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-piezo-resistance type micro mechanical resonant beam sensor

Technical field

The present invention relates to a kind of input and closed-loop system principle of the sensor based on micromechanics single-beam harmonic oscillator.

Background technology

Resonance beam is one of fundamental measurement parts in the MEMS device, measurand q is converted into the natural frequency f of resonance beam _nAnd measurement f _nCan obtain q.In order to measure f _n, resonance beam must have vibrational excitation (exciting) and vibration pickup (pick-up) function, and constitutes closed-loop system with control circuit, makes resonance beam be in resonant condition, because resonance frequency f _rWith f _nCan be considered equal, measure f _rCan obtain f _nTherefore, having the resonance beam of exciting and pick-up function and control circuit is two key elements that realize resonance beam type measuring principle.

For the scheme that adopts the single-beam harmonic oscillator, for avoiding on beam, processing vibrator or vibro-pickup adverse effect in addition to the beam mechanical property, can adopt electromagnet-piezo-resistance formula structure, promptly utilize the pressure drag film of beam surface total length to realize electromagnetic exciting and pressure drag pick-up simultaneously: on the one hand it to be used as lead, by ac-excited current i _e(t)=I _eCos2 π f _eT also is placed in the magnetic field, utilizes distributed alternating electromagnetic force (Amp) ψ that is produced _e(x, t)=Ψ _e(x) cos2 π f _eT encourages vibration of beam (coordinate on the x fingerboard length direction); On the other hand it is used as pressure drag component, utilizes its resistance under the alterante stress field action on beam surface, to change and the AC resistance component that produces Pick up vibration of beam.For r _sDetection, be by normal value reference current I according to a conventional method in the pressure drag film _REF, with r _sBe transformed to and exchange pick-up voltage v _s=I _REFR _s

But ac-excited current i _eNormal value resistive component R at the pressure drag film _SThe very strong undesired signal v of last generation _e=i _eR _S, the vibration of beam in magnetic field produces induction electromotive force e simultaneously at two ends _iv _e, e _iAnd v _sFrequency is f _e, v _d=v _e+ e _iBe called co-channel interference, its amplitude is generally more than the 100mV, and v _sBe μ V magnitude, obviously v _sWill be by v _dFlood and be difficult to and detect, must address this problem and make electromagnet-piezo-resistance formula structure really obtain practicality.

Summary of the invention

The technical problem to be solved in the present invention: the AC resistance component r that solves pressure drag film under the above-mentioned soaking disturbed condition _sThe detection problem, provide a kind of and can effectively solve the electromagnetic-piezo-resistance type micro mechanical resonant beam sensor of the detection of faint pick-up signal under the strong jamming background.

Technical solution of the present invention: electromagnetic-piezo-resistance type micro mechanical resonant beam sensor, its characteristics are: two fixed ends single-beam harmonic oscillator is placed in along in the magnetic field of its Width, its resonance frequency f _rControlled by measurand q; Control circuit by controller, signal source and low-pass filter LPF form, signal source in the control circuit is to the pressure drag film output AC exciting current i on single-beam harmonic oscillator surface _eAnd detect its both end voltage, i _eProduce electromagnetic force and make the vibration of single-beam harmonic oscillator; The pressure drag film produces respective quadrature leakage resistance component r to single-beam harmonic oscillator single order mode sensitivity _si _eWith r _sThe pick-up signal V that multiplies each other and obtain modulating _s, obtain direct current signal V through low-pass filter LPF _S, according to V _SAdjust i _eFrequency f _e, make it to continue to follow the tracks of f _r, can realize the continuous coverage of q.Above-mentioned signal modulated process v _s=i _eR _sConstituted phase sensitive detector PSD, realized cross-correlation detection, can effectively solve faint pick-up signal V under the strong jamming background and PSD and follow-up LPF promptly constitute lock-in amplifier LIA (based on the cross-correlation detection principle) _sDetection.

