Abstract:
Disclosed is a microelectromechanical sensor ( 10 ) with an element ( 40 ) that is driven into oscillations with drive forms (φ 1, φ2, φ3, φ4 ) through the use of arms ( 50 ), comb-drives ( 55 A,  55 B,  55 C, and  55 D) and corresponding comb-fingers ( 51, 61 ) and wherein a sense signal is transduced with capacitive sense electrodes ( 26, 26 ). The driveforms (φ 1, φ2, φ3, φ4 ) are provided in four-phases and are applied in pairs (φ 1, φ3  and φ 2, φ4 ) that are  180  degrees out of phase with respect to one another such that the driveforms are substantially self-canceling with regard to any driveform energy that feeds through any parasitic capacitance ( 99 ) that connects the comb-drives ( 55 A,  55 B,  55 C, and  55 D) to the capacitive sense electrodes ( 26, 26 ).

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to Micro-Electro-Mechanical Systems (MEMS) and, more particularly, to a MEMS sensor with a balanced four-phase comb drive.  
         BACKGROUND OF THE RELATED ART  
         [0002]    MEMS sensors are often used in modern day devices because of their small size and low cost. Typical MEMS sensors include accelerometers, angular rate sensors, and pressure sensors, but there are many more.  
           [0003]    MEMS sensors often use electrostatic comb-drives that use AC drive signals to drive some part of the system into oscillation and capacitive sense circuits that provide an output signal. The AC drive signals are usually of relatively high voltage (e.g. 5 volts) as compared with the voltages produced by the capacitive sense circuits (e.g. 1 nanovolt). Due to this large disparity in magnitude, the known MEMS sensors often suffer from parasitic capacitance, or “feed-through”. In particular, given current drive methodologies, signals applied to the electrostatic comb-drives are transmitted through the parasitic capacitance that connects the drive-combs to the sense capacitors and thereby swamp the tiny, capacitively-induced sensor voltages. The industry has not adequately addressed this problem prior to this invention.  
           [0004]    The problem may be best understood with reference to an exemplary MEMS sensor disclosed in U.S. Pat. No. 5,955,668, a patent that is commonly owned by the assignee of this invention. The ‘668 patent discloses an angular rate sensor, or “micro-gyro,” and hereby incorporated by reference in its entirety.  
           [0005]    As typical of many MEMS sensors, the micro-gyro in the ‘668 patent uses electrostatic comb-drive actuators that each consists of two comb structures with partially overlapping comb fingers. In a rate sensor built according to the ‘668 patent, an electrostatic comb-drive structure is used to oscillate an element or “proof mass” so that it is naturally subjected to coriolis forces whenever the device is rotated about a input axis or “rate” axis at some angular rate of rotation.  
           [0006]    In the particular design shown, the oscillating element is a ring that is driven into oscillation about a drive axis. In more detail, the ring element is driven into oscillation with an arm that extends radially outward from the ring element. The ring element supports four such arms. Each arm moves back and forth in between a pair of electrode pads. As shown FIG. 4 of the ‘668 patent, the arm supports two sets of outwardly extending comb-fingers and each electrode pad supports a set of inwardly extending comb-fingers.  
           [0007]    The ‘668 patent uses a drive methodology that may be regarded as “pull-pull” in that the ring element is repeatedly pulled one way and then pulled the other way using electrostatic forces. In particular, as explained at column 5, lines  11 - 18  of the ‘668 patent, “[t]he oscillation of ring element  20  may be established by applying a differential voltage between the fingers  50  connected to the ring element and the fingers  52  connected to the electrical pads  46  and  48  mounted on the substrate. By alternating the potential on the electrode pads  46  and  48 , ring element  20  can be driven into oscillation around its axis  21 , i.e., motion of each arm  44  back and forth between electrode pads  46  and  48 .” 
           [0008]    As first discussed above with regard to MEMS sensors in general, the micro-gyro of the ‘668 patent relies on capacitive sensing. In particular, an inner disk-shaped element is positioned above a pair of electrodes to form a differential pair of parallel capacitors. The inner element is mechanically constrained to oscillate about an output axis or “sense” axis, in a “teeter-tofter” fashion, above the electrodes. As such, when the inner element is oscillating about the sense axis, its capacitance with respect to one electrode is increasing in value while its capacitance with respect to the other electrode is decreasing in value.  
