Patent Publication Number: US-2005132805-A1

Title: Capacitance accelerometer having compensation electrode

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an accelerometer, more particularly, which has compensation electrodes arranged at both ends of a mass to displace a mass thereby equalizing initial capacitances at the both ends of the mass.  
      2. Description of the Related Art  
      An accelerometer is known as a Micro Electro Mechanical System (MEMS) device. MEMS devices indicate microscale mechanical devices that are electrically controlled and measured, in which the MEMS is a technique for fabricating mechanical and electrical devices through the semiconductor process.  
      Various accelerometers capable of measuring acceleration are being currently developed, and adopted in vehicle air bag systems, Anti-lock Brake Systems (ABS) and general vibrometers. The accelerometers are mainly fabricated through the semiconductor process, and classified into piezoelectric, piezoresistant and capacitance accelerometers. Piezoelectric accelerometers are commercially retrogressing since it is difficult to prepare piezoelectric thin films of excellent properties without static characteristics. Further, piezoresistant accelerometers show a wide range of characteristic change according to temperature variation, which is hardly compensated. Therefore, the current technical trend is inclined to capacitance accelerometers.  
      The capacitance accelerometers have very excellent characteristics: A capacitance accelerometer shows a small level of characteristic change according to temperature variation, allows a field effect transistor of a high integrity to constitute a signal processing circuit without additional processes, and can be prepared at low cost.  
       FIG. 1  is a structural view illustrating a typical accelerometer. As shown in  FIG. 1 , a conventional capacitance accelerometer  1  includes a floating mass  10  as a movable structure, suspension beams  22  and  24  functioning as springs of a mechanical stiffness for elastically supporting both ends of the mass  10 , a plurality of movable electrode fingers  12  and  14  extended outward from the mass  10  into a bilaterally symmetrical configuration seen in the drawing, a plurality of fixed electrode fingers  32  and  34  fixed to both electrode-fixing sections  30   a  and  30   b  and spaced from the movable electrode fingers  12  and  14  to a predetermined gap and beam-fixing sections  20   a  and  20   b  for fixing the suspension beams  22  and  24  to the bottom of an insulation board. The movable electrode fingers  12  and  14  are adapted to maintain a fixed gap from the fixed electrode fingers  32  and  34  unless any acceleration is applied from the outside so as to keep a predetermined value of capacitance.  
      The reference numeral  19  designates an etching hole for introducing etching solution therethrough.  
      Upon application of an external force to the accelerometer  1 , the mass  10  is displaced in the direction of the force or the y-axial direction (i.e., the vertical direction seen in the drawing), pulling the movable electrode fingers  12  and  14  fixed thereto in the y-axial direction. This as a result increases and decreases the gaps g 1  and g 2  from the movable electrode fingers  12  and  14  to the fixed electrode fingers  32  and  34 , indicating the displacement of the mass  10 .  
      This changes the capacitance between the movable electrode fingers  12  and  14  and the fixed electrode fingers  32  and  34 . The change of capacitance is induced in the form of current into the movable electrode fingers  12  and  14  according to a sensing voltage applied to the fixed electrode fingers  32  and  34 , and the current is converted into a voltage and then amplified with an amplifier (not shown) connected to the movable electrode fingers  12  and  14  so that the external acceleration can be measured.  
      In the accelerometer  1 , the movable electrode fingers  12  and  14  alternate with the fixed electrode fingers  32  and  34  in the form of combs to further increase the change of capacitance with respect to the acceleration. With respect to the acceleration in a direction (e.g., the upward direction in the drawing), the movable electrode fingers  12  become nearer to the fixed electrode fingers  32  in the left of the drawing to increase the capacitance C 1  with relation to the initial capacitance Col as expressed in Equation 1 and the movable electrode fingers  14  in the right of the drawing move away from the fixed electrode fingers  34  to decrease the capacitance C 2  with relation to the initial capacitance C 02  as expressed in Equation 2: 
 
 C   1   =C   01   +AC   0   Equation 1, and 
 
 C   2   =C   02   −AC   0   Equation 2. 
 
      Therefore, in order to obtain a differential value ACT twice of the change of capacitance, a differential circuit is provided according to Equation 3 below: 
 
 ACT=C   1   −C   2 =2 AC   0   Equation 3. 
 
