Patent Publication Number: US-2006010978-A1

Title: MEMS gyroscope having coupling springs

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of Korean Patent Application No. 2004-56011, filed Jul. 19, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated in its entirety herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a micro electronic mechanical system (MEMS) gyroscope, and more particularly to a MEMS gyroscope having coupling springs capable of being driven in a vertical direction to synchronize with a driving resonant frequency.  
      2. Description of the Related Art  
      In general, the gyroscopes for detecting a rotational angular velocity have been used as a core part of navigation devices for vessels, aircrafts, and so on, since long ago. With the development of MEMS technology, gyroscopes are used even as navigation devices for cars or devices for vehicles or the anti-shake feature of high-magnification video cameras.  
      Such gyroscopes are devices that use the principle of generating the Coriolis force in the third direction perpendicular to the first and second axis directions, when applied with a rotational force of constant angular velocity in the second axis direction vertical to a mass body invariably vibrating in the first axis direction, and convert displacement of the sensing mass body caused by the Coriolis force into a capacitance change and detect a rotational angular velocity.  
      In order to generate and detect the Coriolis force, the gyroscopes are provided therein with sensing electrodes and mass bodies capable of vibrating in predetermined directions. Hereinafter, ‘driving direction’ refers to a direction in which mass bodies vibrate in the gyroscope, ‘input direction’ refers to a direction in which rotational angular velocity is inputted or applied to the gyroscope, and ‘sensing direction’ refers to a direction in which the Coriolis force generated on the mass body is sensed.  
      The driving, input, and sensing directions are set to be perpendicular to one another in space. Usually, gyroscopes using MEMS technology have coordinate axes in three directions consisting of two directions (hereinafter, referred to as a ‘horizontal direction’) parallel with the surface of a substrate and perpendicular to each other and a direction (hereinafter, referred to as a ‘vertical direction’) perpendicular to the surface of the substrate.  
      In general, gyroscopes are classified into gyroscopes of horizontal type (Z-axis) and gyroscopes of vertical type (X-axis or Y-axis). Z-axis gyroscopes have the horizontal direction as the driving and sensing directions and the vertical direction as the input direction (Z-axis), and X-axis or Y-axis gyroscopes have the X-axis or Y-axis direction as the input direction.  
      Such conventional gyroscopes have a problem in that the external application of acceleration is treated in the same way as the application of angular velocity. In order to solve such a problem, tuning fork gyroscopes have been proposed as shown in  FIG. 1 . If angular velocity is externally applied, a plurality of mass bodies in the tuning fork gyroscope move in different directions, and, if acceleration is externally applied, the tuning fork gyroscope moves the plurality of mass bodies in the same direction. Therefore, the tuning fork gyroscope can prevent the external application of acceleration from being treated in the same way as the external application of angular velocity. However, the tuning fork gyroscope shown in  FIG. 1  has a problem of low yields due to the difficulties in synchronizing the driving frequency for driving the plurality of mass bodies.  
      In order to solve this problem, a tuning fork gyroscope has been proposed that uses coupling beams as shown in  FIG. 2 . However, this tuning fork gyroscope is able to drive only on a plane surface.  
     SUMMARY OF THE INVENTION  
      An aspect of the present invention addresses the above problems and/or disadvantages and provides at least the advantages described below. Accordingly, an aspect of the present invention is to provide a MEMS gyroscope having coupling springs to enable driving in a vertical direction.  
      In order to achieve the above-described aspects of the present invention, there is provided a MEMS gyroscope comprising a plurality of mass bodies symmetrical to one another and relatively movable with respect to a substrate in a direction vertical to a surface of the substrate; and a coupling spring connected between the plurality of mass bodies and moving one of the mass bodies in a vertical direction as another one of the mass bodies moves in an opposite vertical direction.  
      Further, the MEMS gyroscope comprises first and second driving electrodes for vibrating the plurality of mass bodies in the vertical direction; first and second mass bodies relatively movable with respect to the substrate in a horizontal direction, being one of directions parallel with the surface of the substrate, and moving in the horizontal direction by the Coriolis force generated by an application of angular velocity; and first and second sensing electrodes measuring displacement of the first and second mass bodies in the horizontal direction.  
