Dr. Brian Mac Namee (www.comp.dit.ie/bmacnamee)
Mobile Robotics:
3. Actuators
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Acknowledgments
These notes are based (heavily)
on those provided by the authors
to accompany “Introduction to
Autonomous Mobile Robots” by
Roland Siegwart and Illah R.
Nourbakhsh
More information about the book is available at:
http://autonomousmobilerobots.epfl.ch/
The book can be bought at:
The MIT Press and Amazon.com
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Introduction
A robot must be able to interact physically with
the environment in which it is operating
Actuators are the components of a robot that
enable it to affect the environment, say, by
exerting forces upon it or moving through it
We’ll take a look at:
– Electric motors
– Artificial muscles
– Pneumatics & hydraulics
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Robot Joints
Robot joints can be either rotary (also known
as revolute) or prismatic (telescoping)
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Robot Joints (cont…)
Prismatic Cartesian
robot
Rotary SCARA
robot
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Robot Joints (cont…)
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Actuator Control
Robots are classified by control method into
servo and non-servo robots
Non-servo robots are essentially open-loop
devices whose movements are limited to
predetermined mechanical stops
Servo robots use closed-loop computer control
to determine their motion
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Open Loop Controller
An open-loop controller (or non-feedback
controller) is a type of controller which
computes its input into a system using only the
current state and its model of the system
The system does not observe the output of the
processes that it is controlling
Input
Controller
Motor
Output
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Open Loop Controller (cont…)
Open-loop control is useful for well-defined
systems where the relationship between input
and the resultant state can be modeled by a
mathematical formula
For example determining the voltage to be fed
to an electric motor that drives a constant load,
in order to achieve a desired speed would be a
good application of open-loop control
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Open Loop Controller (cont…)
An open-loop controller is often used in simple
processes because of its simplicity and lowcost, especially in systems where feedback is
not critical
Generally, to obtain a more accurate or more
adaptive control, it is necessary to feed the
output of the system back to the inputs of the
controller
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Closed Loop Controller
A closed-loop controller uses feedback to
control states or outputs of a dynamical system
Input
Controller
Motor
Output
Measurement
Output
Feedback
Process inputs have an effect on the process
outputs, which is measured with sensors and
processed by the controller; the result is used
as input to the process, closing the loop
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Closed Loop Controller
Closed-loop controllers have the following
advantages over open-loop controllers:
– Disturbance rejection (such as unmeasured
friction in a motor)
– Guaranteed performance even with model
uncertainties, when the model structure does not
match perfectly the real process and the model
parameters are not exact
– Unstable processes can be stabilized
– Reduced sensitivity to parameter variations
– Improved reference tracking performance
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Types of Actuators
Some of the most common actuators are:
– Electric motors, the most common actuators in
mobile robots, used both to provide location by
powering wheels or legs, and for manipulation
by actuating robot arms
– Artificial muscles of various types, none of
which are very good approximations of living
muscles
– Pneumatic and hydraulic actuators, used in
industry for large manipulation tasks but seldom
for mobile robots
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Electric Motors
Electric motors are the most common source of
torque for mobility and/or manipulation in
robotics
The physical principle of all electric motors is
that when an electric current is passed through
a conductor (usually a coil of wire) placed
within a magnetic field, a force is exerted on
the wire causing it to move
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Components Of An Electric Motor
The principle components of an electric motor
are:
– North and south magnetic poles to provide a
strong magnetic field. Being made of bulky
ferrous material they traditionally form the outer
casing of the motor and collectively form the
stator
– An armature, which is a cylindrical ferrous core
rotating within the stator and carries a large
number of windings made from one or more
conductors
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Components Of An Electric Motor (cont…)
– A commutator, which rotates with the
armature and consists of copper contacts
attached to the end of the windings
– Brushes in fixed positions and in contact
with the rotating commutator contacts.
They carry direct current to the coils,
resulting in the required motion
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Components Of An Electric Motor (cont…)
Stator
(Rotating)
Commutator
Brushes
Armature
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How Do Electric Motors Work?
