1 Proximity Sensors and Motion Detectors Thomas Schlebusch Abstract In this paper I will present the principles of proximity and motion detectors. First I will give an overview about the physical variables which let us assume that something has appeared or moved and then I will sketch the working principles of sensors using those measured variables. As a final step I will provide an overview about the characteristics of each presented sensor and which sensor might be best for which application in attention to needs, complexity and cost. I I.INTRODUCTION N modern life sensing of motion and proximity is widely used. Think about automatic floor lights, hand dryers and water standpipes in public toilets, alarm systems or electronic sliding doors of a store entrance. Everyone of us has to deal with proximity sensors in his daily life. Even more crucial is the sensing of occupation in security applications, for example in areas where robots and humans work in parallel. In that field occupation sensing is used to stop the machine to protect the human worker. II.CHARACTERISTICS OF OCCUPATION There are many possibilities to measure the occupation or movement of an object. One property of an object entering the sensing area is that one can usual see it. This fact is used by most optical sensors, e.g. by using a TV camera and a image processing unit. Not only the visible light can be measured, thermal cameras and Passive Infrared (PIR) Detectors measure the radiation of a warm object (e.g. human or animal) in the invisible infrared spectrum. Besides that the object changes the reflection characteristics of radiation sent out by the sensor. This might be acoustic waves as used in ultrasonic applications, microwaves as in radar applications or infrared radiation as in active infrared sensors. Ultrasonic This paper is a handout form of the talk by Thomas Schlebusch in context of the lecture Sensors and Transducers at NTNU Trondheim, Norway, in October 2007. Author: Thomas Schlebusch, Student of Electrical Engineering at RWTH Aachen, Germany. e-mail: thomas@thomas-schlebusch.de and Radar Systems have the advantage that you can also measure the speed and moving direction of the object due to the doppler effect. In principle all three sensors send out a portion of radiation and measure how much they receive back by reflection. A processing unit can then determine changes in the received energy and assume the occupation of a new object. A similar principle is used by capacitive or inductive sensors. They rely on the fact that an object entering free space has a different dielectric constant or permeability than air. In case of a human mostly in the area of dielectricity and permeability of water. This change can be measured and used for sensing. Besides of those main concepts there are many further possibilities that I don't want to mention in detail here. In model railways conductance changes can be used to determine if a train has entered a specific railway section because the axis shortens the left and the right rail. Alternatively a reed contact could be used: A magnet mounted underneath the train closes the reed contact as moving by. In other applications e.g. in control of illegal immigration at the Canadian border chemical sensors are used to measure the CO 2 concentration or to detect urine in cargo containers (see [1]). A. Opacity sensing III.OPTICAL MEASUREMENTS A very simple and cheap proximity sensor using optical measurements is a light barrier. It consists of a light emitting source on one side of the barrier, most often a source of invisible infrared light, and a detector on the other side of the barrier. When an object crosses the barrier the light beam is interrupted and an alarm is triggered. This system is very simple and has a very narrow sensing area (light beam) that makes it only useful in few cases. B. Visible Light A much broader sensing field can be gained by detecting changes in a projected image in a camera. The first systems for motion detection in
2 visible light range consisted of a lens that focused an image of the sensing area onto a photoresistive element. When contrast in the projected image changed, the electrical current through the sensor changed and could be sensed. Today a motion-picture camera is used in junction with a image processing unit is used in most applications. Depending on the power of the processing unit it is not only possible to detect the occupation of a new object but also to identify it (human, animal, moving tree in the wind, car) and to track the object (Fig. 1). These systems are mostly used in combination with a video cassette recorder that starts recording when an object enters the sensing area. If you are interested in this field a huge amount of code sniplets and examples are available on the internet that you can use for example with your webcam connected to a personal computer with e.g. matlab installed. The advantage of such a system is that it is possible to recognize objects and thus minimize the number of false alarms. On the other hand it does not work in darkness or needs an additional illumination, e.g. by an invisible infrared light source as used in many night-view cameras. C.Passive Infrared Sensors Passive Infrared Sensors, also known as PIR sensors or pyroelectric sensors detect the radiation of warm objects in the far infrared spectral range (radiation of a human body is between 9µm and 14µm wavelength). Pyroelectricity is the ability of certain materials to generate an electrical potential when they are heated or cooled [2]. Thus the appearance of an object that has a different temperature than its surroundings can be detected as an electrical potential change of a pyroelectric sensor. The infrared radiation of, e.g. the human body, enters the casing of the sensors through a fresnel lens that focuses the radiation at the pyroelectric sensor (Fig. 2). A fresnel lens (Fig. 3) is used to minimize the attenuation of the infrared radiation because it is much thinner but has the same effect. Because normal glass is opaque in the spectral range of the far infrared radiation, a special plastic is used as the material of the lens. Fig. 1: Example screenshot of a computer based motion detector [4] Fig. 2: Assembly of a PIR sensor [3] Fig. 3: Fresnel lens [3]. Pyroelectric material has the property that the electrical charge does only change when the projection of the warm object enters the sensor area or leaves it. But movement of the projection of the warm object on the pyroelectric surface does not produce any change in electrical charge and is thus not sensed. To avoid this blindness of PIR sensors, the pyroelectric sensing area is divided into smaller pieces or covered by a grid aperture such that a constantly moving object is