      The first driving electrode, first mass body, and first sensing electrode are symmetrical to the second driving electrode, second mass body, and second sensing electrode, respectively, with respect to the coupling spring.  
      The driving and sensing electrodes are formed in a comb structure.  
      The MEMS gyroscope further comprises a vertical sensing electrode sensing displacement of the plurality of mass bodies in the vertical direction, which are driven by the first and second driving electrodes.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above aspects and features of the present invention will be more apparent by describing certain exemplary embodiments of the present invention with reference to the accompanying drawings, in which:  
       FIG. 1  is a view for schematically showing a conventional tuning fork gyroscope;  
       FIG. 2  is a top view showing a tuning fork gyroscope using coupling beams; and  
       FIG. 3  is a top view showing a MEMS gyroscope provided with coupling springs consistent with an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      Hereinafter, the present invention is described in detail with reference to the accompanying drawings.  
      In the following description, same drawing reference numerals are used for the same elements even in different drawings. The matters described in the description, such as construction details and elements, are those which assist in a comprehensive understanding of the invention. Thus, it is apparent that the present invention can be carried out without these defined matters. Also, well-known functions or constructions are not described in detail since they would obscure the invention with unnecessary detail.  
       FIG. 3  is a top view showing a MEMS gyroscope having coupling springs consistent with an exemplary embodiment of the present invention. In  FIG. 3 , the MEMS gyroscope consistent with an exemplary embodiment of the present invention includes first and second gyroscopes bilaterally symmetrical to each other with respect to a coupling spring.  
      Since the first and second gyroscopes  300   a  and  300   b  in this embodiment are bilaterally symmetrical to each other with respect to the coupling spring  360 , hereinafter, description will be made only on the first gyroscope  300   a  of the first and second gyroscopes  300   a  and  300   b.    
      The first gyroscope  300   a  has a first internal mass body  350   a  and a first external mass body  340   a,  a driving electrode  310   a  for driving the first external mass body  340   a,  horizontal sensing electrodes  320   a  for sensing displacement in the horizontal direction of the first internal mass body  350   a,  vertical sensing electrodes  330   a  for sensing displacement in the vertical direction of the first external mass body  340   a,  and a first external spring  345   a  and a first internal spring  355   a  for supporting the first external mass body  340   a  and the first internal mass body  350   a  respectively, which are disposed on a substrate (not shown).  
      The first external mass body  340   a  has a rectangular shape, and movably floats over the substrate (not shown). The left side of the first external mass body  340   a  is fixed on the substrate by the first external spring  345   a,  and the right side of the first external mass body  340   a  is connected to the second gyroscope  300   b  by the coupling spring  360 .  
      The first internal mass body  350   a  has a rectangular shape smaller than the first external mass body  340   a,  and movably floats over the substrate. The first internal mass body  350   a  is disposed within the inner space of the first external mass body  340   a.  The first internal mass body  350   a  is fixed to the first external mass body  340   a  by the first internal spring  355   a.  The first internal spring  355   a  has a leaf-spring shape, so the first internal mass body  350   a  is supported by the first internal spring  355   a  to relatively move in the Y direction with respect to the first external mass body  340   a.    
      The driving electrodes  310   a  are respectively formed on one side of the first external mass body  340   a  in the X direction, and disposed on the same plane as the first external mass body  340   a  on the substrate. The driving electrodes  310   a  consist of a fixed electrode  311   a  and a moving electrode  312   a  which are combined with each other in a comb structure.  
      The horizontal sensing electrodes  320   a  are formed in the inner space of the first internal mass body  350   a.  Like the driving electrodes  310   a,  the horizontal sensing electrodes  320   a  consist of a fixed electrode  321   a  and a moving electrode  322   a  which are combined with each other in a comb structure. The fixed electrode  321   a  is fixed on the substrate (not shown), and the moving electrode  322   a  is fixed on the first internal mass body  350   a.    
