CMPUT 412
Actuation
Csaba Szepesvári
University of Alberta
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Defining sensors and actuators
Actuators
Sensors
Environment
Sensations
(and reward)
actions
Controller
= agent
2
Actuation
Why? How?
The process of sensing
Characterizing sensors
Some 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 motion
Tad McGeer’s
passive
walking robot
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Active Actuation: How?
 Electromagnetism
 Electric motors
 Pressure
Incomplete!
Direction of motion:
-Rotation
-Linear
 Hydraulics (fluid pressure)
 Pneumatics (air pressure)
 Materials




Photo-reactive materials
Chemically reactive materials
Thermally reactive materials
Pizeoelectric materials (crystals)
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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
Power wires
 Speed: 3K-9K rpm
(50-150rps)
 Problem:
Speed high, force low
shaft
9
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
Output (2)
Input (1)
3:1 gear reduction
 Const ´ power ~ torque * vrot
 vrot,2 = vrot,1/3
===
torque2 = torque1 * 3
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More Gearing
 How to achieve 9:1 gear reduction?
 Use larger gears
 Use multiple gears
output
 Issues:
 Loosiness btw
meshing gears 
Backlash
 No loosiness 
increased friction 
energy waste
input
9:1 gear reduction
with ganged gears
 Solution: “Gearbox”
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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
Wheel encoder
Place for main shaft
Motor
Gears
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NXT Motor: Servo Function
No load, no servo, 9V
No load, no servo, 7.2V
11.5 Ncm load, no servo, 9V NXT
11.5 Ncm load, servo, 9V
11.5 Ncm, load, servo, 7.2V
Target RPM (% of max RPM)
Source: http://www.philohome.com/nxtmotor/nxtmotor.htm
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Moving the Robots
Degrees of Freedom
Controllable 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?
1DOF in 2D
Rigid body in 3D  6DOF
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Joints
Hinge: 1DOF
Ball and socket: 3DOF
Saddle: 2DOF
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
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