Abstract:
A micro-electro-mechanical-system (MEMS) resonant sensor includes: a MEMS unit that generates an output signal corresponding to a vibration component of a mass body vibratable between a first driving electrode and a second driving electrode; an automatic gain control (AGC) unit that automatically generates a comparative voltage by controlling a gain of the output signal; and a bias unit that receives a reference voltage and generates a bias voltage using the comparative voltage and the reference voltage, wherein a sinusoidal driving voltage is applied to the first driving electrode and the second driving electrode, and the bias voltage is applied to the mass body. It can maintain the amplitude of the mass body stably in the MEMS resonant sensor, and prevent malfunction of an electronic circuit by reducing a response error of the MEMS resonant sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority to Korean Patent Application No. 10-2013-0065508 filed on Jun. 7, 2013, the entire contents of which is incorporated herein for all purposes by this reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a micro-electro-mechanical-system (MEMS) resonant sensor and a method of controlling the MEMS resonant sensor. 
         [0004]    2. Description of Related Art 
         [0005]    A resonant sensor calculates a size of a physical quantity input by detecting a resonant characteristic that is changed by applying a physical quantity from the outside. A resonant sensor is generally used as a MEMS resonant sensor because of its wide input range and easy connection through a digital interface. 
         [0006]    The MEMS resonant sensor is modeled by a mass body-spring-damper, and detects a resonant characteristic such as amplitude of the mass body by applying a physical quantity from the outside and a conversion coefficient of resonant frequency. The mass body of the MEMS resonant sensor vibrates by repeating a resonant loop consisted of three parts: the mass body of the MEMS resonant sensor vibrates initially and precisely by applying the physical quantity from the outside; a signal of initial vibration amplifies through an amplifier and generates an electrostatic force; and the mass body will have a broader amplitude according to the electrostatic force. 
         [0007]    Meanwhile, the MEMS resonant sensor can lead to a spring constant error or an amplifier gain error due to a process error of the mass body or an amplifier error of the resonant loop. Thus, a problem such as an irregular amplitude of the mass body occurs. This problem becomes a response error of the MEMS resonant sensor, and it leads to malfunction of an electronic circuit. 
         [0008]    The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art. 
       BRIEF SUMMARY 
       [0009]    Various aspects of the present invention are directed to providing a MEMS resonant sensor and a method of controlling the same having advantages of stable maintenance of the amplitude of the mass body. 
         [0010]    In an aspect of the present invention, a micro-electro-mechanical-system (MEMS) resonant sensor may include a MEMS unit that generates an output signal corresponding to a vibration component of a mass body vibratable between a first driving electrode and a second driving electrode, an automatic gain control (AGC) unit that generates a comparative voltage by automatically controlling a gain of the output signal of the MEMS unit, and a bias unit that receives the comparative voltage and a reference voltage and generates a bias voltage using the comparative voltage and the reference voltage, wherein a sinusoidal driving voltage is applied to the first driving electrode and the second driving electrode, and the bias voltage is applied to the mass body. 
         [0011]    The MEMS resonant sensor may further include a hold amplifier generating an input signal that is input to the MEMS unit by amplifying the output signal of the MEMS unit. 
         [0012]    The bias unit generates a sum or a difference of the reference voltage and the comparative voltage as the bias voltage. 
         [0013]    Another exemplary embodiment of the present invention provides a method of controlling a MEMS resonant sensor using a mass body vibratable by an electrostatic force between a first driving electrode and a second driving electrode, including generating an output signal corresponding to a vibration component of the mass body, generating a comparative voltage corresponding to the output signal by automatically controlling the gain of the output signal, adjusting a level of a bias voltage applied to the mass body by using a reference voltage and the comparative voltage, and controlling the electrostatic force by applying the adjusted bias voltage to the mass body. 
         [0014]    The generation of the comparative voltage corresponding to the output signal may include applying a sinusoidal driving voltage to the first driving electrode and the second driving electrode. The generation of the comparative voltage corresponding to the output signal by automatically controlling the gain of the output signal may include generating a sum or a difference of the reference voltage and the comparative voltage as the bias voltage. 
