AUTOMATED AVOIDING AND MAPPING ROBOT

Tài liệu nghiên cứu khoa học

08ECE , Center of Excellence, Danang University of Technology – The University of Danang

2 Doan Thanh Thien

08ECE , Center of Excellence, Danang University of Technology – The University of Danang

3 Giao Duy Vinh

08ECE , Center of Excellence, Danang University of Technology – The University of Danang

Advisor:

Eng Nguyen The Nghia,

Department of Electronic and Telecommunication Engineering

Danang University of Technology – The University of Danang

Da Nang, 2012

ABSTRACT

The goal of this project is to create a mobile object-avoiding and mapping robot The robot must

be able to detect objects and have enough intelligence to avoid collision, while a computer is responsible to draw a map of the surrounding environments The robot detects obstacles using an ultrasonic sensor This sensor is attached to a servo to enable the spinning of the sensor itself, and update the distance data from three different directions This data is returned to computer via a wireless transceiver A piece of software available on computer will do the job of analyzing this data and draw a map of the areas in which the robot is moving

MEMBERS’ TASKS ASSIGNMENTS

Tran Hoang An:

- Write programs for the MSP430FG4618 microcontroller to control the operations of ultrasonic sensor, servo and motors via the driver circuit

- Come up with the avoiding algorithm and write programs for the MSP430FG4618

microcontroller to fulfill the avoiding function

Giao Duy Vinh:

- Design and assembly driver circuit using L298 IC

- Build robot (Connect all hardware with each other such as servo, motors, driver circuit, MCU, ultrasonic and design appearance of the robot

Doan Thanh Thien:

- Make printed circuits

- Design mapping software with Visual Studio 2010

- Design RF module

INTRODUCTION

Robots have been very popular in our daily lives Robots have replaced humans in the assistance

of performing those repetitive and dangerous tasks which humans prefer not to do or are unable to do Robots applications are easily found in surgery, transportation, automatic manufacturing systems, etc Even though it’s going to take engineers and scientists a long time to design robots that can actually do many of the things that humans can do, robots have been considered as one of the greatest inventions of all time

One of the most important features of any robots is the capability to move around and navigate For our project, we decided to build a robot system that can smoothly run in a particular environment, detecting objects from various directions, and avoiding them when it comes close to them The locomotion is enabled by two motors which drive the two wheels An ultrasonic sensor is deployed to give the robot awareness of the surroundings The intelligence to process information is provided by TI’s MSP430FG4618 microcontroller

Mapping function: The data from MSP430FG4618 will be sent to computer through a RF module This module consists of an nRF24L01 transceiver and MSP430G2553 Launchpad After that, the data is sent

to computer through COM port and we use Visual Studio 2010 and C# language to draw a map of robot’s surrounding environment

3 Full Schematic

Figure 3: Full Schematic

IC, requires a voltage source of 5 V, which is provided from one of the L7805CV regulators while a 6 V source needed to feed the motors Since the ultrasonic sensor needs 5 V source and the servo works properly under a voltage source from 4.8 to 6 V, we decided to use output from a L7805CV regulator as the sources for these parts

4.2.2 Servo Motor

A Servo consists of 5 main parts: a three wire DC motor, a gear train, a potentiometer, a control circuit and an output shaft bearing Three wires from DC motor have three different colors with different functions The positive and negative power wires are colored red and black The control signal is sent to the servo via a yellow-colored wire to regulate the shaft’s angular positions

Figure 7: Main components of a servo [2]

The following figure shows three cases associated with three different positions of the shaft

Figure 8: Three operation phases of servo

Detailed Specifications:

Control system: +Pulse width control 1500 usec neutral

Operating voltage range: 4.8 to 6.0 V

Operating speed: 0.19 sec/60

Idle current: 7.4 mA at stop

Running current: 160mA/ 60 at no load

Dimensions: 40x20x36.5mm (1.57x0.78x1.43in)

Weight: 43g

Figure 9: Servo motor [3]