           [0009]    In operation, when the ring element is being driven but the gyro is not rotating about the rate axis, the oscillating ring simply moves back and forth in the same plane and no energy is transferred to the inner element. When the gyro is rotating about the rate axis, however, the oscillating ring begins to tip and tilt as well oscillate about its axis. The ring&#39;s tip and tilt energy is dynamically coupled to the inner disk-shaped element such that it begins to rock about the sense axis and change the value of its capacitance with respect to the underlying electrodes.  
           [0010]    The ‘668 patent discloses that the ring and the disk are held at a reference value or “virtual ground” of 5 v and that the voltages on the electrodes  46  and  48  are alternated between values above and below the reference value in order to drive the ring  20  into oscillation in an electrostatic “pull-pull” fashion. The voltage that is applied to the electrodes that pull in the counterclockwise direction is always 1 volt and the voltage that is applied to the electrodes that pull in the clockwise direction is 9 volts. At any moment, therefore, there is an “unbalanced” situation which results in feed-through into the capacitance electrodes associated with the sense disk. Under worst case conditions, the sense signal may be completely overpowered by the drive signal.  
           [0011]    Electrostatic comb-drive methods are inherently incompatible with capacitive sensing methods because there is always some degree of parasitic feed-through capacitance between the drive-combs and the sense capacitors and because the voltages are so different in terms of magnitude. There remains a need, therefore, for a drive method that minimizes this feed-through problem.  
         SUMMARY OF THE INVENTION  
         [0012]    In a first aspect, the invention resides in a method of vibrating a proof mass in a microelectromechanical sensor at a desired motor frequency wherein the proof mass is flexibly supported above a substrate with first, second, third and fourth moveable electrodes connected to the proof mass and adjacent to first, second, third and fourth fixed electrodes connected to the substrate, respectively, the method comprising the steps of: applying to the first and third fixed electrodes first and third periodic driveforms that operate to periodically pull the proof mass in the one direction; applying to the second and fourth fixed electrodes second and fourth periodic driveforms that operate to periodically pull the proof mass in the opposite direction; and phasing the first, second, third and fourth periodic driveforms relative to one another to cause the first and third periodic driveforms to pull the proof mass in the one direction during one period of periodic proof mass movement and to cause the second and fourth periodic driveforms to pull the proof mass in the opposite direction in a subsequent period of periodic proof mass movement.  
           [0013]    In a second aspect, the invention resides in a method of vibrating a proof mass in a microelectromechanical sensor at a desired motor frequency wherein the proof mass is flexibly supported above a substrate with first, second, third and fourth moveable electrodes connected to the proof mass and adjacent to first, second, third and fourth fixed electrodes connected to the substrate, respectively, the method comprising the steps of: applying to the first and third fixed electrodes first and third periodic driveforms that periodically pull the proof mass in the one direction, the first and third periodic driveforms being 180 degrees out of phase with respect to one another; and applying to the second and fourth fixed electrodes second and fourth periodic driveforms that periodically pull the proof mass in the opposite direction, the second and fourth periodic driveforms being 180 degrees out of phase with respect to one another.  
           [0014]    In a third aspect, the invention resides in a method of driving a proof mass at a desired motor frequency wherein the proof mass is flexibly supported above a substrate in a microelectromechanical sensor, the method comprising the steps of: providing a first movable electrode that is connected to the proof mass and a first fixed electrode for pulling the proof mass in one direction when a voltage differential exists between the first movable electrode and the first fixed electrode; and providing a second movable electrode that is connected to the proof mass and a second fixed electrode for pulling the proof mass in an opposite direction when a voltage differential exists between the second movable electrode and the second fixed electrode; providing a third movable electrode that is connected to the proof mass and a third fixed electrode for helping the first fixed and moveable electrodes pull the proof mass in said one direction when a voltage differential exists between the third movable electrode and the third fixed electrode; providing a fourth movable electrode that is connected to the proof mass and a fourth fixed electrode for helping the second fixed and movable electrodes pull the proof mass in said opposite direction when a voltage differential exists between the third movable electrode and the third fixed electrode; applying to the first fixed electrode a first periodic driveform at a one-half motor frequency that operates to periodically pull the proof mass in the one direction; and applying to the second fixed electrode a second periodic driveform at the half motor frequency that operates to periodically pull the proof mass in the opposite direction; applying to the third fixed electrode a third periodic driveform at a one-half motor frequency that operates to periodically pull the proof mass in the one direction; and applying to the fourth fixed electrode a fourth periodic driveform at a one-half motor frequency that operates to periodically pull the proof mass in the opposite direction, wherein the first and third periodic driveforms are 180 degrees out of phase with respect to one another, wherein the second and fourth periodic drives are 180 degrees out of phase with respect to one another, and wherein the first and second periodic drive forms are substantially ninety degrees out of phase with respect to one another and the third and fourth periodic drive forms are substantially ninety degrees out of phase with respect to one another such that the proof mass is repetitively and alternately pulled back and forth by the first and second periodic driveforms and by the third and fourth periodic driveforms at the motor frequency.  
