DC Brushed Motors & Power Transmission Ken Stafford 2014
FRC
Slide 2
Permanent Magnet Brushed? 2 Stator (Field) made of permanent
magnets Rotor (Armature) made with wire coils (electromagnets)
Commutator has brushes to pass current
Slide 3
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
Slide 4
The more basic BasicsBasics 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 designonly decreases
through transmission
Slide 5
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)
Slide 6
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
Slide 7
Extrapolate Motor Performance
Slide 8
Performance Map Note: This is at full rated voltage
Slide 9
Sowhat 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
Slide 10
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
Slide 11
Typical FRC Motors Sealed vs Fan-Cooled Thermal Protection
Anti-backdrive vs backdrive resistant Built in transmissions
Slide 12
Motor Selection Criterion 1Power Requirement 2Weight of Motor
& Transmission 3Physical Size of Motor & Transmission
4Backdrive Characteristics 5Continuous vs Intermittent Operations
6Efficiency 7Availability
Slide 13
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
Slide 14
Recommendations Cont. Fan-cooled motors (eg FP) Very high
power/low weight/ intolerant of high load Applications:
Shooters/fans
Slide 15
Recommendation Cont. Worm Gear Motors (eg Van Door) Thermal
protection, backdrive resistant, low power, heavy Applications: Arm
shoulder, turret Low in chassis
Slide 16
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
Slide 17
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 its ~45% efficient; at
275, its ~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
Slide 19
Design Result Drum RadiusMotor ConditionTime to Lift 3.0 secs
=1.0 inWorking easy=3.0 secs >1.0; 3.0 secs
Slide 20
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
Slide 21
Transmission Essentials Unless you are VERY luckyyou 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
Slide 22
Transmission Essentials Unless you are VERY luckyyou 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
Slide 23
More Basics For spur gears and chain/sprockets, its 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
Slide 24
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
Slide 25
Torque Example Same transmission: e =.19 Losses occur at each
stage Typically 5% loss ( =.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
Slide 26
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: 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
Slide 27
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)
Slide 28
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
Slide 29
Spur Gear Design Details Gear sizes Recommend 12T to Infinity!
(rack) Smaller (pinions) are weak from under- cuttingesp 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
Slide 30
Other Gear Stuff Worm Gears Antibackdrive (Helix angle
dependent, Window--yes; Van Door--no) Woefully inefficient:
=.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
Slide 31
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!
Slide 32
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 manufactures
dataautomotive use 10.5V commonly