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1. General Description
The photo-electric beam detector consists of an emitter and a receiver which face each
other at a distance of between 5 and 100 meters.
In the event of fire the smoke generated will decrease the amount of near infra-red light
incident on the receiver. This decrease is electronically interpreted to identify the
occurrence of fire. An important feature of the detector is that it monitors the protected
space linearly. This enables the detector to identify a fire before it spreads, even when the
smoke is scattered over a large area.
The fire detection sensitivity is switch selectable in 3 settings of 25%, 50% and 70%
beam obscuration.
1.1. Detection principle
A near infra-red pulsed beam generated by the emitter is sensed by the photo-diode of
the receiver, where it is converted into an electrical signal. This signal is then amplified
and applied via an A/D converter to a micro-processor. The normal state signal (the initial
beam data) once stored in the micro-processor is used as a reference for comparison with
subsequent beam signals.
When there is sufficient difference between actual beam strength and stored reference
data to indicate the occurrence of a fire, then a fire signal is produced.
If an obstacle
blocks the beam, thus abruptly reducing the received beam level, a fault signal is emitted.
(The detector can be set to emit a fire signal after emitting a fault signal.) The same error
signal is emitted when reception signals increase abruptly.
The micro-processor also provides compensation for a change in received signal value
with time, caused by contamination of the optics or slight alignment changes. The
processed signal is adjusted at a rate of ±1% towards the reference data every 20
minutes. When the limit of compensation is reached the micro-processor will automatically
produce a fault signal.
In order to improve the performance of the detector and to enhance the rejection of noise
the emitter and receiver are synchronised together
．
The emitter and receiver are
automatically synchronized by using near-infrared beams produced by the emitter. The
emitter and receiver therefore do not need to be interconnected.
（
Wireless
）
