IMTC 2007 - Instrumentation and Measurement
`Technology Conference
`Warsaw, Poland 1-3 May 200
`
`Design of a Low-cost Physiological Parameter Measurement and Monitoring Device
`
`, S.C. Mukhopadhyal, B.S. Devlin2 and S. Demidenko 2
`G. Sen Gupta1
`1School of Electrical and Electronic Engineering, Singapore Polytechnic, 500 Dover Road, Singapore
`2Institute ofinformation Sciences and Technology, Massey University, Palmerston North, New Zealand
`Em ail: s. c.m ukhopadhyay@m assey. ac.nz, SenGupta@sp. edu. sg, b. s. dev lin@m assey. ac.nz, s. demidenko@m assey. ac. nz
`
`Abstract- In this paper we present the design of a low-cost system
`that can be used to monitor physiological parameters, such as
`temperature and heart rate, of a human subject. The system consists
`of an electronic device which is worn on the wrist and finger, by an
`elderly or at-risk person. Using several sensors to measure different
`vital signs, the person is wirelessly monitored within his own home.
`An impact sensor has been used to detect falls. The device detects if
`a person is medically distressed and sends an alarm to a receiver
`unit that is connected to a computer. This sets off an alarm, allowing
`help to be provided to the patient. The device is battery powered for
`use indoors. The device can be easily adapted to monitor athletes
`and infants. The low cost of the device will help to lower the cost of
`home monitoring of patients recovering from illness. A prototype of
`the device has been fabricated and extensively tested with very good
`results.
`
`-
`physiological
`Keywords
`transmission, home monitoring
`
`parameters,
`
`sensors, wireless
`
`I.
`
`INTRODUCTION
`
`Many elderly people dread the idea of being forced to live
`with their adult children, or in a rest home or in other
`sheltered
`living
`arrangement. They want
`to
`live
`independently and keep control of their own lives. Yet at the
`same time they know there is a high risk of injury or even
`death because of a fall or stroke. With the population aging
`in most developing countries, there will be more and more
`elderly people living alone in future. Such people need to be
`monitored continuously and provided with
`immediate
`medical help and attention when required.
`The cost of hospitalization is ever increasing, so is the cost
`of rehabilitation after a major illness or surgery. Hospitals are
`looking at sending people back as soon as possible to recoup
`at home. During this recovery period several physiological
`parameters need
`to be continuously measured. Hence
`telemedicine and remote monitoring of patients at home are
`gaining added importance and urgency [1-3]. Today, the
`progress in science and technology offers miniaturization,
`speed, intelligence, sophistication and new materials at lower
`cost. In this new landscape, micro-technologies, information
`technologies and telecommunications are the key factors in
`inventing devices to assist mankind. Patients are being
`monitored using a network of wireless sensors [4]. A system
`to monitor the overall health of welfare facility residents, who
`need constant care, has been reported in [5]. This system [5]
`has been designed with wireless sensors, wireless repeaters
`and a host computer. The system consists of a piezoelectric
`
`1-4244-0589-0/07/$20.00 © 2007 IEEE
`
`sensor, a 2 axis accelerometer, a microcontroller and a low
`It records respiration activity and
`power
`transceiver.
`indicators of posture for 24 hours. These data are transmitted
`to the wireless repeater by the transceiver. The wireless
`repeaters, which are installed throughout the welfare facility,
`send data, including the repeater's ID, to the host computer.
`The ID is used to detect the resident's location in the welfare
`facility. The host computer stores the data, which can be used
`to analyze the resident's overall health condition. When the
`resident is in an emergency situation, such as falling or in an
`inactive state for more that the allotted time, the host
`computer automatically alerts the situation to the care staff by
`an alarm sound and also by mobile phone. However, all
`reported systems are relatively expensive.
