Abstract:
A vibratory rotational rate gyroscope has a suspended assembly isolated from external vibrations by an arrangement of helical springs. This isolated assembly includes both the active components of the rotational rate gyroscope and a digital processing circuit. The digital processing circuit includes digital storage for both externally determined and internally determined unit-specific calibration values. These values provide seed values for startup processes, which improves loop startup time, and values for unit-specific electronic calibration. The digital processing circuit further converts all data to digital form. A digital communications protocol is used to transmit the calibration information and the outgoing data to and from the isolated assembly on only two conductors. Two additional conductors used for power. Four of the helical springs used in the suspension arrangement are used for these conductors such that no additional wiring is required.

Description:
This application claims the benefit, under 35 U.S.C. §365 of International Application PCT/US2005/008361, filed Mar. 11, 2005, which was published in accordance with PCT Article 21(2) on Sep. 29, 2005 in English and which claims the benefit of U.S. provisional patent application No. 60/552,652, filed Mar. 12, 2004. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of rotational rate sensors that include vibrating resonators. 
     BACKGROUND OF THE INVENTION 
     Rotational rate sensors with vibrating resonators, also referred to as “vibratory-rate gyroscopes,” measure rotational rates directly by sensing forces generated by the vibrating elements in response to rotation of the sensor. Various configurations of vibratory elements have been developed for use in vibratory-rate gyroscopes, including suspended tuning-fork structures, vibrating beams and vibrating rings. These elements are driven on resonance and the motion of the elements in response to rotation is measured to determine the forces on the elements and the rotation of the sensor. 
     An illustrative vibratory-rate gyroscope having a tuning fork element is taught in U.S. Pat. No. 5,698,784, Vibratory Rate Gyroscope and Methods of Assembly and Operation, issued to Steven P. Hotelling and Brian R. Land, Dec. 16, 1997. The Hotelling-Land gyroscope utilizes two vibratory elements, one to detect motion about each of two different rotational axes. However, not only does this design require the use of two tuning forks, the two tuning forks must operate at different frequencies in order to minimize crosstalk between the units. From a perspective of complexity and compactness, it is desirable to have a gyroscope capable of sensing rotation about two axes that requires only one vibrating element. 
     One difficulty with vibratory rate sensors arises from the fact that the driven vibratory motion is very large compared to the forces and motion resulting from rotation. Small amounts of mechanical transducer misalignment can result in the large driven motion causing errors in the small signals being sensed on the other axes. These errors are typically corrected mechanically, by adjusting sensors and/or by trimming material from the vibrating elements However, such mechanical trimming and adjustment is time consuming and expensive. It is desirable to provide automatic error correction electronically and to further provide correction that compensates over a wide variation in operating conditions. 
     It is also desirable to provide a rotational rate sensor that is small, inexpensive to produce, is adaptable to a wide range of applications, and is easily integrated with microelectronics. Such adaptability would preferably include the ability to adjust the bandwidth of the sensor and to provide for uniform output from a number of sensors. 
     Vibrating sensors are sensitive to vibrations, both external vibrations and self-generated vibrations that can be reflected back into the sensor. It is therefore desirable to isolate the rotational rate gyroscope from such vibrations. Preferably this isolation is done with a simple and effective suspension system having a minimum of components, and not having non-essential wires for communications and power to complicate the isolation function and compromise the longevity of the sensor. 
     It is further desirable to provide a rotational rate sensor having provision for storing unit-specific calibration values and seed values to speed up the startup settling times 
     The present invention is directed to providing these advantages. 
     SUMMARY OF THE INVENTION 