      The change of capacitance of the accelerometer is doubled with the differential circuit to obtain a larger positive output signal. Based upon this, the capacitance can be converted with a C-V converter into voltage, and amplified if necessary to obtain an amplification signal.  
      Also, as shown in  FIG. 2 , initial capacitances C 01  and C 02  between the movable electrodes  12  and  14  and the fixed electrodes  32  and  34  can be expressed as in Equation 4 below: 
 
 C   01  or  C   02 ={(ε× h×L/d   1 )−(ε× h×L/d   2 )}× N   Equation 4, 
          wherein ε is permittivity, h is the height of the electrode fingers, L is the length of an intersecting portion of the electrode fingers, d 1  and d 2  are the distances between adjacent electrode fingers, and N is the number of the electrode fingers.        

      As can be seen from Equation 4, the initial capacitances C 01  and C 02  are proportional to the height h, the length L and the electrode number N, and inverse proportional to the finger-to-finger distances d 1  and d 2 .  
      If errors are made in the distances d 1  and d 2  between the movable electrode finger  12  and the fixed electrode finger  32  and between the movable electrode finger  14  and the fixed electrode finger  34  during the fabrication of the accelerometer  1 , the left and right initial capacitances C 01  and C 02  become different from each other.  
      If the initial capacitances C 01  and C 02  become different from each other, an offset of a reference voltage V ST  and an output voltage V OUT  is generated where the mass  10  having the movable electrodes  12  and  14  is stopped because the output voltage of the accelerometer circuit is obtained according to Equation 5 below: 
 
 V   OUT   =V   ST   +{V   ST ×( C   01   −C   02 )/ C   F   }×G   Equation 5, 
          wherein V OUT  is an output voltage, V ST  is a reference voltage, C 01  and C 02  are left and right initial capacitances, C F  is the capacitance of a feedback capacitor which is provided in an amplifier to influence amplification rate as well as to function as a filter, and G indicates the gain of the amplifier connected to a circuit output terminal.        

