JP2005315805A - Sensor system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an acceleration sensor system for vehicles, which is capable to correct zero point drift induced from temporal variations of the acceleration sensor and temperature variability at an arbitrary timing regardless of horizontal situation of the vehicles, while minimizing dependence on vehicle signals during the period of correction and having function in which even sensor simplex can nearly complete corrective actions. <P>SOLUTION: In the sensor system, for example, before its shipment, both ends of a negative feedback capacitor 21b of a charge-voltage translate circuit 21 outputting acceleration signals, based on charge output variations of sensing capacitors 10a and 10b, are short-circuited and a reference value of offset outputs from an acceleration signal generating circuit measured in that state is memorized as a reference offset output amplitude. After that, both ends of the negative feedback capacitor 21b is kept in a short-circuited state at a predetermined checking time during the period of usage of the sensor, the offset outputs from the signal processing circuit are detected as the current offset output value at the state concerned, and the zero point drift correction of acceleration signals is implemented based on comparing the current offset outputs concerned with the referring offset output value. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a capacitance type acceleration sensor system used for vehicle control.

JP 2001-336618 A

  Capacitance type acceleration sensors are widely used for vehicle control. This type of acceleration sensor has a sensing capacitor with a variable distance between the electrode plates, and the charge / voltage conversion circuit converts the charge accumulation state of the capacitor based on the change in the distance between the electrode plates due to the application of acceleration by a charge-voltage conversion circuit. Thus, an acceleration signal is output.

  In recent years, the acceleration sensor for vehicles has been required to have a higher sensitivity. However, since the charge-voltage conversion circuit is used for the output part of the sensor, the main part of the charge-voltage conversion is improved as the sensitivity of the sensor is improved. As a result, the zero point drifts over time or due to temperature changes due to deterioration or temperature of the operational amplifiers and peripheral discrete elements.

  In Patent Document 1, as a solution, a condition for horizontally detecting the state of the vehicle (transmission neutral + brake release + wheel) in order to automatically correct errors due to the temperature and aging of the acceleration sensor attached to the vehicle. And a method of correcting the acceleration sensor only when the condition is met is disclosed.

However, the method of Patent Document 1 has the following problems.
(1) Since the zero point cannot be corrected unless the vehicle is in a horizontal state, the correction timing is limited.
(2) Since a signal from the vehicle is essential to acquire the horizontal detection condition, automatic correction cannot be performed with the acceleration sensor alone.

  The problem of the present invention is that it is possible to correct the aging of the acceleration sensor and the zero point drift accompanying the temperature at any timing regardless of whether the vehicle is in a horizontal state, and minimize the dependence on the vehicle signal during the correction. Therefore, an object of the present invention is to provide a vehicular acceleration sensor system having a function capable of almost completing correction processing with a single sensor.

Means and actions / effects for solving the problem

In order to solve the above problems, an acceleration sensor system for a vehicle according to the present invention includes:
A sensing capacitor that changes the distance between the electrode plates with the application of acceleration;
A charge-voltage conversion circuit comprising an operational amplifier having a negative feedback capacitor for charge detection, and inverting the charge output change of the detection capacitor to output it as an acceleration signal in a voltage-converted form;
A standard offset output value storage unit for storing a standard offset output value, which is a standard value of an offset output of an acceleration signal generation circuit including a charge-voltage conversion circuit when both ends of the detection capacitor are short-circuited;
The negative feedback capacitor is short-circuited at a predetermined check timing while the sensor is in use, and the offset output of the acceleration signal generation circuit in this state is detected as the current offset output value, and the standard offset output value and the current offset are detected. And zero point correction means for correcting the zero point of the acceleration signal based on the comparison with the output value.

