Abstract:
A device is described for generating an electric voltage by which a body of a capacitive and/or inductive sensor capable of vibration, such as a capacitive micromechanical rotational rate sensor in particular, is induced to vibrate. In order to reduce the manufacturing cost of the sensor, a voltage generating device is provided which induces a constant mechanical deflection of the body capable of vibration, this deflection being independent of the manufacturing tolerances of the sensor.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device for generating an electric voltage. 
     BACKGROUND INFORMATION 
     A known rotational rate sensor produced by microsystem technology has an oscillating weight which oscillates about its axis of rotation. The oscillating weight has a comb structure, i.e., it is formed by a comb structure which alternately meshes with a first stationary comb structure and with a second stationary comb structure of the sensor as it oscillates. This arrangement forms two capacitors whose capacitances change in opposite directions over time. If the rotational rate sensor experiences a rotational rate perpendicular to the axis of torsional vibration of the oscillating weight, one side of the oscillating weight moves toward the substrate of the rotational rate sensor and the other side moves away from it. These changes in distance are measured capacitively by electrically conducting surfaces beneath the oscillating weight. The comb structures which are stationary with respect to the sensor and the comb structure which is provided on the oscillating weight are acted upon by an alternating voltage, thereby inducing oscillation of the oscillating weight. 
     To obtain a high signal-to-noise ratio of the test signal which represents the rotational rate, the deflection of the moving structure of the sensor must be maximized. 
     In the case of a known capacitive micromechanical sensor, such as a rotational rate sensor manufactured by planar silicon processes in particular, the change in capacitance depends not only on the deflection of the moving structure but also on the gap distance. Gap distance is understood to refer to the average distance between the “teeth” of the movable comb structure and the two stationary comb structures in the case of a stationary oscillating weight. Since the gap distance may vary from one sensor to the next due to the manufacturing technology, each sensor must be adjusted individually to achieve maximum deflection, i.e, maximum vibration amplitude of the movable structure. Not only is this complicated, but it may also result in the movable structure striking against the stationary structure, which could damage the sensor. 
     SUMMARY OF THE INVENTION 
     The device according to the present invention has the advantage over the related art in particular that, regardless of the manufacturing tolerances, it automatically adjusts a predefined deflection of the oscillating weight of a capacitive or inductive sensor. This eliminates individual manual adjustment of each sensor for setting a virtually maximum deflection of the oscillating weight in order to obtain a maximum signal-to-noise ratio. This makes it possible to manufacture capacitive and inductive sensors such as rotational rate sensors in particular less expensively. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the first part of a schematic diagram of a first embodiment of a sensor-independent vibration amplitude regulating device according to the present invention. 
     FIG. 2 shows the second part of the schematic diagram of the first embodiment of a sensor-independent vibration amplitude regulating device according to the present invention. 
     FIG. 3 shows the first part of the schematic diagram of the second embodiment of a sensor-independent vibration amplitude regulating device according to the present invention. 
     FIG. 4 shows the second part of the schematic diagram of the second embodiment of a sensor-independent vibration amplitude regulating device according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     For the sake of simplicity, the schematic diagram of a sensor-independent vibration amplitude regulating device according to the present invention has been divided into FIGS. 1 and 2 plus  3  and  4 . An output of a first part of the schematic diagram, labeled as A in FIGS. 1 and 3, is connected electrically to an input of a second part of the schematic diagram, labeled as E in FIGS. 2 and 4. 
