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DC Brushed Motors & Power Transmission Ken Stafford 2014 FRC

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Permanent Magnet Brushed? 2 Stator (Field) made of permanent magnets Rotor (Armature) made with wire coils (electromagnets) Commutator has brushes to pass current

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The Basics… Imperfect Transducers –Electrical Power to Mechanical Power –Electrical Power to Thermal Power! Electrical Power (input) –Volts times Amps (Watts) –EG: CIM @ 40A has 480W input @12V Mechanical Power (output) –Work divided by Time or –RPM times Torque (Watts or Hp) –EG: CIM (40A/12V) 3800rpm/6.15 inlbs=275W

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The more basic Basics…Basics Torque “twisting effort” –EG: shaft turning, force at the end of an arm, force at the circumference of wheel… “pushing/pulling strength” –Unlimited torque available through any motor with appropriate transmission Power “rate of doing work” –EG: speed of lifting, torque times rpm, force times velocity… “robot/mechanism speed” –Maximum is set by motor design—only decreases through transmission

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Motor Parameters Manufacturers provide varying data Not too difficult to obtain experimentally with basic lab equipment You need only four values to predict ideal performance –At full speed (no load) Motor Speed (rpm) Current (Amps) –At maximum torque (stall) Torque (inlbs) Current (Amps)

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Example: Taigene (Van Door) Motor clamped in vise hooked to calibrated power supply Free-running rpm by timed counting Stall torque by linear force balance at end of measured arm Current measured directly from power supply Results: –Free running: 47.5 rpm @ 1.23 A –Stall: 360 in lbs @ 24.2 A

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Extrapolate Motor Performance

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Performance Map Note: This is at full rated voltage

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So…what does this mean? Max Torque occurs at zero rpm (stall) –Also produces zero Mech Power and Max Thermal Power –Lightweight, air-cooled motors will smoke in seconds

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More on Motors… Max Power occurs at 50% Stall Torque, ~ 50% Stall Current, and 50% Free-running speed Any sub-maximum power is available at 2 different operating conditions –High speed/low torque –Low speed/high torque Max Efficiency occurs at ~25% Stall Torque or ~60% Max Power

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Typical FRC Motors Sealed vs Fan-Cooled Thermal Protection Anti-backdrive vs backdrive resistant Built in transmissions

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Motor Selection Criterion 1Power Requirement 2Weight of Motor & Transmission 3Physical Size of Motor & Transmission 4Backdrive Characteristics 5Continuous vs Intermittent Operations 6Efficiency 7Availability

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Specific Recommendations Sealed motors (eg CIM) –High torque, can handle intermittent high loads –Heavy Application: –Driveline or other high power accessories –Must be kept LOW in chassis

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Recommendations Cont. Fan-cooled motors (eg FP) –Very high power/low weight/ intolerant of high load Applications: –Shooters/fans

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Recommendation Cont. Worm Gear Motors (eg Van Door) –Thermal protection, backdrive resistant, low power, heavy Applications: –Arm shoulder, turret –Low in chassis

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Design Example Build a winch to lift a 100 lb robot 3 ft in 10 secs: –Mech Power = Work/Time * Conversion –Required Power = ((100 lb)(3 ft)/10 sec)(746 W/550 ft-lb/sec) = 40W –12 of the 20 allowed 2014 FRC motors > 40W

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Taigene Performance Map

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Design Example (cont) From the Performance Map –It produces 40 W at either 100 or 275 in-lb At 100 in-lbs it’s ~45% efficient; at 275, it’s ~18%! –Design your drum radius so it develops 50 lbs of force with 100 in-lbs of torque Radius = 100in-lbs/100 lbs = 1 in 2 in 100 lbs 100 in-lbs

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Design Result Drum RadiusMotor ConditionTime to Lift <1.0 inHappy—cool>3.0 secs =1.0 inWorking easy=3.0 secs >1.0; <2.75 inGetting warm…<3.0 secs =2.75 inWorking hard=3.0 secs >2.75 inUnhappy—hot!>3.0 secs