`In the back drop of the
`importance of continuous
`monitoring of vital physiological parameters of a patient, our
`research was undertaken
`to design a
`low-cost smart
`monitoring device. It aims to provide peace-of-mind to users
`who have medical problems, but are not placed in a hospital
`for monitoring. Caregivers, who look after patients with
`mental or physical disabilities, can use this device when they
`are not able to visually supervise the patients.
`Currently there are monitoring products in the market that
`are aimed to provide emergency assistance to senior citizens,
`rehabilitation patients,
`and medically or physically
`challenged individuals, but these have limitations. St John's
`and Medic Alert's Lifelink™ [6] allows the user to set off an
`alarm manually if they are under medical stress, which will
`then dial designated contact phone numbers. The fundamental
`problem with this system is that when medical emergencies
`happen to the user, they are often unconscious and unable to
`press an 'emergency alert button'. There is no product on the
`market which does not require manual activation of the alarm
`and monitors a user's vital signs smartly. This is the novel
`design goal of the work presented in this paper.
`The reported device consists of a wrist strap and a finger
`glove. This allows the sensors to be mounted around the wrist
`and finger. A battery and a microcontroller, with built-in RF
`transceiver, are mounted within the wrist strap as well. In
`Section II we present the complete system overview. All the
`sensors are explained in Section III. The hardware details are
`in Section IV and the algorithms in Section V. The prototype
`and test results are discussed in Section Vl. The paper ends
`with a discussion on future developments
`
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`

`

`II. SYSTEM OVERVIEW
`
`B. Heart Rate Sensor
`
`The system has been designed to take several inputs from
`a human subject to measure physiological parameters such as
`temperature and heart rate. Figure 1 shows the functional
`block diagram of the system hardware. The inputs from the
`sensors are processed and the results transmitted to a receiver
`unit, which is connected to a computer placed in the home,
`using Radio Frequency (RF) wireless technology. The
`receiver unit decodes and analyses the data. If it is inferred
`that the person is medically distressed, an alarm is generated.
`The design is modular which makes it rather easy and straight
`forward to add extra sensors for measuring and monitoring
`other parameters. The hardware blocks are explained in full
`details in a later section.
`
`Fig. 1 Functional block diagram ofthe system hardware
`
`III. SENSORS AND INTERFACE
`
`The system consists of three sensors- a temperature sensor,
`heart rate sensor, and an impact sensor. All the sensor
`circuitries used in the design generate analog voltages which
`are fed to the ADC (Analog-to-Digital) inputs of the micro(cid:173)
`controller. The ADC
`inputs are
`time-multiplexed and
`sampled at different rates. The description of individual
`sensors follows.
`
`A. Temperature Sensor
`
`The temperature measurement is done using a LM35 [7]
`precision integrated-circuit temperature sensor. It provides an
`accuracy of +/- 0.25°C within the desired temperature
`measurement range of 20-40°C. It has a very low current
`drain of 60 ~A. This sensor is mounted within the wrist strap,
`positioned in such a way that it is in contact with the skin,
`allowing it to measure the external temperature of the skin.
`From the skin temperature, the body temperature is estimated.
`Because an exact measurement of body temperature is not
`required, this method is suitable. Rather, relative changes are
`monitored within set thresholds, which set off the alarm. This
`allows the device to detect changes in body temperature that
`could indicate the patient is undergoing any of the following
`conditions:
`trauma,
`injury, heart attack, stroke, heat
`exhaustion, and bums [8]. The temperature sensor is sampled
`once every 3 seconds.
`
`2
`
`A custom heart rate sensor was designed to read the
`patient's beats per minute (bpm). The designed sensor is very
`small and inexpensive. The technique used to measure the
`heart rate is based on near-infrared (NlR) spectroscopy. NIR
`spectroscopy involves using light in the wavelength of 700-
`900nm to measure blood volume. At these wavelengths most
`tissues do not absorb light- other than hemoglobin (which is
`what we are interested in). This allowed for designing a non(cid:173)
`invasive and low cost method of measuring the pulse. A
`silicon phototransistor and a GaAs infrared emitting diode
`were used in the sensor, moulded into a flat side-facing
`package. The amount of light that was detected by the
`phototransistor varied with the patients heart pulse, as the
`amount of absorbed IR light changed with the flow of blood,
`which is directly linked to the heart rate. This signal was then
`amplified, filtered, and sent to the microcontroller to be
`analyzed. The heart rate sensor was mounted in the finger
`glove as this position proved to give the best response.
