Page 139 - Sensors and Control Systems in Manufacturing

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T w o
Cha p te r

The DC input power for this stage (8 V at 150 mA) should be well-
regulated, since the oscillator is voltage-sensitive. The sensitivity of
the system can be significantly reduced by noise (interference).
At the end of the waveguide assembly, a flange is fastened to the
antenna. The antenna focuses the microwave energy into a beam, the
characteristics of which are determined by the application. Antennas
are specified by beam width or gain. The higher the gain, the longer
the range and the narrower the beam. An intrusion alarm protecting
a certain domain would require a wide-beam antenna to cover the
area, while a traffic control microwave sensor would require a nar-
row-beam high-gain antenna to focus down the road.
Regardless of the antenna selection, when the beam of microwave
energy strikes an object, some of the microwave energy is reflected
back to the module. The amount of energy will depend on the com-
position and shape of the target. Metallic surfaces will reflect a great
deal, while Styrofoam and plastic will be virtually transparent. A
large target area will also reflect more than a small one.
The reflected power measured at the receiver decreases by the
fourth power of the distance to the target. This relationship must be
taken into consideration when choosing the transmitted power, antenna
gain, and signal processing circuitry for a specific application.
When the reflected energy returns to the transceiver, the mixer
diode will combine it with a portion of the transmitted signal. If the
target is moving toward or away from the module, the phase relation-
ships of these two signals will change and the signal out of the mixer
will be an audio frequency proportional to the speed of the target.
This is called the Doppler frequency. This is of primary concern in mea-
suring velocity and direction of motion. If the target is moving across
in front of the module, there will not be a Doppler frequency, but
there will be sufficient change in the mixer output to allow the signal
processing circuitry to detect it as unqualified motion in the field.
The signal from the mixer will be in the microvolt to millivolt range
so that amplification will be needed to provide a useful level. This ampli-
fication should also include 60-Hz and 120-Hz notch filters to eliminate
interference from power lines and fluorescent light fixtures, respectively.
The remaining bandwidth should be tailored to the application.
Besides amplification, a comparator and output circuitry relays
are added to suit the application (Fig. 2.97).

2.10.3 Detecting Motion with Microwave Sensors
The presence of an object in the microwave field disturbs the radiated
field. There may be a Doppler frequency associated with the distur-
bance. The signal from the mixer to the signal processing circuitry
may vary with a large amplitude and long duration so it can be
detected. The amplitude gain and the delay period are of specific
importance in tailoring the device for particular application, such as