GB2250821A - Accelerometers - Google Patents

Abstract

An accelerometer comprises a housing 10 and an inner core 11 defining an annular chamber 17 therebetween, a magnetisable fluid 17A within the chamber, an electrically conductive and substantially non-magnetic hollow cylindrical proof mass 16 suspended within the chamber so as to be displaceable from a null position along a displacement axis by an applied acceleration, a sensing arrangement including electrodes 20, 21, 22 on the inside wall of the chamber for detecting displacement of the proof mass 16 along the displacement axis and for supplying an electrical signal indicative of the applied acceleration, and restoring coils 24, 25 for restoring the proof mass 16 to its null position. A permanent magnet 18 is provided within the core 11 for magnetising the fluid 17A to cause the proof mass 16 to be suspended within the chamber 17 by the fluid. Since there is no requirement for the proof mass 16 to itself be magnetic, it follows that there is no magnetic interaction between the magnetised particles of the fluid 17A and the proof mass 16. Also, since the magnet 18 is surrounded by the annular chamber 17, a relatively small magnet may be used, and the quantity of magnetisable fluid required is kept to a minimum. <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. 1492142 discloses an accelerometer comprising a housing defining a chamber having an axis, a magnetisable fluid within the chamber, a proof mass in the form of a permanent magnet magnetically suspended within the chamber by the magnetisable fluid with its poles orientated along a displacement axis, sensing means for detecting displacement of the proof mass from a null position along the displacement axis in response to an applied force, and restoring means responsive to detection of displacement of the proof mass for causing the proof mass to be restored to its null position.

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 conditions of high temperature and vibration encountered down-hole, and this can result in inaccuracy in measurements. Such drift is caused by changes in the effective mass of the proof mass of the accelerometer due to changes in the distribution of magnetisable particles within the fluid and due to magnetic interaction between these particles and the magnet.

U.K. Patent Application No. 2222679A discloses an improved accelerometer in which the proof mass is electrically conductive and substantially non-magnetic, and in which magnets are provided outside the chamber and radially outwardly of the housing for magnetising the fluid to cause the proof mass to be suspended by the fluid away from the walls of the chamber. Such an arrangement has the advantage that, since the proof mass is substantially non-magnetic, the required calibration does not drift under down-hole conditions due to changes in the magnetic interaction between the particles of the fluid and the proof mass.

It is an object of the invention to provide an accelerometer having an electrically conductive and substantially non-magnetic proof mass and which is of a more advantageous design than the previously proposed arrangement.

According to the present invention there is provided an accelerometer comprising a housing defining a chamber having an axis, a magnetisable fluid within the chamber, an electrically conductive and substantially nonmagnetic proof mass in the form of a hollow cylinder coaxially disposed in the chamber, magnet means for magnetising the fluid to cause the proof mass to be suspended by the fluid away from the walls of the chamber, sensing means for detecting displacement of the proof mass from a null position along the axis of the chamber in response to an applied acceleration, and restoring means responsive to detection of displacement of the proof mass for causing the proof mass to be restored to its null position, wherein the chamber is of annular cross-section and surrounds an inner core incorporating said magnet means.

With this arrangement 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 center the proof mass within the chamber. Thus there is no requirement for the proof mass to itself be magnetic as in the accelerometer of U.K.

Patent Specification No. 1492142, and there is no magnetic interaction between the magnetised particles of the fluid and the proof mass.

Furthermore, the technically novel and surprising step of incorporating the magnet means in an inner core surrounded by the annular chamber provides a number of technical advantages as compared with the arrangement of U.K. Patent Application No. 2222679A.

Only a relatively small magnet is required and furthermore the volume of the chamber can be kept to a minimum. This is important in reducing the cost of production of the accelerometer, particularly in keeping the quantity of magnetisable fluid to a minimum since such fluid is expensive. The invention also offers potential for overall size reduction of the accelerometer.

Preferably the inner core is in the form of a substantially non-magnetic former surrounding the magnet means which is in the form of a permanent magnet. However it is also possible for the inner core to be formed by a permanent magnet which is supported only at its ends so that the inner wall of the chamber is formed by the outside of the magnet itself.

Furthermore the inner core preferably comprises an intermediate portion of a diameter less than the internal diameter of the proof mass and two outer portions, at opposite ends of the intermediate portions of a diameter greater than the internal diameter of the proof mass, the proof mass surrounding the intermediate portion for displacement between extreme limiting positions determined by the outer portions.

The inner core may be supported centrally within the housing by end faces of the outer portions so that the annular chamber surrounds the outer portions as well as the intermediate portion of the proof mass.

