Abstract:
Certain disclosed accelerometer sensors and methods employ a proof mass that is acted upon by multiple feedback paths. One illustrative sensor embodiment includes an electrode arrangement proximate to a proof mass, the electrode arrangement providing multiple electrostatic force centroids on the proof mass. The sensor embodiment further includes multiple feedback paths, each feedback path independently controlling an electrostatic force for a respective centroid, and an output unit that converts signals from the multiple feedback paths into an acceleration-responsive output signal. An illustrative method embodiment derives multiple feedback signals from at least one displacement signal, applies the multiple feedback signals to an arrangement of electrodes that capacitively couple the proof mass to a substrate, and converts the multiple feedback signals into an acceleration signal.

Description:
BACKGROUND 
       [0001]    The term microelectromechanical systems (“MEMS”) refers to devices having very small components (typically in the range of 1 μm to 1 cm) which often operate as sensors. Electronics may be integrated into the devices to convert physical effects (such as displacement of a proof mass) into electrical output signals. High performance MEMS sensor systems are typically based on closed-loop architectures to provide good linearity and large dynamic range. As one example of such architectures, many systems reported in the literature use sigma-delta modulated single-bit feedback with a MEMS proof mass as an integrated part of the loop filter. 
         [0002]    H. Paulson, “MEMS-Based Capacitive Sensor”, U.S. Pat. No. 8,104,346, Jan. 31, 2012, employs this architecture in a capacitive-coupled MEMS accelerometer and provides a lucid explanation of the underlying principles. Naturally, the single-bit feedback system creates a substantial amount of quantization noise which others have attempted to address through various means. For example, the mechanical part of a voltage driven micro-machined inertial sensor can be described as a harmonic oscillator described by a second-order characteristic function. To reduce the in-band quantization noise, the loop filter can add electrically determined poles to the system transfer function. This approach has been demonstrated by Y. Dong, M. Kraft, C. Goilasch and W. Redman-White, “A high-performance accelerometer with a fifth-order sigma-delta modulator”, J. Micromechanics &amp; Microengineering, 15 (2005) pp 22-29. They created a fifth order system by introducing three electrical poles. 
         [0003]    A similar architecture is employed in a state-of-the-art device by H. Kulah, J. Chae, N. Yazdi, K. Najafi, “Noise analysis and characterization of a sigma-delta capacitive microaccelerometer”, IEEE J. Solid-State Circuits, 41 (2006), pp 352-361. They conclude that mass-residual motion is the dominant noise source at low sampling frequencies for systems operated in closed-loop mode, and suggest that the best way to overcome this limitation is by increasing the sampling frequency. 
         [0004]    Another state-of-the-art example can be found in P. Zwahlen, A. Nguyen, Y. Dong, F. Rudolf, M. Pastre, H. Schmid, “Navigation-grade MEMS accelerometer”, published by Colibry, www.colibrys.com. They present both simulated and measured data, showing what they claim is excellent matching between simulation and measurement. However, careful inspection of their results indicates that in the frequency range from 500 Hz to 1 kHz they have “smearing” of high frequency quantization noise down into the upper part of the passband. We attribute this discrepancy to mass-residual motion. 
         [0005]    Thus the conventional techniques for providing a high-performance MEMS accelerometer include the use of a single-bit sigma-delta feedback architecture with a high-order loop filter and/or a substantially elevated clock rate. These techniques add an undesirable performance requirements and complexity for the integrated electronics and, for the reasons explained further herein, can provide only a limited amount of performance improvement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Accordingly, there are disclosed herein methods for improving MEMS accelerometer performance through the use of multiple feedback architecture for a given proof mass, and accelerometers employing such methods. In the drawings: 
           [0007]      FIG. 1  shows a first illustrative MEMS proof mass configuration. 
           [0008]      FIG. 2  shows a second illustrative MEMS proof mass configuration. 
           [0009]      FIG. 3  is a cross-section of an illustrative MEMS accelerometer configuration. 
