VEX Motors:
Presented by:
Mr. Harder & Thuan Doan
Robotics Training Week 2010
General Topics
Torque
Speed
Example problems
Torque curves and specification sheets
Gear ratio
What is a
Motor/Servo?
Converts electrical energy into
mechanical energy


Provides motion for robot
Has specifications that must be
designed for
VEX Motor

spins 360 degrees
VEX Servo

can only turn 180 degrees
Torque
Force at a distance


Rotational strength of a motor
Measured in Newton-meters or foot-pounds
 Be aware of unit conversions
Speed
Rotations per minute (RPM)
 May be measured as radians/second
Power
Power = angular velocity * torque


Prot =  τ*ω
Measures how strong and how fast a motor is
spinning
 Measured in Watts (radians -Newton-
meters/second)

This relationship shows that speed and torque
are inversely related
 Less torque = more speed
 More torque = less speed
Gears, Chains &
Sprockets
Gearing can be used to change the
output torque/speed

Sprockets and chains can be used for
same purpose
 Slightly less efficiency loss
See drive presentation for details
Power Loss in the
Mechanism
Some power from the motor is lost due to
friction in the mechanism



Gears, belts, cables
Bearings, guides
Tires, balls, or other deformable item
Power loss is heat
Heat and friction are usually not your
friends
transmission efficiency
is important
Spur gears: 90% per pair
Worm and gear: 10%-60%
Chain: 85%-95%
Rack and pinion 50%-80%
Motor Parameters
Applied voltage ( V )
 Voltage of your power source (7.5V
for VEX)

Stall torque
( stall )
 Torque at zero speed
 Highest torque that motor can
output
Stall current

i
( stall )
Amount of current drawn by motor
when stalling

Free speed
(
free )
 Speed of a motor under no load
 Fastest speed of a motor
VEX Motor Specifications
Stall torque

Stall current
istall = 1.6 A
stall = 6.5 in-lbs
Free speed

Reference
voltage
V = 7.5 Volts
free = 100 rotations/minute
Current vs. Torque
Fisher-Price motor
160
148 A
140
Current, A
120
100
80
60
40
20
0
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
Torque (Nm)
• Higher torque = higher current
• More torque you put on your motor, the more
current it draws ->
• Stall current is reached at stall torque (remember?)
Speed vs. Torque
• Inversely related
•Review: (important
concept)
• Stall torque ->
speed = 0
• Stall speed ->
torque = 0
•More torque -> Less
speed
Max Power
Fisher-Price motor
Speed (rad/s); Power (W)
2500
Output power, W
2000
Speed, rad/s
1500
1000
500
407 W
0
0.00
0.10
0.20
0.30
0.40
Torque (Nm)
0.50
0.60
Stall
0.70
• Max power = max output
of the motor
• This is the power you
should design for
• Torque at max power is
half of stall torque
• Speed at max power is
half of free speed
Peak Efficiency
Fisher-Price motor
100
90
80
76%
Efficiency, %
70
60
50
40
30
20
10
0
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
Torque (Nm)
 Peak efficiency is about at about 1/3 – 1/6 of stall
torque
 Occurs at about 60% of Max Power
 After max efficiency -> efficiency crashes
After this exhausting
presentation…
Definitions of all terms discussed
How to calculate power, max power,
torque, and speed (including unit
conversions)
Relationship between torque and speed
Relationship between torque and current
What causes power loss in a system
Basic understanding of gear ratios