Medical Electronics

Final Report

July, 30th 2010

Group 3: Mitsuo Yoshimoto Yasemin Örter Viral Ladhani Yousuf Alkanan

Contents

1 Introduction.......................................................................................................... 2 2 Derivation and description of requirements ......................................................... 2 3 Description of design HW, SW, mechanics with respect to fulfilment of

requirements ........................................................................................................... 6

3.1 Description of Hardware design ................................................................... 6 3.2 Description of Software design................................................................... 11 3.3 Description of Mechanics ........................................................................... 13

4 Description of testing HW, SW .......................................................................... 20

4.1 Description of Hardware testing ................................................................. 20 4.2 Description of Software testing................................................................... 28

5 Description of experiment and result ................................................................. 33

5.1 Experiments with Hardware equipment...................................................... 33 5.2 Experiments with Software equipment ....................................................... 34

6 Discussion and conclusion ................................................................................ 38 7 Literature ........................................................................................................... 38

1

1 Introduction Non-invasive Acquisition of Blood Pulse is one of the innovative techniques in the field of Bioelectronics which may count the blood pulse of humans. Non-invasive blood Pulse can be acquired by many techniques for example, light absorption, acoustic, a magnetic effect. The Aim of this project was the Verification of an Experiment on Blood Pulse Detection by Magnetic Disturbance Technique conducted by Chee Teck Phua, Gaëlle Lissorgues at World Academy of Science, Engineering and Technology 54, Dec 2009. Even though by referring the publication and communicating with its author, some points were still not clear such as the origin of the magnetic disturbance, there is no information about the applied gain and hence of the magnitude of the detected signal, there is suspicion of faulty results which could be due to pulse correlated movement of the sensor setup. So as to verify the experiment, some strategy was followed where intended design and experimental setup was created that has the ability to detect a signal smaller or at least similar than the smallest theoretically expectable, Give experimental evidence of the detection threshold of the experimental setup by applying a known magnetic field strength, Repeat the published experiment to verify the result.

2 Derivation and description of requirements After analyzing and discussing the experiment, a list of requirements was created to design the experiment setup of the GMR sensor, calibration setup of GMR sensor and the software. List of requirements have been set and analyzed as following: GMR sensor Power supply:

• Supply voltage: Vss GMR = +15V DC, stabilized. ‐ Specification of the GMR sensor.

• Noise voltage superposed on Vss GMR Vnoise = < 150μV. ‐ The signal is around 150uV, therefore we need lower noise.

• Minimum current: Iss GMR > 20mA.

- Specification of the GMR sensor.

GMR Sensor(NVE) and Magnet: • The sensor and the magnet shall be fixed together. ‐ The magnet has been used to shift out put the GMR sensor, output to the

linear range there for it has been fixed to the GMR sensor with adjustable distance.

• Distance between sensor and magnet shall be adjustable between 0.5 – 3.0 cm.

‐ This distance was estimated in the original experiment.

2

• Distance between sensor and skin shall be 0 – 2 mm. ‐ The distance between the sensor and the proband skin must be as minimal as

possible to ensure the acquisition of the signal. • No use of ferromagnetic materials ‐ In order to avoid not to disturb the magnetic field.

• The permanent magnet has to have a magnetic field strength in the range of

20 –35 Oe.

‐ The magnetic field with that strength has been used to shift the GMR sensor

output to the linear range.

• The measurements at the proband shall be taken while he is sitting or lying.

• The proband has to put his arm in 90 – 180° to his forearm in a relaxed condition, on a reference firm plane for a stable positioning.

‐ To avoid moving artefacts which could magnetic field.

• The distance between GMR sensor and filter or gain block shall be less than 20mm or shielded .

‐ twisted pair cable shall be used to connect sensor and filter/gain.

Power supply for gain and filter:

• Vss = +15V

‐ Standard power supply for OP amp.

