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. 1. 人體熱釋電紅外傳感器PIR原理詳解 在電子防盜、人體探測器領(lǐng)域中，被動式熱釋電紅外探測器的應用非常廣泛，因其價格低廉、技術(shù)性能穩(wěn)定而受到廣大用戶和專業(yè)人士的歡迎。 　　被動式熱釋電紅外探頭的工作原理及特性： 　　人體都有恒定的體溫，一般在37度，所以會發(fā)出特定波長10μm左右的紅外線，被動式紅外探頭就是靠探測人體發(fā)射的10μm左右的紅外線而進行工作的。人體發(fā)射的10μm 左右的紅外線通過菲涅爾濾光片增強后聚集到紅外感應源上。紅外感應源通常采用熱釋電元件，這種元件在接收到人體紅外輻射溫度發(fā)生變化時就會失去電荷平衡，向外釋放電荷，后續(xù)電路經(jīng)檢測處理后就能產(chǎn)生報警信號。 　　(1)這種探頭是以探測人體輻射為目標的。所以熱釋電元件對波長為10μm 左右的紅外輻射必須非常敏感。 　　(2)為了僅僅對紅外輻射敏感，在它的輻射照面通常覆蓋有特殊的菲涅爾濾光片，使環(huán)境的干擾受到明顯的控制作用。 　　(3)被動紅外探頭，其傳感器包含兩個互相串聯(lián)或并聯(lián)的熱釋電元。而且制成的兩個電極化方向正好相反，環(huán)境背景輻射對兩個熱釋元件幾乎具有相同的作用，使其產(chǎn)生釋電效應相互抵消，于是探測器無信號輸出。 　　(4)一旦人侵入探測區(qū)域內(nèi)，人體紅外輻射通過部分鏡面聚焦，并被熱釋電元接收，但是兩片熱釋電元接收到的熱量不同，熱釋電也不同，不能抵消，經(jīng)信號處理而報警。 　　(5)菲涅爾濾光片根據(jù)性能要求不同，具有不同的焦距(感應距離)，從而產(chǎn)生不同的監(jiān)控視場，視場越多，控制越嚴密。 　　被動式熱釋電紅外探頭的優(yōu)缺點： 　　優(yōu)點： 　　本身不發(fā)任何類型的輻射，器件功耗很小，隱蔽性好。價格低廉。 　　缺點： 　　◆容易受各種熱源、光源干擾 　　◆被動紅外穿透力差，人體的紅外輻射容易被遮擋，不易被探頭接收。 　　◆易受射頻輻射的干擾。 　　◆環(huán)境溫度和人體溫度接近時，探測和靈敏度明顯下降，有時造成短時失靈。 　　抗干擾性能： 　　1.防小動物干擾 　　探測器安裝在推薦地使用高度，對探測范圍內(nèi)地面上地小動物，一般不產(chǎn)生報警。 　　2.抗電磁干擾 　　探測器的抗電磁波干擾性能符合GB10408中4.6.1要求，一般手機電磁干擾不會引起誤報。 　　3.抗燈光干擾 　　探測器在正常靈敏度的范圍內(nèi)，受3米外H4鹵素燈透過玻璃照射，不產(chǎn)生報警。 　　紅外線熱釋電傳感器的安裝要求： 　　紅外線熱釋電人體傳感器只能安裝在室內(nèi)，其誤報率與安裝的位置和方式有極大的關(guān)系，正確的安裝應滿足下列條件： 　　1.紅外線熱釋電傳感器應離地面2.0-2.2米。 　　2.紅外線熱釋電傳感器遠離空調(diào), 冰箱，火爐等空氣溫度變化敏感的地方。 　　3.紅外線熱釋電傳感器探測范圍內(nèi)不得隔屏、家具、大型盆景或其他隔離物。 　　4.紅外線熱釋電傳感器不要直對窗口，否則窗外的熱氣流擾動和人員走動會引起誤報，有條件的最好把窗簾拉上。紅外線熱釋電傳感器也不要安裝在有強氣流活動的地方。 　　紅外線熱釋電傳感器對人體的敏感程度還和人的運動方向關(guān)系很大。熱釋電紅外傳感器對于徑向移動反應最不敏感, 而對于橫切方向 (即與半徑垂直的方向)移動則最為敏感. 在現(xiàn)場選擇合適的安裝位置是避免紅外探頭誤報、求得最佳檢測靈敏度極為重要的一環(huán)。 2. 菲涅爾透鏡 2.1 概述 菲涅爾透鏡多是由聚烯烴材料注壓而成的薄片，鏡片表面一面為光面，另一面刻錄了由小到大的同心圓。菲涅爾透鏡的在很多時候相當于紅外線及可見光的凸透鏡，效果較好，但成本比普通的凸透鏡低很多。菲涅爾透鏡可按照光學設計或結(jié)構(gòu)進行分類。菲涅爾透鏡作用有兩個：一是聚焦作用；二是將探測區(qū)域內(nèi)分為若干個明區(qū)和暗區(qū)，使進入探測區(qū)域的移動物體能以溫度變化的形式在PIR（被動紅外線探測器）上產(chǎn)生變化熱釋紅外信號。 2.2 作用 菲涅爾透鏡利用透鏡的特殊光學原理，在探測器前方產(chǎn)生一個交替變化的“盲區(qū)”和“高靈敏區(qū)”，以提高它的探測接收靈敏度。當有人從透鏡前走過時，人體發(fā)出的紅外線就不斷地交替從“盲區(qū)”進入“高靈敏區(qū)”，這樣就使接收到的紅外信號以忽強忽弱的脈沖形式輸入，從而強其能量幅度。 菲涅爾透鏡 菲涅爾透鏡，簡單的說就是在透鏡的一側(cè)有等距的齒紋，通過這些齒紋，可以達到對指定光譜范圍的光帶通(反射或者折射)的作用。傳統(tǒng)的打磨光學器材的帶通光學濾鏡造價昂貴。菲涅爾透鏡可以極大的降低成本。 典型的例子就是PIR。PIR廣泛的用在警報器上。如果你拿一個看看，你會發(fā)現(xiàn)在每個PIR上都有個塑料的小帽子。這就是菲涅爾透鏡。小帽子的內(nèi)部都刻上了齒紋。這種菲涅爾透鏡可以將入射光的頻率峰值限制到10微米左右（人體紅外線輻射的峰值）。 菲涅耳透鏡可以把透過窄帶干涉濾光鏡的光聚焦在硅光電二級探測器的光敏面上，菲涅爾透鏡不能用任何有機溶液(如酒精等)擦拭，除塵時可先用蒸餾水或普通凈水沖洗，再用脫脂棉擦拭。 3. How Infrared motion detector components work Infrared Radiation_____________ Infrared radiation exists in the electromagnetic spectrum at a wavelength that is longer than visible light. It cannot be seen but it can be detected. Objects that generate heat also generate infrared radiation and those objects include animals and the human body whose radiation is strongest at a wavelength of 9.4um. Infrared in this range will not pass through many types of material that pass visible light such as ordinary window glass and plastic. However it will pass through, with some attenuation, material that is opaque to visible light such as germanium and silicon. An unprocessed silicon wafer makes a good IR window in a weatherproof enclosure for outdoor use. It also provides additional filtering for light in the visible range. 