25-08-2017, 09:32 PM
Small wireless ECG with Bluetooth™ communication to a PDA
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Abstract
The Electrocardiogram (ECG) is en essential diagnostic tool that measure and record the
electrical activity of the heart. A wide range of heart conditions can be detected when
interpreting the recorded ECG signals. These qualities make the ECG a perfect
instrument for patient monitoring and supervision.
The commonly used ECG-machine used for diagnosis and supervision at the present is
expensive and stationary. The aim of this project is to develop a small wireless sensor
system to make the patient more mobile without losing the reliability of the ECG sensor.
Wireless patient monitoring has become a more established technology and a natural step
in this progress is to develop a reliable ECG system that contributes to the cable
reduction in medical and physiotherapy environments. The main focus of this thesis is to
create a reliable small wireless ECG sensor system at low cost.
This thesis investigates the possibilities to create a small sized ECG sensor system that
can be wirelessly connected to a handheld device that can graphically presents the
ECG-signals.
A small embedded ECG sensor system prototype has been developed. Using Bluetooth™
technology the ECG sensor system can connect to a personal digital assistant (PDA).
Software for the PDA has been developed for presentation of the 2-channel ECG-sensor.
With the use of a microprocessor the analogue signal is digitally converted at a specific
sample rate that based on the resolution of the ECG-signals. The prototype is well suited
for patient monitoring were a low noise and power efficient system has been created to be
powered by a cellular phone battery.
Background
Wireless ECG monitoring using Bluetooth™ has become a more and more established
and acknowledged technology in the medical environment as in physical therapy. A
natural step in the development is to make this technique more available and make it
more user-friendly. One way to do this is to create a small wireless ECG that can be
connected to a personal digital assistant (PDA) that will act as a data acquisition system
(DAQ).
The development of this type of system has great advantages; the patient will be more
mobile and the technique is relative low cost.
By creating an embedded sensor system with Bluetooth™ it can contribute to the cable
reduction within the medical environment and therefore make the patient monitoring
more efficient [15].
Aim
The first part of the project will be to create an electrocardiograph (ECG-sensor) with two
channels that can be wirelessly connected using Bluetooth™ that will act as a data
acquisition system (DAQ).
The ECG-sensor will be an embedded sensor system that contains a sensor, digital-toanalog
processor and a Bluetooth™ module. This will be powered by a small battery
which is normally used for cellular phones. The main focus of this development will be to
create a reliable ECG- amplifier.
The second part will be to program the PDA so it will connect to the ECG-amplifier and
work as a DAQ and present the ECG signals on its display. The system must be able to
be connected continuously for supervision abilities.
Piconets
The Bluetooth wireless technology allows either communication one-to-one or up to
seven different Bluetooth devices to connect and interact with a 10 meter radius, this is
called Piconet or PAN, personal area network. A requirement for the devices within a
piconet is that they all have the same Bluetooth profile. A Bluetooth device can however
be a member of endless numbers of Piconets. Each piconet has one master that initializes
the connection the other Bluetooth devices are slaves. Bluetooth enabled devices can
establish piconets dynamically and automatically as they enter and leave the masters
radio proximity [12].
Limitations
Making the system wireless has its limitations, foremost it is the size and the weight of
the ECG amplifier that has its limitations due mostly to the battery. The ECG will be
placed on the chest on patient and should interfere as little as possible with the patient’s
mobility. The use of Bluetooth™ will limit the ECG-sensor system primary in the
maximum data rate of 750 kbps and the 10 metre range.
The data rate will automatically limit the bandwidth of the ECG-sensor to 150Hz
according to Nyquist sampling theorem that states that the sample frequency must be at
least twice the bandwidth of the signal and the maximum sample frequency that can be
used is 400Hz due to Bluetooth maximum data rate. This leads to that this system is more
suited for patient monitoring than diagnostic purposes.
Other limitations are the battery time for the PDA as well as the ECG sensor system.
Operation amplifier
The operation amplifiers used in the application was chosen based on the electrical
characteristics. The requirement that were essential for the operation amplifier that had to
be fulfilled was;
• Single supply voltage at 3.3 V. The Mitsumi Bluetooth module is restricted to
3.3 V maximum; therefore the entire system will have the same supply voltage.
• Quad operation amplifier, due to the size requirement of the ECG sensor system
size.
• High output current, the operation amplifier should be able to put out enough
current so it can drive the Right Leg Drive function for an efficient reduction of
50Hz noise.
• Rail to Rail input and output, essential for high resolution output.
When deciding on operation amplifiers other characteristics were looked upon, not
essential for the application, but important for functionality of the ECG amplifier. The
most important characteristics here were;
• Low noise, eliminating disturbances in every step will make the ECG more
reliable and make a high resolution possible.
• Low input offset, DC offset on the input will escalade and disturb the base line off
ECG-signal.
• Low power consumption, the application will be power by battery and less power
leads to longer battery life time.
Programming
The programming part of this project is divided into to separate part, the microcontroller
and LabView 8.0 for pocket PC.
The microcontroller, that will handle the output signal from the ECG amplifier, tasks
consist of:
• Analog to digital conversion.
• Serial port configuration.
• Controlled sample rate.
• Sending data via serial UART to Bluetooth™ Module.
LabView on the PDA is required to handle the output signal from the ECG sensor
system. The main tasks for the PDA will be:
• Connect to the ECG sensor system.
• Send start signal.
• Read output from ECG sensor system.
• Graphical display of the two separate signals.
• Send stop signal before exiting program.
Problem description
During the development progress several problems has occurred most of them easy to
handle and what could be described as expected problems. In this chapter of the report
some of the more challenging problems will be described and how they were solved.
Power consumption
The greatest challenge of developing this wireless electrocardiograph is the power
consumption. When constructing a reliable ECG-sensor that will be powered by a battery
usually used for cellular phones the power consumption must be as low as possible
without interfering with the performance. For example there are operation amplifiers that
have a lower consumption than the TS924, but it will not be able to supply the right leg
drive function with the acquired output current and the noise reduction will not be
efficient enough.
The Mitsumi Bluetooth module consumes about 45 mA when connected and
programmed as spp_slave as required in this project, the spp_master configuration
consumes less than half with 20 mA.
Noise reduction
The wanted electric potential that the ECG sensor is measuring is relative small in
comparison to the noise that both the body and electrical wires absorbs form the
surroundings. Mostly 50 Hz noise is absorbed from the power cables and all electrical
equipment that is power by them. The ECG signals are specified to have a resolution 0-
150 Hz therefore a low-pass filter that eliminates the 50Hz noise cannot be used.
The DC-offset is an additional problem in developing bioelectrical amplifiers.
To avoid 50 Hz noise as thin and short wires as possible will be used for connecting the
leads to the ECG sensor system, the electrical components will be surface mount devices
(SMD) and the embedded system will be powered with a cellular phone battery.
But the most efficient way to reduce all unwanted noise is to use a Right Leg Drive loop
to eliminate the noise absorbed by the wires and body by inverting the noise and feeding
it back to the body via the right leg.