• Vcc = -15V

‐ Standard power supply for OP amp

• Iss cc > 50mA

‐ Required for gain and filter.

• Overvoltage protection is absolutely necessary to protect the proband from harm.

• Noise/Ripple: Noise-voltage < 3mV

‐ As decided by Professor.

Filters:

• Preamble: Filtering shall mainly be accomplished by software. • Hardware filter shall only be used to avoid any saturation at filter/gain blocks

and ADC input.

3

Hardware Filter:

• Hardware filter block has to be put after gain block. ‐ It acts as an anti-aliasing filter.

• If active filter, the supply voltage shall be +-15V ‐ use standard OP available via in-stock RS-Components)

• Output impedance has to be low in comparison to ADC input impedance ‐ To insure the acquisition of the low signal

• A high pass filter has to be realized inside the gain block. ‐ To filter the signal before it enters the gain block, we don’t have to amplify the

noise.

• Input impedance has to be high in comparison to gain block output impedance ‐ Characteristics of Op-Amp. • Aliasing filter: low pass of first order, fc = 100 Hz ‐ To avoid aliasing in the reconstructed signal (2*highest frequency component

of the signal).

• Operating temperature range: 21 - 25 °C. Software Filter:

• 50 Hz notch filter, mid attenuation of 100dB, bandwidth < 2 Hz. Gain

• Bandwidth (-3dB): 0,5 Hz – 100 Hz ‐ It’s the width of the notch filter.

• Gain shall be adjustable between 10 – 200. ‐ This gain is considered from publication , but since we are using a less

sensitive NVE sensor we are using and adjustable gain of 10 - 200

• Differential input => use integrated instrumentation amplifier ‐ Available via in stock RS Components.

• Supply voltage: +- 15V ‐ It’s standard power supply from OP.

• Input impedance large in comparison to output impedance of the GMR ‐ Characteristic of OP. • Sufficient common mode input voltage range with respect to sensors DC

output offset voltage. ‐ If active filter are being used then there has to be offset correction.

4

• CMRR > 80 dB ‐ The higher the rejection ration, better is the signal.

• Operating temperature range: 21 - 25 C.

A/D Converter:

• The analog to digital converter shall have a sampling rate of 500 Hz for each channel

• The analogue to digital converter shall have a 14 bit resolution. • 8 bit is not compatible, also 14 bit resolution is easily available in Lab.

• Vfull scale = 10V, single ended Driving voltage.

ECG: • ECG reference voltage available in the laboratory. ‐ It’s been used to reduce the noise in the GMR signal.

Digital signal processing:

• Computer to be able to run software.

• Software should be able to make a noise reduction by correlating ECG and GMR signals.

• It should be able to display raw data (ECG and GMR) over time in real time

and offline.

• The result has to be represented in graph and also by numbers in a table format.

• Should be able to give the spectrum of signal (that is in frequency domain), to

be able to do FFT of stored data (offline) or in real time.

• Should be able to use and add filters to raw data in software. Calibration coil:

• Certain magnetic field: As large as the signal from magnetic Disturbance. Given (19mV)/(120*1.3mV/V/Oe) = 0.12 Oe. [Amplitude of the reported signal/(Gain*Sensitivity of the sensor)].

5

• Distance between the Calibration coil and the GMR sensor shall be between 0 – 2 mm.

• Mechanical fixation for the calibration coil which should not disturb the field strength.

• The input current entering in the coil according to [B=Iμ(N/l)] with B according to the first bullet point.

• Ohmic resistance of the coil has to be negligible in comparison to 50 Ohms

output resistance of signal generator. Calibration signal:

• Shall be produced by a standard signal generator. ‐ The output of Signal generator is 50 ohms

Function Generator:

• +- 10V output voltage ‐ Standard specification of Function Generator. • 50 Ohms output resistance ‐ Standard specification of Function Generator. • Frequency 0.5 – 1000 Hz ‐ Standard specification of Function Generator.