4.3 Microcontroller

The robot uses TI’s MSP430FG4618 microcontroller, which goes with an experimenter’s board,

as its intelligence unit The microcontroller is used to trigger the inputs to and process the output from the ultrasonic sensor, generate a PWM signals to drive the wheels via a driver circuit and generate another signal to spin the servo for sensing purposes

The SRF05 ultrasonic sensor can detect objects from a maximum distance up to 4 meters The operating mode of the sensor can be set up via a control pin called Mode, which we decided to leave open, to specify an operating mode where Trigger and Echo pins are separated

Figure 11: SRF05 Ultrasonic Sensor

Minimum detected distance: 0.3cm

Five pins: VCC, Trigger, Echo, Out, GND

Figure 12: Connections for Trigger/Echo Mode

Figure 13: SRF05 Timing Diagram

4.5 Driver Circuit Using the H-Bridge (IC L298)

An H bridge is an electronic circuit that enables a voltage to be applied across a load in either direction These circuits are often used in robotics and other applications to allow DC motors to run forwards and backwards H bridges are available as integrated circuits, or can be built from discrete components.[4] A simple H-Bridge has the form shown in the following figure

Figure 14: A simple H-Bridge

In the above picture, we have a power line Vin, ground line and 4 switches, S1, S2, S3, S4 When S1 and S4 are closed, the current goes from Vin, through S1, Motor and then S4, making the motor turn right Similarly, when the S1 and S4 are open, S3 and S4 are closed; the current will go from Vin, S3, motor, S2, causing the motor to turn left Four switches correspond to 16 possible situations However, only 4 situations are considered as valid in our case, with other situations leading to short circuits Therefore, the circuit requires careful designs to avoid unwanted situations

1 0 0 1 Motor moves right

0 1 1 0 Motor moves left

0 1 0 1 Motor brakes (stop)

1 0 1 0 Motor brakes (stop)

1 1 0 0 Short circuit

0 0 1 1 Short circuit

Figure 16: Pin Connection Diagram

Table 2: Specifications of L298 (from datasheet)

Io

Peak Output Current (each Channel)

- Non Repetitive (t = 100us)

- Repetitive (80% on -20% off, ton = 10ms)

- DC Operation

3 2.5

2

A

A

A

Tstg, Tj Storage and Junction Temperature -40 to 150 0C

The nRF24L01 is a single chip 2.4GHz transceiver with an embedded baseband protocol engine

designed for ultra -low power wireless applications The nRF24L01 is designed for operation in the worldwide ISM frequency band at 2.400 - 2.4835GHz An MCU (microcontroller) and very few external passive components are needed to design a radio system with the nRF24L01

The nRF24L01 is configured and operated through a Serial Peripheral Interface (SPI.) Through this inter-face the register map is available The register map contains all configuration registers in the nRF24L01 and is accessible in all operation modes of the chip

The air data rate supported by the nRF24L01 is configurable to 2Mbps The high air data rate is combined with two powers saving modes makes the nRF24L01 very suitable for ultra-low power designs Internal voltage regulators ensure a high Power Supply Rejection Ratio (PSRR) and a wide power supply range.[5]

4.6.2 nRF24L01 Features

• Worldwide 2.4GHz ISM band operation

• Up to 2Mbps on air data rate

• Ultra low power operation

• 11.3mA TX at 0dBm output power

• 12.3mA RX at 2Mbps air data rate

• 900nA in power down

• 22µA in standby-I

• On chip voltage regulator

• 1.9 to 3.6V supply range

• Automatic packet handling

• Auto packet transaction handling

• Air compatible with nRF2401A, 02, E1 and E2

• Low cost BOM

• ±60ppm 16MHz crystal

• 5V tolerant inputs

• Compact 20-pin 4x4mm QFN package

4.6.3 nRF24L01 Applications

• Wireless PC Peripherals

• Mouse, keyboards and remotes

• 3-in-one desktop bundle

• Advanced Media center remote controls

• VoIP headsets

• Game controllers

• Sports watches and sensors

• RF remote controls for consumer electronics

• Home and commercial automation

• Ultra low power sensor networks

4.7 Visual Studio 2010

In our project, we use Visual Studio 2010 and C# to write the mapping software which displays

the robot movement’s map The above figure is the software’s interface

Figure 20: Mapping software’s interface

This software has several basic functions:

- Draw a mapping

- Choose COM port to connect

- Display the received data

3 distances and makes decisions all over again

5.1.2 Motor Control

Figure 22: Motor control

The automotive vehicle has three wheels: two back wheels and one front wheel to make it balanced The robot can move forward, backward, turn left, and turn right to avoid blocking items To perform these tasks, two motors in the two back wheels are controlled by PWM signals from the MCU via the driver circuit To turn left and turn right, each of wheels will move backward while the other still keeps moving forward For example, when the robot is going forward and is to turn left, the H-Bridge circuit changes the current direction through the left wheel and holds the direction of current in the right

wheel the same, causing the robot to turn left immediately

Figure 23: Left and Right Movement

Since the two motors are different, to make the robot go forward satisfactorily, we need to properly adjust the PWM signals feeding the two motors to make sure they have the same speed For our

of the robot So far we have been able to transmit distance data generated by ultrasonic sensor wirelessly

to computer and display the distances over a GUI environment

[14] Datasheet MSP430G2553 mix signal microcontroller SLAS735G April 2011 REVISED August

2012 -by Texas Instruments Incorporated

[15] nRF24L01 Single Chip 2.4GHz Transceiver Datasheet by the Nordic Semiconductor

1 Tra Khue, trakhue08dt3@gmail.com

08DT3, Department of Electronic and Telecommunication Engineering, Danang University of Technology – The University of Danang

2 Lai Thi Kim Phung, phung08dt3@gmail.com

08DT3, Department of Electronic and Telecommunication Engineering, Danang University of Technology – The University of Danang

3 Nguyen Duy Khiem, ngdkhiem89@gmail.com

08DT3, Department of Electronic and Telecommunication Engineering, Danang University of Technology – The University of Danang

Advisor:

Eng Nguyen Van Hieu, vanhieu.1608@gmail.com

Department of Electronic and Telecommunication Engineering

Danang University of Technology – The University of Danang

Da Nang, 2012

We shall be failing in our duties if we do not express our thanks towards Mr Hung T Le, Texas Instruments, Vietnam and Texas Instruments Incorporated for providing us the excellent and appropriate devices for this project, as well as sharing their experiences to help us deal with the problem causing in reality when designing this project

We are also thankful to all the staff members of the Electronic Faculty, Da Nang University of Technology for the enthusiasm of helping us to complete this thesis

Danang, 23 Sep 2012

Khue Tra Phung K Lai Khiem D Nguyen

ABSTRACT

The Electrocardiogram (ECG) is a diagnostic tool which measure and record the information about the electrical activity of the heart When the recorded ECG signals are monitored, many heart conditions can be detected These qualities make the ECG a perfect instrument for patient monitoring and supervision There are many implementations carried out with ECG measurement, especially with ECG monitoring devices [1] However, most of them were designed by using either special ASIC or custom built integrated circuits which are too expensive to be commonly used with patients in hospital

or in their houses In this paper, the design of a wireless ECG monitoring circuit using MSP430 microcontroller is presented This project is at the aim designing a small, portable and ultra-low power wireless ECG monitoring circuit which is built from commercially available electronic components to support doctors to track patients with strokes It consist of an amplifier, filtering, MSP430 microcontroller, wireless Technology and MATLAB as a platform for processing the received digital signals With the use of a MSP430 microcontroller, the analog signal is digitally converted at a specific sample rate that based on the resolution of the ECG signals The digital data signal can be processed in MATLAB and graphically can be displayed

Keywords: Electrocardiogram (ECG), monitoring system, MSP430, wireless ECG

TEAMWORKS

With the aim of creating the complete device, helping to diseases treatment, and our group also want to help each member to develop knowledge and skills Moreover if task of each member matches their expertise and capacity, work will finish best and quickly For those purposes, our group tries to divide the work appropriately and help every member of the group understand all the aspects of the thesis The specific contributions of the members are listed as follows:

