CN100498236C - Electromagnetic-piezo-resistance type micro mechanical resonant beam sensor - Google Patents

Abstract

The electromagnetic-piezoresistance type micro mechanical resonant beam sensor has single beam harmonic oscillator with ends fixed and set inside magnetic field in the width direction, and resonance frequency controlled by the measured physical quantity. The control circuit outputs exciting AC current to the piezoresistance film on the surface of the harmonic oscillator and detects the voltage across it, and the exciting current produces electromagnetic force to make the harmonic oscillator to vibrate. The piezoresistance film senses the first modal shape to generate corresponding AC resistance component. The exciting current and the AC resistance component are multiplied to obtain modulated vibration pick up signal, which is low pass filtered to obtain DC signal reflecting the resonance frequency changing tendency of the harmonic oscillator. The present invention can realize the continuous measurement of physical quantity and detect weak vibration signal in strong interference background.

Description

Electromagnetic-Piezoresistive Micromachined Resonant Beam Sensor

technical field

The invention relates to a signal detection and closed-loop system principle of a sensor based on a micromechanical single-beam harmonic oscillator.

Background technique

The resonant beam is one of the basic measurement components in MEMS devices. The measured physical quantity q is converted into the natural frequency f _n of the resonant beam and measured f _n to obtain q. In order to measure f _n , the resonant beam must have the functions of vibration excitation (excitation) and vibration pickup (vibration pickup), and form a closed-loop system with the control circuit, so that the resonant beam is in a resonant state. Since the resonant frequency f _r and f _n can be regarded as equal, f _n can be obtained by measuring f _r . Therefore, the resonant beam with the functions of exciting and picking up vibration and the control circuit are two elements to realize the resonant beam measurement principle.

For the scheme using a single-beam resonator, in order to avoid the adverse effect of additionally processing the exciter or vibration pickup on the beam on the mechanical properties of the beam, an electromagnetic-piezoresistive structure can be used, that is, the piezoresistive film on the entire length of the beam surface is used Simultaneously realize electromagnetic excitation and piezoresistive vibration: on the one hand, it is used as a wire, through the AC excitation current i _e (t) = I _e cos2πf _e t and placed in the magnetic field, using the generated distributed alternating current The electromagnetic force (ampere force) ψ _e (x, t) = Ψ _e (x) cos2πf _e t excites the vibration of the beam (x refers to the coordinate on the length direction of the beam); The AC resistance component produced by the resistance value changing under the action of the alternating stress field on the surface of the beam Picks up the vibration of the beam. For the detection of _rs , the conventional method is to pass a constant value reference current I _REF in the piezoresistive film, and transform _rs into an AC pickup voltage v _s =I _REF · _rs .

But the AC excitation current i _e produces a strong interference signal _ve = _ie · _RS on the constant resistance component R _S of the piezoresistive film, and the vibration of the beam in the magnetic field generates an induced electromotive force e _i at both ends. The frequencies of v _e , e _i and v _s are all _fe , v _d = v _e + e _i is called co-channel interference, and its amplitude is generally above 100mV, and v _{s is} in the order of μV. Obviously, v _s will be suppressed by v _d Submerged and difficult to detect, this problem must be solved to make the electromagnetic-piezoresistive structure truly practical.

Contents of the invention

The technical problem to be solved by the present invention is to solve the detection problem of the AC resistance component r _s of the piezoresistive film under the above-mentioned strong excitation and interference conditions, and provide an electromagnetic-piezoresistive device that can effectively solve the detection of weak vibration pickup signals under the background of strong interference micromechanical resonant beam sensor.

The technical solution of the present invention: electromagnetic-piezoresistive micromechanical resonant beam sensor, which is characterized in that: the single-beam resonant oscillator fixed at both ends is placed in the magnetic field along its width direction, and its resonant frequency f _r is affected by the measured physical quantity q Control; the control circuit is composed of a controller, a signal source and a low-pass filter LPF. The signal source in the control circuit outputs an AC excitation current i _e to the piezoresistive film on the surface of the single-beam resonator and detects the voltage across it, and i _e generates The electromagnetic force makes the single-beam resonator vibrate; the piezoresistive film is sensitive to the first-order mode of the single-beam resonator, and generates the corresponding AC resistance component r _s ; the multiplication of i _e and r _s gives the modulated vibration pickup signal V _s , which is passed through the low-pass The filter LPF gets the DC signal V _S , adjusts the i _e frequency f _e according to V _S , and makes it continuously track f _r , so that the continuous measurement of q can be realized. The above signal modulation process v _s =i _e r _s has constituted a phase-sensitive detector PSD, and the PSD and the subsequent LPF constitute a lock-in amplifier LIA (based on the principle of cross-correlation detection) to achieve cross-correlation detection, which can effectively solve the strong interference background The detection of the weak pickup signal V _s .

