Abstract:
Quadrature error occurs in Corolis based vibrating rate sensors because of manufacturing flaws that permit the sensing element to oscillate either linearly along or angularly about an axis that is not orthogonal to the output axis. This creates an oscillation along or about the output axis that is a component of the sensing element&#39;s vibration acceleration. This output axis oscillation is in phase with the driven acceleration of the sensing element and is called quadrature error since it is ninety degrees out of phase with the angular rate induced Coriolis acceleration. Rather than applying forces that reorient the axis of the driven vibration to be orthogonal to the output axis to eliminate the output axis oscillation, the present invention applies sinusoidal forces to the sensing element by means of a quadrature servo to cancel the output oscillation. In order to avoid the phase uncertainty associated with electronic modulation, the quadrature servo feeds back a DC signal that is modulated mechanically by means of an interdigitated variable area electrostatic forcer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inertial instrument and more specifically pertains to vibrating accelerometers used as multi-sensors for measuring linear acceleration and rate of rotation of a moving body. 
     2. Description of Prior Art 
     Gyroscopes are well known for use as angular velocity and acceleration sensors for sensing angular velocity and acceleration which information is necessary for determining location, direction, position and velocity of a moving vehicle. 
     SUMMARY OF THE INVENTION 
     The present invention utilizes two masses in tandem, a dither mass and a proof mass, or pendulum. Each mass has only a single degree of freedom. It is desired to have the dither mass move along an axis that is parallel to the plane of the housing. The driving forces on the dither mass causing its vibration do not act directly on the pendulum. These forces, however cause the dither mass to move out of the plane of the housing due to dither beam misalignments. This out-of-plane motion generates error signals which are in quadrature with the signals generated by rate inputs. Therefore, a high degree of phase discrimination is required to separate the rate signal from the quadrature signal. This invention uses a new quadrature nulling technique which eliminates the requirement for accurate phase and relaxes control of the dither beam alignment tolerances which generate out of plane motion. The present invention applies vibration driving signals to the dither mass to vibrate the dither mass and the proof mass at a combined resonant frequency, and applies a restoring force to the proof mass which is in phase with its dithered displacement. In an alternate embodiment, vibration driving signals are applied to the dither mass to vibrate the dither mass and proof mass which are in an X-Y plane at a combined resonant frequency about the Z axis of the X-Y plane. A restoring torque is applied to the proof mass which is in phase with its dithered displacement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The exact nature of this invention as well as its objects and advantages will become readily apparent from consideration of the following specification in relation to the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein: 
     FIG. 1 is a top plane view of the driven and sensing element of an accelerometer according to the present invention; 
     FIG. 2 is a cross-section of the quadrature nulling projection on the end of the pendulum of FIG. 4; 
     FIG. 3 is a diagrammatic illustration of the functional relationship of portions of the accelerometer of FIG.  1  and FIG. 4; 
     FIG. 4 is a diagrammatic illustration of the pendulum and dither mass or vibrating structure of the present invention; 
     FIG. 5 is a top plan view of an alternate configuration for the proof mass and dither mass of the present invention; 
     FIG. 6 is a partial broken-away perspective of the configuration of FIG. 5 showing the relationship between the disc-shaped proof mass and the ring-shaped dither mass. 
     FIG. 7 is a partial perspective showing the relationship between the proof mass of FIG. 5 with its top cover and the quadrature nulling electrodes; 
     FIG. 8 is a top plan view of the disc-shaped proof mass with multiple teeth formed by etching grooves around its circumference; 
     FIG. 9 is a left side plan view showing the edge of the disc-shaped proof mass between its top and bottom covers, each cover containing quadrature nulling electrodes; and 
     FIG. 10 is a right side plan view showing the edge of the disc-shaped proof mass between its top and bottom covers, each cover containing quadrature nulling electrodes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The accelerometer gyro disclosed in an application for Micromachined Silicon Gyro Using Tuned-Accelerometer having U.S. patent application Ser. No. 09/778,434 filed on Feb. 7, 2001 and assigned to the same assignee as the present application illustrates a micromachined accelerometer-gyro having a pendulous mass or proof mass suspended within a dither mass that provides improved performance as the result of a considerable decrease in manufacturing flaws that affects performance of the accelerometer gyro. 
     The present invention goes beyond structural improvement of the proof mass and dither base assembly and the manufacture thereof by providing a means for nulling the error created from manufacturing tolerances and electronic phase uncertainty. Referring to FIG. 4, a conceptual schematic of the structure of the pendulum or proof mass  87  in association with the dither mass  93  is shown. The proof mass  87  is attached to the dither mass  93  by pendulum flexures points  89 . As illustrated, the dither mass  93  moves back and forth in the direction  109 . Because of the manufacturing flaws, the dither mass motion  109  is not exactly along the X axis  103  of the pendulum  87 . The actual dither mass motion  109  is off-axis by a dither misalignment angle  111 , causing slight oscillation in the Y direction  105 . 
