Pyroelectric infrared detectors, or simply pyroelectric detectors, are a new type of thermal detector that has seen significant development in the field of thermal detection over the past decade. These detectors are reportedly widely used in fields such as radiation temperature measurement and infrared spectroscopy.
Infrared radiation has a wavelength of approximately 0.77–1000 pm, and different bands of infrared radiation have different applications. From military applications such as infrared-guided missiles and infrared imaging night vision devices, to infrared meteorological satellites incorporating cutting-edge technologies, from industrial applications such as infrared photoelectric counters, infrared thermometers, and infrared gas analyzers, to civilian applications such as wave-type infrared burglar alarms and automatic infrared human body lighting switches, infrared sensing technology has been widely used in various fields including space, industry, agriculture, and civilian use. Pyroelectric infrared detectors, or simply pyroelectric detectors, are a new type of thermal detector that has seen significant development in the field of thermal detection over the past decade. It has been reported that this type of detector can be widely used in radiation temperature measurement, infrared spectroscopy measurement, laser parameter measurement, non-contact temperature measurement, automatic monitoring of industrial processes, security alerts, infrared imaging, and space technology. Although my country's research in this area started later, significant research results have been reported in recent years. Compared with commonly used thermal detectors such as radiation calorimeters and thermoelectric piles, this type of detector has the following characteristics:
It has good frequency characteristics. Other thermal detectors output maximum value after reaching thermal equilibrium, and the irradiation time during operation must be greater than the thermal equilibrium time constant (typically several milliseconds to tens of milliseconds). Pyroelectric detectors, however, operate in a non-thermal equilibrium state, and have no output at thermal equilibrium. Therefore, the irradiation time during operation must be less than the thermal equilibrium time constant (typically 0.1 to 1 second). In other words, its operating speed is not limited by thermal equilibrium; its upper frequency limit mainly depends on its equivalent capacitance and subsequent circuitry.
It operates at room temperature and achieves high sensitivity without the need for refrigeration, comparable to high-sensitivity radiometric calorimeters operating at low temperatures.
The output impedance is purely capacitive, with extremely high DC impedance.
Small in size, light in weight, and sturdy.
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 an infrared thermometer is based on the blackbody radiation law. As is well known, all objects in nature above absolute zero continuously radiate energy. The amount of energy radiated and the distribution of its wavelengths are closely related to its surface temperature; the higher the temperature of an object, the stronger its infrared radiation. The monochromatic radiant exitance of a blackbody at temperature T at wavelength λ is determined by Planck's formula.
Pyroelectric infrared sensors consist of ceramic oxide or piezoelectric crystal elements, with electrodes formed on two surfaces. When the temperature changes by a value T within the sensor's monitoring range, the pyroelectric effect generates charges on the two electrodes, resulting in a weak voltage V between them. The pyroelectric detector detects the temperature change, which is then converted into an AC voltage signal by photoelectric conversion for processing by the signal processing circuit. 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; ε2 is the emissivity of the environment; σ is the Stefan-Boltzmann constant, and σ = 5.6703 × 10⁻⁸ W·m⁻²·K⁻⁴; K represents the sensitivity of the detector, and has
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; and α is a constant related to the atmospheric attenuation distance.
2. Overall Design of Temperature Measurement System
The infrared thermometer system mainly consists of an optical system, photoelectric conversion, signal processing, and display output. The optical system determines the field of view. The pyroelectric detector converts the infrared energy focused on the detector into an electrical signal. After amplification and filtering, the signal is converted from analog to digital and sent to the microcontroller for signal processing. The LCD display unit shows the temperature value of the measured target.
3. Signal Processing Circuit Design
When the light signal passes through the pyroelectric sensor, it becomes an alternating pulse signal. The human radiation signal received by the pyroelectric sensor is very weak, only on the order of microvolts or nanomicrovolts, so it needs to be amplified before signal processing can be performed.
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 internally includes two independent, high-gain, internally frequency-compensated dual operational amplifiers. It is suitable for single-supply applications with a wide power supply voltage range, and also for dual-supply operation. The system's signal amplification circuit is shown in Figure 1.
The LM358 operational amplifier is characterized by low offset, low noise, and low drift, and is widely used in precision instrument amplifiers, sensor amplifiers, and other applications. The infrared sensor signal enters the amplification circuit through pin 3 of the LM358. Capacitor C1 filters out DC signals from the signal, and the adjustable potentiometer Rv1 adjusts the gain of the sensor output signal. The amplified infrared sensor signal is then sent to the signal acquisition circuit unit for analog-to-digital conversion.
