Abstract:
Dithering mechanism and method for eliminating the effects of zero-rate bias in a rate sensor or gyroscope. Both continuously moving and indexing embodiments are disclosed. The mechanism includes a first part mounted in a fixed position centered about a dither axis perpendicular to the input axis of the gyroscope, a second part disposed coaxially of the first part and affixed to the sensing element of the gyroscope, and a plurality of piezoelectrically driven quartz flexure beams extending radially between the first and second parts for dithering the second part about the dither axis. In some embodiments, the dithering mechanism is formed separately from and affixed to the sensing element of the gyroscope, and in others it is formed integrally with the sensing element. In the indexing embodiments, radial arms and fixed stops limit movement of the mechanism between two fixed positions, and drive signals and holding potentials are applied alternately to dither the mechanism between the two positions and to hold it alternately in those positions during successive data acquisition periods.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    This invention pertains generally to angular rate sensors or gyroscopes and, more particularly, to a dithering mechanism and method for eliminating the effects of zero-rate bias in a rate sensor or gyroscope. 
         [0003]    2. Related Art 
         [0004]    Angular rate sensors or gyroscopes exhibit a non-zero output in the absence of rotation about the input axis. This non-zero output is called bias, and it can cause significant errors in inertial measurements. At the time of manufacture, the bias for each individual gyroscope can be measured and subtracted from the output to set the zero-rate output to zero. However, bias is not fixed in time, and it tends to drift with changes such as temperature and aging of the materials employed in the sensor. This may require periodic re-zeroing of a sensor in the field. 
         [0005]    Bias cancellation, or “washout”, mechanisms are employed in order to reduce or eliminate the effects of biases that slowly change with time in an unpredictable manner. By dithering the input, or sensitive, axis of a gyroscope, such mechanisms are able to provide automatic adjustment for changes in zero-rate output due to temperature, time or other errors. 
         [0006]    Dithering of gyroscopes is well known in the art. A ring laser gyroscope, for example, is dithered around its sensitive axis for the purpose of avoiding frequency lock-in. However, rotating a sensor about its input axis is undesirable because it provides an angular rate input other than the one to be detected. 
         [0007]    Dithering the input axis of a gyroscope about an axis perpendicular to the input axis to cancel bias is also well known. Continuously rotating the input axis to eliminate the effects of bias is known as carouseling, and periodically moving the input axis between discrete locations to remove or cancel bias is known as indexing. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0008]    It is in general an object of the invention to provide a new and improved dithering mechanism and method for eliminating the effects of zero-rate bias in a gyroscope. 
         [0009]    Another object of the invention is to provide a dithering mechanism and method of the above character wherein the mechanism is fabricated by micromachining techniques. 
         [0010]    These and other objects are achieved in accordance with the invention by providing a dithering mechanism and method in which a first part is mounted in a fixed position centered about a dither axis, a second part is disposed coaxially of the first part and affixed to the sensing element; and a plurality of piezoelectrically driven flexure beams extend radially between the first and second parts. A drive signal is applied to the flexure beams to rotate the sensing element toward a first fixed position, a first electrostatic holding potential retains the sensing element in the first fixed position during a first data acquisition period, and at the end of the first data acquisition period the first electrostatic holding potential is removed and another drive signal is applied to the flexure beams to assist the flexure beams in rotating the sensing element back toward a second fixed position. A second electrostatic holding potential retains the sensing element in the second fixed position during a second data acquisition period. The drive signals and holding potentials continue to be applied alternately to swing the sensing element back and forth and hold it in alternate ones of the two fixed positions during successive data acquisition periods. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a top plan view of one embodiment of a dithering mechanism incorporating the invention. 
           [0012]      FIG. 2  is a top plan view of one embodiment of a tuning fork mounted on the dithering mechanism of  FIG. 1 . 
           [0013]      FIG. 3  is a fragmentary bottom plan view of the embodiment of  FIG. 2 . 
           [0014]      FIG. 4  is a vertical sectional view of the embodiment of  FIG. 2 . 
           [0015]      FIG. 5  is a top plan view of another embodiment of a tuning fork with a dithering mechanism incorporating the invention. 
           [0016]      FIG. 6  is a vertical sectional view of the embodiment of  FIG. 5 . 
           [0017]      FIG. 7  is a vertical sectional view of another embodiment of a tuning fork with a dithering mechanism incorporating the invention. 
           [0018]      FIG. 8  is a fragmentary bottom plan view of another embodiment of a tuning fork with a dithering mechanism incorporating the invention. 
           [0019]      FIG. 9  is a top plan view of the dithering mechanism in the embodiment of  FIG. 8 . 
           [0020]      FIG. 10  is an enlarged, fragmentary, horizontal sectional view of the dithering mechanism in the embodiment of  FIG. 9 . 
           [0021]      FIG. 11  is a vertical sectional view of the embodiment of  FIG. 8 . 
           [0022]      FIG. 12  is a graphical representation of the dithering motion of the tuning fork in the embodiment of  FIG. 8 . 
