Virtual Instrumentation For Acquisition Computer Science Essay

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The Electrooculography is a noninvasive technique for measuring the resting potential of the retina. The resting potential is changed when the eye is moved and the movement of the eye is translated into electrical change of potential. Besides the clinical research and laboratory usage, EOG is also extensively used in developing assistive technologies based on the eye movement. EOG is traditionally acquired and measured using dedicated bio-signal measuring equipment developed by some manufacturers. However, the increased performance of personal computers and their reduced cost has made it possible for development of PC based signal processing systems. This research work exploits the benefits of PC based signal acquisition, and analysis. PC based data acquisition and analysis is an efficient and cost effective method for EOG signal acquisition and monitoring. The system developed utilizing virtual instrumentation largely decreased the cost and increased the flexibility of the instrument. The proposed EOG virtual instrument is built by LabVIEW software, DAQ card and additional hardware circuits designed on PCB.

INTRODUCTION

Eye movement detection has attracted a large number of researchers from many fields. From as early as the 1950's, many researchers began to investigate eye movements. Several methods of eye movement detection have been explained in the literature [1-5]. These methods are either expensive or bulky or cause lot of discomfort to the subject. Electrooculography (EOG) is widely used in ophthalmic research, ophthalmological diagnosis and clinical laboratories because it provides a noninvasive method for recording full range of eye movements. Besides the clinical research and laboratories usage, EOG is also broadly used in developing assistive devices [1]. There have been much efforts to develop EOG based assistive devices, inspired by the fact that the physical energy drained in moving eyes is much lesser when compared to other gestures such as nodding head (dumb people), speaking or writing etc. Electrooculograms (EOGs) occurring as a result of eye movements, are found to be most advantageous and convenient [1-3]. It causes least discomfort to the patient and recording is done with minimal interference with subject activities and minimal discomfort. EOG based method is easy to apply and can be used for long term monitoring. With all these advantages the Electrooculogram is chosen as a suitable method for recording eye movement, for conducting this research work which focused at acquisition and analysis of eye movement signals. The recording of the EOG signal has traditionally been associated with several problems. The signal is seldom deterministic, even for the same person in different experiments. It is a result of a number of factors, including eyeball rotation and movements, eyelid movement. The EMG produced by the muscle of the eye, eyelid movement, the eye blinks, electrode placement, head movements, influence of luminance, etc. The repeatability and flexibility are the two major requirements from EOG measuring equipment. Considering the cost and availability of the dedicated EOG acquisition instrument, it was decided to develop a virtual instrument for the bio signal acquisition. This virtual instrument should be economical, portable and easy to use. The virtual instrument developed had all these features available in it.

Hospitals need several measurement systems that can measure physiological parameters of the patient. Measurement systems should be able to measure accurately the vitals of patient like heart conditions, body temperature electrical activity of the heart, electrical activity of the brain etc. This information should be readily available to the doctors for diagnosis and proper treatment. PC based signal acquisition, and analysis is an efficient and cost effective method for biomedical signal acquisition and monitoring. Since the bio signal level is very low, amplification of signals is important. Hence, a PC based system consists of additional circuits for isolation and amplification of the signals. A data acquisition card and software for signal processing is important. The signals acquired contain useful information. But extracting useful information from signals in their raw form is a difficult task. This has inspired biological signal analysis in extracting useful information from the biological signals. In many cases, it is observed that the frequency content of the waveform provides more useful information than the time domain representation. Many biological signals show diagnostically extremely useful properties when viewed in the frequency domain. In this work we have developed a virtual instrument for acquiring, processing and analysis of EOG signal in horizontal and vertical channels. PC based instrumentation can easily bypass the need for standalone instruments by using the PCs currently available and some inexpensive acquisition equipment. There are several software packages that can be used for the purpose. In this work National Instruments, LabVIEW has been used because of the powerful tools available. LabVIEW is user friendly, its highly interactive user interface developing tools and built in procedures for handling data acquisition have made it a convenient choice.

