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FIGURE 2.100 Velocity sensing module.

for heavy and high-speed machine tools. Also, a microwave sensor
will detect an overvelocity condition (Fig. 2.100).
A microwave sensor, mounted on a tractor or other farm equip-
ment to measure ground speed, will play an important role in reduc-
ing excessive distribution of seeds and fertilizer per acre. The ordinary
wheel driven–speedometer is not sufficiently accurate because of
wheel slippage. Accurate speed measurement is necessary in these
vehicles so seeds and fertilizer are spread at a specific rate by the
accessory equipment. An over- or underestimate of the speed will
result in the wrong density per acre.

2.10.4 Detecting Presence with Microwave Sensors
A static object can be detected in the field of a microwave sensor. The
purpose of this detection is to determine that the object is still in the
field of interest and has not departed. This is particularly desirous in
control systems where the controller is performing other tasks and
then accesses the sensor to determine whether there is a sensed object
at that particular time. In this situation, presence sensing is especially
advantageous since the output can be verified by further interroga-
tions to eliminate false sensing.
To detect the presence of an object, a microwave sensor with a
separate transmitter and receiver must be used. A transceiver in this
application is not adequate, although the transmitter and the receiver
can be mounted in the same enclosure. The receiver must not sense
any energy unless the object is present in the field. A means to modu-
late the transmitter is needed, and the receiver should be narrowband
to amplify and detect the modulated reflection. The sensitivity of the
receiver must be adjustable to allow for ambient reflections.
Microwave sensors have been extensively and successfully tested
at various fast-food drive-through vending locations. Other types of
sensors, such as ultrasonic and photoelectric sensors, were also tested,