Patent Application
20160231119
The field of this invention relates to a mechanical resonator for use within a system, such as a micro-electro-mechanical-system (MEMS) device, and method therefor. The invention is applicable to, but not limited to, a mechanism for reducing or compensating for any quadrature error generated in the system, for example at boot-up of the MEMS device.
A vibrating micro-electro-mechanical-system (MEMS) gyroscope is one application of a mechanical resonance system and is often used where an angular rotation rate is to be measured. A vibrating MEMS gyroscope includes a movable gyroscope mass (sometimes referred to as a proof mass) that is connected by springs to a substrate. A drive force applied to the proof mass provokes and maintains a constant linear momentum of the proof-mass along a driving position axis, which is needed to generate a Coriolis force ‘Fc’. A Coriolis effect is based on conservation of momentum, whereby the Coriolis force ‘Fc’ is proportional to the product of the proof-mass ‘m’, the input rate ‘Ω’, the proof mass velocity ‘v’, and its angular rate of rotation perpendicular to the direction of movement of the proof mass. The Coriolis force acting on the proof mass, in the presence of an angular rotation, can be induced as a capacitive force by applying a voltage to the capacitor plates of the drive actuation unit. In response to the induced force, the proof mass is moved.
An induced drive force is supplied and controlled using a drive actuation unit, a drive measurement unit and associated circuitry, which in combination is sometimes referred to as a drive-mode oscillator. The drive actuation unit typically includes a capacitive coupling along the driving position axis between a capacitor plate on the substrate and an opposite capacitor plate on the proof mass.
The drive measurement unit includes a similar pair of capacitor plates. The capacitance between the capacitor plates of the drive measurement unit can be measured and indicates a displacement of the proof mass along a sensing position axis that is perpendicular to the driving position axis. Measurement of the displacement of the proof mass along the sensing position axis can be used to obtain a measure of the Coriolis force and thus a measure of the angular rate of rotation.
A sense measurement unit is also sometimes provided, which, similar to the drive measurement unit, can include a capacitive coupling along the sensing position axis between a sense capacitor plate on the substrate and an opposite sense capacitor plate on the movable proof mass. The sense measurement unit can measure any induced sinusoidal Coriolis force due to a combination of the drive oscillation and any angular rate input. The capacitance between the sense capacitor plates of the sense measurement unit is measured as a sense measurement signal and forms an indication of the displacement of the proof mass along the sensing position axis.
U.S. Pat. No. 7,290,435 B2 describes a way to compensate for mechanical quadrature errors by determining a digital code at a production stage, storing the digital code in a non-volatile memory in a one-time programmable (OTP) manner and using the digital code to set an amplitude of a quadrature error compensating signal. Hence, the solution proposed in U.S. Pat. No. 7,290,435 suffers from practical limitations when applied in the field, particularly in that a quadrature error compensating signal is only identified during the production stage of the MEMS gyroscope.
Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the drawings. In the drawings, like reference numbers are used to identify like or functionally similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.
Although examples of the invention are described with reference to use with a MEMS device, the concepts herein described may be applied to any system or device employing a mechanical resonator, and are thus not limited to the specific components or circuits or architecture of
In examples of the invention, a digital quadrature controller is introduced into a system employing a mechanical resonator, such as MEMS device having a MEMS proof mass. The system comprises an analog circuit, coupled to the mechanical resonator, is arranged to receive a mechanical resonator measurement signal having a quadrature error from the mechanical resonator, and extract the quadrature error signal from the mechanical resonator measurement signal using a quadrature clock. The digital quadrature controller is arranged to generate a quadrature error compensation signal from the extracted quadrature error signal. A quadrature error compensation signal is applied to the mechanical resonator or the mechanical resonator measurement signal to reduce quadrature error in the mechanical resonator measurement signal.
