Life Detection System Using L and S Band of Microwaves

Abstract

A new sensitive microwave life-detection which can be used to locate human subjects buried 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 the 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 tile 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  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.

TABLE OF CONTENTS

v Chapter 1

·       Introduction

v Chapter 2

·       Introduction of microwaves

·       Properties of microwaves

·       Advantages of microwaves

·       Disadvantages of microwaves

v Chapter 3

·       Block diagram of Life Detection System

v Chapter 4

·       Antenna System

·       Principle of operation

·       Working and Range

·       Examples

v Chapter 5

·       Conclusion

·       References

Chapter 1

 

INTRODUCTION

Existing methods for searching and rescuing human victims buried under earthquake rubble or collapsed building debris 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 feet, 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 clutter cancellation 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 rubble of up to 10-ft thickness.

Chapter 2

INTRODUCTION OF MICROWAVES

Microwaves are electromagnetic waves with wavelength ranging from as long as one meter to as short as one mili-meter, or equivalently, with frequencies between 300 MHz(0.3 GHz) and 300 GHz. This broad definition includes both UHF and EHF waves and various sources uses different boundaries. In all cases, microwave  includes the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum, with RF engineering often putting the lower boundary at 1 GHz (30cm), and the upper around 100 GHz(3mm).

Properties of Microwaves

Following are the main properties of Microwaves.

·        Microwaves are the waves that radiate electromagnetic energy with shorter wavelength.

·        Microwaves are not reflected by Ionosphere.

·        Microwaves travel in a straight line and are reflected by the conducting surfaces.

·        Microwaves are easily attenuated within shorter distances.

·        Microwave currents can flow through a thin layer of a cable.

Advantages of Microwaves

There are many advantages of Microwaves such as the following −

·        Supports larger bandwidth and hence more information is transmitted. For this reason, microwaves are used for point-to-point communications.

·        More antenna gain is possible.

·        Higher data rates are transmitted as the bandwidth is more.

·        Antenna size gets reduced, as the frequencies are higher.

·        Low power consumption as the signals are of higher frequencies.

·        Effect of fading gets reduced by using line of sight propagation.

·        Provides effective reflection area in the radar systems.

·        Satellite and terrestrial communications with high capacities are possible.

·        Low-cost miniature microwave components can be developed.

·        Effective spectrum usage with wide variety of applications in all available frequency ranges of operation.

Disadvantages of Microwaves

There are a few disadvantages of Microwaves such as the following −

• Cost of equipment or installation cost is high.
• They are hefty and occupy more space.
• Electromagnetic interference may occur.
• Variations in dielectric properties with temperatures may occur.
• Inherent inefficiency of electric power.

Chapter 3

BLOCK DIAGRAM OF LIFE DETECTION SYSTEM

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 cancelling signal. However, the small reflected wave from the subject’s body cannot be cancelled by a pure sinusoidal, cancelling 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 cancelled, and the output of the 3-dB directional coupler consists only of the small reflected wave from the subject 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 double balanced mixer to control the phase of the local reference signal for the purpose of increasing the system sensitivity is explained below.

 

Chapter 4

ANTENNA SYSTEM

We have designed and constructed three types of antennas for the microwave life-detection system. They are:

 1) The reflector antenna;

 2) The patch antenna;

 3) The probe antenna.

 Each antenna simultaneously acts as the radiating element and the receiving element. It radiates EM wave through the earthquake rubble to reach the trapped human subjects and at the same time it receives the reflected EM wave from the rubble and the human subjects. The antenna can perform two functions simultaneously with the help of a circulator, which separates the radiating EM wave from the received EM wave. The reflector antenna was constructed with two aluminum plates as the reflectors and an adjustable dipole antenna as the driving element. The two aluminum plates with the dimensions of 21 in × 11in form a corner reflector with the dipole antenna as its primary radiator. The angle between the two aluminum plates is adjustable and they are folded together when it is not used. The dipole antenna is a conventional, half-wavelength electric dipole. The reflector antenna is a simple, lightweight, and ragged structure and it performs very well in the most of situations. The gain of the reflector antenna is difficult to define and measure because the antenna is placed directly over a rubble pile and the scattered field of the antenna is strongly dependent on the nature of the rubble material.

