Abstract:
The present invention particularly relates to an inertial force sensor used in various electronic equipment, for example, used for altitude control, navigation, or the like, of a mobile body such as an aircraft, an automobile, a robot, a watercraft and a vehicle in which the detection accuracy is enhanced. Therefore, a sense circuit constituting the inertial force sensor includes a sigma-delta modulator converting a sense signal output from a sense electrode into a one-bit digital signal, and a signal processing circuit (i) comparing a monitor signal output from a monitor electrode with predetermined reference amplitude information, (ii) carrying out operation processing of the one-bit digital signal based on this comparison information, and (iii) adjusting an output level of the output signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention particularly relates to an inertial force sensor used in various electronic equipment, for example, used for altitude control, navigation, or the like, of a mobile body such as an aircraft, an automobile, a robot, a watercraft and a vehicle. 
     2. Background Art 
     In a conventional inertial force sensor, a tuning fork-shaped sense element is allowed to oscillate and an inertial force applied to the sense element is detected by using a Coriolis force. Therefore, in order to make the amplitude of the sense element constant, an AGC circuit is provided in a drive circuit for oscillating the sense element. Prior art information relating to the invention of this application includes, for example, Japanese Patent Unexamined Publication No. H9-281138. 
     However, since the AGC circuit includes many analog elements, the temperature properties of the elements are accumulated and the size of the AGC circuit is increased. It is therefore difficult to make the amplitude of the sense element constant. As a result, the detection accuracy of the inertial force sensor is affected. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the problems discussed above, and has an object to improve the detection accuracy of an inertial force sensor. 
     In order to achieve the object, in the present invention, a sense circuit constituting an inertial force sensor is configured to have a sigma-delta modulator for converting a signal output from a sense electrode into a one-bit digital signal; and a signal processing circuit for comparing a monitor signal output from a monitor electrode with predetermined reference amplitude information, carrying out operation processing of the one-bit digital signal based on the comparison information, and adjusting an output level of the output signal. 
     Furthermore, in the present invention, a drive circuit constituting an inertial force sensor is configured to: (i) analog-to-digital convert a monitor signal output from a monitor electrode so as to generate a digital value and amplitude information of the monitor signal; (ii) compare the amplitude information with predetermined reference amplitude information so as to generate comparison information; (iii) carry out operation processing of the digital value of the monitor signal based on the comparison information so as to convert the digital value of the monitor signal into a multi-bit signal; (iv) convert the multi-bit signal into a predetermined output signal; and (v) output the output signal to the drive electrode. 
     Furthermore, in the present invention, a drive circuit constituting an inertial force sensor is configured to: (i) sigma-delta convert a monitor signal output from a monitor electrode into a one-bit digital signal so as to generate amplitude information; (ii) compare the amplitude information with predetermined reference amplitude information so as to generate comparison information; (iii) carry out operation processing of the monitor signal based on the comparison information so as to convert the monitor signal into a multi-bit signal; (iv) convert the multi-bit signal into a predetermined output signal; and (v) output the output signal to a drive electrode. The operation processing includes operating a rectangular wave signal formed from the monitor signal with the comparison information. 
     With such configurations, the detection accuracy of the inertial force sensor can be enhanced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit block diagram showing an angular velocity sensor in accordance with a first embodiment of the present invention. 
         FIG. 2  is a view showing a sense element constituting an angular velocity sensor in accordance with the first embodiment of the present invention. 
         FIG. 3  is a circuit block diagram showing an angular velocity sensor in accordance with a second embodiment of the present invention. 
         FIG. 4  is a circuit block diagram showing an angular velocity sensor in accordance with a third embodiment of the present invention. 
         FIG. 5  is a diagram showing a flow of a signal processing in a sigma-delta modulator constituting a circuit block in accordance with the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, an inertial force sensor in accordance with embodiments of the present invention is described with reference to drawings. 
     First Embodiment 
       FIG. 1  shows a circuit block of an angular velocity sensor as an inertial force sensor in accordance with a first embodiment of the present invention. The configuration of the sensor roughly includes sense element  1 , drive circuit  2  for oscillating sense element  1 , and sense circuit  4  for electrically processing sense signal  3  output from sense element  1 . 
