Method and apparatus for synchronous demodulation of multiply modulated signals

[0002] The present invention relates to a method and an apparatus for synchronous demodulation of multiply modulated signals.

[0003] An electromechanical rotation rate sensor, for example a micromechanical rotation rate sensor, constitutes a spring-mass system in which the Coriolis effect may be used to measure the rotation rate of the sensor. Upon rotation of the sensor or the system, the masses of the system are deflected and the deflection may be ascertained capacitatively in order to determine the rotation rate. While the rotation rate sensor is in operation, the spring-mass system of the rotation rate sensor oscillates at its natural resonant frequency f_z. Absent the effect of an external rotation rate, the capacitance of so-called sensing capacitances provided in the rotation rate sensor does not change. If the spring-mass system is deflected “out-of-plane” in the context of a rotation of the sensor, the capacitance changes at the natural resonant frequency f_z. The amplitude of this oscillation is an indication of the external rotation rate presently acting on the rotation rate sensor.

[0004] The deflection of the spring-mass system during its rotation as a result of the Coriolis effect may be sensed, by way of a capacitance change associated therewith in capacitances provided in the rotation rate sensor, by way of a capacitance/voltage (C/U) conversion.

[0005] When capacitance changes occur as a result of the rotation of the sensor, those changes may bring about voltage jumps at the input of the charge amplifier. The output signal U_outof the charge amplifier is proportional to the quotient of the useful capacitance C_NUTZand feedback capacitance C_RKof the sensor in question, multiplied by the amplitude of the high-frequency voltage jump U_HF, that is:

[0006] U_OUT=(C_NUTZ/C_RK)*U_HF.

[0007] To ensure that signal processing of the output signal U_OUTof the charge amplifier can occur in phase with the natural resonant frequency f_zof the spring-mass system, a phase-locked loop (PLL) that synchronizes itself to the natural resonant frequency f_zof the spring-mass system may be used. The sampling frequency for analyzing capacitance changes by the switched capacitor technology may be referred to as f_a, and the modulation frequency, or frequency of the high-frequency voltage jump U_HF, may be referred to as f_HF.

[0008] To achieve a greater signal-to-noise ratio than would be possible in the baseband, the voltage jumps that are brought about by the capacitance changes may be alternated as to sign. The baseband is thereby transformed to half the sampling frequency f_a, that is, to the frequency f_HFof the voltage jump U_HF; thus f_HF=f_a/2. In same analysis devices, the natural resonant frequency f_zof the rotation rate sensor may be used as the intermediate frequency. In a first demodulation step in synchronous demodulation, the raw signal is multiplied by the frequency f_HFof the voltage jump U_HF. In a second demodulation step of synchronous demodulation, the rotation rate signal of the rotation rate sensor is then convoluted into the baseband. In a subsequent filtration step, the high-frequency convolution products are suppressed and the output signal of the filtration step or of a corresponding filter stage is limited to the desired bandwidth.

[0009] For cost reasons, according to one exemplary embodiment and/or exemplary method of the present invention, a charge amplifier of so-called switched capacitor technology may be used as the C/U converter.

[0010] According to an exemplary embodiment and/or exemplary method of the present invention the high-frequency signals of frequency f_HF, which may serve for analysis of the capacitance change resulting from the Coriolis effect upon rotation of the rotation rate sensor, may be configured by way of a PLL as a multiple of the signal, of natural resonant frequency f_z, of the spring-mass system of the rotation rate sensor. A fixed dependence between the phase position of the signals and frequencies f_HFand f_zmay thereby obtained. As a result of a logical association between these two signals, signals for controlling a synchronous demodulator may be generated. This may be done with the exemplary circuit according to the present invention depicted in FIG. 1. This circuit may create an in-phase multiplication of the two frequencies f_HFand f_zand may effect a synchronous demodulation that transforms the rotation rate signal to be sensed, or the rotation rate signal of the rotation rate sensor, into the baseband.