A patch antenna was constructed for radiating and receiving EM wave for the microwave life-detection system. The patch antenna consists of an aluminum ground plane, which is supported by four legs and a strip plate of about a half-wavelength, which is attached to the ground plane and fed by a coaxial line. The strip plate is insulated from the ground plane. The coaxial cable is attached to the ground plane through a connector. The performance of the patch antenna is not better than that of the reflector antenna. It only serves as alternative type of antenna and may be useful in some situations.

A probe antenna was designed to insert through boreholes or naturally occurring fissures into the earthquake rubble to seek for the trapped victims. Physically, a probe antenna should have a cylindrical wire structure and its radius be kept as small as possible. We have designed a probe antenna, which is essentially a sleeve antenna. The radiating element is a half-wavelength dipole, which is loaded with an inductor at the centre. The inductance of the inductor was determined numerically in the design. One half of the dipole is connected to the centre conductor of the coaxial cable via the inductor. The other half of the dipole is a quarter wavelength section of the outer surface of the coaxial cable. A quarter-wavelength choke, which is cylindrical tubing of larger radius than that of the coaxial cable, is soldered to the coaxial cable at one end and kept open at the other end. This choke is acting as a shorted, quarter-wavelength transmission line, which provides very high input impedance at the end point of the radiating dipole. Thus, this choke will stop the unbalanced current leaking to the outer surface of the connecting cable. A parasitic element, a wire of slightly shorter than half-wavelength, is placed next to the radiating dipole to increase the bandwidth of the antenna. The selection of dimensions of the parasitic element was made empirically through an experiment with a network analyzer. The whole structure of the probe antenna is encased in rugged plastic tubing.

The dual antenna system has two antennas, which are energized sequentially by an electronically controlled microwave single-pole double-throw (SPDT) switch. The SPDT switch turns on and off at a frequency of 100 Hz which is much higher than the frequency range of the breathing and heartbeat signals between 0.2 Hz and 3 Hz. Thus, we can consider that the two antennas essentially sample their respective objects at the same time. In this dual-antenna system, the two antenna channels are completely independent.

PRINCIPLE OF OPERATION

The basic physical principle for the operation of 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 the 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.

WORKING AND RANGE

A phase-locked oscillator generates a very stable EM wave at 1150 MHz with an output power of 400Mw (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 frequency of the microwave falls under two categories, depending on the type and nature of the collapsed building.

They are :-

 L (or) S band frequency say 1150 MHz

 UHF band frequency say 450 MHz An electromagnetic 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 EMwave of 450 MHz may penetrate deeper into a rubble without metallic wire mesh than that of 1150 MHz. 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.

EXPERIMENTS AND EXAMPLES

Experimental Results Obtained with the 450-MHz System at a Simulated Rubble in MSU Laboratory The 450-MHz microwave life-detection system was tested in simulated rubble constructed in the Electromagnetics Laboratory of Michigan State University. The rubble is depicted in Fig. 3. It was constructed with bricks, cinder blocks, and steel re-bars. The dimension of the rubble was about 5 ft wide, 6 ft long, and 6 ft high. Two layers of steel re-bars separated by 8 in are placed perpendicularly through bricks as shown in Fig.