     Sense element  1  includes drive electrode  6 , sense electrode  7  and monitor electrode  8  formed on tuning fork-shaped silicone board  5  as shown in  FIG. 2 . By applying drive signal  9  of a predetermined frequency to drive electrode  6  from drive circuit  2 , drive arm  10  of sense element  1  is allowed to oscillate in the left and right direction in  FIG. 2 . When an angular velocity is applied to sense element  1  in this state, a Coriolis force is generated and drive arm  10  is bent in the back and force direction in  FIG. 2 . With this bending, sense signal  3  is output from sense electrode  7 . Although not particularly shown, drive electrode  6 , sense electrode  7  and monitor electrode  8  have a structure in which a PZT thin film is sandwiched between the upper and lower electrodes. Note here that monitor electrode  8  detects a state of the oscillation of sense element  1  and outputs a signal corresponding to the oscillation of the tuning fork as monitor signal  11 . 
     This angular velocity sensor includes: sigma-delta modulator (hereinafter, referred to as “ΣΔ”)  12  for converting sense signal  3  into a one-bit digital signal; corrected information generating circuit  13  for detecting the peak-to-peak of monitor signal  11  and comparing the amplitude information of the monitor signal  11  with predetermined reference amplitude information so as to generate corrected information; signal processing circuit  14  for carrying out operation processing of the one-bit digital signal output from ΣΔ  12  with the corrected information; digital filter  15  for filtering a multi-bit signal formed in signal processing circuit  14 ; and communication circuit  16  for storing this filtered multi-bit signal and communicating to the outside. In particular, this structure does not include an AGC circuit that was conventionally provided in order to make the amplitude of sense element  1  constant. 
     Note here that ΣΔ  12  oversamples an analog signal output from sense electrode  7  and converts the signal into a one-bit digital signal by sigma-delta conversion. 
     Then, signal processing circuit  14  converts the one-bit digital signal formed from sense signal  3  in ΣΔ  12  into a multi-bit signal according to the magnification obtained in corrected information generating circuit  13 . For example, when the magnification of the corrected information is “5” and the one-bit digital signal is “ . . . 0011010 . . . ,” the signal is converted into a multi-bit signal, “ . . . 0055050 . . . .” Herein, since the one-bit digital signal is a one-bit signal, 0 or 1, when the magnification of the corrected information is decided and the magnification is substituted for the signal “1,” the signal can be converted into a multi-bit signal. Therefore, multiplication processing and the like, which is generally carried out, is not required, thus enabling the processing circuit to be simplified. 
     By configuring the angular velocity sensor in this way, an AGC circuit in which the temperature property in drive circuit  2  is large is eliminated. Accordingly, the above-mentioned output level processing is introduced and carried out by digital processing. Therefore, it is possible to suppress a signal fluctuation due to a temperature change. Consequently, drive circuit  2  can be simplified, and the detection accuracy of the angular velocity sensor can be enhanced. 
     Since ΣΔ  12  converts a signal into a one-bit digital signal, the signal can be converted into a multi-bit digital signal by an A/D converter without using ΣΔ  12  mentioned above. However, since ΣΔ  12  has an integrator function inside thereof as a circuit configuration, it is possible to reduce the size of the circuit. 
     Note here that in the above-mentioned first embodiment, as a processing carried out in sense circuit  4 , controlling of the output level according to the amplitude fluctuation with respect to sense signal  3  is described. However, the processing carried out in sense circuit  4  is not limited to this alone. Sense circuit  4  is a portion also carrying out additional processing for eliminating undesired signals caused by variation in the shape of sense element  1  and undesired connection between electrodes such as drive electrode  6  and sense electrode  7 . As to this elimination of undesired signals, various processings such as that disclosed in Japanese Patent Unexamined Publication No. H9-281138 are carried out. 
     Furthermore, in the above-mentioned first embodiment, an angular velocity sensor is described as an example of the inertial force sensor. However, the inertial force sensor can be applied to an acceleration sensor and the like as long as sense element  1  is oscillated and a Coriolis force is used. 