           [0010]      FIG. 4  is a schematic of an illustrative single-feedback architecture. 
           [0011]      FIG. 5  is a schematic of an electrical portion of an illustrative loop filter. 
           [0012]      FIG. 6  is a schematic of an illustrative multi-feedback architecture. 
           [0013]      FIG. 7  shows an illustrative electrode configuration. 
           [0014]      FIG. 8  is a flowchart of an illustrative sensing method. 
           [0015]      FIG. 9  shows an illustrative marine seismic survey system. 
       
    
    
       [0016]    It should be understood that the specific embodiments given in the drawings and detailed description below do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill in the art to discern the alternative forms, equivalents, and other modifications that are encompassed in the scope of the claims. 
       DETAILED DESCRIPTION 
       [0017]      FIG. 1  shows a first illustrative microelectromechanical systems (“MEMS”) component that can serve as a proof mass for embodiments of an accelerometer disclosed herein. A layer  105  is supported over an upper surface of a substrate  102  by one or more supports  104 . (Note that “upper” and “lower” are used herein as descriptive terms with respect to the Drawings, and such terms are not intended to limit the orientation of embodiments of the invention in practice.) A movable proof mass  106  and one or more deformable arms or beams  108  are formed in the layer  105  such that the proof mass  106  is suspended over the upper surface of the substrate  102  by the arms or beams  108 . A single electrode  110  is formed on the upper surface of a substrate  102  adjacent the proof mass  106 . The supports  104  and the top layer  105  define a cavity over the electrode  110 . As described in more detail below (see  FIG. 3 ), a second electrode is formed on a layer (e.g., a “cap layer”) positioned above the proof mass  106  to provide an additional degree of sensitivity and control. It is emphasized that this particular proof mass configuration is merely illustrative—other designs employing cantilevers, springs, and gas cushions are also known and may be suitable depending on the chosen application. The contemplated dimensions of the proof mass  106  are 3×2×0.5 mm, though this may vary based on design constraints. The substrate  102  may be or include, for example, silicon, ceramic, sapphire, silicon carbide, glass, metal, and/or plastic. 
         [0018]      FIG. 2  shows a second illustrative MEMS component that can also serve as a proof mass for embodiments of an accelerometer. Elements of the MEMS proof mass configuration shown in  FIG. 1  and described above are labeled similarly in  FIG. 2 . In the MEMS proof mass configuration of  FIG. 2 , an array of electrodes  202  is formed on the upper surface of a substrate  102  adjacent the proof mass  106 . As described in more detail below (see  FIG. 3 ), a second array of electrodes is formed on a layer (e.g., a “cap layer”) positioned above the proof mass  106  to provide an additional degree of sensitivity and control. 
         [0019]    To provide capacitive coupling with the electrodes, the moveable proof mass  106  is made from a conductive material or at least coated with a layer of conductive material. The electrodes and the proof mass  106  are coupled to electronics that sense and control the relative voltages between the electrodes and the proof mass  106 . Such electronics are expected to be included in the packaging for an electronic component. In some embodiments, the electronics may be integrated on substrate  102  with the proof mass  106 . 
         [0020]      FIG. 3  shows a cross-section of an accelerometer&#39;s MEMS component having a proof mass  302  positioned between a cap layer  304  and a substrate  306 . An upper array of electrodes  310  are positioned on the cap layer  304  at a distance D U  from a conductive surface on the proof mass  302  and a lower array of electrodes  312  are positioned on the substrate at a distance D L  from the conductive surface on the proof mass  302 . D U  and D L  are nominally the same, though subject to change by displacement of the proof mass  302 . In some embodiments, the contemplated values for D U  and D L  are on the order of 2 μm. 