           [0023]      FIG. 13  is a graphical representation of the drive voltage applied to the piezoelectrically driven quartz flexure beams of the dithering mechanism in the embodiment of  FIG. 8 . 
           [0024]      FIGS. 14 and 15  are graphical representations of the electrostatic holding potentials applied respectively to the clockwise and counter-clockwise stops of the dithering mechanism in the embodiment of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION  
       [0025]    As illustrated in  FIG. 1 , the dithering mechanism includes a mounting post  11  and an outer ring  12  which is disposed coaxially about the post. Both the post and the ring are hexagonal in shape and centered about a dither axis  13 , with the post in a hexagonal opening  14  in the ring. The ring is mounted to the post by radially extending flexure beams  16 , and mounting pads  17  are provided on the upper side of the ring for attachment to the sensing element of a gyroscope. 
         [0026]    The dithering mechanism is formed as a unitary structure of a piezoelectric material such as crystalline quartz and fabricated by micromachining techniques such as photo lithography, etching and surface deposition. The flexure beams are driven as piezoelectric actuators by signals applied to electrodes (not shown) mounted on the beams to dither the ring through a small angle, typically on the order of 10 degrees. In order to maintain a stable angle of dither, the drive signals preferably have an amplitude which varies constantly and continuously such as that of a pure sinusoidal waveform. A suitable sinusoidal drive signal can, for example, have an amplitude on the order of 100 volts or less and a frequency on the order of 10 to 100 Hz. 
         [0027]    Thus, the flexures function as piezoelectric actuators as well as suspension beams. In addition, electrical conductors or traces can be mounted on them for carrying the drive signals to the actuators and drive and pick-up signals for the sensing element dithered by the mechanism. 
         [0028]    The hexagonal structure of the mechanism is particularly compatible with the trigonal crystal symmetry of quartz, and the six flexural members can be aligned along axes which provide optimal piezoelectric coupling for actuating the dithering motion, e.g., along either the crystallographic x-axis or the crystallographic y-axis of the quartz. 
         [0029]    If desired, the mechanism can be fabricated of a material other than quartz. Other suitable materials include other piezoelectric crystals such as lithium niobate or lithium tantalate, piezoelectric ceramics such as lead zirconium titanate (PZT), and thin-film piezoelectric coatings such as PZT film on single-crystal silicon. 
         [0030]    In  FIGS. 2-4 , the dithering mechanism is illustrated in conjunction with a sensing element in the form of a tuning fork  19 . The tuning fork has a pair of drive tines  21 ,  22  and a pair of pickup tines  23 ,  24  which extend in opposite directions from a central body or base  26  and are disposed symmetrically about the input axis  27  of the device. The body includes a frame  28  which surrounds a central opening  29 , with a mounting pad or base  31  within the opening connected to the frame by relatively thin bridges  32 . Like the dithering mechanism, the tuning fork is formed as a unitary structure of a piezoelectric material such as quartz. Drive and pickup electrodes (not shown) are mounted on the tines in a conventional manner. 
         [0031]    The tuning fork is mounted on the dithering mechanism, with the base  31  of the tuning fork resting on mounting pads  17  and the lower end of post  11  affixed to the base  33  of the package in which the gyroscope is housed. Base  31  is bonded to mounting pads  17  and post  11  is affixed to the package by suitable means such as solder, epoxy or other adhesive. Dither axis  13  is perpendicular to the input or sensitive axis  27 , of the gyroscope, and dithering of ring  11  is transferred to the base of the tuning fork so that the input or sensitive axis of the tuning fork is dithered about the dither axis. 
         [0032]    In the embodiment of  FIG. 5 , the dithering mechanism is formed integrally with the tuning fork. In this embodiment, the tuning fork and the dithering mechanism are formed as a unitary structure of crystalline quartz, with a hexagonal mounting post  34  disposed in a hexagonal opening  35  in the central body  36  of the tuning fork. The tuning fork  37  is suspended from the mounting post by flexure beams  38  which are similar to flexure beams  16 . The post extends below the under side of the tuning fork and is affixed to the package  39  in which the gyroscope is housed. 
         [0033]    As in the embodiment of  FIG. 2 , the tuning fork has drive tines  41 ,  42  and pickup tines  43 ,  44  which extend in opposite directions from central body  36  and are disposed symmetrically about an input axis  46 . When driven piezoelectrically, flexure beams  38  cause the tuning fork to dither about the axis  47  of mounting post  34  which is perpendicular to input axis  46 . 
         [0034]    In this embodiment, the flexures once again function both as piezoelectric actuators and as suspension beams, and electrical conductors or traces can be mounted on them for carrying the drive signals to the actuators as well as for carrying drive and pick-up signals to and from the electrodes on the tines of the tuning fork. 
         [0035]    Means is provided for monitoring the instantaneous rotational position of the tuning fork relative to the base on which it is mounted, and in the embodiment illustrated, the scale  48  of an optical encoder is applied to the upper surface of the tuning fork and disposed concentrically of dither axis  47 . Alternatively, another suitable type of encoder, such as a capacitive encoder, can be utilized, if desired. 
