Wireless Physiological Parameter Monitoring and

IOSR Journal of Electronics and Communication Engineering (IOSR-JECE)
e-ISSN: 2278-2834, p- ISSN: 2278-8735. Volume 5, Issue 1 (Jan. - Feb. 2013), PP 25-35
Wireless Physiological Parameter Monitoring and Recording
M. Manojprabu
M.E, Embedded System Technologies University college of Engineering BIT campus, Trichy
Abstract: The design and development of a Zigbee smart non-invasive wearable physiological parameters
monitoring device has been developed and reported in this paper. The system 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 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 user. The device is battery
powered for use outdoors. 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.
IN Recent Times, wireless sensors and sensor networks have become a great interest to research,
scientific and technological community. Though sensor networks have been in place for more than a few
decades now, the wireless domain has opened up a whole new application space of sensors. Wireless sensors
and sensor networks are different from traditional wireless networks as well computer networks and, therefore,
pose more challenges to solve such as limited energy, restricted life time, etc. [1].
Wireless sensing units integrate wireless communications and mobile computing with transducers to
deliver a sensor platform which is inexpensive to install in numerous applications. Indeed, co-locating
computational power and radio frequency (RF) communication within the sensor unit itself is a distinct feature
of wireless sensing. Today, the progress in science and technology offers miniaturization, speed, intelligence,
sophistication, and new materials at lower cost, resulting in the development of various high-performance smart
sensing system. Many new research is focused at improving quality of human life in terms of health [2] by
designing and fabricating sensors which are either in direct contact with the human body (invasive) or indirectly
(noninvasive). One of the reasons for more development in this area is the global population and rise in ageing
population [3], one statistic provided by the U.S. Department of Health that by 2050 over 20% of the world's
population will be above 65 years of age. This results in a requirement for medical care, which is expensive for
long-term monitoring and long waiting lists for consultations with health professionals. 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 [4]-[6]. Patients are being monitored using a
network of wireless sensors [7]. 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. Such people need to be monitored continuously and provided with immediate medical help and
attention when required. We seek to come up with solutions, which help to remove anxiety. As a result, there is
a need for an accurate, flexible, noninvasive, comfortable, reliable, and low-cost monitoring unit that unites all
these demands.
A system to monitor the overall health of welfare facility residents, who need constant care, has been
reported in [5]-[8]. This system [8] has been designed with wireless sensors, wireless repeaters and a host
computer. The system consists of a piezoelectric sensor, a two-axis accelerometer, a microcontroller, and a lowpower transceiver. It records respiration activity and 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. After researching work related to fall detection, falls are the most
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Wireless Physiological Parameter Monitoring and Recording System
widespread domestic accidents among the elderly. Furthermore, it frequently happens that elderly people who
have previously experienced a fall fear a new fall and sink gradually into inactivity. Due to less mobility it leads
progressively to an increase in the risk of a fall [17]. Literature review reveals that reliable fall detection based
raw sensor data is much discussed in literature and requires algorithm development of wide scope based on
deeper knowledge of specific [18] application principles as outlined in and to monitor a range of human
movement. However, all reported systems are relatively expensive and the cost depends on the number of
sensors used. So, there is an effort to develop the home monitoring system using optimum number of sensors
These facts show an increasing demand for long-term health monitoring which is affordable,
continuous, and unobtrusive [9], which will result in considerable impact on annual medical costs [2] and health
management [10], [19]. Wearable systems for continuous health monitoring are a key technology in helping the
transition to more practical and affordable healthcare. It not only allows the user to closely monitor changes in
her or his physiological parameters but also provides feedback to help maintain an optimal health status.
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™ [12] 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, though research is currently undergoing [22]. This is the novel
design goal of the work presented in this paper.
The reported device consists of a wrist strap and a finger ring (circuitry). This allows the sensors to be
mounted around the wrist and finger and the 8051 microcontroller unit connected via ribbon cable. 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 VI. This
paper ends with a discussion on future developments
System Overview
Fig. 1 shows the functional block diagram of the system hardware. The system has been designed to
take several inputs to measure physiological parameters of human such as temperature, heart rate, and detection
of any fall. The inputs from the sensors are integrated and processed. The results are sent through the XBee
Module to a host computer, which stores the data into an Access Database. The values can then be displayed on
the Graphical User Interface (GUI) running on a computer. If it is inferred that the person is medically
distressed, an alarm may be generated. The program is a user interface, allowing a report on the current status of
the individual. Once the user has connected to the receiver unit, data is automatically updated.