- Khue Tra: researched and designed software

- Phung K Lai: researched theory of this project and designed hardware

- Khiem D Nguyen: designed layout and implemented circuit

Following is timeline for the thesis:

START Jun, 2012 July, 2012 August, 2012 September, 2012 FINISH

Research Theory

and ECG Issues

Explore and Design Circuit Schematic

Implement Hardware and Softwave

Finish the rest works

INTRODUCTION

Health care is a very important issue in every country around the world Novel methods in medical which are created by the application of technology science have no small influence on characteristics of healthcare system Nowadays, along with the population which is getting much older and the increasing in number of those living alone, especially the patients with history of strokes, stroke events are one of the greatest dangers for them There are many methods for stroke treatment For example, using the Thrombolytic (fibrinolysis) drugs to reestablish blood flow against blocking the flow [2], preventing blood clots from forming by aspirin [3], treatment by the stem cells [4], etc However, early diagnosis and precaution is the best treatment method

The EKG (Electrocardiogram-ECG) is a diagnostic method based on which diseases such as cardiovascular disease, hypertension, diabetes, etc can be detected much earlier However, an ECG sometimes just is a tool for measuring and recording the electrical activity of the heart, such as the speed and regularity of heartbeats A heart evaluation using an ECG is important in any patient with a stroke or suspected stroke According to the World Health Organization (WHO), stroke is the third leading causes

of death after cardiovascular disease and cancer in Vietnam and around the world In the United States, every 45 seconds someone suffers a stroke, and some patients die of the stroke every 3 to 4 minutes [5] According to Professional Thanh V Le, chairman of the Vietnam stroke association, there is more than 50% of 200,000 cases of stroke occurring annually in Vietnam causing death (around 100,000 cases per year) [6] However, just only 16 stroke treatment centers are in operation in Vietnam Compared Vietnam population of over 87 millions with the number of those who died from stroke every year, this number is too scaring to match with medical needs of patients Moreover, probability of stroke recurrence is very high, which can be seen from statistics of National Stroke Association that recurrence rate after a one-month treatment was 3 - 10%, after the first year treatment was 5 - 14%, and led to 25 – 40% after a five-year treatment [7] Recurrent stroke is really a horrible prognosis which directly leads

to the risk of death and disability Hence, it is time for monitoring devices, detection and timely treatment applications to come into being However, it is not easy for one to afford an ECG monitoring device due to its expensive price A portable handheld ECG monitor of ReadMyHeart, for example, costs five million VNĐ; an ECG monitor and InstantCheck Handheld ECG-EKG Monitor FP-ICH cost more than ten million VNĐ [8] Therefore, with a desire to design a recorded ECG device with a moderate price of about one million VND, in accordance with the affordability of small hospitals, patients can afford an ECG monitoring circuit that they can use in their houses Besides, device is designed with small and light circuit by using three electrode measure a lead Patients carry it along and are followed while doing their normal activities without any feeling of trouble Instead of forcing one to lye on a position to measure and record the ECG signal in a long time, transmission of signals based on wireless technology allow people to move and act freely The ECG signal is still recorded as normal within a relatively wide range (several tens of meters) Especially, instead of pressing the cumbersome control button or difficult mode selection, the circuit is easy to use (just replace the battery after a period

of monitor time), which is more convenient for the elderly, stroke patients In addition, extremely low power consumption will help to save power for a long-time using, which reinforces the feasibility of our project

From the above causes, we believe our project has the large application if this project is developed into a complete product

The paper is organized as follows: In section 2, the theory of the ECG signal is presented Afterward, the hardware description is shown with the monitor node mainly in section 3 Analysis of software is carried out in section 4 Conclusion and future work conclude the last paper

1 THE ELECTROCARDIOGRAM

1.1 The Electrocardiogram signal theory

The Electrocardiogram (ECG or EKG) is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body ECG is used to measure the rate and regularity of heartbeats, the presence of any damage to the heart, and the effects of drugs or devices used to regulate the heart

Figure 1: Human Heart Anatomy (left) and ECG Waveform (right)