     First part  100  of the schematic diagram of the first embodiment of the vibration amplitude regulating device according to the present invention, as illustrated in FIG. 1, shows at the left a schematic diagram  101  of another comb structure arrangement having a comb structure movable with the oscillating weight and two stationary comb structures of the type described above. These additional comb structures are used to sense the deflection of the oscillating weight. Diagram  101  shows two capacitors  102  and  103 , which are formed by. the two comb structures, these comb structures being stationary with respect to the sensor and having the function of sensing the deflection, and by the movable comb structure oscillating between the two former comb structures. 
     Furthermore, first part  100  of the schematic diagram shows a first signal path  107 , a second signal path  108 , an adder  110 , a demodulator  111 , an amplifier  121  and a common-mode regulating apparatus  109 . 
     First signal path  107  has a terminal  104 , a C/U converter  112  and an amplifier  113 . Terminal  104  is connected to the input of C/U converter  112 , whose output is connected to the input of amplifier  113 , and the output of amplifier  113  is connected to a first input of adder  110 . In an identical manner, second signal path  108  has a terminal  106 , a C/U converter  114  and an amplifier  115 . Terminal  106  is connected to the input of C/U converter  114  whose output is connected to the input of amplifier  115 , and the input of amplifier  115  is connected to a second input of adder  110 . The output of adder  10  is connected to a first input of demodulator  111  and its output is connected to third input of amplifier  121 . 
     C/U converters  112  and  114  are preferably optical amplifiers wired as inverting amplifiers having on-chip capacitance C RK  in the feedback; these are charge amplifiers. 
     Common-mode regulating apparatus  109  (CMRA) has an adder  120 , a regulator  119 , preferably an I regulator, a modulator  118 , a capacitor  116  having a capacitance C I  and a capacitor  117  also having capacitance C I . A first input of adder  120  is connected to the output of C/U converter  112 , i.e., the input of amplifier  113 , and a second input of adder  120  is connected to the output of C/U converter  114 , i.e., the input of amplifier  115 . The only output of adder  120  is connected to the input of regulator  119 , and the output of regulator  119  is connected to both the input of modulator  118  and to a regulating terminal of amplifier  12 l. 
     The output of modulator  118  is connected to a first terminal of capacitor  116  and to a first terminal of capacitor  117 . The second terminal of capacitor  116  is connected to the input of C/U converter  112 , i.e., terminal  104 , and the second terminal of capacitor  117  is connected to the input of C/U converter  114 , i.e., terminal  106 . 
     The second part of the schematic diagram of the first embodiment of the vibration amplitude regulating device of a rotational rate sensor, as shown in FIG. 2, shows input E connected to output A shown in FIG. 1, a phase quadrature device  201 , an output stage  203 , a terminal  204 , a terminal  205 , an adder  208 , an amplifier  209 , a rectifier  206  and a regulator  207 , where regulator  207  forms part of an automatic gain control (AGC). 
     Input E of the second part of the schematic diagram of the vibration amplitude regulating device of a rotational rate sensor shown in FIG. 2 is connected to the input of the phase quadrature device  201 , the output of phase quadrature device  201  being connected to the input of amplifier  202 , the output of amplifier  202  being connected to an input of output stage  203 , and one output of output stage  203  being connected to terminal  204  and another output of output stage  203  being connected to terminal  205 . The input of phase quadrature device  201  is also connected electrically to the input of rectifier  206 , whose output is connected to the first input of adder  208 , whose output is in turn connected to the input of regulator  207 , and finally, the output of regulator  207  is connected to an additional input of output stage  203 . The second input of adder  208  is connected to the output of amplifier  209 . 
     A setpoint voltage U setpoint  is applied to the input of amplifier  209  and sets the desired maximum deflection of the oscillating weight for all sensors of the same type. 
     The function of the vibration amplitude regulation of a rotational rate sensor according to the present invention is described in detail below. It is assumed that the oscillating weight oscillates about its resting position. 
     The time-dependent capacitance (C(t)) of capacitor  102  or capacitor  103  for identical capacitors, i.e., comb structures, is described in first approximation as: 
     
