GB2222680A

GB2222680A - Accelerometers

Info

Publication number: GB2222680A
Application number: GB8806114A
Authority: GB
Inventors: Kenneth Stephen Evans
Original assignee: Baroid Technology Inc
Current assignee: Baroid Technology Inc
Priority date: 1988-03-15
Filing date: 1988-03-15
Publication date: 1990-03-14
Anticipated expiration: 2008-03-15
Also published as: GB2222680B; DE3906678A1; NL8900631A; GB8806114D0; NO891106D0; FR2628844A1; NO891106L

Abstract

An accelerometer comprises a ferromagnetic housing 10, a proof mass 15 including a coil 18 suspended within the housing 10 so as to be displaceable from a null position along a displacement axis by an applied force, and a sensing arrangement including electrodes on a fixed electrode support 33 for detecting displacement of the proof mass 15 along the displacement axis and for supplying an electrical signal indicative of the applied force. The proof mass 15 is mounted on an arm 20 mounted on a shaft 21 which is pivotally supported by rotary bearing assemblies 22 and 23. Each bearing assembly 22, 23 comprises a rotatable bearing member 26 and a fixed bearing member 28 incorporating a magnet 29 and a magnetisable fluid 30 is interposed between the bearing members 26 and 28. With this arrangement the proof mass 15 is supported so as to be capable of virtually frictionless movement under an applied force by virtue of the bearing arrangement. The magnetisable fluid 30 and the magnets 29 can be considered as providing permanent magnetic levitation forces acting between the bearing members 26 and 28. Circuitry using a phase-sensitive detector (45, Fig. 3 not shown) applies a current to the coil 18 to restore the latter to its null position. <IMAGE>

Description

"Accelerometers" This invention relates to accelerometers, and is more particularly, but not exclusively, concerned with accelerometers for use in down-hole instrumentation for surveying a borehole.

U.K. Patent Specification No. 1,492,142 discloses an accelerometer comprising a housing defining a chamber, a magnetisable fluid within the chamber, a permanent magnet magnetically suspended within the chamber by the magnetisable fluid with its poles oriented along a displacement axis and displaceable from a null position along the displacement axis by an applied force, and sensing means for detecting displacement of the permanent magnet along the displacement axis and for supplying an electrical signal indicative of the applied force.

Such an accelerometer requires to be calibrated prior to use. However,it is found that the required calibration of the accelerometer can tend to drift under the conditions of high temperature and vibration encountered down-hole, and this can lead to inaccuracy in measurement. It is believed that such drift is caused by changes in the effective mass of the proof mass of the accelerometer due to change in the distribution of the magnetic particles within the fluid and in the magnetic interaction between these particles and the magnet.

It is an object of the invention to provide a novel form of accelerometer which is capable of improved performance under such conditions.

According to the present invention, there is provided an accelerometer comprising a housing, a proof mass suspended within the housing so as to be displaceable from a null position along a displacement axis by an applied force, and sensing means for detecting displacement of the proof mass along the displacement axis and for supplying an electrical signal indicative of the applied force, wherein the proof mass is supported by bearing means spaced from the proof mass and having a movable bearing member movable with the proof mass and relative to a fixed bearing member, a magnetisable fluid being interposed between the bearing members and one of the bearing members including magnet means for magnetising the magnetisable fluid.

With this arrangement the proof mass is supported so as to be capable of virtually frictionless movement under an applied force by virtue of the bearing means. The bearing means is such that the magnetisable particles within the fluid are magnetised by the magnet means and the resultant magnetic interaction of the particles with one another and with the magnet means produces a "magnetic pressure" which tends to keep the adjacent surfaces of the bearing members apart. The magnetisable fluid and the magnet means can be considered as providing permanent magnetic levitation forces acting between the bearing members. Furthermore, no special measures are required to seal the magnetisable fluid within the space between the bearing members, since the magnetisable fluid is maintained within this space by magnetic attraction.

Such an accelerometer is less subject to calibration drift than the accelerometer of U.K. Patent Specification No. 1,492,142 since the proof mass is supported by bearing means spaced from the proof mass rather than by a magnetisable fluid surrounding the proof mass and capable of affecting the effective mass of the proof mass by magnetic interaction therewith. The bearing means is extremely rugged and substantially non-wearing,, as well as being economical to produce. In this regard the bearing means compares favourably with other forms of bearing means, such as quartz hinges which are expensive to produce and are easily damaged.

