Abstract:
In some implementations, a control system for a resonating element comprises: a resonating element being driven by an oscillating drive signal and configured to generate a sense signal proportional to an amplitude of motion; a phase comparator coupled to the resonating element and to an oscillating drive signal, the phase comparator configured to compare the sense signal and the oscillating drive signal and to generate an error signal proportional to the phase difference; an oscillator coupled to the phase comparator and configured for generating the oscillating drive signal, the oscillator configured to receive the error signal and to adjust a phase of the oscillating signal based on the error signal; and an automatic gain control coupled to the resonating element and the oscillator, the automatic gain control configured to adjust the gain of the oscillating drive signal based on the signal generated by the resonating element.

Description:
TECHNICAL FIELD 
     This disclosure relates generally to systems and methods for controlling resonating elements in electronic circuits, such as inertial sensors that include micro-electrical-mechanical systems (MEMS) technology. 
     BACKGROUND 
     A typical vibratory gyroscope that utilizes MEMS technology includes a proof mass that is suspended above a cavity in semiconductor substrate by a mechanical suspension system that includes flexible beams. The proof mass is driven into resonance in a drive direction by an external periodic electrostatic or electromagnetic force. When the gyroscope is subjected to an angular rotation, a sinusoidal Coriolis force is induced in a direction orthogonal to the drive-mode oscillation at the driving frequency. The Coriolis force is proportional to the amplitude of the drive motion and to precisely determine the rotation rate around the rotation axis, a feedback control system is necessary to assure a constant-amplitude oscillation of the proof mass in the drive direction. Since the output of the gyroscope is typically very small, the control system also forces the proof mass to vibrate at resonance to achieve maximum sensitivity. 
     Conventional gyroscopes use a self-oscillating loop architecture including several filtering stages and a phase-locked loop (PLL) to produce a clean signal with the same frequency as the drive frequency. The performance of these conventional gyroscopes, however, may be degraded due to noise and parasitic signals. Also, conventional gyroscopes can be more costly to manufacture due to the incorporation of complex filter stages. 
     SUMMARY 
     A drive signal control system and method for resonating elements is disclosed. 
     In some implementations, a control system for a resonating element comprises: a resonating element being driven by an oscillating drive signal and configured to generate a sense signal proportional to an amplitude of motion; a phase comparator coupled to the resonating element and to an oscillating drive signal, the phase comparator configured to compare the sense signal and the oscillating drive signal and to generate an error signal proportional to the phase difference; an oscillator coupled to the phase comparator and configured for generating the oscillating drive signal, the oscillator configured to receive the error signal and to adjust a phase of the oscillating signal based on the error signal; and an automatic gain control coupled to the resonating element and the oscillator, the automatic gain control configured to adjust the gain of the oscillating drive signal based on the signal generated by the resonating element. 
     In some implementations, a method of controlling a drive signal for a resonating element comprises: receiving a signal from a resonating element; comparing a phase of the signal with a phase of an oscillating drive signal; generating an error signal based on the comparing; adjusting a phase of the oscillating drive signal based on the error signal; adjusting a gain of the oscillating drive signal based on an amplitude of the received signal; and driving the resonating element to its resonant frequency using the oscillating drive signal. 
     In some implementations, an apparatus comprises: a motion sensor including a resonating element; a control system coupled to the resonating element and configured to: receive a signal from the resonating element; compare a phase of the signal with a phase of an oscillating drive signal; generate an error signal based on the comparing; adjust a phase of the oscillating drive signal based on the error signal; adjust a gain of the oscillating drive signal based on an amplitude of the received signal; and drive the resonating element to its resonant frequency using the oscillating drive signal. 
     Particular implementations disclosed herein provide one or more of the following advantages. The disclosed implementations provide a drive control system and method for resonating elements that reduces the need for complex filters while offering better noise and parasitic signal rejection, a potentially faster and more reliable start-up and an ability to record the drive frequency. 
     The details of the disclosed implementations are set forth in the accompanying drawings and the description below. Other features, objects and advantages are apparent from the description, drawings and claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is block diagram of an example control system for resonating elements. 