The classic DC motor has a rotating armature in the
form of an electromagnet
A rotary switch called a commutator reverses the
direction of the electric current twice every cycle, to
flow through the armature so that the poles of the
electromagnet push and pull against the permanent
magnets on the outside of the motor
As the poles of the armature electromagnet pass the
poles of the permanent magnets, the commutator
reverses the polarity of the armature electromagnet.
During that instant of switching polarity, inertia keeps
the motor going in the proper direction
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How Do Electric Motors Work? (cont…)
A simple DC electric motor: when the coil is
powered, a magnetic field is generated around
the armature. The left side of the armature is
pushed away from the left magnet and drawn
toward the right, causing rotation
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How Do Electric Motors Work? (cont…)
The armature continues to rotate
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How Do Electric Motors Work? (cont…)
When the armature becomes horizontally
aligned, the commutator reverses the direction of
current through the coil, reversing the magnetic
field. The process then repeats.
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Electric Motors
Electric motors usually have a small rating,
ranging up to a few horsepower
They are used in small appliances, battery
operated vehicles, for medical purposes and in
other medical equipment like x-ray machines
Electric motors are also used in toys, and in
automobiles as auxiliary motors for the
purposes of seat adjustment, power windows,
sunroof, mirror adjustment, blower motors,
engine cooling fans and the like
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Stepper Motors
When incremental rotary motion is required in a
robot, it is possible to use stepper motors
A stepper motor possesses the ability to move
a specified number of revolutions or fraction of
a revolution in order to achieve a fixed and
consistent angular movement
This is achieved by increasing the numbers of
poles on both rotor and stator
Additionally, soft magnetic material with many
teeth on the rotor and stator cheaply multiplies
the number of poles (reluctance motor)
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Stepper Motors
This figure illustrates the design
of a stepper motor, arranged
with four magnetic poles
arranged around a central rotor
Note that the teeth on the rotor
have a slightly tighter spacing
to those on the stator, this ensures that the two
sets of teeth are close to each other but not
quite aligned throughout
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Stepper Motors (cont…)
Movement is achieved when
power is applied for short
periods to successive magnets
Where pairs of teeth are least
offset, the electromagnetic
pulse causes alignment and a
small rotation is achieved, typically 1-2o
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How Does A Stepper Motor Work?
The top electromagnet (1) is charged, attracting the
topmost four teeth of a sprocket.
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How Does A Stepper Motor Work? (cont…)
The top electromagnet (1) is turned off, and the
right electromagnet (2) is charged, pulling the
nearest four teeth to the right. This results in a
rotation of 3.6°
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How Does A Stepper Motor Work? (cont…)
The bottom electromagnet (3) is charged; another
3.6° rotation occurs.
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How Does A Stepper Motor Work? (cont…)
The left electromagnet (4) is enabled, rotating again by
3.6°. When the top electromagnet (1) is again charged, the
teeth in the sprocket will have rotated by one tooth
position; since there are 25 teeth, it will take 100 steps to
make a full rotation.
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Stepper Motor
Stepper motors have several advantages:
– Their control is directly compatible with digital
technology
– They can be operated open loop by counting
steps, with an accuracy of 1 step.