3 Description of design HW, SW, mechanics with respect to fulfilment of requirements

3.1 Description of Hardware design Introduction In order to be able to measure the blood pulse of a human being with the magnetic disturbance technique, electronically a lot of considerations have to be taken into account. It has been decided that the small magnetic changes will be detected with the help of the Giant Magnetoresistance sensor (GMR) that is being positioned on a blood artery at the wrist of a human being, and an additional permanent magnet to bias the signal. First of all, the problem arises due to the small signal size that is being measured on a blood artery. This signal has to be amplified in a proper way and the correct filters have to be applied in order to avoid noise within the sensitive signal. Moreover the ripple factor that appears in a power source has to be reduced as well.

6

This is an important precondition for a circuit design, especially when working with small signal sizes. Block diagram The functional block diagram of the whole circuit design has the following order:

Power Source

GMR-Sensor, represented

by a Wheatstone

bridge

Instrumentation Amplifier and its power source to

amplify the measured signal

First order passive

Lowpass-Filter with a

cut-off frequency of fc = 100 Hz

First order passive

Highpass-Filter for offset correction

Ripple rejection with

Linear Regulator

AD-Converter

with a sampling

rate fs = 500

Additional Capacitor for offset correction

Digital Signal

Processin

7

Schematic diagram

Figure 1: Schematic diagram of the complete electrical circuit

Power source and ripple rejection In order to ensure a power source for the GMR sensor with low noise and disturbance, it was decided to use a voltage regulator (7815 as standard voltage regulators). For the simulation in the semi-final report another voltage regulator (LT1086 for the positive source, same characteristics as the 7815) from the manufacturer Linear Technology was taken. The data sheet of this component [1, page 5] ensures a ripple rejection of a minimum 65 dB in the area of f = 10 Hz to f = 1 kHz. The simulation proved this result. A ripple source as a sine wave with an amplitude of Vripple,in = 2 mV and a frequency range of f = 10 Hz to f = 1 kHz was simulated and the output voltage was observed. It could be seen that the ripple factor of Vripple,in = 2 mV was reduced to a value below Vripple,out = 100 µV, as it was required. The input voltage of the voltage regulator must be higher than the output voltage. It was chosen to be 20 V, in order to achieve a 15 V power source for the GMR sensor. The theoretical ripple rejection is calculated with the following equation that is given in [2]:

Output

Input

RippleRipple

PSRR log20 ⋅= (1)

with PSRR = power supply ripple rejection ratio in dB. In the calculations PSRR was assumed to be 60 dB and the ripple input to be Vripple,in = 2 mV. The ripple output was then calculated to be Vripple,out = 2 µV. At the input of the voltage regulator a poled capacitor with C = 10 µF was positioned, at the output a ceramic capacitor with 100 nF was placed in order to minimize oscillating effects coming from the linear regulator. Complete circuit The complete circuit will be explained stepwise: As shown in the schematic diagram (figure 1) the GMR sensor is mainly presented by four pins and its behaviour can be simulated by a Wheatstone bridge with two changeable resistors. The positive input of the sensor is connected to the output of the Linear Regulator. The negative input is connected to ground. The differential outputs are then attached to the amplifier with the Highpass-Filter in-between. These steps will be explained in the following text. The outcome of the experimental report, which is the aim to be proven, had a signal amplitude of V = 19 mV. With an amplification of 120, the signal was back-calculated

