Design of non-contact measurement and control system for pyroelectric sensors

Introduction
    The wavelength of infrared radiation is about 0.77 to 1 000 pm, and different bands of infrared radiation have different applications. From infrared guided missiles and infrared imaging night vision devices in the military to infrared meteorological satellites containing high-tech cutting-edge technologies, from infrared photoelectric counters, infrared thermometers, and infrared gas analyzers commonly used in industry to civilian wave-type infrared anti-theft alarms and human infrared automatic lighting switches, infrared sensing technology has been widely used in various fields such as space, industry and agriculture, and civilian use. Pyroelectric infrared detectors, also known as pyroelectric detectors, are a new type of thermal detector that has made important developments in the field of thermal detection in the past decade. It is reported that this detector can be widely used in radiation temperature measurement, infrared spectrum measurement, laser parameter measurement, non-contact temperature measurement, automatic monitoring of industrial processes, safety alerts, infrared photography and space technology. Although China's research in this area started late, important research results have been reported in recent years. Compared with the commonly used thermal detectors such as bolometers and thermoelectric piles, this detector has the following characteristics:
    ① Good frequency characteristics. Other thermal detectors have the maximum output after thermal equilibrium, and the irradiation time must be greater than the time constant of thermal equilibrium (generally several ms to tens of ms) when working. Pyroelectric detectors work in a non-thermal equilibrium state, and have no output when thermally balanced. Therefore, the irradiation time must be less than the time constant of thermal equilibrium (generally 0.1 to 1 s) when working. In other words, its working speed is not limited by thermal equilibrium, and its upper limit of frequency mainly depends on its equivalent capacitance and subsequent circuit.
    ② Working at room temperature, it can obtain high sensitivity without refrigeration, which is comparable to high-sensitivity bolometers at low temperatures.
    ③ The output impedance is purely capacitive, and the DC impedance is extremely high.
    ④ Small size, light weight, and strong.
    Therefore, pyroelectric detectors are an ideal infrared radiation detector and occupy a very important position in the field of thermal detection.

1 Measurement principle
    The temperature measurement principle of infrared thermometers is the blackbody radiation law. As we all know, all objects above absolute zero in nature are constantly radiating energy outward. The size of the energy radiated outward by the object and the distribution of its wavelength are closely related to its surface temperature. The higher the temperature of the object, the stronger the infrared radiation capacity emitted. When the black body is at temperature T,
the monochromatic radiation output at wavelength λ is determined by Planck's formula, that is
   
    , the pyroelectric infrared sensor is composed of ceramic oxide or piezoelectric crystal elements, and the two surfaces of the element are made into electrodes. When the temperature changes by △T within the monitoring range of the sensor, the pyroelectric effect will generate charges on the two electrodes, that is, a weak voltage △V is generated between the two electrodes. The pyroelectric detector detects the changing temperature, and after photoelectric conversion, it becomes an AC voltage signal for the signal processing circuit to process. The formula for the temperature of the radiator can be derived from Planck's formula:
   
    Where T1 is the temperature of the target; T2 is the ambient temperature; ε1 is the emissivity of the target to be measured; ε2 is the emissivity of the environment; σ is the Stefan-Boltzmann constant, and σ=5.670 3×10-8W·m-2·K-4; K represents the sensitivity of the detector, and
    K=R·α·ε·σ1R
    , where R represents the sensitivity of the detector; ε is the emissivity of the radiator, which is generally taken as 1 during calibration; α is a constant related to the atmospheric attenuation distance.

2 Overall design of temperature measurement system
    The infrared thermometer system is mainly composed of optical system, photoelectric conversion, signal processing, display output and other parts. The optical system completes the determination of the field of view. The pyroelectric detector converts the infrared energy focused on the detector into an electrical signal, which is amplified and filtered for analog/digital conversion and sent to the single-chip microcomputer for signal processing. The LCD display unit displays the temperature value of the measured target.

3 Signal processing circuit design
3.1 Signal amplification circuit
    When the light signal passes through the pyroelectric sensor, it becomes an alternating pulse signal. The human body radiation signal received by the pyroelectric sensor is very weak, only in the order of microvolts or nanovolts, so it needs to be amplified before signal processing.
    This system divides the signal amplification circuit into a preamplifier circuit and a post-amplifier circuit for processing. The preamplifier must be high gain and low noise. High gain is used to amplify weak signals to a certain level for further processing, and low noise is to maintain the highest possible signal-to-noise ratio.
    The amplifier circuit uses the low-offset precision operational amplifier chip LM358, which includes two independent, high-gain, internal frequency-compensated dual operational amplifiers, which are suitable for single power supply with a wide power supply voltage range, and are also suitable for dual power supply working mode. The signal amplifier circuit of the system is shown in Figure 1.

    The integrated operational amplifier LM358 features low offset, low noise, and low drift, and is widely used in precision instrument amplifiers, sensor amplifiers, and other occasions. The infrared sensor signal enters the amplifier circuit from the pin 3 port of the LM358. The capacitor C1 in the figure is used to filter out the DC signal in the signal, and the circuit adjustable potentiometer Rv1 is used to adjust the gain of the sensor output signal. The pin 7 port sends the amplified infrared sensor signal to the signal acquisition circuit unit for analog/digital conversion. [page]

3.2 Signal Acquisition Circuit
    The analog electrical signal amplified by the amplifier circuit needs to be converted into a digital signal before it can be sent to the microcontroller for data processing. The system data acquisition uses the TLC549 integrated chip for analog/digital conversion as shown in Figure 2.