         [0036]    The embodiment of  FIG. 7  is generally similar to the embodiment of  FIG. 5 , and like reference numerals designate corresponding elements in the two embodiments. In this embodiment, however, a second tuning fork  51  is positioned beneath tuning fork  37 . Tuning fork  51  is identical to tuning fork  37  and is stacked congruently with it, with mounting post  34  extending through a central opening  52  and flexure beams  53  mounting the fork to the post. The flexure beams for the two tuning forks are driven out of phase so that the tuning forks dither in an anti-phase manner about dither axis  47 . With a common dither frequency and with equal and opposite angular displacements, the net angular momentum is zero, and little or no vibration is coupled to the package. If desired, a dummy mass can be utilized instead of the second tuning fork and still provide a balancing of angular momentum within the system. 
         [0037]    With the embodiments described thus far, the tuning fork swings continuously back and forth through a small angle. In the embodiment of  FIG. 8 , however, the tuning fork is periodically swung between and locked in two predetermined positions for data acquisition. As in the embodiment of  FIG. 1 , the dithering mechanism is formed as a unitary structure of a piezoelectric material such as crystalline quartz with a substrate  54 , a mounting post  56  affixed to the substrate, an outer ring  57  disposed coaxially of the post, and flexure beams  58  extending radially between the post and the ring. The post is mounted in a fixed position, and a tuning fork  59  is mounted on the ring by mounting pads  61  for movement about the axis  62  of the post. Post  56  and ring  57  are hexagonal in shape, and flexure beams  58  are driven as piezoelectric actuators by signals applied to electrodes (not shown) on the beams. 
         [0038]    Arms  63  extend out in radial directions from the corners or vertices of the ring  57 , and stops  64  are mounted in fixed positions on opposite sides of the arms to limit rotation of the ring. The confronting surfaces of the arms and the stops intercept a small angle, typically about 10 degrees, when the ring is in a neutral position. 
         [0039]    Electrodes  66 ,  67  are mounted on the confronting faces of arms  63  and stops  64  and energized to produce electrostatic forces for locking the ring in the two extreme positions with the arms abutting against the stops. Small protrusions  68  on the stops extend through openings in electrodes  67  and abut against electrodes  66  to prevent the electrodes from contacting each other and shorting out. The length of the protrusions determines the width of the gaps between the electrodes when the mechanism is in the two locked positions, and with current photo lithography technology, the protrusions can have a length on the order of 2 microns. Since the thickness of the electrodes is negligible compared to that dimension, the width of the gaps is substantially equal to the length of the protrusions. 
         [0040]    The movement of the dithering mechanism and the tuning fork mounted thereon is illustrated in  FIG. 12 , where the rotational position of the mechanism is shown as a function of time. This figure shows a start-up phase which occurs during the interval between 0 and 1 unit of time, a first data acquisition phase during the interval between 1 and 2 units of time, a second data acquisition phase during the interval between 2.1 and 3 units of time, and a third data acquisition phase during the interval between 3.1 and 4 units of time. 
         [0041]    Because of the relatively high Q of the dither-gyro system (Q&gt;1,000) it takes time to pump enough energy into oscillatory system in order to reach the desired amplitude of oscillation. This process takes place during the start-up phase and manifests itself as a rise of amplitude of oscillation. 
         [0042]    As illustrated in  FIGS. 12-14 , when the amplitude of the oscillation reaches the distance between the stops, the drive voltage is removed from piezoelectric actuators  58 , and an electro-static locking potential is applied to the electrodes  66  on the clockwise faces of arms  63  and to the electrodes  67  on the counter-clockwise faces of stops  64 . The locking potential is maintained during the first data acquisition interval, and the mechanism and tuning fork are thus held in the clockwise position during that interval. 
         [0043]    At the end of the first data acquisition phase, the first locking potential is removed, and energy stored in the flexure beams or actuators swings the dithering mechanism back in the counter-clockwise direction. A small amount of mechanical energy is lost in the system, and a drive pulse is applied to the piezoelectric actuators, as shown in  FIG. 13 , to ensure that the mechanism swings to the full counter-clockwise position. When that position is reached, a second holding potential is applied to the electrodes on the counter-clockwise faces of arms  63  and to the electrodes on the clockwise faces of stops  64 , as shown in  FIG. 15 , to lock the mechanism in the counter-clockwise position. This locking potential is maintained during throughout the second data acquisition interval, and the mechanism and tuning fork are thus held in the counter-clockwise position during that interval. 
         [0044]    At the end of the second data acquisition phase, the second locking potential is removed, and the dithering mechanism swings back in the clockwise direction. Another drive pulse is applied to the piezoelectric actuators to ensure that the mechanism swings to the full clockwise position. When that position is reached, the first holding potential is once again applied to the electrodes on the counter-clockwise faces of arms  63  and to the electrodes on the clockwise faces of stops  64  to lock the mechanism in the counter-clockwise position throughout the third data acquisition interval. 
         [0045]    This process continues, with the tuning fork being held first in one data acquisition position and then in the other. 
         [0046]    For a given mechanism, the required electrostatic holding potential can be determined from the following equation for balancing the electrostatic attraction force (on the left) and the elastic restoring force (on the right): 
         [0000]    
       