Thus, in contrast to the known prior art of U.S. Pat. No. 7,290,435, where a quadrature compensating signal solution is only identified during a one-time programming operation performed at the production stage of the MEMS gyroscope, examples of the present invention propose that the MEMS system itself, extracts a quadrature error signal from the mechanical resonator measurement signal using a quadrature clock. Thereafter, the digital quadrature controller identifies and generates a quadrature error compensation signal that can be advantageously applied to the mechanical resonator or the mechanical resonator measurement signal to reduce quadrature error in the mechanical resonator measurement signal. In some examples, the generation of a quadrature error compensation signal can be performed, for example, when the system is not measuring a Coriolis force. Similarly, applying a compensation signal to reduce any quadrature error can be implemented when the system is not measuring a Coriolis force, and therefore whilst the system is operational in the field. In an exemplary embodiment, the MEMS system itself can identify and generate a quadrature error compensation signal to be applied at system boot.
Notably, in U.S. Pat. No. 7,290,435, a digital code that is determined at a production stage is stored in a non-volatile memory in a one-time programmable (OTP) manner and used to set the amplitude of the quadrature compensating signal. However, as this code is determined at production, it does not cope for any subsequent mechanical stress that can occur, once the MEMS device is active in the field. Furthermore, the subsequent soldering operation of the gyroscope on a printed circuit board generates new mechanical stress that modifies the already compensated-for quadrature error.
Referring to
In accordance with examples of the invention, a digital quadrature controller 370 is coupled to the analog circuit 340 and generates a quadrature error compensation signal, which in some examples can be a quadrature error cancellation digital word. In some examples, the generated quadrature error compensation signal is based on an extracted or determined quadrature error or extracted sign. Examples of the invention describe a mechanism whereby displacement of the proof-mass 310 is sensed by electrodes within the DMU 360, in a sensing position axis that is orthogonal to the drive position axis. The sensed signal 392 is generated by sensing electrodes 362 placed orthogonal to the driving electrodes and arranged to identify or pick up vibrations of the proof-mass 310. The sensed signal 392 is passed to a quadrature readout processing module 395.
In some examples, the quadrature readout processing module 395 can include a demodulator running on a quadrature (Q) clock signal, i.e. one that is in phase with the quadrature signal that is being demodulated. In this manner, by using a synchronised quadrature (Q) clock signal, the demodulator can extract the amount of quadrature error from the proof-mass measurement signal 365. For example, by mixing a quadrature (Q) clock signal with the mechanical resonator measurement signal having a quadrature error, the quadrature error is automatically output. In an alternative example (not shown), a direct sampling of the measurement signal with the proper phase can be performed. A signal 397 identifying an amount of quadrature error is passed to the digital quadrature controller 370.
In some examples, the digital quadrature controller 370 includes a signal processor or logic in a form of a state machine. The digital quadrature controller 370 is arranged to cancel or reduce quadrature errors via a feedback loop, for example via a quadrature error compensation signal 398 that is applied to DAU 350 (or other components as illustrated and described further with respect to
In some examples, the digital quadrature controller 370 may be arranged to create or adapt a binary search algorithm stored in memory, such as non-volatile memory 380. The binary search algorithm is able to identify suitable or optimum quadrature compensating signals or settings to compensate for, or reduce, any of various determined quadrature errors. The quadrature error compensation signal is based on an extracted quadrature error from the mechanical resonator measurement signal, or in the example of
A binary search is a relatively easy approach to implement digitally. In other examples, one or more alternative sequence algorithms, such as: a linear search, a Fibonacci search technique, etc., can be used to identify suitable or optimum quadrature compensating signals or settings to reduce any of various quadrature errors that may occur in the mechanical resonator measurement signal.
In some examples, the digital quadrature controller 370 may be arranged to perform a series of tests during production of the MEMS device, or during post-board mounting, in order to determine quadrature compensating signals for various determined quadrature errors. The results of these tests, which in some examples can be in a form of multiple digital codes or codewords, can be stored in a non-volatile memory 380 for later use. In some examples, the digital quadrature controller 370 may be arranged to calibrate the system during a period when the system is not measuring a Coriolis force (e.g., when the circuit is effectively ‘OFF’ and not measuring the rate).
In some examples, the digital quadrature controller 370, and in some examples the non-volatile memory 380, may be implemented in an integrated circuit 390.
