optical mouse
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ABSTRACT
Topic OPTICAL MOUSE
Every day of our computing life, we reach out for our mouse whenever we want to move our cursor or
activate something. Our mouse senses our motion and our clicks and sends them to the computer so
that it can respond appropriately. It is amazing how simple and effective a mouse is, and it
is also amazing how long it took Mice to become a part of everyday life. Given that people
naturally point at things -- usually before they speak -- it is surprising that it took so long for a
good pointing device to develop. Although originally conceived in the 1960s, it took quite
some time for mice to become mainstream. In the beginning there was no need to point because
computers used crude interfaces like teletype machines or punch cards for data entry. The early text
terminals did nothing more than emulate a teletype (using the screen to replace paper), so it was many
years (well into the 1960s and early 1970s) before arrow keys were found on most terminals. Full
screen editors were the first things to take real advantage of the cursor keys, and they offered humans
the first crude way to point.
In this paper on WORKING OF OPTICAL MOUSE Ill take the cover off of
this important part of the human-machine interfaces and see exactly what makes it tick!
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INTRODUCTION
Optical Technology uses an optical sensor to track movement, rather than the standard ball and
moving parts. Optical Technology provides increased control and precision and works on most
surfaces. This superior technology translates into precise cursor movement and unmatched
responsiveness.
It is amazing how simple and effective a mouse is, and it is also amazing how long it took mice to
become a part of everyday life. Given that people naturally point at things -- usually before they
speak -- it is surprising that it took so long for a good pointing device to develop. Although
originally conceived in the 1960s, it took quite some time for mice to become mainstream.
In the beginning there was no need to point because computers used crude interfaces like
Teletype machines or punch cards for data entry before arrow keys were found on most terminals.
Full screen editors were the first things to take real advantage of the cursor keys, and they offered
humans the first crude way to point.
Light pens were used on a variety of machines, as a pointing device for many years, and graphics
tablets, joysticks and various other devices were also popular in the 1970s. None of these really
took off as the pointing device of choice, however, when the mouse hit the scene, it was an
immediate success. There is something about it that is completely natural. Compared to a
graphics tablet, mice are extremely inexpensive and they take up very little desk space. In the PC
world, mice took longer to gain ground, mainly because of a lack of support in the Operating
system. Once Windows 3.1 made Graphical User Interfaces (GUIs) a standard, the mouse became
the PC-human interface of choice very quickly.
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Mice first broke onto the public stage with the introduction of the Apple Macintosh in 1984, and
since then they have helped to completely redefine the way we use computers. Every day of our
computing life, we reach out for our mouse whenever we want to move our cursor or activate
something. Our mouse senses our motion and our clicks and sends them to the computer so it can
respond appropriately.
DATA INTERFACE
Most mice in use today use the standard PS/2 type connector, as shown here
These pins have the following functions (refer to the above photo for pin numbering):
1. Unused
2. +5 volts (to power the chip and LEDs)
3. Unused
4. Clock
5. Ground
A typical PS/2 connector: Assume that pin 1 is
located just to the left of the black alignment pin, and
the others are numbered clockwise from there.
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6. Data
Whenever the mouse moves or the user clicks a button, the mouse sends 3 bytes of data to the
computer. The first byte's 8 bits contain:
1. Left button state (0 = off, 1 = on)
2. Right button state (0 = off, 1 = on)
3. 0
4. 1
5. X direction (positive or negative)
6. Y direction
7. X overflow (the mouse moved more than 255 pulses in 1/40th of a second)
8. Y overflow
The next 2 bytes contain the X and Y movement values, respectively. These 2 bytes contain the
number of pulses that have been detected in the X and Y direction since the last packet was sent.
The data is sent from the mouse to the computer serially on the data line, with the clock line
pulsing to tell the computer where each bit starts and stops. Eleven bits are sent for each byte (1
start bit, 8 data bits, 1 parity bit and 1 stop bit). The PS/2 mouse sends on the order of 1,200 bits
per second. That allows it to report mouse position to the computer at a maximum rate of about
40 reports per second. If we are moving the mouse very rapidly, the mouse may travel an inch or
more in one-fortieth of a second. This is why there is a byte allocated for X and Y motion in the
data protocol.
Some mice use serial or USB type connectors.