         FIG. 2  is a flow diagram of an example process performed by the control system of  FIG. 1 . 
         FIG. 3  is example apparatus that includes the control system, as described in reference to  FIGS. 1-2 . 
     
    
    
     The same reference symbol used in various drawings indicates like elements. 
     DETAILED DESCRIPTION 
     The example implementations disclosed below include a capacitive MEMS angular rate sensor (gyroscope) which includes a control system that assures constant-amplitude oscillation of the proof mass at the drive resonant frequency. The disclosed implementations, however, can be used with any sensor or circuit that includes a resonating element (e.g., a piezoelectric MEMS sensor) and where the frequency and amplitude of the drive signal is controlled. 
     Example Control System 
       FIG. 1  is block diagram of an example control system  100  for resonating elements. In some implementations, system  100  can include resonating element  101 , signal conditioning circuit  102 , drive signal generator  103  and automatic gain control (AGC)  104 . System  100  provides advantages over conventional control systems for resonating elements by reducing the need for complex filters while offering better noise and parasitic signal rejection, a potentially faster and more reliable start-up and an ability to record the drive frequency. 
     In some implementations, signal conditioning circuit  102  can include capacitance-to-voltage (C/V) converter  105  and filter  106  and drive signal generator  103  includes phase comparator  107 , loop filter  108  and oscillator  109 . In the example shown resonating element  101  is a capacitive MEMS structure and C/V converter  105  converts a change in capacitance induced by resonating element  101  into a voltage signal. In other implementations, resonating element  101  is a piezoelectric MEMS structure. Filter  106  is an optional bandpass filter for conditioning the voltage signal output of C/V converter  105  (e.g., reduce out-of-band noise) before the voltage signal is input to phase comparator  107 . 
     Phase comparator  107  compares the voltage signal output by C/V converter  105  (or the output of filter  106  if used) with an oscillating drive signal output by oscillator  109  and produces an error signal which is proportional to the phase difference of the input signals. The error signal is then filtered by loop filter  108  which provides a control signal to oscillator  109 . The control signal causes oscillator  109  to adjust the phase of the oscillating drive signal by speeding up or slowing down the oscillating drive signal. The oscillating drive signal is looped back to the input of phase comparator  107  thereby creating a negative feedback loop. If the phases of the input signals to phase comparator  107  drift apart, the error signal will increase or decrease, and the control signal will drive the phase of the oscillating drive signal in the opposite direction to reduce the error. 
     In some implementations, oscillator  109  is a voltage controlled oscillator (VCO) and loop filter  108  is a low-pass filter. Loop filter  108  assures loop stability and reduces ripple in the error signal appearing at the output of phase comparator  107 . The oscillating drive signal generated by oscillator  109  is input to AGC  104  together with the voltage signal output by C/V converter  105 . AGC  104  adjusts the gain of the oscillating drive signal according to the output of C/V converter  105  so that the oscillating drive signal has a substantially constant amplitude. In some implementations, AGC  104  includes a variable gain amplifier (VGA). 
     Example Process 
       FIG. 2  is a flow diagram of an example process  200  performed by control system  100  of  FIG. 1 . In some implementations, process  200  can begin by receiving an input signal from a resonating element being driven by an oscillating drive signal ( 202 ). The input signal can be, for example, a sense signal output by an inertial sensor (e.g., gyroscope). Optionally, the input signal can be conditioned ( 204 ). For example, if the resonating element is a capacitive MEMS structure, the input signal is a change in capacitance induced by the capacitance MEMS structure, which is converted to a voltage signal by a C/V converter. In some implementations, a bandpass filter can be applied to the voltage signal to reduce out-of-band noise on the voltage signal. 
     Process  200  can continue by comparing the phases of the input signal and the oscillating drive signal to generate an error signal proportional to the phase difference ( 206 ). The error signal is used to adjust the phase of the oscillating drive signal and the amplitude of the input signal is used to adjust the gain of the oscillating drive signal ( 208 ). For example, a VCO can generate the oscillating drive signal and adjust the phase of the oscillating drive signal based on a control signal. A VGA can adjust the gain of the oscillating drive signal based on the voltage signal output by the C/V converter so that the oscillating drive signal has a substantially constant amplitude. Process  200  can continue by driving the resonating element with the adjusted oscillating drive signal ( 210 ). 
