Patent Publication Number: US-6981415-B2

Title: Reduced start time for MEMS gyroscope

Description:
FIELD 
   The present invention relates generally to MEMS gyroscopes, and more particularly, relates to an improved start time of a MEMS gyroscope. 
   BACKGROUND 
   Microelectromechanical systems (MEMS) integrate electrical and mechanical devices on the same silicon substrate using microfabrication technologies. The electrical components are fabricated using integrated circuit processes, while the mechanical components are fabricated using micromachining processes that are compatible with the integrated circuit processes. This combination makes it possible to fabricate an entire system on a chip using standard manufacturing processes. 
   One common application of MEMS is the design and manufacture of sensor devices. The mechanical portion of the device provides the sensing capability, while the electrical portion processes the information obtained by the mechanical portion. One example of a MEMS sensor is a MEMS gyroscope. 
   A type of MEMS gyroscope uses a vibrating element to sense angular rate through the detection of a Coriolis acceleration. The vibrating element is put into oscillatory motion in the X-axis (drive plane), which is parallel to the substrate. Once the vibrating element is put in motion, it is capable of detecting angular rates induced by the substrate being rotated about the Z-axis (input plane), which is parallel to the substrate. The Coriolis acceleration occurs in the Y-axis (sense plane), which is perpendicular to both the X-axis and the Z-axis. The Coriolis acceleration produces a Coriolis motion that has an amplitude that is proportional to the angular rate of the substrate. 
   The start time of a device is the time required to produce a usable output after power application. A typical MEMS gyroscope takes between one and two seconds to start. There are MEMS gyroscope applications that require a faster start time. For example, some inertial measurement units (IMUs) that include one or more MEMS gyroscopes may require a start time of one second or less. 
   Therefore, it would be desirable to have a MEMS gyroscope that starts in one second or less. 
   SUMMARY 
   A MEMS gyroscope system and method for improving the start time of a MEMS gyroscope comprising of a MEMS gyroscope and a bias power source providing a first value of voltage to a first side of the MEMS gyroscope and a second value of voltage to a second side of the MEMS gyroscope is disclosed. The bias power source may also provide a voltage to drive electronics connected to the MEMS gyroscope. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Presently preferred embodiments are described below in conjunction with the appended drawing figures, wherein like reference numerals refer to like elements in the various figures, and wherein: 
       FIG. 1  is a plan view of a MEMS gyroscope, according to an exemplary embodiment; 
       FIG. 2  is a plan view of a MEMS gyroscope system, according to an exemplary embodiment; and 
       FIG. 3  is a plan view of a MEMS gyroscope system, according to another exemplary embodiment. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a plan view of a microelectromechanical system (MEMS) gyroscope  100  according to an exemplary embodiment. While  FIG. 1  shows the MEMS gyroscope  100  as a tuning fork gyroscope, other MEMS gyroscopes that use the Coriolis acceleration to detect rotation, such as an angular rate sensing gyroscope, may also be used. The MEMS gyroscope  100  may be formed on a substrate and may include at least one proof mass  102   a ,  102   b ; a plurality of support beams  104 ; at least one cross beam  106   a ,  106   b ; at least one motor drive comb  108   a ,  108   b ; at least one motor pickoff comb  110   a ,  110   b ; at least one sense plate  112   a ,  112   b ; and at least one anchor  114   a ,  114   b.    
   The at least one proof mass  102   a ,  102   b  may be any mass suitable for use in a MEMS gyroscope system. In a preferred embodiment, the at least one proof mass  102   a ,  102   b  is a plate of silicon. Other materials that are compatible with micromachining techniques may also be employed.  FIG. 1  shows two proof masses; however, one or more proof masses may be employed. 
