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CMPUT 412Actuation

Csaba SzepesváriUniversity of Alberta

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Defining sensors and actuators

Environment

actions

Sensations(and reward)

Controller= agent

Sensors Actuators

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Actuation

Why? How?The process of sensingCharacterizing sensorsSome sensors

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Actions

Effectors, actuators Motors Wheels Wheeled locomotion

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Actions for Moving Things

What moves? Robot moves locomotion Objects move manipulation

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What Makes an Action Possible?

Effector = device on a robot that has an effect (impact or influence) on the environment e.g. leg, wheel, arm, finger

Actuator = Mechanism that enables the effector to work e.g. electrical motors, hydraulic or

pneumatic cylinders

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Types of Actuation

Passive actuation Utilizes potential energy Examples

Nature: flying squirrels Robots: walking

Active actuation External energy

transformed into motionTad McGeer’s

passive walking robot

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Active Actuation: How?

Electromagnetism Electric motors

Pressure Hydraulics (fluid pressure) Pneumatics (air pressure)

Materials Photo-reactive materials Chemically reactive materials Thermally reactive materials Pizeoelectric materials (crystals)

Incomplete!

Direction of motion:-Rotation-Linear

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Direct Current (DC) Motors Advantages: Simple, inexpensive,

easy to use, easy to find Input: Voltage

“Right range” -> current drawn ~ work work = force * distance

Powerout ~ torque * vrot

Free running/stalled: p=0 Speed: 3K-9K rpm

(50-150rps) Problem:

Speed high, force lowshaft

Power wires

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Operation: Brushed DC Motor

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Gearing: The Challenge

Purpose: Change the torque output of motors

Wheels: torqueout ~ torquein/radius

why? Can decrease torque!

Problem: How to increase torque?

Solution: Gears

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Gearing

Const ´ power ~ torque * vrot

vrot,2 = vrot,1/3 ===

torque2 = torque1 * 3

3:1 gear reduction

Input (1)

Output (2)

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More Gearing

How to achieve 9:1 gear reduction? Use larger gears Use multiple gears

Issues: Loosiness btw

meshing gears Backlash

No loosiness increased friction energy waste

Solution: “Gearbox”

9:1 gear reductionwith ganged gears

input

output

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Servo Motors

Purpose: Instead of continuously rotating, move to a given position Servo (Motors)

Components DC Motor Gear reduction Position sensor Controller

Input signal: pulse-width modulated Position control vs. torque control

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NXT Motor

Motor

Wheel encoder

Gears

Place for main shaft

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NXT Motor: Servo Function

Target RPM (% of max RPM)

No load, no servo, 9VNo load, no servo, 7.2V11.5 Ncm load, no servo, 9V NXT11.5 Ncm load, servo, 9V11.5 Ncm, load, servo, 7.2V

Source: http://www.philohome.com/nxtmotor/nxtmotor.htm

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Moving the Robots

Degrees of FreedomControllable Degrees of Freedom

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Moving the Robot

“Degrees of Freedom”: How many variables are needed to describe the configuration of the system in space?

Rigid body in 3D 6DOF

1DOF in 2D

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Joints

Hinge: 1DOF Saddle: 2DOF

Ball and socket: 3DOF Plane: 1DOF

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Explosion of the DOF

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Controllable Degrees of Freedom

Controllable vs. uncontrollable DOF

Can cars get to anywhere?

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Classification of Systems

TDOF = CDOF Holonomic e.g. helicopter

TDOF > CDOF Nonholonomic e.g. car

CDOF > TDOF Redundant e.g. human arm

without hand 7DOF 3 shoulder (ball&socket joint) 1 elbow 3 wrist

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Summary

Effectors & actuators enable robots to produce movement: manipulation or locomotion

Actuators: many types, motors most common

Gears: change speed, torque Servo motors: Complement DC

motors DOF != CDOF