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PRIMARY AREAS OF APPLICATION
Severe Environments
Where the environment is too hostile to permit
reliable operation of photo-electric controls.
Microwave 320 is unaffected by dust, smoke, fog or
vibration.
High Temperature Environments
Model MT 861 and MR 861 transmitters and
receivers are designed for high temperature service.
Please note: These sensors are water cooled to
maintain an operating temperature of +140°F
(+60°C) or less at the pre-amplifier.
Abrasive Materials
Eg: Crushed coal, sand, ore. Rugged construction.
No moving parts. Sensors do not protrude into flow
stream.
Corrosive - Liquid Products
Eg: Water, acids, diluted acids (within the limits of
transmitter and receiver housing and insulation
materials), water with impurities.
Liquids - Interface
Eg: Petroleum products and water.
Liquids - With Solids
Eg: Municipal waste (sewage).
PRINCIPLE OF OPERATION
These controls are non-contact sensing microwave based
controls. The transmitter (source) consists basically of a power
supply, pulse modulator, Gunn oscillator, and directional
antenna. The receiver consists of a directional antenna, a
microwave mixer cavity with a Schottky barrier diode detector,
a high gain, low noise amplifier, a pulse coding network, a
voltage comparator circuit and a relay driver circuit.
In the transmitter, power is converted to a well regulated and
filtered 12 Volts DC supply. It is then pulsed at about 1 kHz by
the pulse modulator circuit. This circuit is included to permit
pulse discrimination circuitry to be used. In addition, pulsing at
a 10% duty cycle safely permits peak transmitted power levels
10 times greater than permitted under continuous wave
operation. The pulsed DC is fed to a Gunn oscillator in the
antenna assembly, where the 12 Volts DC 1 kHz square wave
is converted to a pulsed X bank (10.525 gHz) microwave signal.
The signal is radiated by the directional antenna, which is
typically a 10dB gain horn with a beam spread of approximately
40°.
In the receiver, the signal is received by a directional antenna
and coupled to a mixer cavity containing a Schottky detector
diode. This diode converts the low level microwave signal to a
low level pulsed DC, which is then amplified by an adjustable
gain — low noise IC amplifier to a 0-10 Volts DC control
signal. This system is interconnected and uses pulse
discrimination coding. In these systems the receiver is on only
when the transmitter is on, thus the system is virtually immune
to false triggering from stray microwave interference. The level
of the amplified received signal (0-10 Volts DC) is compared
with a preset value in a voltage comparator circuit. When the
signal received exceeds the comparator set-point, an output
signal in initiated which is processed through time delay circuits
to drive the output relay.
A microwave system has been described in the above, based
on a fixed power transmitter and a receiver/detector with
adjustable gain to discriminate between various signal power
levels received at its antenna. Materials in the industrial
environment have various effects of microwave signals. For
example, low level microwaves cannot penetrate metals, but
are reflected by them. They are absorbed almost entirely by
water, and to varying degrees by water based solutions or
products that have a significant moisture content such as grain,
wood products, etc.
Transmission losses increase with increasing dielectric
constants and increase with increasing conductivity. For
example, air (dielectric constant of 1 and conductivity of zero)
transmits microwave with no loss while sea water (dielectric
constant of 55 at X-band and conductivity of 4 mhos/meter)
provides extreme attenuation of the microwave energy. It is the
material’s dielectric constant and conductivity that determine
whether or not the material is a good candidate for microwave
control.
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