   The at least one proof mass  102   a ,  102   b  may be located substantially between the at least one motor drive comb  108   a ,  108   b  and the at least one motor pickoff comb  110   a ,  110   b . The at least one proof mass  102   a ,  102   b  may contain a plurality of comb-like electrodes extending towards both the at least one motor drive comb  108   a ,  108   b  and the at least one motor pickoff comb  110   a ,  111   b . While the at least one proof mass  102   a ,  102   b  has ten electrodes as depicted in  FIG. 1 , the number of electrodes on the at least one proof mass  102   a ,  102   b  may be more or less than ten. 
   The at least one proof mass  102   a ,  102   b  may be supported above the at least one sense plate  112   a ,  112   b  by the plurality of support beams  104 . While eight support beams  104  are depicted in  FIG. 1 , the number of support beams used may be more or less than eight. The plurality of support beams  104  may be beams micromachined from a silicon wafer. The plurality of support beams  104  may act as springs allowing the at least one proof mass  102   a ,  102   b  to move within the drive plane (X-axis) and the sense plane (Y-axis). (See  FIG. 1  for axis information.) 
   The plurality of support beams  104  may be connected to at least one cross beam  106   a ,  106   b . The at least one cross beam  106   a ,  106   b  may be connected to at least one anchor  114   a ,  114   b  providing support for the MEMS gyroscope  100 . The at least one anchor  114   a ,  114   b  may be connected to the underlying substrate. While two anchors  114   a ,  114   b  are depicted in  FIG. 1 , the number of anchors may be more or less than two. The at least one anchor  114   a ,  114   b  may be positioned along the at least one cross beam  106   a ,  106   b  in any manner that provides support to the MEMS gyroscope  100 . 
   The at least one motor drive comb  108   a ,  108   b  may include a plurality of comb-like electrodes extending towards the at least one proof mass  102   a ,  102   b . While the at least one motor drive comb  108   a ,  108   b  has four electrodes as depicted in  FIG. 1 , the number of electrodes on the at least one motor drive comb  108   a ,  108   b  may be more or less than four. The number of the electrodes on the at least one motor drive comb  108   a ,  108   b  may be determined by the number of electrodes on the at least one proof mass  102   a ,  102   b.    
   The plurality of interdigitated comb-like electrodes of the at least one proof mass  102   a ,  102   b  and the at least one motor drive comb  108   a ,  108   b  may form capacitors. The at least one motor drive comb  108   a ,  108   b  may be connected to drive electronics, not shown in  FIG. 1 . The drive electronics may cause the at least one proof mass  102   a ,  102   b  to oscillate at substantially a tuning fork frequency along the drive plane (X-axis) by using the capacitors formed by the plurality of interdigitated comb-like electrodes of the at least one proof mass  102   a ,  102   b  and the at least one motor drive comb  108   a ,  108   b.    
   The at least one motor pickoff comb  110   a ,  110   b  may include a plurality of comb-like electrodes extending towards the at least one proof mass  102   a ,  102   b . While the at least one motor pickoff comb  110   a ,  110   b  has four electrodes as depicted in  FIG. 1 , the number of electrodes on the at least one motor pickoff comb  110   a ,  110   b  may be more or less than four. The number of the electrodes on the at least one motor pickoff comb  110   a ,  110   b  may be determined by the number of electrodes on the at least one proof mass  102   a ,  102   b.    
   The plurality of interdigitated comb-like electrodes of the at least one proof mass  102   a ,  102   b  and the at least one motor pickoff comb  110   a ,  110   b  may form capacitors, which may allow the MEMS gyroscope  100  to sense motion in the drive plane (X-axis). 
   The at least one sense plate  112   a ,  112   b  may form a parallel capacitor with the at least one proof mass  102   a ,  102   b . If an angular rate is applied to the MEMS gyroscope  100  along the input plane (Z-axis) while the at least one proof mass  102   a ,  102   b  is oscillating along the drive plane (X-axis), a Coriolis force may be detected in the sense plane (Y-axis). The parallel capacitor may be used to sense motion in the sense plane (Y-axis). The output of the MEMS gyroscope  100  may be a signal proportional to the change in capacitance. The at least one sense plate  112   a ,  112   b  may be connected to sense electronics, not shown in  FIG. 1 . The sense electronics may detect the change in capacitance as the at least one proof mass  102   a ,  102   b  moves towards and/or away from the at least one sense plate  112   a ,  112   b.    