         [0015]    Yet another embodiment of the present invention provides a method of controlling a MEMS resonant sensor, including applying a sinusoidal driving voltage to a first driving electrode and a second driving electrode, applying a bias voltage to a mass body which vibrates in a first direction between the first driving electrode and the second driving electrode by an electrostatic force, generating an output signal corresponding to a vibration component of the mass body, and calculating a size of a physical quantity input from the outside in a second direction by detecting the output signal, wherein the applying the bias voltage to the mass body may include generating a comparative voltage corresponding to the output signal by automatically controlling the gain of the output signal, adjusting a level of the bias voltage applied to the mass body by using a reference voltage and the comparative voltage, and controlling the electrostatic force by applying the adjusted bias voltage to the mass body. 
         [0016]    The generation of the comparative voltage corresponding to the output signal by automatically controlling the gain of the output signal may include generating a sum or a difference of the reference voltage and the comparative voltage as the bias voltage. 
         [0017]    According to the present invention, the amplitude of a mass body can be stably maintained in a MEMS resonant sensor, and a malfunction of an electronic circuit can be prevented by reducing a response error of the MEMS resonant sensor. 
         [0018]    In addition, a rising time of the mass body whose amplitude is in an initial weak condition in the MEMS resonant sensor can be considerably reduced. 
         [0019]    The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a block diagram schematically illustrating a MEMS resonant sensor according to an exemplary embodiment of the present invention. 
           [0021]      FIG. 2  is a block diagram schematically illustrating a MEMS unit according to an exemplary embodiment of the present invention. 
           [0022]      FIG. 3  is a flowchart showing a method of controlling the MEMS resonant sensor according to an exemplary embodiment of the present invention. 
       
    
    
       [0023]    It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
         [0024]    In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
       DETAILED DESCRIPTION 
       [0025]    Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that the present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
         [0026]    Parts that are irrelevant to the description are omitted in order to clearly describe the present invention, and like reference numerals designate like elements throughout the specification. 
         [0027]    Throughout this specification and the claims that follow, when it is described that an element is “coupled” to another element, the element may be “directly coupled” to the other element or “electrically coupled” to the other element through a third element. In addition, unless explicitly described to the contrary, the word “comprise” and variations such as “comprises” or “comprising” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. 
         [0028]      FIG. 1  is a block diagram schematically illustrating the MEMS resonant sensor and  FIG. 2  is a block diagram schematically illustrating the MEMS unit according to an exemplary embodiment of the present invention. 
         [0029]    Referring to  FIG. 1 , the MEMS resonant sensor  100  includes a MEMS unit  110 , an AGC (automatic gain control) unit  120 , a hold amplifier  130 , and a bias unit  140 . 
         [0030]    Referring to  FIG. 2 , a MEMS unit  110  may include a first driving electrode  111 , a second driving electrode  112 , and a vibratable mass body  113  between the first driving electrode and the second driving electrode. A sinusoidal driving voltage is applied to the first driving electrode  111  and the second driving electrode  112 , and a bias voltage Vbias is applied to the mass body  113 . The sinusoidal driving voltage generates an electrostatic force of the mass body  113  between the first driving electrode  111  and the second driving electrode  112 , and the mass body  113  vibrates between the first driving electrode  111  and the second driving electrode  112  according to the electrostatic force. 
         [0031]    When the mass body  113  vibrates in a first direction between the first driving electrode  111  and the second driving electrode  112 , an elastic coefficient of vibration of the first direction is changed by inputting a physical quantity from the outside to the second direction different from the first direction, so that resonant frequency of the MEMS unit  110  is changed. In this case, a size of the physical quantity input is calculated by measuring the change of resonant frequency. 
         [0032]    The MEMS unit  110  generates an output signal Do corresponding to a vibration component of the mass body  113 . 
         [0033]    The AGC unit  120  generates a comparative voltage Vo by automatically controlling a gain of the output signal Do. That is, when the output signal Do is a pulse signal which has amplitude and frequency depending on vibration of the mass body  113 , the AGC unit  120  maintains the comparative voltage Vo automatically by controlling the gain of the output signal Do. The comparative voltage Vo is transmitted to the bias unit  140 . 
