Pyroelectric sensor principle and application - Database & Sql Blog Articles

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I. Introduction Pyroelectric infrared sensor is a very promising sensor. It detects infrared rays emitted by people or certain animals and converts them into electrical signal outputs. As early as 1938, it was proposed to use the pyroelectric effect to detect infrared radiation, but it was not taken seriously. Until the 1960s, with the rapid development of laser and infrared technology, the research on the pyroelectric effect and the application development of pyroelectric crystals were promoted. In recent years, with the rapid development of integrated circuit technology and in-depth study of the characteristics of the sensor, related ASIC processing technology has also grown rapidly. This article first introduces the principle of pyroelectric sensors, and then describes the related ASIC processing technology.

2. Pyroelectric effect When some crystals are heated, an equal number of oppositely charged charges will be generated at both ends of the crystal. This phenomenon of polarization due to thermal changes is called pyroelectric effect. Usually, the bound charge generated by the spontaneous polarization of the crystal is neutralized by free electrons from the air attached to the surface of the crystal, and the spontaneous polarization electric moment cannot be expressed. When the temperature changes, the center of gravity of the positive and negative charges in the crystal structure is relatively shifted, the spontaneous polarization changes, the charge is depleted on the surface of the crystal, and the state of charge depletion is proportional to the degree of polarization. Figure 1 shows the pyroelectricity. The principle of effect formation.


A crystal that can generate a pyroelectric effect is called a pyroelectric or pyroelectric element, and its commonly used materials are single crystal (LiTaO3, etc.), piezoelectric ceramic (PZT, etc.), and polymer film (PVFZ, etc.).
The pyroelectric sensor utilizes the pyroelectric effect and is a temperature sensitive sensor. It consists of ceramic oxide or piezoelectric crystal elements. The two surfaces of the element are made into electrodes. When the temperature of the sensor is within the range of ΔT, the pyroelectric effect will produce a charge ΔQ on the two electrodes. A weak voltage ΔV is generated between the electrodes. Because of its extremely high output impedance, there is a FET in the sensor for impedance transformation. The charge ΔQ generated by the pyroelectric effect disappears in combination with ions in the air. When the ambient temperature is stable, ΔT=0, and the sensor has no output. When the human body enters the detection zone, the human body temperature and the ambient temperature are different, and ΔT is generated, and there is a signal output; if the human body does not move after entering the detection zone, the temperature does not change, and the sensor does not output, so the sensor can detect the human body. Or animal activities. The structure and internal circuit of the pyroelectric infrared sensor are shown in Figure 2. The sensor mainly consists of a casing, a filter, a pyroelectric element PZT, and a field effect transistor FET. Among them, the filter is arranged at the window to form a window through which the infrared rays pass. The filter is a 6mm multilayer film interference filter that filters out short wavelengths (about 5mm or less) for sunlight and fluorescent light. The pyroelectric element PZT converts the weak change of the infrared signal with a wavelength between 8mm and 12mm into an electrical signal. In order to be sensitive only to the infrared radiation of the human body, the radiation surface of the pyroelectric element is usually covered with a special Fresnel filter. The environmental interference is significantly inhibited.



