The present application claims the benefit under 35 U.S.C. § 119 of German Patent Application No. DE 102020203851.1 filed on Mar. 25, 2020, which is expressly incorporated herein by reference in its entirety.
The present invention is directed to a method for determining a detection sensitivity of a rotation rate sensor.
Conventional rotation rate sensors are limited in their performance by adverse effects such as, for example, the drift of the detection sensitivity since, if not compensated or corrected for, this may lead to a false interpretation of the measured results. For this reason, rotation rate sensors frequently include compensation mechanisms, which counteract the drift of measured results.
Such a drift of sensitivity is dependent on a multitude of factors such as, for example, environmental influences such as temperature or atmospheric humidity or also on mechanical stresses, aging and many more. The actual physical cause of the drift is often due to a mechanical influence on the spring mass system (SMS) of the system of the sensor. Such a mechanical influence in turn results in a change of the amplitude response (amplification characteristic curve) and/or in the phase response (phase characteristic curve). Amplitude response and phase response are determined by the transfer function (also transfer function or system function) of the spring mass system, which characterizes the frequency-dependent response of the system, in that it indicates the correlation between the input signal (for example, drive signal) and the output signal (for example, the amplitude of the drive mode or detection mode, or of the associated electrical signal). The transfer function for various models of oscillatory systems is known in an analytical form. The following dependency of the frequency ω of the input signal, in particular, results for a damped oscillation of a system made up of a mass and a linear spring (the dimensional prefactor of the amplification G(ω) is omitted here):
In this case, ω0 refers to the inherent frequency (determined by the mass and the spring constant) and Q refers to the quality factor of the oscillatory system. In the event of a change, for example, of the spring constant (and thus the inherent frequency) of the system, the amplification characteristic curve and phase characteristic curve are influenced by the aforementioned effects in terms of their slope and in their position relative to the frequency axis. The profile of the characteristic curves may on the other hand also be influenced by a change in the quality of the system due to a temperature change. A change in the amplification or in the phase in turn influences the measured signal (cf. equation 3 further below), without this being based on an actual change of the rotation rate, so that here a measuring error results and the detection sensitivity is reduced. In order to identify a drift without resorting to a reference stimulus (i.e., a known external rotation rate) in the process, the quadrature (identified in the formulas as Quad) present in each real micromechanical system may be utilized. The quadrature produces a force effect independent of rotation rate Ω, or an additional contribution to the detection signal, which is phase shifted by 90° with respect to the useful signal generated by the rotation rate. Both signal contributions are, however, proportional to sensitivity G. Due to the phase shift, it is possible to separate quadrature signal sQuad via demodulation from detection signal sΩ.
s
Ω
=G*Ω+G*Quad*ϕ (3)
s
Quad
=G*Quad (4)
This fact is exploited in conventional methods for determining a sensitivity change. This is achieved by temporarily artificially amplifying by electrostatic means the quadrature of the oscillating detection apparatus via electrodes also integrated in the sensor. The output signals changed as a result are then compared during operation with a reference value, which has been established via a previous calibration. An indication about the sensitivity change may then be derived from the relationship of the output signal to the reference value. One disadvantage of such a method, however, is the additional design complexity associated therewith and the space requirement for the additional electrodes on the sensor. It is therefore desirable to obtain a measurement of the sensitivity drift without additional mechanical outlay. Moreover, with the conventional method described above, it is possible to determine only a temperature-dependent change in the amplification characteristic curve and phase characteristic curve of an oscillatory system. By contrast, a shift of the characteristic curve as a result of additional disruptive effects, such as mechanical stresses in the micromechanical structure that forms the sensor, may be ascertained to only a limited extent.
It is an object of the present invention to provide a possibility of obtaining pieces of information about a potential drift of the detection sensitivity without the disadvantages described above occurring in the process.
The method according to an example embodiment of the present invention may have the advantage that it is possible to dispense with an artificial influencing of the quadrature of the oscillatory system and, in particular, with the additional electrodes required for this purpose. The method allows for an efficient way of establishing a change of sensitivity of an oscillatory system (also referred to hereinafter as spring mass system or abbreviated SMS). The SMS may, in particular, be the detection apparatus of a rotation rate sensor. In this case, the detection of this change is independent of a potential change of the quadrature and is also virtually temperature-independent. In accordance with an example embodiment of the present invention, the method allows for the determination of the position of the transfer function (TF) or the position of the operating point on the amplification characteristic curve or phase characteristic curve determined by the transfer function. In this way, it is possible to determine not only the detection sensitivity, but also the phase change (regardless of the temperature influence). This provides, in particular, the possibility of establishing a correction factor or a correction factor function or correction factor table, which may be utilized during the operation of the sensor for compensating for the drift of sensitivity and phase. Based on the measured results determined in the method, it is possible to determine a temperature-dependent absolute phase difference and a temperature-independent relative phase difference, which may serve as a further indicator for additional influence factors.
