Abstract:
A sensing circuit for a vibration type of angular rate sensor comprises a vibrator, driving unit, follow-up signal forming unit, normal voltage-range setting unit, and determining unit. The driving unit drives the vibrator to vibrate at a predetermined amplitude by using, as a feedback signal, an error voltage signal in which an amplitude of vibration of the vibrator is reflected. The follow-up signal forming unit forms, by using the error voltage signal, a follow-up signal following up the error voltage signal at changes which are gentler than changes in the error signal. The normal voltage-range setting unit sets a range of a normal voltage for the error voltage signal by using the follow-up signal. The determining unit determines whether or not the sensor circuit is in a malfunctioning condition, by using an estimation as to whether or not the error voltage signal is within the range of the normal voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based on and claims the benefit of priority from earlier Japanese Patent Application No. 2004-301351 filed on Oct. 15, 2004, the description of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a sensing circuit for vibration type of angular rate sensor (i.e., angular velocity sensor) provided with a malfunction detector. In particular, the present invention relates to a sensing circuit provided with a malfunction detector for detecting, with high accuracy, variation of driving impedance caused such as by attachment of foreign matter to a vibrator, the sensing circuit being applied, for example, to a vibration type of angular rate sensor used such as for vehicle control. 
   2. Description of Related Art 
   As a system for performing vehicle control by means of an angular rate sensor, stable control systems for vehicle, for example, are known in which vehicle sideslip is detected to optimally control a brake and torque of a vehicle to thereby maintain the vehicle in a normal condition. As such a system, four-wheel rudder angle control systems, for example, are also known in which a rudder angle of rear wheels or front wheels of a vehicle is controlled. 
   These types of systems typically detect malfunction conditions of a vehicle, such as sideslip, with a yaw rate signal, i.e. by means of an angular rate sensor. Malfunction of a yaw rate signal means a possible unstableness of the traveling characteristics of a vehicle, which may cause unexpected behavior of the vehicle. 
   One example of a technique for resolving the problem described above is disclosed in Published Japanese Unexamined Patent Application No. H11-051655. This technique includes a device for examining whether or not an angular rate sensor is in a normal operation. The device is so arranged to detect that a drive voltage corresponding to a driving force for a vibrator, has gone off a specified voltage, by means of a constant voltage circuit and a comparator, and to output self-diagnosis as a diagnostic detection signal. 
   Japanese Patent No. 2084567 discloses a technique in which the amplitude of a vibrator in a driving direction is detected, and amplitude control and 90-degree phase shift are effected in order to stabilize zero point/sensitivity of an angular rate signal, so that feedback control is effected as a drive signal. 
   A malfunction detection circuit, which is the combination of the techniques disclosed in the above publications, has been provided to a sensing circuit in an angular rate sensor. Specifically, a sensing circuit provided with a malfunction detection circuit, as shown in  FIG. 4 , has generally been employed. 
   As shown in  FIG. 4 , a sensing circuit in an angular rate sensor is so configured that it comprises a vibrator  30 , an amplitude control circuit  40 , and a malfunction detection circuit  50 . 
   The vibrator  30  is provided with a driving sensor element and a pair of sensor elements for detecting yaw (not shown), and is so configured that, if yaw occurs when the driving sensor element is effecting driving vibration, the pair of detection sensor elements are vibrated by the Coriolis force. Since the vibrator  30  generates outputs corresponding to the respective vibrations of the pair of detection sensor elements, yaw can be detected based on the outputs. The vibrator  30  is so configured that it also generates an output corresponding to the driving vibration in order to detect that the driving sensor element is adequately effecting driving vibration. 
   The output from the vibrator  30  corresponding to the driving vibration, i.e. a vibration amplitude in a driving direction, is converted to a voltage in an i/v converter circuit (or C-V converter circuit)  41  and then passed to a rectification circuit  42  in order to obtain a DC voltage, which is equivalent to the vibration amplitude. Then, in a differential amplifier  44 , an error voltage with reference to Vref 1 , which is generated by a first reference-voltage generation circuit  43 , is detected as an error signal S 61 . 