Principle of the present invention: for periodic AC signals, the most fundamental feature is frequency, and if useful signals and noises have the same frequency, they cannot be distinguished in principle. Therefore must try to make v _s and v _d have different frequencies. One of the key improvements of the present invention is to use the AC reference current i _ref with the frequency f _ref instead of the DC reference current I _REF , so as to realize signal modulation, and obtain v _s =i _ref · _rs with a frequency other than f _e : According to the signal Modulation principle, the product i _ref · _rs is the sum of two side frequencies f _ref -f _e and f _ref +f _e .

However, this method requires an additional i _ref signal source, and the circuit is more complicated; it is necessary to apply i _e and i _ref to the piezoresistive film 11 at the same time, which increases the resistance thermal effect; and i _ref will also generate electromagnetic excitation force, which may Disturb the vibration state of the beam. The second key improvement of the present invention is to combine i _e and i _ref into one, and take i _e =I _e cos2πf _e t as a reference signal at the same time. Since f _ref =f _e , the two side frequencies at this time are 0 Hz (direct current) and 2f _e (double frequency) respectively.

因此可得v_s＝i_e·r_s＝0.5I_eR_ssin2π(2fe)t，即v_s只包含2f_e边频，直流分量为V_s＝0。当f_r随q变化使得谐振子1暂时脱离谐振状态(Δf≠0)时，由于谐振子1及压阻薄膜11均为线性系统，故r_s频率仍为f_e：但r_s相对i_e的相位不再是

由谐振子1传递函数When the harmonic oscillator is in the resonance state, that is, Δf=f _e -f _r =0, not only the amplitude reaches the maximum, but also the excitation force ψ _e and its instantaneous velocity according to the principle of vibration In phase; according to the principle of piezoresistive effect, r _s is in phase with instantaneous displacement z; and i _{e is} in phase with ψ _e , lead z90°, so i _e leads r _{s by} 90°, namely

Therefore, it can be obtained that v _s = _ie · _rs =0.5I _e R _s sin2π(2fe)t, that is, v _s only includes 2f _e side frequency, and the DC component is V _s =0. When f _r changes with q so that the resonator 1 temporarily leaves the resonance state (Δf≠0), since the resonator 1 and the piezoresistive film 11 are both linear systems, the _rs frequency is still f _e : But the phase of r _s relative to i _e Is no longer

By harmonic oscillator 1 transfer function

$\mathrm{<span\; class="notranslate">\; G</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; A</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="no"\; filenames="C200610114276E00091.png"\; state="\{\{state\}\}">no</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="s"\; filenames="C200610114276E00091.png"\; state="\{\{state\}\}">s</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; \&xi;\&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="no"\; filenames="C200610114276E00091.png"\; state="\{\{state\}\}">no</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}$

由信号调制原理容易得出：V_S>0，显然：

因此可将传递函数G(s)、信号调制运算i_ref·r_s和LPF构成的整体视为以f_r为正输入端、以f_c为负输入端、以V_S为输出端的“频率比较器”，如图2所示。其输出-输入特性为非线性，如图3所示。It can be seen that:

It is easy to get from the principle of signal modulation: V _S >0, Obviously:

Therefore, the whole composed of transfer function G(s), signal modulation operation i _ref · _rs and LPF can be regarded as a "frequency comparison" with f _r as the positive input terminal, f _c as the negative input terminal, and V _S as the output terminal. device", as shown in Figure 2. Its output-input characteristic is non-linear, as shown in Figure 3.

The control circuit 2 adopts the following negative feedback control law to maintain the resonant state of the resonator 1: judge the polarity of Δf according to V _S , and adjust f _e in real time accordingly: if V _S >0, f _e is increased, and if V _S <0, it is decreased f _e , repeat this until Δf→0.

Therefore, the frequency comparator and the above-mentioned negative feedback control law (that is, the resonant oscillator 1 and the control circuit 2) constitute a negative feedback control system with f _r as the input and f _e as the output, as shown in Figure 4, and its control target is Δf=f _e -f _r →0, so it belongs to the following system.

