Many physically disabled individuals are deterred from using computers due to their inability to utilize a hand-controlled mouse. However, if directional discrimination of an icon can be achieved, these individuals would be able to take on the functions of a mouse without the use of hands.
We implemented a module that consist of Electrooculogram (EOG) biopotential amplifier in order to obtain a physiological signal due to eye movements and to use this signal to show directional discrimination. Our design can also be used as a model for future advancements in human-computer interactions.
The EOG bio potential amplifier should be capable of detecting frequencies between dc-10 Hz, the range at which most ocular movements operate. The EOG signal is in the microvolt range (50-3500 ÂµV). Therefore, when the DC offset is removed, it will be challenging to obtain a strong, usable signal given the minute nature of the recorded signal. Our choice of an EOG over other possible methods was selected based on the ease of usage and the low cost of production.
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The objective of this project is to build a module that consists of an EOG amplifier to control the mouse pointer in the system monitor. This module will be very useful for physically challenged people.
The first stage of our design is the electrodes and electrooculogram (EOG) biopotential amplifier.
Stages 2 and 3 encompass the detection of horizontal and vertical movements of the eye, respectively. The second and third stages (for horizontal and vertical discrimination) detect lateral movements at the periphery of each eye. The hardware in these stages consists of the EOG bio-potential amplifier. The filters are implemented using IC LM358.Two ICs are used in order to produce a six pole lowpass filter. The cutoff frequency of the filter is 15Hz and roll off rate is 120dB/decade.
Stage 4 consist of Analog to Digital converter and microcontroller to convert the analog signal into digital signal and to send this digital signal to the system. Analog to digital counter is implemented using IC0809.Separate analog to digital converters are used for horizontal and vertical electrode outputs. For the microcontroller part 8051 is used.
The last and stage 5 is interfacing the whole circuit with system using the PS/2 mouse port to access the mouse pointer by programming the microcontroller to synchronize with PS/2 port.
Fig:1 Design model
Right upper electrode.
Left upper electrode.
Right lower electrode.
Left lower electrode.
MEASUREMENT AND ANALYSIS OF ELECTROOCULOGRAPHY:
The eye is a seat of a steady electric potential field that is quite unrelated to light stimulation. In fact, this field may be detected with the eye in total darkness and/or with the eyes closed. Between the cornea and retina exists a differential potential. This potential is because there are established ionic currents between external segment of the photoreceptors and its base (darkness currents).These currents relatively change when the photoreceptors are excited. This potential can be considered as an electric dipole which rotation center is in the center of the eye and it rotates when the eye rotates in its orbit. This potential can be registered in electoretinogram or electrooculogram.
The continuous corneal-retinal potential can be used to measure eye's position, this can be done by placing superficial electrodes in both sides of the eye (left and right) or in the top and bottom and a third electrode on the forehead as a reference. This type of registration is called electrooculogram. When the sight is centered, the dipole is symmetrically localized between the two electrodes, so the output of the EOG is zero.
When the person looks to the right the positive cornea gets closer to the left electrode that becomes more positive. There is an almost linear relation between the horizontal angle and the output of the EOG, by approximately +30 degree or -30 degree from the visual arc. When the electrodes are located on the top and bottom, the movements register vertically. The magnitude of this cornea-retinal potential is in the range 0.05-3.5 mV.
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Fig2:Section of Eye
It is not generated by excitable tissue but, rather, is attributed to the higher metabolic rate in the retina. The polarity of this potential difference in the eyes of invertebrates is opposite to that of vertebrates. This potential difference and the rotation of the eye are the basis for a signal measured at a pair of surface electrodes. The signal is known as the electro-oculogram, (EOG).
The electrodes were chosen with the concern of protecting the eyes from hazardous elements. ECG disposable electrodes were used because of their easy availability. Silver/Silver-Chloride electrodes were chosen because the half-cell potential was the closest to zero. Electrodes with the smallest amount of half-cell potential are desirable because they cause the least amount of offset. By definition, the hydrogen electrode has a zero half-cell potential, but due to the gaseous nature, they cannot be feasibly used. Although lead electrodes have a lower half-cell potential than the Ag/AgCl electrodes, lead is hazardous to the health and thus it is avoided. Thus our choice of electrodes takes into account a low cost and proper signal pick-up.
