Measuring device and measured quantity sensor having coupled processing and excitation frequencies

A measuring device for determining a measured quantity having an oscillatory structure where an oscillation signal is detectable. The measuring device further includes a device for exciting the oscillatory structure by an excitation frequency to result in an oscillation of an oscillation frequency. The measuring device further has a device for processing the oscillation signal by a frequency depending on the oscillation frequency and the excitation frequency. Furthermore, the measuring device includes an evaluator for determining the measured quantity based on the oscillation signal processed.

RELATED APPLICATIONS

This application claims priority from German Patent Application No. 10 2006 055 589.9, which was filed on Nov. 24, 2006, and is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a measuring device and to a measured quantity sensor for determining a measured quantity, such as, for example, a rate of rotation or acceleration, as are present in various fields of application.

BACKGROUND

A way of measuring rotational speeds and accelerations is, for example, using micro-mechanical gyroscopes, i.e. rotational rate sensors. These rotational rate sensors may exemplarily make use of the Coriolis force to find out a rotational speed or acceleration. In conventional rotational rate sensors, micro-mechanical resonators are, for example, put in controlled oscillation for this purpose which is also referred to as excitation oscillation. The oscillating structure formed in this way additionally comprises another resonant mode which is coupled to the excited mode, for example proportional to the Coriolis force and thus the rotational rate. The result is the possibility of utilizing an oscillation of the second resonant mode which is also referred to as detection oscillation for measuring the rotational rate.

In conventional technology, sigma-delta modulators are used for measuring the rotational rate, i.e., for example, for detecting the amplitudes of excitation and detection oscillations, the noise suppression characteristic of which can be adjusted such that they achieve the best quantizing noise suppression possible in the excitation frequency. Deviations occurring between the frequency of the maximum attenuation of the sigma-delta noise transfer function and the true oscillation frequency may result in a deterioration in the system performance due to increased noise in the signal band which cannot be attenuated or suppressed, not even by filtering.

SUMMARY

According to an embodiment, the present invention may include a measuring device for determining a measured quantity including an oscillatory structure in which an oscillation signal is detectable. In addition, the measuring device may comprise means for exciting the oscillatory structure by an excitation frequency to result in an oscillation of an oscillation frequency. In addition, the measuring device may include means for processing the oscillation signal synchronous to the oscillation frequency or the excitation frequency and evaluating means for determining the measured quantity based on the oscillation signal processed.

DETAILED DESCRIPTION

Before embodiments will be discussed in greater detail below referring to the drawings, it is to be pointed out that same elements in the figures are provided with same or similar reference numerals and that a repeated description of these elements is omitted.

Frequencies of discrete-time signal processing circuits, such as, for example, a system clock frequency, are defined in a digital or analog manner, exemplarily by capacitance ratios. In order for coefficients, such as, for example, of digital filters, to be well controllable, there is the possibility of coupling all frequencies proportionately to the frequency of an excitation oscillation of a sensor, such as, for example, of a gyroscope or rotational rate sensor. The frequencies of the signal-processing components are thus dependent on the excitation frequency, which offers the possibility of increasing the precision of the coefficient setting and achieving better adjustment between an oscillation frequency of a sensor which may be subject to tolerances and subsequent signal processing. Exemplarily, the zero values of a noise-shaping function of an analog-to-digital or digital-to-analog transducer and center frequencies of subsequent filters can be matched well to the excitation frequency. This generally is also true for the entire further signal processing which may, for example, be composed of regulators for controlling the excitation oscillation and compensation of the detection oscillation. The clock may, for example, be used for the digital demodulation of the detection signal, and maybe also for subsequent filters and, for example, also for compensation in a force-feedback circuit.

Embodiments using sensors for determining measured quantities the excitation frequency of which is coupled to a clock frequency of subsequent signal processing are described subsequently. This, for example, allows using filters, DA and AD transducers, regulators etc. having a smaller tolerance range in the frequency range than in conventional systems since the tolerance range is no longer fixed but may shift with the excitation frequency. A more preferable filter characteristic is achieved by coupling the excitation frequency to the clock frequency of the signal processing, also resulting in improved noise suppression. Measured quantities preferably used here are, but are not limited to, rotation rates, accelerations, forces, pressures etc.

