人体热释电红外传感器PIR原理

.1. 人体热释电红外传感器PIR原理详解在电子防盗、人体探测器领域中，被动式热释电红外探测器的应用非常广泛，因其价格低廉、技术性能稳定而受到广大用户和专业人士的欢迎。被动式热释电红外探头的工作原理及特性：人体都有恒定的体温，一般在37度，所以会发出特定波长10m左右的红外线，被动式红外探头就是靠探测人体发射的10m左右的红外线而进行工作的。人体发射的10m 左右的红外线通过菲涅尔滤光片增强后聚集到红外感应源上。红外感应源通常采用热释电元件，这种元件在接收到人体红外辐射温度发生变化时就会失去电荷平衡，向外释放电荷，后续电路经检测处理后就能产生报警信号。(1)这种探头是以探测人体辐射为目标的。所以热释电元件对波长为10m 左右的红外辐射必须非常敏感。(2)为了仅仅对红外辐射敏感，在它的辐射照面通常覆盖有特殊的菲涅尔滤光片，使环境的干扰受到明显的控制作用。(3)被动红外探头，其传感器包含两个互相串联或并联的热释电元。而且制成的两个电极化方向正好相反，环境背景辐射对两个热释元件几乎具有相同的作用，使其产生释电效应相互抵消，于是探测器无信号输出。(4)一旦人侵入探测区域内，人体红外辐射通过部分镜面聚焦，并被热释电元接收，但是两片热释电元接收到的热量不同，热释电也不同，不能抵消，经信号处理而报警。(5)菲涅尔滤光片根据性能要求不同，具有不同的焦距(感应距离)，从而产生不同的监控视场，视场越多，控制越严密。被动式热释电红外探头的优缺点：优点：本身不发任何类型的辐射，器件功耗很小，隐蔽性好。价格低廉。缺点：容易受各种热源、光源干扰被动红外穿透力差，人体的红外辐射容易被遮挡，不易被探头接收。易受射频辐射的干扰。环境温度和人体温度接近时，探测和灵敏度明显下降，有时造成短时失灵。抗干扰性能：1.防小动物干扰探测器安装在推荐地使用高度，对探测范围内地面上地小动物，一般不产生报警。2.抗电磁干扰探测器的抗电磁波干扰性能符合GB10408中4.6.1要求，一般手机电磁干扰不会引起误报。3.抗灯光干扰探测器在正常灵敏度的范围内，受3米外H4卤素灯透过玻璃照射，不产生报警。红外线热释电传感器的安装要求：红外线热释电人体传感器只能安装在室内，其误报率与安装的位置和方式有极大的关系，正确的安装应满足下列条件：1.红外线热释电传感器应离地面2.0-2.2米。2.红外线热释电传感器远离空调, 冰箱，火炉等空气温度变化敏感的地方。3.红外线热释电传感器探测范围内不得隔屏、家具、大型盆景或其他隔离物。4.红外线热释电传感器不要直对窗口，否则窗外的热气流扰动和人员走动会引起误报，有条件的最好把窗帘拉上。红外线热释电传感器也不要安装在有强气流活动的地方。红外线热释电传感器对人体的敏感程度还和人的运动方向关系很大。热释电红外传感器对于径向移动反应最不敏感, 而对于横切方向 (即与半径垂直的方向)移动则最为敏感. 在现场选择合适的安装位置是避免红外探头误报、求得最佳检测灵敏度极为重要的一环。2. 菲涅尔透镜2.1 概述菲涅尔透镜多是由聚烯烃材料注压而成的薄片，镜片表面一面为光面，另一面刻录了由小到大的同心圆。菲涅尔透镜的在很多时候相当于红外线及可见光的凸透镜，效果较好，但成本比普通的凸透镜低很多。菲涅尔透镜可按照光学设计或结构进行分类。菲涅尔透镜作用有两个：一是聚焦作用；二是将探测区域内分为若干个明区和暗区，使进入探测区域的移动物体能以温度变化的形式在PIR（被动红外线探测器）上产生变化热释红外信号。2.2 作用菲涅尔透镜利用透镜的特殊光学原理，在探测器前方产生一个交替变化的“盲区”和“高灵敏区”，以提高它的探测接收灵敏度。当有人从透镜前走过时，人体发出的红外线就不断地交替从“盲区”进入“高灵敏区”，这样就使接收到的红外信号以忽强忽弱的脉冲形式输入，从而强其能量幅度。菲涅尔透镜菲涅尔透镜，简单的说就是在透镜的一侧有等距的齿纹，通过这些齿纹，可以达到对指定光谱范围的光带通(反射或者折射)的作用。传统的打磨光学器材的带通光学滤镜造价昂贵。菲涅尔透镜可以极大的降低成本。典型的例子就是PIR。PIR广泛的用在警报器上。如果你拿一个看看，你会发现在每个PIR上都有个塑料的小帽子。这就是菲涅尔透镜。小帽子的内部都刻上了齿纹。这种菲涅尔透镜可以将入射光的频率峰值限制到10微米左右（人体红外线辐射的峰值）。菲涅耳透镜可以把透过窄带干涉滤光镜的光聚焦在硅光电二级探测器的光敏面上，菲涅尔透镜不能用任何有机溶液(如酒精等)擦拭，除尘时可先用蒸馏水或普通净水冲洗，再用脱脂棉擦拭。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 sensorThe 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 sensorsA.1 Pyroelectric infrared sensorsA.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 lensA.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 lensSince 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 sensorThis 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 lensAnother 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 windowA 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.