      If the initial capacitance C 01  between the movable electrode fingers  12  and the fixed electrode fingers  32  in the left becomes different from the initial capacitance C 02  between the movable electrode fingers  14  and the fixed electrode fingers  34  in the right owing to process errors in the fabrication of the accelerometer  1 , it is necessary to perform compensation in order to equalize the capacitances so that the difference between the initial capacitances C 01 −C 02  becomes zero.  
      However, a conventional compensation approach for equalizing the initial capacitances C 01  and C 02  arrays very small capacitances of capacitors in a circuit of the accelerometer, and the capacitors are trimmed through switching on/off to perform compensation. Therefore, this approach complicates an array structure of additionally arranged elements such as the capacitors in the circuit as well as an adjustment operation for equalizing the initial capacitances C 01  and C 02 .  
     SUMMARY OF THE INVENTION  
      Therefore the present invention has been made to solve the foregoing problems of the prior art.  
      It is an object of the present invention to provide a capacitance accelerometer capable of simply compensating initial capacitances measured between movable and fixed electrodes arranged at both ends of a mass into an equal value in order to obtain a correct output voltage.  
      According to an aspect of the invention for realizing the object, there is provided an accelerometer capable of compensating initial capacitances comprising: a horizontally movable floating mass; support beams extended from a beam-fixing section to elastically support both ends of the mass; movable electrodes extended outward from both sides of the mass to a predetermined length; fixed electrodes extended from electrode-fixing sections to a predetermined length, and alternating with the movable electrodes with a predetermined gap; and compensation electrode sections for displacing the mass in a moving direction of the mass to equalize an initial capacitance between the movable and fixed electrodes at one side with that between the movable and fixed electrodes at the other side.  
      It is preferred that the support beams are elastic bodies for connecting the mass with the beam-fixing section which is arranged in an opening formed in a central portion of a body of the mass.  
      It is preferred that the support beams are elastic bodies for connecting the mass with the beam-fixing sections arranged adjacent to the both ends of the mass.  
      It is also preferred that the compensation electrode sections include: at least one movable compensation electrode extended outward from the both ends of the mass to a predetermined length; at least one fixed compensation electrode arranged parallel with the movable compensation electrode at a predetermined gap to generate electrostatic force for attracting the movable compensation electrode at application of electric power; and compensation electrode-fixing sections fixed adjacent to the both ends of the mass to power the fixed compensation electrode extended toward the mass to a predetermined length.  
      It is more preferred that the movable and fixed compensation electrodes are comb-shaped electrode members which are extended to a predetermined length in the moving direction of the mass.  
      It is more preferred that the movable and fixed compensation electrodes are comb-shaped compensation electrode members which alternate with each other with a uniform gap.  
      It is also preferred that the compensation electrode sections include a control unit for controlling the movement of the mass, wherein the control unit includes a comparison section for comparing the initial capacitance between the movable and fixed electrodes at one side with that between the movable and fixed electrodes at the other side and a voltage-applying section for selectively applying voltage to a pair of compensation electrode-fixing sections until the comparison value becomes zero.  
      It is preferred that the compensation electrode sections are separately provided adjacent to the both ends of the mass.  
      It is more preferred that one of the movable and fixed compensation electrodes has at least one projection which contacts a body of an opposed electrode in the deformation of thereof.  
      It is more preferred that the projection is extended in the form of a prism to perform point contact with the corresponding movable or fixed compensation electrode.  
      Also, it is more preferred that the projection is extended in the form of a semicylinder to perform line contact with the corresponding movable or fixed compensation electrode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a structural view illustrating a typical accelerometer;  
       FIG. 2  is an enlarged perspective view illustrating the gap variation between movable electrode fingers and fixed electrode fingers in a general accelerometer;  
       FIG. 3  is a structural view illustrating a capacitance accelerometer having compensation electrodes according to a first embodiment of the invention;  
       FIG. 4  is a perspective view of the accelerometer taken along a line A-A′ in  FIG. 3 ;  
       FIG. 5  is a structural view illustrating a capacitance accelerometer having compensation electrodes according to a second embodiment of the invention; and  
       FIGS. 6A and 6B  are perspective views illustrating projections in the capacitance accelerometer having compensation electrodes according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Hereinafter the present invention will now be described in detail with reference to the accompanying drawings.  
       FIG. 3  is a structural view illustrating a capacitance accelerometer having compensation electrodes according to a first embodiment of the invention, and  FIG. 4  is a perspective view of the accelerometer taken along a line A-A′ in  FIG. 3 .  
      As shown in  FIGS. 3 and 4 , an accelerometer  100  of the invention is designed to compensate initial capacitances at both ends if different owing to design errors into the same value in order to more precisely measure external acceleration in the movement of the mass, and includes a mass  110 , movable electrode fingers  112  and  114 , support beams  122  and  124 , fixed electrode fingers  132  and  134  and compensation electrode sections  140   a  and  140   b.    