The ECG is a medical standard for testing the human heart for defects and diseases Figure1 shows the anatomy of the human heart and the waveform of the ECG signal This signal is usually very small about 100mV at heart and about 1mV on the surface of the skin [9] A standard cardiac cycle is 0.8 seconds with five peaks are P, Q, R, S, T peak and U peak is very small The largest R wave has amplitude about 1mV This amplitude measurement often depends on the location and condition of the patient ECG signal has a wide frequency range but the useful information of ECG signal in medical is usually in the range of frequency from 0.05Hz to 100Hz [10]

The human body is a conductive environment, so the current which the heart creates can be transmitted throughout the body When two electrodes placed on two points of body, there are obtained voltage between these points which is called the lead Each lead have a different waveform People usually use the 12-lead: Three bipolar limb leads, three augmentedleads, six chest leads [11]

Due to the signal from the heart is tiny, noise from the environment will easily effective The leads connecting the electrode to the amplifier act like an antenna Therefore, it can receive unwanted signals, such as signals are the 50Hz from power lines and other noise which are generally quite difficult

to remove Perhaps filtering noise is the most difficult problem in ECG signal detection and our project

For the purpose of tracking patients, we decided that a standard lead - Lead III of the ECG signal

A detailed introduction of our project will be discussed in the following sections of the report

1.2 The Electrocardiogram in stroke

According to many research around the world [12][13], the ECG relates to stroke From the received signals, we calculate information then predict the stroke from received value In [14], the author record the ECG signals of 150 patients, to assess the relative frequencies of ECG abnormalities among the pathophysiologic categories of stroke Of the 150 stroke patients, 138 have ECG abnormalities The most common abnormalities such as QT prolongation (45%), ischemic changes (35%), U waves (28%), tachycardia (28%), and arrhythmias (27%) Stroke patients have an increased frequency of pathologic Q waves (20%) and left ventricular hypertrophy (26%) The wave which gets from stroke patients is depicted in the Figure 2

Figure 2: Electrocardiogram of patient with stroke

2 HARDWARE DESCRIPTION

2.1 Block diagram of the Wireless ECG monitor

Figure 3: Detailed block scheme of an ECG recording device

2.2 Circuit components

2.2.1 Monitor Node

Since the ECG signal has small amplitude under the influence of many kind of noise in the recorded environment and in the transceiver, receipt of the correct signal is very necessary When ECG signal is detected correctly, we can make the behind work properly That ECG signal is fed into filters for noise suppression and amplifiers to increase the signal amplitude as much as possible while keeping

it within the input voltage range of the analogue-digital converter (ADC) The ADC with a resolution high enough represent the original signal is necessary Because the transmission of analog signal creates much noise comparing with digital transmission These value will be collected by a microcontroller (MCU) which maintains the connection with the wireless transmitter When the signal is enter into a computer, it is processed to remove noise by digital filters and show the wave by computer screen

 The Instrumentation Amplifier

Voltage Supply

Receiver Process Show ECG signal

r

Negative feedback drive

(MSP430

F2274)

Wireless Trans- mission

(CC2500)

Wireless Receiver

(CC2500)

Interface with CC2500

(MSP430

F2274)

Voltage Supply

(TUSB-3410UF)

ECG signal is detected at the electrode is very small Theory shows that ECG signal at the electrode about 1mV but with actual work, it can smaller Moreover, in ECG measurement the biggest problem is the elimination of the body's DC offset and the 50Hz hum Differential amplifier perfectly fits this application Its differential inputs with very high common-mode rejection ratio remove much of the DC offset and the interference picked up from the AC mains Besides differential inputs with very high impedance and extremely tiny current are also characteristic which is well worth considering Implementation of signal amplification often consists of many stages, such as two stages is the simplest ones The first differential amplifier gets rid of a substantial part of the noise while amplifying the signal Then it is filtered before is entered into the second amplifier This helps the signal is amplified with amplifier of small noise [14]

