Page 140 - Sensors and Control Systems in Manufacturing

Classification and Types of Sensors

FIGURE 2.97 Typical microwave motion sensor module. 101

motion detection. These sensors are primarily used in intrusion alarm
applications where it is only necessary to detect the movement rather
than derive further information about the intruder. The sensitivity
would be set to the minimum necessary level to detect a person-sized
object moving in the protected domain in order to prevent pets or
other nonhostile moving objects from causing a false alarm. In addi-
tion, some response delay would be introduced for the same reason,
requiring continuous movement for some short period of time.
Other applications include parts counting on conveyer belts;
serial object counting in general; mold ejection monitoring, particu-
larly in hostile environments; obstacle avoidance in automated
guided vehicle systems; fill indication in tanks; and invisible protec-
tion screens. In general, this type of sensor is useful where the objects
to be sensed are moving in the field of interest (Fig. 2.98).
Other devices that compete for the same applications are ultra-
sonic, photoelectric, and infrared sensors.
In the intrusion alarm manufacturing industry, microwave sen-
sors have the advantages of longer range and insensitivity to certain
environmental conditions. Ultrasonic sensors are sensitive to drafts
and high-frequency ambient noise caused by bells and steam escap-
ing from radiators. Infrared sensors are sensitive to thermal gradients
caused by lights turning on and off. The effectiveness of infrared sen-
sors is severely reduced at high ambient temperatures. However, uti-
lizing dual technologies is recommended to minimize false alarms—
combining microwave technology with infrared technology, for
example. It is necessary for the intruder to be sensed by both tech-
nologies before an alarm is given. In other applications, microwave
sensors can show advantages over photoelectric sensors in the areas