  According to the above configuration of the present invention, for example, when the sensor system is shipped from the factory, both ends of the negative feedback capacitor of the charge-voltage conversion circuit that outputs the acceleration signal based on the change in the charge output of the detection capacitor are short-circuited and measured in that state. The standard value of the offset output of the acceleration signal generation circuit is stored as a standard offset output value. Then, both ends of the negative feedback capacitor are short-circuited at a predetermined check timing during use of the sensor, the offset output of the acceleration signal generation circuit in this state is detected as a current offset output value, and the standard offset output value and the Based on the comparison with the current offset output value, zero point correction of the acceleration signal is performed. In this method, the offset output of the current signal processing circuit is measured at any time by short-circuiting the negative feedback capacitor at the timing required for checking, and this is compared with the standard offset output value to measure the acceleration measured in the standard state. From the signal, it is possible to easily and reliably grasp how much the offset output has changed. As a result, it becomes possible to correct the zero point drift caused by various factors such as aging of the acceleration sensor and temperature characteristics regardless of whether the vehicle is in a horizontal state. In addition, as long as the current offset output, that is, the current offset output value and the standard offset output value measured and stored in advance, the correction process itself can be basically completed with the sensor alone, and the vehicle signal for correction can be Can be kept to a minimum. In addition, when the negative feedback capacitor is short-circuited, the capacitance of the sensing capacitor does not affect the output of the acceleration signal generation circuit, and even when gravitational acceleration acts as a background on sloping ground, the offset output No longer has any effect. Therefore, horizontal detection as in Patent Document 1 is also unnecessary.

  As the standard offset output value, it is possible to use an offset output value measured at a reference timing that precedes the check timing in time series (desirably, at a timing when the sensor system is new or close to that at the time of factory shipment, for example). The zero point correction means is configured to correct a drift with time of the offset output value between the reference timing and the check timing. Even if the zero-point drift of the charge-voltage conversion circuit (or the acceleration signal generation circuit including it) occurs due to factors over time from the reference timing at which the standard offset output value was measured, Can be reliably corrected.

  As a simple method for switching both ends of the negative feedback capacitor of the charge-voltage conversion circuit between short-circuit and non-short-circuit for zero point correction processing, a short-circuit path for short-circuiting both ends of the negative feedback capacitor of the charge-voltage conversion circuit A configuration in which a negative feedback capacitor short-circuit switch unit is provided on top can be exemplified. In this case, the zero point correction means is configured to have a switch control means that opens the negative feedback capacitor short-circuiting switch part at the normal time of acceleration detection and closes the negative feedback capacitor short-circuiting switch part at the check timing. That's fine.

  Next, as described above, the temperature of the sensor use environment greatly affects the offset output of the acceleration sensor. Therefore, it is effective to configure the zero point correction means as having a sensor output temperature correction means. In this case, the standard offset output value measured at a reference temperature (constant room temperature: for example, 20 ° C.) can be used. In addition, the temperature correction means includes a temperature sensor, a correction content information storage unit that stores correction content information for sensor outputs at various temperatures, and a correction information storage based on the detected temperature value detected by the temperature sensor at the check timing. In addition to drift components due to secular changes and gravitational acceleration on slopes, temperature drift components can be used at the same time. Correction can be made and the accuracy of acceleration measurement can be further increased.

  FIG. 1 shows an example of the configuration of a capacitive sensor unit used in the acceleration sensor system of the present invention. The sensor unit includes a sensor element 10 and a detection circuit 20. In the sensor element 10, an inertial displacement body 2c that generates a displacement by sensing acceleration is coupled to a substrate 1 functioning as a sensor frame by an anchor portion 2a via a beam portion 2b. A movable electrode 2d is integrated with the inertial displacement body 2c, and fixed electrodes 3 and 4 are formed on the substrate 1 so as to face the movable electrode 2d. The inertial displacement body 2c, the beam portion 2b, the anchor portion 2b, and the movable electrodes 2d and 2d form an integral beam structure 2.

  In the present embodiment, the entire beam structure 2 is formed in a plate shape. The inertial displacement body 2c has a rectangular shape, and the beam portion 2b is formed so as to extend from each corner of the inertial displacement body 2c in the short side direction, and its end is an anchor portion 2a. On the pair of long sides of the inertial displacement body 2c, a plurality of movable electrodes 2d are arranged in a comb shape at regular intervals. On the other hand, the fixed electrodes 3 and 4 on the substrate 1 side are opposed to each other so as to form a pair on both sides of each movable electrode 2d. The beam structure 2 is an integral conductor, and the potentials of the individual movable electrodes 2d are the same. Further, the set of fixed electrodes 3 facing the first side of each movable electrode 2d and the set of fixed electrodes 4 facing the second side are insulated from each other on the substrate 1 between the sets. Commonly connected. Thereby, the fixed electrodes 3 and the fixed electrodes 4 have the same potential.