         [0021]      FIG. 4  is a schematic of an illustrative sigma-delta (“ΣΔ”) feedback architecture suitable for use with a conventional MEMS transducer  402 . The illustrated transducer  402  has a proof mass  404  positioned approximately midway between an upper electrode  406  and a lower electrode  408 . A driver  410  applies feedback voltages along feedback paths to the upper and lower electrodes  406 ,  408 . The proof mass  404  may initially be set at ground voltage by switch  412  and thereafter maintained there by charge amplifier  414 , though in some embodiments the switch can also operate to isolate the charge amplifier momentarily to minimize transients as the drivers  410  change the electrode polarity. The proof mass  404  may experience a net upward or downward electrostatic force based on the polarity of the feedback signal (which alternates). As the proof mass  404  experiences inertial forces due to physical acceleration of the transducer, the feedback architecture modifies the feedback signal to counteract (on average) any displacement of the proof mass  404 . 
         [0022]    Charge amplifier  414  employs an op-amp  416  with a feedback capacitance  418  and feedback resistance  420  to convert any capacitance changes into a voltage input signal for an electrical portion of a loop filter  422 . Herein below, the electrical portion of the loop filter  422  will be referred to simply as “loop filter  422 .” In the embodiment of  FIG. 4  the loop filter  422  includes a one-bit quantizer  424 , and provides a digital feedback signal via feedback path  426  to the loop filter  422  and the driver  410 . We note here that the feedback resistance  420  is optional, and the order of the blocks in  FIG. 4  can be varied. In particular, it is not a requirement that the quantizer  424  be included in the loop filter  422 . In some embodiments the quantizer  424  may be downstream from the loop filter  422 , or at the input to the loop filter  422 . 
         [0023]    The feedback signal on feedback path  426  may be a binary pulse signal having a sliding mean value (or equivalently, a duty cycle or pulse density) proportional to an average force required to maintain the proof mass  404  at a neutral position, which in turn is proportional to the physical acceleration being applied to the transducer  402 . An output unit  428  converts the feedback signal into a digital representation of the acceleration operating on the proof mass  404 . The output unit  428  may include, for example, a decimation filter. In an alternative embodiment, the output unit  428  produces an analog voltage proportional to the feedback signal&#39;s sliding mean value. 
         [0024]      FIG. 5  shows an illustrative (electrical portion of) loop filter  422  suitable for use with the architecture of  FIG. 4 , though other filter architectures are known and can also be used. As described above, the loop filter  422  includes the quantizer  424  that generates the feedback signal on feedback path  426 . The quantizer  424  quantizes the output of a summation node  502  that combines together: a scaled version of the input signal; a scaled version of the output from a first integrator  503 ; a scaled version of the output from a second integrator  504 ; and a scaled version of a pseudorandom binary sequence. The first integrator  503  integrates the sum of: the input signal; a scaled version of the output from the second integrator  504 ; and a scaled version of the feedback signal. The second integrator  504  integrates the sum of the output from the first integrator  503  with a scaled version of the feedback signal on feedback path  426 . The various scale factors are set based on desired performance criteria and in some cases may be adaptive. 
         [0025]    Having outlined the structure and operation of an illustrative MEMS accelerometer, we recall from the background discussion that the residual motion of the proof mass (i.e., proof mass displacement noise) is a dominant source of error for high performance systems. The reason that residual motion has such a negative impact on voltage-driven capacitive MEMS system performance is the fact that displacement results in non-linear feedback. The electrostatic force F between the electrodes of a plate capacitor is proportional to the charge Q squared 
         [0000]        F=Q   2 /2∈ A   (1)
 
         [0000]    where ∈ is the dielectric permittivity, A is the plate area, and charge Q is given by 
         [0000]        Q=CV   (2)
 
         [0000]    with V being the voltage difference between the plates and the capacitance C given by 
         [0000]        C=∈A/d   (3)
 
         [0000]    where d is the distance between the plates. 
