Method and apparatus for generating a proof-mass drive signal

A drive-mode oscillator module generates at least one proof-mass drive signal for use within a micro-electro-mechanical system (MEMS) device. The drive-mode oscillator module comprises at least one gain control component arranged to receive at least one proof-mass motion measurement signal, and to generate a digital modulation control signal based at least partly on the at least one proof-mass motion measurement signal, and at least one modulation component arranged to receive the digital amplitude modulation control signal, and to output at least one proof-mass drive signal. The at least one modulation component is arranged to digitally modulate the at least one proof-mass drive signal based at least partly on the received digital amplitude modulation control signal.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for generating a proof-mass drive signal, and in particular to a drive-mode oscillator module for generating at least one proof-mass drive signal for use within a micro-electro-mechanical system (MEMS) device and method therefor.

BACKGROUND OF THE INVENTION

A gyroscope is a sensor that measures the rate of rotation of an object. The concept of a vibrating MEMS (Micro-Electro-Mechanical System) gyroscope is to generate momentum of a proof-mass to induce and detect the Coriolis force. A Coriolis force is applied to the proof-mass in motion when an angular rate is applied. The Coriolis force Fc is the product of the proof-mass m, the input rate Ω and the mass velocity v. The direction of the Coriolis force is perpendicular to the motion of the proof-mass.

The basic architecture of a vibratory gyroscope is comprised of a drive-mode oscillator circuit that generates and maintains a constant linear momentum of the proof-mass, and a sense mode circuit that measures the sinusoidal Coriolis force induced due to the combination of the drive oscillation and any angular rate input. The majority of vibratory gyroscopes utilize a vibratory proof-mass suspended by springs above a substrate. The objective being to form a vibratory drive oscillator coupled to an orthogonal sense system detecting the Coriolis force.

Since the Coriolis Effect is based on conservation of momentum, the drive-mode oscillator circuit is implemented to provoke the oscillation of the proof-mass which is the source of this momentum.

FIG. 1illustrates a simplified block diagram of an example of a MEMS gyroscope implementation100. In such a MEMS gyroscope implementation100, a drive-mode oscillator circuit110vibrates the proof-mass120, causing the proof-mass120to oscillate. When an angular rate is applied to the proof-mass120, the motion of the proof-mass120is deflected in a direction perpendicular to the direction of oscillation of the proof-mass (sense mode). The amount of deflection may then be measured via sense electrodes and used to determine the angular rate that was applied to the proof-mass.

Due to the mechanical properties of such MEMS devices, the drive-mode oscillation circuit110is required to operate at a resonance frequency of the proof-mass120. In a typical MEMS gyroscope implementation, the drive-mode oscillator circuit110is based on a self-oscillating loop principle in which the proof-mass motion is detected, phase-shifted, amplified and used as an electrical stimulus to drive the proof-mass oscillation.

FIG. 2illustrates a simplified block diagram of an example of such a conventional drive-mode oscillator circuit110. The drive-mode oscillator circuit110in the illustrated example comprises a capacitance to voltage (C2V) circuit210arranged to convert a capacitance change of differential MEMS drive measurement units (DMUs)200,205caused by the displacement of the proof-mass to a differential voltage measurement signal215. An integrator220receives the voltage measurement signal and phase shifts it by, for example, 90° to compensate for the phase lag of the system. A voltage gain amplifier (VGA)230receives the phase shifted voltage signal225and outputs a proof-mass drive signal235to differential drive actuation units (DAUs)240,245, which vibrate the proof-mass120accordingly. An automatic gain control (AGC) circuit250provides a control signal255to the VGA230to control the amplitude of the proof-mass drive signal235output thereby. Conventionally, such an AGC circuit250is implemented using complex analogue circuitry for providing the necessary gain correction, which tends to require high current consumption and a large die-size, and is prone to temperature and process variations.