  With such an electrode arrangement, as shown in FIG. 2, the fixed electrodes 3 and 4 form a pair of detection capacitors 10a and 10b connected in series with the movable electrode 2d as a common electrode. The opposing direction of the movable electrode 2d and the fixed electrodes 3 and 4 (the long side direction of the inertial displacement body 2c in FIG. 1) is the acceleration detection direction (D1 and D2 in FIG. 1). When acceleration in the detection direction is applied to the sensor part, the inertial displacement body 2c is displaced by elastically bending and deforming the beam part 2b by the inertial force, and the movable electrode 2d and the fixed electrodes 3 and 4 , That is, the capacitances of the detection capacitors 10a and 10b change. Therefore, if a detection bias voltage is applied to the detection capacitors 10a and 10b, a change in the capacitance of the detection capacitors 10a and 10b appears as a change in the accumulated charge state, as shown in FIG. This can be output as an acceleration signal Vs by voltage conversion by the charge-voltage conversion circuit 21.

  In addition, the sensor part including the beam structure 2 and the substrate 1 as a main part is combined with a comb-like electrode structure and the like, and is formed as an integral structure by fine processing of the Si substrate by dry etching technology such as high energy plasma etching. One-chip (this is a typical application of so-called MEMS (Micro-Electro Mechanical Systems) technology).

  Hereinafter, the configuration of the detection circuit 20 will be described in more detail with reference to FIG. The detection circuit 20 has a charge-voltage conversion circuit 21, a signal processing circuit 22, a control signal generation circuit 24, and a microcomputer 53 as main parts. The operational amplifier 21a is provided with a negative feedback capacitor 21b for charge detection. The charge output change of the detection capacitors 10a and 10b is inverted and input to output it as an acceleration signal in the form of voltage conversion. The inverting input terminal of the operational amplifier 21a is connected to the movable electrode 2d. Further, assuming that the detection bias voltage of the detection capacitors 10a and 10b is V, a voltage of V / 2 is input to the non-inverting input terminal of the operational amplifier 21a. Further, a short-circuit path 21s for short-circuiting both ends of the negative feedback capacitor 21b is provided, and a negative feedback capacitor short-circuit switch unit 21c (hereinafter also simply referred to as a switch 21c) is provided on the short-circuit path 21s.

  The signal processing circuit 22 includes a sample and hold circuit (hereinafter also referred to as S & H circuit) 22a, an amplifier circuit (AMP) 22b, and a low-pass filter (LPF) circuit 22c. The S & H circuit 22a samples the output voltage of the charge voltage conversion circuit 21 and holds it for a certain period, and the amplifier circuit 22b amplifies the output voltage of the S & H circuit 22a to a predetermined sensitivity. The low-pass filter circuit 22c serves to extract only a component in a predetermined frequency band from the acceleration output voltage of the amplifier circuit 22b. The S & H circuit 22a has a well-known configuration including operational amplifiers 221a and 221b, a switch 221b, and a capacitor 221c that constitute a voltage follower. The signal processing circuit 22 and the charge / voltage conversion circuit 21 constitute an acceleration signal generation circuit.

  The control signal generation circuit 24 switches the pulse carrier signals P1 and P2 for applying a detection bias voltage whose polarity is inverted and inverted periodically to both ends of the detection capacitors 10a and 10b, and the switch 221b for sampling the acceleration signal. The switch drive signal S1 for driving and the switch drive signal S1 for driving the negative feedback capacitor short-circuiting switch portion 21c in order to periodically discharge the charge of the negative feedback capacitor 21b of the charge-voltage conversion circuit 21 as necessary. Are generated based on the clock signal CK from the clock circuit 25, and are constituted by a known pulse counter circuit.