           [0015]    In a fourth aspect, the invention resides in a method of generating drive waveforms for excitation of an oscillating mass driven by electrostatic actuation comprising the steps of: detecting a periodic motion of the oscillating mass with sense electrodes; producing a periodic waveform that is coherent in phase with the periodic motion of the oscillating mass and with a period of even multiple of the periodic motion of the oscillating mass; generating four orthogonal waveforms with phases of 0°, 90°, 180°, and 270°, and whose edges are coincident with a peak amplitude of the oscillating mass; and summing the orthogonal waveforms together to form a four-phase set of drive signals that produce torque over the entire sensor motor duty cycle.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The just summarized invention can be best understood with reference to the following description taken in view of the drawings of which:  
         [0017]    [0017]FIG. 1 is a top plan view of a micro-gyro  10  that uses the four-phase drive method of this invention;  
         [0018]    [0018]FIG. 2 is a block diagram of a preferred motor drive control circuit  200  used to drive the micro-gyro of, FIG. 1 according to this invention;  
         [0019]    [0019]FIG. 3 is a graph of the proof mass or motor response (position versus time) relative to the periodic driveforms (voltage versus time) used to drive the proof mass where the periodic driveforms are presented as sinusoids;  
         [0020]    [0020]FIG. 4 is a graph that is comparable to FIG. 3 except that the periodic driveforms are stair-stepped approximations of a sinusoidal waveforms;  
         [0021]    [0021]FIG. 5 is a graph of the presently preferred method of producing the driveforms of FIG. 4;  
         [0022]    [0022]FIG. 6 is a simplified diagram of a ring-based embodiment driven according to this invention;  
         [0023]    [0023]FIG. 7 is a simplified diagram of a single-plate embodiment driven according to this invention; and  
         [0024]    [0024]FIG. 8 is a simplified diagram of a two-plate embodiment driven according to this invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    The four-phase driving method of this invention can be used with any variety of MEMS sensors. FIG. 1 is a top plan of an exemplary micro-gyro  10  that may be driven with a balanced four-phase comb-drive as disclosed herein.  
         [0026]    The illustrated gyro  10  has three main components: (1) a substrate  20 , (2) a plurality of vibrating elements supported above the substrate  20  from a central anchor  25 , and (3) a plurality of stationary electrodes located on the substrate  20  for driving, sensing, and adjusting the motion of the vibrating elements. The vibrating elements include an outer ring element  40  that serves as a “proof mass” that responds to coriolis forces in the presence of an angular rate of rotation about a rate axis  22  and an inner sense disk  30  that serves as a sense element by interfacing with a pair of electrodes  26 , electrodes  26  are on the substrate  20 , and thereby forming a one half of a pair of differential capacitors as taught in the ‘668 patent.  
         [0027]    In this particular embodiment, the disk  30  is supported by flexures (not numbered) extending from the anchor  25 , and the ring element  40  is thereafter supported by other flexures (not numbered) extending from the disk  30 . The flexures supporting the ring element  40  permit it to vibrate about a drive axis  21  that is perpendicular to the plane of FIG. 1 and, as would be expected, to also allow it to tip and tilt in the presence of a coriolis force when the overall gyro  10  is rotating at some angular rate about the rate axis  22 . The inner disk  30 , by contrast, is mechanically constrained to pivot about sense axis  23  that lies in the plane of FIG. 1. As taught by the ‘668 patent, the tip and tilt energy from the ring element  40  is dynamically coupled to the inner disk  30  by the flexures. As a result, in the presence of an angular rate of rotation, the inner disk  30  will pivot back and forth about the sense axis  23 , above the electrodes  26 ,  26 , and thereby produces a differential capacitance that can be detected with suitable electronics.  