SUMMARY OF THE INVENTION

The present invention provides a drive-mode oscillator module for generating at least one proof-mass drive signal for use within a micro-electro-mechanical system (MEMS) device, a micro-electro-mechanical system (MEMS) device comprising at least one such drive-mode oscillator module, and a method of generating at least one proof-mass drive signal for use within a micro-electro-mechanical system (MEMS) device as described in the accompanying claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described with reference to examples of a method and apparatus for generating a proof-mass drive signal for use within a micro-electro-mechanical system (MEMS) device. It will be appreciated that the present invention is not limited to the specific examples herein described and illustrated in the accompanying drawings. For example, the present invention is herein described with reference to a drive-mode oscillator module comprising a differential architecture and arranged to receive and output differential signals. However, the present invention may equally be implemented within a single-ended architecture whereby a drive-mode oscillator module receives and outputs single-ended signals.

Referring first toFIG. 3, there is illustrated a simplified block diagram of an example of a drive-mode oscillator module300for generating a proof-mass drive signal for use within a micro-electro-mechanical system (MEMS) device, such as the MEMS gyroscope device100illustrated inFIG. 1. The drive-mode oscillator module300may be implemented within an integrated circuit device, for example as illustrated generally at305, comprising at least one die within a single integrated circuit package.

The drive-mode oscillator circuit300in the illustrated example comprises a capacitance to voltage (C2V) circuit310arranged to convert a capacitance change measurement signal312received from differential MEMS drive measurement units (DMUs) (not shown) into a proof-mass motion measurement voltage signal314.

The drive-mode oscillator module300further comprises a gain control component320, which in the illustrated example comprises a comparator. The gain control component320is arranged to receive the converted proof-mass motion measurement signal314, and to generate a digital modulation control signal325based at least partly on the proof-mass motion measurement signal314. For example, and as illustrated inFIG. 3, the gain control component320may be arranged to receive a reference amplitude signal322, compare the proof-mass motion measurement signal314to the reference amplitude signal322, and output the digital modulation control signal325comprising an indication of whether the amplitude of the proof-mass motion measurement signal314exceeds a voltage level of the reference amplitude signal322. In the example differential implementation illustrated inFIG. 3, the differential amplitude of the proof-mass motion measurement signal314is compared with an absolute value between differential components of the reference amplitude signals322(AGC_REFp-AGC_REFn), and an indication of whether the differential amplitude of the proof-mass motion measurement signal314exceeds the absolute value between differential components of the reference amplitude signals322is output as the digital modulation control signal325.

The drive-mode oscillator module300further comprises a modulation component330arranged to receive the digital amplitude modulation control signal325output by the gain control component320, and to output a proof-mass drive signal335. The proof-mass drive signal335may then be provided to differential drive actuation units (DAUs) (not shown), which vibrate a proof-mass of the MEMS device accordingly, such as the proof-mass120of the MEMS gyroscope device100illustrated inFIG. 1. The modulation component330is further arranged to digitally modulate the proof-mass drive signal335based at least partly on the received digital amplitude modulation control signal325, as described in greater detail below with reference toFIGS. 4 to 9.

The drive-mode oscillator module300illustrated inFIG. 3further comprises a phase component340arranged to receive the proof-mass motion measurement signal314output by the C2V circuit310, detect a proof-mass motion phase, and output to the modulation component330a digital phase signal345. The modulation component330may thus be arranged to output the proof-mass drive signal335comprising a phase based at least partly on the received digital phase signal345output by the phase component340.

In the illustrated example, the phase component340comprises a comparator342arranged to receive the differential proof-mass motion measurement signal314, compare a first differential component of the differential proof-mass motion measurement signal314to a second differential component of the differential proof-mass motion measurement signal314, and output a phase detection signal344based on the comparison of the first and second differential components of the differential proof-mass motion measurement signal314. The phase component340illustrated inFIG. 3further comprises a phase shift component346arranged to receive the phase detection signal344output by the comparator342, apply a 90° phase shift to the phase detection signal344, and output the digital phase signal345comprising the phase shifted phase detection signal.