Principle of the present invention: for the cycle AC signal, the most basic feature is a frequency, if useful signal is identical with noise frequency, promptly can't distinguish on principle.Therefore must manage to make v _sAnd v _dHas different frequency.It is f that one of key improvements of the present invention promptly is to adopt frequency _RefInterchange reference current i _RefBut not direct current reference current I _REFThereby, realize the signal modulation, obtain the non-f of frequency _eV _s=i _RefR _s: according to the signal modulation principle, product i _RefR _sBe two side frequency f _Ref-f _eAnd f _Ref+ f _eSum.

Yet this method needs extra i _RefSignal source, circuit are complicated; Need be with i _eAnd i _RefBe applied to simultaneously on the pressure drag film 11, increased the resistance heat effect; And i _RefElectric magnetization power can be produced equally, the vibration of beam state may be disturbed.Two of key improvements of the present invention promptly is i _eAnd i _RefIntegrate, with i _e=I _eCos2 π f _eT is used as reference signal simultaneously.Because f _Ref=f _e, this moment, two side frequencys were respectively 0Hz (direct current) and 2f _e(two frequencys multiplication).

When harmonic oscillator is in resonant condition is Δ f=f _e-f _r=0 o'clock, not only amplitude reached maximum, also has exciting force ψ according to the vibration principle _eWith its instantaneous velocity Homophase; According to piezoresistive effect principle, r _sWith immediate movement z homophase; And i _eWith ψ _eHomophase, Leading z90 °, so i _eLeading r _s90 °, promptly

Therefore can get v _s=i _eR _s=0.5I _eR _sSin2 π (2fe) t, i.e. v _sOnly comprise 2f _eSide frequency, DC component are V _s=0.Work as f _rWhen making harmonic oscillator 1 temporary transient disengaging resonant condition (Δ f ≠ 0), because harmonic oscillator 1 and pressure drag film 11 are linear system, so r with the q variation _sFrequency still is f _e: But r _sRelative i _ePhase place No longer be

By harmonic oscillator 1 transport function

$G\left(s\right)=\frac{A{\mathrm{\&omega;}}_n^2}{{\mathrm{<figure-callout\; id="2"\; label="s"\; filenames="C200610114276E00091.png"\; state="\{\{state\}\}">s</figure-callout>}}^2+2{\mathrm{\&xi;\&omega;}}_{n}s+{\mathrm{\&omega;}}_{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="C200610114276E00091.png"\; state="\{\{state\}\}">n</figure-callout>}}^2}$

As can be known:

Draw easily by the signal modulation principle: V _S0, Obviously:

Therefore can be with transport function G (s), signal modulation operation i _RefR _sThe integral body that constitutes with LPF is considered as with f _rFor positive input terminal, with f _cFor negative input end, with V _SBe " frequency comparator " of output terminal, as shown in Figure 2.Its output-input characteristics is non-linear, as shown in Figure 3.

Control circuit 2 adopts following negative feedback control law to keep the resonant condition of harmonic oscillator 1: according to V _SJudge Δ f polarity, and adjust f according to this in real time _e: if V _S0 item improve f _e, if V _S＜0 is reduced f _e, so repeat, until Δ f → 0.

So frequency comparator and above-mentioned negative feedback control law (being harmonic oscillator 1 and control circuit 2) have constituted with f _rFor input, with f _eBe the negative feed back control system of output, as shown in Figure 4, its controlled target is Δ f=f _e-f _r→ 0, therefore belong to system for tracking.

Above-mentioned signal modulated process v _s=i _eR _sConstituted phase sensitive detector PSD, and PSD and follow-up LPF promptly constitute lock-in amplifier LIA (based on the cross-correlation detection principle), as Fig. 5.LIA not only can thoroughly suppress co-channel interference v _d, and can effectively suppress random noise in the circuit.And different with conventional art is that three of key improvements of the present invention is to utilize " current i _eMultiply by resistance r _sObtain voltage v _s" principle realize multiplying, be " virtual multiplier " promptly with Ohm law.Owing to need not practical devices (analog multiplier or analog switch), thereby circuit is simple, and the performance limitations (noise, imbalance, saturated, leakage current etc.) that does not have practical devices, handle atomic weak (100nV magnitude) and utmost point low signal-to-noise ratio (＜-have original advantage during 100dB) signal.