Referring now to
When a supply voltage is applied, the MEMS device 400 starts by turning on the drive loop and the sense circuit as well as any associated circuitry. Once the drive proof-mass(es) is/are vibrating to the correct displacement and velocity, a sense loop is enabled. The MEMS device 400 includes a MEMS gyroscope 310 providing proof-mass displacement output to a quadrature readout processing module 395 via a sense plate or electrode 462. The quadrature readout processing module 395 is able to operate in either a digital domain, for example with a sigma-delta quadrature demodulator, or an analog domain architecture as shown in
In the illustrated example analog implementation, the quadrature readout processing module 395 includes a ‘sense’ capacitance to voltage (C2V) converter 415 arranged to convert a sensed capacitance measure, associated with the sense proof-mass displacement, to a voltage. The C2V converter 415 outputs a mechanical resonator quadrature measurement signal, based on the measured sense proof-mass displacement, to a sense demodulator 420. The sense demodulator 420 is arranged to demodulate the mechanical resonator quadrature measurement signal using a quadrature (Q) clock signal 425 that is synchronous to the mechanical resonator quadrature measurement signal. All signals within the MEMS gyroscope 310 are effectively induced by the drive motion (e.g. drive-mode oscillator). Thus, the mechanical resonator measurement signal, for example as measured at a DMU output, becomes a natural reference signal for the system, and is in phase with quadrature (Q) clock signal 425. In some examples, the quadrature clock signal 425 comprises one quadrature (I) clock to a drive circuit (not shown) in order to obtain the rate of the proof mass displacement and the other quadrature (Q) clock 425 is provided to the sense circuit (e.g. the (quadrature) sense demodulator 420 or mixer) to obtain or extract the quadrature error signal.
In this example, the sense demodulator 420 is arranged to output a sign that is representative of the quadrature error of the mechanical resonator quadrature measurement signal. In the example of using a binary search algorithm, only the sign of the quadrature error is needed.
However, the sense demodulator 420 is able to provide the full value of the remaining quadrature error, provided that this error does not exceed the total range of the sense circuit. With the sign only of the remaining quadrature error, the binary search algorithm is able to toggle each compensation bit, one by one, and work its way down from the most significant bit (MSB) to the least significant bit (LSB). In this manner, the binary search algorithm progressively and iteratively (as described with reference to
Thus, in this example, a quadrature error signal output from the sense demodulator 420 is in a form of an extracted sign 428 that is representative of the quadrature error, such that a simple binary search algorithm may be employed. In this manner, and as described further with respect to
The extracted sign 428 that is representative of the quadrature error is input to a threshold comparator 430. The threshold comparator outputs a binary signal to the digital quadrature controller 370 based on whether the input extracted sign 428 exceeds or falls below one or more threshold(s). A digital quadrature controller 370 manages the MEMS device 400 and is coupled to an output of the threshold comparator 430. As with all synchronous digital systems, the digital quadrature controller 370 includes a clock input for pulsing its digital operations. The digital quadrature controller 370 runs an algorithm, for example the algorithm that is described with reference to
In some instances, the quadrature signal may easily be 100 to 1000 times higher than the largest rate signal. In such a situation, the quadrature signal will saturate the sense C2V converter 415. Hence, the quadrature error must be reduced to a level that is lower than the full scale range of the rate signal.
In some examples, the digital quadrature controller 370 can be arranged to generate a quadrature cancellation digital word, based on the extracted quadrature error from the mechanical resonator measurement signal using a quadrature clock, or the extracted sign that is representative of the quadrature error in this example of
In some examples (not shown), the digital quadrature controller 370 can include a signal processor or logic in a form of a state machine that is arranged to cancel quadrature errors via a feedback loop 442. The feedback loop between the digital quadrature controller 370 and the MEMS gyroscope 310 includes a cancellation digital to analog converter (CDAC) 450 arranged to receive a digital word 445 and convert the digital word to a quadrature error compensation signal 455.
