As people become increasingly concerned about health and the environment, temperature sensing has become more important. Many devices have added temperature sensing capabilities, such as medical thermometers and smart wearable health monitoring devices, and the application areas are becoming increasingly widespread. This article will explore the principles and smart solutions behind non-contact temperature sensing, as well as the relevant solutions offered by Melexis.
Non-contact temperature sensing using MEMS thermopile technology
Non-contact temperature sensing can detect energy emitted in the infrared (IR) wavelength range. Every object emits energy in this way, so we can calculate the temperature of an object by measuring this energy. However, as sensor sizes get smaller, they are more susceptible to thermal shocks, which can lead to measurement errors and thermal noise.
Currently, the mainstream technology for non-contact temperature sensing is integrated MEMS thermopile technology. A thermopile is an electronic sensor that can convert thermal energy into an electrical signal. Its working principle is based on the fact that all objects emit thermal far-infrared (FIR) radiation.
From an electronic perspective, a thermopile consists of multiple thermocouples connected in series. The voltage generated by these thermocouples is proportional to the temperature difference between two points, and this temperature difference can be used to measure relative temperature. A MEMS thermopile sensor ICs incorporate a thermal isolation membrane. Because this thermal membrane has low thermal mass, it can quickly absorb the incoming heat flux, generating a temperature difference that the thermopile can report. By integrating a reference thermistor into the MEMS system, an absolute temperature measurement can be obtained.
Wearable devices require a significant reduction in the size of temperature sensors
The role of temperature sensing has become increasingly important in various applications, leading many devices to incorporate this functionality. These devices include health monitors and wearable devices like smart glasses, smart wristbands, and devices worn inside the ear, often referred to as "hearables." However, contact-based temperature measurement solutions frequently encounter issues with poor thermal contact with the site of interest. Non-contact temperature sensing based on the FIR principle is well-suited for these new applications. Still, it requires a significant reduction in the size of temperature sensors.
The applications of temperature measurement are expanding rapidly, especially with the integration of temperature measurement into portable devices like smartphones and wearables as part of home care. However, temperature measurement faces two major challenges. First, sensor IC components must be small enough for various applications. Second, sensor IC components need to be installed in large metal case to provide sufficient thermal capacity to mitigate the impact of rapid thermal shocks.
If small FIR sensor ICs are mounted on a PCB, they may be exposed to heat from nearby heat-generating components, such as microprocessors or power transistors. FIR sensor IC manufacturers attempt to overcome this issue by placing the sensing element in large metal can (e.g., TO-can package). While the significant heat storage and high thermal conductivity of metal can provide some protection against rapid thermal gradients and shocks, this approach may not be highly effective in environments where thermal characteristics change dynamically. Additionally, one challenge is that TO-can packages are relatively large and not suitable for small devices like wearables and hearables.
The application of infrared temperature sensors on PCR
I believe everyone should be very familiar with the term Polymerase Chain Reaction (PCR). PCR's main function is to amplify (duplicate) DNA, and its most widespread application is in the detection of infections. Pathogens such as viruses or bacterium in DNA/RNA can be detected in patient samples after amplification. This application has flourished under the impact of the COVID-19 pandemic, and PCR technology can also be used to detect many other pathogens.
Many biochemical processes are used in medical diagnostics, and PCR is just one example. To expedite temperature-sensitive biochemical reactions, a "thermal cycler" is needed. The thermal cycler is equipped with one or multiple thermal blocks with holes into which tubes containing reactants can be inserted. The purpose of the thermal cycler is to subject these tubes to a predefined temperature program, allowing for fast and accurate temperature sweeps. Some models support controlling the temperature gradient of the thermal block to place different samples at different temperatures. This functionality is mainly used in the research phase to optimize certain critical steps of temperature cycling.
During testing, specimens are often replaced, making it challenging for manufacturers to reliably measure the temperature of the tubes by direct contact method. Strict control of temperature cycling relies on accurate sensor inputs, and this is where the role of infrared temperature sensors comes in. They enable non-contact temperature measurements, which is a significant advantage compared to contact-based thermometers. Furthermore, avoiding direct contact significantly reduces the risk of cross-contamination between specimens.
Currently, medical diagnostic testing is rapidly evolving, shifting from the need to send specimens to specialized medical laboratories and waiting weeks for results to the point-of-care. In this transformation, infrared temperature sensors play a crucial role. By using infrared temperature sensors, temperature control can be more stringent, further fine-tuning the biochemical reaction processes, leading to faster, more accurate, and more reliable diagnostics.
A versatile and highly integrated infrared thermometer
The Melexis MLX90614 is an infrared thermometer designed for non-contact temperature measurements. It integrates the IR-sensitive thermopile detector chip and the signal conditioning ASIC within the same TO-39 can package. The MLX90614 features a low-noise amplifier, a 17-bit ADC, and powerful DSP unit, providing both high accuracy and resolution temperature measurements.