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Design Details Cont. If holding a lifter/arm in position is important do not rely upon motor torque (overheating) Previous example: ~30W to hold @ 1.0 in drum; ~180W to hold @ 2.75 in drum! Design a mechanical one-way clutch/ retractable ratchet or balance mechanism

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Transmission Essentials Unless you are VERY lucky…you will need ‘em Transmissions can: 1) Modify output speed/torque. 2) Change direction of rotation 3) Physically separate motor from device They will ALWAYS 4) Reduce power through losses

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Transmission Essentials Unless you are VERY lucky…you will need ‘em Transmissions can: 1) Modify output speed and torque 2) Change direction of rotation 3) Physically separate motor from device They will ALWAYS 4) Reduce power through losses

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More Basics For spur gears and chain/sprockets, it’s all about the number of teeth! “Gear Train”, aka “Speed Ratio” e = Product of Drivers / Product of Driven = Speed out / Speed in = (Torque in / Torque out) * Sys efficiency

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Speed Example Driving elements: –16 T gear –18 T sprocket Driven elements: –54 T gear –28 T sprocket e = (16 * 18)/(54 * 28) =.19 With motor @ 5000 rpm Roller speed = 5000 *.19 = 950 rpm

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Torque Example Same transmission: e =.19 Losses occur at each “stage” –Typically 5% loss (η =.95) You calculate you need 20 lbs force at the 2 inch dia roller Torque out = 20 lbs * 1 in = 20inlb Torque in = (.19 * 20 inlb) / (.95 *.95) = 4.2 inlb Note: this would result in a 28 A load with the small CIM as shown

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“Roller” Chain and Sprockets Parameters (ANSI) –Chain Number: 1 st digit = pitch in 1/8 inches 2 nd digit = “roller” (0), “lightweight” (1), or “bushed” (5) EG: #25: ¼ inch pitch without rollers, #35: 3/8 inch pitch without rollers –Strength (breaking/working): #25 = 875/140 lbs #35 = 2100/480 lbs –“Working” implies industrial duty/life expectancy

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Chain Design Details Sprocket sizes –Recommend 12T to 75T Smaller causes vibration & excess wear Larger leads to chain loss Chain Wrap –120 degree minimum for any drive or driven sprocket –Change routing, add idlers if necessary Design in adjusters for long runs –Adjustment range should ideally be 2 pitch lengths (to avoid half-links)

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Gears, gears, gears! Parameters (ANSI) –(Diametral) Pitch: P = Number of Teeth/ Pitch Diameter Integer number, normally divisible by 4 EG: 32, 28, 24, 20, etc Larger the P, the smaller/weaker the teeth –Pressure Angle: the actual off-tangent angle force transmission Typically 14.5 or 20 degrees 14.5 degrees weaker, more efficient

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Spur Gear Design Details Gear sizes –Recommend 12T to Infinity! (rack) Smaller (“pinions”) are weak from under- cutting—esp 14.5 degree PA –All gears with same P and PA will fit Transmission axle separation –An exact, easily computed distance D = (Total number of teeth/P) / 2 –EG: 16T & 42T, 24 Pitch gears D = ((16 + 42) / 24) / 2 = 1.208 inches Can be 95-98% efficient/stage 16T 42T

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Other Gear Stuff Worm Gears –Antibackdrive (Helix angle dependent, Window--yes; Van Door--no) –Woefully inefficient: η =.25-.75 –Very compact e = Number leads on worm / Teeth on Worm Gear = 4 / 50 =.08 –DO NOT USE FOR HI-POWER –Requires VERY secure bearing support 4 50T

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General Suggestions Operate motors on left side of perf map Air-cooled motors cannot operate near stall for more than a few seconds Control top speed of operation by suitable gearing not by reduced voltage Avoid powered anti-backdrive Driveline ROT: no wheel-spin when blocked? = TOO HIGH GEARED!

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Overall Caveats Real world motors in robots will not operate at the peak values predicted on the performance maps –Batteries will sag, voltage will be lost through conductors, etc You need to consider mechanical transmission efficiency when calculating motor requirements Be careful to note reference voltage in manufacture’s data—automotive use 10.5V commonly

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Questions?

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