Transcript Power, Torque and robot Arms

Power, Torque and
Robot arms
An intro
VEX Arms
Robert’s ARL robot in 2006
Outline
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Rotating Arms
Torque
Power
Multi-Jointed Arms
Challenge
Limit Switches
Rotating Arms
gear
Torques are large
bolted to arm
motor
Use counterweights and gears to
compensate
Attach the gear to the arm
Attach the motor to the robot
driven gear
Rotating Arm
Carrier Robot
posted on www.vexforum.com by VexLABS
Torque (T)
T = F┴ d
force
perpendicular
F
d
pivot point
distance
Which arm has more torque on it?
10 lbs
Arm 2
Arm 1
10 lbs
d1
d2
Which arm has more torque on it?
T=FxD
D1 > D2
10 lbs
- so Arm 2
T1 > T2
Arm 1
10 lbs
D1
D2
Two Equations for Power

Force Distance 
Power 
Time
- or -
Power  Torque Rotational Velocity 
Which arm would require a more
powerful motor?
Torque  ForceDistance 
Power  Torque Rotational Velocity 
Arm 1
D
Force: 10 lbs
Rotational Velocity: 100 RPM
Gear Ratio: 4 to 1
Arm 2
D
Force: 10 lbs
Rotational Velocity: 200 RPM
Gear Ratio: 4 to 1
Which arm would require a more
powerful motor?
Torque  ForceDistance 
Force & distance are the same so torque is the same.
Power  Torque Rotational Velocity 
Arm 2 needs a more powerful motor since its rotational velocity is greater.
Arm 1
D
Force: 10 lbs
Rotational Velocity: 100 RPM
Gear Ratio: 4 to 1
Arm 2
D
Force: 10 lbs
Rotational Velocity: 200 RPM
Gear Ratio: 4 to 1
Multi-Jointed Arms
Put multiple sections together to increase dexterity.
www.vexrobotics.com
Posted on www.vexforum.com by juniorVEXbot
Vex Challenge #4:
Arms
Due Date: Friday
Vex Inventor’s Guide: Stability & Center of Gravity in the Structure section (p. 27-31)
Forums: www.vexforums.com  Search the forum or post questions
Chief Delphi Forum: www.chiefdelphi.com  Search the forum or post questions
C Level Challenge (70 points):
 Build a robot that uses a single stage arm to lift an empty soda can 6 inches
 Make a design drawing of your arm
Arms
Challenge
B Level Hardware Challenge (85 points):
 Build a robot that uses a two stage arm to raise a soda can 10 inches
 Make a design drawing of your arm
A Level Hardware Challenge (100 points):
 Build a robot that uses a multi stage arm to pick an empty soda can off the floor and place
it on a chair
 Make a design drawing of your arm
A Level Software Challenge (100 points):
 Complete the C Level Challenge
 Implement limit switches to limit the arm’s range of motion in software
 Make a design drawing of your lift
CHALLENGES LIKE THESE ARE GOOD FOR HIGH SCHOOL
STUDENTS, because they are well-specified for existing robot kits.
• In our case we work on open-ended projects, more similar to real life.
• For instance we can assume nothing when designing a humanoid robot arms.
• It is however good to be realistic about costs.
Advice
• Balance arms with counterweights
– not always possible
• Use gears to get extra torque
– not always necessary with balanced arms
• Use sensors
– limit switches to stop arms from over rotating
– potentiometers or encoders to control location
Limit Switches
Limit switches tell the robot controller when a device has gone far
enough.
Software can stop the servos and motors moving the device.
use limit switches
&
stop arms mechanically
mechanical stop: something the arm hits to physically stop it
Optical Shaft Encoders
*
• Detects 90 ticks per shaft rotation
• Useful for measuring speeds
*Old encoders (they only have one PWM cable) can’t tell direction of rotation
Potentiometers
(Variable Resistors)
• Resistance depends on shaft rotation
• Useful to accurately measure angles
• Limited range of rotation
Sources
• J.M. Gabrielse
• Greg Needel - Designing Competitive
Manipulators: The Mechanics & Strategy
(www.robogreg.com)