         [0028]    The ring element  40  is vibrated at a desired frequency with a plurality of driven arms  50  and feedback arms  60  that extend radially outward therefrom. Each driven arm  50  extends between a pair of drive electrodes  55  and each feedback arm  60  extends between a pair of feedback electrodes  65 . Finally, in order to form an electrostatic comb-drive structure, the arms  50 ,  60  and the electrodes  55 ,  65  include partially overlapping comb-fingers  51 ,  56  and  61 ,  66 .  
         [0029]    As shown, the preferred micro-gyro  10  has ten arms  50 ,  60 . These arms are symmetrically arranged for mechanical balance and clustered left and right so that the overall micro-gyro  10  requires less area. The ten arms include eight driven arms  50  that are used for vibrating the ring element  40  at a desired frequency and two feedback arms  60  that are used to provide position feedback to suitable drive circuitry.  
         [0030]    As described above in the background section, and as symbolically suggested by the lumped capacitor shown in dashed lines, a parasitic capacitance  99  may exist between the drive electrodes  55  and the sense electrodes  26 ,  26  (only one is shown). The parasitic capacitance  99  can be very troublesome because the high drive voltages can “feedthrough” to the sense electrodes  26 ,  26  and drown out or swamp the much lower sense voltages induced by the rocking disk  30 . The present invention offers a balanced four-phase driving method that uniquely addresses this problem.  
         [0031]    The drive electrodes  55  that surround the eight driven arms  50  are provided in four different groups that correspond to four different driveform phases that are uniquely designed to minimize feedthrough voltages. The drive electrodes in the four groups have been suitably designated with different letters, i.e.  55 A,  55 B,  55 C, and  55 D. In the FIG. 1 gyro embodiment, there are four members of each group, but there could be as few as one arm in each group in less complex embodiments.  
         [0032]    [0032]FIG. 2 is a block diagram of a preferred motor drive control circuit  200  used to drive the micro-gyro of FIG. 1 according to this invention. As a starting point, it is assumed that the sensor motor has been set into low level motion by a conventional positive feedback scheme. A phase lock loop  230  detects this low level of motion and produces a phase coherent signal at half the frequency of sensor motor motion. In this unique motor control design, motor motion is pendulous and excitation can be provided at both ends of the pendulum.  
         [0033]    A key advantage of the four-phase drive circuit  200  and underlying drive methodology is that it can increase the motor drive efficiency by driving the gyro motor pull-pull as opposed to “pull-release”. This results in a lower drive voltage that translates to a lower cost device. In addition, the drive frequency is only one-half the motor frequency instead of at the full motor frequency.  
         [0034]    As shown, the preferred motor drive control circuit  200  includes a motor sense amplifier  210 , a zero crossing detector  220 , a phase lock loop  230 , a four-phase driveform generator  240 , a start/run multiplexer  250 , motor drive circuits  261 - 264 , and automatic gain control startup circuits  270 . The PLL  230  is not required to generate the four-phase motor drive in an all-analog approach. The PLL approach was required to generate high frequency signals, used in other circuits, synchronized with the motor frequency. The four-phase driveform generator  240 , with the PLL  230 , is implemented using two flip-flops (not shown). When in the normal operating mode, the start/run multiplexer  250  inputs motor drive signals from the four-phase driveform generator  240  and PLL  230 . At start up, the PLL  230  requires time to lock onto the motor frequency. The PLL  230 , therefore, is bypassed and the motor signal is inputted directly via the zero crossing detector  220  and a 90 degree phase shift circuit  280 . In addition, at startup the drive is at the motor frequency therefore only two signals (at 180-degree phase separation) of the four phases are used. The start/run multiplexer  250  is implanted using analog FET switches in a conventional multiplexer design. The start detector  272  in the ACG startup circuits  270  controls the switches in the multiplexer  250 . When the motor is running, the AGC signal is within normal operating range, and the PLL  230  is locked onto the motor drive, the multiplexer  250  switches in the four-phase drive at one-half the motor frequency. The integrated ACG signal  276  controls the gyro sensor displacement by amplitude modulating the four-phase motor drive. The motor drive control circuit  200  can be implemented with operational amplifiers or switched capacitor circuits because the motor input is represented by very small value capacitors and no DC gain is required.  