Referring now toFIG. 4, there is illustrated a simplified block diagram of a first example of the modulation component330. In the example illustrated inFIG. 4, the modulation component330is arranged to apply amplitude shift keying (ASK) modulation to the proof-mass drive signal335. The modulation component330comprises, in the illustrated example, a flip-flop410arranged to receive at a data input thereof the digital amplitude modulation control signal325output by the gain control component320. The flip-flop410is further arranged to receive at a clock input thereof the digital phase signal345output by the phase component340. In this manner, the flip-flop410is arranged to capture and hold the digital amplitude modulation control signal325upon each rising edge of the digital phase signal345(e.g. at the start of each cycle thereof). The flip-flop410is further arranged to output a modulation selection signal415comprising the captured and held digital amplitude modulation control signal325.

It will be appreciated that the flip-flop410is one possible implementation for capturing and holding the digital amplitude modulation control signal325. For example, it is also possible to perform a capture and hold on both rising and falling edges of the digital phase signal345using, say, two flip-flops working in parallel, or an equivalent solution. In this manner, the refresh of the modulation selection signal415may be performed at twice the clock frequency.

The modulation component330further comprises a multiplexer component420arranged to receive at a control input thereof the modulation selection signal415. The multiplexer component420comprises a first (differential) input arranged to receive the digital phase signal345, and a second (differential) input arranged to receive, in the example illustrated inFIG. 4, a constant voltage signal440. In the illustrated example, an inverter430is used to provide an inverted version of the digital phase signal345; in this manner a differential form of the digital phase signal345to be provided to the first input of the multiplexer component420is created, with a first differential component comprising the original digital phase signal345and a second differential component comprising the inverted version of the digital phase signal345. The multiplexer component420is arranged to selectively output, as the proof-mass drive signal335, one of the (differential) digital phase signal345and the constant voltage signal440.

FIG. 5illustrates a simplified timing diagram500of an example of the modulation component330ofFIG. 4digitally modulating the proof-mass drive signal335based on the received digital amplitude modulation control signal325and digital phase signal345. The differential proof-mass motion measurement voltage signal314is illustrated with reference to the reference amplitude signal322. As can be seen inFIG. 5, the digital phase signal345comprises a 90° shifted representation of the phase of the proof-mass motion measurement voltage signal314.

In an initial drive phase510of the digital modulation illustrated inFIG. 5, the proof-mass motion measurement voltage signal314comprises an amplitude less than the reference amplitude signal322. As a result, the digital amplitude modulation control signal325output by the gain control component320comprises, in the illustrated example, a low logical level. Accordingly, the modulation selection signal415output by the flip-flop410also comprises a low logical level. In the example illustrated inFIG. 4, the multiplexer component420is arranged to selectively output the signal received at its first input upon receipt of a low logical signal at its control input. Thus, in the initial drive phase510illustrated inFIG. 5, the multiplexer component420outputs, as the proof-mass drive signal335, the differential form of the digital phase signal345, outputting a 90° phase shifted oscillating proof-mass drive signal when the proof-mass motion measurement signal314is less than the reference amplitude signal322.

Such a 90° phase shifted oscillating proof-mass drive signal will have the effect of accelerating the oscillations of the proof-mass, causing a gradual increase in the magnitude of the proof-mass oscillations. In this manner, as the oscillations of the proof-mass gradually increase in magnitude, the amplitude of the proof-mass motion measurement voltage signal314will gradually increase. When an amplitude of the proof-mass motion measurement voltage signal314subsequently exceeds the reference amplitude signal322, as illustrated at525, the gain control component320detects as such and outputs high logical level pulses within the digital amplitude modulation control signal325corresponding to instances when the amplitude of the proof-mass motion measurement voltage signal314exceeds the reference amplitude signal322, as indicated at527. As can be seen inFIG. 5, because of the 90° phase shift in the digital phase signal345relative to the phase of the proof-mass motion measurement voltage signal314, the high logical level pulses occur on each of a rising and falling edge of the digital phase signal345. Accordingly, and as described above in relation toFIG. 4, the flip-flop410captures and holds the high logical level of the digital amplitude modulation control signal325that occurs during, in the illustrated example, the rising edge of the digital phase signal345. As a result, the modulation selection signal415changes from a low logical level to a high logical level, causing the multiplexer component420to selectively switch to outputting, as the proof-mass drive signal335, the signal received at its second input; i.e. the constant voltage signal440in the example illustrated inFIGS. 4 and 5. As a consequence, the digital modulation illustrated inFIG. 5changes into a second, no-drive phase520in which the proof-mass drive signal335output by the modulation component300comprises a constant voltage.

