DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.
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Transcript of DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.
![Page 1: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/1.jpg)
DC Brushed Motors
FRC Kick-off WorkshopsKen Stafford
2009
![Page 2: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/2.jpg)
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)– RPM times Torque (Watts/Hp)– EG: CIM (40A/12V) 3800rpm/6.15 inlbs=275W
![Page 3: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/3.jpg)
The more basic 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 distance… “robot/mechanism speed”– Maximum is set by motor design—only
decreases through transmission
![Page 4: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/4.jpg)
Motor Parameters
• Different 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 Draw
– At maximum torque (stall)• Torque• Current Draw
![Page 5: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/5.jpg)
Example: 2008 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
![Page 6: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/6.jpg)
Extrapolate Motor Performance
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Performance Map
![Page 8: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/8.jpg)
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
![Page 9: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/9.jpg)
More…
• 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
![Page 10: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/10.jpg)
Recent FRC Motors• Sealed vs Air-Cooled• Thermal Protection• Anti-backdrive vs backdrive
resistant• Built in transmissions
![Page 11: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/11.jpg)
Selection Criterion
1 Power Requirement
2 Weight of Motor & Transmission
3 Physical Size of Motor & Transmission
4 Backdrive Characteristics
5 Continuous vs Intermittent Operations
6 Efficiency
7 Availability
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Specific Recommendations
• Big and Little CIMs:– High power, can handle intermittent high
loads, very heavy
• Application:– Driveline, or high power accessories located
low in the chassis– To avoid overheating, rule of thumb says
that you should be geared sufficiently low enough to spin your wheels when blocked
![Page 13: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/13.jpg)
Recommendations Cont.
• Fisher-Price/BaneBots– Very high power/low weight/
intolerant of high load
• Applications:– Shooters/fans
![Page 14: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/14.jpg)
Recommendation Cont.
• Van Door–Mid power, thermal
protection, backdrive resistant, heavy
• Applications:– Arm shoulder, turret– Low in chassis
![Page 15: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/15.jpg)
Recommendations Cont.
• Globe – Small size, integral
transmission, thermal protection
• Applications:–Wrist joint, end effectors,
high on chassis
![Page 16: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/16.jpg)
Recommendations Cont.
• Window Motors– Low power, anti-
backdrive, thermal protection, heavy
• Applications:– Nothing else left, gates,
low powered arms/ accessories
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Design Details• EG: Build a winch using the Taigene to
lift a 50 lb weight 3 ft in 5 seconds:– Power = ((50 lb)(3 ft)/5 sec)(746 W/550
ft-lb/sec) = 40W– 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/50 lbs = 2 in
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Design Details Cont.
• If holding a lifter in position is important do not relay upon motor torque (overheating)
• Design a mechanical one-way clutch or retractable ratchet
![Page 19: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/19.jpg)
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
![Page 20: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/20.jpg)
General Suggestions
• Operate motor on left side of performance 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
![Page 21: DC Brushed Motors FRC Kick-off Workshops Ken Stafford 2009.](https://reader035.fdocuments.net/reader035/viewer/2022062718/56649e795503460f94b78691/html5/thumbnails/21.jpg)
Questions?