      The mass  110  has a horizontally movable structure which is suspended by an underlying sacrificial layer, and the support beams  122  and  124  are arranged at both ends of the mass  110  to elastically support the mass  110  in a fashion movable in the y-axial direction in the drawing. The support beams  122  and  124  are of elastic bodies such as a leaf spring of a desired mechanical elastic modulus, and extended toward the mass  110  from a beam-fixing section  120  fixed in position to the bottom.  
      The mass  110  has an opening  111  perforated in a central portion thereof as shown in  FIG. 3 , and the support beams  122  and  124  may be of elastic bodies for connecting the beam-fixing section  120  in the opening  111  with the mass  110 .  
      Further, the support beams  122  and  124  may be provided in an alternative accelerometer  10   a , as shown in  FIG. 5 , which includes beam-fixing sections  120   a  and  120   b  adjacent to both ends of a mass  110 , the support beams  122  and  124  of elastic bodies extended from the beam-fixing sections  120   a  and  120   b  to the mass  110  to connect between the same, and compensation electrode sections  140   a  and  140   b  arranged at the both ends of the mass  110 .  
      The movable electrode fingers  112  and  114  moving along with the mass  110  are of a plurality of comb-shaped electrode members which are extended outward from both sides of the mass  110  to a predetermined length in a direction perpendicular with respect to the displacement of the mass  110  (e.g., the y-axial direction in the drawing).  
      The fixed electrode fingers  132  and  134  alternating with the movable electrode fingers  112  and  114  are of a plurality of comb-shaped electrode members which are extended from electrode-fixing sections  130   a  and  130   b  fixed at both sides of the mass  110  toward the mass  110  to a predetermined length, and have a predetermined gap from the movable electrode fingers  112  and  114 .  
      The movable electrode fingers  112  and  114  and the fixed electrode fingers  132  and  134  alternate with each other along the moving direction of the mass  110 , and are so structured that the upward movement of the mass  110  under the external force narrows the gap d 1  between one of the movable electrode fingers  112  and  114  and an adjacent one of the fixed electrode fingers  132  and  134  to increase the capacitance while widening the gap d 2  between the fixed electrode finger  132  or  134  and another one of the movable electrode fingers  112  and  114  to decrease the capacitance. The change of capacitance between the movable and fixed electrode fingers  112  and  132  placed in the left of the drawing shows an opposite aspect from that between the movable and fixed electrode fingers  114  and  134  in the placed in the right of the drawing.  
      The compensation electrode sections  140   a  and  140   b  are adapted to displace the mass  110  in the y-axial direction so that the initial capacitance C 01  between the left side movable and fixed electrode fingers  112  and  132  becomes the same as the capacitance C 02  between the right side movable and fixed electrode fingers  114  and  134 .  
      The compensation electrode sections  140   a  and  140   b  are separately provided adjacent to upper and lower ends of the mass  110  to potentially displace the mass  110  supported by the support beams  122  and  124  upward or downward in the drawing.  
      The compensation electrode sections  140   a  and  140   b  are provided at the both ends of the mass  110  to generate external force capable of displacing the mass  110  upward or downward when electric power is applied. Each of the compensation electrode sections  140   a  and  140   b  includes at least one movable compensation electrode  141  extended outward from the end of the mass  110  to a predetermined length, at least one fixed compensation electrode  142  extended toward the mass  110  to a predetermined length and arranged parallel with the movable compensation electrode  141  at a predetermined gap to generate electrostatic force for attracting the movable compensation electrode  141  when powered, and a compensation electrode-fixing section  143  fixed adjacent to the end of the mass  110  to apply electric power to the fixed compensation electrode  142 .  
      The movable and fixed compensation electrodes  141  and  142  are of comb-shaped electrode members which are extended in the moving direction of the mass  110  to a predetermined length, in an alternating fashion at a uniform gap.  
      The compensation electrode sections  140   a  and  140   b  include a control unit  150  for controlling bias voltage as external electric power applied to the compensation electrode-fixing sections  143  for displacing the mass  110  at compensation of the initial capacitances C 01  and C 02  measured in the left and right sides.  
      The control unit  150  includes measuring sections  151   a  and  151   b  for measuring the initial capacitance C 01  generated between the movable electrode fingers  112  and the fixed electrode fingers  132  in the left from the mass  110  movable in the y-axial direction and the initial capacitance C 02  generated between the movable electrode fingers  114  and the fixed electrode fingers  134  in the right from the mass  110 , a comparison section  152  for comparing the measured initial capacitances C 01  and C 02  received from the measuring sections  151   a  and  151   b  to obtain a comparison value and voltage-applying sections  153   a  and  153   b  for selectively applying voltages to the compensation electrode-fixing sections  143  of the upper and lower compensation electrode sections  140   a  and  140   b  to displace the mass  110  in the y-axial direction until the comparison value obtained in the comparison section  150  becomes zero.  
      The compensation electrode sections  140   a  and  140   b  are separately arranged adjacent to the both ends of the mass  110  to receive desired levels of electric power from the voltage-applying sections  153   a  and  153   b  to displace the mass  110  forward or backward in the axial direction. If the comparison value between the initial capacitances C 01  and C 02  becomes zero, uniformly adjusted electric power is supplied through the voltage-applying sections  153   a  and  153   b  without additional change of voltage.  
