Download MOH Goat Autonomous Operation

DC Brushed Motors
2009
FRC Kick-off Workshops
Ken Stafford
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
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
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
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
Extrapolate Motor Performance
Performance Map
60
50
40
Speed (rpm)
Power (Watts)
30
Efficiency (%)
Current (Amps)
20
Heating (Watts)
10
0
0
50
100
150
200
250
Torque (in lbs)
300
350
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
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
Recent FRC Motors
• Sealed vs Air-Cooled
• Thermal Protection
• Anti-backdrive vs backdrive
resistant
• Built in transmissions
Selection Criterion
1
2
3
4
5
6
7
Power Requirement
Weight of Motor & Transmission
Physical Size of Motor & Transmission
Backdrive Characteristics
Continuous vs Intermittent Operations
Efficiency
Availability
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
Recommendations Cont.
• Fisher-Price/BaneBots
– Very high power/low weight/
intolerant of high load
• Applications:
– Shooters/fans
Recommendation Cont.
• Van Door
– Mid power, thermal
protection, backdrive
resistant, heavy
• Applications:
– Arm shoulder, turret
– Low in chassis
Recommendations Cont.
• Globe
– Small size, integral
transmission, thermal
protection
• Applications:
– Wrist joint, end effectors,
high on chassis
Recommendations Cont.
• Window Motors
– Low power, antibackdrive, thermal
protection, heavy
• Applications:
– Nothing else left, gates,
low powered arms/
accessories
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 ftlb/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
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
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
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
Questions?