     The preferred embodiment of the present invention comprises a digital processing circuit integrated onto the isolated of the rotational rate sensor. The digital processing circuit includes digital storage for both externally determined and internally determined unit-specific calibration values. These values provide seed values for startup processes, which improves startup time, and provides values for unit-specific electronic calibration. The digital processing circuit further converts all outgoing data to a digital format. Digital data is transmitted to and from the isolated assembly using a digital communications protocol which requires only a minimum of conductors. This allows for an isolation system that is simple and has a minimum number of components. Four helical spring suspension elements are used to conduct data and power such that no additional wiring is required from the digital communications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  an exploded view of vibratory assembly  100 ; 
         FIG. 2  is an illustration of a top view of beam element  150 ; 
         FIG. 3  illustrates an assembled vibratory assembly; 
         FIG. 4  is a side view of vibratory assembly  100  illustrating counter-phase motion; 
         FIG. 5  is a perspective drawings illustrating the motion of vibratory assembly  100  in response to rotation about the x and/or y axes. This motion is referred to as the “sense mode;” 
         FIG. 6  is a side view illustrating the motion of vibratory assembly  100  in the “in-phase” mode; 
         FIG. 7  is a perspective drawing of the vibratory assembly  100  mounted on mount plate  700 ; 
         FIG. 8  is a perspective drawing of drive side assembly  800  in an exploded view; 
         FIG. 9  is a perspective drawing of sense side assembly  900  in exploded view; 
         FIG. 10  is an exploded view of signal PCB  1010 ; 
         FIG. 11  is an exploded view of the suspended assembly  1110 ; 
         FIG. 12  is an exploded view of final assembly  1200 ; 
         FIG. 13  is a logical schematic diagram of ASIC  1030 ; 
         FIG. 14  is a detailed schematic of combining and scaling DACs  1340  and adjoining circuitry. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The characteristics and advantages of the present invention will become apparent from the following description, given by way of example of an exemplary rotational-rate sensor according to the principles of the present invention. 
       FIG. 1  is an exploded view of vibratory assembly  100 , which includes cylindrical permanent magnets  110  and  120 , magnet holders  130  and  140 , and a planar beam structure  150 . Magnets  110  and  120  are preferably aligned as illustrated, with poles oriented in a common direction, so that their magnet fields reinforce each other. 
     A top view of beam structure  150  is provided in  FIG. 2 . Beam structure  150  has an axially symmetric hexagon shape and is composed of several winding serpentine beams symmetrically folded about the z-axis. Beams  210 - 265  operate as spring arms to provide restoring forces in vibratory assembly  100 . Contacts  210   c - 235   c  on the ends of the beams provide mounting locations the two magnet holders  130  and  140 . Mounting points  240   m - 265   m  provide locations for fixing to an external assembly. 
     Referring now to  FIGS. 1-3 , three serpentine beams  210 ,  215  and  220  are connected to magnet holder  130  at contacts  210   c ,  215   c  and  220   c . Magnet holder  140  is connected in a similar fashion to three interleaved beam ends  225   c ,  230   c  and  235   c  on the opposite side of beam structure  150 . This mounting, and the relief in magnet holders  130  and  140 , allows magnet holders  130  and  140  to move freely with respect to each other both in and out of the plane of beam structure  150 . Specifically, magnet holders  130  and  140  can move towards each other along the z-axis without interference for a distance sufficient for the operation of vibratory assembly  100 . 
     Magnet holders  130  and  140  are formed from a non-magnetic material, such as certain stainless steels, and are attached to beam structure  150 . Magnets  110  and  120  are fixed inside magnet holders  130  and  140 . The remaining 6 serpentine beams are mounted to an external assembly at mounting points  240   m ,  245   m ,  250   m ,  255   m ,  260   m  and  265   m.    
     In operation, magnets  110  and  120  are driven into a counter-phase motion along the z-axis as illustrated in  FIG. 4 . Counter-phase motion is a forced-resonant sinusoidal motion along the z-axis wherein magnets  110  and  120  sequentially move apart, then together, along the z-axis. 
     In general, vibratory rotational-rate sensors measure the rate of rotation of the sensor by sensing the force exerted on a mass moving in a linear direction within the rotating frame-of-reference of the sensor. This force is commonly referred to as the “Coriolis force,” and is described by the equation:
 
