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As evidenced by the Covid-19 pandemic, quick and non-invasive techniques to assess body temperature have become necessary. In various locations, including hospitals, airports and schools, non-contact infrared thermometry, which employs an infrared sensor to measure surface temperature without physical contact, has become popular and is commonly used for taking body temperature. Infrared thermometers are non-invasive and provide quick, reliable readings.
It is important to remember that variables like the surface being measured and its surroundings might impact how accurate infrared thermometers are. This article will demonstrate how these problems have been successfully resolved: attaining medical-grade accuracy and temperature compensation while lowering the size, using a miniaturized infrared temperature sensor developed by Melexis Microelectronic Integrated Systems as a case study.
With headquarters in Belgium, Melexis specializes in microelectronics sensors and ICs for various applications, including automotive, consumer, digital health, energy management and smart devices. One product that has seen a recent deployment in Samsung’s GW5 smartwatch series, is the medical-grade version of its MLX90632 temperature sensor based on far-infrared (FIR) technology. The non-contact temperature measurement with enhanced accuracy enables menstrual-cycle tracking. Reliable continuous temperature monitoring opens a wide range of new applications in sports, health and other domains.
FIR sensor
The FIR sensor is a surface-mount device (SMD) that measures an object’s infrared radiation to report the temperature accurately. Its SMD packaging (see Figure 1) makes the sensor suitable for a variety of applications, including wearables, particularly advanced in-ear devices (so-called hearables), and clinical point-of-care applications in which highly accurate human body temperature measurement is required.
Because it enables temperature sensing without directly touching the measured object, non-contact temperature measurement offers advantages over traditional contact methods. This can be helpful in various circumstances in which making physical contact with the object is undesirable, such as when it is fragile, moving or located in a dangerous area. When a quick response is required, or when good thermal contact between the sensor and object under test cannot be guaranteed, non-contact temperature measurement can be more accurate and yield more reliable results than contact temperature measurement.
“Melexis is basically a design house for chips and also for packaging,” said Joris Roles, marketing manager at Melexis. “In the design of each of our products, there is considerable development and lots of protected IP. We prefer to outsource the manufacturing to our wafer fab partners, but all testing and calibration are handled in-house to guarantee the final quality.”
The block diagram of the sensor is shown in Figure 2. The extremely small device is a full-solution 3 × 3 × 1-mm3 QFN package that incorporates the sensor element, signal processing, digital interface and optics, enabling quick and easy integration into a wide range of modern applications with limited space.
The MLX90632 is factory-calibrated, ensuring high accuracy, while electrical and thermal precautions have been taken internally to compensate for thermally harsh external conditions. As shown in Figure 2, the thermopile sensing element voltage signal is amplified and digitized. After digital filtering, the raw measurement result is stored in the RAM. A state machine controls all the functions. The result of each measurement conversion is accessible via an I2C interface that also allows access to the control registers of the internal state machines, the RAM for pixel and auxiliary measurement data and the E2PROM for storing the trimming values, calibration constants and various device/measurement settings.
The external unit can calculate the sensor and the object’s temperature using the measurement and calibration data. If an optical window or impediments are present, the user can modify the temperature calculation algorithm for their particular application.
According to Melexis, due to its compact size, high thermal stability and optimization for human body temperature, the medical MLX90632 is perfectly suited for ultra-compact wearable health-monitoring devices, such as portable diagnosis instruments that can continuously detect body temperature. Repeated vital-sign monitoring is a key element of preventive medicine techniques to spot serious health conditions early on. The sensor is also perfect for more conventional medical devices, such as thermometers worn on the forehead or ears.
Samsung is using the MLX90632 as a miniature medical-grade infrared temperature sensor embedded into its Galaxy Watch 5 series. Combined with a third-party app, the smartwatch’s high-accuracy temperature sensor also enables menstrual-cycle tracking, helping women to assess their fertility period.
“We developed the MLX90632 sensor to address the customer’s need for skin temperature measurement,” Roles said. “We added an additional feature; that is, the fact it’s a non-contact-based solution.”
By definition, contact thermometers need good thermal contact to operate. Additionally, it has been demonstrated that contact temperature measurement affects the measured value; that is, the skin is disturbed by the sensor system. Using non-contact technology overcomes both of these issues.
Thermal stability
Thanks to sophisticated in-factory calibration procedures, the MLX90632 can provide a medical-grade accuracy of ±0.2°C within the normal human body temperature range. This high accuracy is achievable when object temperatures are in the range of 35°C to 42°C and the ambient temperature is between 15°C and 40°C.
Thermal gradients can be a common phenomenon in wearables and consumer applications. Miniaturized FIR (contactless) sensors are usually sensitive to thermal gradients, but Melexis has mitigated this effect in the MLX90632 by adopting advanced compensation algorithms.
Melexis has conducted a test experiment using the MLX90632 and a standard infrared temperature sensor to measure the temperature of an object at 40°C. They heated both with a heat gun to assess their thermal stability. The graph at the bottom of Figure 3 (green curve) shows the rapid heating (1°C/s) forced on both chip sensors. The graph on top of Figure 3 demonstrates the excellent thermal stability achieved by the MLX90632 (blue line). It provides almost the same temperature measurement regardless of the external forced heating. The red curve shows instead how, under the same conditions, the measure provided by a standard sensor is heavily affected by errors.
“Standard FIR sensors have a large package because it helps to smooth out thermal gradients,” Roles said. “When you try to make smaller sensors, you naturally face instability issues. What we did was let the thermal gradient happen but compensated it in real time to maintain measurement accuracy.”
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