Post on 05-Jan-2016
description
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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