Abstract:
A micromachined device for filtering mechanical vibrations caused by an external disturbance is disclosed. The device can include a first electrostatic vertical comb drive assembly having a first array of stationary elements and a second array of movable elements correspondingly interspersed with the first array. The device can also include a plurality of springs, each springs coupled between a support frame and a proof mass. The first drive assembly can be configured for motion in the z-direction. The device can include a sensor for sensing a position of the proof mass relative to the support frame by measuring displacement between each of the stationary and movable elements. The device can further include a second electrostatic vertical comb drive assembly. The device can have multiple electrostatic comb drive assemblies. An optional feedback network signal processes a displacement measurement to control one of the drive assemblies.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is based on and claims priority under 35 U.S.C. §119(e) of the now abandoned U.S. Provisional Patent Application, Ser. No. 60/579,115, filed Jun. 12, 2004, and entitled “MULTI-AXIS ELECTROSTATICALLY TUNED MICROFABRICATED VIBRATION FILTER.” The U.S. Provisional Patent Application, Ser. No. 60/579,115 filed Jun. 12, 2004, and entitled “MULTI-AXIS ELECTROSTATICALLY TUNED MICROFABRICATED VIBRATION FILTER” is also hereby incorporated by reference. This application also claims priority under 35 U.S.C. §119(e) of the now abandoned U.S. Provisional Patent Application, Ser. No. 60/579,116, filed Jun. 12, 2004, and entitled “ELECTROSTATIC COMB DRIVE TUNED MICROFABRICATED VIBRATION FILTER.” The U.S. Provisional Patent Application, Ser. No. 60/579,116 filed Jun. 12, 2004, and entitled “ELECTROSTATIC COMB DRIVE TUNED MICROFABRICATED VIBRATION FILTER” is also hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   This invention relates to vibration filtering techniques for miniaturized systems. More specifically, this invention relates to micromachined devices for filtering vibrations caused by external disturbances by utilizing electrostatic vertical comb drives to measure displacement between a support frame and a proof mass, and to generate restoring forces. 
   BACKGROUND OF THE INVENTION 
   Vibration isolation techniques are used to reduce the motion transmitted from a vibratory base to a device or system. The basic components are illustrated in  FIG. 1 . The device, represented by the rigid mass, m, is connected to the base with a linear spring and damper pair, k and c, respectively. The governing equation for this system is:
 
 m{umlaut over (x)}+c ( {dot over (x)}−{dot over (y)} )+ k ( x−y )=0
 
or
 
 {umlaut over (x)}+ 2ζω n ( {dot over (x)}−{dot over (y)} )+ω n   2 ( x−y )=0
 
where 2ζω n =c/m and ω n   2 =k/m.
 
   For practical purposes, transmissibility is defined as the ratio of the amplitude of the device motion (x) to that of the base motion (y). Assume that y(t) is sinusoidal of frequency ω. Some algebraic manipulation gives: 
           TR   =       (       1   +       (     2   ⁢   ζβ     )     2             (     1   -   β     )     2     +       (     2   ⁢   ζβ     )     2         )       1   /   2             
where β=ω/ω n .
 
   The damping ratio, ζ, is equal to 0.5 c/(mk) 1/2 . A plot of TR for various levels of the damping ratio, ζ, is shown in  FIG. 2 . 
   A passive mechanical spring-mass-damper system with external vibrational excitation is analogous to a passive electrical network consisting of resistors, capacitors and inductors that is excited by a voltage waveform. Both systems can be utilized as second order filters with the following Laplacian characteristic equation:
 
 s   2 +2 sζω   n +ω n   2 =0
 
where ω n  is the system natural frequency and ζ is the damping coefficient. Such systems can be utilized to spectrally pass, reject, or attenuate frequency components of the external forcing function&#39;s bandwidth, whether a voltage signal for an electrical filter or a vibrational waveform for a mechanical filter.
 