The present invention's advantage compared with prior art: adopt the reference signal that exchanges, thoroughly solved faint pick-up problem of signal detection under the soaking jamming pattern; With pumping signal double as reference signal, effectively simplified circuit and avoided the adverse effect of reference signal; With Ohm law is that " virtual multiplier " realizes phase sensitive detector and lock-in amplifier, and circuit is simple, superior performance.

Description of drawings

Fig. 1 is a composition frame chart of the present invention;

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

Fig. 3 is the output-input characteristics synoptic diagram of frequency comparator of the present invention;

Fig. 4 is the theory diagram of negative feed back control system of the present invention;

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

Fig. 6 is the structural representation of the typical sensitive structure that the present invention supported;

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

Fig. 8 is the schematic diagram of concrete enforcement of the present invention.

Embodiment

As shown in Figure 1, the two fixed ends single-beam harmonic oscillator 1 of electromagnetic-piezo-resistance type micro mechanical resonant beam sensor of the present invention is placed in along in the magnetic field of its Width, its resonance frequency f _rControlled by measurand q; Control circuit 2 by controller 3, signal source 4 and low-pass filter LPF5 form, signal source 4 in the control circuit 2 is to the pressure drag film 11 output AC exciting current i on single-beam harmonic oscillator 1 surface _eAnd detect its both end voltage, i _eProduce electromagnetic force and make 1 vibration of single-beam harmonic oscillator; 11 pairs of single-beam harmonic oscillators of pressure drag film, 1 single order mode sensitivity produces respective quadrature leakage resistance component r _si _eWith r _sThe pick-up signal v that multiplies each other and obtain modulating _s, obtain direct current signal V through low-pass filter LPF5 _S, according to V _SAdjust i _eFrequency f _e, make it to continue to follow the tracks of f _r, can realize the continuous coverage of q; Multiply each other and the combination of LPF has realized cross-correlation detection simultaneously, can effectively solve faint pick-up signal v under the strong jamming background _sDetection.

Support of the present invention typical sensitive structure as shown in Figure 6, the monocrystalline silicon one-piece construction of this structure for processing based on SOI wafer and epitaxy technology.Monocrystalline silicon single-beam harmonic oscillator 1 two fixed ends is on elastic matrix 30, and measurand q controls the resonance frequency of elastic matrix 30 deformation and then control single-beam 1.Harmonic oscillator 1 surface coverage has complete doped epitaxial film 31, and the stage casing surface coverage of doped epitaxial film 31 has the conductive layer 32 of low-resistivity.Because the piezoresistive effect of doped semiconductor materials and the bypass effect of conductive layer 32, and the regularity of distribution of harmonic oscillator 1 surperficial alterante stress, the all-in resistance between the electrode 33 at two ends changes with harmonic oscillator 1 single order modal vibration.The typical structure dimensional data of harmonic oscillator 1 is length * wide * thick ≈ 800 * 80 * 8 μ m ³

But the invention is not restricted to this size, also be not limited to this structure.Anyly place the pressure drag component that has total length on magnetic field, the resonance beam (to can be the structure of whole processing the two fixed ends resonant beam structure, also can be the independent lead and the series network of pressure drag component), only there are two extraction electrodes at the pressure drag component two ends, and, be the object construction that the present invention supports by these two sensitive structures that electrode carries out electromagnetic exciting and pressure drag pick-up.

Control circuit 2 of the present invention comprises three major parts: controller 3, signal source 4 and LPF5, as shown in Figure 1.Controller 3 is realized above-mentioned negative feedback control law, promptly according to V _SJudge Δ f polarity, and adjust f according to this in real time _e: if V _S0, then improve f _e, if V _S＜0, then reduce f _e, so repeat, until Δ f → 0, signal source 4 produces required accumulation signal i _eLPF5 cooperates formation LIA with " virtual multiplier " PSD.