MICE: HOW DO THEY WORK?
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Open up a mouse and inside it we will find two wheels, each one similar to the first drawing. The
wheel is usually made of black plastic with rectangular slots punched in it. I have shown only 6
slots at 60 spacing but they are a lot closer and many more. Shining through the slots are two
LEDs (light Emitting Diodes) shown by the black dots. Each LED shines on to a light sensitive
transistor. The two emitters are spaced so that, when one transistor can 'see' its LED through the
centre of its window, the other LED is looking at an edge and is therefore switching on or off. In
my illustration the LEDs are spaced at 105 (60 x 1.75). The output voltage from the transistor is
processed to switch rapidly from high to low as the LED's light is transmitted or occluded so that
the voltage is low when the transistor is lit and high when it is in darkness. In the diagram LED A
is fully illuminated and LED B is switching. Note that LED B may be switching from light to
dark or from dark to light - this depends on the rotation direction.
Now consider the second drawing. Here the wheel is shown in 4 different states, each 15 rotated
from the last. Diagram E is equivalent to diagram A, being 60 rotated. For clockwise rotation the
states follow each other in order A-B-C-D-E from left to right but if we read the states from right
to left, E-D-C-B-A, then these correspond to anticlockwise rotation.
Notice that LED 2 is changing state from light to dark in diagram A for clockwise rotation and in
diagram C for anticlockwise rotation. So if we measure LED1 every time LED 2 goes from light
LED A
LED B
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1) A ball inside the mouse touches the desktop and rolls when the mouse moves.
2) Two rollers inside the mouse touch the ball. One of the rollers is oriented so that it detects
motion in the X direction, and the other is oriented 90 degrees to the first roller so it
detects motion in the Y direction. When the ball rotates, one or both of these rollers rotate
as well.
3) The rollers each connect to a shaft, and the shaft spins a disk with holes in it. When a
roller rolls, its shaft and disk spin.
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4) On either side of the disk there is an infrared LED and an infrared sensor. The holes in
the disk break the beam of light coming from the LED so that the infrared sensor sees
pulses of light. The rate of the pulsing is directly related to the speed of the mouse and
the distance it travels.
5) An on-board processor chip reads the pulses from the infrared sensors and turns them
into binary data that the computer can understand. The chip sends the binary data to the
computer through the mouse's cord.
In this optomechanical arrangement, the disk moves mechanically, and an optical system counts
pulses of light. On this mouse, the ball is 21 mm in diameter. The roller is 7 mm in all is 21 mm
in diameter. The roller is 7 mm in diameter. The encoding disk has 36 holes. So if the mouse
moves 25.4 mm (1 inch), the encoder chip detects 41 pulses of light.
It is noticed that each encoder disk has two infrared LEDs and two infrared sensors, one on each
side of the disk (so there are four LED/sensor pairs inside a mouse). This arrangement allows the
processor to detect the disk's direction of rotation. There is a piece of plastic with a small,
precisely located hole that sits between the encoder disk and each infrared sensor.
This piece of plastic provides a window through which the infrared sensor can "see." The window
on one side of the disk is located slightly higher than it is on the other -- one-half the height of
one of the holes in the encoder disk, to be exact. That difference causes the two infrared sensors
to see pulses of light at slightly different times. There are times when one of the sensors will see a
pulse of light when the other does not, and vice versa.
THE OPTICAL MOUSE
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With advances in mouse technology, it appears that the venerable wheeled mouse is in danger of
extinction. The now-preferred device for pointing and clicking is the optical mouse.
Developed by Agilent Technologies and introduced to the world in late 1999, the optical mouse
actually uses a tiny camera to take 1,500 pictures every second.
Able to work on almost any surface, the mouse has a small, red light-emitting diode (LED) that
bounces light off that surface onto a complimentary metal-oxide semiconductor (CMOS)
sensor. The CMOS sensor sends each image to a digital signal processor (DSP) for analysis.
The DSP, operating at 18 MIPS (million instructions per second), is able to detect patterns in the
images and see how those patterns have moved since the previous image. Based on the change in
patterns over a sequence of images, the DSP determines how far the mouse has moved and sends
the corresponding coordinates to the computer. The computer moves the cursor on the screen
based on the coordinates received from the mouse. This happens hundreds of times each second,
making the cursor appear to move very smoothly.