   However, unlike electrical filters where the system components can be easily tunable, mechanical filter components are difficult to tune. As such, the filter characteristics (ω n  and ζ) are difficult to adjust in mechanical filters (vibration filters). This is particularly complicated in micromachined or MEMS devices, which are usually fabricated in crystalline silicon, because it is difficult to obtain sufficient damping to prevent ringing. This is often accomplished by hermetically packaging the MEMS device in a fluid at a prescribed pressure, and tailoring the device design to utilize squeeze-film or sheer resistance damping. These techniques are both expensive to implement and limited in application. Without sufficient damping, mechanical devices will oscillate (ring) for an unacceptably long length of time when externally excited. 
   In macroscale devices, electromagnetic actuators (such as DC or AC motors) are far more efficient and practical than electrostatic actuators. However, as devices are shrunk to the micro level, electromagnetic forces shrink faster than electrostatic forces because electromagnetic forces tend to be proportional to volume while electrostatic forces tend to be proportional to area, for the same amount of applied energy. Therefore for micro (i.e. MEMS) devices, electrostatic forces tend to be stronger than electromagnetic forces. Hence, electrostatic actuators are often used in MEMS applications. 
   A commonly used MEMS electrostatic actuator is the comb drive actuator, which consists of two comb shaped structures aligned to interdigitate the comb teeth. One of the combs is spatially fixed, while the other one is allowed to move so that its interdigitated teeth can move into or out of the teeth of the fixed comb. When a voltage is applied across the two combs, the resulting electrostatic force is equal to: 
             F   T     =       n   ⁢           ⁢   β   ⁢           ⁢   h   ⁢           ⁢     ɛ   r     ⁢     ɛ   o     ⁢     V   2         d   O             
where F T  is the tangential force pulling the combs together, n is the number of active teeth in the moveable comb, β is the fringe effect correction factor, h is the overlapping height between comb teeth, ε r ε o  is the permittivity of the dielectric and d O  is the fixed distance between a moveable comb tooth and a stationary comb tooth. Note that the force is proportional to the applied voltage squared, and is not proportional to the distance the movable comb has traveled.
 
