Power, Torque and Robot arms An intro

VEX Arms Robert’s ARL robot in 2006

Outline Rotating Arms Torque Power Multi-Jointed Arms Challenge Limit Switches

motor Rotating Arms Torques are large Use counterweights and gears to compensate Attach the gear to the arm Attach the motor to the robot driven gear gear bolted to arm counter weight

Rotating Arm Carrier Robot posted on by VexLABSwww.vexforum.com

T = F ┴ d perpendicular distance Torque (T) F pivot point d force

10 lbs d2d2 d1d1 Which arm has more torque on it? Arm 1 Arm 2

10 lbs D2D2 D1D1 Which arm has more torque on it? Arm 1 Arm 2 T = F x D D 1 > D 2 - so - T 1 > T 2

Two Equations for Power - or -

Which arm would require a more powerful motor? Force: 10 lbs Rotational Velocity: 100 RPM Gear Ratio: 4 to 1 Force: 10 lbs Rotational Velocity: 200 RPM Gear Ratio: 4 to 1 D D Arm 1 Arm 2

Which arm would require a more powerful motor? Force: 10 lbs Rotational Velocity: 100 RPM Gear Ratio: 4 to 1 Force: 10 lbs Rotational Velocity: 200 RPM Gear Ratio: 4 to 1 D D Force & distance are the same so torque is the same. Arm 2 needs a more powerful motor since its rotational velocity is greater. Arm 1 Arm 2

Multi-Jointed Arms Put multiple sections together to increase dexterity. Posted on by juniorVEXbotwww.vexforum.com

Arms Challenge 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 gearsUse 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 & StrategyDesigning Competitive Manipulators: The Mechanics & Strategy (