9.4um infrared will also pass through polyethylene which is usually used to make Fresnel lenses to focus the infarared onto sensor elements. Pyroelectric Sensors_____________ The pyroelectric sensor is made of a crystalline material that generates a surface electric charge when exposed to heat in the form of infrared radiation. When the amount of radiation striking the crystal changes, the amount of charge also changes and can then be measured with a sensitive FET device built into the sensor. The sensor elements are sensitive to radiation over a wide range so a filter window is added to the TO5 package to limit detectable radiation to the 8 to 14mm range which is most sensitive to human body radiation. Typically, the FET source terminal pin 2 connects through a pulldown resistor of about 100 K to ground and feeds into a two stage amplifier having signal conditioning circuits. The amplifier is typically bandwidth limited to below 10Hz to reject high frequency noise and is followed by a window comparator that responds to both the positive and negative transitions of the sensor output signal. A well filtered power source of from 3 to 15 volts should be connected to the FET drain terminal pin 1. The PIR325 sensor has two sensing elements connected in a voltage bucking configuration. This arrangement cancels signals caused by vibration, temperature changes and sunlight. A body passing in front of the sensor will activate first one and then the other element whereas other sources will affect both elements simultaneously and be cancelled. The radiation source must pass across the sensor in a horizontal direction when sensor pins 1 and 2 are on a horizontal plane so that the elements are sequentially exposed to the IR source. A focusing device is usually used in front of the sensor The figure below shows the PIR325 electrical specifications and layout in its TO5 package. Note the wide viewing angle without an external lens. This is a typical application circuit that drives a relay. R10 and C6 adjust the amount of time that RY1 remains energized after motion is detected. Download PDF drawing. Fresnel Lens_____________ A Fresnel lens (pronounced Frennel) is a Plano Convex lens that has been collapsed on itself to form a flat lens that retains its optical characteristics but is much smaller in thickness and therefore has less absorption losses. Our FL65 Fresnel lens is made of an infrared transmitting material that has an IR transmission range of 8 to 14um which is most sensitive to human body radiation. It is designed to have its grooves facing the IR sensing element so that a smooth surface is presented to the subject side of the lens which is usually the outside of an enclosure that houses the sensor. The lens element is round with a diameter of 1 inch and has a flange that is 1.5 inches square. This flange is used for mounting the lens in a suitable frame or enclosure. Mounting can best and most easily be done with strips of Scotch tape. Silicone rubber can also be used if it overlaps the edges to form a captive mount. The FL65 has a focal length of 0.65 inches from the lens to the sensing element. It has been determined by experiment to have a field of view of approximately 10 degrees when used with a PIR325 Pyroelectric sensor. This relatively inexpensive and easy to use Pyroelectric Sensor and Fresnel Lens can be used in a variety of science projects, robots and other useful devices. 3.1 Focusing devices for pyroelectric infrared sensors A.1 Pyroelectric infrared sensors A.2 First we will look at a pyroelectric infrared sensor and see how it is made and why a focusing device is necessary. A commonly used pyroelectric infrared sensor has two sensing elements internally connected in a voltage bucking configuration. A pyroelectric sensor has an infrared filter window that admits IR within the 5 to 15 micrometer wavelength range. One end of the two series-connected elements in an analog sensor is connected to pin 3 that is normally grounded. The other end connects internally to the gate of a Field Effect Transistor and to a very high value pulldown resistor. Power is applied