The magnet means may comprise either one or more than one cylindrical permanent magnet having opposite poles at its two axial ends and coaxially disposed within the proof mass. If two magnets are provided these may be spaced apart so that each lies at a respective end of the proof mass. If more than two magnets are provided these are preferably regularly spaced apart along the axis of the proof mass, possibly by means of non-magnetic shims.

Advantageously, in this case, each magnet has a polar orientation which is opposite to that of the or each adjacent magnet, so that adjacent axial ends of adjacent magnets are of like poles.

The sensing means may include an electrode assembly attached to the outer wall of the chamber and comprising at least three electrodes spaced apart in the direction of the chamber axis and positioned such that the extent to which the electrodes are overlapped by the proof mass varies in dependence on the displacement of the proof mass from the null position.

The sensing means may also include at least one electrode on the inner core internally of the proof mass.

If electrodes are provided both internally and externally of the proof mass an improved signal level can be obtained due to the increased capacitance provided by such an arrangement.

The or each electrode may have substantially the shape of a hollow cylinder which is prevented from forming a complete cylinder by the provision of an axial slot.

The sensing means preferably also 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.

The restoring means may comprise two coils coaxial with the chamber axis and wound on the housing so as to be symmetrically disposed on opposite sides of the null position.

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 an axial section through the accelerometer taken along the line I-I in Figure 2; Figure 2 is a cross-section through the accelerometer taken along the line II-II in Figure 1; and Figure 3 is a simplified block diagram of the control circuitry of the accelerometer.

Referring to Figures 1 and 2 the illustrated accelerometer comprises a non-metallic cylindrical housing 10 formed with four spaced annular flanges 12, 13, 14 and 15. An inner core 11 is positioned coaxially within the housing 10 and defines therewith a chamber 17 of annular cross-section within which a non-magnetic proof mass 16 in the form of a hollow cylindrical electrically conductive tube, made for example of copper, is suspended by magnetisable fluid 17A within the chamber 17. The magnetisable fluid 17A is a ferrofluid comprising a colloidal suspension of very small ferromagnetic particles in a liquid, such as a synthetic hydrocarbon carrier.

The inner core 11 comprises a non-metallic and non-magnetic former 19 surrounding a cylindrical permanent magnet 18, and having an intermediate portion 18A of diameter less than the internal diameter of the proof mass 16 and two outer portions 18B and 18C at opposite ends of the intermediate portion 18A of diameter greater than the internal diameter of the proof mass 16. The proof mass 16 is thus confined for movement along the intermediate portion 18A with the outer portions 18B and 18C defining the extreme limiting positions of the proof mass 16. The magnet 18 is disposed coaxially within the proof mass 16 with its North pole at one end of the proof mass and its South pole at the other end of the proof mass.The magnet 18 magnetises the ferromagnetic particles within the fluid 17A in such a manner as to cause the particles to magnetically interact with one another and with the magnet 18 so as to center the proof mass 16 within the chamber 17 out of contact with the walls of the chamber 17. Thus the proof mass 16 is suspended within the chamber 17 in such a manner as to allow the proof mass to be displaced from its null position (shown in Figure 1) along the central axis by an applied force.

Such movement is detected by a sensing arrangement comprising three electrodes 20, 21 and 22 axially spaced apart on the outer wall of the chamber 17.

As may be seen from the electrode 21 in Figure 2, each electrode 20, 21 or 22 does not quite form a complete cylinder but is interrupted by an axial slot 23. This is done so as to ensure that induced currents are not caused to flow continuously around the electrodes. The electrodes 20, 21 and 22 are electrostatically coupled to the proof mass 16 so that the capacitance across adjacent electrodes varies in dependence on the extent to which the proof mass 16 overlaps the end electrodes 20 and 22.

Two restoring coils 24 and 25 are wound on the housing 10 between the annular flanges 12 and 13 and the annular flanges 14 and 15 respectively. Each of these coils 24 and 25 when supplied with alternating current induces alternating current flow in the proof mass 16, which may be considered as a single turn coil, and as a result the proof mass 16 is repelled by the coil 24 or 25.

In this manner the restoring coils 24 and 25 can serve to restore the proof mass 16 to its null position when the proof mass is displaced from the null position.

The control circuitry of the accelerometer will now be described briefly with reference to Figure 3. A more detailed description may be obtained by reference to the disclosure of U.K. Patent Application No. 2222679A.

As shown the electrodes 20, 21 and 22 in association with the proof mass 16 can be considered as constituting two variable capacitors 26 and 27 connected together and earthed at their common point. An oscillator 30 is connected to supply the capacitors 26 and 27, and a phase-sensitive detector 33 detects the outputs from the capacitors 26 and 27 and provides a pulsed d.c. output voltage which is positive or negative depending on whether the output from the capacitor 26 leads or lags the output from the capacitor 27. Thus the polarity of the output of the phase-sensitive detector 33 depends on whether the proof mass 16 is displaced to the left or to the right in Figure 1.