 F   Coriolis =−2 m (ω× y )  (Equation 1)
 
     Where m is the mass of the moving object, v is the velocity vector of the moving object, and ω is the angular rate of rotation of the system. 
     When vibratory assembly  100  is rotated about the x or y axis (or about any axis in the x-y plane), a force is exerted on magnets  110  and  120  in a direction orthogonal to both the axis of rotation and the axis of vibration, as given in Equation 1. This force is proportional to angular rate of rotation ω and results in movement of magnets  110  and  120  in the x-y plane. This motion is detected to resolve the rotational rates about the x and y-axes. More specifically, referring to the coordinate system in  FIG. 4 , the axis of forced counter-phase vibration is the z-axis. When magnet  110  moves in the +z direction as shown, forces are produced on the moving magnets  110  and  120  when vibratory assembly  100  is rotated about either the x or y-axis. If vibratory assembly  100  is rotated about the x-axis in the direction “R” shown in  FIG. 4 , the angular rate ω will be a vector along the +x axis. This results in a force on magnet  110  in the +y direction (according to the “right hand rule”), and movement of magnet  110  in the +y direction. A similar analysis resolves the forces on magnet  120  as it moves (simultaneously) in the −z direction. As vibratory assembly  100  rotates about the x-axis in the direction R shown in  FIG. 4 , the force on magnet  120  is in the −y direction, resulting in movement of magnet  120  in the −y direction. 
     As illustrated in  FIG. 5 , rotation of vibratory assembly  100  about the y-axis results in motion of magnets  110  and  120  parallel to the x-axis and the amplitude of this motion is sensed to provide a measurement of the rate of rotation about the y-axis. Specifically, magnets  110  and  120  are illustrated as having a component of motion parallel to, or along, the x-axis, which corresponds to a rotation of the sensor about the y-axis. Motion of magnets and  110  and  120  in the x-y plane is referred to as “sense motion,” since motion in the x-y plane is sensed in order to measure the rotational rates of the vibratory assembly  100 . 
     As discussed above and illustrated in  FIG. 4 , to establish the required resonant counter-phase motion of vibratory assembly  100 , magnet  110  is driven along the z-axis at the resonant frequency of the counter-phase mode of 1800 Hz. The amplitude of this motion is about 50 microns peak-to-peak and the resonance is characterized by a quality factor “Q” of about 2000. The high Q, which is an indication that the system loses only a small fraction of its energy over time, and the matching of the drive frequency to the resonance of the vibratory assembly allow the counter-phase motion to be driven with a relatively small forcing input. 
     Magnet  120  acquires a symmetrical sympathetic motion in response to the motion of magnet  110 . The motion of the non-driven magnet  120  in the x-y plane is sensed to detect the rotation of vibratory assembly  100  about the x and y-axes. Alternatively, the same magnet or mass could both driven and sensed by using appropriate time sequencing. However, the preferred embodiment allows for more design in the placement of the drive and sense functions. 
     Referring again to  FIG. 2 , beam structure  150  provides the restoring forces required to establish the desired resonant frequencies of vibratory structure  100 . It also is designed to provide for radial symmetry in the x-y plane, such that magnets  110  and  120  move symmetrically in the x-y plane in response to rotation. Specially, beam structure  150  is 6-fold axially symmetric. That is, if beam structure  150  is divided into 6-sixty degree segments, the segments are identical, and they remain identical with rotation. This symmetrical design minimizes drive trajectory misalignment errors, cross-axis errors, and facilitates equal movement of the magnets in any direction in the x-y plane. 
     Six serpentine beams  210 ,  215 ,  220 ,  225 ,  230  and  235  couple to the magnet/magnet holder pairs at contacts  210   c ,  215   c ,  220   c ,  225   c ,  230   c  and  260   c . These beams provide a restoring force that establishes a counter-mode resonance of about 1800 Hz. The serpentine shapes of the beams allow for the longer beam lengths required to lower the resonant frequency to the desired 1800 Hz in a compact design. 
     Beam structure  150  also includes design features that reduce undesirable modes of oscillation. More specifically, the design minimizes the “in-phase” mode of vibration illustrated in  FIG. 6 . In the in-phase mode, magnets  110  and  120  both move in the +z direction, then both move in the −z direction. This mode of operation is undesirable because it couples unbalanced forces in and out of vibratory assembly  100  at mounting points  240   m - 265   m . The in-phase mode is minimized by designing beam structure  150  such that the resonant frequency of the in-phase mode is well separated from the counter-phase resonance. This is accomplished by the proper choice of beam lengths for the six beams  240 - 265  which couple beam structure  150  to an external assembly at mounting points  240   m - 265   m . These beams are minimally involved in the counter-phase mode but are involved in the in-phase mode, in a manner analogous to the involvement of the handle on a tuning fork when the arms of the tuning fork move in phase. Lengthening these 6 beams lowers the resonant frequency of the in-phase mode, which reduces the energy coupled between the in-phase mode and the driving frequency, which is picked to coincide with the desired counter-phase mode. The lengths of beams  240 - 265  have been chosen to result in an in-phase resonant frequency of 1400 Hz, well below the drive frequency. This separation of greater than 20% is more than sufficient to allow filters to eliminate in-phase frequencies. 
     Beam structure  150  has a planar, single-element design that lends itself to easy manufacturing and has a number of preferable mechanical characteristics. The planar beam-spring configuration can be easily manufactured by chemical etching, or by various semiconductor or micro-machine manufacturing techniques, or by etching, fine blanking, stamping or electro-discharge machining. Physical vapor deposition processes, such as sputtering, may also be used to produce the desired beam shapes. It is desirable that the material beam structure  150  maintain a constant modulus of elasticity over temperature so that the vibration frequency remains suitably constant over the operating temperature of the sensor. The material is preferably homogenous and suitable materials for beam element  150  include metals such as elinvar, stainless steel, beryllium copper, spring steel or other suitable alloys. Alternatively, quartz or silicon may be used and shaped through conventional photolithographic etching processes. 
