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Design of a Wearable Electronics Package for Firefighter Monitoring


admin - September 27, 2018 - 1 comment

Michael F. Masterman, mfm@extreme-endeavors.com

Todd D. Leonhardt

Alton G. Dunn

Extreme Endeavors and Consulting

Philippi, West Virginia 26416, USA

Abstract

Wearable electronics using “e-fabrics” or electronic textiles are being developed for military and law enforcement personnel.  Also, the popularity and expansion of mobile communications has spurred interest in numerous personal electronic devices.  The design of an electronic lifeline system integrated with a firefighter’s personal protective equipment (PPE) is described in this paper.  The purpose of this electronic PPE system is to monitor the vital signs of active firefighters, while they are performing strenuous activity inside a burning structure, and transmit this data to the incident command center located outside of the building on the fireground.  The primary goal is to reduce firefighter fatalities, which are mainly the result of heart attacks.  Secondarily, the system may be used to provide voice communications, and the location of trapped or downed personnel.  The entire electronic monitoring system, including the sensors, processor, and communications equipment is embedded within the turnout coat, and is transparent to the user.  Operation of the system is automatic, and requires no actions or inputs from the individual.

Introduction

When firefighters don their turnout gear the electronics automatically turn on, requiring no input from the users to initiate measurements or communications.  Each coat communicates with a base station, either located on the truck or in a small briefcase.  Charting of the firefighter’s pulse begins almost immediately, as the firefighter climbs onto the vehicle to respond to the scene.  The jacket has embedded in it all of the hardware for monitoring physiology and the environment around the firefighter. 

The sensors embedded in the cuffs of the coat detect the radial pulse from the heart beat, while temperature sensors both inside the coat and outside begin take baseline thermal recordings, showing the temperature gradient through the coat.  Utilizing the latest in Texas Instrument Digital Signal Processing technology the data from the sensors is processed to conserve valuable transmission time and bandwidth, creating a useable and effective monitoring tool.

The transmission of data is performed through a software-defined radio that transmits and receives via a wearable antenna embedded in the sleeves of the turnout coat.   This antenna is a conformal metallized fabric that eliminates the problems of an external whip antenna that can be hooked by obstacles.  This embedded radio also allows for relaying of data via a custom-designed ad-hoc network.  If a firefighter becomes lost or injured, the data relay can direct firefighters to the location by indicating which firefighter is the closest.

The Safety officer can quickly look at a view screen, evaluate multiple firefighters, organize reinforcements, and assess the overall safety of the team.  Statistics show that overexertion is one of the most common causes of death for a firefighter.  The Electronic Life Line is a system that is intended to reduce this factor all while allowing more efficient and effective operations on the fire ground.

The greatest design challenge of this coat, the user, the firefighter will never even know the monitoring systems are there and in operation.  The coat will resemble a normal, everyday turnout coat, dawning the gear will be the same as always done!

The decision to place all of the system components on the upper torso, excluding the head, was to satisfy the design goal of “self-containment.”  We wanted to avoid interconnections between different parts of the PPE, such as the helmet, bunker pants, boots, gloves, hood, etc.  Although conceptually possible, using a wireless network to connect the electronics to the sensors and communication device was less desirable than maintaining everything in one location, i.e., within the turnout coat.

Monitoring

The primary goal for this system is the remote monitoring of the heart rate and other physiological parameters for an active individual in one of the most demanding applications known, the fire industry.  The noise artifacts induced from motion can easily be many orders of magnitude larger than the heart beat signal.  During times of extreme physical exertion, the PQRST complex of pulses can get close enough to confuse ECG-based electronics into misinterpreting the waveform, thus effectively miscalculating the measured heart rate.

Extreme Endeavors began research on physiological monitoring systems while collaborating with Army Research Labs under a Cooperative Research And Development Agreement (CRADA) with the Natick Soldier Center. We have experience using acoustical sensors in conjunction with advanced wearable signal processing electronics.  Combined with the firefighting experience of Extreme Endeavors engineers, we have developed a tool ideally suited for use in this application.  Figure 1 shows pictures from our proof of concept system.  During this testing we acquired raw data as shown in Figure 2A which provides us with design parameters for a real time system that indicates the firefighters physiology

Figure 1: Photographs showing the proof-of-concept hardware.
Figure 2: (A) Joint Time Frequency Domain Representation of Acoustic Data; (B) Raw Acoustic Data Taken Under Laboratory Conditions from the Right Wrist

With these sensors we are able to measure an electrical signal derived from the sounds created by the heart beat at any pulse point.  During initial testing, both 10 and 12-bit resolution analog-to-digital converters were used to sample the acoustical signal.  This resolution was found to provide an inadequate dynamic range, so the current system will use a 24-bit analog-to-digital converter followed by processing with an advanced state-of-the-art digital signal processing algorithm.  This algorithm has been shown to be capable of extracting the heart rate from a signal that is 100 times smaller than the noise it is embedded in.  This method involves direct motion artifact measurements, which provides an added benefit of knowing when the firefighter is moving and a measurement of how vigorously.

 

The acoustic sensors can also be used to measure respirations, oral intake, impact, systolic blood pressure, as well as operating as a voice microphone in extremely noisy environments.  One of the additional benefits to the sensing and processing method utilized is that such things when a firefighter is spraying water and the general activities of the firefighter can be determined.  Due to the complexity of the processing required for this, Extreme Endeavors is only working on determining pulse and movement during the initial release of this product.  A sixteen second strip of data is shown in Figure 2B from the improved data sampling system, this was taken in a laboratory environment; once the firefighters begins normal duties, the signal-to-noise-ratio  can decrease dramatically.  This is the reason very complex and computationally intensive signal processing techniques are required. 

