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TRANSDUCERS
Presentation
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Hall Effect
The effect of generating a measurable voltage
by using a magnetic field is called the Hall
Effect after Edwin Hall (1879)
In its most common application, a Hall
effect transducer serves to measure a
magnetic field and convert that
measurement into voltage
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Hall Voltage
Working Principle: If a strip of conducting
material carries a current in the presence
of a transverse magnetic field, a potential
difference is produced between theopposite edges of the conductor.
The output Hall voltage is given as:
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Hall Effect Transducers
Hall Effect Sensors consist of a thin piece ofrectangular p-type semiconductor material
such as Ga-As, In As passing a continuous
current through itself. When the device is
placed within a magnetic field, the magnetic
flux lines exert a force on the semiconductor
material which deflects the charge carriers,
electrons and holes, to either side of thesemiconductor slab
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Hall Effect Transducers
To ensure maximum sensitivity themagnetic lines of flux must always be
perpendicular to the sensing area of
the device and must be of the correctpolarity
To ensure linearity, high field strength
magnets are required that produce alarge change in field strength for the
required movement.
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Applications of Hall Effect
Transducers
Magnetic to Electrical Transducer
Measurement of displacement
Measurement of current
Measurement of power
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Typical Applications
Switched Mode Power Supplies Uninterruptible Power Supplies (UPS)
Over voltage protection
Feedback of control systems Electric power network monitoring
AC frequency conversion servo-
motors Various power supplies
Power supply for welding applications
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Ways of Detecting a magnetic
field Head-on Detection:
It requires that themagnetic field isperpendicular to thesensing device and that it
approaches it straight ontowards the active face.This head-on approachgenerates an output signal,VH which in the lineardevices represents the
strength of the magneticfield, the magnetic fluxdensity, as a function ofdistance away from thesensor.
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Ways of Detecting a magnetic
field Sideways Detection:
This requires movingthe magnet across theface of the Hallelement in a sideways
motion. Sideways orslide-by detection isuseful for detecting thepresence of amagnetic field as itmoves across the faceof the Hall elementwithin a fixed air gapdistance for example,counting rotationalmagnets or the speed
of rotation.
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Digital Transducers
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Contents
Shaft Encoders
Incremental Optical Encoder
Absolute Optical Encoder
Encoder Error
Digital Resolvers
Digital Tachometers
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INTRODUCTION
What is a Digital Transducer ? Any transducer that presents information as discrete
samples and that does not introduce a quantization errorwhen the reading is represented in the digital form may beclassified as a digital transducer
What is an encoder ? Any transducer that generates a coded of a measurement
can be termed an encoder
SHAFT ENCODERS
They are Digital Transducersthat are used for measuring ANGULARDISPLACEMENTS and ANGULAR VELOCITIES.
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Applications Of Shaft Encoders
1.) Control of robotics manipulators2.) Machine tools
3.) Digital tape-transport mechanisms
4.) Servo plotters
5.) Printers
6.) Satellite mirror positioning system
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Encoder Types
Shaft encoders can be classified into threecategories
1. Incremental Encoders
2. Incremental Optical Encoders3. Absolute Optical Encoders
Incremental Encoders
1.Optical (photo sensor) method2.Sliding contact (Electrical conducting) method
3.Magnetic saturation (Reluctance) method
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Optical Encoder
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Optical Encoder
The optical encoder uses an opaque disk that hasone or more circular tracks, with somearrangement of identical transparent windows.
A parallel beam of light is projected to all tracks
from one side of the disk The light sensor could be a silicon photodiode, a
phototransistor, or a photovoltaic cell.
The light from the source is interrupted by theopaque areas of the track, the output signal fromthe probe is a series of voltage pulses
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Incremental Optical Encoders
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Incremental Optical Encoder
The disk has a single circular track with
identical and equally spaced transparent
windows.
The area of the opaque region between
adjacent windows is equal to the window
area.
Two photodiode sensors (pick offs 1 and
2) are positioned facing the track a
quarter-pitch
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Absolute Optical Encoders
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Absolute Optical Encoders
The disk has a circular track withidentical and equally spaced transparent
windows.
In absolute optical encoders photo sensorsare not used.
The output can be binary, gray code,
natural binary code.
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Encoder Error
Any transducer that generates a coded readingof a measurement is known as Encoder.
The primary sources of errors in shaft
encoder are:1) Quantization error
2) Assembly error
3) Coupling error
4) Structural limitations
5) Manufacturing tolerances
6) Ambient effects
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Digital Resolvers
Digital resolvers or mutual induction encodersoperate using the principle of mutual induction.
They are commercially known as Inductosyns
A digital resolver has two disks namely statorand rotor which is coupled to the rotating object.
The rotor has fine electric conductor foilimprinted which is connected to a highfrequency AC supply.
The stator has two separate printed patternsidentical to the rotor pattern but are shifted by aquarter-pitch from one another
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Digital Tachometers
As Shaft encoders are also used for measuring angularvelocities, they can be considered as Tachometers.
A Magnetic induction tachometer of variable-reluctance
type is shown in figure.
Teeth on the wheel are made of ferromagnetic material.
Two magnetic induction proximity probes are placed
facing the teeth radially, a quarterpitch apart.
Speed is computed either by counting pulses over asampling period or by timing the pulse width.