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Effectors

CSCI545Introduction to Robotics

Hadi Moradi

Previous LectureIntroduction to RobotsIntroduction to RobotsBasics of Manipulators

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Effector vs. ActuatorEffector: Any device affectingEffector: Any device affecting the environment.

Actuator: The actual h i th t b i timechanism that brings action

Actuator

Degree Of Freedom (DOF)The number of independent motions aThe number of independent motions a device can make

Examples:Examples:Simple sliding

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Degree Of Freedom (DOF)Examples:Examples:

Rotation + translation:

Degree Of Freedom (DOF)Examples:Examples:

A free body in space: 6DOF

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Degree Of Freedom (DOF)Q: How many DOF areQ: How many DOF are needed to position parts on a planar surface?

The Use of EffectorsLocomotion: Moving robot aroundLocomotion: Moving robot around

Manipulation: Moving other objectsExample: (courtesy of Martin Rohrmeier)

Puma 560Puma 560Kr6

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LocomotionTypes:Types:

Legs

WheelsArmsFlippers…

ActuatorsExample:Example:

Pneumatic (air pressure)Hydraulics (fluid pressure)

Give another example!

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ActuatorsExample:Example:

Motors (current)

DC MotorsThe most used in mobile robotsThe most used in mobile robots

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DC Motors

DC Motor: Permanent Magnet

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DC Motors

Different DC Motor Confs

Image: Courtesy of Paul Hill

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DC MotorsEfficiency: Loss on frictionEfficiency: Loss on friction

90% high end models50% low end models

Operating Voltage:

DC MotorsCurrent: proportional to the loadCurrent: proportional to the load

Pushing toward a wall

Free rotation:

T t ti l fTorque == rotational forceMore current =>

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Torque and PowerStall torque: The amountStall torque: The amount of rotational force when stalled.Power= torque * rotational velocity

Torque vs. Power

Image: Courtesy of Paul Hill

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DC Motor Specs: Iskra 6-pole

Torque and SpeedHow do you measure torque?How do you measure torque?How do you measure speed?

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Stall Torque calculation

Speed3 000 to 9 000 RPM3,000 to 9,000 RPM

Or 50 to 150 RPS

High speed =>Example: fans

In robotics:Load should be carried + not very high speed

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GearingToo much speed:Too much speed:

Cars:1000 RPM standby1000 RPM standby3000-6000 Operational

GearingWhat is the idea?What is the idea?

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GearingTwo gears meshed togetherTwo gears meshed together

Gear TeethBacklash: moving back and forthBacklash: moving back and forthReducing backlash => tight meshing

Problem?

Designing a gear box:3:1 speed reduction3:1 speed reduction

Q: How much reduction can we get? Infinite?

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Gear BoxesMultiple gears in seriesMultiple gears in series

8 24 40 8 24

Servo MotorsGoal:Goal:

Gear reductionPosition sensorPosition control

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Inside a Servo Motor

Top cover removed Gears are removed

Inside a servo motor

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Pulse Width Modulation

10% duty cycle

50% duty cycle

90% duty cycle

PWM10% of 9 volts => 0 9 v10% of 9 volts => 0.9 v50% of 9 volts => 4.5 v

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PWMHow frequently the pulses should beHow frequently the pulses should be repeated?Working frequency: 1KHz – 200 KHzNote:

Stepper Motor vs. Servo MotorServo motor: DC motor with positionServo motor: DC motor with position control (feedback/closed loop)Stepper motor:

Open loopMany coils:Many coils:

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Stepper vs. DC vs. AC

DC motor Stepper motor AC motor

Continuous Rotation MotorsUse DC motorsUse DC motorsUse servo motors:

Remove mechanical limitsRemove position sensorApply two resistors to fool the controlApply two resistors to fool the control circuit