     Other mechanisms could be developed to provide the restoring forces for certain embodiments of the present invention, however, the planar beam structure, and more particularly the serpentine beam structure with beam-springs of varying lengths, provides unique advantages to the preferred embodiment of the present invention, such as the extremely high-Q which is a result of the low-loss homogeneous design of beam structure  150 . 
     Referring again to  FIG. 5 , the sense motion is responsive to the torques caused by rotation of vibratory assembly  100 . The sense motion is a sinusoidal motion that is driven at the frequency of the counter-phase motion and has an amplitude proportional to both the rotational rate and the amplitude of the counter-phase motion. The natural resonant frequency of the sense motion, referred to as the “sense mode,” is chosen close to the drive frequency of the counter-phase mode. The natural resonant frequency of the sense mode is preferably close to the drive frequency of the counter-phase mode, about 1700 Hz in the preferred embodiment. Picking the sense mode resonance to be close to the frequency of the driven counter-phase motion magnifies the amount of sense motion, which is desired because of the very small magnitude of the motion to be sensed. This frequency is close enough to the drive frequency of the counter-phase motion to achieve a significant physical resonance magnification multiple of approximately 10. A higher magnification and sensitivity is possible if the sense mode resonance is chosen to be closer to or coincident with the frequency of the driven counter-phase motion. However, in the present design it was preferable to calibrate a number of sensors to respond identically to like inputs, thus a frequency was chosen wherein the slope of the magnification curve was small enough so that unit-to-unit variations could be more readily compensated. 
     Beam structure  150  thus provides a restoring force for the desired counter-phase mode and permits vibrations in the plane orthogonal to the vibratory axis, (referred to as the “sense motion”) which are detected to indicate rotation of the assembly. Further, beam structure  150  establishes a high-Q resonance (Q is about 2000) for the counter-phase mode, and establishes the resonance of the sense mode close to the driven frequency of the counter-phase mode. 
       FIG. 7  illustrates the beam element mounted to an external metallic mount plate  700 . Beam structure  150  is attached to mount plate  700  by fastening mounting points  240   m - 265   m  (See  FIG. 2 ) of the 6 serpentine beams  240 - 265  to raised protrusions on mount plate  700 . These protrusions (not shown) are formed as semi-piercings in mount plate  700  and hold beam structure  150  approximately 0.23 mm from mount plate  700  so that all beams of beam structure  150  can vibrate freely. 
       FIG. 8  illustrates drive side assembly  800  in an exploded view. Drive side assembly  800  locates drive coil  810  proximate to magnet  110  and magnet holder  130  (illustrated in previous  FIGS. 1 ,  3 - 7 ) and includes a plastic drive mold  820 , which incorporates a location feature for drive coil  810 . A magnetically permeable iron plate  830  is attached to the rear of drive mold  820  to help channel the magnetic flux lines generated by magnet  110  and to improve the coupling between drive coil  810  and magnet  110 . Plate  830  also provides additional inertia to the system to reduce unwanted motion. Pins  840  are attached to drive mold  820  and carry electrical signals to and from drive side assembly  800   
       FIG. 9  illustrates a sense side assembly  900  in an exploded view. Sense side assembly  900  is mounted proximate to magnet  120  on the side of beam structure  150  opposite drive assembly  800 . Two pairs of sense coils  910  and  915  are attached to sense mold  920 . The two pairs of flat, oval, sense coils are oriented 90 degrees apart and, when completely assembled, are located in close proximity to magnet  130 . Coils  910  and  915  coils are each mounted in the x-y plane and are used to detect motion of magnet  120  in the x-y plane, one pair sensing motion along the x-axis, the other pair sensing motion along the y-axis. Specifically, with reference to the x-y axis illustrated in  FIG. 9 , sense coils  910  are oriented such that motion magnet  120  along the x-axis will generally increase the flux in one coil, and decrease the flux in the other, signal corresponding to the x position of magnet  120 . The oval shape of the coils makes them more sensitive to motion in the x-direction. Each coil pair is connected with opposite polarity (wound in opposite directions) so that flux changes resulting from the driven z-axis motion of magnet  120  generally cancel out, but motions along the x-axis will be additive. 
       FIG. 10  is an exploded view of signal printed circuit board (PCB)  1010 . Attached to signal PCB  1010  are Electronically Erasable Programmable Read Only Memory (EEPROM)  1020  and Application Specific Integrated Circuit (ASIC)  1030 . A capacitive shield plate  1040  is attached to signal PCB  1010  and prevents capacitive coupling between sense coils  910  and  915  and the various traces and pins on signal PCB  1010 . Capacitive shield plate  1040  is a made from an electrically conducting and non-magnetic material, such as phosphor bronze. AGC coil  1050  is attached to the back side of signal PCB  1010 . As described in more detail below, AGC coil  1050  senses the amplitude and precise phase of the driven counter-phase motion along the z-axis and provides feedback for the drive, error correction and signal demodulation circuitry. Coils  810 ,  910 ,  915  and  1050  are manufactured by winding insulated electrically conductive wire, such as copper, around a spindle which is later removed. The coils can also be formed by spiraling traces on several PCB layers to generate a coil structure or by depositing metal films onto a substrate and then etching coil turns with photolithographic methods. One advantage of the present design is that it is amenable to the use of flat coils, which are inexpensive and easily manufactured. Alternatively, other types of transducers could be used for driving and sensing the vibratory motion. 
