Download Industrial Variable Reluctance Sensor

ME8843
ME 8843
Advanced Mechatronics
Instructor: Professor I. Charles Ume
Lecture #1
Ultrasonic Sensors (Sonic Distance Sensors)
Photo Interrupt
Pressure Sensors
Accelerometers
Hall effect Sensors
Variable Reluctance Sensors
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• Ultrasonic transducer (piezoelectric transducer)
is device that converts electrical energy into
ultrasound
• Upon receiving sound echo (pressure wave)
back from surface, ultrasound transducer will
turn sound waves into electrical energy which
can be measured and displayed
• Ultrasound are sound waves above normal
range of human hearing (greater than 20K
hertz).
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Since piezoelectric crystal generates voltage when force is
applied to it, same crystal can be used as an ultrasonic
detector
 Some systems use separate transmitter and receiver
components while others combine both in single
piezoelectric transceiver
Alternative methods for creating and detecting ultrasound
include magnetostriction and capacitive actuation.
Pulse echo
sensor
Transmit-Receive
sensor
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• Sound is transmitted through propagation of
pressure in air
• Speed of sound in air is normally 331 m/sec at
0oC and 343 m/sec at 20oC for dry air
• Digital signal processor embedded in sensor
calculates distance between sensor and object
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 x = vsound . t
 Where Vsound is known, t = 0.5 (time of flight), x is
distance between sensor head and object
 Range of sensor varies between 5 cm to 20 m
 Sensor is not appropriate for very short distance
measurements
 Frequency response (distance measurement
update rate) varies with distance measured
– In general, it is about 100 Hz
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• Piezoelectric crystals have property of changing
size when voltage is applied
• Applying alternating current (AC) across them
causes them to oscillate at very high frequencies,
thus producing very high frequency sound waves.
• Ultrasonic sensors work on principle similar to
radar or sonar
–Evaluate attributes of target by interpreting
echoes from radio or sound waves respectively
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Ultrasonic sensors generate high frequency
sound waves and evaluate echo which is received
back by the sensor
Sensors calculate time interval between sending
signal and receiving echo to determine distance to
object.
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Applications
Medical:
 Medical ultrasonic transducers (probes) come in
variety of different shapes and sizes for use in making
pictures of different parts of body.
 Transducer may be passed over surface of body or
inserted into body opening such as rectum or
woman’s reproductive organ
 Clinicians who perform ultrasound-guided procedures
often use a probe positioning system to hold the
ultrasonic transducer.
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 Technology can be used for measuring:
– Often used in robots for obstacle avoidance
– Wind speed and direction (anemometer),
– Fullness of tank, and speed through air or
water
– Measuring amount of liquid in tank, sensor
measures distance to surface of fluid.
– Other applications include: burglar alarms,
non-destructive testing, and etc
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Photo Interrupt
• Uses emitter and detector
photo diode pair
• With no obstruction
detector is high
• When an object blocks the
light the detector is low
• Advantages
– Simple to interface
– Inexpensive
– Reliable
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Photo Interrupt
Types
• Wide variety of packages
and orientations
• Types
– Logic (digital ±5 volts)
– Transistor/diode (analog)
• Manufacturers
– Fairchild
– Honeywell
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Photo Interrupt
Applications
• Encoder wheel for angular
measurements.
– Computer mouse with a ball
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Photo Interrupt
Applications
• Detect holes or slots for
positioning of liner slides
– Elevators
• Detect the location of
products
– Assembly line
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Pressure Sensors
• Used to detect pressure of
fluids or gasses.
• Technologies (many)
– Strain gage
– Piezoresistive
– Microelectromechanical
systems (MEMS)
• Each sensor has a pressure
range that it works in.
• Most have analog outputs
that need amplification
– Some have built-in amplifiers
for direct connection into
microcontroller
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Pressure Sensors
Types
• Differential Pressure
– Difference between two or more
pressures introduced as inputs to
the sensing unit
– 2 input
• Absolute/Gage Pressure
– Pressure relative to perfect
vacuum pressure or set pressure
(such as pressure at sea level)
– 1 input
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Pressure Sensors
Applications
• Measure pressure of gas or fluids
• Measure altitude
– For planes or weather balloons
• Measure flow
– pressure sensors in conjunction
with the venturi effect to measure
flow
• Measure depth of water
– When measuring liquids, most
sensors are not rated to have
unclean liquids contact the sensor
components. A small amount of air
in the tube right before the sensor
will create a barrier from the liquid.
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Accelerometers
• Used to measure acceleration
– Common SI units
meters/second2 (m/s2) or
popularly in terms of g-force (1 g
is earth’s gravity)
• At rest an acceleration will
measure 1 g in the vertical
direction
• They can come in 1, 2 or 3
axis configurations
– With 3 axis it gives a vector of
the accelerations direction (after
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accounting Advanced
for gravity)
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Accelerometers
• Because of earth’s gravity, the
sensor will read 1 to 0 g as the
sensor is rotated from being
vertical to horizontal.