To record the ECG, EEG, EMG, EOG, etc. electrodes must be used as a transducers to convert an ionic flow of current in the body to an electronic flow along a wire. Two important characteristics of electrodes are electrode potential and contact impedance. Good electrodes will have low stable figures for both of the above characteristics. Electrode potential arises because a metal electrode in contact with an electrolyte (body fluids) forms a half cell with a potential dependent upon the metal in use and the ions in the electrolyte.
All electrodes suffer from variations in contact resistance due to movement, and the drying out of any coupling medium. This is improved by setting the electrode back slightly from the surface of the skin on a quantity of coupling jelly (electrolyte paste).
MEASURMENT OF ELECTROOCULOGRAPHY
The acquisition system employs Ag - AgCl surface electrodes for signal pickup which requires application of sufficient electrolyte gel to reduce the skin impedance. As we said earlier human eye can be described as a dipole with positive pole at cornea and negative pole with retina. Two surface electrodes are placed surrounding the eye.
Fig3: Formation of dipole moment due to eye ball movement
When the eye turns to right side, right surface electrode becomes positive when compared with the other surface electrode. Likewise when the eye turns to left side, left surface electrode becomes positive than the other surface electrode.
Fig4: Typical EOG waveform
DESIGN OF ELECTRO-OCULOGRAM AMPLIFIER
The first stage of any EOG biopotential amplifier is the instrumentation amplifier which provides the initial amplification while reducing the effect of signals such as Power-line interference and skin muscle artifacts owing to its high Common Mode Rejection Ratio (CMRR). Two instrumentation amplifiers are employed for this purpose, one for each of the two channels. Since the EOG signal content varies between DC and 10 Hz, a low pass filter is used after the signal pickup stage, with cutoff frequencies of 15 Hz as shown in fig. 5.
Fig 5: EOG measurements
Here we use AD620 instrumentation amplifier. The main objective of this stage is to eliminate the dc drift between cornea and retina in our eyes. It is a low power amplifier. If we use IC741 we need three op amps, so we finish it with this single op amp. It is also an 8pin IC, but it consumes very low power. Minimum CMRR value is 100dB, so it has the greater tendency to eliminate the common mode signal, so it provides difference signal which we need exactly.
The input current is 3mA.The output of this amplifier is the difference between the two electrodes voltage outputs. So this voltage contains the information of the eye movement.
LOW PASS FILTER
Usually noise frequency is in the range of KHz or MHz, but our signal is in the range 0-15Hz. But, the output voltage is very low. So if noise interrupts, then the information may lose. To avoid this, we designed a low pass filter for 16Hz.It is a six pole filter (Three second order filters are cascaded). It provides 120dB/decade. So the noise will be completely eliminated at the end of this stage. Here feedback resistors are adjusted to provide the required gain. Actually, this stage can be implemented using IC 741.But, three ICs. So, we designed this filter part using IC LM358. It has two comparators inside the IC
ANALOG TO DIGITAL CONVERTER:
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The next stage is to convert the analog signals to digital signals. ADC 0809 is used to do this operation. Since two analog signals are to be converted to digital signals ADC 0809 is used which has 2 channels. The ADC0809 data acquisition component is a monolithic CMOS device with an 8-bit analog-to-digital converter, 8-channel multiplexer and microprocessor compatible control logic. The 8-bit A/D converter uses successive approximation as the conversion technique. The converter features a high impedance chopper stabilized comparator, a 256R voltage divider with analog switch tree and a successive approximation register. The 8-channel multiplexer can directly access any of 8-single-ended analog signals. The device eliminates the need for external zero and full-scale adjustments.
Easy interfacing to microprocessors is provided by the latched and decoded multiplexer address inputs and latched TTL TRI-STATE outputs.
The design of the ADC0809 has been optimized by incorporating the most desirable aspects of several A/D conversion techniques. The ADC0809 offers high speed, high accuracy, minimal temperature dependence, excellent long-term accuracy and repeatability, and consumes minimal power. These features make this device ideally suited to applications from process and machine control to consumer and automotive applications.