     If this displacement along the Y axis is differentiated twice, this acceleration is known as quadrature error. Quadrature error and Coriolis acceleration are very similar in that both are sinusoidal signals centered at the frequency of oscillation. However, quadrature error can be distinguished from Coriolis acceleration by the phase relative to the driven oscillation. 
     A prior art approach to solving the problem presented by quadrature error is discussed in an article entitled  Surface Micromachined Z-Axis Vibrating Rate Gyroscope  authored by William A. Clark, Roger T. Howe, and Roberto Horwitz and published as a paper in Solid State Sensor And Actuator Workshop held in Hilton Head, S.C., Jun. 2-6, 1996. The approach suggested in the paper to null quadrature error is to apply a balancing force that is exactly proportional to position of the proof mass. The paper suggests that this can be achieved by using interdigitated position sensing fingers that sense position of the proof mass. As this proof mass oscillates, the position sensing fingers, which are position sense capacitors, change proportionately. A slight modification in the d.c. bias voltage applied to these fingers results in a net force applied to the proof mass that is directly proportional to the position of the proof mass thereby forcing the proof mass to vibrate along the desired dither axis, in other words, parallel to the X axis which is the housing axis for the proof mass. 
     FIG. 4 illustrates in schematic form the structure of the dither mass or vibrating structure and the proof mass or pendulum structure of the present invention. In normal operation, the dither mass  93  which is the vibrating structure for the pendulum  87  oscillates off-axis by an angle  111  along direction  109 . This off-axis vibration of the dither mass  93  is inherent in the construction of the rate sensor of the present invention. This is the best that can be done in the manufacturing process. This means that the dither mass and pendulum will naturally dither in the direction which is primarily along the X axis  103  but will also dither along Y axis  105  because of a misalignment angle  111  caused by mechanical imperfections of the dither driving beams during the etching phase of the fabrication. 
     As will be noted, the pendulum  87  is attached to the dither base  93  by the pendulum flexures  89 . In operation, the pendulum  87  senses Coriolis acceleration causing the pendulum to rotate about the flexure  89 . The present invention, in contrast to the approach in the above noted article, rather than forcing the dither base  93  and pendulum  87  to vibrate along the desired dither or X axis  103 , which is parallel to the housing axis, allows the dither base  93  to vibrate along the misaligned direction  109 . The quadrature control electrodes  107  of the present invention do not force the dither base  93  to vibrate along the X axis  103 . The quadrature control electrodes  107  only exert forces on the pendulum mass  87  to cause pendulum motion about the flexure axis  89  so that the pendulum continuously centers within the housing  81  as sensed by the pickoff. In this manner, neither the dither base  93  nor the pendulum mass  87  are coerced to move along a certain axis, like X axis  103 . 
     As shown in FIG. 1, the angular rate sensor according to the present invention is constructed to have a pendulum or sensing element  87  which is attached by flexures  89  to the dither mass or vibrating structure  93  which has dither drive and pickoff electrodes elements  83  mounted thereon. The dither mass  93  is mounted for motion within the plane of the paper of FIG. 1 within a frame  81 . The dither mass  93  has a plurality of flexure suspensions  85  therein to permit the dither motion along the X axis  103 . 
     The mechanical misalignment illustrated graphically in FIG. 4, along with phase error of the dither reference signal are the major source of bias instability, nonrepeatability and temperature sensitivity of tuned Coriolis angular rate sensors. The present invention provides a method to servo the quadrature error signal to null. Since the servo signal is d.c., there is no resulting phase sensitivity. The result is improved bias stability, repeatability and reduced temperature sensitivity, in addition to relaxing the tolerance requirements on etching the dither beams and the tolerance requirements on the system digital electronics phase stability. The present invention is contrary to the traditional manner of controlling bias error. The traditional approach was to attempt exceptionally close tolerances on the etching of the dither beams and attempt to achieve exceptionally close system tolerances on the digital electronics phase stability circuitry. 
     The concept of the invention is to introduce a torque to the sensing element or pendulum  87  by the application of d.c. signals which results in an a.c. restoring force that is in phase with the dither displacement. Such a torque or forcer can be used to servo the quadrature error signal to null because the quadrature signal is in phase with acceleration which in turn is in phase with the dither displacement. 
     FIG. 2 is a diagram of a quadrature nulling forcer as envisioned by the present invention. FIG. 2 is a cross-section of the present invention taken through the region containing the top electrodes  95 ,  97 , and bottom electrodes  99 ,  101  and the scalloped edge  91  of the sensing element  87 . 
