Page 138 - Sensors and Control Systems in Manufacturing

Classification and Types of Sensors
monitor the speed of power-plant generator shafts, continu- 99
ously monitoring acceleration and deceleration in order to
maintain a constant rotational speed. Microwave sensors can
effectively penetrate nonmetallic surfaces, such as fiberglass
tanks, to detect liquid levels. They can also detect objects in
packaged cartons.
• Rugged. Microwave sensors have no moving parts and have
proven their reliability in extensive military use. They are
packaged in sealed industrial enclosures to endure the rigors
of the production environment.
• Environmental reliability. Microwave sensors operate reliably
in harsh inhospitable environments. They can operate from
–55°C to +125°C in dusty, dirty, gusty, polluted, and poisonous
areas.
• Intrinsically safe. Industrial microwave sensors can be
operated in an explosive atmosphere because they do not
generate sparks due to friction or electrostatic discharge.
Microwave energy is so low that it presents no concern about
hazard in industrial applications.
• Long range. Microwave sensors are capable of detecting
objects at distances of 25 to 45,000 mm or greater, depending
on the target size, microwave power available, and the
antenna design.
• Size of microwave sensors. Microwave sensors are larger than
inductive, capacitive, and limit switch sensors. However, use
of higher microwave frequencies and advances in microwave
circuit development allow the overall package to be signifi-
cantly smaller and less costly.
• Target size. Microwave sensors are better suited to detect large
objects than smaller ones, such as a single grain of sand.

2.10.2 Principles of Operation
Microwave sensors consist of three major parts: (1) transmission
source, (2) focusing antenna, and (3) signal processing receiver.
Usually the transmission and receiver are combined together in
one module, which is called a transceiver. A typical module of this
type is used by intrusion alarm manufacturers for an indoor alarm
system. The transceiver contains a Gunn diode mounted in a small
precession cavity that, upon application of power, oscillates at micro-
wave frequencies. A special cavity design will cause this oscillation to
occur at 10.525 GHz, which is one of the few frequencies that the U.S.
Federal Communications Commission (FCC) has set aside for motion
detectors. Some of this energy is coupled through an iris into an
adjoining waveguide. Power output is in the 10- to 20-mW range.