IEEE-RWEP_Feedback Controlled _Faculty-Proj-Desc.pdf

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    Faculty Project Description for Brushless DC Motor Project

    Project Description The brushless DC motor project has been used for at least two years in a first-semesterintroduction to electrical engineering (EE) course. More than 125 first-year EE students havesuccessfully built the project during that time. The simple-to-construct motor kits provide

    exposure to elementary circuits, magnetics, power electronics, and feedback systems.Feedback signals are generated using magnetic and optical sensors. The feedback signalcauses an electromechanical reed switch or a power transistor to actuate an electromagneticcoil. Project benefits are an animated, physical manifestation of electrical engineering and arecruiting tool for future students. Assessment results indicate a high degree of studentsatisfaction with the project.

    Educational objectives of the project are to gain an understanding of the operation andapplication of brushless DC motors, particularly in transportation systems; to comprehend theelectrical, magnetic, and mechanical principles of operation of the brushless DC motor; tocorrelate the physical system components and layout with their abstract schematicrepresentation; to use physical sensors as feedback elements; and to compare the use of an

    operationally visible mechanical reed switch as a coil commutator with the use of a solid statepower transistor.

     An assignment to analyze the potential impact of personal electric vehicles on energy use andcarbon dioxide emissions is included to illustrate a real-world application and its benefit tosociety. This assignment depends on at least two lecture periods for introduction andclarification. The students write a final report that: defines the problem, shows construction andfinal stage pictures of their motor, presents their analysis results, and draws conclusions. Thisreport is a major outside effort for the students. You may want them to work in pairs to reducethe work. Make sure a digital camera is available during motor construction or that somestudents can bring theirs and share.

     A schematic of the system showing all electrical components is shown in Figure 1. In this

    1.5V

    1.5V

    1.5V

    1.5V

    TIP106Reedswitch

    1N4001

    X

    3121E

    SW1

    OPB831W55Optical switch

    SW2

    SW1 and SW2 areconceptual only.

    Implemented as wirehook-ups on breadboard.

    Hall-effectswitch

          C      O      I      L

    Figure 1. Schematic of brushless DC motor

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    system, the position of the rotor may be sensed by a Hall-effect sensor switch, a slotted opticalswitch mechanically configured as a rotary encoder, or a reed switch. The Hall sensor or theoptical switch signal drives the base of a Darlington PNP transistor. The transistor operates asa switch to energize the coil whenever a rotor permanent magnet is facing the coil.

     Alternatively, the reed switch may be used to combine the sensor and coil switching functions.The diode allows the coil current to circulate during times that the coil is disconnected from the

    battery supply. This reduces voltage stress on the transistor and reed switch. Thefreewheeling diode provides opportunity for further exploration by students.

    The motor is built in two laboratorystages. An unassembled motor kit isshown in Figure 2. Although studentsbuild their own individual motors, theywork in pairs to assist each other inassembly.

    The first operational stage, built in thefirst-week laboratory, uses a mechanicalreed switch to commutate theelectromagnetic coil. Students can seethe reed switch actuate as they move therotor permanent magnets. During thesecond-week laboratory, the reed switchis replaced with a bipolar junctiontransistor (BJT). The remainder of theconstruction is completed and a fully assembled motor is shown in Figure 3. The transitionfrom the mechanical reed switch to the solid state transistor switch effectively introduces theconcepts of current flow and transistor switching. The students’ physical understanding ofswitch operation provides a basis for understanding transistor operation as a switch. It isrewarding when first-year EE students comprehend the functional operation of the transistoras a switch. The concept of magnetic field attraction and repulsion is also observed.

    Having optical and Hall-effect feedbackavailable invariably raises the studentquestion, “Which is better?” This yields agreat opportunity to discuss their relativemerits and applications. Students also cansee the effects of inductive switching byremoving the freewheeling diode from thecircuit and seeing the reed switch arc as itoperates. Students may observe the switchvoltage on an oscilloscope with and withoutthe freewheeling diode in the circuit.

    Students may measure the speed of theirmotor using a stroboscope or anoscilloscope attached to the rotor positionsignal. If these instruments are available,speed measurement is a very satisfyingconclusion to the laboratory. Some studentsmodified their motors (e.g., by adjusting encoder phasing or by adjusting the position of Hallsensor) to demonstrate the effects on motor speed.