   On the other hand, in order to permit the vibrator  30  to vibrate at a frequency of normal mode of vibration (resonance frequency) of the vibrator  30 , a signal corresponding to the vibration amplitude, i.e. the output, of the i/v converter circuit  41  is passed to a 90-degree phase-shift circuit  46 . The signal is then multiplied with the error signal S 61 , which is an output from the differential amplifier  44 , in a multiplier  45  in an amplitude control circuit  40 , for feeding back to the vibrator  30  as a driving voltage. As a result, the vibrator  30  can effect vibration at the resonance frequency, so that the amplitude is kept at a constant level. 
   If malfunction occurs, such as an attachment of foreign matter to the vibrator  30 , to increase driving impedance, the driving amplitude of the vibrator  30  is decreased, which in turn decreases the output of the rectification circuit  42 , making larger the difference between the rectifier output and the Vref 1  generated by the first reference-voltage generation circuit  43 . As a result, the error signal S 61 , which is an output of the differential amplifier  44 , is magnified, and thus the driving voltage, which is an output of the multiplier  45 , is increased. In this way, the driving amplitude is controlled so as to keep the driving amplitude of the vibrator  30  at a constant level. 
   In the vibrator  30  and the driving circuit  40  configured as described above, in order to detect malfunction of the vibrator  30  caused such as by the attachment of foreign matter, all that is required is to detect whether or not the error signal S 61 , which is an output of the differential amplifier  44  and is equivalent to the driving impedance, is within a specified range. 
   Thus, as shown in  FIG. 4 , an arrangement is made such that the error signal S 61  is inputted to a window comparator  53  in a malfunction detection circuit  50 , and that the error signal S 61  is detected as to whether or not the voltage thereof is within a voltage ranging from Vref 2  generated by a second reference-voltage generation circuit  51  to −Vref 2  formed by an inverting circuit  52 . If the voltage of the error signal S 61  does not fall in this voltage range, a diagnostic detection signal S 62  is adapted to be outputted. 
   The driving impedance of the vibrator  30 , however, is unavoidably varied depending such as on the dimensional accuracy of the vibrator  30  and due to temperature variation and aging variation. Accordingly, circumstances have been such that, as a criterion for detecting malfunction of the vibrator  30  for the error signal which corresponds to the driving impedance of the vibrator  30 , such values as described above in consideration of errors had to be set. Because of this, setting of values which are essentially required for malfunction detection have not been enabled, and thus no high-accuracy malfunction detection has been enabled. 
   Moreover, with the recent trend of downsizing of vibrators, vibrators are being replaced by those of semiconductor type which carry out detection in terms of capacitance. This, in turn, has come to give impact on zero point and sensitivity of an angular rate sensor with even the attachment of very small foreign matter which conventionally caused no problem. Thus, there is an increasing need for a malfunction detector having high accuracy and high reliability. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in light of the points described above, and has as its object to provide a sensing circuit for vibration type of angular rate sensor provided with a malfunction detector for detecting variation of driving impedance caused such as by the attachment of foreign matter to a vibrator, enabling absorption of vibrator variation, temperature variation and aging variation, and/or enabling high-accuracy detection. 
   In order to achieve the above object, as one aspect, the present invention provides a sensing circuit for a vibration type of angular rate sensor, comprising: a vibrator; a driving unit driving the vibrator to vibrate at a predetermined amplitude by using, as a feedback signal, an error voltage signal in which an amplitude of vibration of the vibrator is reflected; a follow-up signal forming unit forming, by using the error voltage signal, a follow-up signal following up the error voltage signal at changes which are gentler than changes in the error signal; a normal voltage-range setting unit setting a range of a normal voltage for the error voltage signal by using the follow-up signal; and a determining unit determining whether or not the sensor circuit is in a malfunctioning condition, on the basis of an estimation as to whether or not the error voltage signal is within the range of the normal voltage. 
   Preferably, the normal voltage-range setting unit is formed, by using the follow-up signal, to set both of an upper reference voltage and a lower reference voltage which define the normal voltage range. 