      The vertical sensing electrodes  330   a  are formed on the first external mass body  340   a  at the corners furthest from the coupling spring  360 . The vertical sensing electrodes  330   a  consist of a fixed electrode  331   a  and a moving electrode  332   a  which are combined with each other in a comb structure. The fixed electrode  331   a  is fixed on the substrate (not shown), and the moving electrode  332   a  is fixed on the first external mass body  340   a.    
      Hereinafter, description will be made of the operations of the MEMS gyroscope having the above structure consistent with an exemplary embodiment of the present invention.  
      If the driving electrodes  310   a  are applied with a voltage changing with time, the first external mass body  340   a  vibrates in the Z direction driven by an electrostatic force generated by the driving electrodes  310   a.  Because the first internal mass body  350   a  is relatively fixed by the first internal spring  355   a  to the first external mass body  340   a  in the Z direction, the first internal mass body  350   a  vibrates in the Z direction together with the first external mass body  340   a.    
      When the first external mass body  340   a  and first internal mass body  350   a  vibrate in the Z direction, the coupling spring  360  operates like a seesaw so as to move the second external mass body  340   b  and second internal mass body  350   b  in the -Z direction.  
      Thus, the first and second external mass bodies  340   a  and  340   b  vibrate in the vertical direction, having opposite phases, so as to have the mass bodies driven with the same resonant frequency.  
      Although the description has been made of the above exemplary embodiment in which the first external and internal mass bodies  340   a  and  350   a  move in the Z direction and the second external and internal mass bodies  340   b  and  350   b  move in the -Z direction, it is possible to move the first external and internal mass bodies  340   a  and  350   a  in the -Z direction whereas the second external and internal mass bodies  340   b  and  350   b  move in the Z direction.  
      The vertical sensing electrodes  330   a  measure the displacement of the first external mass body  340   a  in the Z direction, and the measured value is supplied to a controller (not shown). The controller controls an electromagnetic field applied to the driving electrodes  310   a  so that the first external mass body  340   a  can effectively vibrate in the Z direction based on the measured value of the vertical sensing electrodes  330   a.    
      If angular velocity is applied to the first external mass body  340   a  in the X direction during the vibrations of the first external and internal mass bodies  340   a  and  350   a,  the first internal mass body  350   a  moves in the X direction together with the first external mass body  340   a  by the first internal spring  355   a.  At this time, the Coriolis force is applied to the first internal mass body  350   a  in the Y direction so that the first internal mass body  350   a  moves in the Y direction. Thus, the distance varies between the fixed electrode  321   a  and the moving electrode  322   a  of the horizontal sensing electrodes  320   a,  which causes the change of capacitance of the horizontal sensing electrodes  320   a  according to such a change of the distance. The controller (not shown) calculates the Coriolis force using the change of the capacitance of the horizontal sensing electrodes  320   a,  thereby enabling calculation of the angular velocity or angular acceleration caused by an external force applied in the X direction.  
      In the present exemplary embodiment, the vibration of the first external mass body  340   a  in the Z direction is controlled by the driving electrodes  310   a  having the fixed electrode  311   a  and moving electrode  312   a  formed on the same plane as that of the first external mass body  340   a.  Thus, the driving electrodes  310   a  can be simultaneously formed during the manufacturing process of the other components such as the first external mass body  340   a,  the first internal mass body  350   a,  and so on. Thus, whole components of the gyroscope can be formed by use of one mask, simplifying the manufacturing process of the gyroscope.  
      Further, since the fixed and moving electrodes  311   a  and  312   a  are disposed on the same plane, the intervening distance can be easily formed to become shorter. Thus, it can be precisely controlled to drive the first external mass body  340   a  and detect the displacement of the first internal mass body  350   a.    
      As described above, the MEMS gyroscope having coupling springs can synchronize with the resonant frequency of the mass bodies that are vertically driven, so as to bring out an advantage of higher yield.  
      The present invention has an advantage of completely removing the influence of the linear acceleration and angular acceleration by disposing the mass bodies to be coincident with the sensing direction.  
      While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.