JP6309113B2 - Acceleration sensor system - Google Patents

Description

  The present invention relates to acceleration sensor technology. The present invention also relates to a capacitance type and servo type acceleration sensor system.

  There is an acceleration sensor system that detects a target acceleration using a sensor and outputs useful information by a predetermined analysis process using the acceleration. The acceleration sensor system is used for various applications and products. Examples of applications include earthquake measurement and detection devices, automobile airbag devices, abnormality detection and failure detection devices for automobile parts, resource exploration systems such as petroleum and veins, portable devices and game devices. For example, in the case of earthquake measurement detection, the acceleration sensor system detects an earthquake sign or the like from the acceleration of vibration and outputs the earthquake sign detection information as useful information.

  As an acceleration sensor system, there are a capacitance type and a servo type acceleration sensor system as a configuration for realizing high-sensitivity, that is, high-accuracy acceleration detection. The following description is mainly directed to capacitance type and servo type acceleration sensor systems.

  Japanese Patent Laid-Open No. 4-337468 (Patent Document 1) is given as an example of prior art related to a capacitance type and servo type acceleration sensor system. Patent Document 1 describes the following as an acceleration sensor. This acceleration sensor includes an acceleration detecting element having a movable electrode and a fixed electrode, a capacitance detecting means for detecting a capacitance difference between the electrodes, and an electrostatic force that can return the movable electrode to a reference position based on the detection. Electrostatic force generating means for generating the The electrostatic force generating means applies a rectangular wave voltage by pulse width modulation (PWM) to the electrode at a predetermined repetition period, and changes the ratio of voltage application time and the repetition period in accordance with the capacitance difference.

JP-A-4-337468

  Acceleration sensor systems are increasingly required to achieve higher sensitivity and lower power consumption as device performance improves.

  Capacitance type and servo type acceleration sensor systems realize high sensitivity by so-called servo control in which an electrostatic force is applied to an electrode of an acceleration detection element that is an acceleration sensor to bring the electrode into an equilibrium state. In this acceleration sensor system, a servo signal, which is an electrical signal for generating an electrostatic force, is applied to, for example, a fixed electrode of the acceleration detection element in accordance with the acceleration detected from the acceleration detection element. This electrostatic force is an electrostatic force for canceling out the force applied to the movable electrode by acceleration, in other words, displacement.

  There is a method of performing servo control using a voltage signal by PWM as the servo signal. In this servo-type acceleration sensor system, the pulse density of the PWM voltage signal, which is a servo signal, is adjusted according to the detected capacitance difference or acceleration. In other words, the pulse density corresponds to the number of switching of the rectangular wave pulse in the repetition period and the ratio of the voltage application time. In this method, when the acceleration is large, the pulse density of the PWM voltage signal is set to a low density to give a large electrostatic force, and when the acceleration is small, the pulse density becomes a high density to give a small electrostatic force. Adjust as follows.

  In the conventional capacitance type and servo type acceleration sensor systems, the level of the PWM voltage signal, in other words, the amplitude of the pulse is set to a constant value in advance. This constant value corresponds to a value capable of generating an electrostatic force that can maintain the movable electrode in an equilibrium state corresponding to the maximum input acceleration. The power consumption (W) by the PWM voltage signal is the level of the PWM voltage signal (V), the capacitance between the sensor electrodes (C), and the pulse of the PWM voltage signal per second. It can be calculated using the number of times of switching (assuming N). The power consumption (W) increases in proportion to the capacitance (C), the square of the level (V), and the number of switching times (N). In the conventional servo control, the number of times of switching (N) increases when the acceleration is small while the level of the PWM voltage signal is kept constant. Therefore, the power consumption (W) is relatively large when the acceleration is small compared to when the acceleration is large.

  For this reason, when the conventional acceleration sensor system is applied to various uses, it may not be efficient in terms of power consumption. For example, in the case of seismic measurement detection, as a typical example of the time change of acceleration, the acceleration is steady and minute in normal times, the acceleration is large at the beginning of the vibration due to the earthquake, and the acceleration decreases as time passes after the initial vibration. . In this case, although it can be controlled efficiently with a large PWM voltage signal level at the beginning of vibration, the PWM voltage signal level is unnecessarily high while the acceleration is small and the required electrostatic force is small after normal or after a lapse of time. . In other words, the longer the time during which the acceleration is low, the greater the overall power consumption.

  The objective of this invention is providing the technique which can implement | achieve low power consumption, ensuring high sensitivity regarding an acceleration sensor system.

  A typical embodiment of the present invention is an acceleration sensor system that detects acceleration, and has the following configuration.

  An acceleration sensor system according to an embodiment includes a sensor that outputs a signal representing a change in capacitance according to acceleration, a signal detection unit that receives the signal and detects it as a detection signal, and a detection for obtaining the signal A detection signal generator that generates a signal for application and applies it to the sensor; a servo signal generator that generates a PWM voltage signal as a servo signal for servo control based on the detection signal; An output unit that outputs information including acceleration based on the detection signal, wherein the servo signal generation unit sets the level of the PWM voltage signal to a first level according to the magnitude of the capacitance change. The adjustment is performed by switching between at least two levels including a level and a second level lower than the first level.

  According to a typical embodiment of the present invention, low power consumption can be realized while ensuring high sensitivity for an acceleration sensor system.

It is a figure which shows the structure of the acceleration sensor system of Embodiment 1 of this invention. FIG. 2 is a diagram illustrating a sensor structure in the first embodiment. FIG. 3 is a diagram illustrating an example of a circuit configuration of a multilevel servo signal generation unit in the first embodiment. FIG. 6 is a diagram illustrating a first servo control operation and waveform in the first embodiment. FIG. 6 is a diagram illustrating a second servo control operation and waveform in the first embodiment. FIG. 10 is a diagram illustrating a third servo control operation and waveform in the first embodiment. FIG. 3 is a diagram illustrating a circuit configuration of an acceleration output unit in the first embodiment. It is a figure which shows the structural example at the time of applying to automotive component failure detection as a use in Embodiment 1. FIG. It is a figure which shows the operation | movement and waveform of servo control in the acceleration sensor system of Embodiment 2 of this invention. FIG. 10 is a diagram illustrating a processing flow of servo control operation in the second embodiment. It is a figure which shows the structure of the acceleration sensor system of Embodiment 3 of this invention. It is a figure which shows the structure of the sensor in Embodiment 3. FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiment, and the repetitive description thereof will be omitted. The acceleration sensor system of each embodiment is a capacitance type and servo type acceleration sensor system.

(Embodiment 1)
The acceleration sensor system according to the first embodiment of the present invention will be described with reference to FIGS.

[Acceleration sensor system]
FIG. 1 shows the configuration of the acceleration sensor system of the first embodiment. The acceleration sensor system according to the first embodiment includes a sensor 101, a detection signal generation unit 110, a signal detection unit 102, a servo signal generation unit 120, an output unit 130, and an operation mode switching unit 111.