`
`C. Impact Sensor
`
`An ADXL311 accelerometer was used as an impact
`sensor. It provides a 2-axis response, measuring accelerations
`up to +/- 2g. It was fitted into the wrist strap. The
`accelerometer provides an analog voltage, the amplitude of
`which is directly proportional to acceleration. This signal was
`scaled down to bring it within the acceptable input range of
`the micro-controller, and then analyzed. Software algorithms
`were used to detect sharp impacts, while allowing slower
`movements, such as walking, to be ignored. The purpose of
`this sensor was to detect sudden impacts that could indicate
`the patient had fallen over.
`
`IV. HARDWARE DESIGN
`
`The hardware was built in three separate blocks. A sensor
`card was designed
`to house
`the
`temperature sensor,
`accelerometer and the connections for the NIR emitter and
`detector. A separate analog card was designed for all the
`analog processing circuitry needed for the sensors, primarily
`for processing the heart rate signal. The temperature and
`acceleratim output was fed directly to the micro-controller.
`The micro-controller was mounted on a separate card which
`also had the antenna connection. The cards were connected
`by ribbon cables within the wrist strap. Figure 2 shows the
`circuit schematics of the sensor units and Figure 3 shows the
`analog processing circuit for measuring heart beat rate.
`
`A. Lock-In Amplification for heart rate measurement
`
`The micro-controller modulates the infra-red emitter
`signal at 1 KHz through a npn transistor (Figure 2). This is
`then mixed with the signal obtained from the IR sensors
`(back scattered light). This technique, known as Lock-In
`[9],
`involves phase sensitive detection.
`Amplification
`
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`Fig. 2. Sensor Units
`
`The signal of interest (1-2 Hz) is moved into a much higher
`frequency band together with the noise that is picked up by
`the sensors. Demodulation is performed by an analog switch,
`which demodulates at the same frequency and in phase with
`the 1 KHz modulating signal. The resulting signal is then
`passed through a low-pass filter with a cut off frequency of
`6Hz. This allows the signal of interest to pass through, while
`filtering out all noise that originated from the sensor. This is
`illustrated in figure 4.
`
`B. Controller Card
`
`The controller used in the wrist strap unit (and also in the
`receiver unit) is a Nordic nREF24E1. This takes inputs from
`
`Fig. 3. Signal conditioning forHemt Rate Monitoring
`
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`
`the sensor circuits in the form of analog voltages. Each sensor
`has a dedicated ADC channel which is multiplexed by the
`microcontroller. Each sensor's signal is sampled at a pre(cid:173)
`defined rate,
`through
`interrupt-driven algorithms. This
`microcontroller was chosen because of its small footprint
`(6mm x 6mm),
`low power consumption, and built-in
`
`3
`
`

`

`transceiver. The microcontroller is responsible for producing
`the carrier signal that modulates theIR emitting diode, which
`is used in demodulation too. It is powered by a 9V battery,
`which is regulated down to 5V. The controller card contains
`the microcontroller, crystal oscillator, RF antenna connection,
`and header pins which connect analog and digital pcrts to the
`other components of the unit
`
`C. REceiver Unit
`
`The hardware of the receiver unit is housed in a plastic
`box and consists of a nREF24EI microcontroller, antenna, a
`5V AC adaptor and serial interface port. The receiving unit is
`connected, via a RS232 serial port, to a personal computer
`(PC) and is constantly receiving information about the
`patient's medical status. A program, running on the PC,
`receives the packetized information from the serial port,
`decodes the packet and then displays this informaticn on the
`PC monitor. The program offers several opticns to set
`thresholds of the allowable range of heart rate, skin
`temperature and impact sensor. WhEn the readings from the
`patient move outside of the set range, an alarm is raised. The
`software can depict the change in data over time in a
`graphical plot. One receiver unit can be inta·faced with
`multiple wrist units. In the software, one can tab through the
`information received from each wrist unit.