       
           C   102 ( t )= n ∈*(((1 0 +δ1( t )))* h )/ d=C    0   +δC ( t )  (1) 
       
     
     
       
           C   103 ( t )= n *∈*(((1 0 +δ1( t )))* h )/ d   
       
     
     
       
         =C 0   −δC ( t )  (2) 
       
     
     where: 
       1   0 : basic overlapping of the movable comb structure with the corresponding stationary comb structure; 
     δ 1 : deflection of the movable comb structure; 
     h: height of the movable comb structure; 
     d: gap distance of the movable comb structure from the stationary comb structure, i.e., the distance (ideally always identical) between adjacent “teeth” or fingers of movable and stationary comb structures; 
     n: number of overlapping fingers of movable and stationary comb structures; 
     ∈: dielectric constant of the medium, air in particular, between the movable and the stationary comb structures; 
     δC: time-dependent change in capacitance as a function of the deflection of the movable comb structure relative to the stationary comb structure; 
     C 0 : resting capacitance, i.e., the capacitance of the capacitor formed by the movable comb structure and the stationary comb structure when the movable comb structure is stationary. 
     It holds that: 
     
       
         δ C/C   0 =δ1/1 0   (3) 
       
     
     i.e., the relative change in capacitance due to deflection of the movable comb structure is equal to δ 1 / 1   0 . The movable comb structure is acted upon by an alternating voltage U HF  from a device (not shown) at frequency f HF  via terminal  105 . Frequency f HF  of alternating voltage U HF  is much higher than operating frequency f sensor  supplied to the sensor via the driving comb structures. For example, frequency f HF  of alternating voltage U HF  corresponds approximately to 16 times operating frequency f sensor , operating frequency f sensor  amounting to approx. 1.5 kHz, for example. It is self-evident that this information applies only to examples of one specific sensor. 
     An alternating voltage having a frequency f HF  is applied to terminals  104  and  106 , frequency f HF  being amplitude-modulated with the operating frequency of sensor f sensor . 
     The time-dependent capacitance of first capacitor  102  is converted by C/U converter  112  into a corresponding electric voltage, amplified by amplifier  113  and sent to adder  110 . The capacitance of second capacitor  103  showing an inverse time dependence in comparison with the capacitance of the first capacitor is converted by C/U converter  114  into a corresponding electric voltage, amplified by amplifier  115  and also sent to adder  110 . 
     The alternating voltage delivered by adder  110  is sent to demodulator  111 . Demodulator  111  demodulates, i.e., multiplies the alternating voltage delivered by adder  110  by the sign of alternating voltage U HF . 
     Adder  110  forms the difference between the electric signals in first signal path  107  and second signal path  108 , amplified by gain factor g by amplifier  113  and amplifier  115 ; therefore, the alternating voltage delivered by demodulator  111  at its output is: 
     
       
         U FE =2 *g*δC/C   RK   *U   HF =2 * g*δ 1 / 1 0   *C   0   /C   RK   *U   HF   (4) 
       
     
     where: 
     g: gain factor; 
     C RK : feedback capacitance of C/U converter  112  and identical C/U converter  114 ; 
     U HF : alternating voltage U HF ; 
     U FE : the alternating voltage delivered by demodulator  111  after demodulation, i.e., multiplication by sign U HF , 
     this means that, due to the differentiation of the electric signals at the output of first signal path  107  and second signal path  108  performed by adder  110 , the common-mode component caused by resting capacitance C 0  is eliminated. 
     An essential aspect of the present invention is providing measures so that U FE  is independent of the resting capacitance C 0  of the sensor, which is subject to certain fluctuations due to manufacturing tolerances. 
     According to a preferred embodiment of the present invention, both electric voltage U LV1  between the output of C/U converter  112  and amplifier  113  and electric voltage U LV2  between the output of C/U converter  114  and amplifier  115  are picked up, electric voltage U LV1  being sent to the first input of adder  120  and electric voltage U LV2  being sent to the second input of adder  120 . 
     The electric voltage delivered by C/U converters  112  and  114  at their outputs is: 
     
       
           U   LV1,LV2 =( C   0   +/−δC )/ C   RK   +U   HF   (5) 
       
     
     The result of addition of the electric voltages performed by adder  120  is an output voltage U add  of adder  120 , for which it holds that: 
     
       
           U   add   =f (( C   0   +δC )+( C   0   −δC ))= f ( C   0 )  (6) 
       
     
     i.e., the output voltage of adder  120  is a function of resting capacitance C 0 . 
     Output voltage U add  of adder  120  is sent to regulator  119 , preferably an I regulator delivering an output voltage U I  which is sent to an input of modulator  118  and also to the regulating terminal of anplifier  121 . 
     Modulator  118  also receives alternating voltage U HF , and the output signal delivered by modulator  118  goes to a first terminal of each capacitor  116  and  117 , both having a capacitance C I . The second terminal of capacitor  116  is connected to the input of C/U converter  112  in signal path  107 , and the second terminal of capacitor  117  is connected to the input of C/U converter  114  in signal path  108 . 
     Capacitors  116  and  117  receive a voltage via regulator  119  such that the output signal of adder  120  has an amplitude of approx. 0 volt, i.e., capacitors  116  and  117  almost completely compensate resting capacitance C 0  of the respective sensors. 
     Common-mode regulating apparatus  109  (CMRA) therefore responds only to common-mode signals, i.e., direct voltage signals, at the input end. The output of regulator  119  changes its voltage in regulating operation until there is no longer a common mode signal at the input of adder  120 . This condition is met when the following holds: 
     
       
           U   HF   *C   0   =−U   I   *C   I   (7) 
       
     
     
       
         i.e.,  U   I   =−C   0   /C   I   *U   HF   (8) 
       
     
     i.e., voltage U 1 , is directly proportional to resting capacitance C 0 . 
     Amplifier  121  performs an amplification g var  of voltage U FE  as a function of the particular resting capacitance via voltage U I  applied to amplifier  121 , for which the following equation holds: 
     
       
           g   var   =C   I   /C   0   (9) 
       
