17-09-2016, 03:29 PM
Microwave Life-Detection Systems for Searching
Human Subjects Under Earthquake Rubble or Behind
Barrier
1455180913-praveenseminar.pdf (Size: 170.67 KB / Downloads: 11)
Abstract—A new sensitive microwave life-detection system
which can be used to locate human subjects buried under
earthquake rubble or hidden behind various barriers has been
constructed. This system operating at 1150 MHz or 450 MHz
can detect the breathing and heartbeat signals of human subjects
through an earthquake rubble or a construction barrier of about
10-ft thickness.
The basic physical principle for the operation of a microwave
life-detection system is rather simple. When a microwave beam of
appropriate frequency (L or S band) is aimed at a pile of earthquake
rubble covering a human subject or illuminated through
a barrier obstructing a human subject, the microwave beam can
penetrate the rubble or the barrier to reach the human subject.
When the human subject is illuminated by a microwave beam, the
reflected wave from the human subject will be modulated by the
subject’s body movements, which include the breathing and the
heartbeat. If the clutter consisting of the reflected wave from stationary
background can be completely eliminated and the reflected
wave from the human subject’s body is properly modulated, the
breathing and heartbeat signals of the subject can be extracted.
Thus, a human subject buried under earthquake rubble or hidden
behind barriers can be located.
This system has been tested extensively in a simulated earthquake
rubble in the laboratory and also in a field test using realistic
earthquake rubble conducted by a Federal Emergency Management
Agency (FEMA) Task Force.
Index Terms—Breathing and heartbeat signals, clutter cancellation,
earthquake rubble, laboratory and field tests, microwave
life-detection systems.
I. INTRODUCTION
E XISTING methods for searching and rescuing human victims
buried under earthquake rubble or collapsed building
debris are the utilization of dogs, or seismic or optical devices.
These existing devices are not effective if the rubble or debris
covering the human victims is thicker than a few teet, especially
for the case when the victims are completely trapped or
too weak to respond to the signal sent by the rescuers. Thus,
there is great demand for constructing a new sensitive life-detection system which can be used to locate human victims trapped
deep under earthquake rubble or collapsed building debris. Especially,
the system needs to be sensitive enough to detect the
breathing and heartbeat signals of passive victims who are completely
trapped or too weak to respond to the existing seismic
detection system. We have constructed a sensitive life-detection
system for such purposes using microwave radiation. In this
paper we will describe a microwave life-detection system constructed
at Michigan State University supported by the National
Science Foundation (NSF).
The construction of the microwave life-detection system for
the post earthquake rescue operation is a spin-off of a research
project conducted by us for constructing a microwave life-detection
system for sensing the physiological status of soldiers
lying on the ground of a battlefield. We constructed a microwave
life-detection system which operates at the X-band (10 GHz)
for such a purpose [1], [2] in 1980. Such an X-band microwave
beam cannot penetrate earthquake rubble or collapsed building
debris sufficiently deep to locate the buried human victims. For
an electromagnetic (EM) wave to penetrate deep (up to 10 ft)
into the rubble or the debris, the frequency of the electromagnetic
wave need to be in the L or S band range. For this reason,
we have constructed two systems, one operating at 450 MHz
and the other at 1150 MHz [3]–[7]. Each system has advantages
and disadvantages depending on the nature of the rubble as will
be explained later.
The basic physical principle for the operation of a microwave
life-detection system is rather simple. When a microwave beam
of appropriate frequency (L or S band) is aimed at a pile of earthquake
rubble or collapsed building debris under which a human
subject is buried, the microwave beam can penetrate through the
rubble or the debris to reach the subject. When the human subject
is illuminated by the microwave beam, the reflected wave
from the subject will be modulated by the subject’s body movements,
which include the breathing and the heartbeat. If the reflected
wave from the stationary background can be cancelled
and the reflected wave from the subject’s body is properly demodulated,
the breathing and heartbeat signals of the subject can
be extracted. Thus, a human subject buried under the rubble or
the debris can be located.
The system operating at 450 MHz was constructed first. This
system was tested on simulated earthquake rubble constructed
at the Electromagnetics Laboratory at Michigan State University,
and it was also tested in a field test using realistic earthquake
rubble consisted of layers of reinforced concrete slabs
with imbedded metallic wire mesh at a test site in Rockville,
MD, with the cooperation of the Maryland Task Force of the
Federal Emergency Management Agency (FEMA). The results
of these tests will be described. The second system operating
at 1150 MHz was constructed after the field test at Rockville,
MD. In that field test, it was found that an EM wave of 450 MHz
is difficult to penetrate layers of reinforced concrete slabs with
imbedded metallic wire of 4-in spacing. Through a series of
experiment, we selected the operating frequency of 1150 MHz
for the second system with the goal of penetrating such earthquake
rubble. After the construction of the 450-MHz and the
1150-MHz systems and an extensive series of experiments, we
found that an EM wave of 1150 MHz can penetrate a rubble
with layers of reinforced concrete slabs with metallic wire mesh
easier than that of 450 MHz. However, an EM wave of 450 MHz
may penetrate deeper into a rubble without metallic wire mesh
than that of 1150 MHz.
