Suman Mashruwala Micro Mechanical Systems Laboratory Department of Mechanical Engineering IIT Bombay

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Suman Mashruwala Micro Mechanical Systems
LaboratoryDepartment of Mechanical
EngineeringIIT Bombay
HOW CDROM DRIVE WORKS
By Yonas Tegegn M.Tech 2nd year
Prof P.S Gandhi
FOCUSING SYSTEM
TRACKING SYSTEM
CD layer
CD layer
Voice coil moves into the page
Voice coil
Voice coil
Diffraction grating
Diffraction grating
Beam splitter
Laser
Beam splitter
Laser
Collimator
Collimator
CylindericalLens
Cylindrical Lens
Photodiode
http//naraya.phy.qub.ac.uk
Photodiode
Beam shape on Photodiodes
E and F wont get signal
E
A
A
F
F
F
F
E
E
E
F
D
D
B
B
READING SYSTEM
C
C

• Focusing system maintains constant focus by
  moving the voice coil up and down
• The Beam shape will be elliptical or circular
  depending on the position of disc
• The error signal will be given to voice coil for
  correction(AC)-(BD)

• The beam is kept on a single track all the time
  by Tracking system
• Different amount of intensity of light obtained
  in the photodiode (E and F)
• Voice coil moves sideways to correct it

000001000010001000010001001001001000

• Laser light emitted from source
• Pass through Diffraction grating and beam
  splitter
• Focused by objective lens
• The Light is reflected by the reflecting surface
  inside the Disc
• A photodiode array detect the reflected light
• Whenever there is change in pits and land its
  detected as 1 otherwise 0

vertical
In focus
Horizontal
Three spot on track