2.0 LITERATURE REVIEW2.1Alarm systemsAlarm systems are usually adopted whenever the needcalls for sensitization mostly in the need for protection and security oflives,. It may also be for a wakeup call for an action to be taking concerningan arising situation. Where alarm system is used in the area of security,there are detectors that are installed to trigger such aarm. In this report,motion and smoke detectors are case studies.
2.2Motion detectorMotiondetectors as the name implies are aimed a sensing motion or movement. Their usein security system is tremendous because of its ability to detect movement ofintruders to a secured area.
Overyears many of these detectors had been so developed. Most of these are electronicmotion detectors which are used to detect any physical movement in a given areaand convert the motion into electronic signals. The sensors may optical sensor,sound sensor, ultrasonic sensor or an infrared sensor.Infraredsensors are widely used in the area of intrusion detection and in fire or smokedetection because of their advantages over the other one. Atypical infrared sensor includes a radiation source(transmitter) and aninfrared sensor (receiver) which is sensitive to interruptions of the radiationfrom the source. APassive infrared (PIR) motion detection senses heat energy emitted by anobject, such as a body of a person, moving across a field of view of a heatsensor of the motion detection system.
It is generally use an opticalcollection system and multiple sensing elements of alternating polarity tocreate a detection pattern in the volume of interest.PIRdetectors employ many radiation sensors coupled through amplifiers to a logiccircuit. The radiation sensors detect changes in ambient infrared radiation.Thedetection system has an electronic circuit coupled to the heat sensor toproduce a detection signal in relative to the heat sensed detecting a change oftemperature caused by the body heat of a person entering the detection view.PIRmotion detectors are perhaps the most frequently used home security device.
Basedon the improvement informed the use of the passive infrared motion detectorthis project work. A simple design is needed to build an effective motiondetection system based on motion detection. 2.3Smoke detectorThedecade following 1970 was a period of tremendous growth in thepopularity of smoke detectors. A growth in research and the general knowledgebase regarding the operation of smoke detectors accompanied this. Mostof the practical means of estimating the response of smoke detectorswere derived from this era and have remained largely unchanged.
Byitself, this fact is not significant. However, there have beensignificant advances in detector technology since that time, including moreuniform smoke entry characteristics among detector technologies, reducedsensitivity to nuisance (i.e.
, non-fire) sources, algorithm-baseddetection and multi-sensor, multi-criteria detection. Research into thecurrent trend toward the development of fire detection algorithms andmulti-sensor, multi-criteria fire detectors is prevalent in the literature inthe last decade e.g. Gottuk, et al., 1999; McAvoy, et al., 1996; Milke,1995; Milke and McAvoy, 1996; Milke and McAvoy, 1997; Rose-Pehrsson, etal.
, 2000; Wong, et al., 2000. However, advancement in the researchbehind predicting the response of common spot-type ionization andphotoelectric detectors has been minimal. More fundamental approachesexist to model the detectors, though these methods have not been advanced sufficientlyto prove practically useful for modeling smoke detectors.Thischapter will address many of the issues mentioned in the previous paragraph inmore detail as well as some fundamental topics related to the prediction ofsmoke detector response. 2.
3.1Detector Operating PrinciplesBeforeany attempt is made to understand the means by which smoke detector response ispredicted, an understanding of the fundamental operating principles of smokedetectors is required. For this study, only spot-type ionization and photoelectricdetectors are considered and are therefore the only technologies addressed inthis section. For the sake of brevity, from this point forward the use of thephrase smoke detectors will refer only to spot-type ionization andphotoelectric smoke detectors. More information on detector operatingprinciples, both those included here and some that are not, are available fromBukowski & Mulholland, 1978; Schifiliti & Pucci, 1996.Ionizationsmoke detectors operate as a result of the reduction of electrical current intheir ionization chamber below a given threshold in the presence of smoke.
Theionization chamber consists of a tiny amount of radioactive material (typicallyAmericium-241) located between two metal plates, one with a positive electricalcharge and one with a negative electrical charge. The voltage across the twoplates is maintained via a 9-volt battery or 120-volt alternating current(typical household current). The Americium contained in the chamber emits alphaparticles, positively-charged ions consisting of two protons and two neutrons,which ionize molecules in air (e.g.
, nitrogen and oxygen molecules).Ionization of the oxygen and nitrogen molecules simply means that electronsfrom these molecules are “knocked off” by the positively charged alphaparticles. As a result of this collision, the neutral atoms that lose anelectron become positively charged and the free electrons (i.e. the ones thatwere knocked off) attach to neutral gas molecules to form negative ions.
Theions are then drawn to the metal plate containing the opposite charge of theion. A small current (on the order of 10-11 amperes) occurs as the result ofthis normal transfer of charge between these ions and the metal plates of theionization chamber Bukowski and Mulholland, 1978. When smoke particles enterthe chamber they become attached to ions (just as occurred with the gasmolecules of the clean air).However,since these particles are significantly larger than the ions formed from thegas molecules, the velocity at which they are drawn to the metal plates isorders of magnitude slower, which allows the ionized smoke particles to becarried out of the sensing chamber by convection before they reach the metalplates Bukowski and Mulholland, 1978. As a result, a reduction in the currentbetween the metal plates occurs and the smoke detector triggers an alarm whenthis current falls below a preset level. Photoelectricsmoke detectors operate on a significantly different principle than ionizationsmoke detectors – light scattering.
Light scattering results from theinterference of smoke particles with a beam of light. Photoelectric smokedetectors contain a light source, typically a light-emitting diode (LED), and alight receiver, such as a photocell. Meacham reported that two photoelectricdetector manufacturers use LEDs with peak wavelengths in the range of 880 – 950nm Meacham, 1992. The photocell is arranged at such an angle that it does notnormally receive any light from the LED. The volume defined by the intersectionof the viewing angles of the light beam from the LED and the photocell istermed the scattering volume Bukowski & Mulholland, 1978. As smoke entersthe scattering volume, light from the LED is scattered onto the photocell.
Photocellsgenerate a current when a luminous flux (the scattered light) is applied. Theluminous flux received by the photocell increases in proportion with the smokeconcentration in the scattering volume. When the amount of scattered lightreaching the photocell exceeds a preset threshold, an alarm is triggered. Thesignal produced by photoelectric detectors is sensitive to a number of physicalcharacteristics of both the detector and smoke including the numberconcentration, size distribution, shape, and refractive index of the smokeparticles as well as the scattering volume and wavelength of light used in thedetector Schifiliti, et al.