     FIG. 2  shows a plan view of a MEMS gyroscope system  200 . The MEMS gyroscope system  200  may include a MEMS gyroscope  216  and a bias power source  218 . The MEMS gyroscope system may also include sense electronics, drive electronics, a system power source, and other typical operational electronics, which are not shown in  FIG. 2  for the sake of simplification. The drive electronics and the system cower source are depicted in  FIG. 3 . The MEMS gyroscope  216  may be substantially the same as the MEMS gyroscope  100  as depicted in  FIG. 1 . The bias power source  218  may be a battery, a super capacitor, or any other power source operable to provide a substantially continuous source of power. In a preferred embodiment, a long life battery is employed. 
   To start the MEMS gyroscope system  200 , the system power source may provide power to the MEMS gyroscope  216 . The system power source may be any power source used to power a typical MEMS gyroscope. For example, the system power source may be the power source for an avionics system that includes at least one MEMS gyroscope. The system power source may provide power based upon the system application. The system power source typically provides power in the range of 5 to 1000 volts; however, this embodiment is not limited to that range. 
   When the system power source is applied to the MEMS gyroscope  216 , the parallel capacitor formed by the at least one sense plate  212   a ,  212   b  and the at least one proof mass  202   a ,  202   b  may begin charging. The charge time of the parallel capacitor may be inversely proportional to the product of the circuit resistance and the circuit capacitance. This charge time may impact the start time of the MEMS gyroscope system  200 . For example, the longer it takes for the parallel capacitor to charge, the longer the delay may be from the time when the system power source is applied to when the MEMS gyroscope system  200  may provide meaningful angular rate detection data. 
   To reduce the start time of the MEMS gyroscope system  200 , the bias power source  218  may provide a substantially continuous source of voltage to the MEMS gyroscope  216 . The bias power source  218  may provide a first value of voltage to a first side of the MEMS gyroscope  216  and a second value of voltage to a second side of the MEMS gyroscope  216 . In a preferred embodiment, the first value of voltage has a magnitude equal to and a polarity opposite of the second value of voltage. For example, the first value of voltage may be +5 volts and the second value of voltage may be −5 volts. However, the first value of voltage may be a different magnitude than the second value of voltage, and the first and second voltages may have the same polarity. 
   In a preferred embodiment the first value of voltage may be applied to a first sense plate  212   a  of the MEMS gyroscope  216  and the second value of voltage may be applied to a second sense plate  212   b  of the MEMS gyroscope  216 . However, other components of the MEMS gyroscope  216  may receive the substantially continuous source of voltage, such as the at least one motor drive comb  208   a ,  208   b  or the at least one motor pickoff comb  210   a ,  210   b . Alternatively, the bias power source  218  may apply the first value of voltage to more than one component on the first side on the MEMS gyroscope  216  and may apply the second value of voltage to more than one component on the second side of the MEMS gyroscope  216 . For example, the first value of voltage may be applied to a first motor drive comb  208   a  and the first sense plate  212   a , and the second value of voltage may be applied to a second motor drive comb  208   b  and the second sense plate  212   b.    
   By keeping the substantially continuous voltage applied to the at least one sense plate  212   a ,  212   b , the charge time of the parallel capacitors formed by the at least one sense plate  212   a ,  212   b  and the at least one proof mass  202   a ,  202   b  may be reduced. The charge time of the parallel capacitors may be substantially eliminated if the bias power source  218  provides power that is substantially equal in magnitude and polarity as the system power source to the first sense plate  212   a , and substantially equal in magnitude and opposite polarity as the system power source to the second sense plate  212   b . For this example, assume that the system power source provides +5 volts to the MEMS gyroscope system  200 . The charge time of the parallel capacitors may be substantially eliminated if the bias power supply  218  applies +5 volts to the first sense plate  212   a  and −5 volts to the second sense plate  212   b.    