 A human subject to be tested can lie down in the cavity at the bottom of the rubble. A reflector antenna or a patch antenna can be placed on the top of the rubble, while a probe antenna can penetrate into the rubble through a hole in the rubble. Typical experimental results of the breathing and heartbeat signals of a human subject lying in the rubble cavity obtained with the 450-MHz system are shown in Figs. Given below:-

 Fig. 1 shows a breathing signal superimposed with a heartbeat signal recorded for a female human subject. A reflector antenna was used and the radiated power was about 300 mW. The upper graph is the time domain measured signal and the lower graph is the fast Fourier transform (FFT) of the time-domain signal, which shows the frequency components of the time-domain signal. The upper graph clearly shows the breathing and heartbeat signals. The frequency domain FFT results show that the time-domain signal has a breathing signal of 0.3 Hz (the dominant peak) and a heartbeat signal of 1.36 Hz (the second largest peak). The other peak at 0.6 Hz (the third largest peak) is the second harmonic of the breathing signal. Other small peaks are due to noise or harmonics of the breathing and heartbeat signals. From a signal as shown in Fig. 4, it is easy to identify the breathing and heartbeat signals from either the time-domain signal or the frequency domain FFT results, and a buried human subject is easily detected. Fig. 5 shows the same measurement conducted on the same subject when she was holding her breath. The time-domain signal (upper graph) shows only the heartbeat signal and the frequency domain FFT results (lower graph) shows only a single dominant peak of heartbeat signal at 1.36 Hz. Other small peaks are probably due to noise. It is noted that when the signals of Figs. 4 and 5 are compared, the amplitude of heartbeat signal is found to be significantly smaller than that of the breathing signal as expected. Fig. 6 shows the background noise recorded when no human subject was in the rubble cavity. It is noted that the amplitude of the noise is lower than that of the breathing signal and the noise has wide spread frequency components as indicated in its FFT results. It is easy to distinguish the noise from the breathing and heartbeat signals from the amplitude and the frequency contents of the recorded signals.

Chapter 5

CONCLUSION

A new sensitive life-detection system using microwave radiation for locating human subjects buried under earthquake rubble or hidden behind various barriers has been constructed. This system operating at 1150 or 450 MHz can detect the breathing and heartbeat signals of human subjects through the earthquake rubble or a construction barrier of about 10-ft thickness. This system has been tested extensively with satisfactory results in simulated earthquake rubble constructed at the Electromagnetics Laboratory of Michigan State University. It has also been tested in a field test using realistic earthquake rubble at Rockville, MD, conducted by Maryland Task Force of FEMA. The possible shortcoming of this system is the effects of the background noise created by the environment and operators. A sophisticated signal processing scheme may further improve the system performance.

Reference

[1] K. M. Chen, D. Misra, H. Wang, H. R. Chuang, and E. Postow, “An X-band microwave life-detection system,” IEEE Trans. Biomed. Eng., vol. BME-33, pp. 697–702, July 1986.

 [2] H. R. Chuang, Y. F. Chen, and K. M. Chen, “Automatic clutter-canceller for microwave life-detection system,” IEEE Trans. Instrum. Meas., vol. 40, pp. 747–750, Aug. 1991.

 [3] K. M. Chen, J. Kallis, Y. Huang, J. T. Sheu, A. Norman, C. S. Lai, and A. Halac, “EM wave life-detection system for post-earthquake rescue operation,” presented at the 1994 URSI Radio Science Meeting, Seattle, WA, June 19–24, 1994.

[4] K. M. Chen, Y. Huang, A. Norman, and R. Ilavarasan, “EM wave lifedetection system for post-earthquake rescue operation,” presented at the 1995 IEEE/APS and Radio Science Int. Symp., Newport Beach, CA, June 18–23, 1995.

[5] K. M. Chen, Y. Huang, A. Norman, and Y. Yerramille, “EM wave life-detection system for post-earthquake rescue operation-field test and modifications,” in Proc. 1996 IEEE/APS-URSI Int. Symp., Baltimore, MD, July 21–26, 1996.

[6] K. M. Chen, Y. Huang, A. Norman, and J. Zhang, “Microwave life-detection system for detecting human subjects through barriers,” in Proc. Progress in Electromagnetic Research Symp., Hong Kong, Hong Kong, Jan. 6–9, 1997. [7] Y. Huang, K. M. Chen, J. Zhang, and Y. Dai, “Fast accurate measurement of phase and amplitude using EM microprocessor-based zero-balance system,” presented at the 4th Int. Symp. Antennas and EM Theory, Xi’an, China, Aug. 19–22, 1997.