me 8843 advanced mechatronics

Advanced Mechatronics, Georgia TechAdvanced Mechatronics, Georgia Tech

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ME 8843Advanced MechatronicsInstructor: Professor I. Charles Ume

Hall effect SensorsVariable Reluctance Sensor

Ultrasonic Sensors (Sonic Distance Sensors)Photo Interrupt

Pressure SensorsAccelerometers


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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

http://farm1.static.flickr.com/62/227729006_fab88c1668.jpg?v=0

Hall Effect Sensor Sensing a Shaft Speed


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How they work• Presence of magnetic field

deflects electrons flowing through a conductive material

• 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/Hall_effect_A.png

Depiction of the Hall Effect


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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– South field turns latch off


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Packaging and Manufacturers

• ICs– Analog Devices:

• AD22151G from Analog Devices

– Allegro MicroSystems, Inc.• Wide range of linear, latching and

switching sensors• Great sampling policy

– Many, many more

• Packaged units– Honeywell– Many, many more

http://sensing.honeywell.com/client_asset/document/1/5/4/0/3/5/document_C3697B35-

C930-CB7C-FE090DFFCE61FB22.jpg

http://www.allegromicro.com/en/Products/Part_Numbers/1120/pinout.gif

SOT23

SIP

Hall Effect Sensor Module


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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 measure speed and/or position of a moving metallic object

• Sense the change of magnetic reluctance (analogous to electrical resistance) near the sensing element

• Require conditioning circuitry to yield a useful signal (e.g. LM1815 from National Semi.)

http://www.motionsensors.com/railwithoring2.jpg

Industrial Variable Reluctance Sensor


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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

http://www.instronics.com/images/sensoronix/image.ds.drawing.vr.jpg

Variable Reluctance Sensor Construction Typical Configuration


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Typical Application• Shaft velocity sensor for ABS/traction

control

• Crank and cam shaft position sensors

http://www.me.gatech.edu/mechatronics_lab/Projects/Spring07/Group1/dorthy6.JPG

Sensor Schematic Installed on CV axle


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Interfacing Concerns

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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• 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 transceiverAlternative 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 –They 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.


ME8843ME8843 ApplicationsMedical: 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 InterruptTypes

• Wide variety of packages and orientations

• Types– Logic (digital ±5 volts)– Transistor/diode (analog)

• Manufacturers – Fairchild– Honeywell


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Photo InterruptApplications

• Encoder wheel for angular measurements.– Computer mouse with a ball


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Photo InterruptApplications

• Detect holes or slots for positioning of liner slides – Elevators

• Detect the location of products on and 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 SensorsTypes

• 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 SensorsApplications

• 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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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 accounting 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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AccelerometersHow 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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AccelerometersApplications

• 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.

me 8843 advanced mechatronics

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