As will be appreciated, such a constant voltage drive signal will have the effect of stopping the oscillating driving force being applied to the proof-mass, which will subsequently continue to vibrate as a result of momentum, but at a gradually reducing magnitude. Accordingly, during such a no-drive phase520, the amplitude of the proof-mass motion measurement voltage signal314will gradually decrease as the magnitude of the proof-mass oscillations decreases, resulting in the proof-mass motion measurement voltage signal314falling below the reference amplitude signal322. Upon the proof-mass motion measurement voltage signal314falling below the reference amplitude signal322, the gain control component320ceases detecting the proof-mass motion measurement voltage signal314exceeding the reference amplitude signal322, and thus ceases outputting high logical level pulses within the digital amplitude modulation control signal325. As a result, the digital amplitude modulation control signal subsequently comprises a substantially continuous low logical level. Thus, upon the next rising edge of the digital phase signal345, the flip-flop410captures and holds the low logical level of the digital amplitude modulation control signal325, causing the modulation selection signal415to change from a high logical level to a low logical level. As a result, the multiplexer component420selectively reverts back to outputting the differential form of the digital phase signal345. As a consequence, the digital modulation illustrated inFIG. 5reverts back to a drive phase530in which the proof-mass drive signal335output by the modulation component comprises, in the illustrated example, the differential form of the digital phase signal345.

In the simplified example illustrated inFIG. 5, the no-drive phase has been illustrated as comprising a single oscillation cycle of the proof-mass motion measurement voltage signal314. However, it will be appreciated that more than one cycle may occur before the amplitude of the proof-mass motion measurement voltage signal314falls below the reference amplitude signal322.

This technique of digitally modulating the proof-mass drive signal335introduces a ripple caused by the ‘switching’ between drive phase and no-drive phase. Such a ripple may be minimised by configuring the ‘strength’ of the proof-mass drive signal335to minimise the rate at which the magnitude of the proof-mass oscillations increases during each drive phase. In the example illustrated inFIG. 4, each component of the differential form of the digital phase signal345is passed through a level shifter450. Each level shifter450may be arranged to receive the respective component of the differential form of the digital phase signal345, apply a voltage level shift thereto, and output the respective component of the differential form of the digital phase signal345at a voltage level configured to reduce the rate at which the magnitude of the proof-mass oscillations increase, and thus to reduce the ripple introduced by the digital modulation.

In some examples, the level shifters450may be configurable, for example by way of a level shift signal332(FIG. 3), to apply a configurable voltage level shift to the respective component of the differential form of the digital phase signal345. In this manner, the voltage level at which the level shifters450output their respective component of the differential form of the digital phase signal345may configured as required. For example, the level shifters450may be configured to compensate for variations in temperature and/or process. Additionally/alternatively, the level shifters450may be configured to allow the drive-mode oscillator module300to be used with different MEMS devices having different drive signal requirements.

Referring now toFIG. 6, there is illustrated a simplified block diagram of a further example of the modulation component330. In the example illustrated inFIG. 6, the modulation component330is arranged to apply binary phase shift keying (BPSK) modulation to the proof-mass drive signal335. As for the example illustrated inFIG. 4, the multiplexer component420inFIG. 6is arranged to receive at its first input the (level shifted) differential form of the digital phase signal345. However, in the example illustrated inFIG. 6, the multiplexer component420is arranged to receive at its second input the inverse640of the differential form of the digital phase signal345, instead of the constant voltage signal440inFIG. 4. In this manner, the multiplexer component420is arranged to selectively output, as the proof-mass drive signal335, one of the digital phase signal345and the inverse640of the digital phase signal345.