   Still preferably, the determining unit includes a comparing member comparing the error voltage signal with the range of the normal voltage and outputting a diagnostic detection signal indicting whether or not an malfunction occurs in the error voltage signal, wherein the diagnostic detection signal indicates that the error voltage signal is free from the malfunction when the comparison shows that the error voltage signal is within the range of the normal voltage and the diagnostic detection signal indicates that the malfunction has occurred in the error voltage signal when the comparison shows that the error voltage signal is outside the range of the normal voltage. 
   It is preferred that the follow-up signal forming unit is a low-pass filter applying low-pass filtering to the error voltage signal so as to output a low-pass filtered signal serving as the follow-up signal. 
   It is also preferred that the determining unit further includes an estimating member estimating whether or not the sensor circuit is either in a normal condition or in the malfunctioning condition, on the basis of the diagnostic detection signal. 
   In this configuration, preferably, the estimating member comprises a trigger generating element generating a trigger in response to the diagnostic detection signal; a counting element counting a predetermined period of time responsively to the generation of the trigger; a determining element determining whether or not the sensor circuit is in the malfunctioning condition, on the basis of the trigger and the predetermined period of time. 
   In this configuration, by way of example, the trigger generating element and the counting element are composed of a timer and the determining element is composed of a latch circuit, wherein the timer is configured to (i) generate a latch signal serving as the trigger when the diagnostic detection signal given to the timer changes from a first state where the error voltage signal is free from the malfunction to a second state where the malfunction has occurred in the error voltage, (ii) count the predetermined period of time when the latch signal is generated, and (iii) reset the counting when the diagnostic detection signal given to the timer returns from the second state to the first state before the predetermined period of time is counted, and wherein the latch circuit is configured to latch the diagnostic detection signal unless the predetermined period of time is counted without receiving the diagnostic detection signal returns from the second state to the first state and to output a signal showing that the sensor circuit is in the malfunctioning condition. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a diagram illustrating a block configuration of a sensing circuit for a vibration type of angular rate sensor according to a first embodiment of the present invention; 
       FIG. 2  is a timing diagram in case long-last malfunction has occurred in the angular rate sensor; 
       FIG. 3  is a timing diagram in case momentary malfunction has occurred in the angular rate sensor; and 
       FIG. 4  is a diagram illustrating a block configuration of a sensing circuit of conventional vibration type of rate sensor. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. 
     FIG. 1  shows a sensing circuit for a vibration type of angular rate sensor (i.e., angular velocity sensor) applied to an embodiment of the present invention. The configuration of the sensing circuit of the present embodiment is described hereunder referring to  FIG. 1 . Details and operations related, for example, to a driving circuit  40  including a vibrator  30 , i.e. related to similar parts in the configuration shown in  FIG. 4 , are as described above. Accordingly, only a malfunction detection circuit  10  provided to the sensing circuit is described hereunder. 
   The malfunction detection circuit  10  is arranged such that an error signal (voltage signal) S 21  outputted from a differential amplifier  44  is inputted thereinto. The error signal S 21  corresponds to a driving impedance involving vibrator variation, temperature variation and aging variation. Malfunction detection of high accuracy and high reliability is carried out based on the error signal S 21 . 
   As shown in  FIG. 1 , the malfunction detection circuit  10  comprises a low-pass filter (LPF)  11 , third reference voltage generator  12 , adder (ADD)  13 , subtracter (SUB)  14 , and window comparator  15 . 
   The LPF  11  has a large time constant, receives the error signal S 21 , and generates an output signal moderately following up the error signal S 21 . In particular, the LPF  11  is arranged such that it basically generates an output having a voltage equivalent to the error signal S 21 , however, if the error signal S 21  drastically changes, generates an output not with a follow-up completely matching the variation, but with a little delayed follow-up. Specifically, as the LPF  11 , one having a cut-off frequency at a level of 0.1 Hz or 0.01 Hz is adapted to absorb the temperature variation and aging variation. The output of the LPF  11  is adapted to be inputted to the ADD  13  and the SUB  14 . 