       FIG. 11  is an exploded view of suspended assembly  1110 . Drive side assembly  800  is attached to mount plate  700  via tangs  710 . Mount plate  700  is further attached to sense side assembly  900  via tangs  720 . The resulting compact integrated assembly positions drive coil  810  in close proximity to magnet  110 ; and AGC coil  1050  and sense coils  910  and  915  are positioned in close proximity to magnet  120 . The various cross connecting pins are connected (soldered) into signal PCB  1010  to establish electrical routes through suspended assembly  1110 . 
       FIG. 12  is an exploded view of final assembly  1200 . Base assembly  1210  consists of a plastic injection molded part, base mold  1220 , which has 4 pins  1225  attached to it. Conducting metallic belly plate  1230  is attached to base mold  1220 . Gasket  1240 , molded out of silicon rubber, is inserted between base mold  1220  and belly plate  1230  as shown. Four helical springs  1250  are connected to pins  1225 . Helical springs  1250  are made from electrically conducting material that is wound into a helical spiral shape. Suspended assembly  1110  is positioned onto  4  helical springs  1250 , and attached by soldered, welding, brazing or mechanical fastening. 
     Helical springs  1250  perform two functions. First, the four conductive springs are used to pass electrical signals between signal PCB  1010  and base mold  1220 . Further, springs  1250  provide vibratory isolation between suspended assembly  1110  and base mold  1220 . This isolation prevents unwanted vibrations (linear acceleration, mass mismatch effects) from being coupled to vibratory assembly  1100 . Lastly, can  1260  is then attached to belly plate  1230  to form a closed container to shield the sensor from undesired interference sources. Gasket  1240  forms a seal against the can so as to prevent moisture from entering the assembly. 
     Referring now to  FIG. 13 , ASIC  1030  performs signal processing for the rotational rate sensor. EEPROM  1020  stores various calibration factors and other data used by ASIC  1030 .  FIG. 13  is a logical schematic of the system electronics of ASIC  1030 . 
     Coils  810 ,  910 ,  915  and  1050  in final assembly  1200  are illustrated in transducer section  1305 . Vibratory assembly  100  is driven at its natural resonant frequency by applying an alternating drive current (DP-DM) to drive coil  810 . This produces a sinusoidal motion in vibratory assembly  100  along the z-axis, which is sensed by AGC coil  1050 . AGC coil  1050  produces a sinusoidal AGC signal (AGP-AGM) having an amplitude scaled to provide the desired physical amplitude of vibration of vibratory assembly  100  along the z-axis. The frequency and phase of the drive current is adjusted to coincide with the frequency of vibratory assembly  100  in the counter-phase mode so as to maximize the driven motion. Yaw motion (about the x-axis) is detected by sense coils  910 , which produces yaw signal. (YWP-YWM) Similarly, pitch motion (about the y-axis) is detected by sense coils  915 , which produces pitch signal (PWP-PWM). 
     Yaw and Pitch preamps  1320  and  1330  and the AGC preamp portion of AGC preamp and vibration oscillator  1310  convert the low-level voltage signals from coils  910 ,  915  and  1050  to differential currents utilizing on-chip conversion resistors. These preamps have minimal phase-delay characteristics, which preserves the precise phase relationships between yaw, pitch and AGC signals used by ASIC  1030 . Alternatively, the phase characteristics could be matched. In either case, the phase relationships between the signals are precisely preserved. 
     Differential analog AGC signal (AGP-AGM) is coupled to AGC preamp and vibration oscillator  1310 . A vibration oscillator portion of AGC preamp and vibration oscillator  1310  drives vibratory assembly  100  at its resonant frequency by applying drive current (DP-DM) to drive coil  810  using phase and amplitude feedback from AGC signal (AGP-AGM) from AGC coil  1030 . AGC signal (AGP-AGM) is integrated over each half-cycle and the result is compared with a voltage reference derived from temperature-compensated band gap reference (BG) and the difference is used to create the appropriate magnitude slew-limited drive signal at the DP and DM outputs. Signal (DP-DM) has a generally square-wave like waveform and is nominally in-phase with the AGC signal. The analog differential AGC signal (AGP-AGM) is amplified and output to combining and scaling DACs  1340  as signals (AG 1 P-AG 1 M). A phase reference signal ADPCOMP is also provided to ADC Clock Synthesizer and Counter  1325  in response to zero crossings of the AGC signal. 
     Scaling and combining DACs  1340  condition analog yaw (YWP-YWM) and pitch (PWP-PYM) sense signals to provide for dc offset removal from the preamplifiers, to remove parasitic quadrature error signals, and to remove cross-axis errors to compensate for unit-to-unit variations so that the signals presented to the ADC blocks will be normalized with respect to unit-to-unit variations. DC offset and parasitic quadrature error signals are removed by analog addition of equal and opposite analog signals which are synthesized by programmable digital-to-analog converters (DACs) using calibration values stored in digital registers in serial interface and RAM  1355 . Cross-axis errors are compensated for in a similar fashion but use the sense channel signals directly as a reference. It should be noted that the COMCAL bus line interconnecting scaling DACs  1340  and serial interface and RAM  1355  provides scaling DACs  1340  access to the PINPH, POFST, PCAX, PSF, YINPH, YOFST, YCAX and YSF registers of serial interface and RAM  1355 . The use of these registers is discussed in further detail below. 
       FIG. 14  is a detailed schematic of combining and scaling DACs  1340  and adjoining circuitry. The yaw input signal is the differential voltage across the yaw input pins YWP and YWM. The differential analog yaw signal (YWP-YWM) is a composite signal which includes the desired rate signal for the yaw rotation as well as unwanted error signals. Specifically:
 
