Patent Description:
A resonant sensor is an oscillator whose output resonant frequency is a function of an input measurand. In other words, the output of a resonant sensor corresponds to the shift in resonant frequency of a mechanical microstructure that gets tuned in accordance with a change in a physical quantity to be measured.

There has been an increased interest over the past few years in the development of high precision micromachined 'all-silicon' resonant micro-accelerometers. This interest has been triggered due to the recent growth in demand for miniature high precision motion sensors within the aerospace, automotive and even the consumer-electronics markets. Resonant micro-accelerometers fabricated using silicon micromachining techniques present a number of significant advantages, the biggest being economy. These silicon resonant micro-accelerometers not only boast improved sensitivity and resolution relative to their more traditional capacitive detection based counterparts with similar device footprints, but have also been shown to provide enhanced dynamic range making them ideal candidates for potential application in numerous motion sensing applications within the identified markets. One potential sensing application is gravimetry. Resonant accelerometers can be designed to provide a low-noise response for near-DC measurements (suitable for applications in gravimetry) and a wide dynamic range enabling measurements over the entire +/-<NUM> regime.

Accelerometer designs that exploit mode localization between two or more weakly coupled resonators have been proposed. The variations in the eigenstates (which refer to the relative amplitudes at the resonant frequencies measured from each of the resonators) due to induced strain modulation one of the resonators, yields a measure of the inertial force on the sensor. Measuring such eigenstate variations induced by mode localization offers two key advantages over conventional resonant frequency shift based measurements: insensitivity to unwanted environmental variations; and orders of magnitude enhancement in the output sensitivity and consequently, the resolution of such sensors.

It is an object of the present invention to provide a resonant accelerometer that provides the advantages of using mode localization but that has an improved scale factor when compared to previous designs.

<CIT> discloses a resonant sensor comprising a substrate and a proof mass suspended from the substrate by one or more flexures to allow the proof mass to move relative to the frame along a sensitive axis. First and second resonant elements are connected between the frame and the proof mass. Drive and sensing circuitry is configured to drive the first resonant element in a first mode, drive the second resonant element in a second mode different to the first mode, and determine a measure of acceleration.

<CIT> discloses a MEMS sensor that comprises a substrate and an anchor region coupled to the substrate. At least one support arm is coupled to the anchor region, at least two guiding arms are coupled to the at least one support arm, and a plurality of sensing elements are disposed on the at least two guiding arms to measure motion of the at least two guiding arms relative to the substrate. A proof mass system comprising at least one mass is coupled to each of the at least two guiding arms by a set of springs. The proof mass system is disposed outside the anchor region, the at least one support arm, the at least two guiding arms, the set of springs, and the plurality of sensing elements.

According to the invention an accelerometer according to claim <NUM> is provided. Other embodiments are covered by the dependent claims.

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

<FIG> 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.

The accelerometer comprises a first proof mass <NUM> and a second proof mass <NUM>. The first proof mass is suspended from a frame <NUM> by flexures <NUM> 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 <NUM> is suspended from the frame <NUM> by flexures <NUM> 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.

A first resonant element <NUM> is connected between an anchor <NUM> and the first proof mass <NUM>. The anchor <NUM> is part of the frame <NUM>. The first resonant element extends along the sensitive axis. The first resonant element <NUM> is connected to the first proof mass <NUM> through amplifying levers <NUM>. The amplifying levers <NUM> are fixed to the frame at pivot points <NUM>.

A second resonant element <NUM>, identical to the first resonant element <NUM>, is connected between a second anchor <NUM> and the second proof mass <NUM>. The second proof mass is also identical to first proof mass, and the force amplifying levers <NUM> are identical to the force amplifying levers <NUM>. 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 <NUM> experiences an equal but opposite strain to the first resonant element <NUM>.

The first and second resonant elements <NUM>, <NUM> are coupled by a coupling beam <NUM>. In this example the coupling beam <NUM> is a simple linear beam. The coupling beam is formed from the same silicon wafer as the resonant elements.

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.

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 <CIT>.