   The reference-voltage generation circuit  12  generates a reference voltage (a third reference voltage) Vref 3 . This reference voltage Vref 3  is for determining a range of a determination threshold in the window comparator  15 . The reference voltage Vref 3  is also inputted to the ADD  13  and the SUB  14 . 
   The ADD  13  and the SUB  14  correspond to normal voltage range forming means. 
   The ADD  13  outputs a voltage S 22  which is derived by adding the reference voltage Vref 3  to the output of the LPF  11 . The voltage S 22  outputted from the ADD  13  is adapted to be inputted to the window comparator  15  and to be set as an uppermost reference voltage. The voltage S 22  outputted from the ADD  13  is referred, hereinafter, to the uppermost reference voltage. 
   The SUB  14  outputs a voltage S 23  which is derived by subtracting the reference voltage Vref 3  from the output of the LPF  11 . The voltage S 23  outputted from the SUB  14  is adapted to be inputted to the window comparator  15  and to be set as a lowermost reference voltage in the window comparator  15 . The voltage S 23  outputted from the SUB  14  is referred, hereinafter, to the lowermost reference voltage. 
   The window comparator  15  determines whether or not the voltage of the error signal S 21  falls within a normal voltage range defined by the uppermost reference voltage S 22  outputted from the ADD  13  and the lowermost reference voltage S 23  outputted from the SUB  14 . The window comparator  15  is so arranged as to generate a diagnostic detection signal S 24  depending on whether or not the voltage of the error signal S 21  falls in the normal voltage range. If, for example, the voltage is in the normal voltage range, the diagnostic detection signal S 24  becomes high level which is indicative of normal function, and if the voltage is out of the normal voltage range, the diagnostic detection signal S 24  becomes low level which is indicative of malfunction. 
   In the malfunction detection circuit  10  of the present embodiment, a latch circuit  16  and a timer circuit  17  are provided. 
   The latch circuit  16  latches the diagnostic detection signal S 24  of the window comparator  15 . Specifically, the latch circuit  16  is so arranged as to latch the diagnostic detection signal S 24  based on a latch signal S 25  from the timer circuit  17 , which will be described later. More specifically, the latch circuit  16  is so arranged as to generate a voltage as an output signal S 26  that matches the diagnostic detection signal S 24  of the window comparator  15  until the latch signal S 25  is inputted from the timer circuit  17 , and, upon input of the latch signal S 25 , to latch the diagnostic detection signal S 24  of the window comparator  15  at the time of the input of the latch signal S 25 , for generation as the output signal S 26 . 
   The timer circuit  17  is adapted to start up, as a trigger, the rise and fall of the diagnostic detection signal S 24  of the window comparator  15 . Specifically, the timer circuit  17  is so arranged as to switch on a timer when the fall of the diagnostic detection signal S 24  is detected, to stop and reset the timer when the rise of the diagnostic detection signal S 24  is detected before a specified time interval Tset (e.g., 1 second or thereabouts is preferable) has elapsed in the timer, and to output the latch signal S 25  when the specified time interval Tset has elapsed in the timer. 
   The sensing circuit of the present embodiment is configured as described above. Subsequently, an operation of the malfunction detection circuit in the thus configured sensing circuit is described hereunder. 
   Upon input of the error signal S 21  into the malfunction detection circuit  10 , an output following up the error signal S 21  is generated from the LPF  11 . Then, the uppermost reference voltage S 22  and the lowermost reference voltage S 23  are formed, respectively, in the ADD  13  and the SUB  14  based on the output of the LPF  11  and the reference voltage Vref 3  generated by the third reference voltage generator  12 , and are inputted to the window comparator  15 . Thus, a range of ±Vref 3  centering on the output of the LPF  11  is defined as a normal voltage range. 