`
`D. Communication
`
`Communication between the wrist units and the receiver
`unit is wireless, powered by the nREF24EI, and transmitted
`in the unlicensed 2.4GHz frequency band. Information is
`gathered every 3 seconds from the sensors and thEn encoded
`into a packet. Each packet is 6 bytes long, composed as
`shown in figure 5.
`
`Device ID
`
`Heart Rate
`
`6 Byles loog
`
`Skin T emperatute
`
`Impact Information
`
`The data packet is sent by the nREF24EI using a
`technology termed ShockBurstTM This allows data to be
`clocked into a FIFO buffer at a low data rate, and then
`transmitted all at once at a very high rate. This lowers power
`consumption by minimizing the amount of receiving and
`transmitting time. Data is transmitted by this method at
`IMbps. This also reduces the risk of a data collisicn with
`other wrist units, or devices operating in the same band.
`
`V. SOFTWARE AND ALGORITHMS
`
`the
`on
`implemented
`algorithms were
`Several
`microcontrolla·. These algcrithms take inputs fi·om the
`sensors and process them into meaningful information.
`
`A. Hea1t Rate Algorithm
`
`The heart rate signal is an oscillating signal of I to 2Hz. This
`is sampled by the mia·ocontroller at 50Hz, and then averaged
`over 5 samples, which gives the algorithm an effective data
`input rate of I 0 samples per second. This is stored in a buffer
`which is 32 bytes long. Once this buffer is full, the algorithm
`scans though each value in the buffer to compute the period
`of the signal (see figure 6) Since the signal is not sinusoidal,
`rather has distinct peaks corresponding to the contractions in
`the heart, the period is measured by marking transitions over
`the 25% marl< This is the most reliable part of the received
`signaL Since the sample time is know (0 Is), the number of
`samples between these 'transition points' is used to calculate
`the period and hence the frequency. The frequency is then
`multiplied by 60 to compute the heart rate (in bpm, beats per
`minute) of the patient
`
`Fig. 6 . Heart rate rn easurernent algorithm
`
`B. Impact Algorithm
`
`Start of File -Bit balanced (10 101010)
`End of File -Bit balanced (101 01 010)
`
`Fig. 5. Data packet composition
`
`The heart rate included in the packet is in beats per
`minutes (bpm) which is calculated using the Heart Rate
`Algorithm explained in Section V. The temperature data is
`the result of the ADC conversion, done once every 3 seconds,
`and is decoded into degrees Celsius at the receiver unit. The
`impact informaticn is the maximum output voltage of the
`accelerometer measured in the past 3 seconds. This is also the
`digital value obtained from the ADC conversion.
`
`The signal received from the accelerometer is a DC
`voltage, the amplitude of which is directly related to the
`acceleraticn (62mV/g). This input is sampled at lOHz, and
`stored in a buffer which is I 0 bytes long. Every second, this
`buffer is scanned, and the maximum amplib..ide is recorded.
`At the receiver unit, this data is compared to recorded
`representative values for walking and jogging. This ensures
`that an alarm is set off only for very sharp, heavy impacts.
`
`4
`
`

`

`The reliability of impact detection was very good and the
`walking could easily be differentiated from a fall.
`
`with thumb on carotid artery). Figure 9 shows a segment of a
`received heart rate signal.
`
`VI. PROTOTYPE AND EXPERIMENTAL RESULTS
`
`The Nordic micro-controller development board was used
`to build and test the prototype design. The analog processing
`circuitry and the sensors were assembled on PCBs which
`were placed within the wrist strap. Figure 7 shows the
`prototype hardware. The prototype was powered off a 9V
`battery. The RF transmission has been tested to operate
`successfully at I 0 meters range through obstacles such as
`concrete walls. The receiver unit. without the casing, can be
`seen in figure 8.
