The principle of the Internet celebrity application is revealed: Analysis of the non-contact infrared body temperature measurement solution
Latest update time：2020-03-06
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At the critical moment of the COVID-19 epidemic, infrared thermometers, a non-contact, fast, and intuitive detection method, played a role that cannot be underestimated in the epidemic prevention process. In addition to fully automatic infrared body temperature detectors, related products such as forehead thermometers/ear thermometers have also seen a surge in demand. In addition, their application sites are more flexible and their prices are relatively low, making them "hard to find" in a short period of time. So do you understand the principle of this non-contact infrared temperature measurement? ADI China technical expert Jiang Zhongya explained this question to us in detail.
Some people on the Internet have questioned whether forehead thermometers have radiation and whether they are harmful to the human body. I can tell them seriously: there is radiation, but you are radiating, and it just absorbs the energy you radiate.
All forms of matter emit infrared radiation, called characteristic radiation, as long as the temperature is above absolute zero (-273.15°C). The cause of the radiation is the mechanical motion of the internal molecules. The intensity of this motion depends on the temperature of the object. Since the molecular motion represents the displacement of electric charges, this radiation is electromagnetic radiation (photon particles). These photons move at the speed of light and follow the known laws of optics. They can be deflected, focused with lenses or reflected by reflective surfaces. The spectrum of this radiation can range from 0.7um to 1000μm. Therefore, this radiation is usually not visible to the naked eye.
In 1879, Stephen and Boltzmann discovered that the total power radiated per unit area of a black body surface per unit time (called the radiance or energy flux density of the object) is proportional to the fourth power of the thermodynamic temperature T (also known as the absolute temperature) of the black body itself. This is called the Stephen-Boltzmann law. In 1893, Wien further revealed the law of black body thermal radiation, namely the black body radiation formula and Wien's displacement Law (for which he won the Nobel Prize): As the temperature rises, the maximum radiation of the object will move toward the short-wave direction. As can be seen from Figure 3, as the target temperature increases, the maximum radiation gradually moves to the shorter wavelength area. From the relationship between the radiation energy and the wavelength of the radiation light, the energy contained in the invisible part of the spectrum is up to 100,000 times that of the visible part. This is the theoretical basis of infrared measurement technology.
In theory, when measuring temperature using the blackbody radiation principle, the infrared thermometer is set up in the widest wavelength range possible to obtain the most energy (corresponding to the area under the curve) or the signal emitted by the target. However, in some cases, this is not always effective. For example, in the figure above, when the temperature is relatively high, the increase in radiation intensity at 2 µm is much greater than that at 10 µm, so the greater the radiation difference per unit temperature difference at 2 µm, the higher the measurement accuracy of the infrared thermometer. Similarly, in a low temperature environment, an infrared thermometer working at 2 µm will stop working when the temperature is below 600°C because the radiation energy is too little and almost nothing can be seen.
In practice, the measured object is also different from the black body model. The black body is an ideal model with no transmission and an emissivity of 1. The radiation emissivity of a gray body is less than 1. The emissivity of a non-gray body is not only less than 1, but also varies at different wavelengths.
Based on the above analysis, the wavelength range of sensors used to measure human body temperature is generally around 5µm-15µm.
The sensor used is a thermopile (thermocouple) made by using the thermoelectric effect (Seeback), that is, two different semiconductors or metal conductors are connected, and when the two materials are in a temperature difference, an electric potential difference will be generated. The infrared thermocouple irradiates the energy radiated by the object to be measured to the hot end of the thermocouple, measures the cold end temperature of the thermocouple through NTC, and then obtains the actual temperature of the object to be measured according to the Stephen-Boltzmann law.
The circuit in the upper part of the figure above uses the internal ADC of the MCU. At this time, an op amp with low temperature drift, low offset voltage, and low bias current is needed to condition the sensor signal. It is recommended to use
AD8538, AD8539, ADA4051, AD4528, AD8638, AD8628, AD8571, AD8551, AD8552, LTC2063, LTC2066,
etc.; the reference source should use the low temperature drift
ADR3530, ADR4530
.
The lower part of the above picture shows a highly integrated AFE.
The AD7191
has two ADC channels, an internal integrated PGA, a 24-bit high-precision ADC, and a precision current source for easy interface with the NTC resistor. The reference source recommended is
ADR3530 and ADR4530
. You can also choose AD7124-4, which has an internal integrated 10ppm/C reference source.
Vout = K*e*(Tobj^4 - Ts^4) + Voffset
1. Voffset is the voltage output by the thermopile when the target temperature is the same as the ambient temperature (actually, there is also the offset voltage error generated by the ADC and the operational amplifier in front of it). This value can be measured as follows:
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Place the entire circuit in the environment for a long enough time, and the thermopile inside the sensor will reach thermal equilibrium with the ambient temperature. At this time, the ADC value sampled by the microcontroller is Voffset.
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If the voltage-temperature transfer function of the selected thermopile sensor batch is very consistent, it can be assumed that the Voffset of this batch is the same; if the consistency is poor, then this test must be performed on each product during production to find the correct Voffset.
2. K is a constant, e is the radiation emissivity of the surface of the target being measured (the surface of the forehead of the human body can be considered as a gray body, e<1, and the specific value should be determined based on actual measurement experience). In actual operation, K*e can be treated as a constant. It is equivalent to the gain G. It can be measured using the following steps:
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The ambient temperature Ts is known, and the temperature gun is placed in the environment for a long enough time that the sensor and the environment reach thermal equilibrium.
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Test the target with known temperature (the bold figure in Figure 6 can do this), know Tobj, and read the ADC voltage Vout;
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K*e, i.e., gain G, can be calculated based on Voffset, Ts, Tobj, and Vout obtained in steps 1 and 2 above.
3. Through the above steps, we have learned the relationship between the measured target temperature Tobj and the sensor output voltage Vout, that is:
Tobj = (Vout/G+Ts^4)^(1/4)
In actual use, the MCU can calculate Tobj by reading Vout and Ts. The calculation of Ts is as follows:
Ts is the temperature of the cold end of the thermopile inside the sensor, which can be measured by the size of the NTC resistor inside the sensor. You can use the data provided by the sensor manufacturer for linear fitting, or put these data into the microcontroller and calculate it through methods such as lookup table interpolation;
4. It should be noted that in the above formulas, the unit conversion of each parameter.
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Tobj, Ts temperature unit is thermodynamic temperature, that is, Kelvin temperature. Its conversion relationship with the commonly used Celsius temperature is: T(K)=273.15+t(℃)
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The units of Vout and Voffset are very flexible, but it is important to use the same units. We can use the ADC reading directly.
5. Finally, if the model of Formula 1 we use cannot achieve the temperature accuracy, we need to fit a more appropriate model through experiments.
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