In some examples, the quadrature error compensation signal 455 may take one or more of a number of forms. For example, in applying a quadrature error compensation signal mechanically, the feedback path may be arranged to control an electrostatic force that is applied to the mechanical resonator through one or more additional plates or electrodes 460 associated with the MEMS drive and coupled to the MEMS gyroscope 310, and that force causes a mechanical adjustment that results in quadrature error to be suppressed. In a further example, in applying a quadrature error compensation signal capacitively, the feedback path may be coupled 465 to the input sense C2V converter 415 such that a capacitive signal may be applied that is inversely proportional to the quadrature error. In a yet further example, in applying a quadrature error compensation signal electrically, the feedback path may be coupled 470 to the output of the sense C2V converter 415 such that an electrical signal may be applied that is the opposite of the quadrature error signal. Each one of the above approaches for implementing the compensation has its own advantages and drawbacks and can be selected according to the specific application.
The quadrature error compensation signal 455 is applied to the MEMS gyroscope 310 in such a manner that the quadrature errors generated in the MEMS device 400 are substantially reduced or cancelled based on the determination by the digital quadrature controller 370.
In some examples, the MEMS device 400 is arranged to produce a quadrature cancellation digital word to cancel quadrature errors generated by the MEMS gyroscope 310 in order to auto-trim the quadrature error at a boot-time, sometimes referred to as a ‘power-on-reset time. In this manner, even should the MEMS technology be sensitive to external mechanical stress or temperature stress, the MEMS vibrating part is calibrated at each boot operation. Furthermore, in this example, no trimming may be required at a production level, as successive reduction of the quadrature error may be achieved subsequent to the MEMS gyroscope being board mounted.
In some examples, the digital quadrature controller 370, and in some examples the non-volatile memory 380, may be implemented in an integrated circuit 390.
The quadrature demodulation in 625 produces either: a negative quadrature output, following which the ‘k’ bit is reset at 630, or a positive quadrature output, following which the ‘k’ bit is kept high at 635. Hence, only a sign that is representative of the quadrature error is needed to be determined by the quadrature demodulation. Subsequent to either a negative quadrature output at 630 or a positive quadrature output at 635, a determination is made as to whether the ‘k’ counter is at ‘0’ in 640. If the determination at 640 is that the ‘k’ counter is at ‘0’, then the process loops back to 615 and the counter is again decremented. If the determination at 640 is that the ‘k’ counter is not at ‘0’, then the flowchart reverts to a saving of the best codeword, for example by reverting to 525 of
As an explanatory example of the simplified flowchart 600 of an example of a method to perform a binary search algorithm let us take an example of the counter N=3 (where the CDAC is over 3 bits in length) and the codeword solution is ‘101’. Thus, at 605, counter (‘k’) is set to ‘N’ (e.g. ‘3’) and the quadrature trim set to ‘0’. Upon decrementing the counter, ‘k:=2’ at 615 and the bit number-2 is set to ‘1’ at 620. Subsequently, at 635, the positive quadrature bit number-2 is ‘1’, such that the counter with k=2 is false at 640. The example binary search algorithm then loops back with the counter ‘k’ further decremented to ‘k:=1’ at 615. Thereafter, bit number-1 is set to a ‘1’ at 620. Subsequently, at 635, the negative quadrature bit number-1 is ‘0’, such that the counter with k=1 is false at 640. The example binary search algorithm then loops back with the counter ‘k’ further decremented to ‘k:=0’ at 615. Thereafter, bit number-0 is set to a ‘1’ at 620. Subsequently, at 635, the positive quadrature bit number-0 is ‘1’, such that the counter with k=0 is true, at 640, and the flowchart exits by reverting back to
In some examples, a sense demodulator is arranged to use a quadrature clock to extract a quadrature error signal from a mechanical resonator measurement signal. An output of the sense demodulator can be input to a threshold comparator, such that the threshold comparator outputs a sign that is representative of the quadrature error signal to the digital quadrature controller. The digital quadrature controller can be arranged to employ a binary search algorithm to generate a quadrature error compensation signal to reduce a quadrature error of the mechanical resonator measurement signal.
In some examples, the digital quadrature controller may be arranged to compensate for any quadrature error when the system is not measuring a Coriolis force.