When using fixed frequency characteristics not including coupling between an excitation frequency and a clock frequency, disadvantages may, for example, arise in sigma-delta DA transducers which are, for example, used for generating the excitation oscillation or for adjusting a detection oscillation in force-feedback operation. These problems may also occur in filters which allow selective measurement of the oscillation amplitudes and suppress interferences or noise at other frequencies, such as, for example, quantizing noise shifted by noise shaping. Since the passband and stopband frequencies in filters are sometimes to be close to each other, the filter structures can become very complex due to high quality requirements. The requirements may be even more critical if the range available for the transition between passband and stopband is narrowed further by manufacturing tolerances of frequencies designed by the mechanics of a sensor.

When AD transducers, filters and regulators use fixed frequency definitions, sensors may be optimized such that the result is the most stable frequency characteristic possible. Trimming or temperature compensation of the oscillator frequencies may, for example, be performed in a final test to optimize adjustment of the signal processing to the individual sensors. This, in turn, increases test times and, when the adjustment fails, the yield may be reduced due to discarding.

FIG. 1ashows an embodiment of a measuring device100for determining a measured quantity. The measuring device100includes an oscillatory structure110where an oscillation signal is detectable. In addition, the measuring device100comprises means for exciting the oscillatory structure110by an excitation frequency to result in an oscillation of an oscillation frequency. The measuring device100further includes means130for processing the oscillation signal by a frequency depending on the oscillation frequency or the excitation frequency. The measuring device100additionally comprises evaluating means140for determining the measured quantity based on the oscillation signal processed.

FIG. 1bshows another embodiment of the measuring device100for determining the measured quantity. The measuring device also includes the oscillatory structure110where the oscillation signal is detectable. In addition, the measuring device100comprises the means110for exciting the oscillatory structure by the excitation frequency to result in the oscillation of the oscillation frequency. The measuring device100also includes the means130for processing which in this embodiment is realized as means for sampling135the oscillation signal by a sample frequency depending on the oscillation frequency or the excitation frequency. The measuring device100also comprises the evaluating means140for determining the measured quantity based on the sampled oscillation signal.

In other embodiments, the means for processing130may, for example, comprise a filter the filter characteristic of which depends on the oscillation frequency or the excitation frequency. A possible realization would, for example, be an analog filter the filter characteristic of which depends on an impedance which in turn is adjustable in dependence on the oscillation frequency or the excitation frequency.

In another embodiment, the means120for exciting includes means for regulating the excitation amplitude, an excitation frequency or an excitation phase by a feedback branch via which the oscillation of the oscillatory structure110is detectable. Exemplarily, the oscillatory structure110, together with the means120for exciting and the feedback branch, can form a resonant circuit oscillating at a self-resonance of the oscillatory structure110.

In another embodiment, the means120for exciting may be formed such that the oscillation of the oscillation frequency is excited, while other resonant modes of the oscillatory structure110essentially remain unexcited or are suppressed. In one realization of the oscillatory structure110, this may comprise a mechanical structure capable of oscillating in a first oscillation or vibrational mode and a second oscillation mode, wherein the two oscillation modes comprise a coupling which depends on the measured quantity. The oscillatory structure110can be caused to oscillate in the first oscillation mode by an excitation with the excitation frequency by the means for exciting120and the oscillatory structure110can be coupled to the means for processing130such that a signal becomes available as a detectable oscillation signal for the means for processing130which results from an oscillation of the mechanical structure in the second oscillation mode. The oscillatory structure110can be made up of two oscillators coupled to each other, wherein it may also be conceivable to use a single oscillator capable of oscillating in several oscillation modes which are also coupled.

The evaluating means140in one embodiment may comprise force-feedback means for counteracting the oscillation of the mechanical structure in the second oscillation mode depending on the oscillation signal processed or sampled, wherein the evaluating means140, when determining the measured quantity, can determine same depending on the counteracting degree. In general, determining the measured quantity is also conceivable without a compensation circuit, i.e. force-feedback circuit, and without adjusting the detection oscillation.

