Page 255 - Concise Encyclopedia of Robotics

Presence Sensing
IR motion detector
A common presence-detection system employs an IR motion detector. Two
or three wide-angle IR pulses are transmitted at regular intervals; these
pulses cover most of the zone for which the device is installed. A receiving
transducer picks up the returned IR energy, normally reflected from
walls, floors, ceilings, and furniture. The intensity of the received pulses is
noted by a microprocessor. If anything in the room changes position, or
if a new object appears, there is a change in the intensity of the received
energy. The microprocessor notices this change, and generates a signal.
These devices consume very little power in regular operation, so batteries
can serve as the power source.
Radiant heat detector
Infrared devices can detect changes in the indoor environment via direct
sensing of the IR energy (often called radiant heat) emanating from
objects. Humans, and all warm-blooded animals, emit IR. So does fire. A
simple IR sensor, in conjunction with a microprocessor, can detect rapid
or large increases in the amount of radiant heat in a room. The time
threshold can be set so that gradual or small changes, such as might be
caused by sunshine, will not trigger the signal, while significant changes,
such as a person entering the room, will. The temperature-change (incre-
ment) threshold can be set so that a small animal will not actuate the alarm,
while a full-grown person will. This type of device, like the IR motion
detector, can operate from batteries.
The main limitation of radiant-heat detectors is the fact that they can
be fooled. False alarms are a risk; the sun might suddenly shine directly
on the sensor and trigger a presence signal. It is also possible that a
person clad in a winter parka, boots, hood, and face mask, just entering
from a subzero outdoor environment, might fail to generate a signal. For
this reason,radiant-heat sensors are used more often as fire-alarm actuators
than as presence detectors.
Ultrasonic motion detector
Motion in a room can be detected by sensing the changes in the relative
phase of acoustic waves. An ultrasonic motion detector is an acoustic inter-
ferometer that employs a set of transducers that emit acoustic waves at
frequencies above the range of human hearing (higher than 20 kHz).
Another set of transducers picks up the reflected acoustic waves, whose
wavelength is a fraction of an inch. If anything in the room changes
position, the relative phase of the waves, as received by the various
acoustic pickups, changes. This data is sent to a microprocessor, which
triggers a presence signal. Compare PROXIMITY SENSING.

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