9

to be approximately V = 160 µV. It can be seen that the sensor has to deal with very low signals, which is the challenging task of this project. For this project, the GMR sensor AA004-02 from the manufacturer NVE Corporation, that has a sensitivity between 0.9 and 1.3 (mV/V)/Oe, was taken. For the experiment to be proven a different sensor, the AAH002-02, was being used. Since this sensor has a sensitivity approximately ten times higher (between 11 and 18 (mV/V)/Oe) than the sensor used for the project, special considerations have to be taken for the calculations, for instance for the calibration setup. In order to filter out the DC offset coming from the sensor signal a Highpass-Filter has to be applied to prevent the amplifier to run into saturation. It was decided to build a passive first order one. The application notes of the GMR-sensor recommended this assembly in connection to an instrumentation amplifier. [3, page 103] Since the output of the sensor is differential, as mentioned before, two separate Highpass-Filters on each output (+ and -) are needed. The cut-off frequency for these filters have to be less than fc = 0.5 Hz. The impedance of these passive Highpass-Filters have to be low in comparison to the input impedance of the amplifier (range of GΩ) and high in comparison to the output impedance of the sensor. For that reason it was decided to take resistors in the range of some MΩ. Knowing this, the capacitor could then be calculated with equation (2) and an available value could be chosen. With the correct and available capacitor value C = 220 nF, again the correct resistor value could be calculated and chosen to be R = 2 MΩ. A cut-off frequency of fc = 0.36 Hz was the result.

CfCR

⋅=⋅

π21 (2)

For the instrumentation amplifier the INA118 from the manufacturer Texas Instruments was decided to be the ideal one for this application. It needs a power supply of ± 15 V. At each input a smoothening capacitor of 100 nF was set. In order to have an adjustable gain a 10 kΩ-potentiometer was taken with a fixed resistance in series of 249 Ω at the inputs RG of the INA118. The amplification can then be varied up to a value of 200. To calculate the resistor values necessary for the circuit, following equation was used:

150+

Ω=

RGkG (3)

Since the number of different potentiometers is limited, it was decided to use a fixed resistor with 249Ω that limits the amplification to a factor 200 in combination with a potentiometer of 10kΩ. If the Potentiometer is turned to its maximal value, the smallest amplification with a factor of 5.9 is being reached. [4] A detailed requirement analysis was being done as well in [4, page 6], in which it can be seen that all other requirements for the amplifier are fulfilled by the INA118 in combination to this design.

10

Right after the amplification another capacitor of 10 µF was applied in order to correct the residual offset that was being observed during the first test measurements. To avoid aliasing effects during the analog to digital conversion, an anti-aliasing filter, that means a Lowpass-Filter, has to be applied. The values of the first order passive Lowpass-Filter with a demanded cut-off frequency of fc = 100 Hz were calculated with equation (2) as well. Again the impedance of the resistance had to be high in comparison to the output impedance of the amplifier and low in comparison to the input impedance of the AD-Converter. The resistance was chosen to be in the range of kΩ. The capacitor was then calculated, again with consideration to the available value, to be C = 1 µF and the resistance was then determined to be R = 1.5 kΩ. The capacitor together with the Lowpass-Filter that now build a CRC-element have the behaviour of a Bandpass-Filter. Layout of the board It was decided to have two separate boards for the realization of this experiment. On the first board only the GMR sensor together with the permanent magnet was positioned. This board was kept as small as possible, since it has to fit to a human wrist. The GMR sensor has to be positioned as close as possible to the artery. On the second board the whole electronics was realized. That means, the voltage regulator, the Highpass, the amplifier as well as the CRC-element. With this design, it could also be avoided that the electronics negatively affect the behaviour of the GMR sensor.

3.2 Description of Software design Software was used for the Data acquisition and Noise reduction. Programs were written instinctively through trial and error. 1. Data acquisition We just used a built-in function called DAQ assistant.