[0011] An exemplary embodiment and/or exemplary method of the present invention is directed to providing a method for synchronous demodulation of the multiply modulated rotation rate signal of a rotation rate sensor which includes a spring-mass system that oscillates at its natural resonant frequency (f_z), and at least one capacitor for ascertaining a Coriolis effect acting on the spring-mass system; i.e. a rotation rate, the rotation rate being ascertained by a time-variable capacitance change, brought about by the rotation rate, of the at least one capacitor by multiple demodulation of a multiply modulated electrical signal that includes a time-variable first electrical signal (f_HF) and a second electrical signal, superimposed thereon, at the natural resonant frequency (f_z) of the rotation rate sensor, the amplitude of the second electrical signal of the time-variable capacitance change of the at least one capacitor being correspondingly modulated, wherein the first electrical signal (f_HF) is generated in such a way that it exhibits a time-invariant phase relationship to the natural resonant frequency (f_z) of the rotation rate sensor.

[0012] The following components may be used to provide an exemplary embodiment and/or exemplary method according to the present invention: a rotation rate sensor, a PLL, a logic circuit, and a synchronous demodulator, as depicted, for example, in FIG. 1. Several demodulation sections with mixers, filters, and optionally also intermediate amplifiers may be necessary.

[0013] According to one exemplary embodiment and/or exemplary method of the present invention, the outlay for synchronous demodulation may be almost halved.

[0014] FIG. 1 shows a block diagram of components of an exemplary embodiment of a synchronous demodulation apparatus according to the present invention for demodulating the multiply modulated signals delivered by a known rotation rate sensor.

[0015] FIG. 2 shows an exemplary embodiment of a synchronous demodulator of the synchronous demodulator apparatus of FIG. 1.

[0016] FIG. 3 shows an input signal +U_eof the synchronous demodulator of FIG. 2.

[0017] FIG. 4 shows an inverted input signal −U_eof FIG. 3 of the synchronous demodulator of FIG. 2.

[0018] FIG. 5 shows a first control signal φ1 for switching on or off two first switches of the synchronous demodulator of FIG. 2.

[0019] FIG. 6 shows a second control signal Φ2 for switching on or off two second switches of the synchronous demodulator of FIG. 2.

[0020] FIG. 7 shows an output signal U_aof the synchronous demodulator of FIG. 2.

[0021] FIG. 1 depicts, in the form of a block diagram, an exemplary embodiment of a synchronous demodulation apparatus 1 according to the present invention for determining the external rotation rate of a sensor or rotation rate sensor 2 on the basis of the multiply modulated signals delivered by the sensor. For the sake of clarity, the block diagram emphasizes certain of the components of apparatus 1 in connection with explaining the exemplary embodiment of the present invention.

[0022] The spring-mass system of rotation rate sensor 2 oscillates at its natural resonant frequency as a result of a signal conveyed to rotation rate sensor 2. If the spring-mass system or rotation rate sensor 2 is deflected because of a rotation of rotation rate sensor 2, this may be sensed in the so-called sensing capacitances (not depicted) provided in rotation rate sensor 2. The amplitude of the signal, at natural resonant frequency f_z, delivered by the capacitances is modulated. The amplitude modulation may be an indication of the external rotation rate acting on rotation rate sensor 2, or of the Coriolis effect acting on the spring-mass system as a consequence of a rotation of the spring-mass system and an out-of-plane deflection, associated therewith, of the spring-mass system. From the drive circuit (not depicted) of rotation rate sensor 2, a signal at natural resonant frequency f_zmay be conveyed to a phase-locked loop or PLL 4. PLL 4 synchronizes itself to natural resonant frequency f_zand generates a modulation signal at a frequency f_HF. Frequency f_HFis a multiple of natural resonant frequency f_z, in particular an integral multiple. In sensor 2, the rotation rate signal modulated with natural resonant frequency f_zis modulated with modulation frequency f_HF. This doubly modulated signal reproduces the capacitance changes resulting from the action of an external rotation rate on rotation rate sensor 2.

[0023] The doubly modulated signal may be converted, by a C/U converter 5 (not explicitly depicted) in the form of a charge amplifier using switched capacitor technology, into mutually inverse voltage signals +U_eand −U_e(see FIGS. 3 and 4). The doubly modulated signal may thereby be sampled at a sampling frequency f_athat is twice as high as the modulation or carrier frequency f_HF. This may be done to achieve a higher signal-to-noise ratio than would be possible in the baseband.