     The top electrodes  95 ,  97  and bottom electrodes  91 ,  101  are divided into segments having alternate polarities. Each electrode segment  95 ,  97  on the top, and aid  91 ,  101  on the bottom, are aligned with respect to the scalloped edge  91  of the sensing element so that both polarities of the alternate segments have equal areas overlapping each projection along the scalloped edge  91  of the sensing element  87  when the dither motion is not excited. 
     In operation, the quadrature nulling forcer exerts a force as depicted in the graph of FIG.  3 . Assuming that the quadrature acceleration is in phase with displacement of the sensing element (pendulum  87 ), a bias voltage +V is applied to the sensing element and plus or minus d.c. control voltages v are applied to the top electrodes  95  and  97  and the bottom electrodes  99  and  101 . As the sensing element  87  translates to the right from the position shown in FIG. 2, it will experience an upward force proportional to its displacement, and the control voltage ν on the top and bottom electrodes. Conversely, as the sensing element and its scalloped edge projections  91  translate to the left from the position shown in FIG. 2, it will experience a downward force. The peak force will be experienced at the peak displacement and will be in phase with the peak quadrature force. 
     In operation, a closed loop servo system (not shown), of a type well known in the art is utilized to adjust the control voltages ν on the upper electrodes and lower electrodes to null the quadrature portion of the sensing element pickoff. Because this control voltage is d.c., there is no phase instability. Referring to FIG. 4, this means that the quadrature control  107 , which is adjusted to null the quadrature portion of the sensing element cause the pendulum to move about its flexure axis  89  to be continuously centered within the housing  81 . In this manner, neither the dither mass  93  nor the pendulum sensing mass  87  are coerced to move parallel to the housing axis  103  while still nulling quadrature error. This allows the dither mass  93  to move along its misaligned path  109  relative to the housing generating motion along the Y output axis  105  but still have the resulting quadrature error nulled. This approach to quadrature error nulling is generally applicable to rate sensors having a certain structure. 
     This quadrature error nulling method is possible because the two masses in operation in the present rate sensor structure are in tandem, with each mass having only a single degree of freedom. In other words, the dither mass  93  is attached to the pendulous mass  87  by the flexure  89 . As a result, the dither forces act only on the dither mass and not on the pendulous mass  87 . 
     An alternate preferred embodiment of the present invention is shown in FIGS. 5-10. These figures illustrate a rotationally dithered proof mass which is disc-shaped. This disc-shaped proof mass is mounted within a ring-like dither mass which is suspended within a frame for rotational dither motion. The dither mass dithers about its Z axis which is perpendicular to the X-Y plane within which the ring shaped dither mass is located. A proof mass is mounted within the ring-shaped dither mass in the X-Y plane and rotates about an output axis Y for an input rate about the X axis. In other words, the proof mass oscillates about the Y torsion bar axis for an input rate on the X axis. 
     Referring first to FIG. 5 and 6, which shows the general relationship between the disc-like proof mass  129  mounted within the ring-like dither mass  123  by a pair of torsion bar suspensions  127 , which lie along the Y axis of the proof mass  129  and dither mass  123 . The dither mass  123  is suspended by a plurality of dither drive beams  125  which, in this preferred embodiment, are four in number, to a frame  121 . 
     The ring-like dither mass  123  is driven rotationally about a Z axis which is perpendicular to the X-Y plane, which is the plane of the paper, in a positive and negative direction  124  causing the proof mass  129  to also be rotationally dithered. The proof mass contains a plurality of teeth  131  around its circumference, creating a scalloped edge, the purpose of which will be explained hereinafter. A plurality of electrodes  135  located in the cover for the dither mass  123  forces the dither mass to rotationally dither about the Z axis. 
     FIG. 6 is a three-dimensional partially broken away perspective showing the relationship of the proof mass  129  suspended within the ring-like dither mass  123 . The dither mass  123  is suspended by a plurality of dither drive beams  125  which is the only attachment to a frame  121 . The proof mass  129 , in turn, is attached to the internal circumference of the ring-like dither mass  123  by a pair of torsion bars  127 , which lie along a Y axis  143  of the X-Y plane  147 ,  143  within which the proof mass  129  and dither mass  123  lie. 
     Shown partially broken away is the bottom cover  135  which contains electrodes  138  therein for driving the dither mass in a back and forth dither motion  124  about the Z axis  145 . This rotational dither motion about the Z axis  145  also dithers the proof mass in the directions  149 ,  151 . 
     Also located in the bottom cover  135  are a plurality of quadrature nulling electrodes  137  which interact with the bottom teeth-like grooves  133  located about the circumference of the disc-like proof mass  129 . Teeth  131  are also located on the top surface of proof mass  129  around its circumference. 