 V yaw= V ( YWP )− V ( YWM )  (Equation 2)
 
     The theoretical definition of the yaw signal, which describes the desired and error 
     components, is:
 
 V yaw NOM=Inph Yaw*SIN(2π* Fvib*t )−CrossYaw*RatePitch*COS(1π* Fvib*t )−(RateYaw+ MRO yaw)* YSF *COS(2π* Fvib*t )  (Equation 3)
 
     The SIN term of this equation is an in-phase signal (relative to the AGC signal) which is an undesired error component resulting from transducer misalignment of the driven vibratory motion. InphYaw is defined as the amplitude of this in-phase yaw signal. This error is eliminated by adding an equal and opposite signal provided by YINPH DAC  1410  to the sensed yaw signal (YWP-YWM) at adder  14110 . YINPH DAC  1410  scales its output with AGC signal (AGP-AGM), which is multiplied by the YINPH calibration parameter. By correcting the sense signal prior to digitization, and by using the analog AGC signal directly to preserve the phase relationship between the vibratory motion and the sense motion, the in-phase error component is greatly reduced. 
     The first COS term is an undesired error signal which comes from coupling between the yaw and pitch axes. CrossYaw is defined as the amplitude of this cross-axis (pitch) error signal. This cross-axis error is compensated for by adding an equal and opposite signal to the sensed yaw signal (YWP-YWM) at adder  14110 . The correction signal is provided by YCAX DAC  1430 , which scales its output with pitch signal (PWP-PYM), and is multiplied by the YCAX calibration parameter. By correcting the sense signal prior to digitization, and by using the analog pitch signal directly to preserve the phase relationship between the error source and the correction signal, the cross-axis error component is greatly reduced. 
     The second COS term contains the desired signal, which is modulated by yaw angular rate. This signal indicates the desired angular yaw rate motion of vibratory assembly  100 . However, this term also includes an undesired mechanical rate offset (MROyaw) arising from alignment errors. 
     The operation of the pitch signal portion of combining and scaling DACs  1340  is similar to that described with reference to the yaw signal circuitry. Digital values in the DAC digital calibration registers provide the scaling factors for the DAC. Further descriptions of the errors compensated by each, the compensating signal source, the description of the error root cause, and the correspondence to the steady-state signal definition are more fully provided in Table I. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                   
                   
                   
                 Reference to 
               
               
                 DAC digital 
                 Description of 
                   
                   
                 Steady-State 
               
               
                 calibration 
                 error to be 
                 Compensating 
                 Description of error root 
                 Signal 
               
               
                 registers 
                 compensated 
                 signal source 
                 cause 
                 Definition 
               
               
                   
               
             
             
               
                 &lt;YOFST&gt; 
                 Yaw DC 
                 DC current 
                 Input offset voltages, 
                 Not 
               
               
                   
                 offsets 
                 proportional to 
                 current mirror tolerances 
                 described in 
               
               
                   
                   
                 preamp biasing 
                 in ASIC 1030 (input 
                 steady-state 
               
               
                   
                   
                   
                 signal has negligible DC 
                 signal 
               
               
                   
                   
                   
                 offset) 
                 definition 
               
               
                 &lt;YINPH&gt; 
                 Yaw In phase 
                 AGC signal 
                 Mechanical transducer 
                 SIN term in 
               
               
                   
                 error 
                   
                 misalignment 
                 Vyaw 
               
               
                   
                   
                   
                   
                 formula 
               
               
                 &lt;YCAX&gt; 
                 Yaw Cross 
                 Pitch signal 
                 Mechanical transducer 
                 First COS 
               
               
                   
                 axis 
                   
                 misalignment and 
                 term in 
               
               
                   
                   
                   
                 vibrating element sense 
                 Vyaw 
               
               
                   
                   
                   
                 principle vibration mode 
                 formula 
               
               
                   
                   
                   
                 misalignment 
               
               
                 &lt;YSF&gt; 
                 Yaw Scale 
                 Yaw signal after 
                 Mismatch between raw 
                 Second COS 
               
               
                   
                 Factor 
                 DC offset 
                 scale factor at output and 
                 term in 
               
