Design of thermometer based on ZTPl35S-R sensor

1 Introduction

Nowadays, people are very concerned about their health and hope to have a good body. Body temperature is one of the important physiological parameters of the human body, and its monitoring is very important. The most popular thermometer on the market is the electronic thermometer, especially the non-contact infrared thermometer. At present, the infrared thermometers developed in China mainly include the "Smart Eye: HW-05" human body temperature infrared thermal imager developed by Huazhong University of Science and Technology. Its resolution is as high as 0.06℃; the infrared thermometer developed by the Shanghai Institute of Physics of the Chinese Academy of Sciences and the LHW-I infrared thermometer developed by the Hehua Technology Application Development Center of Lanzhou University. Foreign products include the infrared thermometer developed by the German Braun Group that can measure body temperature in just 1 second; several non-contact infrared thermometers developed by Omron of Japan and the BJ40 non-contact medical infrared thermometer (accuracy is ±t0.2℃, and its main component is the infrared temperature sensor. This paper gives the design of a thermometer using GE's ZTP135S-R infrared temperature sensor.

2 Principle of infrared temperature sensor

All objects in nature have a temperature higher than absolute zero (-273.15℃). Due to the thermal motion of molecules, they are constantly radiating electromagnetic waves, including infrared waves, into the surrounding space. The relationship between the radiation energy density and the temperature of the object itself conforms to Planck's law. The principle of infrared temperature measurement is the same, all based on Planck's principle. It is generally understood that infrared measurement is the temperature of an object. In fact, it measures the difference between the target object and the sensor, or the object and the ambient temperature. The amount of radiation energy of an object is directly related to the temperature of the object. Specifically, it is proportional to the fourth power of the thermodynamic temperature of the object. It can be expressed by the formula: E=δε(T4-T4o) (1)

Where, E is the radiation emittance, the unit is W/m3; δ is the Stefan-Boltzmann constant, 5.67x10-8W/(m2·K4); δ is the emissivity of the object; T is the temperature of the object (K); To is the ambient temperature around the object (K).

The human body mainly radiates infrared rays with a wavelength of 9 μm-10 μm. By measuring the infrared energy radiated by the human body itself, the surface temperature of the human body can be accurately measured. Since the light within this wavelength range is not absorbed by the air, the infrared energy radiated by the human body can also be used to accurately measure the surface temperature of the human body.

Infrared temperature sensors use the thermocouple principle to measure the temperature difference between the target and the sensor or between the object and the environment. The principle of the thermocouple is that two different metals A and B form a closed loop. When the temperatures of the two contact ends are different (T>To), a thermoelectric potential Eab is generated in the loop, where T is called the hot end, working end or measuring end, and To is called the cold end, free end or reference end. A and B are called hot electrodes. The size of the thermoelectric potential is determined by the contact potential (also called the Beltier potential) and the temperature difference potential (also called the Thomson potential).

3 Working principle and performance of ZTPl35S-R

The ZTP135S-R infrared temperature sensor produced by GE is a device specially used for non-contact measurement of body temperature. Its main parameters are as follows:

Chip size: 1.8 mm x 1.8 mm;

Aperture size: 1.4 mm x 1.4 mm;

Electrical pairs: 60 pairs:

Active area: 0.7 mm x 0.7 mm;

Internal resistance: 60 kn ± 30%;

Resistance temperature coefficient: The radiation energy of the measured object is focused on the heat receiving plate of the receiving element (thermopile) through the window and the aperture. There are 60 thermocouples in series on the heat receiving plate. The hot end of each thermocouple is surrounded by a circle in the center of the heat receiving plate and welded together. The sum of the thermoelectric potential of all the thermocouples can be obtained from the lead wire. This structural design has a small thermal inertia and a high sensitivity. The sensor uses a negative temperature coefficient thermistor for ambient temperature compensation.

4 Application of infrared sensors

This design uses AVR single-chip microcomputer for digital signal processing. It is connected to z1"P135S-R to process the collected temperature electrical signal. After filtering and amplification, the standard electrical signal of temperature is extracted for MD conversion and finally displayed on the liquid crystal display (LMD). The 8 data lines of LCD are connected to PB port; the control lines RS, R/W, EN are connected to PD0, PD1, PD3 respectively; the backlight of LCD is controlled by PD4. The buzzer is controlled by PD5. The infrared temperature signal is connected to the PA0 port of the single-chip microcomputer, and the ambient temperature compensation signal is connected to the PAI port of the single-chip microcomputer. Two buttons (key1, key2) are also designed. Key1 is the system control switch and key2 is the reset switch.

5 Conclusion

In the temperature measurement system, the temperature sensor is an important device, and its performance directly affects the accuracy of temperature measurement. It is necessary to select the sensor according to different measurement accuracy. For infrared temperature sensors, because they are non-contact, they have particularly high requirements for the environment. The measurement principle of thermocouples is adopted to compensate for the ambient temperature, so as to obtain accurate measurement results. This design uses ZTPl35S-R type sensor to meet the requirements well. The measured temperature changes linearly, and the infrared sensor used in this device only absorbs infrared radiation from the human body without emitting any rays to the human body. It adopts passive and non-contact measurement methods. It will not cause radiation damage to the human body.