Power, Torque and Power, Torque and Robot armsRobot arms

An intro

VEX Arms

Robert’s ARL robot in 2006

Outline

• Rotating Arms

• Torque

• Power

• Multi-Jointed Arms

• Challenge

• Limit Switches

mo

tor

Rotating ArmsRotating 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 www.vexforum.com by VexLABS

T = F┴ dperpendicular

distance

Torque (T)

F

pivot point

d

force

10 lbs

10 lbs

d2d1

Which arm has more torque on it?

Arm 1

Arm 2

10 lbs

10 lbs

D2D1

Which arm has more torque on it?

Arm 1

Arm 2

T = F x D

D1 > D2

- so -

T1 > T2

Two Equations for Power

Time

Distance ForcePower

VelocityRotationalTorquePower

- or -

Which arm would require a more powerful motor?

VelocityRotationalTorquePower

Force: 10 lbsRotational Velocity: 100 RPM Gear Ratio: 4 to 1

Force: 10 lbsRotational Velocity: 200 RPM Gear Ratio: 4 to 1

DD

DistanceForceTorque

Arm 1 Arm 2

Which arm would require a more powerful motor?

VelocityRotationalTorquePower

Force: 10 lbsRotational Velocity: 100 RPM Gear Ratio: 4 to 1

Force: 10 lbsRotational Velocity: 200 RPM Gear Ratio: 4 to 1

DD

Force & distance are the same so torque is the same.

Arm 2 needs a more powerful motor since its rotational velocity is greater.

DistanceForceTorque

Arm 1 Arm 2

Multi-Jointed ArmsMulti-Jointed ArmsPut multiple sections together to increase dexterity.

Posted on www.vexforum.com by juniorVEXbotwww.vexrobotics.com

Arms Challenge

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

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 gearsgears 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)