Thermal sensors are commonly used in industrial, medical, home appliances, security, entertainment and other fields, especially non-contact sensors have less restrictions and wider applications. The use of non-contact thermal sensors to detect body temperature and sense the presence of the human body has become a popular market demand. This article introduces the types and operating principles of thermal sensors as well as relevant solutions.
The Difference between Contact and Non-Contact Thermal Sensors
The application of thermal sensors is relatively wide. They are not only used for measuring body temperature, but can also in the industrial, medical, home appliances, security, and entertainment industries. For example, thermal sensors can be used to check the temperature of pieces produced by industrial processes; in terms of medical treatment, they can be used to monitor patients’ physiological state; in terms of home appliances, it can detect temperature and human presence in air-conditioning systems; in terms of security, they can be used to detect human body existence and movement; in terms of entertainment, they can be used to sense player movements. The list of applications goes on and on.
Thermal sensors can be divided into two types; contact and non-contact. The following content includes a brief summary of the differences between them and a deeper introduction aimed at non-contact thermal sensors.
Just as its name implies, contact thermal sensors require the touch of the test subject. This means that a good thermal contact must be kept between the sensors and the subject or fluid, indicating that sensors must be in contact or close to objects or fluid with similar temperatures between them. That is to say, sensors must reach a thermal equilibrium with the test subjects. In addition, the expected temperature of the test subject must be lower than 1700°C (3092°F) or higher than about -40°C (-40°F). This is because at 1700°C, the platinum alloy thermocouple begins to rapidly lose calibration and the wire and insulating materials will probably begin to soften. At a temperature lower than -40°C, problems begin to appear, and although many contact thermal sensors can still work below that temperature, its accuracy will be affected. What’s more, contact thermal sensors must be smaller than the test subject, and they should be fixed on the test subject through welding, soldering, clamping or gluing, maintaining physical touch in a reliable manner.
The most common contact thermal sensors are liquid-in-glass thermometers, thermocouples, resistance temperature detectors (RTD) and thermistors. These are usually enclosed in a protective metal or ceramic sheath called a thermowell, thus able to penetrate process barriers and can be easily pulled out for calibration or maintenance without exposing the process and / or maintenance personnel to adverse conditions. The application flexibility of non-contact thermal sensors is wider with more diverse categories. Although non-contact thermal sensors have multiple styles and types with various names, they are all classed as radiation thermometers if they operate according to the Max Planck law of thermal radiation. These are called radiation pyrometers, infrared pyrometers, optical pyrometers, infrared thermometers, thermal imagers etc. They can be battery powered portable devices, fixed-mount or on-line processing monitoring devices.
As the temperature of non-contact thermal sensors does not have to be the same as that of the subject, its application is far wider than that of contact thermal sensors. Non-contact thermal sensors are applied in fields such as detecting moving objects, where subject or sensor damage may occur during contact (extremely hot, corrosive or abrasive conditions), obvious temperature change of subject during contact, where there is a large and observable measuring area, or the object is too far away or difficult to approach, such as in special atmospheres or in space (observations of stars and galaxies etc.).
Non-Contact Thermal Sensors are Suitable for Human Body Detection
The following content takes the D6T series of OMRON non-contact micro electro mechanical system (MEMS) thermal sensors as an example to introduce its functions and characteristics. The D6T series of MEMS thermal sensors is composed of small circuit board on which a silicon lens is installed, thermopile sensors, a specialized analog circuit and a logical circuit used for converting to a digital temperature value. Only one connector is required for combining those modules. The D6T series circuit board measures 14mm x 18mm and more compact 11.6 mm x 12 mm version is also available.
The operating principle of the D6T series MEMS thermal sensors is focusing the radiant heat (far infrared) generated from the object through a silicon lens on the thermopile sensor of the modules. The thermopile sensor can generate electromotive force according to the radiant energy (far infrared) focused on it to measure the value of electromotive force and the interior thermal sensor. The device can calculate the measured value (object temperature) via interpolation calculation, which compares the measured value and the look-up table stored inside. The measured value will be output via an I2C bus and be read on the host system.
Using MEMS thermal sensors to detect human movement can eliminate the problems faced by conventional pyroelectric sensors. This is because pyroelectric sensors are based on the principle of detecting changes in infrared rays’ state and when the human body is inactive, the measuring signals will be lost. However, thermal sensors can still generate measurement signals even when there is no movement.
Non-contact thermal sensors are highly suitable for human body presence detection. They are quite common for home appliance applications. For example, they are used in air conditioning systems or fans to sense the presence of human bodies to make a relative response. The market demand is increasing day by day. It is worth your efforts to invest in product development.
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