Instrumentation amplifier based on the INA118 from Texas instruments is commonly used in ECG applications Its versatile 3-op amp design and small size make it ideal for a wide range of applications The input signal is entered into two opamps in the first stage is used as the buffer to increase signal current Then they is amplified differential at the second stage Furthermore, the INA118 has the high input impedance about 1010 Ohm According to the standard around the world, current load the person is less than 20µA The INA118 FET-Input instrumentation amplifier may provide lower noise and extremely high input impedance, input current approximately zero, it is very satisfiable for medical applications [15] It is a low power, general purpose instrumentation amplifier offering excellent accuracy Its versatile three opamps design and small size make it ideal for a wide range of applications Current-feedback input circuitry provides wide bandwidth even at high gain The INA118 has high CMRR about 110dBm min and low input bias current about 5nA max A single external resistor sets any gain from 1 to 10,000 It operates with power supplies as low as ±1.35V Performance of the INA118 remains excellent with power supplies ranging from ±1.35V to ±18V The INA118 is available in 8-pin plastic DIP, and SO-8 surface-mount packages, specified for the –40°C to +85°C temperature range The scheme of INA118 is depicted in the Figure 4

Figure 4: Block scheme and shape of INA118

 The Microcontroller

The aim of using microcontroller in this project implements ADC and transmits ECG signals With portable application, this design needs small and ultra-low power Furthermore, due to the ECG signal is effect by many type of noise, specially ac noise, we use battery supply provide whole circuit to decrease noise Why does the MSP430F2274 fit this particular application?

The MSP430F2274 is a part of the family MSP430 – designed 16-bit RISC processors for low power applications from Texas Instruments Both the address and data buses are 16-bit by using a Neumann architecture The MSP430 family offers five low-power modes such as active mode, LPM0, LPM3 and LPM4, LMP5 Selection of five low-power modes are processing by software The MSP430F2274 has 10-bit 200ksps Analog-to-Digital Converter (A/D) with internal reference, sample and hold, autoscan, and data transfer controller Besides, MSP430F2274 has 32KB + 256B Flash Memory and 1KB RAM The value is quite high Specially MSP430F2274 supports UART for interface with the computer and SPI protocol for RF transceiver The module can be utilized both as master and as slave, which further broadens its usability The SPI - Serial Peripheral Interface Bus is a synchronous serial data link standard, that operates in full duplex mode Devices communicate in master/slave mode where the master device initiates the data frame Multiple slave devices are allowed with

ultra-individual slave select lines The SPI bus specifies four-wire serial bus - four logic signals: SCLK (serial clock - output from master); MOSI (master output, slave input (output from master)); MISO (master input, slave output (output from slave)); SS (slave select (output from master) Beside SPI has two bus

such as GDO0 and GDO2 (Digital output pin) Model of SPI protocol is depicted in the following figure

Figure 5: Model of SPI

The MSP430F2274 is integrated on eZ430-RF2500 module This is available and makes a convenient for using RF transceiver

This project uses MSP430F2274 whose block diagram is shown in the Figure6

Figure 6: Functional Block Diagram MSP430F2274

 The negative feedback drive circuit

We measure the ECG by connecting two positive and negative electrodes as shown in Figure

7 Besides two wires are connected with the positive and negative signals, we can use isolation amplifiers (iso-amps) for the direct patient connection The body is connected to output of the opamp This negative feedback drives the common mode voltage to a low level The body's displacement current flows not to ground but rather to the opamp output circuit This reduces the pickup as far as the ECG amplifier is concerned and effectively grounds the patient This connection prevent accidental internal cardiac shock and protect to the circuits and equipment from damage If abnormal high voltage should appear between the patient and ground due to electrical leakage or other means, the auxiliary op-amp saturates If there are not this connection, no signal would be obtained Ordinary, people connects the third electrode into the right foot Performance of elimination noise is better than the previous method However, the purpose of this application is the ECG signal is recorded frequently and patients feel comfortable with carrying this device regularly Therefore, the third electrode is connected as the Figure

7

2400 – 2483.5 MHz This is a standard RF Hence project development in the future create a network monitor is implemented easily Block diagram of the CC2500 is drawn in Figure 8

