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1.0 INTRODUCTION
The contents in this instruction manual are designed for
Delavan’s model Microwave 320. Please review the
information contained in this instruction manual carefully to
ensure proper operation.
1.1 Description
Delavan’s model Microwave 320 is a single point electronic
switch for ON/OFF action. It can be used for level control as
well as detection of objects. This sensing principle can “see”
through windows that are transparent to microwave energy.
This makes the Microwave 320 especially useful for
application on very aggressive as well as hot products as
the sensors do not have to come into direct contact with the
process material. Typical system shown in (Figure 1) below.
Figure 1
1.2 Principle of Operation
These controls are non-contact, 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 VDC 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 VDC, 1 KHz square wave is
converted to a pulsed X band (10.525 GHz) microwave
signal. The signal is radiated by the directional antenna,
which is typically a 10 dB gain horn with a beam angle 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 VDC 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 VDC) is compared with a preset value in a voltage
comparator circuit. When the signal received exceeds the
comparator setpoint, an output signal is initiated which is
processed through time delay circuits to drive the output
relay.
Materials in the industrial environment have various effects
on 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 @ 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.
1.3 Typical Applications
Level control of liquids or solids in tanks, bins, hoppers or
chutes are some typical applications. Non-conductive
fiberglass tanks represent minimal losses to X-band
microwaves. The microwave sensors are mounted on the
outside opposite one another on the tank. Losses through
the tank walls and air vapors above the product are low.
When the product level reaches the control position, the
signal is attenuated significantly, causing the output relay to
change state.
Metal tanks or hoppers must have a “window” transparent
to microwave signals. Sightglasses (3”-4” dia.) can be used
on liquid storage tanks, compatible with the pressures,
temperatures and chemical properties of the materials
stored in the tank. For metal vessels storing solid materials,
“windows” can be constructed of materials compatible with
the product contained therein. See Windows page 7.