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Wireless Physiological Parameter Monitoring and Recording System
On the screen. Beat per minute (BPM), body temperature, and impact (in both axes) are given on the display.
The data are also plotted on a time graph which can be customized to show data received from any of the
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.
III. Details of the Sensing System
The current version of the system consists of three sensors: a temperature sensor, heart rate sensor, and
an impact sensor. Temperature sensor circuitry used in the design generates analog voltage which is fed to the
ADC (Analog-to-Digital) inputs of the micro-controller. The ADC input is time-multiplexed and sampled at
different rates. The description of individual sensors follows.
A. Temperature Sensor
The skin temperature measurement is done using an integrated circuit, the DS600 temperature sensor
produced by MAXIM - Dallas Semiconductor [11]. The Sensor gives an analog output depending on the
measured temperature. This voltage has to be measured by the microcontroller using a 12 bit Analog-to-Digital
converter (ADC). Fig. 2 shows the circuit application of DS600 IC, used as temperature sensor 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. There can be
different methods to estimate the exact body temperature from skin temperature [23], but with a rough
estimation usually the body temperature is 5.1 C higher than skin temperature when the body temperature is
measured at the ear by the National DM-T2-A thermometer used by a general practitioner compared to the skin
Fig. 3. Measured transfer characteristics of DS600.
Measured at the wrist. Because an exact measurement of body temperature is not required, this method is
suitable. Rather, relative changes are monitored within set threshold, which sets 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 burns [14].
The IC has an accuracy of ±0.5 C and a linear output with
6.45 C and an offset of 509 mV of DS600 IC factory calibration [11], which is shown by the experimental
transfer characteristics, as in Fig. 3.
The Output of the ADC has to be converted into the right value. The ADC-value is first compared with the
Reference Voltage of 2.4 V (1) and then with the characteristic of the DS 600 to get the Value for the
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Wireless Physiological Parameter Monitoring and Recording System
The sensor has been used for measurement of body temperature at wrist, upper arm and neck for many males
and females and only two results are shown here. Fig. 4 shows the comparison of temperature measured by the
developed temperature sensor with respect to a thermometer (reference temperature). The temperature at three
different positions (wrist, neck and upper arm) was measured for three male and three female persons. The
temperatures were measured at different times with varying ambient conditions. Fig. 5 shows the variations in
temperature measured with respect to different positions. The accuracy of the measurement is shown in Fig. 5
and is seen that at steady state the error is within 0.5 C.
B. Heart Rate Sensor
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
spectroscopy (NIR). NIR involves using light in the wavelength of 700-900 nm to measure blood volume. At
these wavelengths most tissues do not absorb light - other than haemoglobin (which is what we are interested
Fig. 4. The measured temperature of one male and one female.
Fig. 5. The accuracy of measured temperature for males and females.
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Wireless Physiological Parameter Monitoring and Recording System
Fig. 6. Functional block diagram of the heart rate sensor [21].
This allowed for designing a noninvasive and low cost method of measuring the pulse. A silicon
phototransistor, moulded into a flat side-facing package, and a GaAs Infrared Emitting Diode were used in the
Fig. 6 shows the functional block diagram of the heart rate sensor. The amount of light that was detected by the
phototransistor varied with the patient's 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
Fig. 7. The circuit schematic of heart rate measurement circuit.
Fig. 8. The heart rate signal at the collector of the photo-transistor and at the output (Heart_Rate_signal).
Fig. 9. Heart rate measurement algorithm.
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Wireless Physiological Parameter Monitoring and Recording System
Fig. 10. The comparison of heart rate measurement.
Form of pulses is interfaced with microcontroller through its digital port for further processing. The
waveforms at the collector of the photo-transistor and at the final point (Heart_rate_signal) are shown in Fig. 8.
The heart rate is measured by using the hardware interrupt facility of the microcontroller. The heart rate as is
shown in Fig. 8 is a square wave pulse of varying duty ration. The time period of the wave is measured using the
Timer 0 and in combination with hardware interrupt. The measurement algorithm is explained with the help of
Fig. 9. The Timer 0 generates a tick pulse at every 10 . The total tick count in one period (BPM_T_COUNT) is
measured. The frequency is then calculated using the equation Sensor was mounted in the finger ring as this
position proved to give the best response.
The hardware was built in two separate blocks. A sensor PCB was designed to house the temperature
sensor, accelerometer and the connections for the NIR emitter and detector. The temperature, heart rate and
impact sensors output was fed directly to the microcontroller through header pin-ribbon cable connection. The
microcontroller was mounted onto a separate PCB which also had the ZigBee module connection. Fig. 7 shows
the circuit schematics.