       FIGS. 6A and 6B  are perspective views illustrating projections in the capacitance accelerometer having compensation electrodes according to the invention.  
      As shown in  FIGS. 6A and 6B , projections  144  are extended outward from the movable compensation electrode  141  or the fixed compensation electrode  142  formed in the movable mass  110  to locally contact opposed fixed or movable compensation electrodes in the deformation of the electrode bodies under the external environment.  
      It is preferred that the projections  144  are extended in the form of prisms to perform point contact with the corresponding movable or fixed compensation electrode  141  or  152 . Alternatively, the projections  144  may be extended in the form of a semicylinder to perform line contact with the corresponding movable or fixed compensation electrode  141  or  142 .  
      When any of the movable and fixed compensation electrodes  141  and  142  is deformed narrowing the gap therebetween, the projections  144  on one of the movable and fixed compensation electrodes  141  and  142  perform point or line contact with the outside surface of an opposed one of the compensation electrodes  141  and  142  to prevent the adhesion between the electrodes  141  and  142  through surface contact so that the displacement of the mass  110  in the y-axial direction is not obstructed.  
      The movement of the mass  110  is not restricted to the y-axial direction as shown in FIGS.  3  to  5 , but the mass  110  may be displaced in x- and y-axial directions according to the position of the accelerometer  1  mounted on a board. The movable and fixed electrodes  112 ,  114 ,  132  and  134  associated with the mass  110  may be arranged above and under the mass  110 , and the compensation electrode sections  140   a  and  140   b  may be arranged respectively adjacent to both ends of the mass  110  to displace the mass  110  in the x- and/or y-axial directions.  
      If external force is applied to the accelerometer  100 , the mass  110  of a movable structure is displaced in the y-axial direction, that is, upward or downward in the drawing perpendicular with respect to the electrode-fixing sections  130   a  and  130   b  under the force of inertia.  
      As a result, the gap between the movable electrode fingers  112  in the left of the mass  110  and the fixed electrode fingers  132  in the left electrode-fixing section  130   a  is narrowed to increase the capacitance C 1  as in Equation 1 above, but the gap between the movable electrode fingers  114  in the right of the mass  110  and the fixed electrode fingers  134  in the right electrode-fixing section  130   b  is widened to decrease the capacitance C 2  as in Equation 2 above.  
      The change of capacitance generated from the accelerometer is processed with a differential circuit as expressed in Equation 3 above into a differential value ACT twice of the change of capacitance, which in turn is converted with a C-V converter into voltage to measure the external acceleration.  
      In order to obtain the maximum differential value ACT with the differential circuit, the initial capacitances C 01  and C 02  measured in the left and right sides should be equal. Errors generated during the fabrication of the accelerometer  100  cause the movable electrode fingers  112  and  114  and the fixed electrode fingers  132  and  134  to have uneven thickness and thus irregular gap so that the initial capacitance C 01  measured in the left measuring section  151   a  becomes different from the initial capacitance C 02  measured in the right measuring section  151   b.    
      The comparison section  152  compares the measured initial capacitances C 01  and C 02  received from the measuring sections  151   a  and  151   b  to obtain a comparison value. If the comparison value is positive (+) or the left initial capacitance C 01  is larger than the right initial capacitance C 02 , the comparison section  152  widens the gap between the movable electrode fingers  112  and the fixed electrode fingers  132  in the left to decrease the initial capacitance C 01  while narrowing the gap between the movable electrode fingers  114  and the fixed electrode fingers  134  in the right to relatively increase the initial capacitance C 02  so that the comparison value becomes zero.  
      When bias voltage is applied through the voltage-applying section  153   b  that is electrically connected to the compensation electrode-fixing section  143  of the lower one of the compensation electrode sections  140   a  and  140   b  provided respectively above and under the mass  110 , electrostatic force is generated between the fixed compensation electrode  142  of the compensation electrode-fixing section  143  and the movable compensation electrode  141  of the mass  110  to place the mass  110  downward to equally compensate the left and right initial capacitances.  
      If the comparison value of the left and right initial capacitances C 01  and C 02  becomes zero, the comparison section  152  stops application of the bias voltage to the compensation electrode-fixing section  143  via the voltage-applying sections  153   a  and  153   b  so that adjusted voltage is uniformly supplied.  
      According to the present invention as set forth above, the compensation electrode sections capable of displacing the mass in the moving direction thereof at application of voltage are provided respectively at both ends of the mass so that the different initial capacitances measured above and under or in the left and right of the mass resulting from process errors generated during the fabrication of the accelerometer can be simply compensated to an equal value. As a result, unlike the prior art requiring a complicated structure of capacitors for adding capacitances to a circuit in a board to perform compensation or a complicated compensation process, the present invention can simplify the overall structure of the accelerometer as well as perform the compensation more simply.  
      While the present invention has been shown and described in connection with the preferred embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.