Design and application of the testing platform for

2022-07-24
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Design and application of micro mechanical inertial sensor detection platform

Abstract: a detection platform for the research and development of micro mechanical inertial sensor is introduced. We will provide free technical training for operators, technicians and equipment maintenance personnel. The detection principle of the signal, the overall structure of the system, the working principle of each component and the automatic detection method are introduced

key words: micro electro mechanical system (MEMS) micromachined gyroscope (MMG) detection

with the development of science and technology, many new scientific fields have emerged one after another, among which the micron/nano technology is an attractive frontier technology in many fields. Since the 1990s, following the successful application of micro/nano technology in large-scale integrated circuit manufacturing, various micro sensors and micro electro mechanical systems (MEMS) based on integrated circuit technology and micro machining technology have emerged, with an average annual growth rate of 30%. Micromechanical gyroscope is an important part of it. At present, all advanced industrial countries in the world attach great importance to the research and development of MMG, and have invested a lot of human and material resources. Low precision products have come out and are developing towards high precision

1 brief working principle of micromechanical vibratory gyroscope

see Figure 1 for the composition of the gyroscope system, which is composed of sensitive elements, driving circuit, detection circuit and force feedback circuit. AC voltage with DC bias but opposite phase is applied to the differential circuit of comb electrostatic driver. Due to the action of alternating electrostatic driving torque, the mass sheet generates simple harmonic angular vibration around the Z axis of the driving axis in the plane parallel to the substrate. When the spatial angular velocity Ω is input along the direction perpendicular to the detection axis (x direction) in the vibration plane, the detection mass sheet vibrates up and down around the detection axis (Y axis) under the action of Coriolis force. This kind of vibration amplitude is very small. It can be detected by the capacitor plate under the mass sheet and deposited on the substrate, and processed by the charge amplifier, phase sensitive detection circuit and demodulation circuit to obtain a voltage signal proportional to the spatial angular velocity

in the process of scientific research and processing, an important content is to detect the characteristics of the gyroscope, such as the resonant frequency, bandwidth gain, Q value, etc. in the working state. Therefore, the development task of the micromechanical inertial sensor detection platform is proposed. According to the working principle of the gyroscope, the whole instrument includes two parts: the driving signal generation part and the output signal detection part of the meter. The inertial sensor to be tested in the driving signal generation part gives appropriate driving signals to make the sensor in working state. In the signal detection part, it is required to detect the small capacitance change. After amplification and demodulation, the analog quantity is converted into digital quantity and collected in the PC, and the output signal is analyzed to determine the characteristics of the inertia meter

2 micro capacitance detection technology

in MMG detection technology, the input angular rate signal is obtained by using the vibration angular displacement of the capacitive sensor sensitive test mass under the Coriolis force. Due to the small size of the gyroscope, in order to obtain a medium accuracy of 10 °/h, the capacitance measurement resolution is required to reach (0.01 ×)~ (1 ×) Farah. Therefore, it is a key technology to detect the test mass and capacitance change between substrates for micromachined accelerometers and gyroscopes. At present, there are three micro capacitance detection schemes used in MMG: before switching capacitance, there are three micro capacitance detection schemes used in MMG: switching capacitance circuit, unit gain amplification circuit and charge amplification circuit

2.1 switched capacitor circuit

its basic principle is to use the charge and discharge of the capacitor to convert the unknown capacitance change into voltage output. The measuring circuit includes a charge amplifier, a sample and hold circuit and the timing of the control switch, as shown in Figure 2

during the measurement, charge the unknown capacitor (C1, C2) to the known voltage Vref, and then discharge it. In the process of charging and discharging, let's get to know what should be paid attention to when using the cement pressure tester and what are its characteristics? Controlled by a certain time sequence and repeated continuously, the unknown capacitor is always in a dynamic charging and discharging process. C1 and C2 discharge continuously, and the current pulse is converted into voltage through the charge amplifier. Then it passes through the sample holder to obtain the output VC. Transfer formula Δ When c=2c0 · x/d0 is substituted, the transfer function of the capacitance detection circuit can be obtained as follows:

vc/x=-[2vrefc0/cfd0]

2.2 unit gain amplifier circuit

adxl50 (5g micro mechanical accelerometer) jointly developed by ad company and rkeley adopts unit gain amplifier circuit

Figure 3 shows the equivalent circuit of the unity gain amplifier

in Figure 3, CP is the distributed capacitance, CGS is the input capacitance of the pre stage, and RGS is the input resistance. When the carrier frequency is within the pass band of the amplifier, the input resistance of the preamplifier can be ignored. As can be seen from Figure 3, the useful signal output of the pre stage is:

(vs-vo1 quarterly report display UT) J ω (C0+ Δ C)+(-Vs-Vout)j ω (C0- Δ C)

=Voutj ω (Cp+Cgs)+Vout/Rgs

∵ Rgs→∞

∴Vout=(2 Δ C/2c0+cp+cgs) vs

the distribution capacitance CP is about 10PF, and the input capacitance CGS is about 1 ~ 10PF, which is generally greater than the sensor nominal capacitance C0 (about 1pf). It can be seen that their existence greatly reduces the capacitance detection sensitivity. In order to improve the circuit sensitivity, it is necessary to eliminate the influence of CP and CGS. The usual measure is equipotential shielding

