Abstract:
A micromechanical resonator including a motion arresting mechanism to rapidly damp the vibration of a resonator beginning at any given moment in time to remove vibration caused by previous events. An electrostatic clamp uses a bias voltage between an electrode and the resonator to damp the resonator and return it to its equilibrium position. A mechanical clamp includes an actuator that forces the mechanical clamp to contact the resonator. These micromechanical resonators facilitate condition based monitoring of complex electromechanical machines and components by allowing signature analysis in multiple temporal and frequency domains.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of signal analysis, and more particularly to a micromechanical resonator that allows signal detection in multiple time and frequency domains. 
     BACKGROUND OF THE INVENTION 
     Future electro-mechanical machines and structures will increasingly participate in their own service and maintenance using embedded distributed self-diagnostics that are remotely accessible to monitor machine health, detect and isolate subtle performance degradation, and in some cases even reconfigure some machines to adapt to changing operating environments. Traditionally, corrective maintenance and preventative maintenance have been the only two service paradigms. More recently, predictive or condition-based maintenance using micromechanical resonators, enabled by Micro-Electro-Mechanical Systems (MEMS) technology is emerging as an alternative. Condition-based maintenance is just-in-time maintenance based on the actual health of the machine and its components. Since it avoids the cumulative cost of unnecessary service calls associated with preventative maintenance and the occurrence of machine failure and degradation associated with corrective maintenance, condition-based maintenance provides substantial cost savings. 
     Fault manifestation in electro-mechanical systems with multiple moving elements in complex operating regimes, however, is typically non-stationary in that the frequencies describing specific faults vary over time. The multiple actuating elements such as motors and solenoids produce rich mechanical excitation signals at multiple time and frequency domains. Traditional Fourier spectral analysis techniques, such as the Fourier transform, while useful for establishing the signal bandwidth, is unsuitable for analyzing the time-varying properties of the signal that are important for diagnosis purposes. Another problem is that failure modes of system components are difficult to identify and characterize using time-based or frequency-based analysis. Hence, signature analysis with a time-frequency representation, such as that provided by the short-time Fourier transform (STFT) is required for condition monitoring of these systems. 
     A conventional micromechanical resonator for signature analysis is an array of tuning forks. Each tuning fork of the array resonates at a particular frequency while being insensitive to other frequencies. Thus, the entire spectral content of a vibration signal can be covered using a large number of tuning forks with closely spaced resonant frequencies. As shown in FIG. 1, array  10  includes higher frequency tuning forks  12 , mid frequency tuning forks  14 , and lower frequency tuning forks  16 . Conventional arrays typically use a gas to damp out resonant vibration of the tuning forks. 
     Conventional micromechanical resonators provide frequency information, but do not provide the ability to separate this information into specific time intervals or windows of constant length. These tuning fork arrays, however, suffer from two problems. First, the duration of the time interval is limited by the viscous properties of the damping gas. Since elements that move faster will damp out sooner, the duration of time intervals is frequency dependent and varies for different frequency tuning forks. Thus, the amount of damping that may be employed will be limited by the sensitivity of the high frequency components. This affects the ability of the low frequency tuning forks to distinguish new events from old events in situations in which the new event occurs before the old event has been damped out. Second, some condition-based maintenance algorithms, such as short-time Fourier transform, require all the tuning forks to be damped simultaneously while others, such as wavelet transformation, require a hierarchy of time intervals between damping. Conventional tuning fork arrays, however, do not provide this capability. 
     In light of the foregoing, there is a need for a method and a micromechanical resonator for arresting the motion of micromechanical resonators to allow detection of rich electromechanical excitation signals at multiple time and frequency domains. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an micromechanical resonator that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     In accordance with the purposes of the present invention, as embodied and broadly described, the invention provides a micromechanical resonator including at least one resonant mass having an equilibrium position, at least one anchor electrode that provides electrostatic damping of the resonant mass, and a substrate having a planar surface. 
     In another embodiment, the present invention provides a micromechanical resonator including at least one resonant mass having an equilibrium position, a clamp, wherein the clamp provides mechanical clamping of the resonant mass, and a actuator for applying the clamp. 
     In another embodiment, the present invention provides an array of micromechanical resonators including a plurality of resonant masses and a plurality of mechanisms to damp the resonant masses, wherein the damping mechanisms are simultaneously activated to allow the array to measure discrete time intervals. 
     In another embodiment, the invention provides an array of micromechanical resonators including a plurality of resonant masses and a plurality of mechanisms to clamp the resonant masses, wherein the clamping mechanisms are configured to allow the array to measure a hierarchy of time intervals. 
