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MIT 2 007 - Actuators

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2.007 –Design and Manufacturing I Actuators: Electric Motors, Servomotors, and Pneumatics Concept QuestionConcept QuestionSome Notes on Capstans from 2.001What You May Have SeenDC Permanent Magnet MotorDiscussion Question: How can I design a DC motor to provide high stall torque?A Model of a Motor (Steady State)Torque Speed CurvesTorque Current CurvesBack emf versus speedConcept QuestionConcept QuestionDiscussion Question: How do the things I might do to raise stall torque affect back emf?Speed Control for DC MotorsSwitching On/Off a LoadServo MotorsPulse Width Modulation (PWM)Electronics Within the ServoRunning a Servo via PBASICPneumatic SystemPower Comparison (Steady)Concept QuestionConcept QuestionNext StepsMIT OpenCourseWare http://ocw.mit.edu 2.007 Design and Manufacturing ISpring 2009For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.2.007 –Design and Manufacturing IActuators: Electric Motors, Servomotors, and Pneumatics Presented by Dan Frey on 10 FEB 2009Dan FreyV-+ERw+iT, ω-Figure by MIT OpenCourseWare.Concept Question• With this arrangement, I can hold up 1lb of water with less that 1lb of tension in the spring• What will happen if I swap in a smaller diameter of PVC pipe (reduce the OD from 1.5” to 1.25”)?The tension will:1) drop by much more than 20% 2) drop by about 20%3) stay nearly the same4) rise by about 20%5) rise by much more than 20%Concept Question• With this arrangement, I can hold up 1lb of water with less that 1lb of tension in the spring• What will happen if I swap in a smaller diameter of PVC pipe (reduce the OD from 1.5” to 1.25”)?The tension will:1) drop by much more than 20% 2) drop by about 20%3) stay nearly the same4) rise by about 20%5) rise by much more than 20% The answer is 3. The capstan equation indicates that the ratio of the two forces is e^(mu*theta). Radius is not a variable in the formula.Some Notes on Capstans from 2.001Courtesy Carol Livermore. Used with permission.How might I estimate the maximum power available at the output shaft ?The electrical power consumed ?HS-311 Standard Servo spec sheet removed due to copyright restrictions. Please see http://www.hitecrcd.com/product_file/file/45/HS311.pdfHow do you think power provided by a servo scales with its linear dimensions?HS-805BB+ Mega ¼ Scale Servo spec sheet removed due to copyright restrictions. Please see http://www.hitecrcd.com/product_file/file/66/hs805.pdfWhat You May Have SeenCourtesy Gabriella Sciolla and Walter Lewin. Used with permission.DC Permanent Magnet Motor)( BvEqFKKKK×+=Image removed due to copyright restrictions. Please see http://static.howstuffworks.com/gif/motor7a.jpg++..ArmatureAxleCommutatorBrushesParts of an Electric MotorNORTHSOUTHFIELDMAGNETTo BatteryFFigure by MIT OpenCourseWare.Discussion Question:How can I design a DC motor to provide high stall torque?)( BvEqFKKKK×+=Some options include increasing the magnetic field, increasing the radius at which the force acts, increasing the number of windings, and increasing the current flow through each winding (such as by raising the voltage).Image remove due to copyright restrictions. Please see http://static.howstuffworks.com/gif/motor7a.jpg++..ArmatureAxleCommutatorBrushesParts of an Electric MotorNORTHSOUTHFIELDMAGNETTo BatteryFFigure by MIT OpenCourseWare.A Model of a Motor (Steady State)iRVEw−=V-+ERw+iT, ω-“back emf”Torque Speed Curves02000400060008000100001200014000160000 0.005 0.01 0.015 0.02 0.025Torque (Nm)Speed (rpm)7V6V4V3VStall torqueNo load speedNote, conventionally, speed is on the x axis and Torque is on the Y.Image removed due to copyright restrictions. Please see http://www.robotmarketplace.com/images/store_speed400.jpg012345670 0.005 0.01 0.015 0.02 0.025Torque (Nm)Current (Amps)7V6V4V3VTorque Current CurvesStall torqueNo load speedSame behavior at all voltages!Why doesn’t intercept @ the origin?Image removed due to copyright restrictions. Please see http://www.robotmarketplace.com/images/store_speed400.jpgBack emf versus speed012345670 500 1000 1500 2000w (rad/sec)Back EMF (V)7V6V4V3VpredImage removed due to copyright restrictions. Please see http://www.robotmarketplace.com/images/store_speed400.jpgConcept QuestionDC permanent magnet motorsAs the resistance is increased:1) The shaft speed rises monotonically2) The shaft speed drops monotonically3) The shaft speed rises, reaches a maximum, then falls4) The shaft speed falls, reaches a minimum, then risesBATTERYCouplingVariable resistanceDC Permanent magnet motorsVariable resistanceFigure by MIT OpenCourseWare.Concept QuestionDC permanent magnet motorsAs the resistance is increased:1) The shaft speed rises monotonically2) The shaft speed drops monotonically3) The shaft speed rises, reaches a maximum, then falls4) The shaft speed falls, reaches a minimum, then risesvariable resistanceThe answer is 1. As the resistance increases it decreases the current flow. When the resistance is very high, it’s as if the terminals were an open circuit. An electric potential will be present at the terminals, but no current flows. With no current, there are no forces applied to the armature. The motor connected to the battery can turn freely and approximates its “no load” speed. BATTERYCouplingVariable resistanceDC Permanent magnet motorsVariable resistanceFigure by MIT OpenCourseWare.Discussion Question:How do the things I might do to raise stall torque affect back emf?V-+ERw+iT, ωiRVEw−=-Most of the things I can think of to raise the stall torque willalso increase back emf. That applies to increasing the magnetic field, increasing the radius of the armature, and increasing the number of windings.Speed Control for DC MotorsSwitching!Screenshot of a JETI JE300MC controller description removed due to copyright restrictions.Switching On/Off a LoadC = collectorB = baseE = emitterThe symbolic representation of the transistorHow the transistor (as packaged) looks literally Image from Wikimedia Common, http://commons.wikimedia.orgServo Motors• Actuators that attain and hold a commanded position• The type you have are commonly used in radio controlled cars and planesNote the specifications listed on the box.Output shaft Features for mounting - ground (black)- power (red)- signal (yellow)InputPulse Width Modulation (PWM)• The duration of the pulse is interpreted as a commanded positiontime5VVoltage on yellow (or white) wire1000 μs = full


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