         [0026]    With voltage driven feedback, the electrode voltage V is essentially fixed and the variation in capacitance C determines the force. When the proof mass is displaced, the capacitance deviation from its nominal value is different for the upper and lower capacitances since C is proportional to the inverse of the electrode gap d. With a downwards displacement Δd, the resulting electrostatic force upwards and downwards will be 
         [0000]        F   U   =∈AV   U   2 /2( d   0   +Δd ) 2   (4a)
 
         [0000]        F   L   =∈AV   L   2 /2( d   0   −Δd ) 2   (4b)
 
         [0027]    The premise underlying ΣΔ feedback signaling to cancel the input acceleration is the use of a known feedback value, i.e., F U  and F L . However, the actual dynamic gap value, (d 0 +Δd), is not known by the system. Instead the system read-out is based on the known nominal gap d 0 . This means that any displacement will result in an unknown non-linear feedback causing signal distortion and downconversion of high frequency quantization noise. In a high performance voltage driven system it is therefore important that the displacement is modeled and the system optimized to minimize displacement effects. 
         [0028]    An important consideration that appears to be neglected by existing literature is the presence of secondary and higher-order vibration modes. In addition to the primary mode (vertical vibration of the proof mass), there may be secondary modes such as longitudinal rocking, diagonal rocking, and beam modes (standing waves in the support arms), as well as higher-order resonances in each mode. We have discovered that such additional modes can cause substantial residual motion effects, and in some cases may even be the dominant source of proof mass displacement noise. (We note here that while vertical vibration of the proof mass is the primary mode in the embodiments shown and described, it is also possible to use proof mass finger structures and allow the primary mode to be horizontal vibration.) 
         [0029]    Accordingly, we disclose herein certain alternative embodiments that efficiently suppress any such additional noise-producing modes without requiring knowledge of the specific mode geometries or their resonant frequencies. These embodiments employ multiple independent feedback paths that each operate on the same proof mass. 
         [0030]      FIG. 6  is a schematic of an illustrative ΣΔ feedback architecture suitable for use with an enhanced MEMS transducer  502 . Transducer  502  has a proof mass positioned between an upper array of electrodes and a lower array of electrodes as discussed previously with respect to  FIGS. 2 and 3 . The multiple electrodes enable the use of multiple feedback paths that operate on the same proof mass. In  FIG. 6 , three feedback paths are shown but the actual number of systems could be higher or lower. 
         [0031]    Three drivers  410  are shown for applying feedback voltages to oppositely-positioned electrodes. As before, the proof mass experiences a net upward or downward electrostatic force based on the combined effects of the feedback signals. Charge amplifier  414  converts displacement of the proof mass into a voltage signal for three loop filters  422 . At different times, each loop filter  422 , together with its respective quantizer  424 , provides a digital feedback signal on feedback path  426  to a corresponding driver  410 . As the operation of the quantizers  424  is time-multiplexed, the feedback signal changes occur at different times and the operation of the different feedback paths need not correspond even though they share a common input signal. 
         [0032]    The output unit  528  converts the three feedback signals into a combined representation of the acceleration operating on the proof mass. In one implementation, the sliding mean value for each feedback signal is determined and, with appropriate offsets and scaling, the rates are summed to provide the output signal. 
         [0033]      FIG. 7  shows an illustrative configuration for the upper array of electrodes (numbered 1-8) and the lower array of electrodes (numbered 9-16) that may be employed in the MEMS component of  FIGS. 2 ,  3 , and  6 . Arrays of eight elementary electrodes have been chosen here for manufacturing convenience. The elementary electrodes can be grouped as desired into larger effective electrodes by electrically coupling them together. In  FIG. 7 , elementary electrodes  1 ,  5 ,  6  have been grouped together as electrode A, elementary electrodes  2 ,  3 ,  4  grouped as electrode B, and elementary electrodes  7 ,  8  grouped as electrode C. (If desired, the smaller area of electrode C can be compensated by driving the electrode with a slightly larger voltage—see equation (4)). The lower electrode array has been similarly grouped to form electrodes D, E, and F, with D opposite A, E opposite B, and F opposite C. The electrostatic force centroids for each electrode are marked with a small circle. These centroids are the locations at which a point force can be considered equivalent to the electrode&#39;s distributed electrostatic force. The centroids on the upper array, the proof mass, and the lower array are vertically aligned. 