to FET drain pin 1 and the output signal comes from FET source pin 2 which usually connects through an external pulldown resistor to ground and to an amplifier. A digital sensor not shown here, includes internal processing circuits and outputs digital pulses. A.3 The sensor is housed in a TO5 type package. Sensing elements are each 0.039 inch (1mm) wide and are spaced 0.039 inch (1mm) apart. A.4 Environmental conditions such as temperature changes and sunlight will affect both elements simultaneously and will produce the same amount of output from each element but of opposing polarity and will therefore be cancelled. The sensor will only produce a change in its output voltage when one of its elements is exposed to a change in radiation and the other is not exposed. An IR emitting body moving across the front of a sensor will expose first one, then both and then the other sensor element. The output signal waveform from an analog sensor shows that for motion in one direction, first a positive, then zero and then a negative transition results. Motion in the other direction will produce first a negative, then zero and then a positive transition. A.5 When a lens is not used in front of a sensor and an IR emitting body is close to the sensor, about 3 or 4 feet and it moves across the front of the sensor, the radiated IR will expose one element more than the other and a voltage output will result. However, when the IR emitting body is further away from the sensor its radiation pattern becomes blurred and both elements are exposed more equally, resulting in no voltage output. The limited detection range is due to a lack of unequal exposure. Placing a lens in front of the sensor extends its detection range. A.6 The Fresnel lens A.7 A Fresnel lens is a Plano Convex lens that has been collapsed on itself to form a flat lens that retains its optical characteristics but is much thinner and therefore has less absorption loss. A Fresnel lens is usually thin and flexible and is about 0.015 inch (0.38mm) thick with grooves molded into one surface. The groove side of the lens usually faces the PIR sensor. A Fresnel lens both captures more IR radiation and focuses it to a small point. This focal point moves across the sensor as the IR source moves and exposes one element at a time. A Fresnel lens can extend detection range to about 100 feet. A Fresnel lens will give the best possible performance, however other devices can be used to extend range. Although the following devices may not fit the description of a lens, we will call them lenses anyway. This diagram shows IR exposing both elements equally when no lens is used. Shadow lens Since simultaneous exposure of both elements is the cause of limited detection range, all we need is some method of preventing the IR from exposing both elements simultaneously as the IR emitting body moves across the front of the sensor, even at greater distances from the sensor. The sensor elements are 0.039 inch (1mm) wide and are spaced 0.039 inch (1mm) apart. If we place a thin vertical strip of IR opaque material about 0.060 inch (1.5mm) wide centered in front of the sensor we can prevent some of the IR from striking the surface of the sensor by producing a shadow, even if the IR emitting body is at a greater distance from the sensor. The following figure shows such a baffle but in this example the IR still exposes both elements equally. By placing a baffle or mask in front of the sensor, we can block some of the IR and produce a shadow on the sensor This next figure shows what happens when the IR emitting body moves across the front of the sensor even at greater