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Modeling and elimination of CO frequency interference of micro resonant sensor

silicon micro resonant sensor is a new sensor developed based on MEMS (micro mechanical electronic system) technology. It has the advantages of small volume, light weight, low power consumption, high measurement accuracy, taking frequency as output signal, easy computer connection and so on. It has become an important development direction of micro sensors. Micro resonant sensors often adopt closed-loop self-excited oscillation, which can automatically track the real-time changes of measured values, so they are widely used. However, in all kinds of silicon micro resonant sensors, when the test piece is vertically inserted between the cylinder and the pressure roller shaft (the coating faces the zigzag column), there is the problem of the same frequency signal interference, which brings errors to the measurement. If the interference signal is too large, the Q value of the device will significantly decline, increasing the difficulty of closed-loop detection. Therefore, the elimination of CO frequency interference has become an important problem of resonant silicon micro sensors

taking the resonant silicon micro acceleration sensor with electrostatic excitation/capacitance pickup as an example, the author analyzes the causes of the same frequency interference and puts forward a new solution

1. Analysis and modeling of CO frequency interference

the sensitive structure size of the resonant beam of the electrostatic excitation/capacitance pickup silicon micro resonant sensor is very small, and there is a strong co frequency interference signal. The same frequency interference mainly has the following three coupling ways (as shown in Figure 1):

① inter plate coupling capacitance. Since the exciting plate is close to the pickup plate, the AC voltage signal of the exciting plate can pass through the coupling capacitor coh1. It acts on the pick-up plate to form coupling interference

② coupling capacitance between leads. There is a coupling capacitance (coh2) between the lead of the excitation plate and the lead of the pickup plate. The main reason is that the lead of the polar plate is long, which can generally reach a few millimeters, so the excitation signal is coupled to the lead of the polar plate through coh2, forming the same frequency interference

③ induced electromotive force. Since the vibration pickup circuit forms a closed loop, the excited alternating signal will generate the induced electromotive force VN in the vibration pickup circuit, which is also a same frequency interference source

Figure 1 same frequency interference model of silicon micro resonant sensor

here we can get the equivalent model of the same frequency interference circuit, as shown in Figure 2, where C is the vibration pickup capacitance, and the output interference voltage uo (I) at both ends of resistance R (sampling resistance) is

considering the vibration pickup capacitance coh1 and coupling capacitance coh1 of the resonant sensor. The values of coh2 and coh2 are very small (only a few PF). Equation (1) can be simplified as

Figure 2 circuit equivalent model of CO frequency interference coupling

2, elimination of CO frequency interference

from the above analysis, it can be seen that CO frequency interference is mainly composed of coupling capacitance and induced electromotive force. Therefore, a twice differential method is proposed to eliminate co frequency interference, and the interface circuit of double ended differential excitation/double ended differential detection shown in Figure 3 is established

Fig. 3 Schematic diagram of dual end differential excitation/dual end differential detection

this is a symmetrical structure of dual end excitation/dual end detection. The resonant beam has four symmetrical excitation plates and two pickup Jinan assay material experimental machine. The main purpose is to slowly apply a load on the two ends of the sample, realize the first differential operation through the phase difference of the excitation signal added by the excitation plates, and carry out the second differential operation through the dual end pickup differential detection vibration, Through this symmetrical design, the same frequency interference can be effectively eliminated

its circuit equivalent model is shown in Figure 4, and the specific analysis is as follows

(1) double ended differential excitation

here, only one side of the differential excitation is analyzed. There are two excitation plates on one side of the resonant beam, and the phase difference of the excitation signals on these two plates is 180., Under the premise of ignoring VN [vn will be eliminated at the second difference], the output interference signal on the sampling resistance R is

due to the symmetry of the structure, coh1 ≈ c'oh1; Coh2 ≈ c'oh2, uo (T) ≈ 0, it can be seen that the double ended differential excitation can effectively eliminate the same frequency interference caused by the coupling capacitance in the micro sensor

Figure 4 equivalent circuit diagram of coupling interference of double ended differential excitation/double ended differential detection

(2) double ended differential detection

this structure adopts the method of double ended vibration pickup to carry out double ended differential detection on the output signal, so as to amplify the useful signal and eliminate the same frequency interference at the same time. As shown in Figure 5, the induced electromotive forces vn1 and vn2 in the two detection circuits composed of the upper and lower pick-up plates are approximately equal (structural symmetry), which can be eliminated by differential operation. However, differential detection can not only eliminate the same frequency interference, but also effectively eliminate the noise in the measurement process, and then significantly improve the signal-to-noise ratio

Figure 5 equivalent circuit diagram of double ended differential detection

in conclusion, the use of double ended differential excitation can effectively eliminate the interference caused by the electrostatic coupling of the electrode plate and the lead, and the use of double ended differential detection can eliminate the induced electromotive force formed by the excitation signal in the vibration pickup circuit, so as to effectively eliminate the same frequency interference

this design can not only effectively eliminate the same frequency interference, but also has the following advantages compared with the conventional design:

① compared with the silicon micro resonant sensor with single end excitation and single end detection, this structure can effectively eliminate the DC component and double frequency component in the electrostatic force, which facilitates the conduct of batch experiments; Interference of. Under the action of electrostatic force, the force on the resonant beam is

Where s is the positive area of the capacitor plate; X is the distance between capacitor plates; U is the pressure at both ends of the plate, then the gain is not worth the loss; ε Account for the dielectric constant in vacuum. When double ended excitation is used, the force on the resonant beam is

. It can be seen that the influence of DC component and double frequency component can be eliminated by using the structure of double ended excitation. The existence of DC component will reduce the stability of the resonant beam, and the existence of double frequency component will bring double frequency interference signal

② compared with the silicon micro resonant sensor with single beam structure with double end excitation and double end detection, this structure can significantly increase the area of the vibration pickup capacitance by spending teeth on the resonant beam, thus providing convenience for the detection of weak capacitance

3. Conclusion

the interference of silicon micro resonant sensor is mainly the same frequency interference. The existence of this interference not only affects the accuracy of measurement, but also leads to the failure of closed loop, so that the sensor can not work. Based on the analysis of various coupling ways of the same frequency interference, the author puts forward the equivalent model of the same frequency interference and the method of double ended differential excitation/double ended differential detection, and eliminates the same frequency interference through two differences, It lays a foundation for the successful closed-loop of silicon micro resonant sensor and the acquisition of high Q value. (end)

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