   Most comb drive actuators used to date in MEMS devices have been horizontally oriented, and usually implemented in silicon substrates. However, a vertical comb drive that moves the comb in or out of the plane of the silicon substrate in a particular MEMS micro mirror device application is known. 
   Tunable mechanical dynamic systems have been developed that are based around spring-mass-damper systems for a variety of applications. Typically, they consist of a mechanical system that serves some function that has at least one mechanical element that is tunable by moving a mechanical member with an actuator. Additionally, these systems employ a sensor of some kind to detect that the mechanical system needs tuning. Then a feedback mechanism is utilized to generate the actuation drive signal necessary to correct the error detected by the sensor. Often, the feedback mechanism is electronic and involves analog or digital signal processing. Examples include vibration sensing, vibration isolation, mechanically tuned electrical filters and MEMS tunable chaotic oscillators. 
   Some MEMS devices, such as many MEMS gyroscopic sensors, are extremely sensitive to and adversely affected by high frequency vibrations, which may be present in the environment in which the sensors are used. In order to use these kinds of devices in mechanically harsh environments, they must be protected from high frequency vibrations. This can be accomplished by fabricating a MEMS vibration filter and incorporating it into the sensor package to isolate the sensor die from high frequency vibrations. Passive MEMS vibration filters have been investigated for this purpose. Unfortunately, they lack tunability and suffer from excessive ringing due to the difficulties in obtaining sufficient mechanical damping in silicon based MEMS devices. 
   A MEMS spring-mass-damper mechanical system can be fabricated by micromachining silicon or some other material. A simple example of a MEMS system  10  is illustrated in  FIG. 3 . The system  10  consists of a frame  20 , a proof mass  30  and four springs  40  that are fabricated out of the same material through MEMS fabrication processes, although they could be made from different materials. Damping (not shown) is provided through internal mechanical losses and/or external squeeze-film or sheer resistance methods. The springs  40  are designed to allow the proof mass  30  to move with respect to the frame  20  with one or more degrees of freedom, and may be of any shape. Additionally, the springs  40  are usually designed so that the proof mass  30  has much more mass than the spring structures. This structure has a second order low-pass frequency response, where the resonant frequency has been set by the proof mass and the system spring constant. 
   Problems with the type of system presented in  FIG. 3  include the lack of a convenient way to obtain sufficient damping and the lack of tunability of the frequency response. What is needed is a micromachined device utilizing electrostatic actuators to filter vibrations caused by an external disturbance. What is also needed is a micromachined device having vertical comb drives to measure displacement between stationary and movable comb drive elements, and to generate a restoring force. 
   SUMMARY OF THE INVENTION 
   Vibration filtering techniques for micromachined mechanical systems are disclosed. In one embodiment of the present invention, a micromachined device for filtering vibrations caused by an external disturbance is disclosed. The device includes a first electrostatic vertical comb drive assembly. The first electrostatic drive assembly can include a first array of stationary elements and a second array of movable elements correspondingly interspersed with the first array. The device can include a plurality of springs, each spring coupled between a support frame and a proof mass. The proof mass, support frame and springs can be made of a same material. Alternatively, the proof mass, support frame and springs can be made of a different material. 
   In accordance with the present invention, the first array of stationary elements is preferably coupled to the support frame and the second array of movable elements is preferably coupled to the proof mass. Preferably, the first drive assembly is configured for motion in the z-direction, but can be configured alternatively for motion in other directions. An external voltage can be applied across the first array and the second array. 
   The device of the present invention can include a position sensor coupled to the device for sensing a displacement of the proof mass with respect to the support frame. The position sensor can comprise one of: a capacitance sensor, a piezoresistive sensor, a magnetic sensor and an interferometer. 
   In another embodiment, the device can include a second electrostatic vertical comb drive assembly. In one embodiment, the first drive assembly and the second drive assembly can tune the frequency response of the device. The second drive assembly preferably includes a third array of stationary elements and a fourth array of movable elements correspondingly interspersed with the third array. The first array of stationary elements and the third array of stationary elements are preferably coupled to the support frame. The second array of movable elements and the fourth array of movable elements are preferably coupled to the proof mass. At least one of the first drive assembly and the second drive assembly is configured for motion in the z-direction, but can be configured alternatively for motion in other directions. Alternatively, the second drive assembly can be positioned in line with the first drive assembly. 
   In accordance with another embodiment of the present invention, a micromachined device for filtering mechanical vibrations caused by an external disturbance is disclosed. The device includes a plurality of springs, wherein each spring is coupled between a support frame and a proof mass. The device comprises a first electrostatic vertical comb drive assembly for generating a restoring force on the proof mass; and a second electrostatic vertical comb drive assembly for measuring displacement of the proof mass with respect to the frame. The device can further include a feedback network to signal process the displacement measurement to control at least one of the first drive assembly and the second drive assembly. At least one of a momentary velocity and acceleration of the proof mass is estimated by signal processing the displacement measurements. The signal processed displacement measurements can be used to calculate a driving voltage for the first drive assembly. In one embodiment, the second drive assembly can be positioned orthogonally about a z-axis from the first drive assembly. 
   In accordance with another embodiment of the present invention, a micromachined device for filtering mechanical vibrations caused by an external disturbance is disclosed. The device comprises a first set of fixed individual comb drive structure elements having outer fixed electrodes and inner fixed electrodes wherein each fixed electrode is electrically insulated from one another. A drive voltage of each electrode can be independently set by an external voltage. The device can include a second set of movable individual comb drive electrodes. The outer fixed electrodes can determine a relative position of the proof mass with respect to the fixed electrodes by measuring capacitance between each fixed electrode and motion control electrode closest to the fixed electrode. Preferably, the inner fixed electrodes move the proof mass by electrostatic actuation in at least one of: one-dimensional motion, two-dimensional motion, and three-dimensional motion. At least one of the movable electrodes can be electrostatically pulled closer to at least one of the fixed electrodes. The device can further include a feedback network for processing displacement measurements and generating control voltages for the inner fixed electrodes. 
   In accordance with another embodiment of the present invention, a micromachined device is disclosed. The device comprises a support frame, one or more springs, a proof mass, electrostatic actuators; and a feedback network to isolate the proof mass from external vibrations in one of two-orthogonal and three-orthogonal axes in a selectable frequency band by attenuation of the external vibrations in the selectable frequency band. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a prior art vibration isolation system represented by a rigid mass, m, connected to a base with a linear spring and damper pair, k and c, respectively. 
       FIG. 2  is a plot of transmissibility for various damping levels of the damping ratio, ζ, for the vibration isolation system of  FIG. 1 . 
       FIG. 3  is a prior art micromachined mechanical system including a frame, a proof mass and springs for filtering mechanical vibrations. 
       FIG. 4  is an illustration of a micromachined mechanical system coupled with an electrostatic vertical comb drive assembly having a first array of stationary elements and a second array of movable elements correspondingly interspersed with the first array, in accordance with an embodiment of the present invention. 
       FIG. 5  is an illustration of a micromachined mechanical system coupled to two electrostatic vertical comb drive assemblies, each assembly having stationary and movable elements, in accordance with another embodiment of the present invention. 
       FIG. 6  is an illustration of a micromachined mechanical system coupled to two electrostatic vertical comb drive assemblies, each assembly having fixed individual comb drive structure elements that are electrically isolated from one another, in accordance with another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 4  is an illustration of a micromachined (MEMS) mechanical system  100  in accordance with the present invention. The MEMS mechanical system illustrated in  FIG. 4  includes a vertical comb drive assembly including a first array of stationary comb drive elements  110  coupled to a support frame  130  and a second array of movable comb drive elements  120  coupled to a proof mass  140 . The system  100  also includes a plurality of springs  150  wherein each spring is coupled between the frame  130  and the proof mass  140 . In one embodiment, the system  100  is fabricated of the same material, although the system can be fabricated from different materials. In one embodiment, a first set of individual comb drive elements  115  on one side of the proof mass  140  are electrically isolated from a second set of individual comb elements  125  on the other side of the proof mass  140 . 
   In a preferred embodiment of the present invention, the individual comb drive elements  115  and  125  are configured as vertical comb drives for motion in the z direction. Preferably, the individual comb drive elements  115  and  125  can be operated together. When a voltage is applied across the individual comb drive elements  115  and  125 , between the proof mass  140  and the frame  130 , the balanced operation of the individual comb drive elements  115  and  125  results in a restoring force that attempts to keep the proof mass  140  in the x-y plane with the comb drive elements  110  coupled to the frame  130 . Any perturbation in the + or − z direction will result in the proof mass  140  experiencing an electrostatic force attempting to move the proof mass  140  back into plane with the frame  130 . Recall from above, the restoring force is proportional to the voltage across the comb drives squared, and is independent of the displacement in the z direction. The resulting differential equation of motion becomes, for the free response:
 