In this photo, we can see the LED on the bottom of the mouse.
Optical mice have several benefits over wheeled mice:
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No moving parts mean less wear and a lower chance of failure.
There's no way for dirt to get inside the mouse and interfere with the tracking sensors.
Increased tracking resolution means smoother response.
They don't require a special surface, such as a mouse pad.
Although LED-based optical mice are fairly recent, another type of optical mouse has been
around for over a decade. The original optical-mouse technology bounced a focused beam of light
off a highly-reflective mouse pad onto a sensor. The mouse pad had a grid of dark lines. Each
time the mouse was moved, the beam of light was interrupted by the grid. Whenever the light was
interrupted, the sensor sent a signal to the computer and the cursor moved a corresponding
amount.
This kind of optical mouse was difficult to use, requiring that you hold it at precisely the
right angle to ensure that the light beam and sensor aligned. Also, damage to or loss of
the mouse pad rendered the mouse useless until a replacement pad was purchased.
Today's LED-based optical mice are far more user-friendly and reliable.
Inside An Optical Mouse
If we take apart an optical mouse and look inside, we will find a complete imaging system. The
mouse is essentially a tiny, high-speed video camera and image processor. As shown in figure
below, a light-emitting diode (LED) illuminates the surface underneath the mouse. The light from
the LED reflects off microscopic textural features in the area. A plastic lens collects the reflected
light and forms an image on a sensor. If we were to look at the image, it would be a black-and-
white picture of a tiny section of the surface. The sensor continuously takes pictures as the mouse
moves. The sensor takes pictures quickly-1500 pictures (frames) per second or more fast enough
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so that sequential pictures overlap. The images are then sent to the optical navigation engine for
processing.
:Optical mice illuminate an area of the work surface with an LED, to reveal
a microscopic pattern of highlights and shadows. These patterns are reflected
onto the navigation sensor, which takes pictures at a rate of 1500 images per
second or more.
The Basics of Optical Navigation
The optical navigation engine is the brain of the mouse. It identifies texture or other features in
the pictures and tracks their motion. Figure below illustrates how this is done. Two images were
captured sequentially as the mouse was panned to the right and upwards. Much of the same visual
material can be recognized in both frames. Through a patented image-processing algorithm, the
optical navigation engine identifies common features between these two frames and determines
ImageAcquisition
System
DigitalSignal
Processor
SerialParallel
Interface
Block diagram of optical sensing
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the distance between them. This information is then translated into X and Y coordinates to
indicate mouse movement.
The Navigation Engine identifies common features in sequential images to
determine the direction and amount of mouse movement. Image B was taken
while the mouse was moving, a short time after image A. It shows the same
features as image A, only shifted down and to the left.
BENEFITS OF OPTICAL TECHNOLOGY AND OPTICAL MOUSE
Improved tracking speed The most common complaint optical mouse users have is
that the cursor gets lost during periods of quick hand movement. The problem is, most
optical mouse products can only move up to 14 inches per second. But usability research
indicates that computer users can move the mouse up to 30 inches per second - far faster
than the tracking capability of todays mouse products. Microsoft Optical Technology is
remarkable. It enables the mouse to keep up with the hand, tracking up to 37 inches per
second.
Increased accuracy Due to the limits of existing optical mouse technology, the cursor
does not always land where you want it. This causes reduced accuracy and increased
frustration. Optical Technology provides superior accuracy and enables the cursor to
land where you want it.
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Consistent performance With no mouse ball or moving parts to keep clean or wear
out, the optical sensor stays precise over time.
Works on most surfaces The optical sensor works on most surfaces*, so you no longer
need a mouse pad. Use it on your desk, the kitchen counter, or even your pant leg.
Less frequent cleaning The recessed lens is enclosed and never touches your desk,
so less frequent cleaning is necessary.
The optical sensor performs best on surfaces with detail to track. It will not function
on surfaces without visible detail (such as glass) or surfaces on which it has a
reflection (such as mirrors or glossy surfaces). The optical sensor may also have
difficulty tracking on highly repetitive patterns (such as printed magazine or
newspaper photographs).
REFERENCES:
1) Internet sites
2) Electronics for you-02
3) Pc Quest-02
4) Digit-02