  Further, the microcomputer 53 is a main part of the zero point correction means, and detects the temperature around the A / D conversion port AD1, which receives the pre-correction acceleration signal Vg ′ (T) from the signal processing circuit 22, and the sensor unit. The A / D conversion port AD2 to which the detection output from the temperature sensor 55 is input. Independent of the switch drive signal S1 from the control signal generation circuit 24, the zero point of the pre-correction acceleration signal Vg ′ (T) from the charge / voltage conversion circuit 21 (and the signal processing circuit 22 in the lower stage thereof) is also provided. In order to correct the drift, a signal output port D1 that outputs a switch drive signal S1 ′ for checking at a predetermined check timing, a signal output port D2 that outputs a corrected acceleration signal Vg, and the like are also provided.

Further, the microcomputer 53 has a standard value Vk (T0) (standard offset output value: offset from the signal processing circuit 22 in the lower stage) of the offset output Vk of the charge-voltage conversion circuit 21 when the negative feedback capacitor 21b is short-circuited. An output value Vg ′ (T) may be stored), and an EEPROM (Electrically
Erasable Programmable ROM) 54 is connected. The EEPROM 54 also stores a correction table 54a that stores temperature correction coefficients (α and β) of the acceleration signal Vg for each of various temperatures. The acceleration signal Vg correction processing by the microcomputer 53 configured as described above is performed by the CPU executing a control program stored in the ROM in the microcomputer using the RAM as a work area in accordance with flowcharts shown in FIGS.

  The operation of the angular velocity sensor system configured as described above will be described with reference to the signal waveform diagram shown in FIG. As shown in FIG. 2, the carrier signals P1 and P2 that provide the detection bias voltage output from the control signal generation circuit 24 are high level (Hi) and low level (Lo) in four periods (φ1 to φ4). Is a rectangular wave pulse signal having a constant amplitude (voltage amplitude: V), and the carrier wave signal P2 is a signal whose voltage level is inverted with respect to the carrier wave signal P1.

  First, the operation during normal operation will be described with reference to FIG. In the periods φ1 and φ2, the carrier signal P1 is Hi (5 V), and the carrier signal P2 is Lo (0 V). In the period φ1, the switch 21c is closed by the switch drive signal S1 from the control signal generation circuit 24. As a result, a voltage V / 2 is applied to the non-inverting input terminal of the operational amplifier 21a, a voltage V / 2 is applied to the movable electrode 2d, and the capacitor 21b is discharged. At this time, the output Vs of the Danya voltage conversion circuit 21 is reset to V / 2.

  If the capacitance C1 of the detection capacitor 10a and the capacitance C2 of the detection capacitor 10b satisfy the relationship C1> C2, the applied voltage is on the Hi side during the period φ1 in which the detection capacitor 10a fixed electrode 3 side is Hi. Thus, the divided voltage of the detecting capacitor 10a becomes lower, and the movable electrode 2d is in a state where there is much negative charge (the reverse is true when C1 <C2). Next, in the period φ2, the polarity of the detection bias voltage applied to the detection capacitors 10a and 10b remains unchanged, while the switch 21c is opened by the switch drive signal S1. Then, charge transfer occurs between the charge of the movable electrode 2d and the negative feedback capacitor 21b, and a voltage value Vg1 ″ corresponding to the charge balance state is output from the charge-voltage conversion circuit 21. This output value is a specified value. When the switch 221b is turned on / off by the switch drive signal S2 at the timing, it is sampled and held by the S & H circuit 22a.