11

Figure 2: Data acquisition program –front panel-

Figure 3: Data acquisition program –block diagram-

2. Noise reduction Since we couldn’t know the actual waveform of the GMR signal, we assumed the following very simple model for simulating GMR and ECG signals with white noise[9]; V(t) = A (sin(2πft) + |sin(2πf t)| ) The following algorisms were used for the simulation. ( 1) low-pass filtering

( 2) manual phase shift and multiplication of ECG signal in space domain

12

( 3) averaging of the waveform

3.3 Description of Mechanics Introduction In these experiment the mechanical design include: 1- calibration coil setup. 2- fixation of GMR sensor and permanent magnet on the wrist. The idea of the calibration setup, is first of all to produce a magnetic field with the help of a calibration coil that is in the range of the magnetic disturbance of the blood. With this magnetic field being produced, the outcome in voltage by applying the same sensor as applied for the real experiment (GMR sensor) can be observed. This outcome should then be in the same range as the outcome being measured during the experiment. In order to be able to produce a current for the calibration setup that should result in the desired magnetic field a signal generator has to be used. However, the signal generator can only produce a voltage or a frequency. For this reason the needed

13

current has to be transferred in a voltage or a frequency. By using the Ohm’s law this problematic can be easily solved. The only thing that has to be regarded is the resistance of the coil and the output resistance of the signal generator. Requirements and Specifications Calibration Coil features and specifications

Specification Description

Number of turns of the coil N 3 turns

Diameter of the copper-wire 1 mm

Length of the coil L 3 mm

Radius of the coil r 15 mm

Length of the wire l (calculated) 282 mm

Mechanical Construction of the calibration setup The distance between the calibration coil and the GMR sensor shall be between 0 and 2 mm. Furthermore the mechanical fixation of the calibration coil should not disturb the field strength. Therefore non ferromagnetic materials in the constructions have to be used. Design Setup The coil is fixed axially by tighten it laterally to a movable L-Shape plastic plate. a plastic base plate is used to carry the movable L-shaped plastic cover and the PCB board of the GMR sensor setup. another small plastic base is used to elevate the PCB of the GMR sensor setup to the level of the coil. The distance between the coil and the GMR sensor is to be varied manually by sliding the L-Shape plastic cover using measurement scales (0-10mm) that are graded on the plastic base to adjust the distance. In this design, there is no additional fixation required, since the friction between plastic and is supposed to be sufficient.

14

Parts features & Specification

No Part Description

1 L-shape Plastic plate

Rectangular transparent L- shape plastic plate

2 Plastic base

Base plastic plate with two supporters.

3 Tighten fitting Jack

Banana socket

4

Copper wire Winding coil

15

Sketch

Figure 4: Sketch of the design solution for the calibration setup

16

Figure 5: Picture of the design solution for the calibration setup General description and explanations The reason for choice plastic is, materials are cheap and easy to fabricate. Moreover, their conductance of heat and electricity is poor, which is an advantage in this case, and they do not disturb the magnetic field. In order to be able to vary the distance manually, measurement scales on the plastic base are needed. The friction between the plastic base plane and the plastic plate is supposed to be sufficient, therefore no additional fixation is needed. The GMR sensor would be fixed to one edge of the PCB in order to face directly the coil. Since the radius of the coil is very small, a core to shape the wire in the form of a coil has to be used. Afterwards this core can then be removed and an air filled coil without a core can be used. An open question remains, whether the shape of the coil after removing the core will be deformed. This has to be carefully regarded. The reason for using banana socket is the possible connection of the function generator output to the coil. By using cables the coil can be connected as well to the function generator, however this might cause some motion artefacts in the coil when the plastic cover is being moved. Another reason is the possibility of tightening the coil to the plastic cover. Fixation of GMR sensor and permanent magnet In order to be able to perform an accurate measurement on the wrist of a human being, a good fixation of the GMR sensor is needed. Moreover the permanent magnet has to be considered as well, and a desired variation of its distance to the GMR sensor has to be easily achieved.