   The charge time of the parallel capacitors may also be substantially reduced if the bias power source  218  provides voltage that is less in magnitude than the system power source. The bias power source  218  may provide less voltage than the system power source by design or because the bias power source  218  has degraded over time. For example, the MEMS gyroscope system  200  application may require a faster start time, but may also have space and temperature constraints that require a smaller battery. 
   Alternatively, the MEMS gyroscope system  200  application may require the MEMS gyroscope  216  to be placed in storage for many years. The bias power source  218  may be a battery designed to continuously provide voltage substantially equal in magnitude as provided by the system power source. Over time the battery may degrade and may provide substantially less voltage than the system power source provides. For this example, assume that the system power source provides +5 volts to the MEMS gyroscope system  200 . The charge time of the parallel capacitors may be substantially reduced if the bias power supply  218  applies +3 volts to the first sense plate  212   a  and −3 volts to the second sense plate  212   b.    
   By reducing the charge time of the parallel capacitors formed by the at least one sense plate  212   a ,  212   b  and the at least one proof mass  202   a ,  202   b , the start time of the MEMS gyroscope system  200  may be reduced. For a typical MEMS gyroscope with a start time of one to two seconds, the start time may be reduced to one second or less by applying a substantially continuous source of voltage to the MEMS gyroscope  216 . This start time may be beneficial for MEMS gyroscope applications that require the start time to be one second or less. For example, some inertial measurement units (IMUs) that include one or more MEMS gyroscopes may require a start time of one second or less. 
     FIG. 3  shows a plan view of a MEMS gyroscope system  300 . The MEMS gyroscope system  300  may include a MEMS gyroscope  316 , a bias power source  318 , drive electronics  320 , and a system cower source  322 . The MEMS gyroscope system  300  may also include sense electronics and other typical operational electronics, which are not shown in  FIG. 3  for the sake of simplification. The MEMS gyroscope  316  may be substantially the same as the MEMS gyroscope  100  as depicted in  FIG. 1 . The bias power source  318  may be substantially the same as the bias power source  218  of the MEMS gyroscope system  200 . The system cower source  322  applies cower to the drive electronics  320  and to the sense electronics as is well known in the art. 
   The drive electronics  320  may include at least one amplifier. The at least one amplifier may include a resistor-capacitor network. When the system power source  322  is applied to the MEMS gyroscope  316 , the at least one amplifier may saturate. The start time of the MEMS gyroscope system  300  may be increased by the amount of time it takes for the at least one amplifier to become unsaturated. 
   The drive electronics  320  may include at least one amplifier. The at least one amplifier may include a resistor-capacitor network. When the system power source is applied to the MEMS gyroscope  316 , the at least one amplifier may saturate. The start time of the MEMS gyroscope system  300  may be increased by the amount of time it takes for the at least one amplifier to become unsaturated. 
   The bias power source  318  may apply a substantially continuous voltage to the drive electronics  320 , which may prevent the at least one amplifier from saturating. For example, the bias power source  318  may provide substantially 5 volts to the drive electronics  320 . However, other values of voltage may also be provided. By preventing the at least one amplifier in the drive electronics  320  from saturating, the start time of the MEMS gyroscope system  300  may be reduced. The bias power source may  318  may be applied to both the drive electronics  320  and the at least one sense plate  312   a ,  312   b.    
   It should be understood that the illustrated embodiments are exemplary only and should not be taken as limiting the scope of the present invention. While a MEMS tuning fork gyroscope is employed to illustrate the invention, the present invention also applies to other MEMS gyroscopes that use the Coriolis acceleration to detect rotation, such as an angular rate sensing gyroscope. The claims should not be read as limited to the described order or elements unless stated to that effect. Therefore, all embodiments that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.