distances. The baffle allows full exposure of element 1 while blocking the IR so it produces a shadow over element 2 and does not expose it. This simple lens extends detection range up to 20 feet and is easily made from a strip of paper or other material. The baffle can be spaced 1/2 inch (12mm) to 1 inch (25mm) in front of the sensor. The greater spacing gives a narrower detection angle. As the IR source moves, the baffle blocks it from exposing one of the sensor elements while fully exposing the other. This same idea can be expanded to produce a wide angle lens. Multiple baffle strips can be placed in front of the sensor to alternately block IR from one sensor element at a time even when the IR emitting body is at greater angles to the front of the sensor. This multiple baffle shadow lens provides a wide detection angle. The lens should be curved so all baffles are the same distance from the front of the sensor. Pinhole lens Another simple focusing device that will expose one PIR sensor element at a time can be made using a thin piece of IR opaque material with a hole in it that will function as a pinhole lens similar to the lens of a pinhole camera. The function of a pinhole camera lens is described in http://science.howstuffworks.com/question131.htm . The lens hole in a camera is usually small but can be much larger in a sensor application where image detail is not necessary and only the IR radiation from the animal must be focused onto one sensing element at a time. IR in the 5 ~ 15 micrometer wavelength range will not pass through most materials so you can make a lens of paper, plastic or metal foil. The hole should be about inch (6.4mm) diameter. Lens spacing is not critical but it should be about inch (12mm) to 1 inch (25mm) from the front of the sensor. A detection range of up to 20 feet can be obtained with this type of lens. Although a narrow field of view is usually most desirable for animal photos, a wide field of view can be obtained by placing several holes in the lens material and curving the lens in front of the sensor so all holes are an equal distance from the front of the sensor. Another type of lens can be made by placing a tube about 3/8 inch (10mm) diameter and 2 inch (50mm) long over the front of the sensor. This lens will have a narrow field of view. Infrared window A pyroelectric sensor is very sensitive to rapid temperature changes. A rapid change in air temperature due to a breeze from an open window or from an air conditioner or heater can cause false triggering. The shadow and pinhole lenses described above do not offer protection against such air movement because they are actually open lenses. For outdoor use you would also need protection against rain. An IR transparent weatherproof shield or window can be made of polyethylene which is transparent to IR radiation in the 5 ~ 15 micrometer wavelength range. Polyethylene can be identified by burning a small piece. It will burn with a mostly blue flame and melt like wax while other plastics that are opaque to IR will burn with a smoky red flame and leave a black ash. The only problem with polyethylene is that it is waxy and most adhesives will not bond to it but you can hold it in place with scotch tape, silicone rubber or hot melt glue. Plastic milk bottles are made of polyethylene, and although it is whitish and not visibly transparent, it is transparent to IR with some small loss in detection range. The polyethylene milk bottle is thin, easily cut with scissors and makes a good IR window. Many food containers and plastic sheeting are also made of polyethylene. A thinner window will have less absorption loss. .