  Next, in the period φ3 in FIG. 3, the voltage levels of the carrier wave signals P1 and P2 are inverted (P1 is Lo and P2 is Hi). The switch 21c is kept open by the switch drive signal S1. The charge state of the movable electrode 2d is reversed from the period φ2 due to the inversion of the carrier wave signals P1 and P2, and when the relationship C1> C2 is satisfied as described above, the voltage applied to the fixed electrodes 2a and 2b is inverted. The movable electrode 2d has a large positive charge. However, since the movable electrode 2d and the capacitor 21b are in a closed circuit and the electric charge of the period φ1 is held, the electric charge overflowing from the movable electrode 2d due to the breakage of the charge amount balance moves to the capacitor 21b. Stored. If the capacitance of the capacitor 21b is constant, a voltage value Vg2 ″ proportional to the above-described charge transfer amount is output from the charge-voltage conversion circuit 21 from the relationship of Q = CV. During the period φ4, the voltage value Vg2 ″ is output. This is a waiting period for stabilization, and the value is sampled by the S & H circuit 22a at a predetermined timing when the Vs2 is sufficiently stabilized. The difference value between the output value Vg1 ′ sampled in the period φ2 and the output value Vg2 ′ sampled in the period φ4 (both after passing through the amplifier circuit 23b and the LPF circuit 23c) is obtained in the microcomputer 53. It is calculated and output as an acceleration output value Vg corresponding to the displacement of the movable electrode 2d. In the present embodiment, the microcomputer 53 calculates the difference between the first sampling value Vg1 ′ and the second sampling value Vg2 ′. However, the first sampling value Vg1 ′ and the second sampling value are used. A configuration is also possible in which the microcomputer does not perform the difference, such as holding the value Vg2 ′ by a separate S & H circuit and calculating the difference between each hold value by a differential amplifier circuit or the like to generate an acceleration signal.

  By the way, in both Vg1 ′ and Vg2 ′, the offset output serving as the base of the acceleration waveform to be detected is the deterioration with time of the charge-voltage conversion circuit 21 or the amplification circuit 23b or LPF circuit 23c on the downstream side, the sensor environment temperature. Or a bias due to gravitational acceleration on the slope, etc., which becomes an error factor in acceleration detection. In the circuit of FIG. 2, when the switch 21c is closed and both ends of the negative feedback capacitor 21b are short-circuited, the charge-voltage conversion circuit 21 becomes substantially the same circuit as the voltage follower, and theoretically the input voltage V / Since an offset voltage unique to the circuit is output with 2 as a zero point voltage, the current offset output can be known by reading this.

  For example, when the sensor system is shipped from the factory, the offset output value is measured by the above method in a stationary state at the standard temperature T0 (for example, 20 ° C.), and this is written in the EEPROM 54 as the standard offset output value VF (T0). Leave in. Then, after the start of use of the sensor system, if the offset output value changes due to the deterioration of the acceleration signal generation circuit including the charge-voltage conversion circuit 21 with the passage of time or the influence of the environmental temperature change (that is, the zero point drifts). If the offset output value VF (T) is measured at any time and compared with the above standard offset output value VF (T0), the zero point drift amount can be grasped, and according to the grasped zero point drift amount. The acceleration signal can be corrected.

  The offset output VF (T) is measured, for example, when the vehicle is stopped. In this case, as shown in FIG. 2, the vehicle speed information from the vehicle speed sensor 55 can be obtained to determine stop / running. However, if the offset output VF (T) is measured when the engine is started, the vehicle speed information is obtained. Is no longer necessary. Specifically, the microcomputer 53 receives the engine start signal, determines that this is the check timing, and outputs the drift check signal S1 'from the signal output port D1. This drift check signal S1 'is ORed with the charge reset switch drive signal S1 of the negative feedback capacitor 21b at the gate 26, and input to the switch 21c to close it. Since the output of the charge-voltage conversion circuit 21 at this time gives an offset output value at the check timing, that is, a current offset output value VF (T), this may be sampled by the S & H circuit 22a.

The above correction processing is performed by sampling acceleration signal output values Vg1 ′ and Vg2 ′.
(Hereinafter, both are collectively referred to as Vg ′ (T) to indicate the value at the temperature T) can be executed in software in the microcomputer 53. The purpose is to convert the zero-point drift acceleration signal output value Vg ′ (T) at an arbitrary temperature T into an acceleration signal value Vg (T0) at a standard temperature T0 that is not zero-point drifted. . If the zero point drift amount at the current temperature T is Vk (T), the acceleration signal output value Vg ′ (T) before correction is expressed by the following equation:
Vg ′ (T) = α (ΔT) · Vg (T0) + Vk (T) (1)
However, ΔT is a temperature difference between the standard temperature T0 and the current temperature T, and α (ΔT) is a temperature coefficient of Vg (T0) experimentally determined according to the temperature difference ΔT. The value of ΔT can be calculated by obtaining the temperature measurement value T by the temperature sensor 55 of FIG.

