Page 145 - Sensors and Control Systems in Manufacturing

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Cha p te r
T w o

diode to the module. A discriminating effect is generated by the addi-
tional diode, which is located in the waveguide such that the Doppler
outputs from the two mixers differ in phase by one-quarter wave-
length, or 90°. These outputs will be separately amplified and con-
verted into logic levels. The resulting signals can then be fed into a
digital phase-discrimination circuit to determine the direction of
motion. Such circuits are commonly found in motion control applica-
tions in conjunction with optical encoders. Figure 2.102 shows the
phase relationships of the different directions.
Outputs from this module can vary widely to suit the application.
The simplest is two outputs, one for motion and the other for direc-
tion (toward or away). These outputs can be added to a third, which
provides the velocity of the target. The combination of signals could
be analyzed to provide a final output when specific amplitude, direc-
tion, distance, and velocity criteria are met (Fig. 2.101).
In the door-opening field, using the amplitude, direction, distance,
and velocity information reduces the number of false openings. This
extends the life of the door mechanism, besides saving heat if the
door is an entrance to a heated building.
In this case, the measurements by circuitry indicate the following
characteristics:

Characteristic Measurement
Person-sized object Amplitude of return
Moving at walking pace Velocity
Toward or away Direction
Specific time before opening Distance

2.10.7 Detecting Range with Microwave Sensors
An early-warning military radar system depends on costly micro-
wave sensors. A small yacht may use a microwave sensor selling for
less than $1000 to detect targets at ranges up to 5 mi.
Regardless of their cost, microwave range sensors for commer-
cial, industrial, and military applications employ essentially the same
measuring technique. They transmit a narrow pulse of energy and
measure the time required for the return from the target. Since micro-
wave energy propagates at the speed of light, the time for the pulse to
reach the target and return is 2 ns per foot of range. If the range to the
target is 1 mi, the time required is 10.56 μs.
Although the microwave power needed is sufficient to raise the
sensor temperature to 500°F, the design of the signal processing cir-
cuitry to measure the response is not difficult. However, if the target
is very close to the transmitter, then the short response time may pose
a real problem. At 3 ft, the time response is 6 ns. For 1-in resolution,