1. FRC Kick-off Workshops Ken Stafford
DC Brushed Motors
2009
FRC Kick-off Workshops
Ken Stafford

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

3. 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

4. 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

5. 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: A
Stall: 360 in 24.2 A

6. Extrapolate Motor Performance

7. Performance Map

8. 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

9. 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

10. Recent FRC Motors Sealed vs Air-Cooled Thermal Protection
Anti-backdrive vs backdrive resistant
Built in transmissions

11. Selection Criterion Power Requirement Weight of Motor & Transmission
Physical Size of Motor & Transmission
Backdrive Characteristics
Continuous vs Intermittent Operations
Efficiency
Availability

12. 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

13. Recommendations Cont. Fisher-Price/BaneBots Applications:
Very high power/low weight/ intolerant of high load
Applications:
Shooters/fans

14. Recommendation Cont. Van Door Applications:
Mid power, thermal protection, backdrive resistant, heavy
Applications:
Arm shoulder, turret
Low in chassis

15. Recommendations Cont. Globe Applications:
Small size, integral transmission, thermal protection
Applications:
Wrist joint, end effectors, high on chassis

16. Recommendations Cont. Window Motors Applications:
Low power, anti-backdrive, thermal protection, heavy
Applications:
Nothing else left, gates, low powered arms/ accessories

17. 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

18. 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

19. 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

20. 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

21. Questions?