Page 142 - Sensors and Control Systems in Manufacturing

Classification and Types of Sensors

FIGURE 2.99 Angular velocity measurement. 103

where Θ is the angle between the transceiver and the line of move-
ment of the target. Evidently, as the target is moving across the face of
the transceiver, cos Θ = 0, and the frequency is 0. If the angle is kept
below 18°, however, the measured frequency will be within 5 percent
of the center frequency (Fig. 2.99).
Signal processing for this module must include amplification, a
comparison network to shape the signal into logic levels, and a tim-
ing and counting circuit to either drive a display device or compare
the frequency to certain limits. If more than one moving object is in
the microwave field, it may be necessary to discriminate on the basis
of amplitude or frequency bandwidth, limiting it to exclude
unwanted frequencies. Velocities near 3 km/h and 6 km/h are also
difficult to measure with this system since the corresponding
Doppler frequencies are 60 and 120 Hz, which are prime interference
frequencies from power lines and fluorescent fixtures. Extra shield-
ing or isolation will be necessary in this case. False alarm rate may
also be reduced by counting a specific number of cycles before trig-
gering an output. This will actually correspond to the target moving
a defined distance.
Microwave sensors are well-suited for measuring the velocity of
objects, which most other sensors cannot do directly. Inductive, pho-
toelectric, and other sensors can measure radial velocity. For example,
inductive photoelectric sensors measure radial velocity when config-
ured as a tachometer, and if the rotating element is configured as a
trailing wheel, then linear velocity can be defined. Photoelectric sen-
sors can also be set up with appropriate signal processing to measure
the time that a moving object takes to break two consecutive beams.
This restricts the measurement to a specific location. Multiple beams
would be needed to measure velocity over a distance, whereas a sin-
gle microwave sensor could accomplish the same result.
Aside from their use in police radars, microwave sensors can
measure the speed of baseball pitches. These sensors have many
industrial applications as well. Microwave sensors are an excellent
means of closed-loop speed control of a relatively high-speed rotating
shaft (3600 r/min). Other applications include autonomous-vehicle
speed monitoring and independent safety monitoring equipment