Page 144 - Sensors and Control Systems in Manufacturing

Classification and Types of Sensors
but less successfully because they were too sensitive to the environ- 105
ment. It was discovered that frost heaving of the ground would even-
tually cause their buried loop to fail, and the cost of underground
excavation to replace the loop was exorbitant.
Another application of the microwave sensor is the door-opening
market. The microwave sensor will check, for safety reasons, the area
behind a swinging door to detect whether there is an individual or an
object in the path way. Ultrasonic sensors may perform the same task,
yet range and environmental conditions often make a microwave
sensor more desirable.
A microwave sensor can check boxes to verify that objects actu-
ally have been packed therein. The sensor has the ability to see
through the box itself and triggers only if an object is contained in the
box. This technology relies on the sensed object being more reflective
than the package, a condition that is often met.

2.10.5 Measuring Velocity with Microwave Sensors
Microwave sensors are ideally suited to measuring linear velocity.
Police radar is a simple example of a Doppler-frequency-based veloc-
ity sensor. This technology can be applied wherever it is necessary to
determine velocity in a noncontact manner.

2.10.6 Detecting Direction of Motion with
Microwave Sensors
Direction of motion—whether a target is moving toward or away
from the microwave sensor—can be determined by the use of the
Doppler-frequency concept (Fig. 2.101), by adding an extra mixer

FIGURE 2.101 Direction of motion sensor schematic.