Figure 8: Block diagram of the CC2500

CC2500 features a low-IF receiver The received RF signal is amplified by the low noise amplifier (LNA) and down-converted in quadrature (I and Q) to the intermediate frequency (IF) At IF, the I/Q signals are digitized by the ADCs Automatic gain control (AGC), fine channel filtering, demodulation bit/packet synchronization are performed digitally

2.2.2 Receive node

The receiver node receives the digital signal from the transmission node Interface of the receiver node and the transmission node bases on SPI protocol The goal was to have a properly functioning wireless receiver equipped with RS232 connectivity Its USCI interface takes care of the serial communication with the computer, while the USART is responsible for interfacing with the CC2500

The PCB circuit needs the time enough more and must check the reliability operation of the circuit Whereas there is a little time to finish the circuit Therefore the receiver node and transmission

on the monitor node were built up from eZ430-RF2500 Development Kit User in the current version eZ430-RF2500 module is shown in Figure 9

Figure 9: eZ430-RF2500 module

2.2.3 Supply voltage

The supply is large enough to control the operation circuit Firstly, the INA118 needs it provides stable +2.5V for the amplifier and ensures that the negative supply stays at -1.9V Actually, the amplifier should work well until supply absolute value is higher than 1.35V Secondly, the ADC10 module operates properly which needs higher supply voltage (2.2V) than other peripherals in the MCU At minimum voltage is 1.8V, CPU works, but the ADC10 did not Failures like this are annoying and can corrupt the samples sent to the computer

To require a dual voltage from a single battery, the voltage inverter is used as shown in Figure

10 This circuit is simple and inexpensive It is therefore extremely useful for powering opamp and other circuits In this circuit, LM386 is used The LM386 is a power amplifier designed for use in low voltage consumer applications The quiescent power drain is only 24 miliwatts making the LM386 ideal for battery operation The first, R1 and R2 is just a variable resistors Because we have variable resistor quite big, it is not satisfiable with a small application We replace it by two resistor and we have the same result

From 9V supply, we create ±4.5V with the voltage inverter But we need ±3.3V supply The TPS71533 from Texas Instruments is applied This voltage is satisfiable the voltage demands Since VIN

= VOUT(NOM)+ 1, VOUT(NOM)is the nominal voltage at the output

sub-Figure 11: The monitor node.

As for the amplifier and the voltage inverter sub-circuit, the INA118 precision instrumentation amplifier was chosen because of its wide supply range, that allows its operation even at ±1.35V The

- in

3

4

IC2 OP07

6

3 2

R4 33

R5 33

R2 100k

R1 390k R3

390k

R6 47k C1 3.3uF

GND

GND Vcc

Vcc

GND

V-1 2 3

P1 Header 3

Vcc FROM SOURCE

V-DRIVER LEG

INSTRUMENT AMPLIFIER

HIGHPASS FILTER

The ECG signal has a wide frequency range but the useful information of ECG signal in medical

is usually in the range of frequency from 0.05Hz to 100Hz Nevertheless analog filter characteristics do not usually idea The cut-off frequency can be chosen below the lowest frequency component of the ECG signal, which is 0.05Hz in diagnostic mode and under the highest frequency component of the ECG signal, which is 100Hz It is calculated with the following equation:

=

∗ ∗

With the high pass filter, R, C is chosen 47kΩ, 3.3µF, respectively

Once the signal gets amplified, it may contain notable amount of DC component This has to be eliminated before it is fed into the programmable gain amplifier (PGA) Otherwise the second stage would amplify the DC drift as well

There are two reasons to decide selection a voltage regulator in the circuit Firstly, it provides stable voltage for the positive and the negative input of the amplifier Actually, until its absolute value is higher than 1.35V, the amplifier should work well Secondly, without the voltage regulator, while the ADC10 module did not operate (it needs minimum 2.2V), the MCU operates well (needs minimum 1.8V) This can corrupt the samples sent to the computer

Finally, the four connection points for the programmer, the ground and the Vcc of the battery, a connection from patient body to mass and the two connections for the electrodes have to be added to the circuit

2.3.2 Receive node

The receiver node is quite simple We only use eZ430-RF2500 to receive the signal from the monitor node The three following schematics of the eZ430-RF2500 kit are presented in [16]