`
`Fig. 9. Segment of a heart rate signal
`
`B. Impact Sensor Test results
`
`The output of the accelerometer was tested with walking
`and simulated falling. The results showed the difference was
`simple to detect and proved the accuracy of the algorithm.
`Figure I 0 shows the impact sensor output.
`
`Imp ct
`
`Walking
`
`/~
`
`Fig. 10. Impact sensor output for walking and a fall
`
`VII. DISCUSSIONS AND FUTURE DEVELPOMENTS
`
`In this paper we have presented the research, of applied
`nature, done to monitor physiological parameters such as skin
`temperature, heart rate and body impact. A prototype was
`successfully developed and tested to establish the proof of
`concept. The algorithms were tested and found to be accurate
`and reliable. The novel aspect of the design is its low cost and
`detection of medical distress which does not necessitate
`pressing any panic button. This is an enormous improvement
`over existing commercial products.
`An important aspect of the design was miniaturization, so
`that the system was as non-intrusive as possible to the wearer.
`This was achieved by the use of surface-mounted devices on
`the PCBs designed. Low power operational amplifiers were
`used to minimize battery consumption. The price of the unit
`currently is $70. The major costs come from the use of
`precision components, accelerometer and temperature sensor.
`With some modification, the system can be made available
`commercially. Future improvements will focus on the use of
`
`Fig. 7. Device prototype and finger glove
`
`Fig. 8. Prototype receiver connected to a PC
`
`A. Heart Rate Sensor Test Results
`
`A new heart rate is calculated every 3.2 seconds. The
`output of the sensor is a 200-400 m V p-p signal, riding a DC
`signal of 500mY. On human tests it is able to measure within
`±2 bpm of a rested patient (confirmed by measuring pulse
`
`5
`
`

`

`REFERENCES
`
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`[6] LifeLink
`Panic
`http:! lwww .bgehome.com/hs_protecti on .html#lifelink
`[7] National Semiconductor, LM35 - Precision Centigrade Temperature
`Sensor, http:! /www .national. com/pli'LMILM3 5 .html
`[8] Y.C.Sydney, A-Z Health Guide from WebMD: Medical tests, Body
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`[9] Eastern Michigan University, Lock-In Amplification overview,
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`Flow Sensor/Phoenix PiezoPulse.htm
`-
`-
`
`Button,
`
`flexible PCBs to replace the stiff cards, so that it could be
`moulded around the wrist unit, making it more comfortable
`for the wearer.
`The design of the IR sensors could be improved to
`decrease its susceptibility to noise, to a point where it could
`be moved onto the wrist unit. This would provide a much
`more comfortable and less intrusive unit, getting rid of the
`finger glove.
`The addition of a blood-oxygen sensor would allow the
`system
`to detect medical distress more accurately by
`measuring the amount of oxygen in the blood (HbO). This
`could be implemented by the addition of another emitter
`diode operating at a wavelength which is more readily
`absorbed by oxygen, and measuring the light using photo
`detector. Blood pressure can also be measured by a technique
`known as 'pulse delay' [10] which involves calculating the
`time for the heart pulse to travel a known distance. This is
`directly related to the blood pressure of a patient, and allows
`for non-invasive measurement.
`The unit was initially designed for use by the elderly,
`within the house, where a caregiver is present but is not able
`to be constantly in visual contact with the subject. The
`receiver unit would be enhanced so that it can connect to
`either the local or cellular phone network, and in the case of
`an emergency would contact an ambulance. Beyond the
`application for elderly patients is the use by anyone who is at(cid:173)
`risk, with a mental or physical disability. The device could be
`applied to prevent cot deaths in babies, by alerting the parents
`when the infant becomes stressed.
`
`6
`
`