The paper is organized as follows: Section II explains the experimental components including the challenges, electrode configuration and EOG signal acquisition, Section III explains the significance of the proposed work and section VI briefs the conclusion and future considerations.

EXPERIMENTAL COMPONENTS-MATERIAL AND METHODS

The proposed work is conducted in the following steps:

Design and Simulation of Electrooculogram amplifier.

Building of the hardware circuit board including amplifier and filters.

Design of LabVIEW front panel and block diagram.

Data acquisition using NI- USB 6221, the data acquisition card .

Observing the EOG on the virtual instrument designed, processing of the acquired EOG.

Challenges

The frequency content of the EOG signal is very close to DC and hence the separation of DC drifts from the useful signal content is a difficult task. The shift in the DC level during EOG signal recording is much observed using any EOG measuring system. Measurements are affected when eye makes quick movements from one fixation point to another. The artifacts are caused by eyeball rotation and movement, eyelid movement, the EMG signal produced by the muscle of the eye, eye blinks, electrode placement, head movement, influence of luminance and others. Care should be taken to minimize all these artifacts. In the preprocessing, filter to reject out of band frequencies and amplifier to amplify the signals are required. Patient safety is an important issue. The designed amplifier should provide proper isolation of the subject from the hardware circuit.

Electrode Configuration

The literatures on the human eye describe the eye as a fixed dipole. Cornea is the positive pole and retina as the negative pole. This potential ranges from 10-100 microvolts. It was Emil du Bois-Reymond (1848) who observed the cornea as electrically positive with respect to retina. Horizontal EOG is measured as a voltage by means of electrodes strategically placed as close as possible to each eye (on canthi). Similarly, vertical EOG is measured as a voltage by means of electrodes placed just above and below the eye. The reference electrode is placed on the forehead. Solid gel electrodes were chosen over wet gel electrodes as a precautionary measure to prevent the gel from entering the eyes. Based on the half-cell potential Silver/Silver-Chloride electrodes were chosen. These electrodes have half-cell potential almost near to zero. Minimum half-cell potential causes the least amount of offset. The hydrogen and lead electrodes have very low half-cell potential, but since hydrogen being of gaseous nature and lead being hazardous to the health, they are not used. Hence considering an optimal solution for safety and precision, Silver (Ag)-Silver Chloride (AgCl) electrodes are used. Also these electrodes are the best compromise between cost and reliability. After cleaning the surface of the subject's skin, additionally an electrolytic gel is applied to the skin to reduce the skin impedance that is, for better contact and conductivity [3]. A total of 5 electrodes have been used, two for horizontal, two for vertical and one reference electrode placed on the forehead. The electrode placement is shown in figure 1.

Figure 1. Electrode Placement for EOG

Preprocessing -Amplification and filtering

The EOG signals have been obtained for the subject in vertical and horizontal channel. EOG has been recorded for several activities like right and left eye movements, by following a specific object, and also by reading a simple paragraph, keeping eyes closed, eyes open, up and down movements. Typically, the EOG signal has a frequency range between DC and 27 Hz and amplitude between 10 to 100μV. The EOG signals were pre-amplified and filtered initially, the noise including power line interference is suppressed through a 50 Hz notch filter. To further remove the noise, a band pass filter with a cut-off frequency of 0.05Hz and 30Hz (designed using low pass and high pass filters), and FIR filter with Bartlett window has been used for smoothing the waveform. The EOG signal, like most of the other bio-signals is affected by environmental conditions and biological artifacts. The design of the EOG amplifier needs proper attention. Therefore the primary design considerations that have been kept in mind during the design of the EOG bio potential amplifier are proper amplification, sufficient bandwidth, high input impedance, low noise, stability against temperature and voltage fluctuations, elimination of DC drifts and power-line interference.