  The sensor 101 is a capacitance type acceleration detection element that outputs and detects the target acceleration as an electric signal representing a change in capacitance, and is mainly composed of an electrode structure as shown in FIG. The

  The detection signal generation unit 110 generates and outputs a detection signal S8. The detection signal S8 is an electric signal applied to the sensor 101 to detect acceleration, and is a signal for the sensor 101 to detect a change in capacitance as an electric signal. In the first embodiment, the detection signal S8 is a voltage signal and is applied to the two fixed electrodes of the sensor 101. This detection signal S8 is composed of two signals. The first detection signal applied to one fixed electrode is a signal by repeating a rectangular wave pulse, and the second detection signal applied to the other fixed electrode is , A signal obtained by repeating a rectangular wave pulse inverted with respect to the first detection signal.

  The signal detection unit 102 receives the sensor output signal S1 from the movable electrode of the sensor 101, converts the capacitance change indicated by the sensor output signal S1 into a voltage, amplifies it, and detects the analog signal that is the voltage signal Detect and output as signal S2.

  The servo signal generation unit 120 receives the detection signal S2 from the signal detection unit 102, and generates and outputs a PWM voltage signal as the servo signal S7. The servo signal generation unit 120 generates a PWM voltage signal in which both the pulse density and the level are adjusted according to the magnitude of the capacitance change corresponding to the acceleration. The servo signal generation unit 120 includes an ADC 103, a PID control unit 104, a multi-level servo signal generation unit 105, a PWM voltage level switching unit 106, and a comparator 107 as a detailed configuration.

  The ADC 103 is an analog / digital converter, receives the detection signal S2 which is an analog signal from the signal detection unit 102, converts the detection signal S2 into a digital signal S3 by predetermined sampling, and outputs the digital signal S3. The ADC 103 performs conversion into the digital signal S3 at a sampling rate higher than the detected acceleration band. The detected acceleration band corresponds to the band of the detection signal S2 itself. This sampling rate is desirably a sampling rate of 100 times or more of the detected acceleration band.

  The PID control unit 104 receives the digital signal S3 from the ADC 103, and processes the digital signal S3 to generate and output a PID control signal S4. PID control (Proportional-Integral-Derivative Control) is a kind of feedback control, and is a method of controlling an input value by a deviation between an output value and a target value, integration and differentiation thereof. The PID control unit 104 has a function of deriving a value such as a voltage or a digital value corresponding to an electrostatic force necessary for maintaining the movable electrode of the sensor 101 in an equilibrium state using the digital signal S3. The PID control unit 104 generates a PID control signal S4 based on the derived value.

  The PID control signal S4 is input to the comparator 107 and the PWM voltage level switching unit 106, converted to the servo switching signal S6 by the comparator 107, and converted to the PWM level switching signal S7 by the PWM voltage level switching unit 106. .

  The comparator 107 converts the PID control signal S4 from the PID control unit 104 into a binary servo switching signal S6 based on the comparison with the reference signal and outputs it. The servo switching signal S6 is a signal for adjusting the pulse density as servo control. In the first embodiment, the servo switching signal S6 is a binary voltage signal, one of the two values is a value for selecting the high side voltage of the pulse, and the other value is the low signal of the pulse. This is a value for selecting the side voltage. The servo switching signal S6 has a binary application time adjusted in accordance with the PID control signal S4, whereby the pulse density is adjusted in at least two stages.

  The PWM voltage level switching unit 106 generates and outputs a PWM voltage level switching signal S5 by arithmetic processing in accordance with the PID control signal S4 from the PID control unit 104. The PWM voltage level switching signal S5 is a signal for switching the amplitude of the pulse that is the level of the PWM voltage signal. In the first embodiment, the PWM voltage level switching signal S5 is a binary voltage signal, and one of the two values is a value for setting a relatively high level and a large amplitude, and the other value is This is a value for a relatively low level and small amplitude.

  The multi-level servo signal generation unit 105 receives the PWM voltage level switching signal S5 from the PWM voltage level switching unit 106 and the servo switching signal S6 from the comparator 107, and determines the multi-level according to these two types of signals. The PWM voltage signal, which is the servo signal S7, is generated and output. The multi-level servo signal S7 is a signal in which both the level of the PWM voltage signal and the pulse density are adjusted in accordance with two types of signals corresponding to the detected acceleration. In the first embodiment, the multilevel servo signal generation unit 105 has a function of switching the level of the PWM voltage signal at at least two levels.

  The operation mode switching unit 111 receives the PWM voltage signal that is the servo signal S7 from the multilevel servo signal generation unit 105 and the detection signal S8 from the detection signal generation unit 110. The operation mode switching unit 111 switches these two types of signals in a time division manner according to the operation mode, outputs the signals as a signal S9 according to the operation mode, and applies them to the pair of fixed electrodes of the sensor 101. In the first embodiment, the two operation modes include a servo control mode and a detection mode. The servo control mode is a mode for performing servo control, and the detection mode is a mode for detecting acceleration. The signal S9 corresponding to the operation mode is the servo signal S7 when the operation mode is the servo control mode, and is the detection signal S8 when the operation mode is the detection mode. The operation mode time division control in the operation mode switching unit 111 is defined in advance and is automatically executed.

  The output unit 130 includes an acceleration output unit 131 and an analysis unit 132. The output unit 130 receives the PWM voltage level switching signal S5 from the PWM voltage level switching unit 106 and the servo switching signal S6 from the comparator 107.

  The acceleration output unit 131 calculates acceleration using the PWM voltage level switching signal S5 and the servo switching signal S6, and outputs acceleration information S10 including the acceleration to an external user.

  The analysis unit 132 uses the acceleration calculated and output by the acceleration output unit 131 to perform predetermined analysis processing according to the application, extracts predetermined useful information S11 as the analysis result, and provides an external user with the analysis result. Output. The output of the acceleration S10 and the useful information S11 can take various forms such as screen display and data output.

[Sensor]
FIG. 2 shows the structure of the sensor 101 in the first embodiment. As directions for explanation, an X direction and a Z direction are shown. The X direction corresponds to the horizontal direction in FIG. 2 and the direction in which the main surface of the electrode extends, and the Z direction corresponds to the vertical direction in FIG. 2 and the direction in which the electrodes overlap each other at a distance.

  The sensor 101 includes fixed electrodes 201 and 202 that are a pair of two fixed electrodes, and one movable electrode 203. The fixed electrode 201 is a first fixed electrode, and the fixed electrode 202 is a second fixed electrode. The two fixed electrodes are connected to the operation mode switching unit 111 through signal lines. The movable electrode 203 is connected to the signal detection unit 102 through a signal line.

  The movable electrode 203 has a large surface in the X direction as a main surface. Fixed electrodes 201 and 202 are arranged at predetermined positions facing the main surface of the movable electrode 203 at a predetermined distance on both sides in the Z direction so as to sandwich the movable electrode 203. The pair of fixed electrodes includes a surface region that overlaps when viewed in the Z direction with respect to the main surface of the movable electrode 203 in order to generate an electrostatic force on the movable electrode 203.