               
                   
                   
                 compensation, In 
                 desired ADC full-scale 
                 Vyaw 
               
               
                   
                   
                 phase 
                 conversion range 
                 formula 
               
               
                   
                   
                 compensation, 
               
               
                   
                   
                 and Cross axis 
               
               
                   
                   
                 compensation 
               
               
                 &lt;POFST&gt; 
                 Pitch DC 
                 DC current 
                 In put offset voltages, 
                 Not 
               
               
                   
                 offsets 
                 proportional to 
                 current mirror tolerances 
                 described in 
               
               
                   
                   
                 preamp biasing 
                 in ASIC 1030 (input 
                 steady-state 
               
               
                   
                   
                   
                 signal has negligible DC 
                 signal 
               
               
                   
                   
                   
                 offset) 
                 definition 
               
               
                 &lt;PINPH&gt; 
                 Pitch In phase 
                 AGC signal 
                 Mechanical transducer 
                 SIN term in 
               
               
                   
                 error 
                   
                 misalignment 
                 Vpitch 
               
               
                   
                   
                   
                   
                 formula 
               
               
                 &lt;PCAX&gt; 
                 Pitch Cross 
                 Yaw signal 
                 Mechanical transducer 
                 First COS 
               
               
                   
                 axis 
                   
                 misalignment and 
                 term in 
               
               
                   
                   
                   
                 vibrating element sense 
                 Vpitch 
               
               
                   
                   
                   
                 principle vibration mode 
                 formula 
               
               
                   
                   
                   
                 misalignment 
               
               
                 &lt;PSF&gt; 
                 Pitch Scale 
                 Pitch signal after 
                 Mismatch between raw 
                 Second COS 
               
               
                   
                 Factor 
                 DC offset 
                 scale factor at output and 
                 term in 
               
               
                   
                   
                 compensation, In 
                 desired ADC full-scale 
                 Vpitch 
               
               
                   
                   
                 phase 
                 conversion range 
                 formula 
               
               
                   
                   
                 compensation, 
               
               
                   
                   
                 and Cross axis 
               
               
                   
                   
                 compensation 
               
               
                   
               
             
          
         
       
     
     After the error signals are electronically removed by analog addition at adder  14110 , desired yaw sense signal is demodulated at mixer  14140  with a cosine signal (in quadrature with the AGC signal) to demodulate the desired yaw signal and to further remove remaining undesired sine components. Mixer  14140  can further selectively demodulate the yaw sense signal with either a sine signal or a dc signal. These modes are used to determine calibration values, as described below. 
     Yaw analog-to-digital converter (ADC)  1350  and pitch ADC  1360  perform simultaneous analog-to-digital conversions which are synchronized relative to the AGC signal. Pitch ADC  1360  operates in a manner consistent with that of yaw ADC  1350 , which we now describe. The corrected yaw signal is applied to mixer  14140 , where it is demodulated by mixing with a cosine signal (in quadrature with the AGC signal). The demodulated signal is then rectified and converted to digital levels by yaw ADC  1350 . In the preferred embodiment, yaw ADC  1350  utilizes a Sigma Delta Converter  14150  that samples the demodulated yaw signal at a high rate in response to HSCLK signal from ADC clock synthesizer and counter  1325  many times each cycle. Twice each 1.8 kHz cycle the rectified samples are integrated by integrator  14160  in synchronization with the CLK signal from ADC clock synthesizer and counter  1325 . The CLK signal is derived directly from the AGC signal to retain precise phase relationship with the physical oscillator. This synchronized demodulation and conversion further improves signal-to-noise of the sense signals by further removing certain error signals. 
     The digital values applied to YINPH DAC  1410  and PINPH DAC  1460  are derived by auto-correction loops that perform sine demodulation of the sense channels and adjust the scaling YINPH DAC  1410  and PINPH DAC  1460  to minimize the amount of in-phase signal on each of the two sense channels. Specifically, a selectable in-phase trim mode selects the sine input to mixer  14104  and to demodulate the corrected yaw signal with a sine wave. The resulting sine-demodulated digital signal from yaw ADC  1350  establishes the value of the YINPH parameter, which is loaded into YINPH register  1420  and used to scale the YINPH DAC  1410  to remove sine error signals during normal operation. The PINPH auto-correction loop operates in a similar manner. 
     The digital values for YCAX DAC  1430  and PCAX DAC  1480  are derived from factory calibration procedures wherein the completed assembly is rotated and the amount of cross axis on the relative channels is measured. These digital values are then loaded into EEPROM  1020  and digital registers on ASIC  1030 . YOFST and POFST are correction signals for DC offset errors. The digital values for YOFST and POFST are also determined by auto-correction loops. The output from adder  14110  is applied to integrator  14190 , which is synchronized to the CLK signal from ADC clock synthesizer and counter  1320  to integrate the output from adder  14110  over a set number of cycles of the AGC signal. The integrated value is compared to a reference value (nominally zero) in comparator  14200 . The result is applied to counter  1450 . This count adjusts the YOFST value stored in YOFST DAC  1440 , which scales a DC signal applied to adder  14110  to provide DC error correction. The POFST loop operates in an identical manner. In operation, these loops will adjust the scaling of a YOFST and POFST to minimize the amount of DC signal on the sense channels. 
     Sampling and digital filtering processes are user configurable and allow the user to essentially program the bandwidth of the sensor by simply writing to a control register. The results of the ADC conversion are accumulated in a register on the ASIC. The number of accumulated conversions is set by a user configurable control register. By accumulating several samples, the sense signals are averaged which improves the ratio of signal to noise on the signal. Furthermore, by allowing the user to set the number of samples that are averaged, the user is capable of controlling the effective bandwidth of the sensor. 
     Referring again to  FIG. 13 , ADC clock synthesizer and counter  1320  receives the analog AGCCOMP signal from AGC preamp and vibration oscillator  1310 , and provides the digital clock signals CLK and COUNT used to precisely synchronize the digital circuitry and sampling with the oscillations of the vibratory assembly. This is accomplished by using a high frequency oscillator which is divided down to a frequency matching the natural vibration frequency of vibratory assembly  100  and locked to a phase reference (such as the zero crossing point) of AGC signal AGPCOMP. 
     The CLK/COUNT signal is shifted 90 degrees in-phase from the AGCCOMP signal to facilitate extraction of the sense signal angular rate information. Thus, it is precisely synchronized with the physical vibration of vibration assembly  100 . A high speed HSCLK signal is also provided, which is synchronized with the CLK signal and has a higher frequency. During calibration, the 90 degree phase shift is selectively deactivated in order to determine YINPH and PINPH calibration values. 
     ASIC  1030  and EEPROM  1020  communicate with an external microprocessor using a serial  2  wire interface, such as Philips&#39; I2C interface, contained in serial interface and RAM block  1355 . The 2 wire interface conserves the number of connections required between the external package and the suspended assembly. In fact, there are only four electrical connections to the suspended assembly. This enables the external microprocessor to read digital values from result registers and to write calibration values and status values into input registers. 
     Using the suspension members as conductors for the electronic interface provides for improved and simplified isolation and for improved reliability. Extraneous wires can unpredictably affect the isolation provided by the calibrated suspension components and wires are also susceptible to failure in an environment where they are constantly flexing. In an alternative embodiment, these advantages can be obtained by using an alternative communications interface such as wireless data coupling between the suspended assembly and the frame. Examples would include optical and RF coupling to an external controller. 
     During factory calibration, error signals are measured and the appropriate scaling factors stored in EEPROM  1020 . During initialization of normal operation, an external microprocessor reads the stored calibration values from EEPROM  1020  and writes them to registers on ASIC  1030 . ASIC  1030  uses these stored values to set the appropriate DAC levels used for performing the error corrections discussed above. 
     Values YINPH, PINPH, YOFST, POFST, YCAX, PCAX, YSF, PSSF, RCC, CN and AGS are stored in the &lt;PINPH&gt;, and &lt;YINPH&gt;, &lt;POFST&gt;, &lt;YOFST&gt;, &lt;YCAX&gt;, &lt;PCAX&gt;, &lt;YSF&gt;, &lt;PSSF&gt;, &lt;RCC&gt;, &lt;CN&gt;, &lt;AGS&gt; registers respectively. During factory calibration these register values are written by an external processor to EEPROM  1020  for permanent storage. On subsequent start-ups these values are re-loaded (seeded) into the registers on ASIC  1030  to minimize loop settling time. Values for RCC and AGC are seeded into registers associated with AGC preamp and vibration oscillator  1310 . RCC is the time constant calibration used to adjust the oscillator center frequency to compensate for ASIC process variations. AGS is the amplitude calibration value which scales the AGC signal prior to amplitude detection. Thus, altering the value of AGS changes the physical amplitude of the driven counter-phase motion. Seeding (preloading) these values in the active registers in ASIC  1030 , which is on-board suspended assembly  1110 , allows each sensor to be electronically calibrated and improves start-up time. 
     Digital and analog filtering is further performed throughout the circuitry in a conventional manner in order to remove the unwanted low and high-frequency components present on the signals. This circuitry is not illustrated. 
     ASIC  1030  further includes temperature sensor  1380  and voltage level detector  1390  which are fed through voltage/temperature ADC  1370  so that the temperature of the sensor, as well as the supply voltage can be reported. These values are made available to allow subsequent higher order error correction for temperature and voltage dependant phenomena. 
     While the present invention has been described with reference to the preferred embodiments, it is apparent that various changes may be made in the embodiments without departing from the spirit and the scope of the invention, as defined by the appended claims.