         [0034]    The shortcoming of a single electrode system may be understood by comparing the proof mass to the seat of a stool which we desire to keep in a horizontal position even as gravity is acting on it. While it is theoretically possible theoretically possible to achieve this state with a single support (assuming the precise load point is known), the seat will nevertheless tilt with only the smallest of misalignments. If instead the seat is provided with three supports, the horizontal position can be maintained over a wide range of loading conditions, particularly when the supports can provide both upwards and downwards forces. So it is with the proof mass. The electrode arrangement in  FIG. 7  provides a triangular support arrangement (note the arrangement of the centroids) with bidirectional force capability. This non-collinear arrangement operates to maintain the parallel orientation of the proof mass by suppressing any tilting or rotational modes. 
         [0035]    To ensure that the system remains linear regardless of the applied electrode mismatch, each effective electrode is driven as a single-bit system. A single-bit system is per definition linear since it only has two values which always will fall onto a straight line. Nevertheless, the operation of multiple single-bit systems on the same proof mass still offers multi-bit properties to the vertical eigenmodes. In total these modes will be subject to the following normalized forces: +3/3, +1/3, −1/3, −3/3. This multi-bit effect reduces the quantization noise by 6 dB for these modes compared to a true single-bit system. Though substantial accuracy gains are anticipated, the expected penalty for this architecture is the increased power consumption required by the three loop filters. 
         [0036]      FIG. 8  is a flowchart of one an illustrative method embodiment that may be implemented by the disclosed accelerometer configuration. In block  802 , the initial electrode voltages are established by one or more drivers  410 . In block  804  any displacement of the proof mass is converted to a voltage signal by a charge amplifier. In block  806 , a separate feedback signal is determined for each of multiple opposed pairs of electrodes. This can be done using a time-multiplexed approach. In block  808  the drivers  410  apply each feedback signal to a respective electrode pair. In block  810 , an output unit processes the feedback signals to determine an output signal representing the applied acceleration. 
         [0037]    The disclosed accelerometer methods and devices are suitable for use in a wide variety of applications.  FIG. 9  shows one illustrative application: a seismic survey system. In the illustrative seismic survey system of  FIG. 9 , a survey ship  900  is moving along the surface of a body of water  904 , such as a lake or an ocean. A data processing system on board the ship  900  is coupled to at least one seismic source  912  and at least one streamer  910  having an array of seismic sensor modules  914 . As the ship tows the source(s) and streamer(s) through the water, steering devices attached to the source and streamers (e.g., device  918 ) maintain their depth and relative position. 
         [0038]    The seismic source  912  produces acoustic waves  916  under the control of the data processing system, e.g. at selected times. The seismic source  912  may be or include, for example, an air gun, a vibrator, or other device. The acoustic waves  912  travel through the water  904  and into a subsurface  908 . When the acoustic waves  912  encounter changes in acoustic impedance (e.g., at boundaries or layers between strata), portions of the acoustic waves  916  are reflected. The portions of the acoustic waves  916  reflected from subsurface layers are called “seismic reflections”. In  FIG. 9 , one such seismic reflection is shown from an interface labeled  906 . 
         [0039]    Accelerometers in the seismic sensor modules  914  detect these seismic reflections and produce highly accurate output signals indicative of the seismic reflections. The output signals produced by the sensor units are recorded by the data processing aboard the ship  900 . The recorded signals can be processed and later interpreted to infer structure of, fluid content of, and/or composition of rock formations in the subsurface  908 . 
         [0040]    Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, the time-multiplexed operation of the loop filters can be replaced by fully parallel operation if the conductive surface of the proof mass is split into multiple electrodes corresponding to the outer electrode pairs. Each proof mass electrode can then be monitored by a separate charge amplifier, making the multiple feedback paths fully parallel and independent but for their operation on a single proof mass. It is intended that the following claims be interpreted to embrace all such variations and modifications.