  The signal lines of the fixed electrodes 201 and 202 transmit the servo signal S7 in the servo control mode and the detection signal S8 in the detection mode as the signal S9 corresponding to the operation mode. More specifically, the servo signal S7 includes a first servo signal 211 applied to the fixed electrode 201 and a second servo signal 212 applied to the fixed electrode 202. The second servo signal 212 is a voltage signal inverted with respect to the first servo signal 211. Specifically, the detection signal S8 includes a first detection signal 221 applied to the fixed electrode 201 and a second detection signal 222 applied to the fixed electrode 202. The second detection signal 222 is a voltage signal inverted with respect to the first detection signal 221.

  The signal line of the movable electrode 203 transmits the sensor output signal S1. In the sensor 101, the movable electrode 203 is displaced in the Z direction according to the acceleration, whereby the capacitance between the two fixed electrodes and the movable electrode 203 changes. The capacitance between the fixed electrode 201 and the movable electrode 203 is C1, and the capacitance between the fixed electrode 202 and the movable electrode 203 is C2. The capacitance difference between these capacitances is ΔC (ΔC = C1−C2). The sensor output signal S1 output from the movable electrode 203 through the signal line is a voltage signal representing a change in capacitance including the capacitance difference ΔC. The sensor output signal S1 at a certain point in time represents the capacitance difference ΔC corresponding to the displacement state. The sensor output signals S1 at two points in time according to the passage of time represent changes over time in the capacitance difference ΔC.

[Multi-level servo signal generator]
FIG. 3 shows a circuit configuration of the multilevel servo signal generation unit 105 in the first embodiment. The multilevel servo signal generation unit 105 includes PWM level voltage sources 301 and 302, a low level voltage source 303, a level switch 304, and a servo switch 305. PWM level voltage sources 301 and 302 are connected to the input of the level switch 304, and a servo switch 305 is connected to the output. The output of the level switch 304 and the low level voltage source 303 are connected to the input of the servo switch 305.

  The PWM level voltage sources 301 and 302 are voltage sources for generating two high-side voltages, a high level, in the PWM voltage signal. The PWM level voltage source 301 is a first high-side voltage source that generates a first high-side voltage corresponding to the first level. The PWM level voltage source 302 is a second high side voltage source that generates a second high side voltage corresponding to the second level. The first high side voltage of the PWM level voltage source 301 is set to a voltage higher than the second high side voltage of the PWM level voltage source 302. The low level voltage source 303 is a voltage source for generating one low side voltage, low level, in the PWM voltage signal.

  The level switch 304 includes a switch and switches between the PWM level voltage source 301 and the PWM level voltage source 302 in accordance with a PWM voltage level switching signal S5 that is an input control signal. The level switch 304 selects the first high-side voltage of the PWM level voltage source 301 when the PWM voltage level switching signal S5 is the first value, and PWM when the PWM voltage level switching signal S5 is the second value. The second high side voltage of the level voltage source 302 is selected.

  The servo signal generation unit 120 including the multi-level servo signal generation unit 105, the PWM voltage level switching unit 106 and the PID control unit 104 in the previous stage is configured to control the level and amplitude of the PWM voltage as follows. The switch 304 is controlled. That is, when the acceleration is large and the level of the PWM voltage signal needs to be increased based on the detection signal S2, the servo signal generation unit 120 selects the first high-side voltage, reduces the acceleration, and reduces the PWM voltage. When the signal level may be low, the level switch 304 is switched so as to select the second high-side voltage.

  The servo switch 305 includes a switch, and switches between the low level voltage source 303 and the output of the level switch 304 in accordance with a servo switch signal S6 that is an input control signal. The servo switch 305 selects the low-side voltage of the low-level voltage source 303 when the servo switch signal S6 is the first value, and when the servo switch signal S6 is the second value, The first high-side voltage or the second high-side voltage that is the output voltage is selected. The output terminal 306 of the servo switch 305 outputs a servo signal S7. The signal line connected to the output terminal 306 is connected to the pair of fixed electrodes of the sensor 101 through the operation mode switching unit 111 described above.

  The multi-level servo signal generation unit 105 generates a PWM voltage signal in which both the pulse density and the level are adjusted in accordance with the servo switching signal S6 and the PWM voltage level switching signal S5, which are the above two types of signals, and the multi-level servo signal Output as S7. In the first embodiment, the multi-level servo signal generation unit 105 adjusts the pulse density according to the servo switching signal S6, and the PWM adjusts the pulse level and amplitude according to the PWM voltage level switching signal S5. A signal obtained by synthesizing the voltage signal is output as a servo signal S7.

[Servo control (1) _Pulse density switching]
The acceleration sensor system according to the first embodiment is configured to adjust both the pulse density and the level of the PWM voltage signal, whereas the conventional acceleration sensor system is configured to adjust only the pulse density of the PWM voltage signal. The acceleration sensor system of the first embodiment includes a PWM voltage level switching unit 106 and a multilevel servo signal generation unit 107 as components for adjusting the level of the PWM voltage signal. The function of the acceleration sensor system according to the first embodiment is capable of adjusting only the pulse density of the PWM voltage signal, adjusting only the level of the PWM voltage signal, and adjusting both the pulse density and the level. is there.

  The acceleration sensor system according to the first embodiment has the configuration and operation shown in the following first servo control when only adjusting the pulse density of the PWM voltage signal as in the conventional servo control. This first servo control corresponds to the case where the level of the PWM voltage signal is not adjusted in the servo signal generation unit 120 of FIG. In that case, the PWM voltage level switching signal S5 is a constant value indicating no switching.

FIG. 4 shows an operation and time waveform when only the pulse density of the PWM voltage signal is switched as the operation of the first servo control in the acceleration sensor system of the first embodiment. FIG. 4A shows the acceleration [m / s ² ] acting on the sensor 101. FIG. 4B shows the position of the movable electrode 203 of the sensor 101. This position indicates a position corresponding to the displacement in the Z direction in FIG. FIG. 4C shows a time waveform of the voltage [V] of the detection voltage S8. The detection voltage S8 is a voltage obtained by repeating a rectangular wave pulse having a constant application time and level in a constant cycle. FIG. 4D shows a time waveform of the voltage [V] of the PWM voltage signal which is the servo signal S7 based on the binary servo switching signal S6. When the value of the servo switching signal S6 is the first value, the PWM voltage signal is a low voltage, and when the value is the second value, the voltage is a high voltage.

  When acceleration as shown in FIG. 4A is applied to the sensor 101, the position of the movable electrode 203 of the sensor 101 slightly changes as shown in FIG. 4B. In the detection mode, a voltage signal that is a detection signal S8 as shown in FIG. 4C is applied from the detection signal generation unit 110 to the fixed electrodes 201 and 202 of the sensor 101. As a result, a voltage signal proportional to the interelectrode capacitance of the sensor 101 and the detection signal S8 is output to the signal detection unit 102 as the sensor output signal S1.

  The interelectrode capacitance here is an electrostatic capacitance between the movable electrode 203 of FIG. 2 and the fixed electrode 201 and the fixed electrode 202. Specifically, this interelectrode capacitance is the difference between the capacitance C1 between the movable electrode 203 and the fixed electrode 201 and the capacitance C2 between the movable electrode 203 and the fixed electrode 202. This is a capacitance representing the capacitance difference ΔC. This interelectrode capacitance changes on the time axis according to the change in acceleration. That is, the sensor output signal S1 and the detection signal S2 change between a voltage value at a certain time and a voltage value at another time. The sensor 101 is a detection element for detecting the acceleration as a change in the interelectrode capacitance as described above, and outputs a voltage signal representing the change in the interelectrode capacitance as the sensor output signal S1. The signal detection unit 102 detects a change in the interelectrode capacitance as the detection signal S2 from the sensor output signal S1.