The sensor after filtering provided a clean wave that when observed on an oscilloscope confirmed that
the sensor was correctly measuring the patients pulse. To get the best and most accurate results with the heart
rate sensor we chose to measure the pulse at the finger tip like commercial device do. Nevertheless, it was
checked for working on the wrist and the finger, too. The signal (analog) originally was too small to detect, and
without amplification proved to be too noisy to extract the heart rate. Because of this, operational amplifiers
were used to extract the heart rate signal. After amplifying, the signal was fed to comparator, resulting output in
the form of pulses. The signal in the
Fig. 10 shows the comparison of heart rate measurement with a standard instrument and it is seen that the
maximum error is 2bpm.
It was observed that while measuring heart rate with the sensor developed, it showed more accuracy, as shown
in Fig. 10, in terms of continuous monitoring in comparison to a sport watch (WR30M) with a similar sensor
Was used as reference.
Fig. 11. Typical waveform of impact sensor in oscilloscope.
Fig. 12. The typical results of the impact sensor along on e (Y) axis.
+/ - 2 . This was fitted into the wrist strap. This device provided a digital voltage, the amplitude of which was
directly proportional to acceleration. The acceleration can be determined by measuring the length of the positive
pulse width (t1) and the period (t2) see Fig. 8. The nominal transfer function of the ADXL213 is:
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Wireless Physiological Parameter Monitoring and Recording System
Sensitivity = Minimum magnitude of input signal required to produce a specified output signal having a
specified signal-to-noise ratio, or other specified criteria
Where in the case of the ADXL213
Zero g Bias = 50% nominal;
Sensitivity = 30%/g nominal.
The outputs are digital signals whose duty cycles (ratio of pulse width to period) are proportional to
acceleration. This duty cycle can be directly measured using microcontroller.
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. Fig. 11 shows the typical waveform of the impact sensor in oscilloscope, when user is standing in normal
position, value of duty cycle is around 50%.
The signal from the impact sensors is measured in a very similar way like the heart rate signal. In this
case, both the measurements of the duty ratio is important. The impact sensor has been used for different
conditions and the Fig. 12 shows the typical results. In this project, only one axis () is used to analyze
The output of the accelerometer was tested with walking and simulated falling. The output of the
accelerometer was tested with walking, everyday movements like sitting, standing, writing, etc., and simulated
falls. The results showed the difference was simple to detect and proved the accuracy of the algorithm. Fig. 13
shows the impact sensor output. On analysis of the impacts it shows the difference in duty cycle during falls
(under controlled conditions)
Fig. 13. Impact sensor output for walking and a fall.
Microcontroller Interfacing And Communication
The microcontroller used in the wrist strap unit is the Silicon Laboratories, Inc. C8051F020. The
Microcontroller is programmed using "C" language for the operation of the above mentioned tasks in this
project. This takes inputs from the sensors in the form of analog and digital voltages. Each sensor has a
dedicated channel, ADC for temperature sensor, digital port0 for heart rate and impact sensor which is
multiplexed by the microcontroller. Each sensor's signal is sampled at a predefined rate, through interrupt-driven
A. Communication
Communication between the wrist units and the receiver unit is wireless. The data measured by the
sensors is saved by building a network between the sensors and to set up a computer receiving and storing the
values. For the communication ZigBee modules were used, powered by the Silabs C8051F020 microcontroller
and transmitted in the unlicensed 2.4 GHz frequency band. These provide a wide range and a couple of lowpower modes, which could be used to reduce the current consumption of the circuit. In addition, the networkwww.iosrjournals.org
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Wireless Physiological Parameter Monitoring and Recording System
setup is easy and fast, so that an extension of new units is possible without problems. Fig. 14 shows the
connection overview of various sensor units, wirelessly.
The reason this microcontroller was chosen, was because of its low-power consumption, and built-in
UART function for serial transmission of data to ZigBee module for wireless trans-
Fig. 14. System overview for wireless communication
Fig. 15. XBee module electrical connection with microcontroller.
Mission. It is powered by a 9 V battery, and ports uses 3.3 V, from where sensor and ZigBee modules are
powered. B. XBee Module These modules provide a possibility to build an easy to configure network, with a
high data rate up to 230400 Baud/s. They come in a preconfigured mode and establish the communication
automatically. In addition, they are powered by 2.7-3.3 V and can be connected to the C8051F020 without any
additional power-supply circuit.