The MLX90614 thermometer comes factory calibrated and offers temperature measurements across the entire temperature range via digital SMBus output (with a measurement resolution of 0.02°C). Users can configure the digital output as pulse-width modulation (PWM). As a standard, the 10-bit PWM is configured to continuously transmit the measured temperature in range of -20 to 120°C, with an output resolution of 0.14°C.
The MLX90614 boasts several advantages, including its small size, low cost, and ease of integration. It is pre-calibrated across a wide temperature range, including sensor temperatures from -40°C to 125°C and object temperatures from -70°C to 380°C. Its high accuracy within this broad temperature range is up to 0.5°C (within the temperature range of 0°C to +50°C for both Ta and To). It can achieve medical-grade accuracy of 0.2°C within a limited temperature range when required. The field of view options include 5°, 10°, 35°, and 90°, allowing you to determine the measurement range. The MLX90614 offers both single and dual-zone versions, supports a digital interface compatible with SMBus for easy temperature reading and building sensor network, features customizable PWM output for continuous reading, and comes in both 3V and 5V versions. It can be adapted to applications ranging from 8V to 16V with simple adjustments and supports power-saving modes, digital filtering, and various packaging options. Additionally, evaluation kits are available to meet diverse application and measurement needs, and it is rated for automotive-grade applications.
A high-precision non-contact miniature SMD temperature sensor IC
Melexis MLX90632 is a miniature SMD temperature sensor IC that enables high-precision non-contact infrared temperature measurement. It is particularly suitable for environments with dynamic thermal characteristics and space is limited. The product offers high stability and is available in both medical-grade and consumer-grade versions.
MLX90632 operates accurately and reliably in high-temperature environments and comes in a compact 3mm x 3mm x 1mm QFN package, eliminating the need for bulky TO can packages. It utilizes I2C digital interface for factory calibration and features a 50° field of view. The programmable refresh rate ranges from 0.5Hz to 32Hz, and it operates on a 3.3V supply with a current consumption of 1mA. The duty cycle is 50µW with one reading per minute. Its operating temperature range is from -20°C to 85°C, and there are available drivers on GitHub along with datasheets and evaluation kits.
For commercial-grade devices, MLX90632 supports object temperature measurement from -20°C to 200°C with an accuracy of ±1°C. These devices are commonly used in applications such as white goods appliances, standalone smart thermostats for room temperature monitoring, and ambient temperature monitoring products integrated into portable electronic devices.
For medical-grade devices, MLX90632 supports object temperature measurement from -20°C to 100°C with a high accuracy of ±0.2°C. Medical-grade applications include ear thermometers, health monitoring wearable devices, and point of care applications.
Melexis also offers an evaluation board, EVB90632, which includes the MLX90632 infrared temperature sensor chip (SMD package) and provides a simple interface to a PC. This PCB enables users to perform quick and easy tests on the MLX90632 sensor. EVB90632 allows users to access sensor internal settings and adjust the sensor to specific applications by changing optical window compensation constants, refresh rates, and the integrated circuit bus address.
Low-power battery-powered magnetometers for position sensor applications
Melexis has also introduced the MLX90397, a Triaxis® magnetometer designed for cost-effective battery-powered applications. This 3D magnetometer is specifically tailored for position sensor applications, with a magnetic field range of ±50mT and a BZ adaptive range of ±200mT. The MLX90397 offers low power consumption and is suitable for space-constrained applications, making it an ideal choice for battery-powered applications.
The MLX90397 supports dynamically programmable parameters and is a monolithic sensor that can sense magnetic flux density both perpendicular and parallel to the chip's surface. The device can perform magnetic measurements along the 3 axis (X, Y being in a plane parallel to the surface of the die, Z being perpendicular to the surface). Users can choose to measure magnetic fields BX, BY, BZ individually, or temperature, or perform combined measurements. These measurements, along with the chip's temperature, are converted into 16-bit data and transmitted via the I2C communication channel upon request. The device transmits compensated raw measurement data.
The MLX90397 features a typical standby current of 7nA and operates on a voltage range of 1.7 to 3.6 V. The reset pin enables the MLX90397 to achieve ultra-low standby current consumption, making it well-suited for low update rate applications. This simplifies design, reduces bill-of-material costs, and saves PCB space. The sensor measures only 2 mm x 2.5 mm x 0.4 mm and comes in a UTDFN-8 ultra-thin, flat, no-lead 8-pin package, which is advantageous for space-constrained applications, allowing for simpler PCB layouts. Furthermore, the MLX90397 comes pre-programmed, providing a plug-and-play solution that is easy to integrate and operates within an environmental temperature range of -40°C to -105°C. The use case for the MLX90397 magnetometer in wearable devices is knob control.
Conclusion
The introduction of new, miniaturized temperature sensors has made it possible to integrate temperature sensing into highly integrated healthcare wearable devices. These sensors can also be used to speed up PCR testing and reduce costs, offering significant market development potential. Melexis offers a range of miniaturized temperature sensors and magnetometers that can greatly reduce the size and power consumption of wearable devices, making them an ideal choice for manufacturers developing related products. You can visit the Melexis website here to download the temperature sensor selection guide and find the temperature sensor that best suits your needs.