– This can be used to measure
angle the of tilt
• Each sensor has a range that
it works in.
• Most have analog outputs that
need amplification
– Some have built-in amplifiers for
direct connection into
microcontroller
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Accelerometers
How they work
• Mechanically the accelerometer
behaves as a mass-damper-spring
system
– Many use Microelectromechanical
systems (MEMS). Which use very small
cantilever beams with masses on them
• Under the influence of gravity or
acceleration, the proof mass deflects
from its neutral position.
• This deflection is measured in an
analog or digital manner
– Commonly the capacitance between a
set of fixed beams and a set of beams
attached to the proof mass is measured.
– Integrating piezoresistors in the springs
to detect spring deformation is another
method
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Accelerometers
Applications
• Can be used to sense
orientation, vibration and
shocks.
• Used in electronics like the Wii
and iPhone for user input.
• Acceleration integrated once
gives velocity, integrated a
second time gives position.
– The integration process is not
precise and introduces error into
the velocity and position.
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Hall Effect Sensors
• Used to provide a
noncontact means
to detect and
measure a
magnetic field
• Named based on
their use of the
Hall Effect,
discovered by
Edwin Hall in 1879
Hall Effect Sensor Sensing a Shaft Speed
http://farm1.static.flickr.com/62/227729006_fab88c1668.jpg?v=
0
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How they work
• Presence of magnetic field
deflects electrons flowing
through a conductive material
Depiction of the Hall Effect
• As electrons move to one end
of a conductive material, a
potential is developed in the
direction perpendicular to
gross current flow
• This potential indicates the
strength of the magnetic field
http://upload.wikimedia.org/wikipedia/commons/a/ab/Ha
ll_effect_A.png
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Applications
• IC Engine Electronic Ignition Systems
– Used to determine position of cam shaft
• Brushless DC Motor Control
– Sensors determine position of permanent magnet
rotor
• Assembly Lines
– Shaft position and velocity sensors
– Contactless limit switches
• Current Sensing ICs
– Electrically isolated alternative to shunt resistors
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Hall Effect Sensor Types
• Linear Hall Effect Sensors
– Output is proportional to magnetic field
strength
• Hall Effect Digital Switches
– Presence of field above threshold turns switch
on
– Presence of field below threshold turns switch
off
• Hall Effect Digital Latches
– North field turns latch on
Advanced
– South field
turnsMechatronics,
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Packaging and Manufacturers
• ICs
– Analog Devices:
• AD22151G from Analog Devices
SOT23
– Allegro MicroSystems, Inc.
• Wide range of linear, latching and
switching sensors
• Great sampling policy
SIP
http://www.allegromicro.com/en/Products/P
art_Numbers/1120/pinout.gif
– Many, many more
• Packaged units
– Honeywell
– Many, many more
Hall Effect
Sensor Module
http://sensing.honeywell.com/client_asset/do
cument/1/5/4/0/3/5/document_C3697B35C930-CB7C-FE090DFFCE61FB22.jpg
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Implementation and Words of
Warning
• Sensors may be affected by temperature
variation. Some sensors incorporate
circuitry to reduce this error.
• Sensors may be directional, in which case
care must be taken with respect to
orientations of sensor and magnet
• Some Hall Effect sensors detect presence
of ferromagnetic materials, not magnetic
fields
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Variable Reluctance Sensors
• Used to measure speed and/or
position of a moving metallic
object
• Sense the change of magnetic
reluctance/resistance (analogous
to electrical resistance) near the
sensing element
• Require conditioning circuitry to
yield a useful signal (e.g. LM1815
from National Semi.)
Industrial Variable
Reluctance Sensor
http://www.motionsensors.com/railwithoring
2.jpg
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How Variable Reluctance Sensors
Work
• A magnet in the sensor creates a
magnetic field
• As a ferrous object moves by the sensor,
the resulting change in the magnetic flux
induces an emf in the pickup coil
Variable Reluctance Sensor
Construction
Typical Configuration
http://www.instronics.com/images/sensoronix/image.ds.drawing.v
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Typical Application
• Shaft velocity sensor for ABS/traction
control
• Crank and cam shaft position sensors
Sensor Schematic
Installed on CV axle
http://www.me.gatech.edu/mechatronics_lab/Projects/Spring07/Group1/dorthy
6.JPG
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Interfacing Concerns
 Emf is proportional to the rate of change of
the magnetic flux.
 Indictates the ferrous material must be
moving for the sensor to generate a signal.
 Output voltage depends on velocity of
toothed wheel
 Performance may be reduced at slow speeds
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