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The working principle of quartz crystal sensor

The principle is the piezoelectric effect of quartz crystal. When the quartz crystal is under pressure, the positive and negative charges will be gathered like two opposite surfaces. The quartz crystal is a capacitor, which gathers positive charges on one side and negative charges on the other. This amount of charge It is proportional to all pressures. V=Q/C, which is equivalent to generating a voltage.
1. Quartz crystal temperature-frequency sensor The early quartz crystal temperature-frequency sensor was made of a quartz crystal resonator with nonlinear temperature-frequency characteristics. After the discovery of a quartz crystal with linear temperature-frequency characteristics, different manufacturers Quartz crystals of various shapes and sizes are provided, and their performance indicators are also different. These indicators include resonance frequency, resonance mode, load capacitance, series impedance, case capacitance, and drive level.
It is equivalent to be used as a frequency stabilizing component. The quartz crystal equivalent circuit is a circuit in which a capacitor is connected in parallel with an inductor, a capacitor, and a resistor (three in series).
Quartz crystals are generally equivalent to inductance in the circuit and participate in oscillation, and the other is equivalent to a frequency selective loop.
The reactance characteristic curve of a quartz crystal resonator is shown in the figure below. It has two resonant frequencies fs and fp, fs is the series harmonic frequency, and fp is the parallel harmonic frequency. Between the frequency fs and fp, the quartz resonator is inductive , And it is capacitive in other frequency ranges. Because the difference between fs and fp is very small, the reactance curve between fs and fp is very steep. Because quartz crystals only exhibit inductive reactance in the narrow frequency range of fs and fp, they have very A large slope of change, that is, the quartz crystal has a high quality factor when used as an inductive component, which can play a strong frequency stabilization effect in the oscillation circuit. If the quartz crystal is used as a capacitive component, the reactance curve changes slowly , It is not conducive to frequency stabilization, and once the quartz crystal does not work, it still appears as a capacitor C0, so it is impossible to judge whether the quartz resonator is working in the oscillation circuit. Therefore, the quartz crystal cannot be used as a capacitive element.
According to the piezoelectric effect of quartz crystal, the piezoelectric inverse effect and the principle that the change of certain physical quantities and chemical quantities will cause its frequency and Q value (or equivalent resistance) to change, the quartz sensor is made with high precision, It has unique advantages such as good sensitivity, wide measurement range, rapid response, and digital output. Since the crystal is a frequency control component, it can be digitalized (output in frequency). When the absolute frequency deviation is linear with the measured content, its digital processing is simple and convenient, and the output digital quantity is stable and reliable, easy to interact with The computer interface is conducive to the digitization of the secondary instrument. Compared with analog quantity, digital quantity has strong anti-interference and is suitable for long-distance transmission, eliminating the complicated link of analog-digital conversion and the error caused by it. Because the quartz crystal also has the excellent characteristics of short stable frequency and long stable frequency, the resolution of the sensor can be improved by several orders of magnitude, reducing the number of calibrations of the sensor.
Quartz crystal temperature-frequency sensor  The early quartz crystal temperature-frequency sensor was made of a quartz crystal resonator with nonlinear temperature-frequency characteristics. After discovering a quartz crystal cut with linear temperature-frequency characteristics, the resonator of this temperature sensor is made of an LC-cut plano-convex quartz crystal block with a diameter of several millimeters and a convex curvature radius of more than 100 millimeters. The resonator is encapsulated in a helium-filled tube. In the sensor circuit (Figure 1), it uses its piezoelectric effect and the characteristic that the natural vibration frequency changes with temperature to form a thermal oscillator. Its basic resonant frequency is 28 MHz. There is another reference oscillator with an oscillation frequency of 2.8 MHz in the circuit, which outputs a reference frequency of 28 MHz after passing the tenth frequency. The outputs of the two oscillators are added to the mixer through the gate circuit to obtain the difference frequency output signal, which is the difference between the measured temperature and the reference temperature (ie the temperature of the reference oscillator) and 1000 Hz/℃ (temperature coefficient) Product, so the difference frequency output signal records the change of the measured temperature. Different time control signals are generated by the time selection switch as strobe pulses to obtain different resolutions. The linear quartz crystal-frequency sensor can be used in various high-precision temperature measurement occasions where the flow speed of the thermal process is not high and the interval is long, as well as multi-channel remote control systems, underwater detection, etc. It can also be used to make high-resolution direct reading Type digital automatic thermometer.

 

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