– They can be used as holding devices, since they
exhibit a high holding torque when the rotor is
stationary
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Electric Motors: Mounting
When used with rotary joint systems, motors
can produce torque by being mounted directly
on the joints or by pulling on cables
The cables can be thought of as tendons that
connect the actuator (muscle) to the link being
moved
Since cables can apply force only when pulled,
it is necessary to use a pair of cables to obtain
bidirectional motion around a joint, this implies
mechanical complexity
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Electric Motors: Mounting (cont…)
Mounting motors directly on joints allows for
bidirectional rotation, but such mounting may
increase the physical size and weight of the
joint, and this may be undesirable in some
applications
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Electric Motors: Linear Movement
The fact that electric motors produce rotational
motion raises an issue with regard to their use
in robots
For linear translation it is necessary to translate
rotational to linear motion
– For example, prismatic joints require linear
translation rather than rotation from the motor
Leadscrews, belt-and-pulley systems, rackand-pinion systems, or gears and chains are
typically used to transform rotational to
translational motion
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Artificial Muscles
During the past forty years a
number of attempts have been
made to build artificial muscles
Muscles contract when
activated, since they are
attached to bones on two
sides of a joint, the longitudinal
shortening produces joint rotation
Bilateral motion requires pairs of
muscles attached on opposite sides of a
joint are required to produce
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Artificial Muscles: McKibben Type
The McKibben muscle was the
earliest attempt at constructing
an artificial muscle
This device consisted of a
rubber bladder surrounded by
a sleeve made of nylon fibers in a helical
weave
When activated by pressurized air, the sleeve
prevented it from expanding lengthwise, and
thus the device shortened like living muscles
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Artificial Muscles: McKibben Type
In the 1960s there were attempts to use
McKibben muscles to produce movements in
mechanical structures strapped to
nonfunctional arms of quadriplegics
The required compresses air was carried in a
tank mounted on the person’s wheelchair
These experiments were never completely
successful
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Artificial Muscles: McKibben Type
Since the 1960s there has been
several other attempts to develop
improved McKibben type artificial
muscles:
– (Brooks, 1977) developed an
artificial muscle for control of the
arms of the humanoid torso Cog
– (Pratt and Williamson 1995)
developed artificial muscles for
control of leg movements in a biped
walking robot
However, it is fair to say that no artificial muscles
developed to date can match the properties of animal
muscles
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Artificial Muscles: Shape Memory Alloys
Shape memory alloys (SMAs) have unusual
mechanical properties
Typically, they contract when heated, which is
the opposite to what standard metals do when
heated (expand)
Furthermore, they produce thermal movement
(contraction) one hundred times greater than
that produced by standard metals
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Artificial Muscles: Shape Memory Alloys
Because they contract when heated, SMA
provide a source of actuation for robots
After contraction, the material gradually returns
to its original length when the source of
activation is removed and it is allowed to cool
SMAs have two major problems when used as
artificial muscles:
– They cannot generate very large forces
– They cool slowly and so recover their original
length slowly, thus reducing the frequency
response of any artificial muscle in which they
are employed
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Northeastern University’s Robot Lobster
A robot lobster developed
at Northeastern University
used SMAs very cleverly
– The force levels required
for the lobster’s legs are
not excessive for SMAs
– Because the robot is used underwater cooling is
supplied naturally by seawater
More on the robot lobster is available at: http://www.neurotechnology.neu.edu
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Artificial Muscles: Electroactive Polymers
Like SMAs, Electroactive Polymers (EAPs)
also change their shape when electrically
stimulated
The advantages of EAPs for robotics are that
they are able to emulate biological muscles
with a high degree of toughness, large
actuation strain, and inherent vibration
damping
Unfortunately, the force actuation and
mechanical energy density of EAPs are
relatively low
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Electroactive Polymer Examples
Robotic face developed by a group led
by David Hanson. More information is
available at:
www.hansonrobotics.com
Robotic hand developed by a group
led by Graham Whiteley. More
information is available at:
www.elumotion.com
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Pneumatic & Hydraulic Actuators
Large manipulators in industry frequently
employ hydraulic drives, since such drives
provide a higher torque-to-weight ratio than
electric motors
However, because of the maintenance
problems associated with pressurized oil
(including leaks), hydraulic motors are not used
in smaller mobile robots
Pneumatic drives have been used as actuators
in the past but are not currently popular
Air is compressible, resulting in nonlinear
behavior of the actuator
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Summary
Actuators are the components of a robot that
interact physically with the environment in
which it is operating
The key issues with regard to actuators
include:
– Required power (torque etc)
– Power required
– Weight etc
– Speed