Next, the zero point drift amount Vk (T) is a value indicating how much the current offset output value VF (T) has changed with reference to the standard offset output value VF (T0). By correcting the temperature of the value VF (T0),
Vk (T) = VF (T) −α (ΔT) · VF (T0) (2)
Can be expressed as From the above (1) and (2), Vg (T0) can be expressed as follows:
Vg (T0) = {Vg ′ (T) −Vk (T)} / α (ΔT)
= {Vg ′ (T) − (VF (T) −α (ΔT) · VF (T0))} / α (ΔT) (3)

  In equation (3), the value of α (ΔT) is experimentally determined for each value of various temperature differences ΔT, and is in the form of a temperature coefficient table 54a (which constitutes correction content information) as shown in FIG. It is stored in the EEPROM 54. The VF (T0) measured before the shipment of the sensor system is stored in the EEPROM 54. Therefore, if the current offset output VF (T) is measured in a state where no acceleration is generated, Vg ′ (T) obtained directly from the acceleration signal generation circuit is converted into the above α (ΔT), VF (T) and Vk. By (T0), correction calculation can be performed as the acceleration signal value Vg (T0) to be finally obtained.

The schematic diagram which shows the structural example of the sensor part used for the acceleration sensor system of this invention. The circuit diagram which shows an example of the electrical constitution of the acceleration sensor system of this invention. The timing chart which shows an example of operation | movement at the normal time of the acceleration sensor system of this invention. FIG. 3 is a schematic diagram showing the contents stored in an EEPROM of the circuit of FIG. 2. Explanatory drawing of the measurement principle of an acceleration sensor system.

Explanation of symbols

10a, 10b Detection capacitor 21 Charge voltage conversion circuit 21a Operational amplifier 21b Negative feedback capacitor 21c Negative feedback capacitor short-circuit switch unit 21s Short-circuit path 24 Control signal generation circuit (switch control means)
53 Microcomputer (zero point correction means, switch control means)
54 EEPROM (standard offset output value storage)

Claims (5)

A sensing capacitor that changes the distance between the electrode plates with the application of acceleration;
A charge voltage conversion circuit comprising an operational amplifier having a negative feedback capacitor for charge detection, and inverting the charge output change of the detection capacitor to output it as an acceleration signal in a voltage-converted form;
A standard offset output value storage unit for storing a standard offset output value, which is a standard value of an offset output of an acceleration signal generation circuit including the charge voltage conversion circuit when both ends of the detection capacitor are short-circuited;
Both ends of the negative feedback capacitor are short-circuited at a predetermined check timing during sensor use, and the offset output of the acceleration signal generation circuit in this state is detected as a current offset output value, and the standard offset output value and Zero point correction means for correcting the zero point of the acceleration signal based on the comparison with the current offset output value;
A vehicle acceleration sensor system comprising: As the standard offset output value, an offset output value measured at a reference timing that precedes the check timing in time series is used, and the zero point correction unit is configured to perform the zero point correction from the reference timing to the check timing. The acceleration sensor system for a vehicle according to claim 1, wherein the time-dependent drift of the offset output value is corrected. A negative feedback capacitor short-circuit switch unit is provided on a short-circuit path that short-circuits both ends of the negative feedback capacitor of the charge-voltage conversion circuit,
3. The vehicle acceleration sensor system according to claim 1, wherein the zero point correction unit includes a switch control unit that closes the negative feedback capacitor short-circuiting switch unit at the check timing. 4. The acceleration sensor system for a vehicle according to any one of claims 1 to 3, wherein the zero point correction means includes temperature correction means for the sensor output. The standard offset output value measured at a reference temperature is used, and the temperature correction means includes a temperature sensor, a correction content information storage unit that stores correction content information for sensor output at various temperatures, and the temperature sensor. 5. The vehicle according to claim 4, further comprising: a correction calculation unit that calculates a correction value of the sensor output with reference to the correction content information in the correction information storage unit based on the detected temperature value detected at the check timing. Acceleration sensor system.

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