handbook of modern sensors

232 6 Occupancy and Motion Detectors
(A)
(B)
Fig. 6.2. Block diagram (A) and timing diagrams (B) of a microwave Doppler motion detector
with directional sensitivity.
reflector; however, it may render the detector inoperable at angles other than 0 ◦ . Thus,
an angle of = 45 ◦ can completely divert a reflective signal from the receiving antenna.
This method of diversion was used quite effectively in the design of the Stealth
bomber, which is virtually invisible on radar screens.
To detect whether a target moves toward or away from the antenna, the Doppler
concept can be extended by adding another mixing diode to the transceiver module.
The second diode is located in the waveguide in such a manner that the Doppler
signals from both diodes differ in phase by one-quarter of wavelength or by 90 ◦ (Fig.
6.2A). These outputs are amplified separately and converted into square pulses which
can be analyzed by a logic circuit. The circuit is a digital phase discriminator that
determines the direction of motion (Fig. 6.2B). Door openers and traffic control are
two major applications for this type of module. Both applications need the ability to
acquire a great deal of information about the target for discrimination before enabling
a response. In door openers, limiting the field of view and transmitted power may substantially
reduce the number of false-positive detections. Although for door openers a
direction discrimination is optional, for traffic control it is a necessity to reject signals
from the vehicles moving away. If the module is used for intrusion detection, the vibration
of building structures may cause a large number of false-positive detections.
A direction discriminator will respond to vibration with an alternate signal, and it will
respond to an intruder with a steady logic signal. Hence, the direction discriminator
is an efficient way to improve the reliability of the detection.
Whenever a microwave detector is used in the United States, it must comply with
the strict requirements (e.g., MSM20100) imposed by the Federal Communication
Commission. Similar regulations are enforced in many other countries.Also, the emission
of the transmitter must be below 10 mW/cm 2 as averaged over any 0.1-h period,
as specified by OSHA 1910.97 for the frequency range from 100 MHz to 100 GHz.
A quite effective motion detector may be designed by employing micropower
impulse radar (see Section 7.7.1 of Chapter 7). Advantages of such detectors are very
low power consumption and nearly total invisibility to the intruder. The radar may