    Figure 2. Unassembled motor kit 

    Figure 3. Fully-assembled final stage motor

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     The parts list for the project is shown in Table 1. Cost per kit has been $50 for the past twoyears. The supplier at http://www.simplemotor.com/  has been used for all items except thosewith an asterisk.

    Useful tools and equipment:Safety glasses, solder iron, solder, wire cutters, wire stripper, steel cutter, sandpaper,electrical tape

     A complete set of motor assembly instructions follows. This is provided to the students. Thestudents work in pairs to help each other build their motors.

    OPB831W or eq. Slotted optical switch

    A3121E Hall-effect switche.g., Yaskawa R24U Reed switch (1A, 50V)

    TIP106 PNP Darlington transistor (2A, 20V)1N4001 Diode (2A)

    270Ω, 10kΩ  ¼W resistors50 ft, 27 ga. magnet wire4” nail for magnet wire coil4 permanent magnets 0.5” dia. discs5" x 6" press-boardPre-cut 7/8” round PVC pipePre-cut 5/8” square PVC pipe “rotor core”7/8” PVC endcapsOpaque optointerrupter disk 1.3” dia.PushpinT-pinSequins

     Adhesive feltRubber cab to protect sharp end of t-pinSuper glue4 AA battery holder¼” round dowel

    1.25” long square (3/8” sides) wooden stand24 ga. jumper wire*Male header pins*Female headers*TO-220 heat sink*Heat sink grease*2”x3” breadboard*

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    Assembly Instructions:Brushless DC Motor

    I n s t r u c t i o n s ( r e a d a n d u n d e r s t a n d e a c h st e p c o m p l e t e ly

    b e f o r e a c t i n g ! )

    1.  Insert the T-pin into one of the end caps.

    2.  Insert the rotor core into the same cap as shown below.Carefully apply some pressure (avoid poking yourself

    with the sharp T-pin) to push the rotor core

    approximately 1/2" (10-12mm) into the cap.

    3.  Insert the round wooden dowel into rotor core tube.

    4.  Insert the pushpin into the other cap.

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    5.  Complete rotor assembly as shown. Carefully push thecaps towards each other until they cannot move anymore. The T-pin must be secured firmly. This process

    may require some strength. Be careful not to bend the

    T-pin or poke yourself.

    6.  Carefully open the super-glue tube. Make sure you are working over a protectedsurface. Glue the magnets to the flat surfaces of the rotor core with the letter S facing out. Straighten the T-pin if necessary. You will need to press the magnets

    onto the rotor for at least 30 seconds to allow the glue to adhere.

    7.  Cut out the disk.Poke a hole in the center, which ismarked by a cross. Apply some glue to

    the middle of the disk and glue it to thecap with a shorter axle (with the

    pushpin). Slide two sequins as shown

    below. The sequins act as a spacerbetween the disk and the stand and

    work better if their convex surfaces faceoutwards.

    8.  Insert the rotor into the stands marked with blueand silver stars as shown below. Hold the stands

    and test to see if rotor spins freely. Make finaladjustments to the T-pin if necessary.

    9.  Position the stand with the blue star on the board.While covering the corresponding star completely,

    align the marks on the stand with the line on theboard as shown. Keep in mind that super glue bonds

    instantly. Glue the blue star stand to the board.

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    13. Solder the two coil leads to the short end of twomale header pins. You may have more success atsoldering if you work with a partner. Ask for

    instructor help if you have trouble.

    14. Position the coil stand on the board as shown. Turn the rotor slowly to find therotor magnet that comes closest to the coil head. Position the stand so there is a

    1/16" (1.5mm)gap between

    the coil head

    and the closestmagnet on the

    rotor. You maywish to mark

    this position onthe board to

    remember itbefore gluing.

    Glue the stand

    onto the boardin this position.

    15. Attach the green self-sticking felt pad to the reed switch stand as shown.This soft pad decreases the reed switch vibration thus decreasing the sound

    it generates.

    16.  Sand the wire ends of the

    two 8” lengths of wire youcut earlier to remove theinsulation. Clean about 1"

    to 1.5” (4cm) on one end

    and ¼” to ½” on the other end of each wire piece. Be careful not to break the reedswitch, it is very fragile. Wind longer bare ends tightly around outside contacts of

    the reed switch as shown. Solder the wires onto the reed switch.

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    17. Twist the wires as shown.Make sure the reed switch is

    kept securely in the positionshown by the twists of wire

    on the opposite side of thestand.

    18. Make sure the reed switch is oriented as shown in step 15. Solder the ends of thereed switch leads to the short ends of two male header pins.

