5-Linear & Rotational Sensors – I


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Sensors & Actuators
Dr. Hassan SHARABATY
BY
Aleppo – March 2012
Department of Mechatronics Engineering
EEM304 Mechatronics
5- Linear & Rotational Sensors – I
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
2/21
Linear & Rotational Sensors
Themostcommonmotionsinmechanicalsystemsarelineartranslationalongafixedaxisandangularrotationaboutafixedaxis.
Morecomplexmotionsareusuallyaccomplishedbycomposingthesesimplermotions.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
3/21
Infrared sensors
Infraredlightcanbeusedinavarietyofwaystomeasurelinearandrotationaldisplacement.
Ex.OpticalEncoder
Typically,aninfraredlight-emittingdiode(LED),orphotoemitter,isusedasasource,andaninfraredsensitivedeviceisusedtodetecttheemittedlight.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
4/21
Thedetectorcouldbeaphotoresistororphotocell,avariableresistorwhichchangesresistancedependingonthestrengthoftheincidentlight(possiblyinfraredorvisible);aphotodiode,whichallowstheflowofelectricalcurrentinonedirectioninthepresenceofinfraredlight,andotherwiseactsasanopencircuit;oraphototransistor.
Infrared sensors
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
5/21
Inaphototransistor,theincidentinfraredlightactsasthebasecurrentforthetransistor,allowingtheflowofcollectorcurrentproportionaltothestrengthofthereceivedinfraredlight(uptosaturationofthetransistor).
Infrared sensors
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
6/21
Contact Sensors
Thesimplestkindofdisplacementsensorisamechanicalswitchwhichreturnsonebitofinformation:touchingornottouching.
Atypicalmicroswitchconsistsofaleverwhich,whendepressed,createsamechanicalcontactwithintheswitch,whichclosesanelectricalconnection.
contact sensor
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
7/21
Contact Sensors
Microswitchesmaybeusedasbumpsensorsformobilerobots,oftenbyattachingacompliantmaterialtothelevertoprotecttherobotbodyfromimpactwitharigidobstacle.
Anotherpopularapplicationofthemicroswitchinroboticsisasalimitswitch,indicatingthatajointhasreachedthelimitofitsallowabletravel.
contact sensor
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
8/21
Figureshowsatypicalconfigurationforamicro-switch.Thepull-upresistorkeepsthesignalat+Vuntiltheswitchcloses,sendingthesignaltoground.
signal bounce at a closing switch
ContactSensors
Astheswitchcloses,aseriesofmicro-impactsmayleadto“bounce”inthesignal.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
9/21
Switchesmaybedesignatedfornormallyopen(NO)ornormallyclosed(NC),where“normally”indicatestheunactivatedorunpressedstateoftheswitch.
Aswitchmayalsohavemultiplepoles(P)andoneortwothrows(T)foreachpole.
ContactSensors
SPDT and DPST contact sensors
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
10/21
Apolemovesastheswitchisactivated,andthethrowsarethepossiblecontactpointsforthepole.
SPDT and DPST contact sensors
ThusanSPDT(singlepoledoublethrow)switchswitchesasinglepolefromcontactwithonethrowtotheother,andaDPST(doublepolesinglethrow)switchswitchestwopolesfromopentoclosedcircuit.
Contact Sensors
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
11/21
Non-Contact Range sensorsuse a number of technologies including light/optics, microwave, and ultrasonic to measure the distance from a reference point to an object.
Classification:
Sensorradiatessomeformofenergyintothefieldofinterest.
Sometypicalsensorsinthiscategoryincluderadar,sonar
Active
Passive
Sensor relies on energy emitted from objects or targets of interest.
Largerangedistancemeasurementcalledrangesensors.
Non-Contact Range Sensing
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
12/21
Propagation speed v of sound: 0.3 m/ms
Propagation speed v of ofelectromagnetic signals: 0.3 m/ns, i.e. one million times faster.
time of flight t with electromagnetic signals is not an easy task
laser range sensors are expensive.
Non-Contact Range Sensing
The traveled distance of a sound or electromagnetic wave is given by d = c . T
d = distance traveled
c = speed of wave propagation
t = time of flight.
Ultrasonicsensorsaswellaslaserrangesensorsmakeuseofpropagationspeedofsoundorelectromagneticwavesrespectively.
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – I 13/21
Non-Contact Range Sensing
 Figure shows a conceptual diagram of time of flight laser range finder or Pulsed
laser with the Emitter and Receiver both located at the sensor.
 The distance, d, in the diagram is defined as:
 
2
c. TOF
d 
Time Of Flight (TOF) Technique
Where:
c is the speed of wave propagation
TOF is the time of flight measured in seconds.
Target
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
14/21
There are two basic methods for defining the Time Of Flight (TOF)
1.beginning of signal burst to end of returning burst
Non-Contact Range Sensing
Time Of Flight (TOF) Technique
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
15/21
Non-Contact Range Sensing
Time Of Flight (TOF) Technique
There are two basic methods for defining the Time Of Flight (TOF)
2.Beginning of signal burst to maximum amplitude of returning burst .
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – I 16/21
Light Sensor
Phase Measurement
D
L
Reflecting Object
Beam splitter
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 -1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
θ
λ
Non-Contact Range Sensing
Phase shift measurement
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – I 17/21
Non-Contact Range Sensing
Phase shift measurement
 An other method is to use a continuous-beam laser and measure the delay
(phase shift) between the outgoing the returning beam.
 Suppose that a beam of laser light of wavelength λ is split into two beams:
• One is called the reference beam travels a distance L to a phase measuring
device
Light Sensor
Phase Measurement
D
L
Reflecting Object
Beam splitter
• The other travels a distance
D out to a reflecting surface
• The total distance is D’=L+2D
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – I 18/21
• It can be deduced that if θ=360 the two wave forms are again aligned and
we can not differentiate between D’ and L
• In cases, θ<360 or equivalently, 2D<λ, thus D=(θ/360)(λ/2)
Non-Contact Range Sensing
Phase shift measurement
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 -1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
θ
• D’ is determined as follows: λ
D’=L+(θ/360)λ ; λ=c/f
Where c: is the speed of light;
f the modulating frequency
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
19/21
Range sensing by triangulation
Non-Contact Range Sensing
Triangulation
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
20/21
The approach shown in figure explains a simple method for measuring distance using non-contact sensors
An object is illuminated by a narrow beam of light which is swept over the surface
If the detector is focused on a small portion of the surface then when the detector sees the light spot, its distance D to illuminated portion of the surface can be calculated from the geometry.
D=B.tan(θ)
Non-Contact Range Sensing
Triangulation
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -I
21/21
END

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