     The quadrature nulling electrodes  137  located in a semicircle in the cover are located with respect to the bottom teeth-like grooves  133  on the proof mass  129 . The electrodes  137  are preferably deposited titanium and gold electrodes on glass, like pyrex glass, for example. The top and bottom covers for the accelerometer-gyro are preferably made of pyrex glass. A positive d.c. voltage is supplied to half of the electrodes along the perimeter of the proof mass disc on line  139 . A negative d.c. voltage is supplied to the remaining electrodes on line  141 . These d.c. voltages effectuate quadrature nulling in a manner which will be more fully explained hereinafter. 
     In operation, while the ring-like dither mass  123  is rotationally dithered about the Z axis causing the proof mass  129  to also be dithered about the Z axis, an input rate along the X axis  147  will cause the proof mass  129  to oscillate about the Y torsion axis  143  in an oscillatory motion  153  about the Y torsion axis  143 . 
     Because of manufacturing tolerances, the Z dither axis  145  may not be exactly perpendicular to the Y torsion axis  143 , causing unwanted oscillation to act about the Y torsion axis  143  as a result of this misalignment. As shown in FIG. 7, the dither mass  123  is being driven in a rotational dither direction  124  about Z axis  145 , causing the proof mass  129  to be likewise dithered in the direction  149  on its right side, and the direction  151  on its left side. The top cover  136  for the accelerometer-gyro is illustrated as being fabricated from silicon with protruding teeth thereon that interact with the teeth  131  along the circumference of the disc-like proof mass  129 . The displacement of the teeth in the right side top cover  136 B, with respect to the teeth  131  and the displacement of the teeth in the left side top cover  136 A, with respect to the teeth  131  in the proof mass  129  are illustrated for the case of a peak positive dither amplitude. In this situation, the forces  157  parallel to the Z axis on the left side top cover of the proof mass  129  are strong because the teeth are aligned, while the forces  158  on the right side top cover of the proof mass  129  are weak because the teeth are staggered. This differential creates a torque  153  about Y torsion bar axis  143 . Since the Y torsion bar axis  143  is the output axis for the accelerometer-gyro, this torque cancels the effect due to the unwanted oscillation from the misalignment of the Z dither axis  195 . 
     FIGS. 8,  9 , and  10 , illustrate the relationship between the proof mass  129  and its teeth  131  around its perimeter with the electrodes  136  located on the top cover  136 A and  136 B and the electrodes  137  located on the bottom cover  135 A and  135 B. 
     FIGS. 8,  9 , and  10 , illustrate the relationship between the proof mass  129  and the top and bottom covers and their respective electrodes when the dither motion  124  is at zero amplitude as a starting point. In other words, the dither mass  123  is at null about the Z axis  145 . 
     As the dither mass moves from this null position in a positive direction causing the proof mass  129  to also move in a positive direction  149 , the capacitance  179  between electrodes  136  and the teeth  131  of the proof mass  129  gets bigger because the teeth are becoming more aligned with the electrodes. At the same time, the capacitance  177  between electrodes  136  and the teeth  131  of the proof mass  129  on the left side become smaller. This causes the upward force F TR  on the right side acting on the paddle to increase, while the upward force F TL  on the left side becomes quite low. This difference in upward force between the left and right side of the proof mass  129  causes a torque to be developed about the torsion Y axis  143  which, in turn, causes the proof mass  129  to rotate about the torsion Y axis  143 . 
     When the dither mass goes into a negative direction causing proof mass  129  to also go in a negative direction  151 , the capacitance  179  on the right side gets smaller, while the capacitance  177  on the left side gets larger. This causes the upward force F TL  on the left side to become large and the upward force F TR  on the right side to become low, thereby reversing the torque on the Y axis  143  of the proof mass, which causes the proof mass  129  to move in the opposite direction about the Y torsion bar axis  143 . In essence then, a sinusoidal torque acts on the proof mass  129  causing it to oscillate about the Y torsion bar axis  143  exactly in phase with the dither amplitude. That is, peak torque on the proof mass  129  occurs exactly when there is peak displacement for the dither motion  124  about Z axis  145 . 
     These oscillating forces, F TR  and F TL , acting on the proof mass  129  can be servoed by automatically controlling the voltages V Q  on lines  167  and  169  on the bottom cover, and lines  171  and  173  on the top cover, to thereby cancel the torque about Y axis  143 , which is due to misalignment of the Z dither axis  145 . 
     The cancellation of the torque generated about the torsion bar Y axis  143 , as a result of the Z dither axis  145  not being perpendicular to the Y axis  143 , results in considerably improved performance.