Software-defined Radio

Data transmission is not a trivial task when compounded by multipathing and environmental effects created by the surroundings firefighters must work in.  A firefighter continuously encounters blind spots and missed communications while working inside a basement or deep within a structure. We plan to alleviate this problem with a digital software-based radio that switches between multiple separate frequency bands for optimal transmission in varying environments.  By changing the transmission frequency, the effects of multipathing and blockage can be reduced, allowing communications to continue on a different channel.

The intelligence of the radio is provided by a TMS320C6713 Digital Signal Processor (DSP) chip manufactured by Texas Instruments.  The DSP is interfaced with a high-speed analog-to-digital converter that samples the intermediate frequency signal after it has been mixed down from the transmission frequency.  The transmission waveform and the local oscillator are provided by a Direct Digital Synthesizer (DDS) combined with a phase lock loop and/or frequency multiplication scheme.  An RF switch determines if this signal is to be used as the local oscillator or as the transmission signal, thus providing a reduction in power consumption by using less components.

Through the flexibility of the Direct Digital Synthesizer and analog-to-digital converter, the frequency and modulation of transmission can be adapted and configured to allow for multiple environmental factors and adaptation as the situation calls for it.   This in turn will add reliability and integrity, allowing it to serve any departments need.  As the concept is implemented and tested, additional features will be added in the software, which will enable data relaying from firefighter to firefighter.  This way, if a firefighter goes too far inside of a burning structure such that there signal is obscured, another firefighter’s radio can relay the signal out to the base station.

Wearable Antenna Analysis

The communications link between the firefighter and the incident command center located outside of the building operates in the frequency range of 450 to 470 MHz.  (Future development is planned for an additional 5.3 GHz and a 46 MHz link.)  The antenna worn by the firefighter inside the garment is designed to be effective over this 20 MHz band centered at 460 MHz, which is considered to be somewhat above UHF.

Tests have shown that the operation of the antenna, in particular the impedance and the radiation pattern (or antenna “coverage”), is altered by the proximity of a person’s body.  The impedance of an antenna is similar to the resistance of an electronic component.  How much power is radiated or received by the antenna depends on the impedance, similar to how much current flowing through an electric circuit depends on the resistance.  For an antenna, the impedance varies with the frequency.

Two types of antennas were investigated, a “bowtie” and a “folded dipole.”  In fact, both are variations of perhaps the simplest antenna of all, the common dipole.  A basic dipole consists of two wires, each usually one-quarter wavelength, fed at the middle by a transmission line.  The dipole is known for its “doughnut” shaped radiation pattern, which is essentially uniform in one plane.  The coverage drops along the axis of the antenna (which is where the holes would be in the doughnut).  As long as the antenna is worn in the up-and-down position, these nulls would point at the floor and the ceiling, and the main radiation would be to the sides, which is exactly what is desired for this application.

The bowtie is essentially a dipole antenna with fat, flat, tapered ends as shown in Figure 3. 

Figure 3: Bowtie antenna fed by a coaxial cable

The folded dipole, shown in Figure 4 looks like a loop, but it is not a “loop” antenna.  Because of its particular length and current distribution, it behaves like a “double” dipole. 

Figure 4: Folded dipole antenna fed by a parallel wire transmission line


Prototypes of each antenna type were fabricated from thin sheets of copper or brass, and were initially duct-taped, and subsequently attached with Velcro to the coat liner.  The bowtie antenna was small enough to fit on either portion of the sleeve, upper or lower.  At first the lower, or forearm position, seemed preferable since its interaction with the body might conceivably be less.  However, the bending of the elbow and related wrinkling of the material farther down the arm in addition to the effects of wrist motion led to the selection of the upper arm.  The final location of the dipole antenna was the side of the upper arm, just about where an arm patch would be sewn.

Radiation patterns for both antenna types fall off about 20 dB on the opposite side of the body.  However, again because of its location, the pattern from the folded dipole falls of much more rapidly.  The coverage of the bowtie antenna located on the sleeve remains relatively constant over an angular region ± 90° from the centerline and falls to –10 dB to –20 dB on the opposite side.  In contrast, the radiation level of the folded dipole is down –10 dB at ± 90° and goes from –20 dB to -∞ on the far side.  (The 0° reference or “centerline” is the position of the standing person when the antenna is directly facing the base station receiver.)

Ultimately, the bowtie antenna geometry is simpler to fabricate and install than the folded dipole.  Also, the sleeve location is preferable to the jacket front.  Therefore, the bowtie antenna on the upper sleeve was the final design choice.  A bowtie antenna with the same dimensions was made from a fabric material surfaced with a conductive coating.  Test results indicated there was no change from the thin metal prototype.

References

Branson, K. (2002).  Wireless revolution.  Fire Chief Magazine, December 1, 2002: 12.

Haykin, S. (2005).  Cognitive radio: brain-empowered wireless communications.  Selected Areas in Communications, IEEE Journal on, 23(2): 201-220.

Jackson, B., Peterson, D. J., Bartis, J., LaTourrette, T., Brahmakulam, I., Houser, A., and Sollinger, J. (2002).  Protecting emergency responders – lessons learned from terrorist attacks.  ISBN: 0-8330-3149-X.

Scanlon, M. V. (1998).  Acoustic sensor for health status monitoring.  Proceedings, IRIS Acoustic and Seismic Sensing, 1998, Vol. II: 205-222.

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  1. Extreme Endeavors and Consulting LLC

    Firefighter Protection Project Funded by the National Institute of Health

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