FIG. 7illustrates a simplified timing diagram700of an example of the modulation component330ofFIG. 6, modulating the proof-mass drive signal335based on the received digital amplitude modulation control signal325and digital phase signal345. In the example illustrated inFIG. 7, in an initial drive phase710, the proof-mass motion measurement voltage signal314comprises an amplitude less than the reference amplitude signal322. As a result, the digital amplitude modulation control signal325output by the gain control component320comprises, in the illustrated example, a low logical level. Accordingly, the modulation selection signal415output by the flip-flop410also comprises a low logical level. In the example illustrated inFIG. 6, the multiplexer component420is arranged to selectively output the signal received at its first input upon receipt of a low logical signal at its control input. Thus, in the initial drive phase710illustrated inFIG. 7, the multiplexer component420outputs, as the proof-mass drive signal335, the differential form of the digital phase signal345, outputting a 90° phase shifted oscillating proof-mass drive signal when the proof-mass motion measurement signal314is less than the reference amplitude signal322.

As mentioned in relation toFIG. 4, such a 90° phase shifted proof-mass drive signal will have the effect of accelerating the oscillations of the proof-mass, causing a gradual increase in the magnitude of the proof-mass oscillations. In this manner, as the oscillations of the proof-mass gradually increase in magnitude, the amplitude of the proof-mass motion measurement voltage signal314will gradually increase. When an amplitude of the proof-mass motion measurement voltage signal314subsequently exceeds the reference amplitude signal322, as illustrated at725, the gain control component320detects as such and outputs high logical level pulses within the digital amplitude modulation control signal325corresponding to instances when the amplitude of the proof-mass motion measurement voltage signal314exceeds the reference amplitude signal322, as indicated at727. The flip-flop410captures and holds the high logical level of the digital amplitude modulation control signal325that occurs during, in the illustrated example, the rising edge of the digital phase signal345. As a result, the modulation selection signal415changes from a low logical level to a high logical level, causing the multiplexer component420to selectively switch to outputting, as the proof-mass drive signal335, the signal received at its second input; i.e. the inverse640of the digital phase signal345in the example illustrated inFIGS. 6 and 7. As a consequence, the digital modulation illustrated inFIG. 7changes into a second, no-drive phase720in which the proof-mass drive signal335output by the modulation component300comprises the inverse640of the digital phase signal345.

The inverse640of the digital phase signal345comprises a phase shifted −90° relative to the proof-mass motion measurement voltage signal314. As such, the proof-mass drive signal335comprising the inverse640of the digital phase signal345will have the effect of decelerating the oscillations of the proof-mass, causing an accelerated reduction in the magnitude of the proof-mass oscillations. Accordingly, during such a no-drive phase720, the amplitude of the proof-mass motion measurement voltage signal314will decrease as the magnitude of the proof-mass oscillations decreases, resulting in the proof-mass motion measurement voltage falling below the reference amplitude signal322. Upon the proof-mass motion measurement voltage signal314falling below the reference amplitude signal322, the gain control component320ceases detecting the proof-mass motion measurement voltage signal314exceeding the reference amplitude signal322, and thus ceases outputting high logical level pulses within the digital amplitude modulation control signal325. As a result, the digital amplitude modulation control signal subsequently comprises a substantially continuous low logical level. Thus, upon the next rising edge of the digital phase signal345, the flip-flop410captures and holds the low logical level of the digital amplitude modulation control signal325, causing the modulation selection signal415to change from a high logical level to a low logical level. As a result, the multiplexer component420selectively reverts back to outputting the non-inverted differential form of the digital phase signal345. As a consequence, the digital modulation illustrated inFIG. 7reverts back to a drive phase730in which the proof-mass drive signal335output by the modulation component comprises, in the illustrated example, the non-inverted differential form of the digital phase signal345.

As for the example illustrated inFIG. 4, each component of the differential form of the digital phase signal345is passed to the first (differential) input of the multiplexer component420through a level shifter450. In the example illustrated inFIG. 6, the inverse640of the digital phase signal345is provided directly to the second (differential) input of the multiplexer component420substantially without any level shifting. However, it is contemplated that level shifting may also be applied to the inverse640of the digital phase signal345. For example, and as illustrated inFIG. 8, each differential component of the inverse640of the digital phase signal345may be passed to the second (differential) input of the multiplexer component420through a level shifter850, which may be configured to apply the same or a different voltage level shift to that of the level shifters450.