2.3 charge amplifier circuit

charge amplifier circuit is shown in Figure 4. It uses an inverse input operational amplifier with low input impedance. Where CP refers to the distributed capacitance, CF refers to the standard feedback capacitance, and RF is used to provide DC channel for the amplifier to keep the circuit working normally. RF shall be selected so that the time constant rfcf is much larger than the carrier period to avoid output waveform distortion. However, RF is too large, which will bring inconvenience to circuit integration in the future. A small resistance can be used to form a T-shaped complex instead of a large resistance

if the op amp has sufficient open-loop gain and the inverting input is a good virtual ground, the potential difference between the two input terminals is zero. Therefore, the distributed capacitance CP of the inverting input terminal to the ground and the input capacitance CGS of the amplifier will not affect the circuit measurement. Compared with the unit gain amplifier, the charge amplifier has a simple structure and does not need to consider the problem of equipotential shielding; Only the influence of stray capacitance can be transformed into distributed capacitance to the ground, that is, reasonable shielding to the ground can achieve better results

although the input capacitance and the distributed capacitance of the inverting input terminal to the ground can be ignored in the charge amplification circuit, the output still has a large attenuation when detecting small capacitance changes. This is caused by the distributed capacitance CIO at the input and output of the amplifier. When the carrier voltage frequency is greater than 1/(2 π rfcf) and less than the cut-off frequency of the amplifier, the output voltage Vout should be expressed as:

vout=-[(C1-C2)/(cio+cf)]vs=-[(2 Δ C)/cio+cf]vs

3 system composition and working principle of the detection platform

the working principle of the system is shown in Figure 5. After the appropriate excitation signal is applied to the inertial sensor, the moving plate of the sensor is in the vibration state, and the capacitance between the upper and lower plates changes periodically. The charge amplifier circuit is used to extract the signal. After AC amplification and demodulation, it is converted into a digital quantity through a/d conversion and collected in the microcomputer. The output response of the sensor is observed, which is the next step to analyze the time domain Frequency domain characteristics

3.1 excitation signal generator

according to the working principle of micromechanical wheel vibratory gyroscope, up to 4 excitation signals are required. The excitation signal is a sine wave, and the phases of each two channels are opposite. In order to measure the frequency characteristic of gyroscope, it is necessary to change the frequency of excitation signal. At present, the resonant frequency of gyroscopes of different designs ranges from hundreds of hertz to 10 kHz, and the excitation signal also needs to be adjusted within this range. In addition, the driving torque of the gyroscope is equal to the product of the AC component and the DC component of the driving signal, so a positive or negative DC bias must be applied to make the gyroscope work normally. See Table 1 for the combination of AC phase and DC offset. Table 1 combination of AC phase and DC bias

DC bias: ++ -- AC signal: +-+-

the frequency of sine wave generated by general RC oscillation circuit is adjusted by changing R and C values, which cannot be adjusted continuously in a large range. Therefore, analog waveform is synthesized by digital method in the design, and its principle is shown in Figure 6. 8254 in Figure 6 is a software programmable counter. It contains three independent 16 bit counters, with the highest counting frequency up to 8MHz. In the design, 3MHz clock is input, and the two counters are used in series, which can increase the frequency control range. The square wave signal generated by 8254 is input as the counting pulse of the subsequent parallel counter. The parallel counter consists of 2 pieces of 74ls161 to form an 8-bit binary cycle counter. When the 74ls161 counts to the maximum value, it will automatically clear to start counting again, and its output can be used as the address signal of E2PROM 2817a (that is, 256 sampling points in each sine cycle). The data reading time of 2817a is 150ns. When designing the circuit, the chip selection and read signal are set to be effective to improve the data reading speed. Dac-08 current output d/a converter is used for d/a conversion. The circuit output time is 85ns. The amplifier adopts high-speed and high-precision op-37. Similarly, the chip selection and conversion start signal of d/a converter are always effective. Its output changes with the input to improve the conversion speed. The experimental results show that the signal generator can generate frequency adjustable sine wave within 10kHz. Moreover, using the programmable counter 8254, the frequency of the output sine wave can be adjusted by software. If you want to output non sinusoidal waveforms, you can output periodic waveforms of any shape as long as you modify the data of E2PROM

3.2 low pass tracking filter

digital signal generator has the advantage of flexible control, but the output signal is not smooth enough, in which there will be step waves. In the case of high signal requirements, filtering is also required. The frequency range of the signal in this design is very large: hundreds of hertz to 10 kHz. In order to further improve the signal quality, AD633 analog multiplier is used to form a low-pass tracking filter. Its principle is shown in Figure 7

the cut-off frequency of the passband is controlled by the voltage EC, and the output is output. The cut-off frequency:

fc=ec/[(20V) π rc]

output is the direct output end of the multiplier. The cut-off frequency is the same as that of the RC filter:

f1=1/(2 π RC)

this filter has simple structure, no switching capacitor and low noise. Generally, the digital to analog converter is used to control EC, and it is easy to control the passband frequency

3.3 AC amplifier

the micromechanical inertial sensor is in vibration state after the excitation signal is applied. Sensor has differential micro electric capacity change C0+ Δ C and C0- Δ C。 Extracted by charge amplifier circuit Δ C. This voltage signal is still very elastic and needs further amplification. Therefore, the AC amplifier shown in Figure 8 is used

the AC amplifier is composed of four Operational Amplifiers with amplification factors of -1, -2, -5, -10, which further amplifies the measured signal and adjusts the amplitude to adapt to the input of the demodulator. Adg211 analog switch is selected as the switch in Figure 8. By controlling the opening and closing of the analog switch, a certain stage or several stages of amplifiers can be selected to work, so as to realize the integral multiple adjustment of plus or minus 1, 2, 5, 10, 20, 50 and 100 of the amplification factor. For example, analog switches S0, S2, S8

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