  In the first servo control, a servo signal S7 as shown in (d) of FIG. 4 according to the acceleration detected from the sensor 101 as shown in (a) of FIG. 4 and the magnitude of the corresponding capacitance change. The pulse density of the PWM voltage signal is switched and adjusted so as to change in at least two stages. In the first servo control, when the acceleration is relatively large, the pulse density is relatively small, and when the acceleration is relatively small, the pulse density is relatively large.

  A period 401 in FIG. 4A shows an example of a period in which the acceleration is large. Corresponding to the period 401, the servo switching signal S6 generates a signal with little switching between the high-side voltage and the low-side voltage in a certain time. Correspondingly, the pulse density is reduced as in the servo signal S7 in FIG. A period 402 in FIG. 4A shows an example of a period in which the acceleration is small. Corresponding to the period 402, the servo switching signal S6 generates a signal that frequently switches between the high-side voltage and the low-side voltage in a certain time. Correspondingly, the pulse density increases as in the servo signal S7 in FIG.

  In the servo control mode, the servo signal generator 120 generates a servo signal S7 for applying an electrostatic force that cancels the force applied to the movable electrode 203 of the sensor 101 in accordance with the acceleration. For this purpose, the servo signal generation unit 120 generates a servo switching signal S6 for adjusting the pulse density based on a signal representing the change in the interelectrode capacitance according to the acceleration, which is a signal acquired in the detection mode. Generate. The servo signal generation unit 120 generates a PWM voltage signal with the pulse density adjusted as shown in FIG. 4D based on the servo switching signal S6, and outputs it as a servo signal S7.

  This servo signal S7 is applied to the fixed electrodes 201 and 202 of the sensor 101 in the servo control mode. Accordingly, the movable electrode 203 of the sensor 101 is given an electrostatic force through the fixed electrodes 201 and 202 and maintains an equilibrium state. That is, the movable electrode 203 is returned to a predetermined reference position.

  In the first servo control, the level 411 corresponding to the high voltage of the pulse, the level 412 corresponding to the low voltage, and the amplitude 420 are constant values as in the PWM voltage signal of FIG. . As described above, this constant value is determined in advance in consideration that a sufficient electrostatic force can be generated corresponding to the maximum input acceleration that can be detected.

Here, Fa is a force applied to the movable electrode 201 by acceleration. Let A be the acceleration. Let M be the mass of the movable electrode 201. The force Fa is expressed by the following formula 1. Note that “·” indicates multiplication.

  Let the electrostatic force applied to the movable electrode 201 be Fe. The above-described interelectrode capacitance, which is the capacitance between the fixed electrodes 201 and 202 and the movable electrode 203, is defined as C. The inter-electrode potential difference between the fixed electrode 201 and the movable electrode 203 is Vp. This p represents a positive electrode. The inter-electrode potential difference between the fixed electrode 202 and the movable electrode 203 is Vn. N represents a negative electrode.

The electrostatic force Fe is expressed by the following formula 2 using the capacitance C and the interelectrode potential difference Vp, Vn.

  When the electrostatic force is efficiently induced, when one electrode potential difference, for example, Vp, is given, the other electrode potential difference, for example, Vn, is set to zero. In this case, in order to maintain the movable electrode 203 in an equilibrium state, it is necessary to apply an interelectrode potential difference Vp or Vn proportional to the square root of the acceleration A.

  In the servo control using the PWM voltage signal, the level of the PWM voltage signal is set to a value that can generate an electrostatic force that can maintain the movable electrode 203 in an equilibrium state with the maximum input acceleration based on Equations 1 and 2. Is set. A voltage value corresponding to this level is Vmax.

The power consumption by the PWM voltage signal is W, and the number of pulse switching per second of the PWM voltage signal is N. The power consumption W is expressed by the following Equation 3 using the capacitance C, the number of times of switching N, and the voltage value Vmax.

  That is, the power consumption W increases in proportion to the capacitance C, the number N of switching times, and the square of the voltage value Vmax.

  In the conventional servo control that adjusts only the pulse density, when the acceleration is large with the voltage value Vmax kept constant for the PWM voltage signal, the pulse density is lowered to increase the electrostatic force. In this servo control, when the acceleration is small, the pulse density is increased in order to reduce the electrostatic force. When the pulse density is low, the switching frequency N is small, and when the pulse density is high, the switching frequency N is large. That is, when the acceleration is small, the power consumption is relatively larger than when the acceleration is large.

  As an application of the acceleration sensor system, for example, in an application such as seismic measurement detection, as described above, when the period during which the acceleration is low is long, the power consumption increases overall. In this application, the target acceleration is relatively small when measuring the acceleration at the steady state or when the time has elapsed from the beginning of vibration. During this period, the level of the PWM voltage signal has an unnecessarily large voltage value Vmax and amplitude with respect to the necessary level, and the number of times of switching N is large. Therefore, during this period, power consumption increases in proportion to the voltage value Vmax and the number of switching times N.

[Servo control (2) _Level switching]
FIG. 5 shows an operation and time waveform for switching the level of the PWM voltage signal as the second servo control operation in the acceleration sensor system of the first embodiment. FIG. 5A shows a digital signal S3 sampled by the ADC 103 of the servo signal generation unit 120 based on the detection signal S2. Sampling timing is indicated by n1 or the like. The value of the digital signal S3 corresponding to the sampling timing is denoted by Dn. FIG. 5B shows a PWM voltage level switching signal S5 for switching the level of the PWM voltage signal in two stages. Here, the voltage of the PWM voltage level switching signal S5 is represented by Vs, the high side voltage is represented by VH, and the low side voltage is represented by VL. Although not shown, the detection signal S8 in the second servo control is the same as the detection signal S8 in (c) of FIG.

  In the second servo control, the servo signal generation unit 120 compares the value Dn of the digital signal S3 based on the detection signal S2 with a predetermined threshold value, and sets the level of the PWM voltage signal to 2 according to the determination result. Switch in stages. In FIG. 5A, the threshold value is Dth. The PID control unit 104 determines the value Dn of the digital signal S3 at each sampling timing, compares the value Dn with the threshold value Dth, and determines whether the value Dn exceeds the threshold value Dth. When the value Dn exceeds the threshold value Dth (Dn> Dth), the PWM voltage level switching unit 105 sets the level of the voltage Vs of the PWM voltage level switching signal S5 to the high side as shown in FIG. When the voltage VH is set and the value Dn is equal to or less than the threshold value Dth (Dn ≦ Dth), the low-side voltage VH is set. For example, in the period 501 between timings n2 and n4, the value Dn exceeds the threshold value Dth and therefore becomes the high side voltage VH. In the period 502 between timings n5 and n8, the value Dn is equal to or less than the threshold value Dth and thus becomes the low side voltage VH. .

