The smart solution uses Dual-Interface EEPROM which stores the temperature measurement and communicates with the Android Phones using the NFC interface
By Alok Mittal, Group Manager, AMG Central Lab, STMicroelectronics India; Salil Jain, Senior Design Engineer and Technical Marketing, AMG Central Lab, STMicroelectronics India; and Raunaque Mujeeb QUAISER, Senior Manager, AMG Central Lab, STMicroelectronics
Human body temperature is one of the critical parameters used for successful diagnosis in most of the cases viz. OPD (Out Patient Department), ambulatory etc. Customarily, mercury-glass thermometers are more popular; however, many countries have banned the use of mercury based thermometer for medical usage due to possible environmental hazard. In this paper we are introducing non-invasive infrared thermometers for measuring human body temperature.
Transfer of heat between surfaces depends on any of the principles viz. conduction, convection and radiation. It is the radiating heat which has been exploited while studying infrared thermometers. All the objects emit infrared radiations above absolute zero temperature and the intensity of these radiations is directly proportional to the temperature of the object.
Human body temperature under healthy conditions is around 37°C but it may vary from 36.5°C to 37.5°C. Pyrogens flow in the bloodstream and in case of fever these move to the hypothalamus present in the brain. Hypothalamus regulates the human body temperature. Body temperature rises beyond normal when these pyrogens attach to some receptors present in the hypothalamus.
True core temperature can only be measured invasively. Temperature measured on non-invasive sites such as rectum, oral cavity and temporal artery (forehead) varies greatly. But temperature measured at these non-invasive sites provides the actual estimation of core temperature. The data so obtained is post processed digitally.
In this article, a non-invasive thermometer based on sensing of temperature using Infrared (IR) radiations is presented which are also known as IR thermometers. In these thermometers, the sensing of data is done by IR sensor which projects these rays on to a thermal detector and finally converts these into an electrical signal. The amplitude of the electrical signal is proportional to the projected rays’ temperature. This electrical signal defines the temperature of the body obtained non-invasively; however, human body temperature obtained by invasive methods is the actual core temperature. Post processing of the electrical signal obtained non-invasively is done to compensate the non-invasive and temporal offsets and convert it into the core value. The results are fed to a liquid crystal display.
The smart solution uses Dual-Interface EEPROM which stores the temperature measurement and communicates with the Android Phones using the NFC interface.
Infrared Thermometer Architecture: Infrared Thermometer hardware architecture as shown in Figure 3 is a portable battery operated design based on STM8L052. STM8L052xx microcontrollers feature embedded data EEPROM and low power low voltage single-supply program Flash memory. It operates from 1.8 V to 3.6 V power supply (down to 1.65 V at power down) with Brown-Out-Reset and from a 1.65 V to 3.6 V power supply without Brown-Out-Reset option. The reference design for the same is shown in Figure 4.
This application is running at 1MHz to reduce current consumption. The current consumption in run mode is between 1.5 mA to 3.2 mA, whereas it steps down to 1.2 uA in sleep mode. Inbuilt LCD Driver is used for driving of Glass LCD and LSI (low speed internal) Clock is used for RTC management. LSE (Low Speed External) clock can also be used, but it will increase the consumption (Approx. 1.5 uA) in sleep mode.
Infrared rays emitted by skin surface are projected on to the Infrared sensor and the internal state of the sensor is used to calculate the body temperature. The value obtained by infrared sensor is transmitted to the micro-controller unit through I2C communication using SMBus (System Management Bus) protocol. This reading is fed from microcontroller to the LCD display and also saved into Dual Interface EEPROM. The system utilizes 3 V coin cell (CR2032) for its power requirements and the 12-bit ADC present in the STM8L is used to indicate the battery status.
Infrared Temperature Sensor: The IR sensor is infrared thermometer for non-contact temperature measurements. Both the IR sensitive thermopile detector chip and the signal conditioning chip are integrated in the same package. It consists of serially connected thermo-couples with cold junctions placed at thick chip substrate and hot junctions placed over thin membrane. The IR radiation absorbed from the membrane heats (or cools) it.
The corresponding ambient and object temperature can be calculated after measuring both sensors’ outputs. These calculations are done by the internal DSP, which produces digital outputs, linearly proportional to measured temperatures.
Dependence of temperature reading on supply voltage: In this application, Coin cell is used as a power source. The power supply voltage reduces for higher current demand. The IR sensor used in this thermometer is MLX90615/MLX90614. The output of the sensor decimates due to lower supply voltage and vice-versa.
Typically, the temperature variation with supply is 0.6°C/V. This variation can easily be compensated by determining the battery level using in-built 12-bit ADC of STM8L.
The ADC channel available inside the MCU is connected to internal precise reference. The measurement value obtained for this internal reference provides indication of the battery voltage. This feature is used for compensating the battery voltage variation due to life of the battery
Temperature conversion from forehead to Human body core temperature: By using some mathematical function viz. Curve Fitting Method, a sixth degree polynomial is constructed which is a best fit to a series of data points and implemented in firmware to convert forehead temperature into human body temperature.
Key Features:
STM8L low power MCU based reference design to measure the body temperature non-invasively.
Can measure body temperature within a range of 20°C (68°F) to 42.2°C (108°F) with 0.5°C accuracy.
Low battery indication.
Glass LCD with 6 alphanumeric characters to display information.
Different sounds of buzzer are generated to indicate different operations like – ready screen display, temperature scanning, temperature display and shutdown.
Android Application for reading the data logged in the Dual interface EEPROM over NFC Interface.
Can save up to 50 temperature readings along with date and time stamp in onboard dual interface EEPROM.
Memory clear option available. If no switch is pressed for 1 minute, then system goes into sleep mode automatically. Able to show temperature in Centigrade/Fahrenheit. Current Consumption is only 1.2uA (approx.) in Sleep Mode.
STMicrolectronics Tele-health Android Application: An Android based Healthcare Application, ST Health Care, has been developed by STMicroelectronics to communicate with several healthcare platforms including Infrared Thermometer. The Android application read the data from health care design through Near Field Communication (NFC) and saves in the database for each user. The user can share their health information and upload them on Microsoft HealthVault. This application can be directly downloaded from the Google Play Store. The data so obtained could be viewed in textual or graphical form in the domain of time/date; subsequently analyzed and could be shared. In addition, various parameters viz. Real Time Clock Date/Time and manufacturer’s details can also be edited.
Conclusions: Convenient monitoring of user health parameter can be done easily by portable healthcare devices. These give very fast, reliable and real time diagnostic data. Infrared thermometer is an ideal choice for temperature measurements as these provide results with great accuracy of the order of 0.5°C. The results are also obtained in very small time as compared to mercury based thermometers. In addition, these are more rugged and free from environmental hazards. STMicroelectronics has developed several portable personal healthcare designs with NFC connectivity. In addition, STMicroelectronics free Android App ‘ST Health Care’ provides secure health data management and Microsoft HealthVault integration to demonstrate the complete Tele-health ecosystem. Various reference designs are supported by this application viz. IR Thermometer, Iontophoresis and Pulse Oximeter. In addition, the IR thermometer presented here could be used for industrial applications as well e.g. cloud detection in remote telescopic activity, food preparation surfaces, hot-spots detection in electrical circuit board, research & development for materials in the process of thermal change etc. For future healthcare advancements, non-invasive Infrared thermometer could be a milestone.