ME8843
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
0 o C and 343 m/sec at 20 o C for dry air
• Digital signal processor embedded in sensor calculates distance between sensor and object
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 x = v sound
. t
 Where V sound 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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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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• 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 on and assembly line
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• 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
– The pressure relative to perfect vacuum pressure or set pressure (like pressure at sea level)
– 1 input
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Pressure Sensors
Applications
• Measure pressure of gas or fluids
• Measure altitude
– For plains 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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• 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

ME8843
• 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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• 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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• 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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• 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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• 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

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• ICs
– Analog Devices:
• AD22151G from Analog Devices
– Allegro MicroSystems, Inc.
• Wide range of linear, latching and switching sensors
• Great sampling policy
– Many, many more
SOT23
SIP http://www.allegromicro.com/en/Products/P art_Numbers/1120/pinout.gif
• 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_C3697B35-
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• 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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• Used to measure speed and/or position of a moving metallic object
• Sense the change of magnetic reluctance
(analogous to electrical resistance) near the sensing element
Industrial Variable
Reluctance Sensor http://www.motionsensors.com/railwithoring
2.jpg
• Require conditioning circuitry to yield a useful signal (e.g.
LM1815 from National Semi.)
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• 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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• Shaft velocity sensor for ABS/traction control
• Crank and cam shaft position sensors
Installed on CV axle
Sensor Schematic http://www.me.gatech.edu/mechatronics_lab/Projects/Spring07/Group1/dorthy
6.JPG
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The emf is proportional to the rate of change of the magnetic flux. This dictates the ferrous material must be moving for the sensor to generate a signal.
Output voltage dependent on velocity of toothed wheel - performance may be reduced at slow speeds
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