     In yet another embodiment, the invention provides a method of clamping and releasing a mechanical sensor including the steps of measuring the frequency of an event for an initial time interval using a resonant mass having an equilibrium position, clamping the resonant mass, and releasing the resonant mass in its equilibrium position to measure a second time interval. 
     The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description serve to explain the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, serve to explain the objects, advantages, and principles of the invention. 
     FIG. 1 is schematic drawing of a prior art micromechanical resonator consisting of a tuning fork array. 
     FIG. 2A is a schematic side view of a micromechanical resonator with an electrostatic clamp according to one exemplary embodiment of the present invention. 
     FIG. 2B is a schematic side view of a micromechanical resonator with a segmented electrode according to one exemplary embodiment of the present invention. 
     FIG. 2C is a schematic top view of a micromechanical resonator wherein the resonator oscillates parallel to the substrate according to one exemplary embodiment of the present invention. 
     FIG. 4 is schematic side view of a micromechanical resonator with a elliptically shaped mechanical clamp and a rotary actuator according to one embodiment of the present invention. 
     FIG. 5 is a schematic view showing an array of resonators which can detect signal components in both time and frequency domains. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
     FIGS. 2A-C illustrate a micromechanical resonator consistent with one embodiment of the present invention. As shown in FIG. 2A, micromechanical resonator  20  includes resonant mass  22 , substrate  26 , electrode  28 , and voltage source  29 . Insulating layer  21  is only necessary if the range of motion of resonant mass  22  causes it to contact substrate  26 . Resonant mass  22  is mounted on substrate  26 , usually a silicon wafer although any other planar base material can be used. Resonant mass has fixed end  23  anchored to substrate  26  and free end  25 . Electrode  28  is mounted in a fixed position relative to the free end of resonant mass  25 . Alternatively, electrode  28  can be mounted on a moveable mechanism (not shown), so that its location can be tuned to match the equilibrium position of resonant mass  22 . When residual stress in resonator  20 , introduced during the fabrication process, causes the equilibrium position of free end  25  of resonant mass  22  to be located at a slight offset from its intended or true position, electrode  280  is preferably oversized and segmented as shown in FIG.  2 B. This allows energizing of only the portion of the anchor that corresponds to the intended or true equilibrium position. These components are made using known micromachining techniques from materials known in the art. 
     Voltage source  29  applies a bias voltage between resonant mass  22  and electrode  28  to generate an electrostatic force therebetween. Resonant mass  22  can either be in an equilibrium position or vibrating at a low amplitude until it is subject to excitation by an event of interest. If the event of interest has a frequency component close to the resonant frequency of mass  22 , free end  25  of resonant mass  22  oscillates at a higher amplitude in a direction substantially perpendicular to the plane of the substrate. Alternatively, as shown in FIG. 2C, resonator  22  can be configured so that the movement of free end  25  of the resonant member is parallel to the plane of substrate  26 . In this case, anchor electrode  28  is preferably positioned either above or below resonant mass  22 . This can be accomplished by, for example, placing anchor electrode  28  in another layer above or below resonant mass  22  rather than as a separate anchor structure. More preferably, anchor electrodes  28  are positioned above and below resonant mass  22 . 
     Once the presence and amplitude of oscillation is measured by conventional means, free end  25  of resonant mass  22  is electrostatically damped by anchor electrode  28  by application of a bias voltage. Although the terms “damping” and “clamping” are used interchangeably, the electrostatic forces do not physically clamp resonant mass  22  or directly result in energy dissipation. Instead, the electrostatic forces dramatically increase the resonant frequency of resonant mass  22  causing it to move more quickly and therefore damp out vibrations faster from other intrinsic energy dissipation mechanisms, such as for example, atmospheric squeeze-film damping, resistive damping, and mechanical losses. Additionally, sensing electrodes  27  mounted on anchor electrode  28  may be used to capacitively sense the motion of resonant member  22 . Optionally, these may be the same electrodes that are used to damp the resonator. The bias voltage required to damp resonant mass  22  depends on a number of factors including the mass of the resonator, the elastic properties of the resonator, and the amplitude of the oscillations. It is typically less than 300 V. Once damped, resonant mass  22  is returned to its equilibrium position and is ready for another measurement. By repeatedly damping and releasing resonant mass  22  the temporal range over which resonator  20  detects vibration can be broken up into discrete periods analogous to a short-time Fourier transform. Anchor electrode  28  is preferably positioned so that it releases resonant mass  22  in its equilibrium position, so resonant mass  22  is ready for another measurement. It will be apparent to those skilled in the art that anchor electrode  28  may be segmented into multiple smaller, individually actuated electrodes spaced along the range of motion of free end  25  of resonant mass  22 . The voltage distribution to these individually activated electrodes may be selected so that the effect of energizing electrode  28  is to attract free end  25  to its equilibrium position. Optionally, this pattern of energizing electrode  28  may be derived automatically through sensing of the motion of resonant member  22 . It addition, it will also be apparent to those skilled in the art that electrode  28  and member  22  may be segmented with fingers to provide additional surface area and more force to increase damping capability. 