[0024] The mutually inverse signals +U_eand −U_emay be present in temporally discrete fashion at the input of synchronous demodulator 7, as is evident from FIGS. 1, 3, and 4. For the sake of clarity, in FIGS. 3 and 4 frequency f_HFof the modulation or carrier signal made available by the PLL is simply chosen to be four times the natural resonant frequency f_zof the sensor. The rotation rate signal modulated with the natural resonant frequency of the sensor may be apparent in FIGS. 3 and 4 as an envelope of the pulses. Its amplitude is proportional to the instantaneous external rotation rate.

[0025] PLL 4 conveys to logic circuit 9 the modulation frequency f_HFand the amplitude-modulated natural resonant frequency f_zpicked off from the capacitances of rotation rate sensor 2. The logic circuit 9 generates the control signals Φ1 and Φ2 depicted in FIGS. 5 and 6. A comparison of FIGS. 4 and 5 shows that control signal Φ1 always has a high voltage level (i.e. is a switch-on signal for one of the electronic switches described below) when voltage signal −U_eassumes a positive value. A comparison of FIGS. 3 and 6 also shows that control signal Φ2 is always at a high voltage level (i.e. is a switch-on signal for an electronic switch) when voltage signal +U_eassumes a positive value. A zero level of control signals Φ1 and Φ2 constitutes a switch-off signal for the electronic switches described below.

[0026] FIG. 2 shows synchronous demodulator 7 of FIG. 1 in further detail. Synchronous demodulator 7 includes electronic switches 11, 12, 13, and 14, and a summer 15. The input of synchronous demodulator 7 acted upon by voltage signal +U_eis connected to the inputs of switches 11 and 13. The input acted upon by voltage signal −U_e(which is the inverse of voltage signal +U_e) is connected to the inputs of switches 12 and 14. The output of switch 11 is connected to the output of switch 14 and to a first input of summer 15. The output of switch 13 is connected to the output of switch 12 and to the second input of summer 15. Switches 11 and 12 may be opened or closed by control signal Φ1, and switches 13 and 14 by control signal Φ2, in the manner shown in FIGS. 5 and 6.

[0027] At the output of summer 15, synchronous demodulator 7 then delivers output signal U_adepicted in FIG. 7. A subsequent averaging of output signal U_aby lowpass filtration (not depicted) results directly in the rotation rate signal in the baseband that is proportional to the amplitude of the signal previously modulated with f_HFand f_z.

[0028] Operation of the exemplary method according to the present invention described above involves the PLL 4 establishing a temporally fixed phase relationship between frequencies f_zand f_HF.

[0029] In order to generate, using a PLL, an output signal of frequency f_HFat 2ⁿtimes the frequency of the input signal of frequency f_z, a voltage-controlled oscillator (VCO) that oscillates at 2ⁿtimes the frequency of the input signal is used. A subsequent frequency division by a factor of ½ⁿallows the system to continue processing the resulting signal in the PLL. It is assumed that n=1, 2, 3, etc.; in particular, n may be at least 2.

[0030] With an exemplary embodiment of a single synchronous demodulator 7 according to the present invention as shown in FIG. 2 and with an exemplary method according to the present invention, demodulation of the doubly modulated signal may thus be accomplished on one step directly in the baseband.

[0031] The list of reference characters is as follows:

[0032] 1 Synchronous demodulation apparatus

[0033] 2 Rotation rate sensor

[0034] f_zNatural resonant frequency of rotation rate sensor

[0035] 4 Phase-locked loop, PLL

[0036] f_aSampling frequency of rotation rate signal

[0037] f_HFModulation frequency

[0038] 5 C/U converter

[0039] +U_e, −U_eOutput signal of C/U converter

[0040] 7 Synchronous demodulator

[0041] 9 Logic circuit

[0042] Φ1 Control signal

[0043] Φ2 Control signal

[0044] 11 Switch

[0045] 12 Switch

[0046] 13 Switch

[0047] 14 Switch

[0048] 15 Summer

[0049] U_aOutput signal of synchronous demodulation apparatus