Virtual Instrumentation-LabVIEW and Data Acquisition

LabVIEW is an interactive programming language and it is user friendly. It allows to build applications with easy to understand and attractive Graphical User Interface. LabVIEW has a vast built in library of functions for numerical analysis, design and visualization of data. LabVIEW provides analysis and design tools and modules for control, signal processing, system identification etc. Development of any virtual instrument requires a front panel, which is the user interface and the block diagram which defines the functionality of the program through code. While creating any VI, initially elements are placed on the front panel, and then their properties are set. Block diagram is used to implement the VI's functionality.

Data acquisition using DAQ card:

A data acquisition device is required to pre-processed electrical signal to a computer for further processing, analysis and monitoring. Several options of data acquisition devices are available: using a PCI bus, a PCI Express bus, a PXI bus, or the computer USB. National instruments has made available several data acquisition cards, which may be suitably selected based on the required application. LabVIEW provides with ready-made libraries for making interfacing facing easier. Using these libraries, programs for the data acquisition are quickly and easily made for allowing more time to be spent on the processing and analysis of the acquired signals. In this research work, M Series USB-6221 is used as data acquisition interface. Figure 2 below shows the pin out of USB-6221. The device is connected with external measurement circuit on one side and personal computer (Laptop) on the other side.

Figure 2. USB M Series 6221 terminal pin out (courtesy-ni.com)

Signal Processing tools

LabVIEW provides signal processing tools for Fast Fourier Transform (FFT) and spectral analysis. It is possible to acquire time-domain signals, measure the frequency content, make suitable analysis, provide results in the form of table, displays etc. Using plug-in DAQ devices, flexibility is configuration makes it possible to build a lower cost measurement system. Since EOG is a non-stationary signal, time frequency analysis is most suitable. Hence wavelet analysis is most suitable. The wavelet transform has its application to characterizing transient events, eliminating noise, data compression and many others. In addition that biomedical signals are very small in amplitude, they are in danger of added noise. Noise elimination and amplification is done in the hardware designed. However, after the signal reaches the computer, it can still contain noise. Another way to solve the noise problem is to use the filters provided with LabVIEW. LabVIEW offers the choice of Butterworth, Bessel, Chebyshev and digital filters. With a few adjustments these filters can be configured for almost any design that is needed.

Hardware Design -Design of the EOG amplifier

The EOG signal has a frequency range between DC and 30Hz and amplitude between 10 to 100microvolts. The hardware circuit designed should have isolation from the subject, an instrumentation amplifier, band pass filter (0.05-30Hz), amplifier, notch filter to eliminate power line frequency of 50Hz.Amplifiers increase the strength of the signal while retaining high fidelity. The bio potential amplifiers must satisfy the basic requirements [6]. These amplifiers must have high input impedance in order to minimize the loading effect, should include isolation and protection circuitry and the common mode rejection ratio should be high to minimize interference due to the common-mode signal. Isolation and protection circuit, preamplifier, driver amplifier, filters are the required parts of the hardware designed. The preamplifier stage performs the initial amplification of the EOG. This stage should have very high input impedance and a high common-mode-rejection ratio (CMRR). Driver amplifiers amplify the EOG to a level at which it can be recorded on the recorder. This stage also carries out the band pass filtering of the electrocardiograph to give the frequency characteristics of the signal. The block diagram of the complete system developed is shown in figure 3.