In some examples, the quadrature error signal may be removed from the MEMS rate signal at the demodulation process, and notably post-production. In some examples, the digital quadrature controller may be arranged to reduce any quadrature error at system boot or at each system boot, for example each time a user switches on the MEMS device. Thus, in some examples, the digital quadrature controller may remove any quadrature error due to non-orthogonal MEMS masses movement within the MEMS device.
In some examples, the digital quadrature controller may be arranged to perform a series of tests during production of the MEMS device, post-board mounting to determine quadrature compensating signals to reduce various determined quadrature errors. The results of these tests, which in some examples is in the form of multiple digital codes or codewords, may be stored in a non-volatile memory for later use.
In some examples, in addition or in the alternative, the digital quadrature controller may remove any additional quadrature error due to mechanical post-board-mounting stress imposed on the MEMS device and any stress evolution during the life-cycle of the final MEMS device product.
In some examples, the digital quadrature controller may be arranged to avoid any quadrature error trimming during product testing, thereby speeding up the product test time.
In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention described and illustrated in the drawings. It will, however, be evident that various modifications and changes may be made therein, for example implemented using other electronic components and circuits known to those skilled in the art and without departing from the broader spirit and scope of the invention as set forth in the appended claims.
The connections as discussed herein may be any type of connection suitable to transfer signals from or to the respective nodes, units or devices, for example via intermediate devices. Accordingly, unless implied or stated otherwise, the connections may for example be direct connections or indirect connections. The connections may be illustrated or described in reference to being a single connection, a plurality of connections, unidirectional connections, or bidirectional connections. However, different embodiments may vary the implementation of the connections. For example, separate unidirectional connections may be used rather than bidirectional connections and vice versa. Also, plurality of connections may be replaced with a single connection that transfers multiple signals serially or in a time multiplexed manner. Likewise, single connections carrying multiple signals may be separated out into various different connections carrying subsets of these signals. Therefore, many options exist for transferring signals.
Each signal described herein may be designed as positive or negative logic. In the case of a negative logic signal, the signal is active low where the logically true state corresponds to a logic level zero. In the case of a positive logic signal, the signal is active high where the logically true state corresponds to a logic level one. Note that any of the signals described herein can be designed as either negative or positive logic signals. Therefore, in alternate embodiments, those signals described as positive logic signals may be implemented as negative logic signals, and those signals described as negative logic signals may be implemented as positive logic signals.
Those skilled in the art will recognize that the boundaries between logic blocks are merely illustrative and that alternative embodiments may merge logic blocks or circuit elements or impose an alternate decomposition of functionality upon various logic blocks or circuit elements. Thus, it is to be understood that the architectures depicted herein are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality.
Any arrangement of components to achieve the same functionality is effectively ‘associated’ such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as ‘associated with’ each other such that the desired functionality is achieved, irrespective of architectures or intermediary components. Likewise, any two components so associated can also be viewed as being ‘operably connected’, or ‘operably coupled’, to each other to achieve the desired functionality.
Furthermore, those skilled in the art will recognize that boundaries between the above described operations merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments.
Also for example, in one embodiment, the illustrated examples may be implemented as circuitry located on an integrated circuit or within a same 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.
Also for example, the examples, or portions thereof, may implemented as soft or code representations of physical circuitry or of logical representations convertible into physical circuitry, such as in a hardware description language of any appropriate type.
However, other modifications, variations and alternatives are also possible. The specifications and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense.
The word ‘comprising’ does not exclude the presence of other elements or steps than those listed in a claim. Furthermore, the terms ‘a’ or ‘an’, as used herein, are defined as one or more than one. Also, the use of introductory phrases such as ‘at least one’ and ‘one or more’ in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles ‘a’ or ‘an’ limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases ‘one or more’ or ‘at least one’ and indefinite articles such as ‘a’ or ‘an’. The same holds true for the use of definite articles. Also, the use of phrases such as ‘or’ within the description can be interpreted either exclusively or inclusively, depending upon which is broader in terms of the context described. Unless stated otherwise, terms such as ‘first’ and ‘second’ are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.
| Number | Date | Country | Kind |
|---|---|---|---|
| PCT/IB2015/000267 | Feb 2015 | IB | international |