3.3 Ambient temperature compensation circuit
    Since the pyroelectric sensor measures the difference in radiation energy between the human body and the environment, it is necessary to add an ambient temperature detection circuit to the signal processing circuit. The system uses an integrated temperature sensor DS18B20 to detect the ambient temperature. When the DS18B20 is in the memory write operation and temperature conversion operation, the maximum pull-up resistor opening time is 10μs. The collected signal is sent to the microcontroller, and after analysis and processing by the microcontroller, it is converted into a temperature value of corresponding size. The DS18B20 program flow is shown in Figure 3.

3.4 Microcontroller Peripheral Circuit
    The system uses the low-voltage, high-performance 8-bit microcontroller AT89C51 produced by Atmel Corporation of the United States as the controller. It contains 4 KB of fast erasable read-only memory and 128 bytes of random access memory (RAM), built-in 8-bit central processing unit (CPU) and Flash storage unit. The external circuit of the microcontroller in this system mainly includes the following circuits.
3.4.1 Reset circuit
    When the RESET pin of the AT89C51 microcontroller introduces a high level and maintains two machine cycles, the microcontroller performs a reset operation. If the pin is always kept at a high level, the microcontroller is in a cyclic reset state. A circuit that is effective both on power-on and on keystroke is used. The keystroke reset circuit is a manual reset on power-on. When the program runs away, there is no need to restart the microcontroller power supply, which is more convenient to use.
3.4.2 Clock circuit
    This system uses an external crystal oscillator to provide the system with a clock. The clock frequency of the quartz crystal oscillator is 11.059 2 MHz.

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3.4.3 Keyboard circuit
    The keyboard circuit consists of 4 inching switches, which are respectively sent to P1.3, P1.4, P3.2, and P3.3 of the single-chip computer. At the beginning, the potentials of P1.3, P1.4, P3.2, and P3.3 ports are all at high potentials, and the display screen shows the current ambient temperature. When the switches controlling P3.2 and P3.3 are pressed, the upper and lower temperature values ​​of the human body will be displayed immediately. If the set upper and lower limits of the temperature are found to be too high or too low, click the switches controlling P3.2 and P3.3 to lower or raise the set temperature of the upper and lower limits. When the ambient temperature needs to be displayed, click the switch controlling P1.3. When measuring the human body temperature, aim the probe at the forehead of the pedestrian passing by, and click the switch controlling P1.4 to display the human body temperature. The interface circuit between the keyboard and the single-chip computer is shown in Figure 4.

3.4.4 Alarm circuit
    When the human body temperature detected by the infrared sensor exceeds the set temperature upper and lower limits, the speaker will sound an alarm for several seconds. The interface circuit with the single-chip microcomputer is shown in Figure 5.
3.4.5 Display circuit
    The display uses a character-type liquid crystal display chip SMC1602, which can be used to simultaneously display the values ​​of the measured target and ambient temperature. Compared with the LED digital tube, the SMC1602 has a simple circuit connection and convenient observation in terms of temperature display. It has 160 different dot matrix character graphics built in, can display 2 lines of 16 characters, and uses a standard 16-pin interface. VL is the contrast adjustment terminal of the liquid crystal display. When connected to a positive power supply, the contrast is the highest, and when connected to a negative power supply, the contrast is the lowest. The contrast is controlled by a 10 kΩ potentiometer.

4 Analysis of measurement results
    The distance from the target object to the sensor receiving window is changed to explore the influence of the distance on the output signal voltage difference. As shown in Figure 6, it can be seen from the experimental results that at the same temperature, as the measurement distance decreases, the voltage difference corresponding to the same temperature increases. The measured voltage difference deviation is guaranteed to be within 5% within a certain distance range. The optimal measurement distance is 5 to 15 cm in the experiment.

    By using Matlab software to analyze and fit the measured data, the voltage-distance response curve is obtained, as shown in Figure 7.

    It can be analyzed that as the measuring distance increases, the voltage difference of the response gradually decreases. The farther the distance is, the more part of the infrared radiation is lost into the air and is not effectively received by the probe. Some system modules are shown in Figure 8.

Conclusion
    Based on a large number of literatures, a non-contact pyroelectric infrared thermometer based on 51 single-chip microcomputer is designed with pyroelectric infrared temperature measurement technology as a reference. It takes the black body radiation law as its theoretical basis and is the product of the comprehensive development of optical theory and microelectronics. Compared with traditional temperature measurement methods, it has the advantages of short response time, non-contact, no interference with the measured temperature field, and convenient reading.
    In general, the system designed in this paper has the following characteristics: using the pyroelectric effect, when measuring the human body temperature, there is no need to aim the instrument at the person to be measured, and the body temperature can be measured with high accuracy when the person passes by without obstacles. If the surface temperature of the human body exceeds the preset value, the alarm will automatically alarm, indicating that the person's body temperature is abnormal.
    At the same time, circuits and software for automatically counting the number of passers-by are also designed. Communication is established with the host computer via Bluetooth, and the number of pedestrians currently passing through is displayed in real time on the host computer, which can help the detector to grasp various data in real time.