In addition, in another realization the means for exciting120can comprise means for regulating the excitation amplitude, the excitation frequency or the excitation phase by a feedback branch via which a feedback signal resulting from the oscillation of the mechanical structure in the first oscillation mode is detectable, wherein another sampling means for sampling the feedback signal by another sample frequency is connected into the feedback branch, which may also depend on the excitation frequency. Exemplarily, DA or AD transducers employed for this may be operated by a clock frequency which is also based on the adjusting frequency. In another embodiment, a digital filter for filtering the feedback signal sampled may be coupled into the feedback branch.

In another embodiment, the evaluating means140may also comprise a digital filter for filtering the oscillation signal sampled. Embodiments offer a way of operating the digital filters by a clock frequency which may result from one of the oscillation frequencies. In addition, embodiments can comprise means for establishing a fixed clock ratio between the sample frequency on the one hand and the excitation frequency or the oscillation frequency on the other hand. Thus, the measured quantity may generally be a rotational rate of a rotation of the measuring device or an acceleration to be measured so that embodiments can generally also be employed as acceleration sensors.

Gyroscopes or rotational rate sensors may, for example, be used to technically realize the oscillatory structure. Naturally, the detection oscillation, i.e. the oscillation signal, may be of the same frequency as the excitation frequency. Thus, the excitation frequency may coincide with the resonant frequency of the gyroscope or sensor, however, it is in principle also conceivable to operate a gyroscope or a sensor by another frequency. The detection oscillation of a gyroscope or sensor in many cases has the same frequency as the excitation oscillation since the excitation oscillation provides the energy for the detection oscillation via coupling which is proportional to the rotational rate.

With regard to a possible subsequent signal-processing circuit, it is to be mentioned here that in the end even poles and zero values of the regulator transfer functions required for stabilizing the excitation amplitude and for adjusting force-feedback voltages which may be used here are coupled to the frequencies of the sensor, wherein these frequencies, too, may coincide with the resonant frequencies of the sensor, this, however, is not absolutely necessary.

FIG. 2illustrates an embodiment of a measured quantity sensor200.FIG. 2shows a measured quantity sensor200comprising an oscillatory structure210having a first detection terminal212where an oscillation signal is detectable and an excitation terminal214, wherein the oscillatory structure may be excited to result in an oscillation of an oscillation frequency by applying an excitation signal of an excitation frequency to the excitation terminal214. The measured quantity sensor200additionally shows an excitation circuit220having an output224coupled to the excitation terminal214, and a signal processor230having an input232coupled to the first detection terminal212, a frequency input234to which a signal may be applied of a frequency which depends on the oscillation frequency of the oscillatory structure210or the excitation frequency at the excitation terminal214, and an output236. In addition, the measured quantity sensor200comprises an evaluation circuit240having an input246coupled to the output236of the signal processor, and an output248representing a measured quantity sensor output of the measured quantity sensor200.

The signal processor230in one embodiment may, for example, be realized by a sampler the frequency input234of which corresponds to a sample frequency input. In other embodiments, the signal processor230may, for example, comprise a filter the filter characteristic of which depends on the oscillation frequency or the excitation frequency. A possible realization would, for example, be an analog filter the filter characteristic of which depends on an impedance which in turn is adjustable in dependence on the oscillation frequency or the excitation frequency.

In one embodiment of a measured quantity sensor200, the excitation circuit220may form a control loop for regulating the excitation frequency, an excitation amplitude or an excitation phase with a feedback branch, via which the oscillations of the oscillatory structure210are detectable. In addition, the oscillatory structure210may comprise a mechanical structure capable of oscillating in a first oscillation mode and a second oscillation mode. Thus, the two oscillation modes are coupled, wherein the coupling depends on the measured quantity of the measured quantity sensor200to be measured, and the mechanical structure is excitable to result in an oscillation in the first oscillation mode by applying the excitation signal to the excitation terminal214. An oscillation signal resulting from an oscillation of the mechanical structure in the second oscillation mode may be detectable at the first detection terminal212. The oscillatory structure210can be made up of two oscillators coupled to each other, however, it is also conceivable to use a single oscillator which can oscillate in several oscillation modes which are also coupled.

In another embodiment, the evaluation circuit240may include a force-feedback circuit and the oscillatory structure210may additionally comprise a force-feedback terminal where the oscillation of the mechanical structure in the second oscillation mode can be counteracted. Thus, there is a way of connecting a force-feedback circuit between the output236of the signal processor230and the terminal246of the evaluation circuit240. The force-feedback circuit here may comprise and output coupled to the force-feedback terminal of the oscillatory structure210. In general, however, measurement without adjusting the detection oscillation is also conceivable.