 m{umlaut over (z)}+cż+kz−sgn ( z ) qV   2 =0
 
where m is the mass, c is the damping, k is the system spring constant, sgn(z) is the sign of the proof mass displacement in the z direction, q is a constant related to the comb drive and V is voltage applied across the comb drive.
 
   Dynamics of the system  100  are such that an external voltage (not shown) can be applied to change the frequency response of the MEMS mechanical system  100 , therefore making it tunable. Additionally, multiple comb drives could be added to allow for having more than one external voltage that can be used to adjust the frequency response. 
   In addition to being an actuator, comb drive structures can be used to measure displacement of movable combs into or out of stationary combs by measuring a capacitance between movable and stationary combs, since the capacitance is directly proportional to the amount of tooth to tooth meshing (ignoring fringing effects). Therefore additional comb drive structures can be added to the device presented in  FIG. 4  to measure the proof mass displacement in the z direction. 
     FIG. 5  is an illustration of a micromachined mechanical system  200 , in accordance with an alternative embodiment of the present invention. The system  200  includes two vertical comb drive assemblies including a first set of individual comb drive elements  210  and second set of individual comb drive elements  220 . The first set of comb drive elements  210  includes a first array of stationary elements  215  and a second array of movable elements  216 . The first array of stationary elements  215  is coupled to a support frame  230  and the second array of movable elements  216  is coupled to a proof mass  240 . The second set of comb drive elements  220  includes a third array of stationary elements  225  and a fourth array of movable elements  226 . In one embodiment, the first set of comb drive elements  210  is electrically isolated from the second set of comb drive elements  220 . In one embodiment, the first set of comb drive elements  210  is positioned orthogonally about a z-axis from the second set of comb drive elements  220 . Alternatively, the first set of comb drive elements  210  can be positioned in any orientation about the z-axis, including in-line with the second set of comb drive elements  220 . 
   Still referring to  FIG. 5 , the second set of comb drive elements  220  is used to generate a restoring force on the proof mass  240 , as in the system  100  of  FIG. 4 , and first set of comb drive elements  210  is used to measure displacement of the proof mass  240  in the z direction by measuring a capacitance between stationary and movable combs, such as stationary comb element  211  and movable comb element  212 . By recording (sampling) the displacement of the proof mass  240  at a much higher rate than the highest frequency by which the proof mass  240  is externally driven, a process called oversampling, a momentary velocity and acceleration of the proof mass  240  can be estimated by signal processing the displacement measurements made with one of the sets of comb drive elements  210  and  220 . The resulting data can be used to calculate a driving voltage for the other one of the sets of comb drive elements  210  and  220 , so that the driving voltage is a function of proof mass displacement, estimated velocity and or estimated acceleration. Therefore the resulting differential equation of motion becomes:
 
