extreme positions of the movable shaft, whereby said output provides an indication as to whether the stabiliser blades are extended or retracted. 4. A device according to claim 2 or 3, wherein the detector is disposed within a section of the magnetisable member which is internally tapered in the direction of the said central axis so that, in a first relative position of said members, the detector is within a relatively narrow portion of the tapered section and, in a second relative position of said members, the detector is within a relatively wide portion of the tapered section. 5. A device according to claim 4, wherein the tapered section of the outer member has a curved profile in said direction which is adapted to provide the detector with a substantially linear output characteristic. 6. A device according to claim 1 , wherein the magnetisable member is a hollow outer member having a central axis, and the magnet member is an inner member within the outer member, said relative movement between said members being an angular movement corresponding to relative rotational movement between said members. 7. A device according to any preceding claim, wherein the magnet means have opposite magnet poles which are spaced apart transversely of said relative movement between said members. 8. A device according to claim 7, wherein the magnet means is constituted by a bar magnet having a magnet axis and opposite ends, the magnet having opposite poles at its ends and being oriented so that its magnet axis extends substantially perpendicularly to said relative movement between said members. 9. A device according to any one of claims 1 to 8, wherein the flux sensing means is a directional magnetoresistive flux sensor incoφorating a plurality of magnetoresistive elements in a bridge circuit. 10. A device according to any one of claims 1 to 8, wherein the flux sensing means is constituted by a Hall effect sensor. 11. A device according to any preceding claim, wherein the flux sensing means is incorporated in an oscillator circuit so as to provide a variable frequency output having a frequency which is dependent on the relative positions of the said members. 12. A device according to any preceding claim, wherein a reference sensor is provided on the magnet member in order to produce a compensating signal to compensate for any calibration drift which might otherwise occur in the device output. 13. A device according to claim 12 when appended directly or indirectly to claim 2, wherein the reference sensor comprises reference magnet means having magnet poles oriented so as to generate a magnetic field having a portion extending transversely of said relative movement between said members and disposed within a section of the outer member having a constant internal cross-section so that the reference magnet means is disposed within portions of said section of the same internal cross-section regardless of the relative positions of said members, and reference flux sensing means having a sensing axis with a predetermined orientation relative to the magnet poles. 14. A method of detecting the state of an adjustable downhole device in a drill string within a borehole, the method comprising monitoring a variable reluctance magnetic circuit having a reluctance dependent on the relative positions of a magnet member and a magnetisable member, one of which is fixed in relation to the drill string and the other of which is movable relative to the drill string by adjustment of the position of a movable part of the downhole device, the magnet member being provided with magnet poles oriented so as to generate a magnetic field having a portion extending transversely of said movement, wherein monitoring of the variable reluctance magnetic circuit is effected by sensing the angle between a predetermined sensing axis and the magnetic flux in the vicinity of one of the magnet poles so as to provide an output dependent on the position of the movable part of the downhole device.

Inventors: EVANS KENNETH STEPHEN, EVANS, KENNETH, STEPHEN
Applicants: BAROID TECHNOLOGY INC, EVANS KENNETH STEPHEN, BAROID TECHNOLOGY, INC., EVANS, KENNETH, STEPHEN
Application year: 1996
Main classification: E21B17
IPC codes: E21B17, E21B47, G01D5