     
     For electric voltage U delivered at the output of amplifier  121 , this yields: 
       U= 2 *g *δ1/1 0   *C   I   /C   RK   *U   HF   (10) 
     where δC/C 0 =δ 1 / 1   0  (see equation (3)), 
     i.e., the voltage applied at the output of amplifier  121 , i.e., at output A, is independent of resting capacitance C 0  of the particular sensor whose vibrational amplitude is to be regulated. Voltage U and thus change δ 1  in the path of the movable sensor element depend only on low-tolerance voltage U HF , which is determined by the electronic regulation and/or measurement devices, and basic overlap  1   0 . Basic overlap  1   0  is settable with a high precision, however, in particular in the case of a micromechanical sensor manufactured from semiconductor layers by using planar silicon processes. 
     Voltage U delivered by amplifier  121  is sent to phase quadrature device  201 , which sends voltage U, 90° out of phase, to the input of amplifier  202  and sends amplified out-of-phase voltage U to an input of output stage  203 . 
     Furthermore, voltage U delivered by amplifier  121  is sent to the input of rectifier  206  via input E, i.e., the input of the phase quadrature device. Setpoint voltage U setpoint  amplified by amplifier  209  is subtracted by adder  208  from voltage U rectified by rectifier  206 , and the output signal of adder  208  is sent to the input of regulator  207 . Regulator  207  changes the voltage at its output until its input voltage is virtually zero. Regulator  207 , preferably a PI regulator and/or an automatic gain control regulator (AGC) controls output stage  203  so that the output stage delivers a voltage to the drive comb structures of the sensor (not shown) via terminals  204  and  205 , so that the vibrational amplitude of the oscillating sensor element, i.e., the oscillating weight, is constant and virtually at a maximum. 
     The second embodiment of the vibration amplitude regulating device according to the present invention as illustrated in FIGS. 3 and 4 differs from the first embodiment illustrated in FIGS. 1 and 2 in that instead of setpoint voltage U setpoint  voltage U I  delivered at the output of regulator  119  is applied to the second input of adder  208 ; furthermore, voltage U I  is not applied to amplifier  121  in the second embodiment, so the amplifier implements a constant gain g const . The following thus holds for the output voltage of amplifier  121 : 
       U =2 *g *δ1/1 0   *C   0   /C   RK   *U   RF   *g   const   (11) 
     The regulator, i.e., AGC regulator  207  changes its output voltage until output voltage U of amplifier  121  corresponds to AGC reference input variable U I  (or a variable proportional thereto). As in the first embodiment, this also means that the amplitude of vibration of the oscillating sensor element, i.e., the oscillating weight, is independent of resting capacitance C 0 , which is subject to manufacturing tolerances. 
     Gap distance manufacturing tolerances due to overetching now no longer have any effect on the deflection and thus the speed of the movable sensor element. A more complex and thus more expensive adjustment of each finished sensor to adjust the desired deflection is no longer necessary when using the sensor-independent vibrational amplitude regulating device according to the present invention. 
     As explained above, the sensor-independent vibration amplitude regulating device according to the present invention regulates the vibration amplitude of the oscillating weight of a capacitive sensor such as a rotational rate sensor in particular. It is self-evident that the vibrational amplitude regulating device described here may also be used in a modified form to regulate the amplitude of vibration of the oscillating weight of an inductive sensor, e.g., such as a rotational rate sensor in particular. Such a modified form of the vibration amplitude regulating device according to the present invention takes into account in particular the fact that instead of capacitances, there are inductances which are subject to manufacturing tolerances, in an inductive sensor. 
     LIST REFERENCE NOTATION 
       100  first part of the schematic diagram of the vibration amplitude regulating device according to the present invention 
       101  schematic diagram of the comb structures of a capacitive rotational rate sensor for sensing the deflection of its oscillating weight 
       102  capacitor 
       103  capacitor 
       104  terminal 
       105  terminal 
       106  terminal 
       107  first signal path 
       108  second signal path 
       109  common-mode regulating apparatus (CMRA) 
       110  adder 
       111  demodulator 
       112  C/U converter 
       113  amplifier 
       114  C/U converter 
       115  amplifier 
       116  capacitor 
       117  capacitor 
       118  modulator 
       119  regulator 
       120  adder 
       121  amplifier 
       200  second part of the schematic diagram of the vibration amplitude regulating device according to the present invention 
       201  phase quadrature device 
       202  amplifier 
       203  output stage 
       204  terminal 
       205  terminal 
       206  rectifier 
       207  regulator 
       208  adder 
       209  amplifier