 m{umlaut over (z)}+cż+kz+q   1   {umlaut over (z)}+q   2   ż+q   3   z+q   4 =0
 
where q 1  through q 4  are programmable values determined in whole or in part from the displacement measurements through a signal processing feedback network (not shown). This equation can be rewritten as follows:
 
( m+q   1 ) {umlaut over (z)} +( c+q   2 ) ż +( k+q   3 ) z+q   4 =0
 
If q 4  is set to zero, the mechanical system has the following characteristics:
 
ω n =[( k+q   3 )/( m+q   1 )] 1/2  and ζ=0.5( c+q   2 )/[( k+q   3 ) ( m+q   1 )] 1/2 .
 
   Therefore the resulting mechanical system  200  has a tunable frequency response that can be controlled by electronically changing the signal processing in the feedback loop (not shown). The signal processing can be accomplished by analog, digital, time domain, frequency domain or a combination of techniques. 
   The system  200  illustrated in  FIG. 5 , with an appropriate feedback loop, can be used to realize a micromachined vibration filter that filters external vibrations applied in the z direction to the frame  230  with respect to the proof mass  240 , with filter characteristics that are user tunable. 
     FIG. 6  is an illustration of an alternative micromachined mechanical system  300  including a first set of outer fixed electrodes  321 ,  324 ,  325 ,  328 ,  329 ,  332 ,  333 , and  336  coupled to a support frame (not shown) and a second set of inner fixed electrodes  322 ,  323 ,  326 ,  327 ,  330 ,  331 ,  334  and  335  coupled to the support frame (not shown). The one-dimensional electrostatically tunable vibration isolation filters discussed in conjunction with  FIGS. 4 and 5  above can be modified to realize multi-axis electrostatically tunable vibration isolation filters, as shown in  FIG. 6 . The system  300  is similar to the vibration isolation filter presented in  FIG. 5 , except that the fixed electrodes  321 - 336  are electrically insulated from each other and can have their drive voltages independently set by external circuitry (not shown). The system  300  also includes a plurality of anchored springs  340 , spring anchors  350 , and a proof mass  400  which are electrically grounded. The system  300  further includes a third set of movable electrodes coupled to the proof mass  400 . In one embodiment, the first set of outer fixed electrodes  321 ,  324 ,  325 ,  328 ,  329 ,  332 ,  333  and  336  are used to determine the relative position of the proof mass  330  with respect to the fixed electrodes  321 - 336  coupled to the frame (not shown) by measuring the capacitance between each fixed electrode  321 - 336  and a closest electrode protruding from the proof mass  330 , and comparing them to each other. When the proof mass  330  is centered, the measured capacitances are equal. Each pair or combination of electrode pairs will change differentially due to a unique displacement in one, two or three dimensions, yielding the displacement information. The second set of inner fixed electrodes  322 ,  323 ,  326 ,  327 ,  330 ,  332 ,  334  and  335 , referred to as motion control electrodes, are utilized to move the proof mass  400  by electrostatic attraction in one, two or three-dimensional motion. Actuation can take place by at electrostatic parallel plate attraction, where a movable electrode coupled to the proof mass  400  is electrostatically pulled closer to a fixed electrode coupled to the support frame (not shown), resulting from an applied potential difference between the two electrodes. An alternative actuation method is electrostatic comb drive actuation, similar to the one-dimensional case discussed in reference to  FIG. 5 . With either method, a feedback network (not shown) is used to process displacement measurements and to generate appropriate control voltages for the motion control electrodes  322 ,  323 ,  326 ,  327 ,  330 ,  332 ,  334  and  335 , to obtain a desired multi-axis mechanical frequency response. 
   The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modification may be made in the embodiments chosen for illustration without departing from the spirit and scope of the invention.