An integrator 34 integrates the output from the phase-sensitive detector 33, and a polarity switch 35 supplies the output voltage from the integrator 34 to a control line 36 of a variable gain amplifier 37 when the voltage is positive and to a control line 38 of a variable gain amplifier 39 when the voltage is negative. An oscillator 40 supplies respective inputs which are 180 out-of-phase to the variable gain amplifiers 37 and 39 which in turn supply their outputs to the restoring coils 24 and 25. In this manner it can be arranged that, when the proof mass 16 is displaced from the null position in one direction, a net restoring force is exerted by the coils 24 and 25 to restore the proof mass 16 to its null position.

Furthermore the out-of-phase currents passing through the coils 24 and 25 are summed by an adder 43, and the summed output is supplied to a phase-sensitive detector 44 which is synchronised with the output of the oscillator 40 by a reference line 45. The output from the phase-sensitive detector 44 is a d.c. voltage VO which is proportional to the applied force to be measured. Since any displacement of the proof mass 16 due to an applied force is automatically compensated for by the application of a restoring force to the proof mass, only very small displacements of the proof mass will be required in order to obtain an output from the accelerometer.

The provision of the magnet 18 within the proof mass 16 in such an arrangement provides a number of advantages over an arrangement such as that disclosed in U.K. Patent Application No. 2222679A in which magnets are provided externally of the proof mass. The fact that the magnet is located within what would otherwise be dead space within the proof mass reduces the overall size of the accelerometer. Furthermore substantial cost reductions are obtained by virtue of the fact that a smaller magnet is required than previously as well as a smaller volume of ferrofluid. In addition the inner core provides inherent centering of the proof mass within the housing, and the need for matching of magnet strengths as in the prior arrangement is avoided. Also such an arrangement permits closer coupling of the restoring coils with the proof mass.

Claims (12)

1. An accelerometer comprising a housing defining a chamber having an axis, a magnetisable fluid within the chamber, an electrically conductive and substantially nonmagnetic proof mass in the form of a hollow cylinder coaxially disposed in the chamber, magnet means for magnetising the fluid to cause the proof mass to be suspended by the fluid away from the walls of the chamber, sensing means for detecting displacement of the proof mass from a null position along the axis of the chamber in response to an applied acceleration, and restoring means responsive to detection of displacement of the proof mass for causing the proof mass to be restored to its null position, wherein the chamber is of annular cross-section and surrounds an inner core incorporating said magnet means.

2. An accelerometer according to Claim 1, wherein the inner core is in the form of a substantially nonmagnetic former surrounding the magnet means which is in the form of a permanent magnet.

3. An accelerometer according to Claim 1, wherein the inner core is formed by a permanent magnet which is supported only at its ends so that the inner wall of the chamber is formed by the outside of the magnet.

4. An accelerometer according to Claim 1, 2 or 3, wherein the inner core comprises an intermediate portion of a diameter less than the internal diameter of the proof mass and two outer portions, at opposite ends of the intermediate portions, of a diameter greater than the internal diameter of the proof mass, the proof mass surrounding the intermediate portion for displacement between extreme limiting positions determined by the outer portions.

5. An accelerometer according to Claim 4, wherein the inner core is supported centrally within the housing by end faces of the outer portions so that the annular chamber surrounds the outer portions as well as the intermediate portion of the proof mass.

6. An accelerometer according to any preceding claim, wherein the magnet means comprises either one or more than one cylindrical permanent magnet having opposite poles at its two axial ends nd coaxially disposed within the proof mass.

7. An accelerometer according to Claim 6, wherein the magnet means comprises more than two magnets regularly spaced apart along the axis of the proof mass, each magnet having a polar orientation which is opposite to that of the or each adjacent magnet, so that adjacent axial ends of adjacent magnets are of like poles.

8. An accelerometer according to any preceding claim, wherein the sensing means includes an electrode assembly attached to the outer wall of the chamber and comprising at least three electrodes spaced apart in the direction of the chamber axis and positioned such that the extent to which the electrodes are overlapped by the proof mass varies in dependence on the displacement of the proof mass from the null position.

9. An accelerometer according to Claim 8, wherein the sensing means also includes at least one electrode on the inner core internally of the proof mass.

10. An accelerometer according to Claim 8 or 9, wherein the sensing means also 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.

11. An accelerometer according to any preceding claim, wherein the restoring means comprises two coils coaxial with the chamber axis and wound on the housing so as to be symmetrically disposed on opposite sides of the null position.

12. An accelerometer substantially as hereinbefore described with reference to the accompanying drawings.