MODE LOCALISED ACCELEROMETER

Abstract

An accelerometer comprising: a frame; one or more proof masses suspended from the frame by one or more flexures and movable relative to the frame along a sensing axis; a resonant element assembly, the resonant element assembly comprising a first resonant element and a second resonant element coupled to one another, the first resonant element connected between the one or more proof masses and the frame, the second resonant element connected between the one or more proof masses and the frame, such that movement of the one or more proof masses relative to the frame along the sensing axis results in one of the first and second resonant elements undergoing compression and the other of the first and second resonant elements undergoing tension; and drive circuitry configured to drive the resonant element assembly and a sensing circuit configured to determine a measure of acceleration.

Claims

1. An accelerometer comprising: a frame; one or more proof masses suspended from the frame by one or more flexures and movable relative to the frame along a sensing axis; a resonant element assembly, the resonant element assembly comprising a first resonant element and a second resonant element coupled to one another, the first resonant element connected between the one or more proof masses and the frame, the second resonant element connected between the one or more proof masses and the frame, such that movement of the one or more proof masses relative to the frame along the sensing axis results in one of the first and second resonant elements undergoing compression and the other of the first and second resonant elements undergoing tension; and drive circuitry configured to drive the resonant element assembly into one or more resonant modes and a sensing circuit configured to determine a measure of acceleration based on changes in resonant behaviour of the first and second resonant elements.

2. An accelerometer according to claim 1, wherein the first and second resonant elements are substantially identical.

3. An accelerometer according to claim 1 or 2, wherein the one or more proof masses comprises a single proof mass and the first and second resonant elements are both coupled to the single proof mass.

4. An accelerometer according to claim 3, wherein the first and second resonant elements are surrounded by the single proof mass.

5. An accelerometer according to any one of the preceding claims wherein one or both of the first and second resonant elements is connected to the one or more proof masses through an force amplifying lever.

6. An accelerometer according to any one of the preceding claims wherein the first resonant element is coupled to the second resonant element by a mechanical coupling.

7. An accelerometer according to claim 6, wherein the first resonant element is connected to the frame at a first anchor and the second resonant element is connected to the frame at a second anchor, and wherein the mechanical coupling is positioned at or close to a nodal point of a mode of vibration of the resonant element assembly.

8. An accelerometer according to claim 6, wherein the mechanical coupling is a portion of the frame, the first and second resonant elements being connected to the frame at a common anchor.

9. An accelerometer according to claim 6 or 7, wherein the mechanical coupling comprises a serpentine beam.

10. An accelerometer according to any one of the preceding claims, wherein the first and second resonant elements are electrostatically coupled to one another.

11. An accelerometer according to any one of the preceding claims, comprising a third resonant element coupled to one or both of the first and second resonant elements.

12. An accelerometer according to claim 11, wherein the third resonant element has different mechanical properties to the first resonant element and the second resonant element.

13. An accelerometer according to claim 12, wherein the third resonant element has a different stiffness to the first resonant element and the second resonant element.

14. An accelerometer according to any one of the preceding claims, wherein the drive circuitry is configured to provide a parametric pumping signal to the resonant element assembly.

15. An accelerometer according to any one of the preceding claims, wherein the sensing circuitry is configured to provide an output based on the amplitudes of vibration of the first resonant element and the second resonant element.

16. A gravimeter comprising an accelerometer according to any one of the preceding claims.

17. A borehole tool comprising one or more accelerometers in accordance with claims 1 to 15.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

[0032] FIG. 1 is a schematic illustration of a first embodiment of the invention;

[0033] FIG. 2 illustrates drive and sensing circuitry for use with the embodiment of FIG. 1;

[0034] FIG. 3 is a flow diagram of the control process for the circuitry of FIG. 2;

[0035] FIG. 4 is a schematic illustration of a second embodiment of the invention;

[0036] FIG. 5 is a schematic illustration of a third embodiment of the invention;

[0037] FIG. 6 is a schematic illustration of a fourth embodiment of the invention;

[0038] FIG. 7 is a schematic illustration of resonant elements that can be used; and

[0039] FIG. 8 is a schematic illustration of a borehole tool including an accelerometer.

DETAILED DESCRIPTION

[0040] FIG. 1 is a schematic illustration of a top view of an accelerometer in accordance with the present invention. The accelerometer is advantageously fabricated entirely from a single semiconductor wafer, such as a silicon-on-insulator (SOI) wafer.

[0041] The accelerometer comprises a first proof mass 10 and a second proof mass 20. The first proof mass is suspended from a frame 12 by flexures 14 so as to allow movement of the first proof mass along an axis, towards and away from the second proof mass. The second proof mass 20 is suspended from the frame 12 by flexures 14 in the same way, so as to allow movement of the second proof mass along the same axis, towards and away from the first proof mass. The axis is the sensitive axis of the accelerometer.

[0042] A first resonant element 16 is connected between an anchor 8 and the first proof mass 10. The anchor 8 is part of the frame 12. The first resonant element extends along the sensitive axis.