     The inertial force sensor in accordance with the first embodiment of the present invention can enhance the detection accuracy of the inertial force sensor and is useful as an inertial force sensor used in various equipment. 
     Second Embodiment 
     Hereinafter, an inertial force sensor in accordance with a second embodiment of the present invention is described with reference to  FIG. 3 . 
       FIG. 3  shows a circuit block of an angular velocity sensor as an inertial force sensor in accordance with the second embodiment of the present invention. The configuration of the sensor roughly includes sense element  1 , drive circuit  22  for oscillating sense element  1 , and sense circuit  24  for electrically processing sense signal  23  output from sense element  1 . 
     Since sense element  1  is the same as that shown in  FIG. 2  and described in the first embodiment, the description thereof is omitted. 
     As shown in  FIG. 3 , drive circuit  22  of this angular velocity sensor includes: sigma-delta modulator (hereinafter, referred to as “ΣΔ”)  32  for AD converting monitor signal  31  into a one-bit digital signal; digital filter  33  for converting the one-bit digital signal output from ΣΔ  32  into a multi-bit signal; comparator circuit  34  for detecting a zero cross of this multi-bit signal so as to generate phase information; amplitude detection circuit  35  for detecting the peak-to-peak of each cycle from the phase information and the above-mentioned multi-bit signal; corrected information generating circuit  36  for comparing amplitude information obtained by the detection of peak-to-peak with predetermined reference amplitude information so as to generate corrected information; signal processing circuit  37  for carrying out operation processing of the one-bit digital signal output from ΣΔ  32  with the corrected information; digital filter  38  for filtering a multi-bit drive signal formed in signal processing circuit  37 ; and ΣΔ  39  for converting this filtered multi-bit drive signal into one-bit digital signal  29  and outputting it to drive electrode  6 . 
     Note here that ΣΔ  32  for carrying out AD conversion oversamples an analog signal (monitor signal  31 ) output from monitor electrode  8  and converts the signal into a one-bit digital signal by sigma-delta conversion. 
     Then, signal processing circuit  37  converts the one-bit digital signal formed from monitor signal  31  in ΣΔ  32  into a multi-bit signal according to the magnification obtained from the corrected information. For example, when the magnification of the corrected information is “5” and the one-bit digital signal is “ . . . 0011010 . . . ,” the signal is converted into a multi-bit signal, “ . . . 0055050 . . . .” Herein, since the one-bit digital signal is a one-bit signal, 0 or 1, when the magnification of the corrected information is decided and the magnification is substituted for the signal “1,” the signal can be converted into a multi-bit signal. Therefore, multiplication processing and the like, which is generally carried out, is not required, thus enabling the processing circuit to be simplified. 
     By configuring the angular velocity sensor in this way, drive circuit  22  is operated by digital signal processing, and a signal fluctuation due to a temperature change is suppressed. Therefore, it is possible to make the amplitude of sense element  1  constant, thus enabling the detection accuracy of the angular velocity sensor to be enhanced. 
     Since ΣΔ  32  for carrying out AD conversion converts a signal into a one-bit digital signal, a signal obtained by an I/V converter or an integrator can be also converted into a multi-bit digital signal by a successive approximation type A/D converter without using the above-mentioned ΣΔ  32 . However, since ΣΔ  32  has an integrator function inside thereof as a circuit configuration, it is possible to reduce the size of the circuit. 
     Furthermore, since ΣΔ  39  converts a multi-bit signal into a one-bit digital signal, a signal can be converted into an analog signal by using a D/A converter without using ΣΔ  39  and can be output to drive electrode  6 . However, by converting the signal into a one-bit digital signal by ΣΔ  39 , sense element  1  can be oscillated directly with this one-bit digital signal, thus enabling the size of the circuit to be reduced. 
     In the above-mentioned second embodiment, an angular velocity sensor is described as an example in of the inertial force sensor. However, the inertial force sensor can be applied to an acceleration sensor and the like as long as sense element  1  is oscillated and a Coriolis force is used. 
     The inertial force sensor in accordance with the second embodiment can enhance the detection accuracy of the inertial force sensor and is useful as an inertial force sensor used in various equipment. 