17

Sketch

Figure 6: Sketch for fixation of GMR sensor and permanent magnet

18

Figure 7: Picture for fixation of GMR sensor and permanent magnet Description The purpose of the setup is to pick the signal from the artery to the main circuits. two bands have been used to fix the hall setup ,one at the begining of the PCB fixed part which carry the sensor and another band at the end of PCB movable part which carry the permanent magnet . in order to be able to vary the distance manually, measurement scales on the movable PCB part have been used. to carry the signal from the setup to the main circuit tow cables have been used. this construction have availability to vary the distance between the GMR sensor and the permanent magnet of minimum 5mm to maximum 30 mm by using measurement scales. the materials for this setup its available , cheaper and easy to fabricate because of that reasons the materials have been chosen. during the experiment the fixation was so good for the sensor, easy movements for the movable part which carry the magnet, cables soldering was so good and the distance between the magnet and sensor was in the required range.

19

4 Description of testing HW, SW

4.1 Description of Hardware testing Introduction In order to prove the expected outcome of the electrical circuit, a proper testing has to be done. The circuit has to be tested concerning the inputs, outputs, functionality of each step and each component. This test shall be performed by selecting a known input signal as a reference, and analyzing the output signal with the given input signal. Description

Main Electrical

Circuit

Power Supply (± 15 V)

Linear Regulator GMR

sensor

Power Supply (+ 20 V)

Oscilloscope

Function Generator

The main electrical circuit was partially changed, by adding another capacitor after the amplification block and before the Lowpass-Filter, in order to have an additional offset correction. Figure 8 shows the position where the capacitor was placed. Characteristics of Capacitor Value: 10 uF Material: Ceramic capacitor

20

Figure 8: Part of the complete electrical circuit with the added capacitor C4 for additional offset correction Initial Conditions Reference Signal Device: Yokogawa FC200 Type of Signal: Sinusoidal Frequency Range: 50 – 100 Hz Amplitude Range: 160µVpp – 5mVpp Offset: 0 V The visualization of the dc-coupling in the digital oscilloscope is not feasible with very low frequency. Therefore in the testing a frequency of 100 Hz is used. Results The following images show the outcome visualized at the oscilloscope. The red curve represents the differential input (known signal, taking the place of the GMR sensor). The blue curve represents the output signal of the main electrical circuit (ac-coupled). The green curve from figure 10, 11 and 12 represents the same output signal, but dc-coupled. Hint: Only the dc-coupled signal shows the offset effect.

21

Figure 9: Differential input (red curve) and output (blue curve) signal. The output signal has been achieved with the minimum gain. Properties of the blue curve: AC-coupled, Function generator setup: 5mVpp, 100Hz

Figure 10: Differential input (red curve) and output (blue curve: ac-coupled, green curve: dc-coupled) signals. The output signals have been achieved with the minimum gain. The output offset is in the range of 50 mV. Properties of the blue curve: AC-coupled, Function generator setup: 5mVpp, 100Hz

22

Figure 11: Differential input (red curve) and output (blue curve: ac-coupled, green curve: dc-coupled) signals. The output signals have been achieved with the maximum gain. The output offset is in the range of 1 V. Properties of the blue curve: AC-coupled, Function generator setup: 5mVpp, 100Hz

Figure 12: Differential input (red curve) and output (blue curve: ac-coupled, green curve: dc-coupled) signals. The output signals have been achieved with the maximum gain. The output offset is around 1V. Properties of the blue curve: AC-coupled, Function generator setup: 160µVpp, 100Hz

23

The aim of this test was to proof that the requirements concerning our electrical circuit was fulfilled. This covers mainly the amplifier and the filters. From the figures above it could be seen that the amplification did not work properly. The maximum gain could not be achieved. From the signal form it could be observed that the Lowpass-Filter is working. But due to the bad temperature behaviour of the capacitors used for the Highpass-Filter, the offset-correction is not working as expected. To compensate this problem, an additional capacitor was added to the circuit, right after the amplifier. This turned out be efficient. Afterwards the sensor was tested in its saturation behaviour. For that the distance between the permanent magnet and the sensor was changed. The following figures show the sensor in different positions.

Figure 13: Sensor in saturation.

24

Figure 14: Sensor not saturated. The permanent magnet is not being used.

Figure 15: Proof of total saturation.

