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.
Largerangedistancemeasurementcalledrangesensors.
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
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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
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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
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