     Third Embodiment 
     Hereinafter, an inertial force sensor in accordance with a third embodiment of the present invention is described with reference to  FIG. 4 . 
       FIG. 4  shows a circuit block of an angular velocity sensor as an inertial force sensor in accordance with the third embodiment of the present invention. The configuration of the sensor roughly includes sense element  1 , drive circuit  42  for oscillating sense element  1 , and sense circuit  44  for electrically processing sense signal  43  output from sense element  1 . 
     Since sense element  1  is the same as that shown in  FIG. 2  and described in the first embodiment, the description thereof is omitted. 
     As shown in  FIG. 4 , drive circuit  42  of this angular velocity sensor includes: I/V converter  52  into which monitor signal  51  is input; sigma-delta modulator (hereinafter, referred to as “ΣΔ”)  53  for converting the output signal from I/V converter  52  into a one-bit digital signal; digital filter  54  for converting this one-bit digital signal into a multi-bit signal; corrected information generating circuit  55  for comparing amplitude information possessed by this multi-bit signal with predetermined reference amplitude information so as to generate corrected information; comparator  56  for converting the output signal from I/V converter  52  into a rectangular wave signal; signal processing circuit  57  for converting this rectangular wave signal into a discrete value at a predetermined cycle so as to carry out operation processing with the corrected information; digital filter  58  for filtering the multi-bit signal formed in signal processing circuit  57 ; and ΣΔ  59  for converting this filtered multi-bit signal into one-bit digital signal  49  and outputting it to drive electrode  6 . As shown in  FIG. 5 , ΣΔ  53  integrates an analog signal output from I/V converter  52  in accordance with the rectangular wave signal output from comparator  56 . For example, integration is carried out in each period in which the rectangular wave signal shows High (expressed by ‘H’ in  FIG. 5 ). This integrated signal is used as an input signal and sigma-delta conversion is carried out, thereby converting the amplitude information of monitor signal  51  into the one-bit digital signal. 
     Then, signal processing circuit  57  converts the rectangular wave signal formed in comparator  56  into a discrete signal. According to this discrete signal and a magnification obtained from the corrected information, the signal is converted into the multi-bit signal. For example, when the magnification of the corrected information is “5” and the discrete signal is “ . . . 0011010 . . . ,” the signal is converted into the multi-bit signal, “ . . . 0055050 . . . .” Herein, since the discrete signal is a one bit signal, 0 or 1, when the magnification of the corrected information is decided and the magnification is substituted for the signal “1,” the signal can be converted into a multi-bit signal. Therefore, multiplication processing and the like, which is generally carried out, is not required, thus enabling the processing circuit to be simplified. 
     By configuring the angular velocity sensor in this way, drive circuit  42  is operated by digital signal processing, and a signal fluctuation due to a temperature change is suppressed. Therefore, it is possible to make the amplitude of sense element  1  constant, thus enabling the detection accuracy of the angular velocity sensor to be enhanced. 
     Since ΣΔ  53  converts a signal into a one-bit digital signal, the value obtained by an integrator can be also converted into a multi-bit digital signal by a successive approximation type A/D converter without using the above-mentioned ΣΔ  53 . However, since ΣΔ  53  has an integrator function inside thereof as a circuit configuration, it is possible to reduce the size of the circuit. 
     Furthermore, since ΣΔ  59  converts a multi-bit signal into a one-bit digital signal by operation processing, the signal can be converted into an analog signal by using a D/A converter without using ΣΔ  59  and can be output to drive electrode  6 . However, by converting the signal into a one-bit digital signal by using ΣΔ  59 , sense element  1  can be oscillated directly with this one-bit digital signal, thus enabling the size of the circuit to be reduced. 
     In the above-mentioned third embodiment, an angular velocity sensor is described as an inertial force sensor. However, the inertial force sensor can be applied to an acceleration sensor and the like as long as sense element  1  is oscillated and a Coriolis force is used. 
     The inertial force sensor in accordance with the third embodiment can enhance the detection accuracy of the inertial force sensor and is useful as an inertial force sensor used in various equipment.