The above signal modulation process v _s = _ie · _rs has constituted the phase sensitive detector PSD, and the PSD and the subsequent LPF constitute the lock-in amplifier LIA (based on the principle of cross-correlation detection), as shown in Fig. 5 . LIA can not only completely suppress co-channel interference v _d , but also effectively suppress random noise in the circuit. Moreover, different from the traditional technology, the third key improvement of the present invention is to use the principle of "multiplying the current _ie by the resistance _rs to obtain the voltage v _s " to realize the multiplication operation, that is, to use Ohm's law as a "virtual multiplier". Since no actual device (analog multiplier or analog switch) is needed, the circuit is simple, and there is no performance limitation (noise, offset, saturation, leakage current, etc.) It has a unique advantage when it comes to signals with a signal-to-noise ratio (<-100dB).

Compared with the prior art, the present invention has the advantages of adopting the AC reference signal, completely solving the detection problem of the weak vibration pickup signal under the background of strong excitation interference; using the excitation signal as the reference signal, effectively simplifying the circuit and avoiding the interference of the reference signal Adverse effects; using Ohm's law as a "virtual multiplier" to realize phase-sensitive detectors and lock-in amplifiers, the circuit is simple and the performance is superior.

Description of drawings

Fig. 1 is a block diagram of the present invention;

Fig. 2 is the schematic diagram of the frequency comparator of the present invention;

Fig. 3 is the output-input characteristic schematic diagram of the frequency comparator of the present invention;

Fig. 4 is the functional block diagram of negative feedback control system of the present invention;

Fig. 5 is the functional block diagram of lock-in amplifier of the present invention;

Fig. 6 is a structural schematic diagram of a typical sensitive structure supported by the present invention;

Fig. 7 is a functional block diagram of the signal source of the present invention;

FIG. 8 is a schematic diagram of a specific implementation of the present invention.

Detailed ways

As shown in Figure 1, the electromagnetic-piezoresistive micromechanical resonant beam sensor of the present invention has a single-beam resonator 1 fixed at both ends is placed in a magnetic field along its width direction, and its resonant frequency f _r is controlled by the measured physical quantity q ; The control circuit 2 is composed of a controller 3, a signal source 4 and a low-pass filter LPF5. The signal source 4 in the control circuit 2 outputs an AC excitation current i _e to the piezoresistive film 11 on the surface of the single beam resonator 1 and detects the two terminal voltage, i _e generates electromagnetic force to vibrate the single-beam resonator 1; the piezoresistive film 11 is sensitive to the first-order mode of the single-beam resonator 1, and generates a corresponding AC resistance component r _s ; multiplied by i _e and r _s to obtain the modulated Pick up the vibration signal v _s , get the DC signal V _S through the low-pass filter LPF5, adjust the i _e frequency f _e according to V _S , and make it continuously track f _r , so that the continuous measurement of q can be realized; the combination of multiplication and LPF simultaneously The cross-correlation detection is realized, which can effectively solve the detection of the weak vibration pickup signal _vs under the background of strong interference.

The present invention supports a typical sensitive structure as shown in FIG. 6 , which is a monocrystalline silicon overall structure processed based on SOI wafer and epitaxy technology. Both ends of the single-crystal silicon single-beam resonator 1 are fixedly supported on the elastic matrix 30 , and the measured physical quantity q controls the deformation of the elastic matrix 30 to control the resonant frequency of the single-beam 1 . The surface of the resonator 1 is covered with a complete doped epitaxial film 31 , and the middle section of the doped epitaxial film 31 is covered with a low-resistivity conductive layer 32 . Due to the piezoresistive effect of the doped semiconductor material and the bypass effect of the conductive layer 32, and the distribution of the alternating stress on the surface of the resonator 1, the total resistance between the electrodes 33 at both ends changes with the first-order modal vibration of the resonator 1 . The typical structural size data of the harmonic oscillator 1 is length×width×thick≈800×80×8 μm ³ .

But the present invention is not limited to this size, nor is it limited to this structure. Any piezoresistive element with a fixed-supported resonant beam structure at both ends placed in a magnetic field and a full-length piezoresistive element on the resonant beam (it can be an integrally processed structure, or a series network of separate wires and piezoresistive elements), piezoresistive elements There are only two lead-out electrodes at both ends, and a sensitive structure that performs electromagnetic excitation and piezoresistive vibration pickup through these two electrodes is the target structure supported by the present invention.