Figure 3. Block diagram of the overall system

INSTRUMENTATION AMPLIFIER

The first stage of EOG bio potential acquisition system is the instrumentation amplifier. An Instrumentation Amplifier is a type of differential amplifier with input buffers, which eliminate the need for input impedance matching and thus make the amplifier particularly suitable for use in measurement and test equipment. Additional characteristics include very low DC offset, low drift, low noise, very high open-loop gain, very high common-mode rejection ratio, and very high input impedances. Basic circuit of an instrumentation amplifier is shown in figure 4. Description: C:\Users\Patterson D'Mello\Desktop\Op-Amp_Instrumentation_Amplifier.svg.png

Figure 4 . Instrumentation amplifier

Instrumentation amplifiers can be built with individual op-amps and precision resistors, but are also available in integrated circuit  form from several manufacturers (including Texas Instruments, National Semiconductor, Analog Devices, Linear Technology and Maxim Integrated Products). The Analog Devices AD620 is a low cost, high accuracy instrumentation amplifier that requires only one external resistor to set gains of 1 to 10000. Two AD620 amplifiers have been used one for each channel. The AD620 is a monolithic instrumentation amplifier based on a modification of the classic three op amp approach. The AD620 features 8-lead SOIC and DIP packaging that is smaller than discrete designs and offers lower power (only 1.3 mA max supply current), making it a good fit for battery powered, portable (or remote) applications. AD620 has low noise, low input bias current and low power. Due to these features of the AD620, it has been used for medical applications such as ECG and non-invasive blood pressure monitors. The pin configuration of AD620 is shown in figure 4.

Figure 5. AD620 Pin diagram

The design issues of the overall systems are explained below:

Low Pass Filter and High pass Filter

A low-pass filter passes low frequency components in the signal but rejects the high frequency components. The transition from stop band to pass band occurs at cut-off frequency. The High pass filter passes high frequency components and rejects the low frequency components. Band pass filter is designed combining the two filters.

Figure 6. (a) Low pass filter , (b) High pass filter

The designed circuit includes the low pass filter with the cutoff of 30 Hz. Based on the the useful bandwidth of EOG the band pass filter edge frequencies are taken as from 0.5Hz to 30Hz. The second order Butterworth filters are used since it gives better selectivity of signal than in first order filter. The TL084 series of ICs are high speed J-FET input quad operational amplifiers incorporating well matched, high voltage J-FET and bipolar transistors in a monolithic integrated circuit. The devices feature high slew rates, low input bias and offset currents, high input impedance and low offset voltage temperature.

A1. Design procedure for Low pass filter

1. Define the cut-off frequency Fc = 30 Hz, according to the requirement.

2. Choose an appropriate value for the capacitor C1 = 0.1uF. The choice of capacitor was in terms of lower cost and to get better performance with respect to the frequencies at which they worked.

3. Calculate the value of the capacitor C2. C2 = 0.2μF.

4. Calculate the value of a resistance R1=R2=R, which is obtained by using:

= 37.51 KΩ

5. R3 is taken as 2R. R3 = 2 * (37.51KΩ) = 75KΩ

Figure 7. Low pass Filter Circuit

A2. Design procedure for high pass filter

Define the cut-off frequency Fc = 0.5 Hz

Choose C capacitor value, since C = C1 = C2, we proposed C = 0.47μF. The capacitor choice was in terms of lower cost and greater feasibility of achieving better performance with respect to the frequencies at which it worked.

Calculate the value of the resistor R1 and R2 by the equation

=0.95 MΩ

=0.475 MΩ

To establish R3 resistance that is connected to the operational output and capacitors (R3):

R3 =R1/2 = 0.95 /2 = 0.475 MΩ

Figure 8. High pass Filter circuit

Pre Amplifier (i) & (ii)

The Amplifier ensures that adequate signal is passed to the next block.

Gain: = 1+ = 1+ = 2

Reference resistor = 1.1kΩ (1 kΩ)

Figure 9. Preamplifier Circuit

Instrumentation Amplifier

The instrumentation amplifier used is Analog Devices iAD620. Analog devices AD620 data sheet specifies gain up to 10000. However, it is found that for a gain of 991, RG =49.9 Ohms is required. Most suitably gain can be up to 1000 since higher gains will require RG value very low. We have selected a gain of 100 with RG= 499Ω (510Ω).

G = 1 + (49.4 kΩ/RG)

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