Controller 3 can adopt pure analogue technique or digital-to-analogue hybrid technology; consider that control law of the present invention is very not complicated; reasonably embodiment is for adopting the digital-to-analogue hybrid technology of mimic channel and 8/16-bit MCU combination, to realize the balance of performance, power consumption, volume and cost.

As shown in Figure 7, signal source 4 can adopt voltage controlled oscillator VCO or the synthetic DDS of Direct Digital.VCO or DDS produce the AC signal v of corresponding frequencies under the control signal FC control of controller 3 outputs _eIf adopt DDS, mean that then controller 3 parts must have digital control device.For with v _eBe transformed to the accumulation signal i of current forms _e, also need V-I (voltage-to-current conversion) circuit, constitute AC constant-current source with VCO or DDS.

LPF5 adopts common active low-pass filter circuit to get final product.

As shown in Figure 8, be concrete enforcement schematic diagram of the present invention, this figure function and imperfection, but principle of work of the present invention has been described simply.Controller 3 has adopted integral element, to eliminate static error; Because integral element also has the low-pass filtering effect simultaneously, has omitted LPF, so only promptly realized LPF and two links of controller with an integral element; Consider accumulation signal i _eCurrent forms has increased the homophase pre-amplifier, to avoid reducing the signal source equivalent output impedance because of amplifier input impedance; Owing to adopted VCO, the frequency control signal FC of controller 3 outputs is embodied as control voltage V _fFor constituting negative feedback, V _fShould be negative polarity, i.e. V _fThe VCO output frequency reduces during raising.This integral element both can adopt analog integrator circuit, also can adopt the digital integration algorithm; Adopt analog integrator circuit, the main body of entire circuit can realize that with analogue technique the circuit overall simple is reliable fully; Adopt the digital integration algorithm, the easier solution integral operation of factor word integral algorithm DC maladjustment and drifting problem, be easy to realize the amplitude limit computing, can obtain stability preferably.

Claims (3)

1, electromagnetic-piezo-resistance type micro mechanical resonant beam sensor, it is characterized in that: described sensor is made up of two fixed ends single-beam harmonic oscillator (1) and control circuit (2), two fixed ends single-beam harmonic oscillator (1) is placed in along in the magnetic field of single-beam harmonic oscillator (1) Width, single-beam harmonic oscillator (1) resonance frequency f _rControlled by measurand q; Control circuit (2) is made up of controller (3), signal source (4) and low-pass filter LPF (5).Signal source (4) in the control circuit (2) is to pressure drag film (11) the output AC exciting current i on single-beam harmonic oscillator (1) surface _eAnd detection pressure drag film (11) both end voltage, i _eProduce electromagnetic force and make single-beam harmonic oscillator (1) vibration; Pressure drag film (11) produces corresponding pick-up signal, i.e. AC resistance component r to single-beam harmonic oscillator (1) single order mode sensitivity _si _eWith r _sThe pick-up signal v that multiplies each other and obtain modulating _s, obtain direct current signal V through low-pass filter LPF (5) _SControl circuit (2) is with excitation frequency f _eWith resonance frequency f _rDifference Δ f=f _e-f _rBe controlling object, and with V _SPolarity is as the criterion of Δ f polarity, and employing negative feedback control law is adjusted excitation frequency, i.e. i _eFrequency f _e, make f _eContinue to follow the tracks of f _r, to realize the continuous coverage of q.

2, electromagnetic-piezo-resistance type micro mechanical resonant beam sensor according to claim 1 is characterized in that: described negative feedback control law is: if V _S0, then this moment is improved f in Δ f＜0 _e, until V _S→ 0; If V _S＜0, Δ f then〉0, reduce f this moment _e, until V _S→ 0; So repeat to keep V _S→ 0, thus Δ f → 0 or f kept _e→ f _r, promptly keep excitation frequency to follow the tracks of resonance frequency.

3, electromagnetic-piezo-resistance type micro mechanical resonant beam sensor according to claim 1 is characterized in that: described signal source (4) adopts voltage controlled oscillator VCO or the synthetic DDS of Direct Digital.

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