3 projected as an alternating image and thus generating an alternating charge that can be sensed. An other effect one has to deal with is, that also temperature changes of the surroundings (like a house wall cooling down in the evening) produces a change in electrical charge. A second problem is, that all pyroelectric materials are also piezoelectric. Thus mechanical stress in the sensor case (maybe due to thermal expansion) does also produce a change in electrical charge. These problems can be avoided by using a postprocessing circuitry that only triggers alarm when the electrical charge changes rapidly. Alternatively a more common way is to use pairs of pyroelectric sensors. Mechanical stress and temperature change of the surroundings affects both sensors of a pair while motion of objects does mainly affect only one of the sensors. D. Application fields of optical sensors Optical sensors are most often used in burglary alarms and other security applications. Especially in public buildings PIR sensors are used for light regulation so that e.g. light on the toilet is only switched on when it is needed, namely when someone enters the room. IV.CAPACITIVE MEASUREMENTS Another concept is to measure a change in the dielectric constant by measuring a change in capacitance. In usual applications a capacitor is formed between two metal plates. Usually the free space between both plates is filled by air (Fig. 4). Fig. 5: Measurement of capacitance with human in sensing area The resulting capacitance is higher due to the series connection of the two capacitances Ca and Cb. The change of capacitance can be sensed by measuring the change in the time constant τ = RC A simple circuit would feed an oscillating voltage over a resistor into two capacitors: The one capacitor is built by the two plates in the sensing area and the second one is a trimmable condensator with the same capacitance if nothing is in between the two plates. Then the two voltages over both capacitors can be substracted with an operational amplifier and then feed into a comparator. If the capacitance of both capacitors is equal, the resulting voltage is zero. If something enters the sensing area between both plates, the time constant changes and due to that the voltage at both condensators differ from each other what can be sensed and used to trigger an alarm (Fig. 6). Fig. 4: Measurement of capacitance If a human enters the sensing area, it changes the capacitance between two plates because the human body can be seen as a dielectric put between both plates of a capacitor (Fig. 5). Fig. 6: Measurement of condensator voltage The capacitive measurement can be used in the automotive area to determine if someone is sitting on a seat but has not put on his seatbelt. Another field of application are wearout-free switches and
4 some dimmers for home use. In those applications the switch is one plate of the condensator, covered by a insulating finish. When a human touches the surface, the capacitance increases and e.g. light can be switched on. signal is measured. Changes in the received energy indicate the occupation of an object in the sensing area. V.INDUCTIVE MEASUREMENTS Inductive measurements are of less importance in daily life because the permeability of most objects is to less to measure a change. This kind of measurement is more a industrial application. Fig. 7: Inductive Sensor [5] The sensor consists of a coil and a circuitry that senses changes in the magnetic flux, e.g. by a metallic object passing by the sensor. VI.REFLECTIVE MEASUREMENTS A. Active Infrared Reflection Sensors Active Infrared Reflection Sensors are used e.g. in hand dryers or toilets. They consist of an infrared light emitting diode and a photoresistor (Fig. 8). When an object (e.g. a hand) enters the sensing area of the sensor, more light is reflected (Fig. 9). This is sensed by the circuitry and the hand dryer is switched on. Fig. 10: Simple ultrasound sensor [6]. Additionally the frequency of the received signal can be measured. If a object is moving towards the sensor, the received frequency is higher than the transmitted frequency due to the doppler effect. Additionally to the proximity of the object its speed towards the sensor can be measured. C. Radar Sensor Besides of light waves or acoustic waves microwaves can be used to detect objects. Radar sensors send out microwave impulses at about 24 Ghz. The microwave energy is reflected at objects and scattered back to the radar unit that senses the received microwave energy and concludes from the intensity changes to the proximity of objects (Fig. 11). Doppler Radar can be used to determine the speed of the object. Fig. 8: Reflective Infrared Sensor B. Ultrasonic Sensors Fig. 9: Working principle of reflective sensor The working principle of an ultrasound sensor is quite the same like of the Infrared Reflection Sensor. A sound pulse in the area of about 40 khz is sent out and the signal energy of the received Fig. 11: Beam form of a radar system for sliding doors [7]
5 VII.CONCLUSION The presented sensors have different advantages and disadvantages and vary in cost and complexity. The table below provides an overview which sensor can be used for which application. But before using one of these sensors in your application you should check out the datasheets and look at the detailed characteristics of each sensor. REFERENCES [1] Brown, D.A., Human occupancy detection, Security Technology, 1995. Proceedings. Institute of Electrical and Electronics Engineers 29th Annual 1995 International Carnahan Conference on 18-20 Oct. 1995 Page(s):166-174 IEEE Digital Object Identifier 10.1109/CCST.1995.524907 [2] from: Wikipedia, The free encyclopedia; Article about Pyroelectricity. URL: http://en.wikipedia.org/wiki/pyroelectric [3] Datasheet of PIR325 sensor, GLOLAB Corporation, http://www.glolab.com [4] Motion Detection Algorithms, The Code Project, http://www.codeproject.com/cs/media/motion_detection. asp [5] Altech Corp., Catalog of Inductive Sensors [6] POPtronix, Electronix Handbook www. poptronix.com [7] Triton Controls, DS-70 Miniature radar sensor datasheet Table 1: Application fields of presented sensors