2.0 many of these detectors had been so developed.

2.0       LITERATURE REVIEW

2.1
Alarm systems

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Alarm systems are usually adopted whenever the need
calls for sensitization mostly in the need for protection and security of
lives,. It may also be for a wakeup call for an action to be taking concerning
an 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.

2.2
Motion detector

Motion
detectors as the name implies are aimed a sensing motion or movement. Their use
in security system is tremendous because of its ability to detect movement of
intruders to a secured area.

 

Over
years many of these detectors had been so developed. Most of these are electronic
motion detectors which are used to detect any physical movement in a given area
and convert the motion into electronic signals. The sensors may optical sensor,
sound sensor, ultrasonic sensor or an infrared sensor.

Infrared
sensors are widely used in the area of intrusion detection and in fire or smoke
detection because of their advantages over the other one.

A
typical infrared sensor includes a radiation source(transmitter) and an
infrared sensor (receiver) which is sensitive to interruptions of the radiation
from the source.

 

A
Passive infrared (PIR) motion detection senses heat energy emitted by an
object, such as a body of a person, moving across a field of view of a heat
sensor of the motion detection system. It is generally use an optical
collection system and multiple sensing elements of alternating polarity to
create a detection pattern in the volume of interest.

PIR
detectors employ many radiation sensors coupled through amplifiers to a logic
circuit. The radiation sensors detect changes in ambient infrared radiation.

The
detection system has an electronic circuit coupled to the heat sensor to
produce a detection signal in relative to the heat sensed detecting a change of
temperature caused by the body heat of a person entering the detection view.

PIR
motion detectors are perhaps the most frequently used home security device.

Based
on the improvement informed the use of the passive infrared motion detector
this project work. A simple design is needed to build an effective motion
detection system based on motion detection.

 

2.3
Smoke detector

The
decade following 1970 was a period of tremendous growth in the
popularity of smoke detectors. A growth in research and the general knowledge
base regarding the operation of smoke detectors accompanied this. Most
of the practical means of estimating the response of smoke detectors
were derived from this era and have remained largely unchanged. By
itself, this fact is not significant. However, there have been
significant advances in detector technology since that time, including more
uniform smoke entry characteristics among detector technologies, reduced
sensitivity to nuisance (i.e., non-fire) sources, algorithm-based
detection and multi-sensor, multi-criteria detection. Research into the
current trend toward the development of fire detection algorithms and
multi-sensor, multi-criteria fire detectors is prevalent in the literature in
the last decade e.g. Gottuk, et al., 1999; McAvoy, et al., 1996; Milke,
1995; Milke and McAvoy, 1996; Milke and McAvoy, 1997; Rose-Pehrsson, et
al., 2000; Wong, et al., 2000. However, advancement in the research
behind predicting the response of common spot-type ionization and
photoelectric detectors has been minimal. More fundamental approaches
exist to model the detectors, though these methods have not been advanced sufficiently
to prove practically useful for modeling smoke detectors.

This
chapter will address many of the issues mentioned in the previous paragraph in
more detail as well as some fundamental topics related to the prediction of
smoke detector response.

 

 

2.3.1
Detector Operating Principles

Before
any attempt is made to understand the means by which smoke detector response is
predicted, an understanding of the fundamental operating principles of smoke
detectors is required. For this study, only spot-type ionization and photoelectric
detectors are considered and are therefore the only technologies addressed in
this section. For the sake of brevity, from this point forward the use of the
phrase smoke detectors will refer only to spot-type ionization and
photoelectric smoke detectors. More information on detector operating
principles, both those included here and some that are not, are available from
Bukowski & Mulholland, 1978; Schifiliti & Pucci, 1996.

Ionization
smoke detectors operate as a result of the reduction of electrical current in
their ionization chamber below a given threshold in the presence of smoke. The
ionization chamber consists of a tiny amount of radioactive material (typically
Americium-241) located between two metal plates, one with a positive electrical
charge and one with a negative electrical charge. The voltage across the two
plates is maintained via a 9-volt battery or 120-volt alternating current
(typical household current). The Americium contained in the chamber emits alpha
particles, 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 electrons
from these molecules are “knocked off” by the positively charged alpha
particles. As a result of this collision, the neutral atoms that lose an
electron become positively charged and the free electrons (i.e. the ones that
were knocked off) attach to neutral gas molecules to form negative ions. The
ions are then drawn to the metal plate containing the opposite charge of the
ion. A small current (on the order of 10-11 amperes) occurs as the result of
this normal transfer of charge between these ions and the metal plates of the
ionization chamber Bukowski and Mulholland, 1978. When smoke particles enter
the chamber they become attached to ions (just as occurred with the gas
molecules of the clean air).

However,
since these particles are significantly larger than the ions formed from the
gas molecules, the velocity at which they are drawn to the metal plates is
orders of magnitude slower, which allows the ionized smoke particles to be
carried out of the sensing chamber by convection before they reach the metal
plates Bukowski and Mulholland, 1978. As a result, a reduction in the current
between the metal plates occurs and the smoke detector triggers an alarm when
this current falls below a preset level.

 

Photoelectric
smoke detectors operate on a significantly different principle than ionization
smoke detectors – light scattering. Light scattering results from the
interference of smoke particles with a beam of light. Photoelectric smoke
detectors contain a light source, typically a light-emitting diode (LED), and a
light receiver, such as a photocell. Meacham reported that two photoelectric
detector manufacturers use LEDs with peak wavelengths in the range of 880 – 950
nm Meacham, 1992. The photocell is arranged at such an angle that it does not
normally receive any light from the LED. The volume defined by the intersection
of the viewing angles of the light beam from the LED and the photocell is
termed the scattering volume Bukowski & Mulholland, 1978. As smoke enters
the scattering volume, light from the LED is scattered onto the photocell.

Photocells
generate a current when a luminous flux (the scattered light) is applied. The
luminous flux received by the photocell increases in proportion with the smoke
concentration in the scattering volume. When the amount of scattered light
reaching the photocell exceeds a preset threshold, an alarm is triggered. The
signal produced by photoelectric detectors is sensitive to a number of physical
characteristics of both the detector and smoke including the number
concentration, size distribution, shape, and refractive index of the smoke
particles as well as the scattering volume and wavelength of light used in the
detector Schifiliti, et al., 2002.