[0043] The first resonant element 16 is connected to the first proof mass 10 through amplifying levers 18. The amplifying levers 18 are fixed to the frame at pivot points 6.

[0044] A second resonant element 26, identical to the first resonant element 16, is connected between a second anchor 8 and the second proof mass 20. The second proof mass is also identical to first proof mass, and the force amplifying levers 28 are identical to the force amplifying levers 18. The second resonant element extends along the same sensitive axis as the first resonant element, but in an opposite direction. This means that when the accelerometer undergoes an acceleration, the second resonant element 26 experiences an equal but opposite strain to the first resonant element 16.

[0045] The first and second resonant elements 16, 26 are coupled by a coupling beam 22. In this example the coupling beam 22 is a simple linear beam. The coupling beam is formed from the same silicon wafer as the resonant elements.

[0046] Energy can be transferred from one resonant element to the other through the coupling beam. When the resonant elements experience opposite strain to one another, and so have different resonant properties, energy can be localised more in one resonant element than the other. This phenomenon is often referred to as mode localisation.

[0047] The amount of mode localisation is dependent on the degree to which the strain on the first and second resonant elements is different. So the relative amplitude of vibration of the first and second resonant elements can be used to provide a measure of acceleration experienced by the proof masses. In fact, measuring amplitude variations induced by mode localisation provides a high resolution determination of acceleration that is relatively insensitive to environmental variation. This is described fully in WO2011/148137.

[0048] With the arrangement of FIG. 1, because one resonant element undergoes compression while the other undergoes tension, a level of common mode rejection can be achieved using only a single pair of coupled resonant elements. This means that for a given size of accelerometer, the size, and hence mass, of the proof mass can greater than when two pairs of resonant elements are used to provide a differential output. Furthermore, locating the resonant element assembly at the centre of the structure allows for better matching between the two resonant elements, improving common mode rejection.

[0049] The weaker the coupling between the resonant elements the more pronounced the mode localisation and so the high resolution the measurement. However, the weaker the coupling is between the resonant elements the closer the resonant modes are in frequency. The coupling must therefore be non-zero and sufficient for each resonant mode to be resolvable from each other. In other words the coupling must be strong enough that there is no modal overlap in the coupled response of the system. In order to ensure the structure is robust and can be consistently produced, the coupling beam needs to have sufficient thickness.

[0050] The coupling beam between the first and second resonant element is positioned close to the anchors 8. Positioning the coupling beam 22 closer to a node of the mode of vibration in use reduces the strength of coupling when compared to positioning the coupling beam closer to anti-node of the mode of vibration, and so increases the scale factor of the accelerometer.

[0051] The coupling between the first and second resonant element can be achieved electrostatically instead of by a coupling beam. If a mechanical linkage is used, other shapes of beam are possible, as described with reference to FIG. 5 below.

[0052] In the embodiment of FIG. 1 third and fourth resonant elements are shown in dotted line. The third resonant element 17 is coupled to the first resonant element 16 by a mechanical linkage. The fourth resonant element 27 is coupled to the second resonant element 26 by a mechanical linkage. The third and fourth resonant elements are structurally identical to one another (and may be identical to the first and second resonant elements), and are coupled to the frame, but they are not coupled to either of the proof masses. The provision of further coupled resonant elements in this manner can enhance mode localisation and thereby improve the sensitivity of the measurement.

[0053] FIG. 2 illustrates an arrangement for driving the resonant elements of FIG. 1 and for providing an output measure of acceleration. In FIG. 2, third and fourth resonant elements are not provided.

[0054] The first and second resonant elements 16 and 26 are driven by an AC voltage signal from two separate drive electrodes 34 and 38. The same AC signal is applied to each drive electrode. The amplitude of oscillation of the first resonant element 16 is maintained at a constant level by sensing off electrode 36. The output from electrode 36 is fed into a control circuit, and the output of the control circuit fed back to drive electrodes 34 and 38. The first stage of the control circuit is a gain element 33 that provides a fairly large initial gain before feeding the signal into a variable gain amplifier (VGA) 35. The VGA 35 consists of an amplifier that adjusts its gain in accordance to a control signal (from an automatic gain control (AGC) circuit 39) and feeds the output to a buffer 37. The AGC 39 consists of a circuit that detects the output of the first stage gain element using a peak detector (that compares the peak amplitude of the output arising from the first gain stage with that of a reference signal) and accordingly controls the gain of the first stage gain element to maintain the output at a constant peak amplitude. The controlled output signal from the buffer 37 is, in turn, used to drive the resonant elements in the chosen mode of oscillation. The modal amplitudes of the second resonant element (at the resonant mode wherein the oscillations are sustained) is then read out from sense electrode 40.

[0055] Sensing of the amplitude of vibration may be implemented in several ways. In the embodiment shown in FIG. 2, the sensing of the amplitude of vibration of the first and second resonant elements may be achieved by measuring the motional current of the resonant elements as they oscillate from the sensing electrodes 36 and 40 respectively. Silicon also exhibits a strong piezoresistive effect, so the resistance of a silicon resonant element will change as it oscillates which may also be used as an alternative readout mechanism. Alternatively, the sensing means may comprise electrodes mounted adjacent to the first and second resonant elements to allow for capacitive sensing. Other possibilities for the sensing means include optical sensing of the oscillation of the first resonant element or even electro-magnetic transduction.