   Thus, in the window comparator  15 , the voltage of the error signal S 21  is determined as to whether or not it falls between the uppermost reference voltage S 22  and the lowermost reference voltage S 23 , i.e. within the normal voltage range. If the voltage of the error signal S 21  is within the normal voltage range, the diagnostic detection signal S 24  from the window comparator  15  presents a voltage indicative of normal function. Contrarily, if not within the normal voltage range, the diagnostic detection signal S 24  from the window comparator  15  presents a voltage indicative of malfunction. 
   If the diagnostic detection signal S 24  is kept at a high level condition, the output voltage  826  from the latch circuit  16  also remains at a level indicative of the normal function. However, if the voltage of the diagnostic detection signal (detection signal) S 24  changes only momentarily from the one indicative of normal function to the one indicative of malfunction, a voltage indicative of malfunction is outputted as the output signal S 26  from the latch circuit  16 . 
   Thus, when the voltage of the diagnostic detection signal S 24  from the window comparator  15  changes, if only for a moment, to the one indicative of malfunction, such a voltage is externally outputted, so that the occurrence of malfunction in the sensing circuit can be detected. 
   Then, when the voltage of the diagnostic detection signal S 24  changes from the one indicative of normal function to the one indicative of malfunction, such a change serves as a trigger for starting the timer of the timer circuit  17 . 
   In this connection, if the voltage of the diagnostic detection signal S 24  changes from the one indicative of malfunction to the one indicative of normal function before a specified time interval Tset has elapsed in the timer, the timer is stopped and reset. Therefore, no latch signal S 25  is outputted from the timer circuit  17 , and no voltage indicative of malfunction is latched in the latch circuit  16 . Thus, the output signal  826  of the latch circuit  17  reverts to a voltage indicative of normal function. Accordingly, in this case, the malfunction that has occurred in the sensing circuit has been detected as a momentary one. 
   Contrarily, if the voltage of the diagnostic detection signal S 24  remains as the one indicative of malfunction until the specified time interval Tset has elapsed in the timer, the latch signal S 25  is outputted from the timer circuit  17 , and a voltage indicative of malfunction is latched in the latch circuit  16 . Thus, the output signal S 26  of the latch circuit  17  remains as a voltage indicative of malfunction. Accordingly, in this case, the malfunction that has occurred in the sensing circuit has been detected as a long-lasting one. 
   A malfunction detection circuit operates as described above. Timing diagrams of such an operation are shown in  FIGS. 2 and 3 . 
     FIG. 2  shows a timing diagram in case a long-lasting malfunction has occurred in an angular rate sensor.  FIG. 3  shows a timing diagram in case the error signal S 21  has temporarily become abnormal caused such as by noise and impression of an over impact.  FIGS. 2 and 3  also show the operation of the uppermost reference voltage S 22  and the lowermost reference voltage S 23  in case the error signal S 21  has moderately varied being influenced by temperature variation and aging variation. 
   For the period between time T 1  and time T 2  in  FIG. 2 , the uppermost reference voltage S 22  and the lowermost reference voltage S 23  are formed basically centering on the error signal S 21 . During this period the diagnostic detection signal S 24  of the window comparator  15  is at a high level indicative of normal function, and the latch signal  825  of the timer circuit  17  is at a low level. Accordingly, the output signal S 26  of the latch circuit  16  remains at a high level which is also indicative of normal function. 
   When a long-lasting malfunction occurs in the sensing circuit at the time T 2 , the error signal S 21  drastically changes at the very moment the malfunction has occurred, however, both of the uppermost reference voltage S 22  and the lowermost reference voltage S 23  cannot follow up the change. Thus, the error signal S 21  runs out of the normal voltage range defined by both of the uppermost reference voltage S 22  and the lowermost reference voltage S 23 . 
   Thus, the diagnostic detection signal S 24  of the window comparator  15  goes down to a low level indicative of malfunction, and the output signal S 26  of the latch circuit  16  also goes down to a low level indicative of malfunction. At this time, the latch signal S 25  of the timer circuit  17  remains at a low level, while the timer of the timer circuit  17  is started. 
