1、 Programme 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 and non-contact measurement means for measuring human body temperature. Compared with traditional mercury thermometer, this temperature measurement method has the advantages of fast response speed, short measurement time, high precision and simple use. It can be widely and effectively used for temperature screening of dense people. The non-contact infrared thermometer has a good effect on specific groups, such as children or the elderly. With the rapid pace of life, it is very troublesome for parents to take time out to help their children measure their body temperature. Moreover, because children are unstable and active, their body temperature can be measured quickly and accurately by non-contact infrared thermometer; The elderly are inconvenient to move, and it is inconvenient to use the traditional thermometer. Moreover, due to their old eyes, they can’t see the position of the mercury column of the thermometer. The body temperature can be quickly obtained through the non-contact infrared thermometer, and the elderly can be informed by voice that any abnormality can be found in time.
This scheme is based on the fr8016ha development board of freqchip as the control core. The infrared temperature sensor bm43tha80c is used to measure the temperature, which is displayed on the 240 * 240 color screen on the development board, and the measured temperature is broadcast by loudspeaker.
2、 Functional block diagram
Figure 1 functional block diagram of fr8016ha temperature measuring gun scheme
The main functions of the temperature measuring gun designed in this scheme are as follows: after pressing the key, wake up fr8016ha and start the measurement, display and broadcasting of body temperature.
3、 Overall system scheme
The system structure block diagram of the temperature measuring gun is shown in Figure 2, which is mainly composed of fr8016ha, bm43tha thermopile sensor, amplification circuit, color screen display, voice playback, key part and power supply part. This scheme is based on the demonstration of fr8016ha development board and does not reach the product level scheme. Therefore, to make the product scheme, further secondary development is needed, such as optimizing power consumption, calibration accuracy Add mode, etc.
16bit Σ of fr8016ha Δ The sampling frequency rate of type ADC can be up to 48Khz. It is mainly responsible for collecting the amplified signal of the weak voltage signal of the sensor and controlling the calculation, display and broadcasting of temperature. Bm43tha thermopile sensor mainly converts the heat radiated by human body into voltage signal. The amplification circuit is mainly used to amplify the weak voltage signal of the sensor, and the key part is mainly responsible for starting the temperature measurement according to the wake-up processor. The key part mainly consists of power key and measurement key. The measurement key is responsible for temperature measurement and calculation, display and broadcasting.
4、 Main control characteristics of fr8016ha
4.1 introduction to fr8016ha
Fr8016ha is a ble5.0 SOC chip 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 for external lithium battery, 512kbflash and 48kbram. The MCU supports C language programming, is easy to develop, and supports KEIL development environment and JLINK debugging. It has rich peripheral interfaces: GPIO * 15, SPI, IIC, UART, ADC and PWM
4.2、16bit ∑ Δ Type ADC introduction
There is a built-in audio codec in the fr8010h, which includes an ADC channel with 16 bits accuracy and a rate of 48K. The ADC belongs to Σ-Δ ADC, therefore, when the input signal voltage is less than VMID (about avdd / 2), the sampling data is negative, and when it is greater than VMID, the sampling value is positive. After testing, the ADC has good linearity. In use, we suggest taking multiple points (if the sampling rate is 48K, statistics can be made every 16K points), and then taking the average value to improve the 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 the value and the actual voltage needs to be calibrated, and the calibration method is described later)
Due to different production processes, different chips will convert the same input voltage value to different results, which requires a calibration process. Because the ADC works within the effective range and is a linear output, it needs two points to calibrate the ADC. The mathematical relationship between ADC input signal value x and sampling value y is as follows:
y = ax + b
Where a and B are the two parameters to be corrected. Taking the frontal temperature gun as an example, the reference design is shown in the figure below:
Where U4 is thermopile interface, IR_ Out is ADC input signal. The calibration process is as follows:
1. Short circuit to + and to – so that the input voltage of the measured signal is 0. At this time, the sampling value of an ADC is B1. Bring it into the above formula (B1 = a * 0 + b), and the offset of this conversion line B = B1 can be obtained
2. Under the known ambient temperature, probe the thermopile towards the blackbody with the known temperature (at this time, the voltage between to + and to – can be obtained as a by looking up the table). At this time, the sampling value of an ADC is B2, which is brought into the formula (B2 = a * a + B1). It can be obtained that the slope of this straight line is a = (b2-b1) / A.
3. The straight line can be determined by the above two processes. If you want to get better results, it is to replace step 1 with another temperature blackbody for step 2 measurement.
Note: in order to obtain a more accurate value, it requires a large difference between B2 and B1, and B1 is a value near 0. Therefore, the farther away from 0 point (ambient temperature 25 degrees, blackbody temperature 25 degrees) when measuring blackbody, the better.
In practical use, the actual measured voltage is obtained by the formula x = (Y – b) / A, where B and a can be obtained through the above calibration process.
4.3 main control schematic diagram
In order to improve the accuracy, fr8016ha adopts external LDO_ 3v3 power supply, and the internal power supply of the chip is set to bypass mode
5、 Infrared temperature measurement module
The amplifier circuit adopts 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
IR of amplified thermopile output signal_ Out through 16bit Σ Δ Type ADC acquisition
6、 Audio playback module
The audio output interface directly drives the horn after an external power amplifier. Fr8016ha can play the measured body temperature value through the audio output interface.
7、 Display module
Fr8016ha development board adopts a 240 * 240 resolution full-color display screen with SPI interface to display temperature
8、 Ble Bluetooth communication (function expansion)
When the mobile phone establishes a connection with fr8016ha through Bluetooth, fr8016ha can send the measured body temperature to the mobile phone.
The following figure is a physical photo of the infrared temperature measuring gun realized by using the demo board of fr8016ha