  The multilevel servo signal generation unit 106 generates a PWM voltage signal whose level is adjusted in two steps as a multilevel servo signal S7 in accordance with the PWM voltage level switching signal S5 of FIG. That is, when the PWM voltage level switching signal S5 is the high side voltage VH, the multilevel servo signal generation unit 106 selects the first level 511 and the first amplitude 521 by selecting the PWM level voltage source 301 in FIG. Generate a pulse with. When the PWM voltage level switching signal S5 is the low-side voltage VL, the multi-level servo signal generation unit 106 selects a pulse having the second level 512 and the second amplitude 522 by selecting the PWM level voltage source 302 in FIG. Is generated. The first level 511 is higher than the second level 512, and the first amplitude 521 is larger than the second amplitude 522.

  As described above, in the second servo control, the pulse level and amplitude of the PWM voltage signal are switched in two steps according to the magnitude of the capacitance change corresponding to the acceleration based on the division by the threshold value Dth. A signal S7 is generated. In the second servo control, during a period in which the acceleration is relatively small, the level and amplitude of the PWM voltage signal applied to the fixed electrode of the sensor 101 are reduced as in the second level 512, and the acceleration is relatively In the large period, it is increased as in the first level 511. In the second servo control, the voltage value Vmax corresponding to the level is decreased according to the period during which the acceleration is small, and thereby the power consumption W can be reduced based on the above-described calculation formula.

  When only the second servo control is executed, the servo signal generation unit 120 pauses the comparator 107 and outputs a signal indicating that the pulse density is constant as the servo switching signal S6. Then, the multilevel servo signal generation unit 105 generates a servo signal S7 whose level has been switched based on the PWM voltage level switching signal S5.

[Servo control (3) _Pulse density and level switching]
FIG. 6 shows the operation and time waveform for switching both the pulse density and the level of the PWM voltage signal as the operation of the third servo control in the first embodiment, following FIGS. 4 and 5. FIG. 6A shows acceleration in the same manner as FIG. 4A. 6B shows the position and displacement of the movable electrode 203 of the sensor 101 as in FIG. 4B. FIG. 6C shows a time waveform of the PWM voltage signal which is the multilevel servo signal S7. Although not shown, the third servo control detection signal S8 is the same as the detection signal S8 in FIG. 4C.

  The servo signal generation unit 120 simultaneously executes the first servo control for adjusting the pulse density of the PWM voltage signal in FIG. 4 and the second servo control for adjusting the level of the PWM voltage signal in FIG. A third servo control for adjusting both the pulse density and level of the PWM voltage signal in FIG. 6 is performed. That is, the servo signal S7 in FIG. 6C in the third servo control is a PWM voltage signal obtained by combining the servo signal S7 in FIG. 4D and the servo signal S7 in FIG. is there.

  When acceleration as shown in FIG. 6A is applied to the sensor 101, the position of the movable electrode 203 of the sensor 101 slightly changes from the equilibrium state as shown in FIG. 6B. That is, the movable electrode 203 is slightly displaced in the Z direction from the reference position in FIG. The servo signal generation unit 120 generates the electrostatic force so that the movable electrode 203 of the sensor 101 is in an equilibrium state according to the change amount according to the acceleration and the detection signal S2 corresponding to the capacitance change. Servo control is performed.

  The servo signal generation unit 120 generates the servo switching signal S6 for adjusting the pulse density of the PWM voltage signal in accordance with the detection signal S2, as in the first servo control and FIG. Similar to the control and FIG. 5, the PWM voltage level switching signal S5 for adjusting the level of the PWM voltage signal is generated. The multi-level servo signal generation unit 105 generates a PWM voltage signal in which both the pulse density and the level are adjusted, as shown in FIG. 6C, based on the servo switching signal S6 and the PWM voltage level switching signal S5. And output as a multi-level servo signal S7. The output servo signal S7 is applied to the fixed electrodes 201 and 202 of the sensor 101 in the servo control mode.

  On the basis of the division by the threshold value Dth in FIG. 5 described above, when the acceleration is a relatively large period, the first PWM voltage signal having a large level and amplitude, as in the example of the period 601 in FIG. Is output. In this period 601, the level of the PWM voltage signal becomes the first level 611 corresponding to the first high-side voltage and correspondingly becomes the first amplitude 621. In the case where the acceleration is relatively small, a second PWM voltage signal having a small level and amplitude is output as in the example of the period 602 in FIG. In this period 602, the level of the PWM voltage signal becomes the second level 612 corresponding to the second high-side voltage and correspondingly becomes the first amplitude 622.

  Further, in the first PWM voltage signal in the period 601 and the second PWM voltage signal in the period 602, as shown in FIG. 4, the magnitude of the pulse density depends on the magnitude of the acceleration in the period. Adjustments are made in the same manner as in d).

  As described above, the acceleration sensor system according to the first embodiment adjusts both the level of the PWM voltage signal and the pulse density according to the magnitude of the capacitance change corresponding to the acceleration by the third servo control. . In the third servo control, in a period in which the acceleration is relatively small, the level and amplitude of the PWM voltage signal are reduced to reduce the voltage value Vmax, and the pulse density is increased in accordance with the acceleration to set the switching frequency N. Do more. Thereby, during this period, the power consumption (W) that increases in proportion to the voltage value Vmax and the number of times of switching N can be reduced based on the above-described calculation formula.

[Acceleration output section]
FIG. 7 shows a circuit configuration example of the acceleration detection unit 131 as details of the output unit 130. The acceleration detection unit 131 in FIG. 7 includes a digital filter unit 131A and a variable gain unit 131B. The acceleration detector 131 calculates acceleration from the PWM voltage level switching signal S5 and the servo switching signal S6 by a known acceleration calculation process. A PWM voltage level switching signal S5 is input to the digital filter unit 131A. The output signal of the digital filter 131A is input to the variable gain unit 131B, and the servo switching signal S6 is input as a control signal.

  The digital filter unit 131A has a low-pass filter characteristic. The digital filter unit 131A is designed to have a frequency characteristic that allows the frequency component of the acceleration signal band of the PWM voltage level switching signal S5 to pass.

  The variable gain unit 131B switches the gain according to the servo switching signal S6. The variable gain unit 131B switches the amplification factor according to the servo switching signal S6, amplifies the output signal of the digital filter unit 131A, and outputs it as an acceleration signal. This acceleration signal corresponds to the aforementioned acceleration information S10.

[Effects]
As described above, according to the acceleration sensor system of the first embodiment, the high-sensitivity is ensured by the capacitance type and servo type configurations, and the level of the PWM voltage signal and the level according to the magnitude of the acceleration. A third servo control for adjusting the pulse density is performed. Thereby, according to the acceleration sensor system of Embodiment 1, low power consumption is realizable compared with the conventional acceleration sensor system which is not adjusting the level of a PWM voltage signal. In particular, according to the acceleration sensor system of the first embodiment, the power consumption in a period in which the acceleration is small can be reduced by adjusting the level. According to the acceleration sensor system of the first embodiment, the power consumption can be reduced comprehensively even when the period during which the acceleration is small is long depending on the application, and the power saving of the entire acceleration sensor system can be realized.