In another realization, the excitation circuit220can form a control loop with a feedback branch and the oscillatory structure210can additionally comprise a detection terminal where the oscillation of the mechanical structure in the first oscillation mode is detectable. The feedback branch can thus be coupled to the second detection terminal and another sampler having a sample frequency input can be coupled therein, to which a signal of a frequency depending on the excitation frequency at the excitation terminal214may be applied. Exemplarily, DA or AD transducers employed here can be operated by a clock frequency which is also based on the adjusting frequency.

The control loop for controlling a force feedback can be designed such that modes different from one of the desired oscillation modes of the mechanical structure are suppressed or remain unexcited. In addition, a digital filter can be connected into the feedback branch of the further sampler and there is the possibility of the evaluation circuit240comprising a digital filter having an input coupled to the output236of the signal processor230.

In another embodiment, the evaluation circuit220can be operated by a clock derived from one of the oscillation frequencies. In addition, the measured quantity can correspond to a rate of rotation of a rotation of the measured quantity sensor200, however, in general any accelerations or forces are conceivable as measured quantities. In addition, there is the possibility of the measured quantity sensor200comprising a clock multiplier having an input and an output, by means of which it is connected between the excitation circuit220on the input side and the signal processor230which may be realized as a sampler on the output side such that a sample frequency at the frequency input234has a fixed clock ratio to the excitation frequency or oscillation frequency. Thus, it would be conceivable to select a system clock for digital signal processing for processing the sampled oscillation signal to be correspondingly higher than the sample frequency itself. If a fixed clock ratio is known, there will in principle be the possibility of designing the digital signal processing such that the problems and adjusting difficulties caused by manufacturing tolerances of the oscillation structure are reduced considerably.

FIG. 3shows another embodiment of a measured quantity sensor300comprising a mechanical structure310capable of oscillating in a first oscillation mode and a second oscillation mode which comprise coupling depending on a measured quantity of the measured quantity sensor to be measured. Again, gyroscopes or rotational rate sensors may be employed here. In addition, the mechanical structure310includes a first excitation terminal312where an oscillation of the mechanical structure310of an oscillation frequency in the first oscillation mode is excitable by exciting it by an excitation frequency. In addition, the mechanical structure310includes a first detection terminal314where an oscillation of the mechanical structure in the second oscillation mode is detectable. Furthermore, the mechanical structure310comprises a second detection terminal316where the oscillation of the mechanical structure310in the first oscillation mode is detectable. The measured quantity sensor300additionally includes an excitation circuit320having an output322coupled to the first excitation terminal312, wherein the excitation circuit320forms a control loop with a feedback branch coupled to the second detection terminal316. In addition, the measured quantity sensor300comprises a signal processor330having an input334coupled to the first detection terminal314, a frequency input336coupled to a circuit part of the excitation circuit320where the excitation frequency at the first excitation terminal312is detectable or to the second detection terminal316, and an output338. In addition, the measured quantity sensor300includes an evaluation circuit340having an input348coupled to the output338of the signal processor330and an output350representing a measured quantity sensor output of the measured quantity sensor300.

In one embodiment, the signal processor330in turn may be realized by a sampler, the frequency input336corresponding to a sample frequency input. In addition, the signal processor330in other embodiments may, for example, comprise a filter the filter characteristic of which depends on the oscillation frequency or the excitation frequency. A possible realization would, for example, be an analog filter the filter characteristic of which depends on an impedance which in turn is adjustable in dependence on the oscillation frequency or the excitation frequency.

In one embodiment, the evaluation circuit340can comprise a force-feedback circuit, wherein the measured quantity sensor300can additionally comprise a second excitation terminal where the oscillation of the mechanical structure310in the second oscillation mode can be counteracted, wherein the force-feedback circuit may, for example, be connected between the output338of the signal processor330and the input348of the evaluation circuit340and comprises an output coupled to the second excitation terminal.