25

Figure 16: Proof of total saturation. Comparison of figure 15 and 16 shows no change in the output signal, although the position of the sensor was changed.

Figure 17: Try to get in the center of the linear range of the sensor. For that the permanent magnet has to have a distance to the sensor of about 2cm. The best position for the permanent magnet to the GMR sensor proved to be in a distance of 2 cm.

26

Discussion of the results and problems As shown in the first four pictures, figure 9 to 12, the desired maximum gain of 200 could not be reached. After reviewing the circuit, other problems were noticed: For the calibration setup the soldering between the coil and the wires connected to the power supply was not proper due to the fact that the material of the coil has an isolating coat and it was not removed. Therefore it had to be re soldered without the isolating coat. Another problem was too much incoming noise, coming through the unshielded cables connecting the main circuit to the power supply. To reduce this, the cables were changed for some shielded ones. It was also noticed a strong temperature dependence acting on the capacitors. To reduce as this problem as much as possible, the circuit was placed into a closed box. After solving the problems that were mentioned, additional measurements were done. To sum up, the hardware generally is working except for the fact that the whole circuit is not sensitive enough!

27

4.2 Description of Software testing We simulated the performance of the three different algorisms and compared with them. (1) Low-pass filtering was effective to some extent but not sufficient when SNR was

very low (less than -10dB).

Figure 18: Low-pass filtering (0dB)

28

Figure 19: Low-pass filtering (-10dB)

Figure 20: Low-pass filtering (-20dB)

29

(2) Multiplication in space domain did not work properly when SNR is very low. For this case, fake signal was observed even if there was no signal and only noise.

Figure 21: Multiplication in time domain (0dB)

Figure 22: Fake signal generated by multiplication in time domain (no input signal)

30

(3) Averaging was effects on noise reduction even if SNR is very low and signal were appeared only when the original signal and reference signal had the same frequency.

Figure 23: Averaging (window: 1s, sampling:100 s, SNR:0dB)

Figure 24: Averaging (window: 1s, sampling:100 s, SNR:-30dB)

31

Figure 25: Averaging (window: 1s, sampling:100 s, no input signal)

Figure 26: Averaging (window: 1s, sampling:100 s, SNR:0dB, frequency does not match)

32

We expected that averaging would be most effective for noise reduction because we assumed very low SNR due to the low sensitivity of the GMR sensor and (1) low-pass filtering might not be sufficient for noise reduction.

5 Description of experiment and result

5.1 Experiments with Hardware equipment

After analysing and discussing the requirement list for performing this experiment. Mechanical, Electrical and Software setup were assembled. Mechanical setup consist of two sections i.e calibration and GMR sensor setup. Calibrator was used to test the GMR sensor and it produces the same magnetic field as per produced by blood vessels. The input for Calibrator is functional generator which produces certain amount of current to produce magnetic field in the Calibrator setup. The function generator can produce only a frequency or voltage hence due to this reason the current has to be transferred using voltage or frequency. This problem can be solved by using ohm’s law keeping in mind that resistance of coil and output resistance of the function generator is not to be ignored. This magnetic field is detected by GMR sensor and produces certain amount of voltage and this voltage is required for the electrical setup as input. Electrical setup was mainly constructed for amplification of GMR signal. The input parameter’s for electrical setup is GMR signal which is produced by Calibrator. The output of Electrical setup is certain amount of voltage which is displayed in the oscilloscope. Because signal produced by GMR are complex in nature therefore it’s difficult to analyze in oscilloscope, for that reason we had to develop software setup which helps in amplifying the signal and at the same time it reduces the noise using the digital filters. While taking the measurements we have a high correlation of the GMR signal and the ECG, but this correlation was wrong because it was contradiction stating at the same time that GMR signal is affected by ECG. Instead in this case averaging would have been the best possibility as suggested by Prof. Hoffmann. ECG signal is used to reduce the noise from the GMR signal. After having the setup done for experiment, few problems were occurred which disturbed the GMR signal and this was affected due to noise generated due to external magnetic field and electrical field. GMR signal was dominated by noise so original GMR signal was not to be seen. In Our experiment amplification of 200 was used whereas in publication amplification of 2000 was used. Electrical signal contained some noise and this problem was not been solved by software setup. As per the software group peak detection and pulse counting was not working properly due to noise also it was found that GMR signal was very similar to the of inverse to ECG signal. Due to these problems occurred the original and intended signal was not seen. Whereas the original signal should have been amplified GMR signal which resembles the blood pulse of the proband.

