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Digital thermometer based on quartz crystal temperature sensor

temperature is one of the basic physical quantities. In many occasions, it is required to measure the temperature quickly and accurately, and in some occasions, it is even necessary to measure the temperature value. For these, the traditional methods are not competent

for some cut piezoelectric quartz crystals [1] [5], using its temperature frequency characteristics to make a sensor, the change of temperature can be converted into frequency modulation, and the filter can be removed to make the signal; In the process of transmission and reception, the signal has good anti-interference ability, can realize telemetry and remote control, is easy to measure with digital instruments and 39 specific material test methods, and is easy to connect with single chip microcomputer and computer. Therefore, it can be made into a digital thermometer that meets the requirements. According to different frequencies and cut patterns, the temperature sensitivity of quartz crystal temperature sensor can vary from 20Hz/℃ to 2850hz/℃, so that the temperature resolution can reach 1 × 10-4 ℃, and the temperature drift and time drift are very small

1 temperature measurement mechanism

the speed ratio of piezoelectric stone can reach 1million times. When the quartz crystal is excited by the oscillation circuit, it will produce various forms of mechanical vibration. Thickness shear vibration of the wafer. According to Moco, an analyst of lithium industry, Dang [2] is shown in Figure 1. Its oscillation frequency is:

Where, N: overtone number, H: wafer thickness, V: shear wave velocity, ρ： Quartz crystal density, μ： Elastic coefficient of quartz crystal; Where h, ρ，μ All are functions of temperature, so the frequency f becomes the implicit function f (T) of temperature T

experiments show that for any kind of quartz resonator, its temperature frequency characteristic f (T) is a quadratic or cubic parabola or straight line; Within - 200 ～ + 200 ℃, with sufficient accuracy, the frequency characteristics of quartz crystal oscillator can be expressed by cubic polynomial:

this is the temperature sensitive characteristics of quartz crystal, where F0 is the frequency at any reference temperature t0 ℃; a. B and C are the first, second and third-order frequency temperature coefficients respectively, which are related to the cutting type and vibration mode of the wafer. As a temperature sensor, in order to ensure high sensitivity and good linearity of temperature frequency characteristics, the value of a should be large and far greater than the values of B and C (preferably B = C = 0), but in the actual production of wafers, the two cannot be combined. At present, for rotary Y-cut YXL + 5 °, there is a = 95.6 × 10-6/(F = 10MHz), with a resolution of a few tenths of a degree; Ys cutting type: a = 80 × 10-6/℃ (F = 5MHz, n = 3), with a resolution of up to 4 × 10－6℃； NL tangent linearity is very good, and its coefficient a = 63.5 × 10－6／℃，b＝－18．1 × 10－9／℃，c＝－35．9 × 10－11／℃。 The main cutting patterns, temperature coefficients and temperature frequency nonlinearity of common quartz temperature sensors are shown in Table 1 [3]

for quartz crystal with good linearity, the measured temperature is easy to calculate, so I won't repeat it; If C is ignored, the temperature T [4] can be obtained from formula (3):

obviously, the measured temperature can be calculated after ft is measured

in order not to lose generality, the simplified Newton iteration method can be used to solve the cubic function [3] of formula (3), and take t = t0 for the first iteration:

this not only reduces the nonlinear error, but also facilitates the operation of single chip microcomputer

2 temperature measurement principle of single chip microcomputer

because the oscillation frequency f of quartz sensor crystal oscillator used for temperature measurement is high (more than a few MHz), it cannot be measured directly by single chip microcomputer, but after analysis, it is found that the difference between it and the reference frequency can be measured with the help of D trigger Δ f. So as to realize temperature measurement. The temperature measurement circuit is shown in Figure 2. The oscillator composed of temperature sensing crystal oscillator, whose output frequency signal f is shaped and sent to D trigger

take the clock crystal oscillator signal FCP (FCP is slightly lower than F0) of the single chip microcomputer as the reference frequency and as the CP signal of the D trigger, and its waveform is shown in Figure 3. set up Δ T = tcp-t, initially, the rising edge of CP is aligned with the falling edge of the sensing square wave f input at the d end. At this moment, the Q end jumps from high level to low level, and after K CP cycles, when k Δ When t = t/2, the Q terminal becomes high level. The number of CP cycles K can be converted from the count value (i.e. the number of machine cycles) n of the MCU timer: k = - 12n. So, yes Δ T = t/(24N), and Δ f＝fCP－f＝ Δ TfCP／T＝fCP／（24n）。 Here, FCP and F are not at the same crystal oscillator frequency. When at the initial temperature T0, the count value of the MCU timer is N0 (do not overflow)

when the temperature of the sensing crystal oscillator is t, the change of its frequency should be:

F1 is calculated according to formula (6), taking Δ f1＝ Δ F-f1, after substituting into the above formula for the second iteration, the more accurate measured temperature T can be calculated

the framework flow chart of programming is shown in Figure 4

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