    19. Position the reed switch holder on theboard. It should be located at a distanceof about 1/8" (3mm) from the closest

    magnet. Check the rotation of the rotorto make sure that it does not hit the

    reed switch. The three slots cut into the

    stand must line up with the rotormagnets.

    IMPORTANT: The most sensitive part of the reed

    switch is not in the middle of it, but more to the sideas shown to the right. You should hear a clicking

    sound when each magnet passes by the reed switch.

    Move the reed switch a little closer to the magnetsuntil all four can turn the reed switch on.

    Make sure the three slots cut into the stand line up with the rotor magnets.

    Glue the reed switch holder to the board. 

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    20. Tin the battery holder leads withsolder. Solder the battery holderleads to the short ends of two

    male header pins. Attach the

    battery holder to the board in theposition shown.

    You may have more success in

    soldering if you work with a partner.

    21. Set the board aside.Tin the three colored leads with solder. Solder the

    three colored leads to the pins of three-pin femaleheader strip. This is the most difficult solder step.

    Ask for help if you need it!

    The color order is important!Red=right, black=center, blue=left. 

    Tin the other end of these leads with solder.

    22. Get two, two-position female parts. Cutfour 10” leads from the wire in your kit.

    Tin the leads with solder. Solder leads toeach of the four header pins.

    FINISHED SOLDERING!

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    23. Tape the 2”x3” breadboard onto the motor base board in theposition shown. You should have three pairs of male headerpins – from the battery pack, the coil and the reed switch.

    Connect the circuit using the header pins and the breadboard.Make sure the wires cannot get wrapped onto the rotor. Insert

    batteries to verify operation. You may need to reverse the coil.

    1.5V

    1.5V

    1.5V

    1.5V

    Reedswitch

    COIL

     Negative

     battery

    Coil

    Positive

     battery

    Coil

    Reed

    switch

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    24. Insert the Hall-effect sensor into the three-pinfemale header strip. Holding the sensor with theprinted side up, the left side should insert into the

    red-leaded jack. Use needle-nose pliers to carefullybend the Hall-effect sensor to a 90˚ angle with the printed side of the sensor facing outward .

    25. (Optional: Smear a dab of heat-conducting grease to the metallictab-side of the TIP106 transistor.) Attach the transistor to theheat sink fin using the small bolt and nut. Use needle-nose pliers

    to bend the leads as shown.

    26. Insert the Hall-effect assembly onto the reed-

    switch stand. The printed-side of the sensorshould face the rotor magnets. It needs to be

    close, but not touching the rotor magnets.

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    27. Connect the Hall-effect sensed /transistor-switched circuit. You will needto make two jumper wires. If you use

    stranded wire to make jumpers,

    then tin the leads with solder tomake them easier to plug into the

    breadboard. One connects the

    TIP-106 to the Hall switch (seecircuit diagram on right), the other jumper wire connects the TIP-106

    to the negative battery supply.

    Try to get the Hall switch chip as

    close to the rotor magnets withouttouching them.

    Verify operation!

    1.5V

    1.5V

    1.5V

    1.5V

    TIP106

    X

    3121E

    Hall-effectswitch

          C      O      I      L

    Black

    Re dBlue

    10 k

    baseemitter 

    collector 

    10k resistor

    Hall switch Red

    Hall switch Blue

    Hall switch Black

    Jumper wires

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    Locate the optointerrupter pins as shown on the following picture. It is very important to

    identify all four pins properly. Wrong connection in the circuit will destroy theoptointerrupter. There are no spares of this part. Insert the two pairs of long-leaded

    female header strips onto each pair (Emitter and Sensor) of opto pins making sure they

    are not touching each other.

    28. Glue the opto to the square wooden stand aligned withthe guidelines on the stand. The Sensor side should be

    attached to the stand.

    29. Position the optointerrupter stand on the board asshown in the picture. If you rotate the rotor, the diskblades should be in the middle of the slot as deep as

    possible without hitting the optointerrupter. Glue thestand to the board. Wait for the glue to dry. You willneed to experiment with it to find the best position of

    the disk to provide a good start and the best speed.

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    30. Connect the Hall-effect sensed / transistor-switched circuit. Verify operation!

    31.  Clean up the wiring. Arrange breadboard connections so you can demonstrate yourcircuit in its three modes of operation.

    32.  If a tachometer is available, use it to measure the speed of your rotor for itsdifferent modes of operation.