       FIG. 3  is an example apparatus that includes motion sensors  304   a - 304   n  some of which include a control system for resonating elements in motion sensors  304   a - 304   n , as described in reference to  FIGS. 1-2 . In some implementations, the motion sensors  304   a - 304   n  can be implemented in an apparatus, such as smart phone, tablet computer, wearable computer and the like. The apparatus can have a system architecture  300  that includes processor(s), memory interface  302 , peripherals interface  303 , one or more motion sensors  304   a - 304   n , wireless communication subsystem  306 , audio subsystem  315 , Input/Output (I/O) interface  307 , memory  308 , display device  313  and input devices  314 . 
     Motion sensors  304   a - 304   n  (e.g., MEMS accelerometer, MEMS gyro) may be coupled to peripherals interface  303  to facilitate multiple motion sensing functionalities of the apparatus. In one implementation, one or more motion sensors include resonating elements and a control system for the resonating elements as described in reference to  FIGS. 1 and 2 . Location processor  305  can include a global navigation satellite system (GNSS) receiver. Wireless communications subsystem  306  may include radio frequency (RF) receivers and transmitters (or RF transceivers) and/or optical (e.g., infrared) receivers and transmitters. Wireless communication subsystem  306  can operate over a variety of networks, such as global system for mobile communications (GSM) network, GPRS network, enhanced data GSM environment (EDGE) network, IEEE 802.xx network (e.g., Wi-Fi, Wi-Max, ZigBee™), 3G, 4G, 4G LTE, code division multiple access (CDMA) network, near field communication (NFC) network, Wi-Fi Direct network and Bluetooth™ network. 
     I/O interface  307  may include circuitry and/or firmware for supporting wired mediums and implement various communication protocols and include ports for UART, Serial, USB, Ethernet, RS-232 and the like. 
     Memory interface  302  is coupled to memory  308 . Memory  308  may include high-speed random access memory or non-volatile memory, such as one or more magnetic disk storage devices, one or more optical storage devices, or flash memory (e.g., NAND, NOR). Memory  308  may store operating system  309 , such as Darwin, RTXC, LINUX, UNIX, OS X, iOS, WINDOWS, or an embedded operating system such as VxWorks. Operating system  309  may include instructions for handling basic system services and for performing hardware dependent tasks. In some implementations, operating system  309  may include a kernel (e.g., UNIX/LINUX kernel). 
     Memory  308  may also store communication instructions  310  to facilitate communicating with one or more additional devices in a network topology and one or more computers or servers over wired and wireless mediums. Communication instructions  310  can include instructions for implementing all or part of a wireless communications software stack. 
     Memory  308  may include sensor processing instructions  311  to facilitate motion sensor-related processing and functions on motion signals received from motion sensors  304   a - 304   n . For example, processing instructions  311  can include instructions for implementing at least portions of a control system for resonating elements in one or more motion sensors, as described in reference to  FIGS. 1 and 2 . 
     Other instructions  312  can include instructions for a variety of applications that use the motion signals provided by motion sensors  304   a - 304   n . For example, other instructions can include application instructions that take the motion signals from motion sensors  304   a - 304   n  and compute the current location, speed and orientation of the apparatus in a reference coordinate frame (e.g., geodetic, local level). The application instructions can display a map on display device  313  with a marker indicating the location of the apparatus along with other information such as turn-by-turn directions for a route. Audio subsystem  315  can provide speech output for the application that provides, for example, audible turn-by-turn directions. 
     Other applications can make other uses of motion signals from motion sensors  304   a - 304   n  and will benefit from motion signals that are less noisy and have less errors due to the mechanical filter designs disclosed herein. For example, an electronic pedometer application can benefit from improved motion signals provided by the mechanical filter designs disclosed herein. 
     While this document contains many specific implementation details, these details should not be construed as limitations on the scope of what may be claimed but rather as descriptions of features that may be specific to particular implementations. Certain features that are described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable sub combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a sub combination or variation of a sub combination.