To connect the XBee module to the Microcontroller is done using four wires. The Power-Supply (3.3
V), Ground and TX and RX of the Microcontroller are connected to VCC, GND, DIN and DOUT of the XBee
module (Fig. 15).
1) Configuration and Setup: To configure the XBee Modules, the provided software X-CTU is used. To set up a
network the following conditions have to be fulfilled.
Each network needs one Coordinator and several End-De
All modules have to have the same firmware and PAN-ID.
If everything is setup correct, the coordinator establishes a
connection to the End-Devices automatically.
Fig. 16. Data packet composition
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Wireless Physiological Parameter Monitoring and Recording System
Fig. 17. Sensor data displayed from one unit/system in GUI.
The Coordinator sends Broadcast Commands, and the End-Devices can send to Coordinator only.
2) Communication Protocol: To avoid corrupted data and to see which unit was sending the data, an own
communication protocol is needed. The transmission of the XBee Modules does not provide a checksum or any
other possibility to verify the correctness of the received data. The send string for the sensor units contained 27
characters, as shown in Fig. 16. The first three chars are the name of the user, then each divided by a minus the
sensor data. The data is raw; i.e., no processing of data is done here.
Each unit sends their data every 2 s to the coordinator, where the data has to be collected and tested for
Graphical User Interface (GUI): The GUI was programmed in C# and captures the serial
communication. The string received as serial Data is split into five parts (the address and sensors) and saved in
an Access Database. At this stage, the GUI also tests the data for correctness.
The Database contains a Table in which the different Settings for the sensors are stored. That makes it possible
to attach different sensors and to change the position of the sensor data in the send string. It also provides
calibration of each sensor, for example to get the dot in the temperature. To get the right value of the sensor,
sent data is divided by 100 but in the reality this value may diversify to get the right value. In the GUI, it is possible to display whether one or multiple sensors to give the user the chance to show different graphs and to
compare collected data over a time period. Fig. 17 shows the data collected from one sensor unit and displayed
in the computer GUI.
Prototype and Experimental Results
The SiLab microcontroller 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.
Fig. 18 shows the prototype hardware. The prototype was powered off a 9 V battery. The RF transmission using
ZigBee's has been tested to operate successfully at 30 meters range through obstacles such as concrete walls.
Fig. 18. Fabricated and developed prototype wrist unit
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Wireless Physiological Parameter Monitoring and Recording System
Fig. 19. Details of the prototype unit
The receiver unit can be seen in Fig. 19, without the casing. When in operation, the wrist unit consumes
20 mA of current at 3.3 V power supply, supplied from pins of a port of microcontroller. It was also recorded
off DC power supply display. The microcontroller is powered by 9 V battery.
The XBee module connected to microcontroller consumes 40 mA during transmission. However, Xbee
modules have the option of going in sleep mode while not transmitting. In sleep modes, XBee modules poll
XBEe coordinator (their parent) every 100 ms, while they are awake to retrieve buffered data. Pin sleep of XBee
allows external microcontroller to determine when the XBee should sleep and when it should wake by
controlling the Sleep_RQ pin. It saves power when no data is transmitted. By using several power-down modes
that could be used to reduce consumption during times when the wrist band is not transmitting, alternatively, the
architecture could be altered so that packets are only sent when a value goes outside a preset range. This was
noted for future developments.
Using a 9 V battery which is rated at 60 mA for 10 hours, the device could be run continually for 25
hours before needing recharging.
Discussions And Future Developments
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 at this
developed/development stage. 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. A panic button has also been provided in the developed system which can be used under
an emergency situation.
An important aspect of the design was miniaturization, so that the system was as nonintrusive as
possible to the wearer. This was achieved by the use of surface-mounted devices on the PCBs designed. Lowpower operational amplifiers were used to minimize battery consumption. The major cost comes from the use of
ZigBee modules in the current design.
With some modification, the system can be made available commercially. Future improvements will
focus on the use of 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 need of a finger glove.
The addition of a blood-oxygen sensor would allow the system to more accurately detect medical
distress by measuring the amount of oxygen in the blood (HbO). This could be implemented by the addition of
another diode operating at a different wavelength which is more readily absorbed by oxygen, and measuring the
difference of absorption between the two wavelengths.
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 patient. The receiver unit would ideally 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-risk, with a mental or
physical disability. Monitoring of athletes whilst exercising would be possible if the sensitivity to movement
was decreased.
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