Additionally, in another realization of an embodiment, another sampler can be connected into the feedback branch of the control loop of the excitation circuit320, the sample frequency input of which is coupled to the circuit part of the excitation circuit320where the excitation frequency is detectable. In addition, a digital filter can be connected into the feedback branch of the control loop of the excitation circuit320downstream of the further sampler. In addition, the evaluation circuit320may comprise a digital filter having an input which may be coupled to the output338of the signal processor. In this embodiment of a measured quantity sensor300, too, the measured quantity can correspond to a rate of rotation of a rotation or acceleration of the measured quantity sensor.

In another embodiment, the measured quantity sensor300may additionally comprise a clock multiplier having an input and an output by means of which it is connected between the excitation circuit320on the input side and the signal processor330realized as a sampler on the output side such that a sample frequency at the frequency input336has a fixed clock ratio to the excitation frequency at the first excitation terminal312or the oscillation frequency.

Subsequently, it is exemplarily illustrated referring toFIGS. 4 and 5how noise transfer functions and filter functions may be in relation to the resonance of, for example, a micro-mechanical structure. It is at first illustrated inFIG. 4how the tolerances of a bandpass filter may decrease in relation to an excitation frequency if an independent system clock of, for example, an RC oscillator is replaced by a system clock coupled to the excitation resonance.FIG. 4shows a frequency axis400along which the schematic transfer functions are plotted. The oscillation frequency of a micro-mechanical structure of an embodiment is illustrated by an arrow410, coupled thereto is a system clock420of subsequent digital signal processing. Exemplarily, another oscillation mode which may, for example, be caused by mechanical coupling is illustrated inFIG. 4by the arrow430.

In addition, the transfer functions of different bandpass filters are schematically illustrated inFIG. 4, wherein the two transfer functions440and445make clear that, when coupling the system clock to the excitation frequency, a bandpass filter having a lower tolerance, i.e. lower noise coupling-in, may be used. The transfer function440shows the pass bandwidth of a conventional bandpass filter, graph445shows the transfer function of a bandpass filter which may be employed in one embodiment, wherein the two bandpass filter transfer functions440and445refer to the excitation frequency.

It is possible to achieve similar conditions of transfer functions in a detection oscillation which inFIG. 4is illustrated by the arrow450. Here, too, bandpass filters which become more narrow-banded with a corresponding coupling between the system clock and the excitation frequency may be used, which is to illustrate the transfer function455of a conventional bandpass filter compared to the transmit bandwidth of the bandpass filter460which may be employed in one embodiment. Similar effects can be observed with a noise or other oscillation modes the frequency of which is at first exemplarily indicated inFIG. 4by the arrow430. Here, too, it may be observed that the tolerance bandwidth of this noise occurring has become considerably more narrow-banded, wherein the tolerance bandwidth of a conventional system is illustrated by the schematic transfer function465and the tolerance bandwidth in one embodiment is illustrated by graph470.

FIG. 5shows a frequency axis500along which the different spectra and transfer functions are plotted.FIG. 5at first shows the spectrum of an excitation signal510and the spectrum of a detection signal515. Based on an embodiment, a corresponding narrow-banded bandpass filter having a transfer function520may be used both for filtering the excitation oscillation and for filtering the detection oscillation. If there is noise, exemplarily due to undesired mechanical coupling which may cause undesired other oscillation modes, as is exemplarily illustrated inFIG. 5by the spectrum525, they may exemplarily be compensated by the transfer function of analog-to-digital transducers or digital-to-analog transducers since the clocking thereof may also be coupled to the excitation frequency.FIG. 5exemplarily shows the graph530of such an analog-to-digital transducer or digital-to-analog transducer the transfer function of which has a strong attenuation at the undesired resonance525.

FIGS. 4 and 5show that embodiments of measured quantity sensors and measuring devices can also suppress undesired resonances which are not necessarily to be excited by, for example, quantizing noise. Exemplarily, further zero values can be realized in the noise transfer functions of the analog-to-digital or digital-to-analog transducers. The undesired resonances of a mechanical structure already described above, in particular those of a frequency close to the excitation frequency, may be influenced extensively by the same physical parameters as the excitation resonance, such as, for example, by oscillating masses, filter constants, attenuation constants, etc. The result for these resonances is a parameter synchronism which may be utilized by coupling the system clock to the excitation frequency. The result is a decrease in adjusting errors caused by manufacturing tolerances or temperature dependencies.

