How to design FR8016HA infrared temperature measurement program?
Program overview
Body temperature is one of the key indicators of human life activities. Accurate and rapid measurement of body temperature is of great significance for disease diagnosis and treatment. Infrared temperature measurement provides a fast, non-contact measurement method for measuring human body temperature. Compared with traditional mercury thermometers, this temperature measurement method has the advantages of fast response, short measurement time, high accuracy, and simple use. It can be used widely and effectively. To investigate the body temperature of dense crowds. Non-contact infrared thermometers have good results for specific groups of people, such as children or the elderly. As the pace of life becomes faster, it is very troublesome for parents to take time out to help their children take their body temperature. And because children are unstable and active, they can be quickly and accurately measured by a non-contact infrared thermometer. Body temperature: The elderly have inconvenient activities and it is inconvenient to use traditional thermometers. Moreover, due to presbyopia, it is impossible to see the position of the mercury column of the thermometer. The body temperature can be quickly obtained through a non-contact infrared thermometer, and the elderly can be informed by voice , Abnormal situations can be discovered in time.
This solution is based on the FR8016HA development board of FREQCHIP as the control core, and the infrared temperature sensor BM43THA80C is used to measure the temperature. The 240*240 color screen is displayed on the development board, and the measured temperature is broadcast by the speaker.
The main functions of the temperature measuring gun designed in this program are as follows: After pressing the button, the FR8016HA is awakened, and the temperature measurement, display, and broadcast are started.
System overall scheme
The system structure diagram of the temperature measuring gun is shown in Figure 2. It mainly consists of FR8016HA, BM43THA thermopile sensors, amplifying circuit, color screen display, voice playback, button part, and power supply part. This solution is based on the FR8016HA development board demonstration and does not reach the product level solution. Therefore, to make a product plan, further secondary development is needed, such as optimizing power consumption, calibrating accuracy, increasing modes, and so on.
The 16bit ΣΔ ADC of FR8016HA has a sampling frequency rate of up to 48KHz. It is mainly responsible for collecting the signal amplified by the weak voltage signal of the sensor, and controlling the calculation, display and broadcast of the temperature. The BM43THA thermopile sensor mainly converts the heat radiated by the human body into a voltage signal. The amplifying circuit mainly amplifies the weak voltage signal of the sensor, and the button part is mainly responsible for waking up the processor to start measuring the temperature. The key part mainly includes the power key and the measurement key. The measurement key is responsible for temperature measurement and calculation, display and broadcast.
FR8016HA main control features
Introduction to FR8016HA
FR8016HA is a SOC chip that meets the BLE5.0 standard developed and produced by freqchip. The built-in 16-bit audio codec can support analog audio input and output. The built-in PMU can provide 200mA charging current to the external lithium battery, 512kBFlash, 48kBRAM. The mcu supports C language programming for easy development, Keil development environment and Jlink debugging. Rich peripheral interfaces: GPIO*15, SPI, IIC, UART, ADC, PWM
Introduction to 16bit ∑Δ ADC
There is a built-in audio codec in FR8010H, which includes an adc channel with 16 bits accuracy and a rate of up to 48K. This adc belongs to Σ-Δ ADC, so when the input signal voltage is less than VMID (approximately AVDD/2), the sampled data is negative, and when it is greater than VMID, the sampled value is positive. The ADC has been tested to have good linearity. When using it, we recommend taking multiple points (if the sampling rate is 48K, statistics can be performed every 16K points), and then taking the average value to improve accuracy.
1. The effective input range of the ADC is 10%~90% AVDD of the sampling range
2. The sampling output of the ADC is -24000~24000 (The corresponding relationship between this value and the actual voltage needs to be calibrated, the calibration method will be introduced later)
Due to differences in production processes, different chips will convert the same input voltage value to different results, which requires a calibration process. Because the ADC works as a linear output within the effective range, two points are needed to calibrate the ADC. The mathematical relationship between ADC input signal value x and sampling value y is as follows:
y = ax + b
Among them, a and b are two parameters that need to be corrected. Take forehead temperature gun as an example, the reference design is as follows:
Among them, U4 is the thermopile interface, and IR_OUT is the ADC input signal. The calibration process is as follows:
1. Short-circuit TO+ and TO-, then it is equivalent to the input voltage of the signal under test is 0, then the sample value of an ADC is obtained as B1, and the above formula (B1 = a * 0 + b) can be obtained. The offset of the conversion line b = B1
2. At a known ambient temperature, detect the thermopile toward a black body with a known temperature (at this time, the voltage between TO+ and TO- can be obtained as A by looking up the table), and then an ADC sampling value is B2 , Into the formula (B2 = a * A + B1), the slope of this line can be obtained as a = (B2-B1) / A.
3. The above two processes can determine the straight line. If you want to get a better effect, it is to replace step 1 with a black body with another temperature and perform a 2-step measurement.
Note: In order to obtain a more accurate value, this requires that the difference between B2 and B1 is relatively large, and B1 is a value near 0, so when measuring the black body, it is more away from 0 (the ambient temperature is 25 degrees, and the black body temperature is 25 degrees). The farther is better.
In actual use, the actual measured voltage is obtained through the formula x = (y-b) / a, where b and a can be obtained through the above calibration process.
In order to improve accuracy, FR8016HA adopts external LDO_3V3 power supply, and the internal power supply of the chip is set to Bypass mode
Infrared temperature measurement module
The amplifying circuit uses high-precision integrated operational amplifier TP5591, low offset voltage 20uV, zero drift: 0.01uV/℃
The reference voltage VMID is 1/2AVDD
NTC voltage NTC_ADC is collected by 10bitADC
The amplified IR_OUT of the thermopile output signal is collected by 16bit ΣΔ ADC
Audio output interface, directly drive the speaker after an external power amplifier. FR8016HA can display the measured body temperature through the audio output interface.
The FR8016HA development board uses a 240*240 resolution full-color display with SPI interface to display temperature
BLE Bluetooth communication (function expansion)
When the mobile phone establishes a connection with FR8016HA via Bluetooth, FR8016HA can send the measured body temperature to the mobile phone.
The picture below is a physical photo of the infrared thermometer using FR8016HA DEMO board
If you want to know more, our website has product specifications FR8016HA infrared temperature, you can go to ALLICDATA ELECTRONICS LIMITED to get more information