Each of the aforementioned possibilities may be implemented, in particular, in the case of the detection apparatus, without the additional electrodes conventionally used. They are also independent of an applied rotation rate and may thus be used during the operation of the sensor without further restrictions. It is also conceivable to use additional electrodes for influencing the SMS and to employ them complementarily to the method according to the present invention, in order to improve the accuracy of potential compensations and in this way to contribute to a rotation rate-independent absolute determination of the sensitivity.
The features of the present invention are based on initiating a controlled change of the transfer function (i.e., of the amplitude response and/or phase response) and at a fixed drive frequency to ascertain the relative change of the amplification or the phase shift for determining the position of the transfer function in relation to the frequency axis. At a predefined drive frequency, the detection signal allows for conclusions to be drawn about the actual amplification only with knowledge of the applied rotation rate. According to equation 3, the detection signal contains (in addition to the quadrature component determined by demodulation) amplification G and rotation rate Ω in the form of a product, so that based on the detection signal alone, no separation of the two influences is possible. In the method according to the present invention on the other hand, quadrature component sQuad is determined for at least two different amplifications (caused by the controlled change of the transfer function), by which sufficient information is available in order to determine the true amplification.
With Equation 4, the relationship vi of the quadrature signal sQuad,i,Δ=sQuad,i+ΔsQuad belonging to the changed transfer function (variables belonging to the changed transfer function are identified below with an additional index Δ and differences by a Δ placed in front of the variable) and of quadrature signal sQuad,i belonging to the unchanged transfer function results in:
As is apparent from the equation, the relationship thus established is independent of the quadrature. For the output signal of the rotation rate,
s
Ω,i,Δ=(Gi+ΔGi)Ω(ϕi+Δϕi) (6)
is applicable after the change of the transfer function.
The determination of the amplification on the basis of the relationship vi may be described as exemplified by a controlled horizontal shift (in particular, by a corresponding shift of the inherent frequency) of the amplification characteristic curve and phase characteristic curve provided by equations 1 and 2 (cf.
The above explanations that describe in detail the present invention as exemplified by a controlled horizontal shift may be transferred to the general case, in which a largely arbitrary, reproducible and controlled influence of the transfer function of an oscillatory spring mass system may be utilized for ascertaining the operating point on the amplification characteristic curves and phase characteristic curves provided by the transfer function, and thus ultimately for determining the sensitivity change as a result of external influences.
Beyond the application of the shift of the inherent frequency as a compensation method (whether in a mechanical or electrically induced variant), this method may also be utilized for further functions or in further application alternatives.
According to one preferred specific embodiment of the present invention, a controlled change of an inherent frequency of the oscillatory system takes place and/or a controlled change of the quality factor of the oscillatory system takes place in the second step. Inherent frequency ω0 and the quality Q according to equations 1 and 2 determine the shape and position of the amplification characteristic curve and phase characteristic curve. The quality in this case determines essentially the width of the curve, i.e., the drop of the flanks to the left and right of the maximum, whereas the inherent frequency determines the position of the characteristic curves relative to the frequency axis. Both variables may thus be used in the method according to the present invention to change the transfer function in a controlled and reversible manner and enable a determination of the sensitivity drift via the above-described measurement of the quadrature signal.
According to one particularly preferred specific embodiment of the present invention, the change of the inherent frequency takes place via a controlled change of the spring constant of the oscillatory system. The spring constant, together with the mass of the oscillating body, determines the inherent frequency of the oscillatory system, so that the increase or reduction of the spring constant results directly in a shift of the inherent frequency which, as explained above, may be utilized for a determination of the sensitivity drift.
According to one preferred specific embodiment of the present invention, further controlled changes of the transfer function take place in a fifth step following the third step and preceding the fourth step, and further quadrature signals of the rotation rate sensor are determined, the detection sensitivity being determined in the fourth step on the basis of the first, second, and further quadrature signals. In this way, it is possible to advantageously improve the accuracy of the ascertained sensitivity drift via a repetition or multiple repetition of the second and third step of the method according to the present invention. In the simplest case, the sensitivity drift may, for example, be ascertained with each repetition and a suitable selection may be made or an average value may be formed from the values for the drift thus ascertained.
According to one preferred specific embodiment of the present invention, a compensation variable for a detection signal is determined in a fifth step following the fourth step on the basis of the detection sensitivity determined in the fourth step. In order to be able to utilize the relationship vi for compensating, a matrix or function may be established in advance (under initial conditions), which links the changed relationship vi to a compensation factor CF. This may be achieved, for example, by measuring the relationship vi across a particular range and by determining the associated correction factor for each of these values. In the simplest case, the correction factor may be established by setting the quadrature signal belonging to the shifted inherent frequency in relationship to the reference signal:
Thus, a result is achieved as it would have been measured under initial conditions without a measurement of the quadrature or of the quadrature change being necessary.