In a preferred embodiment of the invention the proof mass is a movable coil having a central axis aligned along the displacement axis. Furthermore, the coil preferably surrounds a fixed magnet within the housing. Means may be provided for supplying a current to the coil which interacts with the magnetic field of the magnet to cause a restoring force to act on the coil to restore it to its null position.

The sensing means may include an electrode assembly comprising a movable electrode on the proof mass and a fixed electrode which is positioned adjacent the movable electrode such that the extent to which the electrodes overlap one another varies in dependence on displacement of the proof mass from the null position.

The sensing means may provide a signal for controlling the current to be applied to the coil.

The proof mass may be mounted on one end of an arm which is pivotally supported at the other end by the bearing means so as to enable movement of the proof mass along an arcuate path. In practice the displacement of the proof mass in use will be very small.

The bearing means may include a rotary bearing assembly comprising a rotatable bearing member having a cylindrical portion received within a cylindrical recess in the fixed bearing member, the magnetisable fluid being disposed in the recess in the gap between the two bearing members. Most preferably the bearing means comprises two such rotary bearing assemblies disposed one at each end of a bearing shaft to which the proof mass is coupled.

According to another aspect of the invention there is provided an accelerometer comprising a housing, a movable coil suspended within the housing so as to be displaceable from a null position along a displacement axis by an applied force, and sensing means for detecting displacement of the coil along the displacement axis and for supplying an electrical signal indicative of the applied force, wherein the sensing means includes variable capacitance means having a fixed electrode and a movable electrode, the movable electrode being constituted by the coil.

In order that the invention may be more fully understood, a preferred accelerometer in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a view of the accelerometer from above; Figure 2 is an axial section taken along the line II-II in Figure 1; and Figure 3 is a block diagram of control circuitry of the accelerometer.

Referring to Figures 1 and 2 the illustrated accelerometer comprises a cylindrical housing 10 made of ferromagnetic material and consisting of a cup-shaped member 11 and a top member 12 which is substantially in the shape of an annulus having a slot 13 therein. A permanent magnet 16 is disposed within the housing 10 with one of its poles magnetically coupled to a cylindrical pole piece 19 which is disposed within the annulus formed by the top member 12 and with its other pole magnetically coupled to the bottom of the cup-shaped member 11. The pole piece 19, the magnet 16 and the housing 10 thus form a magnetic circuit producing a radial magnetic field within an annular gap 14 surrounding the pole piece 19.

An annular proof mass 15 suspended within the annular gap 14 comprises a coil former 17 and a coil 18 wound on the former 17. The proof mass 15 is mounted on an arm 20 extending through the slot 13 in the top member 12. The arm 20 is itself pivotally mounted on a bearing shaft 21 which is supported at its end by respective rotary bearing assemblies 22 and 23 supported on the housing 10 by bearing supports 24 and 25.

Each of the bearing assemblies 22 and 23 comprises a cylindrical rotatable bearing member 26 received within a cylindrical recess 27 in a fixed bearing member 28 (shown in section in Figure 1 to show the inner construction of the assembly). The fixed bearing member 28 incorporates a magnet 29, and a magnetisable fluid 30 is disposed in the gap between the rotatable bearing member 26 and the fixed bearing member 28. The magnetisable fluid 30 is a ferrofluid comprising a colloidal suspension of very fine ferromagnetic particles in a liquid, such as a synthetic hydrocarbon carrier.

Magnetic interaction between the magnet 29 and the ferromagnetic particles of the magnetisable fluid 30 causes the rotatable bearing member 26 to be supported by the magnetisable fluid 30 out of contact with the walls of the recess 27. Further magnetisable fluid may be disposed within the annular gap 14, both between the pole piece 19 and the coil former 17 and between the coil 18 and the surrounding top member 12, with a view to centring the coil 18 in the gap 14.

The proof mass 15 is thereby capable of being moved substantially axially within the annular gap 14 by means of an applied force (that is the force to be measured) and the resulting movement causes slight pivoting of the arm 20 and thus slight rotation of the rotatable bearing members 26 of the rotary bearing assemblies 22 and 23. Such movement is detected by a sensing arrangement comprising two electrodes 31 and 32 mounted on an electrode support 33 which, as seen in Figure 1, has substantially the shape of a C from above so that the electrodes 31 and 32 do not form closed loops.