The control circuit 2 of the present invention includes three main parts: a controller 3, a signal source 4 and an LPF 5, as shown in FIG. 1 . The controller 3 implements the above-mentioned negative feedback control law, that is, judges the polarity of Δf according to V _S , and adjusts f _e in real time accordingly: if V _S >0, then increase f _e , if V _S <0, then decrease f _e , and so on Repeat until Δf→0, the signal source 4 generates the required excitation signal i _e ; the LPF5 cooperates with the "virtual multiplier" PSD to form the LIA.

Controller 3 can adopt pure analog technology or digital-analog hybrid technology. Considering that the control law of the present invention is not very complicated, a more reasonable implementation mode is to adopt the digital-analog hybrid technology combined with analog circuit and 8/16-bit MCU. Achieve a balance of performance, power consumption, size and cost.

As shown in FIG. 7 , the signal source 4 can use a voltage-controlled oscillator VCO or a direct digital synthesis DDS. The VCO or DDS generates an AC signal _ve of a corresponding frequency under the control of the control signal FC output by the controller 3 . If DDS is adopted, it means that part 3 of the controller must have a digital control device. In order to transform _ve into the excitation signal i _e in the form of current, a VI (voltage-current conversion) circuit is also needed, which forms an AC constant current source with VCO or DDS.

LPF5 adopts ordinary active low-pass filter circuit.

As shown in FIG. 8 , it is a schematic diagram of the specific implementation of the present invention. The function of this figure is not perfect, but it simply and clearly illustrates the working principle of the present invention. Controller 3 uses an integral link to eliminate static errors; since the integral link also has the function of low-pass filtering, the LPF is omitted, so only one integral link is used to realize the two links of LPF and controller; considering the excitation signal i _e current form, the non-inverting pre-amplifier is added to avoid reducing the equivalent output impedance of the signal source due to the input impedance of the amplifier; due to the use of VCO, the frequency control signal FC output by the controller 3 is embodied as the control voltage V _f ; Negative feedback, V _f should be negative polarity, that is, VCO output frequency decreases when V _f increases. The integration link can either use an analog integration circuit or a digital integration algorithm; if an analog integration circuit is used, the main body of the entire circuit can be completely realized by analog technology, and the circuit as a whole is simple and reliable; if a digital integration algorithm is used, it is easier to solve the integral problem due to the digital integration algorithm. The DC offset and drift problems of the calculation are easy to realize the limiting operation and can obtain better stability.

Claims (3)

1. Electromagnetic-piezoresistive micromechanical resonant beam sensor, characterized in that: the sensor is composed of a single-beam resonator (1) fixed at both ends and a control circuit (2), and a single-beam resonator (1) fixed at both ends Placed in the magnetic field along the width direction of the single-beam resonator (1), the resonant frequency f _r of the single-beam resonator (1) is controlled by the measured physical quantity q; the control circuit (2) is composed of a controller (3), a signal source ( 4) and low-pass filter LPF (5). The signal source (4) in the control circuit (2) outputs an AC excitation current i _e to the piezoresistive film (11) on the surface of the single beam resonator (1) and detects the voltage across the piezoresistive film (11), and i _e generates an electromagnetic The force makes the single-beam harmonic oscillator (1) vibrate; the piezoresistive film (11) is sensitive to the first-order mode of the single-beam harmonic oscillator (1), and generates a corresponding vibration pickup signal, that is, the AC resistance component r _s ; i _{e is} in phase with r _s Multiply the modulated vibration pickup signal v _s , and obtain the DC signal V _S through the low-pass filter LPF (5); the control circuit (2) uses the difference between the excitation frequency f _e and the resonant frequency f _r as Δf=f _e -f _r Control the object, and use the polarity of V _S as the criterion of Δf polarity, and use the negative feedback control law to adjust the excitation frequency, that is, the frequency f _e of i _e , so that f _e keeps tracking f _r , so as to realize the continuous measurement of q. 2. The electromagnetic-piezoresistive micromechanical resonant beam sensor according to claim 1, characterized in that: said negative feedback control law is: if V _S >0, then Δf<0, at this time f _e is increased until V _S →0; if V _S <0, then Δf>0, then reduce f _e until V _S →0; repeat this to keep V _S →0, so as to maintain Δf→0 or f _e →f _r , namely Keep the excitation frequency tracking the resonant frequency. 3. The electromagnetic-piezoresistive micromechanical resonant beam sensor according to claim 1, characterized in that: said signal source (4) is a voltage-controlled oscillator (VCO) or direct digital synthesis (DDS).

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