         
           
             
               
                 
                   
                     ɛ 
                     0 
                   
                    
                   
                     AV 
                     2 
                   
                 
                 
                   2 
                    
                   
                     g 
                     2 
                   
                 
               
               = 
               kg 
             
             , 
           
         
       
     
         [0047]    where ε 0  is the dielectric constant, A is the area of the electrodes, V is the electrostatic locking potential, g is the gap in the locked position, and k is the elastic constant of the flexure beams. 
         [0048]    Thus, for example, with a crystalline quartz structure, flexure beams having a thickness on the order of 50 microns and a length on the order of 19 mm, and electrodes having dimensions on the order of 625 microns×2 mm, and gaps of approximately 2 microns, the holding potential required is on the order of 0.6 volt. 
         [0049]    The advantage of piezoelectric actuation, dynamic switching, and electrostatic locking is readily apparent when the relatively low 0.6 V holding potential is compared with the actuating potential that would be required for a dithering mechanism of similar dimensions without the piezoelectric actuators and dynamic switching. In such a mechanism, the initial gaps between arms  63  and stops  64  (the gaps when the arms are in their neutral positions between the stops) are on the order of 2.5 mm. With electrostatic pull-in occurring when the gaps are about two-thirds of the initial gaps, i.e. about 1.7 mm, the potential required to draw the arms to the stops would be on the order of 13.3 kV, a highly impractical potential compared to 0.6 volt. 
         [0050]    Although the invention has been disclosed in connection with a dithering mechanism which is particularly suitable for zero-rate bias cancellation, other applications are possible as well. By applying a static voltage to the mechanism, the input axis of the gyroscope can be varied to provide a fine adjustment for pointing the gyroscope. This may be done to achieve a more accurate position of the input axis than can be achieved by just the mechanical assembly of the device. In addition, the orientation of the input axis can be varied to adjust for variations in temperature or mechanical drift over time, or it can be adjusted to new positions as required by the needs of the system in which it is used. 
         [0051]    It is apparent from the foregoing that a new and improved dithering mechanism and method have been provided. While only certain presently preferred embodiments have been described in detail, as will be apparent to those familiar with the art, certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.