With the arrangement of <FIG>, 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.

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.

The coupling beam between the first and second resonant element is positioned close to the anchors <NUM>. Positioning the coupling beam <NUM> 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.

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> below.

In the embodiment of <FIG> third and fourth resonant elements are shown in dotted line. The third resonant element <NUM> is coupled to the first resonant element <NUM> by a mechanical linkage. The fourth resonant element <NUM> is coupled to the second resonant element <NUM> 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.

<FIG> illustrates an arrangement for driving the resonant elements of <FIG> and for providing an output measure of acceleration. In <FIG>, third and fourth resonant elements are not provided.

The first and second resonant elements <NUM> and <NUM> are driven by an AC voltage signal from two separate drive electrodes <NUM> and <NUM>. The same AC signal is applied to each drive electrode. The amplitude of oscillation of the first resonant element <NUM> is maintained at a constant level by sensing off electrode <NUM>. The output from electrode <NUM> is fed into a control circuit, and the output of the control circuit fed back to drive electrodes <NUM> and <NUM>. The first stage of the control circuit is a gain element <NUM> that provides a fairly large initial gain before feeding the signal into a variable gain amplifier (VGA) <NUM>. The VGA <NUM> consists of an amplifier that adjusts its gain in accordance to a control signal (from an automatic gain control (AGC) circuit <NUM>) and feeds the output to a buffer <NUM>. The AGC <NUM> 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 <NUM> 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 <NUM>.

Sensing of the amplitude of vibration may be implemented in several ways. In the embodiment shown in <FIG>, 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 <NUM> and <NUM> 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.

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

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.

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 <CIT>.

<FIG> 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 <FIG> and <FIG>. In a first step, step <NUM>, 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 <NUM>, the amplitude of vibration of the first resonant element is detected. In step <NUM> 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 <NUM> 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 <NUM>.

<FIG> and <FIG> illustrate further embodiments that are variations of the accelerometer of <FIG> and <FIG>.

In <FIG> a single proof mass <NUM> is used instead of separate first and second proof masses <NUM> and <NUM>. The single proof mass is connected to both the first resonant element <NUM> and the second resonant element <NUM>. In effect, the first and second proof masses of <FIG> are joined together and surround the first resonant element <NUM>, second resonant element <NUM> and coupling beam <NUM>. This arrangement operates in exactly the same way as the embodiment of <FIG>, but allows the mass of the proof mass to be maximised for a given footprint of the accelerometer.

The embodiment of <FIG> is identical to the embodiment of <FIG> except for the form of the coupling beam between the first and second resonant elements. In the embodiment of <FIG> the coupling beam <NUM> 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.

The embodiment of <FIG> is identical to the embodiment of <FIG> except for manner in which the coupling between the first and second resonant elements is provided. In the embodiment of <FIG>, the first and second resonant elements <NUM>, <NUM> are arranged coaxially along the sensitive axis of the accelerometer and coupling between them is provided by a non-ideal anchor <NUM>. 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.

In the embodiments of <FIG>, <FIG>, the first and second resonant elements <NUM>, <NUM> may take the form of a simple clamped-clamped beam, as illustrated in <FIG> or may take the form of a double ended tuning fork (DETF) as shown in <FIG>. Different modal shapes for the two types of resonant element are illustrated in dotted line in <FIG>.

In the embodiment of <FIG>, in which coupling through the anchor <NUM> is required, it is advantageous to use a simple clamped-clamped beam, as illustrated in <FIG>. 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.

Claim 1:
An accelerometer comprising:
a frame (<NUM>);
one or more proof masses (<NUM>, <NUM>, <NUM>) suspended from the frame by one or more flexures (<NUM>) and movable relative to the frame along a sensing axis;
a resonant element assembly, the resonant element assembly comprising a first resonant element (<NUM>) and a second resonant element (<NUM>) 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,
characterised in that,
the first resonant element is coupled to the second resonant element by a mechanical coupling (<NUM>, <NUM>) or the first and second resonant elements are electrostatically coupled to one another.