  FIG. 8 shows an example in which the acceleration sensor system according to the first embodiment is applied to automobile component failure detection. The acceleration sensor system according to the first embodiment is applicable not only to the above-described seismic measurement detection but also to various uses such as automobile part failure detection.

  In the automobile 700, an acceleration sensor system 704 is installed in the vicinity of the motor 701, and acceleration sensor systems 705 and 706 are installed in the vicinity of the axles 702 and 703. Each of these acceleration sensor systems detects the acceleration of the corresponding part including the change over time. The acceleration sensor system uses the detected acceleration to perform analysis processing according to the application by the analysis unit 132 of FIG. 1, thereby outputting useful information S11 according to the application.

  In the case of this application, the analysis unit 132 detects a failure or a failure sign of the motor 701 or the axles 702 and 703. For example, the acceleration sensor system 704 detects the acceleration of the vibration of the motor 701, and the analysis unit 132 detects a failure or a failure sign related to the motor 701 by the analysis process of the acceleration, and uses useful information S11 including the detection information. Output. The automobile and its system can effectively utilize the acceleration and useful information output by the acceleration sensor system, and can contribute to improvement of safety and fuel consumption, for example, compared to conventional systems. According to the acceleration sensor system of the first embodiment, high-sensitivity acceleration can be used to improve the accuracy of analysis in various applications such as earthquake measurement detection and automobile component failure detection, and to improve the accuracy and effectiveness of useful information. Can do.

  When the use of the acceleration sensor system is, for example, seismic measurement detection, it is as follows. This acceleration sensor system is installed in one or more places according to a measurement object. Note that the acceleration sensor system may include a plurality of sensors 101 and may be configured to input a plurality of data from the plurality of sensors 101 to one output unit 130 and calculate acceleration and useful information.

  In the case of seismic measurement detection, for example, the analysis unit 132 inputs data from the sensors 101 installed at a plurality of locations, and calculates the arrival direction of the seismic wave, the acceleration attenuation rate, and the like based on the analysis processing of the data. The epicenter and the earthquake intensity are identified and output as useful information S11.

  The analysis unit 132 may perform, for example, the following analysis process according to the application. The analysis unit 132 compares the acceleration value and its time waveform with a predetermined threshold range and time waveform pattern. The analysis unit 132 determines a predetermined output value associated with the threshold range or time waveform pattern when the threshold range or time waveform pattern is applicable.

[Modification]
The following is possible as a modification of the acceleration sensor system of the first embodiment.

  (1) In the first embodiment, as shown in FIG. 5 and the like, the method of switching the level and amplitude of the PWM voltage signal in two stages is used as servo control. However, the present invention is not limited to this, and a method of switching the level and amplitude of the PWM voltage signal in a plurality of stages of three or more stages is also possible. For example, the form of switching in three stages is as follows.

  The servo signal generation unit 120 in FIG. 1 uses the first threshold value Dth1 and the second threshold value Dth2 (Dth1> Dth2) as the two threshold values in the comparison determination of the digital signal S3 in FIG. When the value Dn of the digital signal S3 exceeds the first threshold value Dth1, the PID control unit 104 and the PWM voltage level switching unit 106 are within a range that exceeds the first threshold value and the second threshold value Dth2 below the first threshold value Dth1. In this case, the PWM voltage level switching signal S5 is generated so that the second level is obtained and the third level is obtained when the second threshold value Dth2 or less. The circuit of the multi-level servo signal generation unit 105 includes PWM level voltage sources corresponding to the above three levels, and selects from these voltage sources according to the PWM voltage level switching signal S5. The configuration of the acceleration sensor system of this modification example is a little more than the configuration of the first embodiment, but instead, the level of the PWM voltage signal can be adjusted more finely, and lower power consumption can be realized.

  (2) In the first embodiment, the servo control method selected from the first, second, and third servo controls according to the user setting can be used. Not limited to this, it is of course possible to execute only the second servo control or only the third servo control. Furthermore, in another modified example, the servo signal generation unit 120 performs a mode for executing the first servo control, a mode for executing the second servo control, and a mode for executing the third servo control according to the acceleration or the like. , You may use properly.

  (3) The sensor output signal S1 and the detection signal S2 from the sensor 101 are not limited to voltage signals, and can be current signals. Further, the detailed configuration of the servo signal generator 120 may be other than that shown in FIG. For example, not only an analog circuit such as the multilevel servo signal generation unit 105 in FIG. 3 but also a digital signal processing circuit is possible, and a configuration in which software program processing is performed by a CPU or the like is also possible.

  (4) The following is also possible as a modified example related to the second servo control, and the same effect can be obtained. The servo signal generation unit 120 may switch the level of the PWM voltage signal according to a comparison determination with a predetermined threshold, using a difference value between the sampling timings of the digital signal S3. That is, the servo signal generator 120 compares the difference value between the value Dni at a certain timing ni and the value Dni-1 at the previous timing ni-1 with a predetermined threshold value, and the difference value sets the threshold value. The level is switched in two stages depending on whether or not it exceeds.

  The servo signal generation unit 120 calculates a statistical value such as an average value from a plurality of timing values Dn of the digital signal S3, and uses the statistical value to perform PWM comparison according to a comparison determination with a predetermined threshold value. The level of the voltage signal may be switched. For example, the servo signal generation unit 120 calculates an average value of the values Dn of the predetermined timing n at a predetermined number of times, compares the average value with a threshold value, and determines the level depending on whether the average value exceeds the threshold value. Switch in two steps. In addition, a method configured by combining the above methods is also possible.

(Embodiment 2)
The acceleration sensor system according to the second embodiment of the present invention will be described with reference to FIGS. The basic configuration of the acceleration sensor system according to the second embodiment is the same as the configuration of the acceleration sensor system according to the first embodiment, and the PWM voltage signal level switching control method in servo control is further devised as a different component. doing.

  The level switching control method as shown in FIGS. 5 and 6 of the first embodiment can achieve low power consumption, but may not be efficient depending on the acceleration situation. For example, when the value Dn of the digital signal S3 of the ADC 103 corresponding to the detected acceleration has a large value near the threshold value Dth in FIG. 5, the level of the PWM voltage signal may be frequently switched. In that case, the frequent switching of the level may increase the power consumption, and the system operation may become unstable. Therefore, in the acceleration sensor system of the second embodiment, as a servo control method, a method of realizing low power consumption and high stability by reducing the frequency of switching the level of the PWM voltage signal is shown.

[Servo control]
FIG. 9 shows servo control operations and time waveforms in the acceleration sensor system of the second embodiment. FIG. 9A shows a digital signal S3 obtained by sampling in the ADC 104 in the same manner as FIG. FIG. 9B shows a counter value corresponding to the value Dn of the digital signal S3. The counter value is Cn. “N” corresponds to the sampling timing. A predetermined threshold for the counter value Cn is Cth. FIG. 9C shows a time waveform of the voltage Vs of the PWM voltage level switching signal S5.