Referring back toFIG. 3, the digital modulation provided by the drive-mode oscillator module300makes use of digital modulated stimuli within a polar architecture that enables a significantly simplified implementation for the control and driving of a proof-mass within a MEMS device, as compared with conventional analogue architectures. Furthermore, the use of digital components in the manner reduces current consumption and die size as compared with conventional analogue architectures.

In the example illustrated inFIG. 3, the digital stimulus needs to be substantially synchronous with the proof-mass displacement in order to be able to amplify its motion (resonance conditions). In order to achieve such synchronization, the drive-mode oscillator module300uses the phase detection signal344as a reference. However, the phase detection signal344is typically only reliably when the differential output of the proof-mass motion measurement voltage signal314is above, say, a few mV. Since the amplitude of the proof-mass motion measurement voltage signal314is directly proportional to the magnitude of the oscillations of the proof-mass, the drive-mode oscillator module300in the illustrated example requires the proof-mass to be oscillating in order to operate.

Accordingly, in the illustrated example, the drive-mode oscillator module300further comprises an analogue start-up component350selectively arranged to generate a proof-mass drive signal355for driving the proof-mass120of the MEMS device100during a start-up phase. In the illustrated example, the analogue start-up component350comprises an integrator component352arranged to receive the proof-mass motion measurement signal314output by the C2V circuit310and apply a 90° phase shift thereto, and an amplifier component354arranged to receive the phase shifted proof-mass motion measurement signal, and to output the proof-mass drive signal355based at least partly on the phase shifted proof-mass motion measurement signal.

The drive-mode oscillator module300further comprises a multiplexer component360arranged to receive at a first (differential) input thereof the proof-mass drive signal335output by the digital modulator330, and to receive at a second (differential) input thereof the proof-mass drive signal355output by the analogue start-up component350. The multiplexer component360is further arranged to receive a start-up control signal365, and to selectively output one of the proof-mass drive signals335,355in accordance with the start-up control signal365. In this manner, during start-up of the MEMS device100, the drive-mode oscillator module300may be configured, via the start-up control signal365, to output a proof-mass drive signal355generated by the analogue start-up module350. Once the proof-mass120comprises oscillations of sufficient magnitude for the phase detection signal344to be reliable, the drive-mode oscillator module300may then be reconfigured to output a proof-mass drive signal335generated by the digital modulator330. Once the multiplexer component360is arranged to receive the signal335generated by the digital modulator the start-up circuit is switched off reducing significantly the power consumption.

Significantly, in the illustrated example the analogue start-up component350is only required to provide an initial proof-mass drive signal355in order to start the proof-mass oscillating. As such, it is not required to perform gain control for the proof-mass drive signal355output thereby. Consequently, no complex automatic gain control (AGC) circuitry is required, significantly reducing the additional power consumption and die size required.

Referring now toFIG. 9, there is illustrated a simplified flowchart900of an example of a method of generating a proof-mass drive signal for use within a micro-electro-mechanical system (MEMS) device, such as may be implemented within the drive-mode oscillator module300. The method starts at910, and moves on to920where a capacitance change measurement signal from a drive measurement unit (DMU) is received, such as the capacitance change measurement signal312illustrated inFIG. 3. Next, at930, the received capacitance change measurement signal is converted into a proof-mass motion measurement voltage signal, such as the proof-mass motion measurement signal314illustrated inFIG. 3. A proof-mass motion phase is then detected from the proof-mass motion measurement voltage signal, at940, and a 90° shifted phase signal is generated. The proof-mass motion measurement voltage signal is also compared to a reference amplitude value and a digital amplitude modulation control signal is generated at950. A digitally modulated proof-mass drive signal is then generated, at960, based at least partly on the 90° shifted phase signal and the digital amplitude modulation control signal. The method then ends, at970.

Also for example, in one embodiment, the illustrated examples may be implemented as circuitry located on a single integrated circuit or within a same device. For example, all of the components of the drive-mode oscillator module300illustrated inFIG. 3may be implemented within a single integrated circuit device. Alternatively, the examples may be implemented as any number of separate integrated circuits or separate devices interconnected with each other in a suitable manner.