The Fresnel lens (Fig. 3) is made according to the Fresnel principle, which divides the infrared light into a visible region and a blind region, and at the same time has a focusing effect, so that the sensitivity of the pyroelectric human body infrared sensor (PIR) is greatly increased. Fresnel lens refracting and reflecting two forms, the role of which is the focusing effect, the refraction of the infrared signal is reflected (reflected) on the PIR; the second is to divide the detection area into a number of bright and dark areas, The moving object entering the detection zone can generate a modified pyroelectric infrared signal on the PIR in the form of a temperature change, so that the PIR can generate a varying electrical signal.
If we connect the appropriate resistance to the thermoelectric component, when the component is heated, a current flows through the resistor, and a voltage signal is obtained at both ends.
Third, passive pyroelectric infrared {% sensor%} working principle and characteristics In the natural world, any object above the absolute temperature (-273K) will produce infrared spectrum, the wavelength of infrared energy released by different temperature objects is not the same Therefore, the infrared wavelength is related to the temperature, and the amount of radiant energy is related to the surface temperature of the object.
The human body has a constant body temperature, generally around 37 ° C, will emit about 10mm of specific wavelength infrared, passive infrared probe is to detect the infrared light emitted by the human body to work. The infrared rays are reinforced by the Fresnel filter and then collected into the pyroelectric element. When receiving the infrared radiation of the human body, the element loses the charge balance, releases the charge outward, and then generates an alarm signal after the detection process. Passive infrared probe, whose sensor contains two pyroelectric elements connected in series or in parallel, and the two electrodes are made in the opposite direction (Fig. 2 side view C). The environmental background radiation has almost two thermal release elements. The same effect, so that the effect of the discharge of electricity cancels each other, so the detector has no signal output.
4. The advantages and disadvantages of the passive pyroelectric infrared probe are different from the active infrared sensor. The passive infrared sensor itself does not emit any type of radiation, and has good concealment, low power consumption and low price. However, passive pyroelectric sensors also have disadvantages such as:
1 The signal amplitude is small, and it is easy to be interfered by various heat sources and light sources;
2 Passive infrared penetration is poor, the infrared radiation of the human body is easily blocked, and is not easily received by the probe;
3 susceptible to interference from radio frequency radiation;
4 When the ambient temperature and the human body temperature are close, the detection and sensitivity are significantly reduced, sometimes causing short-term failure;
5 The main detection direction of the passive infrared detector is the direction of lateral motion, and the detection ability of objects moving in the radial direction is relatively poor.
V. Pyroelectric Infrared Probe Processing Chip Principle and Application Although passive pyroelectric infrared probe has some shortcomings, it still has broad application prospects in some fields after using special signal processing methods. Therefore, many manufacturers have designed dedicated signal processors based on the characteristics of PIR sensors, such as HOLTEK HT761X, PTI PT8A26XXP, WELTREND WT8072, BISS0001. This article introduces an application example of PTI (Bai Li Tong Electronics Co., Ltd.) special chip PT8A26XXP.
The shaded portion of Figure 4 is the PIR signal processing section with two operational amplifiers, a window comparator, a voltage regulator, a system oscillator, and a logic controller. The other is the control part that depends on the processing result. Here, the PIR signal processing part is mainly introduced, and the control part is simply skipped.
Due to the slow change of PIR signal and small amplitude, the special signal processor is generally divided into three steps for this feature. The specific processing steps are as follows:
1 filter amplification ordinary PIR sensor output signal amplitude is generally very small, about several hundred microvolts to a few millivolts, in order to effectively process the subsequent circuit, taking into account the signal-to-noise ratio of the sensor, usually take the gain of 72.5dB, passband 0.3 Hz to 7 Hz. At the same time, since the analog small signal is processed, in order to ensure the stable and reliable operation of the amplifier, a voltage regulator is specially integrated in the circuit for supplying power to the sensor, the amplifier and the comparator.
2 The window comparator after the amplified signal passes through the window comparator and detects the signal satisfying the amplitude requirement, and then converts into a series of digital pulse signals.



3 Noise suppression digital signal processing According to the long-term research on the characteristics of human motion and the characteristics of the sensor, the effective number of pulses and the width of the pulse are measured in a fixed time to identify the effective human body signal. Here, the clock source is provided by the system oscillator (16 kHz). ). The specific discriminating method is as follows: The discriminating operation is limited to 2s; the pulse width less than 24ms is counted as noise and is not processed; the single effective pulse: the width must be greater than 340 ms; the double pulse, where the width must be greater than 160ms, and the narrower is greater than 24ms; three pulses are active, each must be greater than 24ms.
After the above three steps, the human body signal can be accurately and reliably judged. According to the specific application, the established control, such as the automatic alarm of the alarm, automatically turns on a certain device. The PT8A26XX series is mainly used for automatic delay switches, in which the delay is adjustable, and it can also be set to not work during the day. In addition, several other company's processor functions are basically similar, and they are widely used in the field of energy conservation.

VI. Conclusion With the continuous improvement of the performance and reliability of related signal processors, pyroelectric crystals have been widely used in infrared spectrometers, infrared remote sensing and thermal radiation detectors, and are widely used by users and professionals because of their low price and stable technical performance. The popularity of the people is widely used in various automatic control devices. It can be used as an ideal detector for infrared lasers, and can also be applied to infrared fields such as burglar alarms, visitor notifications and non-contact switches. In addition to being used in the well-known ramp automatic switch and burglar alarm, the application prospects in more fields are promising.