The microwave life-detection system we constructed has four
major components: 1) a microwave circuit system which generates,
amplifies, and distributes microwave signals to various
microwave components; 2) a microprocessor-controlled cluttercancellation
system which creates an optimal signal to cancel
the clutter from the rubble and the background; 3) a dual-antenna
system which consists of two separate antennas energized
sequentially; and 4) a laptop computer which controls the microprocessors
and acts as the monitor for the output signal. The
system is operated by a portable battery unit.
Both the 450-MHz and the 1150-MHz systems are working
well for various types of earthquake rubble and collapsed building debris. They can detect the breathing and heartbeat
signals of trapped human subjects buried under a rubble of up
to 10-ft thickness.
II. CIRCUIT DESCRIPTION OF THE SYSTEM
The basic circuit structures of the 450-MHz and the
1150-MHz microwave life-detection systems are quite similar
and they are operated based on the same physical principle.
In this paper, only the circuit structure of the 1150-MHz
system will be described, while that of the 450-MHz system is
available elsewhere [8].
The schematic diagram of the 1150-MHz microwave life-detection
system is shown in Fig. 1. A phase-locked oscillator generates
a very stable EM wave at 1150 MHz with an output power
of 400 mW (25.6 dBm). This wave is fed through a 10-dB directional
coupler and a circulator before reaching a radio-frequency
(RF) switch, which energized the dual antenna system sequentially.
The 10-dB directional coupler branches out one-tenth of
the wave (40 mW) which is then divided equally by a 3-dB directional
coupler. One output of the 3-dB directional coupler (20
mW) drives the clutter cancellation circuit and the other output
(20 mW) serves as a local reference signal for the double-balanced
mixer.
The wave radiated by an antenna penetrates the earthquake
rubble to reach a buried human subject. The reflected wave
received by the same antenna consists of a large reflected
wave (clutter) from the rubble and a small reflected wave from
the subject’s body. The large clutter from the rubble can be cancelled by a clutter canceling signal. However, the small
reflected wave from the subject’s body cannot be cancelled by a
pure sinusoidal, canceling signal because it is modulated by the
subject’s motions. The dual-antenna system has two antennas,
which are energized sequentially by an electronic switch. Each
antenna acts independently and the final outputs from these two
antennas are combined in some signal processing schemes to
reduce the background noise. This part will be elaborated later.
The clutter cancellation circuit consists of a digitally controlled
phase-shifter (0–360 ), a fixed attenuator (4 dB), a
RF amplifier (20 dB), and a digitally controlled attenuator
(0–30 dB). The output of the clutter cancellation circuit is
automatically adjusted to be of equal amplitude and opposite
phase as that of the clutter from the rubble. Thus, when the
output of the clutter cancellation circuit is combined with
the received signal from the antenna, via the circulator, in a
3-dB directional coupler, the large clutter from the rubble is
completely canceled, and the output of the 3-dB directional
coupler consists only of the small reflected wave from the
subjects body. This output of the 3-dB directional coupler
is passed through a 6-dB directional coupler. The 1/4 of this
output is amplified by a RF preamplifier (30 dB) and then
mixed with a local reference signal in a double-balanced mixer.
The other 3/4 of the output is detected by a microwave detector
to provide a dc voltage, which serves as the indicator for the
degree of the clutter cancellation. When the settings of the
digitally controlled phase-shifter and attenuator are swept by
the microprocessor control system, the output of the microwave
detector varies accordingly. The minimum detector reading
corresponds to the right settings for the digitally controlled
phase-shifter and attenuator. These settings will be fixed for
subsequent measurements.
At the double-balanced mixer, the amplified signal of the reflected
wave from the subject’s body is mixed with a local reference
signal. The phase of the local reference signal is controlled
by another digitally controlled phase-shifter (0 –180 )
for an optimal output from the mixer. (This function will be elaborated
on later.) The output of the mixer consists of the breathing
and heartbeat signals of the human subject plus unavoidable
noise. This output is fed through a low-frequency (LF) amplifier
(20–40 dB) and a bandpass filter (0.1–4 Hz) before being
displayed on the monitor of a laptop computer.
The function of a digitally controlled phase-shifter (0 –180 )
installed in front of the local reference signal port of the doublebalanced
mixer to control the phase of the local reference signal
for the purpose of increasing the system sensitivity is explained
below.
As mentioned before, the reflected signal from the human
subject after amplification by the preamplifier is mixed with the
local reference signal in the double-balanced-mixer. The local
reference signal is assumed to be where
and are the amplitude and the phase, respectively. While the
other input to the mixer, the reflected signal from the human
subject, is assumed to be where
and are the amplitude and the phase, respectively, and
is the phase modulation due to the body movement of the human
subject. is the angular frequency and is the time.