The electrodes 31 and 32 interact electrostatically with the coil 18, which acts as a movable electrode, and can be considered as forming with the coil 18 two capacitors, the capacitance of which varies in dependence on the degree to which each electrode is overlapped by the coil 18. In this regard it will be seen from Fligure 2 that, in the null position illustrated, the coil 18 is symmetrically disposed with respect to the electrodes 31 and 32 and overlaps slightly more than half of the axial extent of each electrode.

Clearly, if the coil 18 moves axially in either direction with respect to its null position, the extent of overlap of one of the electrodes will increase, and hence the capacitance associated with that electrode will increase, whereas the extent of overlap of the other electrode will decrease, and hence the capacitance associated with that electrode will decrease.

Referring to Figure 3 the control circuitry of the accelerometer comprises an oscillator 40 connected to the electrodes 31 and 32 by respective resistors 41 and 42 of equal values. In the circuit diagram shown the electrodes 31 and 32 are considered as forming with the coil 18 two variable capacitors 43 and 44 connected in series and earthed at their common point. When the capacitances of the capacitors 43 and 44 are unequal, due to displacement of the coil 18 from the null position, the outputs from the capacitors 43 and 44 will be out of phase, and this will be detected by a phase-sensitive detector 45 which provides either a positive or a negative pulsed output depending on the direction in which the coil 18 is displaced.

The output from the phase-sensitive detector 45 is integrated by an integrator 46 which supplies an output to the coil 18 which is positively or negatively ramped depending on whether the pulsed input to the integrator 46 is positive or negative. The coil 18 is connected to earth by a resistor 47, and the current supplied to the coil 18 interacts with the magnetic field in the gap 14 to cause a restoring force to act on the coil 18 to restore it to its null position. The output V = IR is proportional to the actual restoring force and hence to the applied force acting on the accelerometer.

Claims

1. An accelerometer comprising a housing, a proof mass suspended within the housing so as to be displaceable from a null position along a displacement axis by an applied force, and sensing means for detecting displacement of the proof mass along the displacement axis and for supplying an electrical signal indicative of the applied force, wherein the proof mass is supported by bearing means spaced from the proof mass and having a movable bearing member movable with the proof mass and relative to a fixed bearing member, a magnetisable fluid being interposed between the bearing members and one of the bearing members including magnet means for magnetising the magnetisable fluid.

2. An accelerometer according to claim 1, wherein the proof mass is a movable coil having a central axis aligned along the displacement axis.

3. An accelerometer according to claim 2, wherein the coil surrounds a fixed magnet within the housing.

4. An accelerometer according to claim 3, wherein means are provided for supplying a current to the coil which interacts with the magnetic field of the magnet to cause a restoring force to act on the coil to restore it to its null position.

5. An accelerometer according to any preceding claim, wherein the sensing means includes an electrode assembly comprising a movable electrode on the proof mass and a fixed electrode which is positioned adjacent the movable electrode such that the extent to which the electrodes overlap one another varies in dependence on displacement of the proof mass from the null position.

6. An accelerometer according to claim 5, wherein the sensing means further includes oscillator means for supplying an alternating input signal to the electrode assembly, and phase sensitive detector means for detecting variation of the phase of the output of the electrode assembly caused by displacement of the proof mass.

7. An accelerometer according to any preceding claim, wherein the proof mass is mounted on one end of an arm which is pivotally supported at the other end by the bearing means so as to enable movement of the proof mass along an arcuate path.

8. An accelerometer according to any preceding claim, wherein the bearing means includes a rotary bearing assembly comprising a rotatable bearing member having a cylindrical portion received within a cylindrical recess in the fixed bearing member, the magnetisable fluid being disposed in the recess in the gap between the two bearing members.

9. An accelerometer according to claim 8, wherein the bearing means comprises two such rotary bearing assemblies disposed one at each end of a bearing shaft to which the proof mass is coupled.

10. An accelerometer comprising a housing, a movable coil suspended within the housing so as to be displaceable from a null position along a displacement axis by an applied force, and sensing means for detecting displacement of the coil along the displacement axis and for supplying an electrical signal indicative of the applied force, wherein the sensing means includes variable capacitance means having a fixed electrode and a movable electrode, the movable electrode being constituted by the coil.