FIG. 6shows another embodiment. InFIG. 6the components of the measuring device100, followingFIGS. 1aand1b, are illustrated again by broken lines.FIG. 6shows a measuring device100for determining a measured quantity by an oscillatory structure110where an oscillation signal is detectable. The oscillatory structure110is realized inFIG. 6by a micro-mechanical gyroscope610, a so-called MEMS (micro-electromechanical system). The gyroscope here comprises a first resonance circuit612and a second resonance circuit614. The first resonance circuit612forms the excitation resonance circuit, the second resonance circuit614forms the detection resonance circuit. In addition, the gyroscope610can also include other resonance modes616.

The excitation resonance circuit612according toFIG. 6is driven by means120for exciting the oscillatory structure110by an excitation frequency to result in an oscillation of an oscillation frequency. The means120for exciting the oscillatory structure110inFIG. 6is realized by an excitation regulator620coupled to a PLL (phase locked loop)622, a digital-to-analog transducer624, an analog-to-digital transducer626and a digital filter628. Correspondingly, the excitation regulator620in the embodiment ofFIG. 6operates in the digital range and defines the excitation frequency for the PLL622. The excitation resonance circuit612of the gyroscope610is coupled to the excitation regulator620via the digital-to-analog transducer624, wherein the excitation signal is again detected via the analog-to-digital transducer626and filtered via the digital filter628, wherein the output of the digital filter628in turn is coupled to the excitation regulator620. Thus, the result is a control loop.

According to the measuring device100in the embodiment ofFIG. 6it further includes means130for processing the oscillation signal with a frequency depending on the oscillation frequency or the excitation frequency. The means130for processing in this embodiment is realized as means135for sampling according toFIG. 1b. The means135for sampling is realized by the AD transducer634which also receives the clock of the PLL622.

The measuring device100inFIG. 6additionally comprises evaluating means140including a digital filter636, a detection regulator630, a DA transducer632and a processor for IQ demodulation and signal processing640, wherein all these components are coupled to the clock of the PLL622. The detection regulator630can influence the detection oscillation of the detection resonance circuit614of the gyroscope612via the digital-to-analog transducers632, wherein the oscillation signal of the resonance circuit614is detected by the analog-to-digital transducer634of the means135for sampling the output of which is coupled to the detection regulator630via the digital filter636, which is how the detection control loop is closed. Thus, it is possible for the detection regulator630to adjust the detection oscillation and determine an adjustment signal based thereon. This type of operation is also referred to as force-feedback operation which, however, is not absolutely necessary for detecting the measured quantity and in the embodiment inFIG. 6is illustrated exemplarily.

The system clock of all the components of the evaluating means140is predefined by the PLL622. Both the excitation regulator620and the detection regulator630can pass on output signals to a processor for IQ demodulation and signal processing640which, based on these signals, determines the measured quantity which inFIG. 6is referred to as yaw rate.

In general, the signal paths of sensors according to embodiments to not deviate from conventional realizations, wherein it can be seen in the embodiment ofFIG. 6that additionally a PLL622is coupled to the oscillator loop of the excitation oscillation, preferably at the digital output of the excitation regulator620. The PLL622generates the system clock which can be higher than the excitation frequency by a constant factor in accordance with well-known methods.

FIG. 7illustrates an embodiment wherein, in accordance withFIG. 1a, the means130for processing comprises an analog filter.FIG. 7shows a section of a delta-sigma transducer710which comprises a loop filter715in its feedback branch. The loop filter715has a filter characteristic which in the embodiment ofFIG. 7depends on an overall capacity C which in turn is realized by a cascade720of switchable individual capacities. The overall capacity thus results as a sum of the individual capacities switched in, wherein the individual switch positions are controlled by a PLL730which has an equivalent cascade735of individual capacities at its disposal. Since the PL730oscillates at the excitation frequency or oscillation frequency, this is regulated via a control circuit and thus the overall capacity of the equivalent cascade735is regulated, the PLL730has the switch positions of the equivalent cascade735pertaining to the oscillation frequency or excitation frequency at its disposal. They may be transferred to the cascade720of the loop filter715, which is how the filter characteristic can be made dependent on the oscillation frequency or excitation frequency or be adjusted to the oscillation frequency or excitation frequency.