  In the second embodiment, the servo signal generation unit 120 compares and determines the value Dn of the digital signal S3 with the threshold value Dth, and when the value Dn exceeds the threshold value Dth, the level of the PWM voltage signal is set to the first high-side voltage. Switch to the first level corresponding to. The servo signal generator 120 continues to count the counter value Cn when the state where the value Dn is less than or equal to the threshold value Dth, and when the counter value Cn is less than or equal to the threshold value Cth, the level of the PWM voltage signal Is maintained at the first level, and when the counter value Cn exceeds the threshold value Cth, it is switched to the second level corresponding to the lower second high-side voltage. In this servo control, even if the value Dn becomes equal to or less than the threshold value Dth, the first level is not immediately switched to the second level, but is switched to the second level when the state continues to some extent.

  FIG. 10 shows a processing flow of the PWM voltage level switching control method corresponding to the operation of FIG. 9 in the servo signal generation unit 120 of the second embodiment. Hereinafter, steps S901 to S911 in FIG. 10 will be described.

  In S901, the servo signal generation unit 120 starts a servo control process. In S902, the servo signal generation unit 120 acquires the value Dn of the digital signal S3. The servo signal generation unit 120 compares the digital signal value Dn acquired in S902 with a predetermined comparison value Dth in S903 and determines. This comparison value Dth is a threshold value for switching the level in two steps, like the threshold value Dth in the first embodiment, and may be the same value as in the first embodiment or a different value.

  When the value Dn is equal to or smaller than the comparison value Dth as a result of the comparison between the value Dn and the comparison value Dth (S903-Y), the servo signal generation unit 120 counts the counter value Cn in S904. That is, the servo signal generation unit 120 substitutes a value obtained by adding 1 to the counter value C (n−1) at the previous sampling timing (n−1) for the counter value Cn. When the value Dn exceeds the comparison value Dth (S903-N), the servo signal generation unit 120 sets the counter value Cn to 0 in S905.

  Next, in S906, the servo signal generation unit 120 evaluates whether the voltage Vs (n) of the PWM voltage level switching signal S5 corresponding to the PWM voltage signal is the high-side voltage VH. The voltage Vs (n) indicates a voltage value at the timing n. When the voltage Vs (n) is the high side voltage VH (S906-Y), the process proceeds to S907. In S907, the servo signal generation unit 120 evaluates whether the counter value Cn exceeds the counter threshold Cth. When the counter value Cn exceeds the counter threshold value Cth (S907-Y), the process proceeds to S908. In S908, the servo signal generation unit 120 switches the voltage Vs (n + 1) of the PWM voltage level switching signal S5 at the next timing (n + 1) from the high side voltage VH to the low side voltage VL. When the counter value Cn is equal to or smaller than the counter threshold Cth (S907-N), the process proceeds to S911, and the high side voltage VH is maintained.

  On the other hand, if the voltage Vs (n) is not the high side voltage VH in S906, that is, if it is the low side voltage VL (S906-N), the process proceeds to S909. In S909, the servo signal generation unit 120 evaluates whether the counter value Cn is 0 or not. When the counter value Cn is 0 (S909-Y), the process proceeds to S910, and in S910, the servo signal generation unit 120 determines the voltage Vs (n + 1) of the PWM voltage level switching signal S5 at the next timing (n + 1). ) Is switched from the low-side voltage VL to the high-side voltage VH. When the counter value Cn is not 0 (S910-N), the process proceeds to S911 and the low-side voltage VL is maintained.

  When the processing up to S910 is completed, in S911, the servo signal generation unit 120 increments the timing n by 1, that is, substitutes (n + 1) for n, returns to S902 as in A, and the subsequent timing n Similarly, the loop process is repeated.

  By the processing in FIG. 10, the operation as in the example in FIG. 9 is realized. In FIG. 9, the PWM voltage signal is first at a first level corresponding to the high side voltage VH. For example, at the timing n5, the value Dn exceeds the threshold value Dth, so the counter value Cn is 0. A period 801 indicates an example of a period in which the value Dn is equal to or less than the threshold value Dth. In the period 802, the state where the value Dn is equal to or less than the threshold value Dth continues, and the counter value Cn is counted up. At timing n10, the counter value Cn exceeds the counter threshold value Cth. Thereby, the PWM voltage signal is switched to the second level corresponding to the low-side voltage VL. In the period 803, the counter value Cn is further counted up. At time n14, the value Dn exceeds the threshold value Dth. As a result, the counter value Cn is returned to 0, and the PWM voltage signal is switched to the first level corresponding to the high-side voltage VH. In the period 803, it is counted up again.

  As described above, in the servo control system according to the second embodiment, only when the value Dn equal to or less than the threshold value Dth in the digital signal S3 is continuously counted a predetermined number of times corresponding to the count threshold value Cth according to the acceleration, the PWM is performed. The level of the voltage signal is switched from the high side voltage VH to the low side voltage VL. On the other hand, when the value Dn exceeds the threshold value Dth, the level of the PWM voltage signal is immediately switched from the low side voltage VL to the high side voltage VH.

  Therefore, according to the acceleration sensor system of the second embodiment, even when the value Dn fluctuates in the vicinity of the threshold value Dth, the switching frequency can be reduced without frequently switching the level of the PWM voltage signal. Thereby, according to Embodiment 2, power saving and high stability are realizable.

(Embodiment 3)
The acceleration sensor system according to the third embodiment of the present invention will be described with reference to FIGS.

[Acceleration sensor system]
FIG. 11 shows the configuration of the acceleration sensor system of the third embodiment. The configuration of the third embodiment has a sensor 101C as a different component from the configuration of the first embodiment in FIG. 1, and does not have the operation mode switching unit 111.

  The sensor 101C is an acceleration detection element as in the sensor 101 of the first embodiment, but the electrode structure is different. The sensor 101C has a separation type structure with respect to a pair of fixed electrodes as an electrode structure capable of simultaneously applying the detection signal S8 and the servo signal S7.

  The acceleration sensor system of the third embodiment does not perform time-sharing control according to the operation mode for the sensor 101C. The detection signal S8 and the servo signal S7 are simultaneously applied to the fixed electrode of the sensor 101. The detection signal S8 from the detection signal generation unit 110 is directly applied to the first pair of fixed electrodes of the fixed electrodes of the sensor 101. The servo signal S7 from the servo signal generation unit 120 is directly applied to the second pair of fixed electrodes of the fixed electrodes of the sensor 101.

  Similarly to the first embodiment, the signal detection unit 102 detects the sensor output signal S1 from the movable electrode of the sensor 101 as the detection signal S2. As in the first embodiment, the servo signal generation unit 120 generates a PWM voltage signal in which both the level and the pulse density are adjusted according to the acceleration indicated by the detection signal S2, and outputs the PWM voltage signal as the servo signal S7.

[Sensor]
FIG. 12 shows an electrode structure of the sensor 101C in the acceleration sensor system of the third embodiment. The sensor 101C includes a movable electrode 203 and fixed electrodes 201A, 201B, 202A, 202B. The four fixed electrodes are separated into two sets, the fixed electrode 201A and fixed electrode 202A that are the first pair of fixed electrodes 230, and the fixed electrode 201B and fixed electrode 202B that are the second pair of fixed electrodes 240. It consists of. In the third embodiment, the fixed electrode 201 of the first embodiment is separated into the fixed electrode 201A and the fixed electrode 201B, and the fixed electrode 202 of the first embodiment is separated into the fixed electrode 202A and the fixed electrode 202B. Has been.