A further example embodiment of the present invention is a further method. Whereas the first method is based on a controlled change of the transfer function of one oscillatory system, two oscillatory systems of the same sensor are used in this variant, which differ in a defined manner with respect to their transfer functions. Relationship vi accordingly from the quadrature signals of both systems, i.e., in equation 5, is the variable sQuad,i provided by the quadrature signal of the first oscillatory system and the variable sQuad,i,Δ provided by the quadrature signal of the second oscillatory system. The observations explained above are, however, directly transferable.
According to one preferred specific embodiment of the present invention, a quality factor of the first oscillatory system is identical to a quality factor of the second oscillatory system, a mass of the first oscillatory system differing from a mass of the second oscillatory system and/or a spring constant of the first oscillatory system differing from a spring constant of the second oscillatory system. In this specific embodiment, the difference between the transfer functions of the two systems is generated either by the mass, the spring constant and/or by the quality factor. The mass in this case, together with the spring constant, determines the inherent frequency, so that by enlarging or reducing the mass, a further possibility results for shifting the inherent frequencies of the two systems with respect to one another.
According to one preferred specific embodiment of the present invention, a compensation variable for a detection signal occurs in a fifth step following the fourth step on the basis of the detection sensitivity determined in the fourth step. The compensation variable is determined in a manner similar to that described above for the case of a single oscillatory system.
The following described specific embodiments may be understood as variants both of the first method, i.e., change of the transfer function of one oscillatory system, as well as of the second method, i.e., two oscillatory systems having different transfer functions.
According to one preferred specific embodiment of the present invention, the rotation rate sensor includes a register that includes a plurality of value pairs of the detection sensitivity and of the compensation variable, the compensation variable occurring via a selection of a value from the register, or the compensation variable is determined by an analytical, in particular, linear correlation between the detection sensitivity and the compensation variable. Thus, a linear correlation between the detection signal relationship v and the compensation factor CF may be, in particular, even approximately assumed as a function of the selection of the operating point. In this way, a factor, instead of a table or the like may be ascertained, so that CF may be calculated by multiplying v by this factor.
According to one preferred specific embodiment of the method according to the present invention, a first detection signal is determined in the first step and a second detection signal is determined in the second step, a temperature effect on a mechanical phase of the oscillatory system being ascertained in a sixth step following the fourth step on the basis of the first and second detection signal and of the first and second detection signal. The following correlation for the absolute phase change may be calculated based on equations 3 and 4 and on a shift of the detection modes:
Since, in the case of an inherent frequency shift, the same SMS is examined and a relationship is formed, the determination of the horizontal position on the amplification characteristic curve with the method according to the present invention is virtually independent of temperature influences. However, thermal disruptive influences are also not identified or compensated for in this way. The calculation of the phase change caused by the shift of the inherent frequency is an absolute value, however, and therefore a function of the temperature. Thus, the absolute phase change in one variant of the method according to the present invention may serve as an indicator for a temperature influence.
A further subject matter of the present invention is a rotation rate sensor, including an oscillatory system and a control unit, the rotation rate sensor, in particular, the control unit being configured to carry out a method(s) in accordance with an example embodiment of the present invention. A further subject matter is a rotation rate sensor including a first and a second oscillatory system and a control unit, the rotation rate sensor, in particular, the control unit being configured to carry out a method in accordance with an example embodiment of the present invention, or being configured to carry out a method variant in accordance with the present invention, and to carry out a method in accordance with another method variant of the present invention. The specific embodiments of the method according to the present invention described further above are each transferable directly to specific embodiments of the rotation rate sensor according to the present invention.
One specific embodiment of the method according to the present invention is based on the approach of comparing the amplification or phase of an oscillatory system 1 at different inherent frequencies.
The initialization is started in block 17 and it is initially established in block 18 whether the desired number of measuring points is achieved. Initially, no measured values are present, so that in block 20 the inherent frequency of the SMS is shifted and the quadrature signal is subsequently measured in block 21 and the relationship vi is determined from the measured value and from the value of the quadrature signal belonging to the unshifted inherent frequency. Correction factor CFi=vi/v is determined in block 22 and stored in block 23 as assignment CFi(vi), for example, in a register. Sequence 18, 20, 21, 22, 23 is repeated until it is determined in block 18 that the desired number of measured points is achieved and the process is ended in block 19.
Number | Date | Country | Kind |
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102020203851.1 | Mar 2020 | DE | national |