Application case of infrared thermometer based on EFM32

Overview

Infrared thermometers measure the radiation energy of infrared rays emitted by an object to obtain the temperature of the object itself. Currently, they can be divided into three categories according to the different objects used:

Human infrared thermometer . Also called forehead thermometer, it is a thermometer that uses the infrared receiving principle to measure the human body. Its measurement range is relatively narrow, generally 30 to 42 degrees Celsius.

Industrial infrared thermometer. Mainly used for industrial temperature collection, the measurement range is generally -40 to 1500 degrees Celsius.

Veterinary infrared thermometer. According to the Planck principle, by accurately measuring the surface temperature of a specific part of the animal's body surface and correcting the temperature difference between the surface temperature and the actual temperature, the individual body temperature of the animal can be accurately displayed.

System Structure

EFM32 is a high-performance, low-power 32-bit microcontroller designed by Energymicro of Norway using the ARM Cortex-M3 core. It has outstanding low-power characteristics and is suitable for "three meters" (electricity meter, water/heat meter, gas meter), industrial control, alarm security system, health and sports application system, handheld medical equipment and smart home control.

The infrared thermometer mainly includes power supply, optical system, infrared sensor, main control processing, interactive interface and communication interface. The block diagram is as follows:

Figure 1 System Block Diagram

Power supply

The operating voltage of EFM32 is 1.8~3.8V, which is a wide range and is conducive to the simple design of the voltage module. Therefore, the smart door lock can be powered by a 3.3V lithium battery system without adding an LDO chip to the front end. Since the chip has a wide operating voltage range, compared with the traditional 2.8~3.6V MCU, it omits the front-end chips such as the boost chip and charge pump.

Optical system

As shown in Figure 2, the optical system of the infrared thermometer is a process of filtering and focusing. The size of the infrared radiation energy of an object and its distribution by wavelength are closely related to its surface temperature. The optical system filters according to different measurement ranges, allowing only the required wavelength of light to enter, and then the lens focuses the light for conversion by the infrared sensor.

Infrared sensor

This is a core device in infrared thermometers. It converts the radiation energy of infrared light into electrical signals through thermoelectric effect, pyroelectric effect and thermistor. Some infrared sensors with ADC can send infrared temperature and ambient temperature data to MCU through I2C.

Figure 2 Infrared sampling block diagram

Main Controller

Different types of EFM32 chips can be configured as the main control MCU according to functional requirements. This application uses EFM32TG822 as the main control, which has a QFP48 package, Flash and RAM resources ranging from 8 to 32KB and 2 to 4KB respectively, with an 8X11 LCD driver interface, 4-channel ADC, 2 comparators and 3 op amps.

Interaction and Communication

The results can be displayed on the LCD through key control. If necessary, the measurement results can be communicated with the PC through the UART port or the USB port, and the results can be displayed and saved on the PC.

Solution Advantages

Compared with the traditional 8-bit and 16-bit MCU-based infrared thermometer solution, this solution based on EFM32 has the following advantages:

Ultra-low power consumption

EFM32 is the world's lowest power 32-bit microcontroller. In EM2 mode, where the LCD controller and DMA can run, the power consumption current is only 900nA. In the mode without running RTC, it can be as low as 600nA, and when not saving RAM data, it is only 20nA. At the same time, the LCD of EFM32 can realize simple animation mode without the participation of CPU. Low-power peripherals are integrated on the chip: the 12-bit ADC consumes only 350μA at a rate of 1Msps; the analog comparator only requires 100nA; the LCD drives 8×36 segment LCD display, which only requires 0.55μA. Infrared thermometers are powered by batteries and have relatively high requirements for power consumption. EFM32 is very suitable for its power consumption requirements.

High integration

EFM32 integrates LCD driver, analog comparator, operational amplifier, AES hardware module and other resources on the chip, which can reduce most of the peripheral devices and save costs.

Summarize

EFM32 has excellent low power consumption characteristics, which is very suitable for the application of infrared thermometers that require low power consumption. The EFM32 core adopts the currently popular Cortex-M3 design, which greatly shortens the development time of developers. EFM32 has rich peripherals, which provides conditions for system expansion functions and cost reduction. Therefore, EFM32 is very suitable for the application of infrared thermometers.