         [0035]    The AGC circuits  270  consists of an amplitude detector or rectifier circuit  271  with inputs from the motor position amplifier  210  and the zero crossing detector  220 . This rectified motor signal is filtered, amplified and input to a start detector  272  and to an AGC integrator  273 . The start detector  272  is a voltage comparator configured as a window detector around the mid-voltage range. Its function is to output a logic ONE when the AGC signal is within operating range, and in other conditions to output a logic ZERO. This logic output controls the start/run multiplexer  250 . The AGC integrator  273  integrates the motor differential amplitude and outputs the integrated signal  276  to the motor drive circuits  261 - 264  to control the motor displacement. The 90-degree shift circuit  280  is required to align the starting force with motor position. The 90-degree shift circuit  280  consists of a first order band pass filter centered at the motor frequency.  
         [0036]    The periodic driveforms may be of any desired shape including, for example, a true sinudosoid, a sawtooth, a square wave, or a series of square wave pulses. In all cases, however, the periodic driveforms will comprise first and third periodic driveforms that periodically pull the proof mass in one direction and second and fourth periodic driveforms that periodically pull the proof mass in the other direction.  
         [0037]    [0037]FIG. 3 is a graph of the proof mass or motor response (position versus time) relative to the periodic four-phase driveforms (voltage versus time) used to drive the proof mass where the periodic four-phase driveforms are presented as sinusoids. As shown, the first and third drive signals φ 1  and φ 3  are 180 degrees out of phase and the second and fourth drive signals φ 2  and φ 4  are 180 degrees out of phase.  
         [0038]    [0038]FIG. 4 is a graph of the preferred driveforms that are provided as square pulse driveforms. They are comparable to the driveforms of FIG. 3 in that they are stair-stepped approximations of sinusoidal waveforms as suggested by the inclusion of the sinusoidal waveforms in dashed lines. In this embodiment, where the system operates on a conventional 5 volt supply, the driveforms are centered about a virtual ground of 2.5 volts and the driveforms are 2.5 volts +1.8 volts. The edges of the square pulse driveforms are coincident with the peak amplitudes of motor motion. This combination of drive excitation voltage provides a composite drive at one-half of the motor frequency, but does not produce any electrical interference at the sense frequency.  
         [0039]    Of significance, the driveforms are applied such that capacitively coupled voltage is opposite in phase and will be self-canceling to a high degree in accordance with this invention. In particular, as suggested by FIG. 1, the first and third drive signals φ 1  and φ 3  are simultaneously applied to drive electrodes  55 A and  55 C in order to pull the ring element  40  in the counterclockwise direction with minimal feedthrough and the second and fourth drive signals φ 2  and φ 4  are simultaneously applied to drive electrodes  55 B and  55 D in order to pull the ring element in the clockwise direction with minimal feedthrough. As a result of the phase cancellation, there will be a relative cancellation of parasitic capacitance or drive tones and, therefore, less distortion of the sensed rate signal generated by the movement of the disk  30  above the electrodes  26 ,  26 .  
         [0040]    [0040]FIG. 5 is a graph of the presently preferred method of producing the driveforms of FIG. 4 wherein a first half-frequency square wave ( 1 ), and a second half-frequency square wave ( 2 ) that is phase shifted relative to the first are subtracted from one another ( 1 )−( 2 ) to generate the basic driveform of FIG. 3.  
         [0041]    [0041]FIG. 6 is a simplified diagram of a ring-based gyro with the minimum number of arms  50  and drive electrodes  55 A,  55 B,  55 C, and  55 D required to implement the drive method of this invention. This figure is offered to clarify that FIG. 1 is but a preferred embodiment.  
         [0042]    [0042]FIGS. 7 and 8 are offered to show that the drive method of this invention may be applied to a variety of geometries. In particular, FIG. 7 is a simplified diagram of a driven plate embodiment wherein the first through fourth driveforms are applied to a MEMS sensor having a plate-shaped proof mass  140 . FIG. 8, on the other hand, is a simplified diagram of a two-mass system wherein the first through fourth driveforms are suitably applied to first and second plate-shaped masses  141 , 142 .