[0056] The output from the sense electrode 40 is fed into a trans-resistance amplifier circuit 42 to convert the current signal from electrode 40 into a voltage signal that may be used to directly calculate an amplified measure of the modal amplitude variation of the second resonant element 26, from which any induced changes in the stiffness of the first resonant element 1 may be evaluated. From the change in stiffness, acceleration can be determined.

[0057] If further coupled resonant elements are provided, the drive and sense arrangement can be extended, for instance by driving only the first and second resonant elements and sensing the response of all the resonant elements. Alternatively, all of the coupled resonant elements can be driven and sensed. Any number of coupled resonant elements can be used.

[0058] Instead of using feedback control to maintain the first resonant element at a constant amplitude, it is possible to read the amplitude of both the first and second resonant elements and determine the ratio of the amplitudes in order to provide a measure of acceleration. This drive and sensing scheme is described in WO2011/148137.

[0059] FIG. 3 is a flow diagram, illustrating the steps carried out in a method in accordance with the present invention using an accelerometer of the type described above with reference to FIGS. 1 and 2. In a first step, step 50, the resonant elements are caused to vibrate in a resonant mode using a drive signal. As described above the drive signal may comprise an AC voltage applied to the resonant elements and a DC biasing voltage applied to adjacent electrodes. In step 52, the amplitude of vibration of the first resonant element is detected. In step 54 the drive signal is adjusted to maintain the amplitude of the first resonant element at a constant level using a feedback loop. The amplitude of vibration of the second resonant element is detected in step 56 to provide a measure of the change in effective stiffness of the first resonant element, from which the acceleration or angular velocity of the proof mass along the axis of sensitivity can be determined in step 58.

[0060] FIGS. 4, 5 and 6 illustrate further embodiments that are variations of the accelerometer of FIGS. 1 and 2.

[0061] In FIG. 4 a single proof mass 60 is used instead of separate first and second proof masses 10 and 20. The single proof mass is connected to both the first resonant element 16 and the second resonant element 26. In effect, the first and second proof masses of FIG. 1 are joined together and surround the first resonant element 16, second resonant element 26 and coupling beam 22. This arrangement operates in exactly the same way as the embodiment of FIG. 1, but allows the mass of the proof mass to be maximised for a given footprint of the accelerometer.

[0062] The embodiment of FIG. 5 is identical to the embodiment of FIG. 4 except for the form of the coupling beam between the first and second resonant elements. In the embodiment of FIG. 5 the coupling beam 22 is a serpentine shaped beam. The coupling strength provided by a serpentine shaped beam can be less sensitive to temperature fluctuations than a simple linear beam and therefore can result in better common mode rejection of noise due to temperature variations.

[0063] The embodiment of FIG. 6 is identical to the embodiment of FIG. 4 except for manner in which the coupling between the first and second resonant elements is provided. In the embodiment of FIG. 6, the first and second resonant elements 16, 26 are arranged coaxially along the sensitive axis of the accelerometer and coupling between them is provided by a non-ideal anchor 70. A separate coupling beam is not required. This arrangement has the advantage that the sensing axis of the two resonant elements are not offset from one another. It also has the advantage that impact of temperature fluctuations on a coupling beam are removed. However, it can be difficult to achieve a consistent level of coupling between the resonant elements from one device to the next in a manufacturing process.

[0064] In the embodiments of FIGS. 1, 4 and 5, the first and second resonant elements 16, 26 may take the form of a simple clamped-clamped beam, as illustrated in FIG. 7a or may take the form of a double ended tuning fork (DETF) as shown in FIG. 7b. Different modal shapes for the two types of resonant element are illustrated in dotted line in FIG. 7.

[0065] In the embodiment of FIG. 6, in which coupling through the anchor 70 is required, it is advantageous to use a simple clamped-clamped beam, as illustrated in FIG. 7a. If DETF elements are employed, they will be operated in the in-phase mode preferentially to enable mechanical coupling between the resonant elements through the anchor.

[0066] An accelerometer as described can be used for many applications. One example is as a gravimeter. A gravimeter can be used for surveying in oil or gas extraction. FIG. 10 illustrates a gravimeter down a bore hole. Measurements of the gravity field down a borehole can provide information about the density of the surrounding layers and so information about the presence of oil or gas reserves and their size and location. This requires measurement of gravitational filed in three dimensions. A shown in FIG. 8, an accelerometer 80 in accordance with the invention is positioned within a borehole tool 82. The borehole tool is placed in the bore 84, suspended from a suitable structure on the surface. Gravimeters of this sort can be used in other applications, such as carbon storage monitoring, monitoring groundwater depletion, discovery of underwater aquifers, monitoring other processes underpinning the hydrological cycle, early-warning systems for earthquake-prone zones or for areas impacted by volcanic activity.