With the continuous improvement of living standards, people gradually began to pay attention to their physical health. A large number of health medical equipment has appeared in recent years. Such as the recent popular non-contact infrared temperature measurement, is used in a large number of temperature guns, ear thermometers, mobile phones, smart charging treasures...
How to accurately measure the temperature of the measured object, here is a high-precision non-contact infrared temperature sensor MLX90614 ESF-DCC, how to design a high-performance infrared temperature sensing product.
First, the schematic design:
MLX90614 ESF-DCC and MCU connection diagram is very simple, as shown in Figure 1, MLX90614 ESF-DCC uses IIC communication mode to exchange data with MCU. The point to note is that the power supply voltage of the MLX90614ESF-DCC is 2.6~3.6V. It is recommended to use 3V power supply. Figure 2 shows the typical temperature deviation of the measured object and the supply voltage. If the system does not have a 3V power supply, it can be compensated by a compensation algorithm:
Figure 1: Connection diagram of MLX90614 and MCU
Figure 2: Error relationship between MLX90614ESF-DCC supply voltage and To
Second, software design
1, temperature calculation
The MLX90614 directly outputs the digital value of the temperature signal, and the formula is: Ta, To = ReadRegx0.02-273.15 °C. Where ReadReg is the digital value read from inside the MLX90614.
2. Modification of emissivity
The temperature of the measured object is obtained on the premise that the emissivity of the object is correct. The MLX90614 has been calibrated in blackbody at the factory. The default firing rate is 1, and the human body's emissivity is approximately 0.95. If it is to measure the temperature of other objects, it needs to be modified according to the emissivity of the object. The address of the emissivity is EEPROM@0x04.
3, the amount of temperature to body temperature algorithm
The four fever detection sites recognized by the World Health Organization are: rectum, underarm, mouth and ear temperature. There is no temperature, because of the different skin color, whether it is white or black; whether it is indoors or outdoors, a gust of wind blows; there are individual differences; the temperature is different, it is not a constant temperature, so it is used in the measurement of the front temperature. The body temperature needs to be calculated by a certain algorithm. Melexis provides a high-precision algorithm that can be contacted by Shiqiang if you need to obtain the algorithm or more technical support.
Third, the structural design
Structural design is also critical, not only determining the appearance of the entire system, but also determining the accuracy of the system. There are two main factors: thermal shock and field of view.
1, thermal shock
Thermal radiation subjects the object to rapid local heating, known as thermal shock. In general terms, the temperature of the cold end (shell) of the sensor is abrupt. For example, when the surface of the sensor touches the skin during measurement, the temperature of the sensor surface rises, which may cause an increase in measurement error. Therefore, in the structural design, try to avoid this thermal shock.
2, the field of view
The field of view is determined by the 50% radiation signal received by the thermopile and is related to the main axis of the sensor. As shown in Figure 3. The MLX90614 data sheet indicates the size of the perspective (FOV) for each version. If the opening is too small, the measured temperature is the temperature-weighted average of the temperature above the structure and the object being measured. Therefore, in the structural design of the product, the opening angle of the structure must be larger than the FOV of the MLX90614.
Figure 3: Definition of field of view FOV
As shown in Figure 4, the structural design of the ear thermometer, such as the MLX90614ESF-DCC version, has a FOV of 35 degrees, and the opening FOV on the structure needs to be designed at least 60 degrees.
Figure 4: Schematic diagram of the ear thermometer
For more information on the program and product information, contact Shiqiang.
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