  In the X direction, the fixed electrode 201A and the fixed electrode 202A, which are the first pair of fixed electrodes 230, are arranged on the left side with respect to the center of the movable electrode 203 so as to face each other with the movable electrode 203 interposed therebetween in the Z direction. The fixed electrode 201 </ b> B and the fixed electrode 202 </ b> B, which are the second pair of fixed electrodes 240, are arranged in the same manner at positions on the right side. Each fixed electrode includes a surface region that overlaps when viewed in plan in the Z direction with respect to the main surface of the movable electrode 203.

  The movable electrode 203 of the sensor 101C is connected to the signal detection unit 102 through the signal line as in the first embodiment, and outputs a change in the interelectrode capacitance according to the acceleration as the sensor output signal S1. Note that various detailed structures of the movable electrode 203 are possible. For example, a structure in which the movable electrode is held by a spring or the like from both the upper and lower sides in the Z direction is also possible.

  The first pair of fixed electrodes 230 is connected to the detection signal generation unit 110 through a signal line, and the detection signal S8 is applied thereto. Specifically, the first detection signal 221 in the detection signal S1 is applied to the fixed electrode 201A, and the second detection signal 222, which is an inverted signal, is applied to the fixed electrode 202A.

  The second pair of fixed electrodes 240 is connected to the servo signal generation unit 120 through a signal line, and the servo signal S7 is applied thereto. Specifically, the first servo signal 211 in the servo signal S7 is applied to the fixed electrode 201B, and the second servo signal 212 that is an inverted signal is applied to the fixed electrode 202B.

  As described above, according to the third embodiment, as in the first embodiment, it is possible to achieve lower power consumption than a conventional acceleration sensor system while ensuring high sensitivity. Since the sensor 101C of the third embodiment has a separation type structure, time-division control relating to the application of the detection signal S8 and the servo signal S7 is unnecessary, and simultaneous application is possible. Therefore, in the third embodiment, as the configuration other than the sensor 101C, the operation mode switching unit 111 and the like can be eliminated, and the configuration can be simpler than the circuit configuration of the first embodiment. In the third embodiment, the switching noise and the switching current generated when the detection signal S8 and the servo signal S7 are switched in a time division manner in the first embodiment can be reduced, and the power consumption can be reduced accordingly.

  Although the present invention has been specifically described above based on the embodiments, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the invention.

  DESCRIPTION OF SYMBOLS 101 ... Sensor, 102 ... Signal detection part, 103 ... ADC, 104 ... PID control part, 105 ... Multi-level servo signal generation part, 106 ... PWM voltage level switching part, 107 ... Comparator, 110 ... Detection signal generation part, DESCRIPTION OF SYMBOLS 111 ... Operation mode switching part, 120 ... Servo signal generation part, 130 ... Output part, 131 ... Acceleration output part, 132 ... Analysis part.

Claims (10)

An acceleration sensor system for detecting acceleration,
A sensor that outputs a signal representing a change in capacitance according to the acceleration;
A signal detector that inputs the signal and detects it as a detection signal;
A detection signal generation unit that generates a detection signal for obtaining the signal and applies the detection signal to the sensor;
A servo signal generator for generating a PWM voltage signal as a servo signal for servo control based on the detection signal and applying the PWM voltage signal to the sensor;
An output unit that outputs information including acceleration based on the detection signal;
With
The servo signal generation unit includes at least two levels of the PWM voltage signal including a first level and a second level lower than the first level according to the magnitude of the capacitance change. Switch and adjust by level,
Acceleration sensor system. The acceleration sensor system according to claim 1,
The servo signal generation unit adjusts the pulse density of the PWM voltage signal by switching between at least two pulse densities according to the magnitude of the capacitance change.
Acceleration sensor system. The acceleration sensor system according to claim 2,
The signal detection unit converts the signal into a voltage and outputs an amplified analog signal as the detection signal,
The servo signal generator is
An ADC that converts an analog signal as the detection signal into a digital signal;
A PID controller that processes the digital signal and outputs a PID control signal;
A comparator that generates a servo switching signal for switching the pulse density of the PWM voltage signal in response to the PID control signal;
A PWM voltage level switching unit that generates a PWM voltage level switching signal for switching the level of the PWM voltage signal according to the PID control signal;
A multi-level servo signal generation unit that generates the PWM voltage signal in which the level and the pulse density are adjusted based on the PWM voltage level switching signal and the servo switching signal;
Including an acceleration sensor system. The acceleration sensor system according to claim 1,
An operation mode switching unit that switches between the servo signal and the detection signal in a time-sharing manner and applies the sensor;
Acceleration sensor system. The acceleration sensor system according to claim 3 , wherein
The output unit is
An acceleration output unit that calculates the acceleration using the PWM voltage level switching signal and the servo switching signal and outputs information including the acceleration;
An analysis unit that outputs useful information by performing an analysis process according to the application using the acceleration, and
Including an acceleration sensor system. The acceleration sensor system according to claim 3 , wherein
The servo signal generator is
A first high-side voltage source and a second high-side voltage source as voltage sources for setting the first level and the second level in the PWM voltage signal;
A low-side voltage source for setting a low-side voltage in the PWM voltage signal;
A level switch for switching and outputting the first high-side voltage source and the second high-side voltage source according to the PWM voltage level switching signal;
In accordance with the servo switch signal, the low-side voltage source and a servo switch that switches and outputs the output of the level switch;
An acceleration sensor system. The acceleration sensor system according to claim 1,
The servo signal generator is
An ADC that converts the detection signal analog signal into a digital signal;
At each sampling timing of the digital signal of the ADC, the value of the digital signal is compared with a threshold value, and if the value exceeds the threshold value, the PWM voltage signal is set to the first level. If the value is less than or equal to the threshold, the PWM voltage signal is set to the second level;
Acceleration sensor system. The acceleration sensor system according to claim 1,
The servo signal generator is
An ADC that converts the detection signal analog signal into a digital signal;
For each sampling timing of the digital signal of the ADC, obtain the value of the digital signal, compare the value with a comparison value,
When the value is less than or equal to the comparison value, the counter value is increased by 1. When the value exceeds the comparison value, the counter value is set to 0.
If the PWM voltage signal is at the first level and the counter value exceeds a counter threshold, the PWM voltage signal is switched to the second level;
When the PWM voltage signal is at the second level and the counter value is 0, the PWM voltage signal is switched to the first level;
Acceleration sensor system. The acceleration sensor system according to claim 1,
The sensor is
A movable electrode that is displaced in a first direction according to the acceleration and outputs the signal;
A pair of fixed electrodes disposed on both sides in the first direction so as to sandwich the movable electrode, to which the detection signal and the servo signal are applied;
Including an acceleration sensor system. The acceleration sensor system according to claim 1,
The sensor is
A movable electrode that is displaced in a first direction according to the acceleration and outputs the signal;
A first pair of fixed electrodes to which the detection signal is applied, arranged at a distance on both sides in the first direction so as to sandwich the movable electrode, and separated in a second direction; and A second pair of fixed electrodes to which the servo signal is applied;
Including an acceleration sensor system.

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