PS/2 INTERFACE AND PROTOCOL
The PS/2 mouse interface originally appeared in IBM's "Personal System/2" computers in the late 80's. Â It still remains a widely-supported interface for the sake of constantly maintaining backward compatibility.
The PS/2 mouse interface uses a bidirectional serial protocol to transmit movement and button-position data to the computer's auxiliary device controller (keyboard controller). Â The computer, in turn, may send a number of commands to the mouse to set the report rate, resolution, reset the mouse, disable the mouse, etc. Â The computer also provides the mouse with an overload-protected 5V power supply.
The standard PS/2 mouse interface supports the following inputs: X (right/left) movement, Y (up/down) movement, left button, middle button, and right button. The mouse reads these inputs at a regular frequency and updates various counters and flags to reflect movement and button states.Â
The standard mouse has two counters that keep track of movement: the X-movement counter and the Y-movement counter.Â TheseÂ are 9-bit 2's complement values and each has an associated overflow flag.Â Their contents, along with the state of the three mouse buttons, are sent to the host in the form of a 3-byte movement data packet (as described in the next section.)Â The movement counters represent the amount of movement that has occurred since the last movement data packet was sent to the host (i.e., they do not represent absolute positions.)
When the mouse reads its inputs, it records the current state of its buttons and checks for movement. If movement has occurred it increments (for +X or +Y movement) or decrements (for -X or -Y movement) its X and/or Y movement counters. If either of the counters has overflowed, it sets the appropriate overflow flag. PS2 ports use synchronous serial signals to communicate between the keyboard or mouse to the computer
Data transmission from the mouse to the computer is done with, each clock period is usually between 70 to 150 microseconds (10 to 25 microseconds for transitions and 30 to 50 microseconds for high or low state), some may feel that these are large margins both this works good since this is a synchronous port (this also helps cut on the cost of high precision clocks). The data line transition is made on the falling edge of the clock signal and is usually sampled when the clock is low. Each data packet is composed of 11 bits, 1 start bit (which is low), 8 data bits, 1 odd parity bit and 1 stop bit (high).
MOVEMENT DATA PACKET:
The standard PS/2 mouse sends movement/button information to the host using the following 3-byte packetÂ
Y sign bit
X sign bit
MOUSE RESOLUTION AND TRACKING RATE
The standard PS/2 mouse (with Logitech mouse) defaults to 160 CPI and can be switched to 40, 80, 160 or 320 CPI with software. Microsoft mouse driver for Windows 3.x and Windows 95 defaults to 160 counts per inch. The maximum tracking rate for PS/2 mouse is 40 report/second * 255 counts per report = 10200 counts per second. For 100 CPI mouse this would indicate maximum tracking rate of 102 inches per second and for 400 CPI mouse only 25.2 inches per second.Bi-directional transmission is controlled by the CLK and DATA lines. Both are fed by an open collector device which let either host or mouse force the line to "0". During non-transmission, CLK is at "1" and DATA can be at "0" or "1".
The host can inhibit mouse transmission by forcing CLK to "0". If the host inhibits the mouse while it is transmitting, the byte must be retransmitted (if the inhibit state arrived before the 11th clock).There is a simple description in old IBM PS/2 model 50/60 technical reference. The mouse interface is the same as the keyboard interface. The Intel 8042 supports two channels, one for keyboard and one for the auxiliary device (mouse, trackball, touchpad). Pin 1 is the data, pin3 ground, pin 4 +5V and pin 5 clocks.
Receiving data: Check "clock". If inactive, there is a bit on the "data" line. Each transmission unit is one start bit, eight data bits, odd parity and one stop bit. Start bits are low, stop bits high. Each clock active or inactive period is 30 to 50 microseconds. Data transition to falling edge of clock is 5 to 25 microseconds.
Sending data: Check that both clock and data are high. Pull down data for start bit, and start clocking.
APPLICATION OF EOG SIGNAL:
Computer mouse control.
Guidance of a wheelchair.
The paper includes various applications Computer mouse control, Robotics, Guidance of a wheelchair. The paper will be very much useful to the handicapped people to operate their pc using their eyes alone.