     Sensing of the motion of free end  25  of resonant mass  22  may be coupled in a closed-loop manner to the voltage applied to anchor electrode  28 . This coupling will allow an electrostatic attraction to be active when free end  25  of resonant member  22  is moving away from anchor electrode  28 , thus, taking energy out of the system while allowing the electrostatic attraction to be inactive (V=0) when free end  22  is moving towards anchor electrode  28 . The closed-loop control of voltage V enhances the ability of the system to stop the motion of free end  25  by preventing anchor electrode  28  from accelerating free end  25  during those portions of the vibration cycle when it is moving towards anchor electrode  28 . This permits electrostatic attraction, as well as an increase in gaseous damping to decelerate free end  25  of resonant mass  22 . 
     The present invention is not limited to a vibrating beam resonant mass as shown in FIGS. 2A-C. Resonator  20  may be, for example, proof mass structures, doubly fixed beams, or any other structure used in resonators. 
     FIG. 3 illustrates a micromechanical resonator with a mechanical clamp consistent with another embodiment of the present invention. Micromechanical resonator  30  includes resonant mass  32 , substrate  36 , mechanical clamp  38 , and actuator  39 . Resonant mass  32  is attached to substrate  36 . Substrate  36  is typically a silicon wafer, but can be any semiconductor base material. Resonant mass  32  is shown in FIG. 3 as a vibrating beam having a fixed end  33  and a free end  35 . Other embodiments of the present invention include a resonant mass such as, for example, a proof mass, a doubly fixed beam or any other structure used in resonators. 
     Mechanical clamp  38  comprises an flexible material that is preferably viscoelastic, to absorb energy, and hydrophobic, to reduce stiction. Clamp  38  is, for example, the tri-block copolymer polyethylene-oxide-poly-propylene-oxide-polyethylene-oxide. Alternatively, in engineering situations where a visco-elastic material is not suitable, and elastic material, such as a member of the siloxane family, e.g. polydimethylsiloxanes, may be employed. As with the other components, clamp  38  is made using known micromachining techniques. Actuator  39  and clamp  38  are preferably configured so that clamp  38  contacts resonant mass  32  at a point. This reduces problems with stiction. Actuator  39  is shown in FIG. 3 as a comb drive motor that moves clamp  38  into contact with resonant mass  32  when activated. Alternatively, clamp  48  is ellipse shaped and actuator  49  is rotary drive, As shown in FIG. 4, actuator  49  rotates clamp  48  so that it contacts resonant mass  42  at a point. Clamp  48  can also be a thermally activated clamp comprising a two-layer (bimorph) structure where one material had a greater coefficient of thermal expansion than the other material. When heated, one material expands more quickly than the other causing the clamping member to bend towards resonant mass  32 . 
     In another embodiment consistent with the present invention, an array of micromechanical resonators produces a description of what frequencies are present in the signal during specific time intervals of interest. FIG. 5 shows how an array of resonators  50  comprising a plurality of resonators  52  can detect signal components in both time and frequency domains. Individual resonators preferably detect different frequencies over a desired frequency spectrum as represented by the vertical axis. Each of the plurality of resonators  52  further includes a clamping mechanism. Clamping mechanisms can be mechanical, electrostatic, or a combination of both types. After detection of the frequency components of the signal at a first discrete time interval t 1 , clamping mechanisms reset each of the resonators by removing vibrations from the first time interval. Array  50  then measures the frequency components of the signal at a subsequent discrete time interval t 2 . The clamping mechanisms continue to reset the resonators for subsequent measurements as desired to time interval t n  where n is an integer and t n  represents the n th  time interval. In another embodiment, a plurality of arrays can be used to measure the time and frequency variations of a signal. In this embodiment, each array can be configured to measure the frequency components of a signal for different time intervals. 
     For signal analysis algorithms, such as time/frequency methods utilized in condition-based maintenance applications, the plurality of clamping mechanisms can be configured to simultaneously clamp the plurality of resonators  52  as shown in FIG.  5 . In this manner, micromechanical resonator  50  detects the temporal and frequency variations of a signal. 
     For other condition-based algorithms, such as wavelet transformation, the plurality of clamping mechanisms can be configured to provide a hierarchy of time intervals between damping. The hierarchy of time intervals can be realized by activating the clamping mechanisms at the end of preset time intervals. The time intervals could be measured by electronic or mechanical counters. The configuration preferably includes multiple resonators detecting the same frequency during time intervals of varying duration. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the micromechanical resonators including clamping mechanisms of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.