         [0034]    The hold amplifier  130  receives the output signal Do, and outputs an input signal Di by amplifying the output signal Do. The input signal Di is input to the MEMS unit  110 , and the MEMS unit  110  maintains an output of the output signal Do by using the input signal Di. 
         [0035]    The bias unit  140  receives a reference voltage Vdc, and generates a bias voltage Vbias by using the reference voltage Vdc and the comparative voltage Vo. For example, the bias unit  140  may generate a sum or a difference of the reference voltage Vdc and the comparative voltage Vo as the bias voltage Vbias. The bias voltage Vbias is transmitted to the mass body  113  of the MEMS unit  110 . 
         [0036]    The gain of the AGC unit  120  increases in the case that the mass body  113  vibrates initially and precisely according to mechanical or electrical noise generated naturally from non-vibrated initial condition of the mass body  113 . Therefore, a higher bias voltage Vbias is applied to the mass body  113  of the MEMS unit  110 . That is, the bias voltage Vbias applied to the mass body  113  of the MEMS unit  110  is adjusted corresponding to the comparative voltage Vo automatically controlling the gain of the output signal Do of the MEMS unit  110 , so that the adjusted bias voltage Vbias controls the electrostatic force of the MEMS unit  110 . The time that the mass body  113  vibrates with stable amplitude from the initial vibration is reduced considerably according to the controlled electrostatic force of the MEMS unit  110 . 
         [0037]    In addition, when the amplitude of the mass body  113  is not maintained stably after the vibration of the mass body  113  because of factors such as electrical noise, mechanical noise from the outside, temperature change, and so on, the amplitude of the mass body  113  is stably maintained by controlling the electrostatic force of the MEMS unit  110  according to automatic control of the gain of the AGC unit  120 . 
         [0038]      FIG. 3  is a flowchart showing a method of controlling the MEMS resonant sensor according to an exemplary embodiment of the present invention. 
         [0039]    Referring to  FIG. 3 , the MEMS unit  110  vibrates the mass body  113  between the first driving electrode  111  and the second driving electrode  112  by applying the sinusoidal driving voltage to the first driving electrode  111  and the second driving electrode  112  and the bias voltage to the mass body  113 , and generates the output signal corresponding to a vibration component of the mass body  113  (S  110 ). When the mass body  113  vibrates in the first direction between the first driving electrode  111  and the second driving electrode  112 , the elastic coefficient of vibration in the first direction is changed by inputting a physical quantity from the outside to the second direction different from the first direction, so that resonant frequency of the MEMS unit  110  which is the vibration component of the mass body  113  is changed. The size of the physical quantity input is calculated by detecting the output signal corresponding to the vibration component of the mass body  113 . 
         [0040]    The AGC unit  120  generates a comparative voltage Vo corresponding to the output signal Do by automatically controlling a gain of the output signal Do (S 120 ). The output signal Do is a pulse signal which has amplitude and frequency depending on vibration of the mass body  113 . The comparative voltage Vo may have a level of voltage corresponding to an envelope characteristic of the output signal Do. 
         [0041]    The bias unit  140  adjusts the level of the bias voltage Vbias by using the reference voltage Vdc and the comparative voltage Vo (S 130 ). The bias voltage Vbias may be generated as a sum or a difference of the reference voltage Vdc and the comparative voltage Vo. That is, the bias voltage Vbias applied to the mass body  113  of the MEMS unit  110  is adjusted corresponding to the comparative voltage Vo automatically controlling the gain of the output signal Do of the MEMS unit  110 . 
         [0042]    The bias voltage Vbias is applied to the mass body  113  of the MEMS unit  110 , and the electrostatic force of the MEMS unit  110  is controlled by the bias voltage Vbias (S 140 ). The amplitude of the mass body  113  is stably maintained by controlling the electrostatic force of the MEMS unit  110 . 
         [0043]    When the amplitude of the mass body  113  is stably maintained and the mass body  113  vibrates in the first direction between the first driving electrode  111  and the second driving electrode  112 , an elastic coefficient of vibration in the first direction is changed by inputting a physical quantity from the outside to the second direction different from the first direction, so that resonant frequency of the MEMS unit  110  is changed. In this case, a size of the physical quantity input is calculated by measuring the change of resonant frequency. 
         [0044]    The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.