   If the diagnostic detection signal S 24  remains in the state of low level indicative of malfunction until time T 3 , the latch signal S 25  of the timer circuit  17  then goes up to a high level to latch the diagnostic detection signal S 24  in the latch circuit  16 . Accordingly, the output signal S 26  of the latch circuit  16  remains at a low level indicative of malfunction. Thus, even when the follow-up to the uppermost reference voltage S 22  and the lowermost reference voltage S 23  is completed at time T 4  to have the error signal S 21  fallen within the range between the uppermost reference voltage S 22  and the lowermost reference voltage S 23  and to have the diagnostic detection signal S 24  reverted to a high level, the output signal S 26  of the latch circuit  16  still remains at a low level indicative of malfunction. 
   In  FIG. 3 , for the period between time T 1  and time T 2 , the uppermost reference voltage S 22  and the lowermost reference voltage S 23  are formed following up the error signal S 21  just as the period between the time T 1  and the time T 2  in  FIG. 2 . 
   When momentary malfunction occurs in the sensing circuit at the time T 2 , the error signal S 21  again drastically changes just as at T 2  in  FIG. 2 , however, the uppermost reference voltage S 22  and the lowermost reference voltage again cannot follow up the change. Thus, the error signal S 21  runs out of the normal voltage range defied by the uppermost reference voltage S 22  and the lowermost reference voltage S 23 . 
   Therefore, as in the case shown in  FIG. 2 , the output signal S 26  of the latch circuit  16  goes down to a low level indicative of malfunction, while the timer of the timer circuit  17  is started. However, since the error signal S 21  reverts to a voltage of normal function at time T 3 , the output signal S 26  of the latch circuit  16  also immediately reverts to a high level indicative of normal function to stop and reset the timer of the timer circuit  17 . 
   In such a case, therefore, although a low-level signal indicative of malfunction is momentarily outputted from the latch circuit  16 , a high-level signal indicative of normal function is soon outputted thereafter. 
   As described above, in the present embodiment, the uppermost reference voltage S 22  and the lowermost reference voltage S 23  are formed with reference to the error signal $ 21 . For this reason, even when the error signal S 21  is varied by the variation of the vibrator  30 , temperature variation and aging variation, the uppermost reference voltage S 22  and the lowermost reference voltage S 23  can be formed with reference to the error signal S 21 . Accordingly, the uppermost reference voltage S 22  and the lowermost reference voltage S 23  can be the ones that have absorbed the variation of the vibrator  30 , temperature variation and aging variation. 
   In the present embodiment, the arrangement is made such that the uppermost reference voltage S 22  and the lowermost reference voltage S 23  moderately follow up the error signal S 21 . Thus, when the error signal S 21  is varied induced by abnormalities other than the variation of the vibrator  30 , temperature variation and aging variation, detection of such abnormalities is enabled. Accordingly, the malfunction detection circuit  10  is enabled to absorb the variation of the vibrator  30 , temperature variation and aging variation, and to accurately carry out detection. 
   Further, in the present embodiment, the latch circuit  16  and the timer circuit  17  are adapted to be used. It is possible, therefore, to make a distinction between the occurrence of a momentary malfunction and the occurrence of a long-lasting malfunction. 
   OTHER EMBODIMENTS 
   In the embodiment described above, it is so arranged that the uppermost reference voltage S 22  and the lowermost reference voltage S 23  moderately follow up the error signal S 21  by means of the LPF  11 . This, however, is only an example, and the LPF  11  does not necessarily have to be used. For example, although the LPF  11  is comprised of an integration circuit, a differentiation circuit may be generally applied to detect variation. However, in case a vibration type of angular rate sensor is used in a high-noise environment, as in the case of a gyro sensor for use in a vehicle, an integration circuit may preferably be used to sensitively detect the noise. 
   The uppermost reference voltage S 22  and the lowermost reference voltage S 23  may also be constituted with reference, for example, to an average of the error signal S 21  in a predetermined period of time. In this case, a normal voltage range may be specified within ±Vref 3  from the average of the error signal S 21  in the predetermined period of time. 
   The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments and modifications are therefore to be considered in all respects as illustrative and not restrictive, the scope of the present invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein,