Abstract:
The electrostatic elements already present in a vibratory gyroscope are used to simulate the Coriolis forces. An artificial electrostatic rotation signal is added to the closed-loop force rebalance system. Because the Coriolis force is at the same frequency as the artificial electrostatic force, the simulated force may be introduced into the system to perform an inertial test on MEMS vibratory gyroscopes without the use of a rotation table.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Application No. 60/141,918, filed Jun. 30, 1999. 
    
    
     ORIGIN OF INVENTION 
     The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. §202) in which the Contractor has elected to retain title. 
    
    
     TECHNICAL FIELD 
     This invention relates to vibratory gyroscopes, and more particularly to silicon micromachined vibratory gyroscopes. 
     BACKGROUND 
     Multi-axis sensors are highly desirable for inertial sensing of motion in three dimensions. Previously, such sensors were constructed of relatively large and expensive electromagnetic and optical devices. More recently, micromechanical sensors have been fabricated using, semiconductor processing techniques. Microelectrical mechanical or “MEMS” systems allow formation of physical features using semiconductor materials and processing techniques. These techniques enable the physical features to have relatively small sizes and be more precise. Specifically, micromechanical accelerometers and gyroscopes have been formed from silicon wafers by using photolithographic and etching techniques. Such microfabricated sensors hold the promise of large scale production and therefore low cost. 
     In a vibratory gyroscope, the Coriolis effect induces energy transfer from the driver input vibratory mode to another mode which is sensed or output during rotation of the gyroscope. Silicon micromachined vibratory gyroscopes are integratable with silicon electronics. These devices are low cost, capable of achieving high Q factors, can withstand high g shocks due to their small masses, are insensitive to linear vibration and consume little power. 
     As the cost of manufacturing the vibratory gyroscopes decreases, other costs such as the cost of calibration of the gyroscope become a more significant portion of the total cost. Prior calibration using inertial testing required the use of a precision rotation table to perform various tumble and rotation maneuvers. What is desired is a system that calibrates a vibratory gyroscope at a significantly reduced cost. 
     SUMMARY 
     The present invention enables the simulation of the Coriolis forces with electrostatic elements already present in the vibratory gyroscope. For vibratory gyroscopes, the Coriolis force may be at the same frequency as the vibrational frequency of the gyroscope, which may be small in magnitude. An artificial electrostatic rotation signal is added to the closed-loop force rebalance system. Because the Coriolis force is at the same frequency as the artificial electrostatic force, the simulated force may be introduced into the system to perform an inertial test on MEMS vibratory gyroscopes without the use of a rotation table. Magnetic force, piezoelectric actuators or any other actuator used for vibratory gyroscopes may be used to introduce a simulated Coriolis force into the sensor. 
    
    
     DESCRIPTION OF DRAWINGS 
     These and other features and advantages of the invention will become more apparent upon reading the following detailed description and upon reference to the accompanying drawings. 
     FIG. 1 is a block diagram of a microgyroscope system according to one embodiment of the present invention. 
     FIG. 2 is a top view of the microgyroscope of FIG. 1 according to one embodiment of the present invention. 
     FIG. 3 is a side view of the microgyroscope of FIG. 1 according to one embodiment of the present invention. 
     FIG. 4 is a block diagram of a portion of the control circuitry of FIG. 1 according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a block diagram of a microgyroscope system  100  according to one embodiment of the present invention. The microgyroscope system  100  includes a vibratory gyroscope  110  and associated control electronics  125 . The control electronics  125  are typically included on a circuit board electrically connected to the vibratory gyroscope  110 . The control electronics may be embodied in an application specific integrated circuit (ASIC). 
     FIG. 2 is a top view of a vibratory gyroscope  100  constructed according to one embodiment of the present invention. The vibratory gyroscope  100  detects forces in the x-direction  205 , the y-direction  210 , and in the z-direction  215 . A vertical post  220  is supported by a plurality of silicon suspensions  225 . The suspensions  225  connect the vertical post  220  to a frame  235 . Although suspensions  225  are shown connecting the vertical post  220  to the frame  235 , it can be appreciated that the vibratory gyroscope  100  may be constructed without the use of suspensions without departing from the spirit of the invention. 
     Vertical capacitors  230  surround the vertical post  220 . The vertical capacitors  230  provide electrostatic actuation of the vertical post  220  and allow for capacitive detection of the motions of the vertical post  220 . The vertical capacitors  230  are positioned to allow the suspensions  225  to connect the vertical post  220  to the frame  235 . 
     FIG. 3 shows a side view of the vibratory gyroscope  100  according to one embodiment of the present invention. The vertical post  220  is connected to the frame  235  via the silicon suspensions  225 . As can be seen in FIG. 3, the silicon suspensions  225  are positioned at approximately the mid-point of the vertical post  220 . Because the vertical post  220  is connected to the frame  235  via the suspensions  225  at approximately the mid-point, the vertical post  220  is free to move in a rocking motion in the x-direction  205  and the y-direction  210 . Under input rotation, the Coriolis force causes the vertical post  220  to move in the orthogonal direction to the drive motor. The rotation rate sensitivity is proportional to the input-rotation rate, the drive amplitude, and the quality factor of the resonator. 
     FIG. 4 is a block diagram of the rotation detection and calibration loop  400  of the control electronics  125  for the vibratory gyroscope system according to one embodiment of the invention. The rotation detection and calibration loop  400  includes a demodulation circuit  405 , a drive motion excitation circuit  410 , a MEMS vibratory gyroscope  415 , a sense motion closed-loop  420 , a signal adder  425 , and a rotation simulator  430 . 
     The drive motion excitation circuit  410  measures the drive motion and provides an excitation force (τ drive) to the vibratory gyroscope  415  to sustain a constant vibratory motion. The excitation force (τ drive) resonates the gyroscope  415  along one axis at a predetermined motion. The gyroscope  415  then provides a drive signal (θ drive) representative of the motion in the drive resonance. The drive signal (θ drive) is fed back to the drive motion excitation circuit  410  and is also provided to the demodulation circuit  405 . 
     The gyroscope  415  also provides a sense signal (q sense) to the sense motion closed-loop  420 . The sense signal (q sense) represents the motion of the gyroscope in the sense direction. The sense motion closed-loop  420  provides electrical damping of the sense motion of the gyroscope. Thus, the sense motion closed-loop  420  detects the sense motion and generates an output signal (negative feedback signal) to null any motion of Coriolis force from the gyroscope  415 . This output signal is provided to the signal adder  425 . If no rotation simulation signal is provided, the output signal would be the sense force (t sense) provided to the gyroscope  415 . The output signal from the sense motion close-loop  420  is also provided to the demodulation circuit  405 . 
     The rotation simulator  430  generates a rotation signal which is supplied to the signal adder  425 . The rotation signal is an electrostatic force designed to simulate the Coriolis forces that may act on the gyroscope  415 . The rotation signal generated by the rotation simulator  430  is at the same phase and frequency as the Coriolis forces. The rotation simulator  430  is capable of generating a rotation signal which can simulate any motion of the gyroscope  415 . 
     The signal adder  425  combines the rotation signal with the output signal from the sense motion closed-loop  420  to create the sense force (τ sense) provided to the gyroscope  415 . The sense force (τ sense) is then rebalanced by the sense motion closed-loop  420  as if the rotation signal were an actual rotation force. 
     The demodulation circuit  405  receives the output signal from the sense motion closed-loop and the drive signal (θ drive) from the gyroscope  415 . The demodulation circuit.  405  demodulates these signals to generate a rotation output signal. The rotation output signal is proportional to the simulated Coriolis force generated by the rotation simulator  430 . Thus, the gyroscope  415  may be calibrated without the use of a rotation table. Of course, the present system may be used with a single gyroscope  415  or with a complete three-axis system. 
     Numerous variations and modifications of the invention will become readily apparent to those skilled in the art. Accordingly, the invention may be embodied in other specific forms without departing from its spirit or essential characteristics.