VEX Motors:

Presented by:Mr. Harder & Thuan

DoanRobotics 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

TorqueForce 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 & SprocketsGearing 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 heatHeat and friction are usually not your friends

transmission efficiency is important

Spur gears: 90% per pairWorm and gear: 10%-60%Chain: 85%-95%Rack and pinion 50%-80%

Motor ParametersApplied 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 ( istall ) Amount of current drawn

by motor when stalling

Free speed ( free ) Speed of a motor under no

load Fastest speed of a motor

stall = 6.5 in-lbs

istall = 1.6 A

free = 100 rotations/minute

VEX Motor Specifications

Stall torque

Stall current

Free speed

Reference voltage

V = 7.5 Volts

Current vs. Torque Fisher-Price motor

0

20

40

60

80

100

120

140

160

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

Torque (Nm)

Cur

rent

, A

148 A

• 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• 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

Fisher-Price motor

0

500

1000

1500

2000

2500

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

Torque (Nm)

Spee

d (ra

d/s)

; Po

wer

(W)

Output power, W

Speed, rad/s

407 W

Stall

Peak Efficiency Fisher-Price motor

0

10

20

30

40

50

60

70

80

90

100

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

Torque (Nm)

Eff

icie

ncy,

%

76%

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 discussedHow to calculate power, max power, torque, and speed (including unit conversions)Relationship between torque and speedRelationship between torque and currentWhat causes power loss in a systemBasic understanding of gear ratios