33

Figure 27: Calibration setup+ Mechanical Fixation of Sensor and magnet

Figure 28: As mentioned in the report ECG signal and GMR signal with noise

5.2 Experiments with Software equipment We measured the signal 6 times (60 seconds per trial). The last three measurements were failure. We found peaks at the same position of ECG signal without any noise reduction in the acquired data.

34

Figure 29: ECG signal and GMR signal (Raw signal, peaks were not detected correctly and averaging was not calculated correctly due to noise) We also implemented simple low pass-filtering of the wave form and found that the wave form of GMR signal was very similar to ECG signal.

Figure 30: Noise reduction for actual signal -block diagram

35

Figure 31: Trial 1 (91bpm)

Figure 32: Trial2 (105bpm)

36

Figure 33: Trial 3 (94bpm) We calculated the correlation of ECG and GMR concerning the peak position, ROI area (ROI: region of interest = window width), the difference of peak and bottom amplitude in the ROI.

Correlation coefficients Pulse rate (bpm) Peak positions ROI area Max. – min. amp.

Trial 1 91 0.9999 -0.9373 0.5381 Trial 2 105 1.0000 -0.9065 0.7782 Trial 3 94 1.0000 -0.9045 0.6097

The above results shows that both GMR and ECG signals are strongly correlated concerning two features (Peak positions and ROI area). On the other hand, we could not observe any peaks which are correlated to ECG signal in the output of oscilloscope. Therefore the GMR signal is possibly not considered the original output of the GMR sensor but the cross-talk of ECG signal though additional measurements are necessary for confirmation.

37

38

6 Discussion and conclusion The experiment has been done under different circumstances to get optimistic results as possible. The mechanical fixation of the sensor was acceptable, the setup help the movements of the permanent magnet to be easy and with helping measurement scales the distance between the permanent magnet and the sensor has been controlled. The experiment has been done with and without proband, low signal has been observed in the output, practically there was no signal depending on the original signal which should be observe in the output. Due to improper connections of the hardware and the movements of cables, high amplitude of noise signal has been observed along the low signal. That means the filtration of the signal was not acceptable. Figure 17 illustrate the output signal. Recommendation for future to obtain the blood pulse signal using the magnetic disturbance effect, GMR sensor with high sensitivity has to be taken in to account, method of filtration can be changed instead of passive filters active filters can be use. To minimize and reduce the noise from original signal more shielded cables and connections have to be taken in to account, in addition to use ideal design for hardware to amplify the signal to acquire the ideal data acquisition as possible.

7 Literature [1] http://docs-europe.origin.electrocomponents.com/webdocs/078f/0900766b8078f516.pdf [2] http://focus.ti.com/lit/an/slyt202/slyt202.pdf [3] http://www.nve.com/Downloads/apps.pdf [4] Semi-Final Report of Group 1: Report circuit layout [5] Semi-Final Reports of Group 1 [6] Semi-Final Reports of Group 4 [7] Lindner H, Brauer H, Lehmann C: Taschenbuch der Elektrotechnik und Elektronik, München, Carl Hanser Verlag, 2008 [8] http://www.wikipedia.de [9] Chee Teck Phua, et al., Modeling of Pulsatile Blood flow in a weak magnetic field, World Academy of Science, Engineering and Technology 54 740-743 (2009)