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Page OFC1
PIEZO-DRIVEN STAGE MAKES MOVES IN MICROMETERS, PAGE 18
Special
issue on
A helping hand from
robotics, page 56
MOTION CONTROL
Thwarting the time
bandits: How to
overcome actuator
delays, page 64
THE BASICS OF
FIELD-ORIENTED
CONTROL,
page 74
Better embedded
controls through
graphical-data flow, page 82
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MOTION CONTROL
Pumping iron
Artificial
muscles mimic
the action of
their human
counterparts to
synthesize
novel kinds of
motion.
Leland Teschler
Editor
S
Servomotors and hydraulics are great when
you need precision and
large amounts of rigid
force. These technologies are less desirable for situations that demand a soft
touch and some compliance. The latter
needs have motivated research now underway on artificial devices that mimic
the action of human muscles.
Many of the artificial muscles that researchers are experimenting with today
are pneumatic. They work like inverse
bellows, contracting as they inflate. The
force they apply depends on their degree
of inflation and operating pressure. They
can be quite lightweight yet they can
transfer as much energy as a pneumatic
cylinder of the same volume operating at
the same pressure.
These attributes make pneumatic muscles candidates for mobile robotics. Also,
the fact they operate with air makes them
relatively safe, and they can be replaced
easily. All in all, they have attracted the
interest of several research groups.
The Humanoid robot can synthesize movements to about 1° of
precision. Ten Festo Fluidic muscles go into each side of its torso
and 16 more small prototype muscles in each wrist let the robot
pick up and hold a 1-kg object in each of its hands.
The pneumatic muscles made commercially available so far all have the same
basic structure. Their main element is
some kind of flexible reinforced membrane with fittings at each end. Application of gas pressure forces the membrane
to bulge outward and thus shortens the
muscle. The resulting action pulls on a
load attached to the end of the muscle.
In operation, artificial muscles typically generate motion the same way the
real ones do, by working in what’s called
an antagonistic setup. In other words, as
one muscle moves a load, another acts
as a brake to stop the load at the right
position.
The classic example of an antagonistic
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MOTION CONTROL
Electronics and pneumatic
valves sit inside the
waterproof head of the
Airacuda devised by Festo AG
as a demonstration. The
100-cm-long fish weighs about
4 kg. Its internal ribs are lasersintered polyamide and the
skin is silicone. Lithiumpolymer batteries provide
electrical power for the valves
controlling two Fluidic
muscles for the tail and two
more for steering.
setup consists of two artificial
muscles each with one end connected to a rope. The other end
of the muscles connect to a common base. The rope is strung
over a pulley. Then a contraction
of one muscle and a relaxation of
the other rotates the pulley to
synthesize rotary motion. Similarly, two muscles can mount opposite each other to move a platform back and forth in a linear
motion.
The force that each muscle
generates is proportional to the
pressure of the gas in its membrane. Thus the position of the
effector that the antagonistically
coupled muscles drive will be determined by the ratio of their
two gage pressures.
There have been several different kinds of pneumatic muscles developed over the years.
But the type most widely used
today is called a McKibben muscle, named for a researcher who
introduced the design as an orthotic actuator in the late 1950s.
It consists of a gas-tight elastic
tube or bladder surrounded by
braided sleeving. Braid fibers
run helically around the long
axis of the device at some angle.
Both braid and sleeving
terminate in fittings at
both ends of the muscle.
The braid works against
the inflated tube to make
the muscle contract as inflation pressure rises.
McKibben-type muscles were originally conceived as components for
prosthetic devices. But
they are now more likely
to be used in robotic manipulators. One recent
project in this area uses
McKibben-type muscles
made by Festo AG in Germany. Dubbed Fluidic muscles,
they go into a Humanoid robot
that is a joint project with EvoLogics GmbH and the Bionics
and Evolution Technology Dept.
of the Technical University of
Berlin. Starting with a first functional study of a simple robot
arm in 2000, the project has now
developed into a torso with two
anthropomorphic arms and fivefingered hands.
The Festo muscles attach to
cables made of tough Dyneema
fibers, material that is quite
lightweight, keeping the mass of
moved parts to a minimum. Re-
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MOTION CONTROL
Antagonistic connections with artificial muscles
Resulting
pulley
movement
Rotational
movement
Pressurized
McKibben
muscles
Unpressurized
searchers built a controller that
switches two actuators together
as an antagonistic muscle pair.
The Humanoid robot can bend,
stretch, and turn in much the
same way as a human with a total of 48 degrees of freedom.
The Humanoid has almost the
same radius of action as a human
with similar dimensions. It can
grasp objects thanks to a set of
small Fluidic muscles in its
wrists. The finger muscles for
gripping are actually return
springs that the Fluidic wrist
muscles control through Bowden cables. (The second finger
joints are not powered in this
version of the Humanoid.)
Linear movement
Load
An artificial pneumatic muscle
gets shorter and contracts against
a load if the pneumatic pressure
increases. Exerted force is
proportional to the change in
actuator volume divided by the
change in actuator length.
Artificial muscles are generally set
up to work in antagonistic pairs.
Typical configurations either
rotate a pulley or move a load
linearly. In the case of the
Airacuda fish developed by Festo
AG, a pair of muscles deflect a tail
fin to propel the device through
the water.
Airacuda movement
Fluidic muscles
Alternate pressurization
of muscles bends the
corresponding rib to
make tail flap.
Tail rib
Resulting load movement
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MOTION CONTROL
Magnetic sensors resolve the
motion of each joint to about 1°.
Sensor data go to one of two microprocessors in each arm and
hand. One reads pressure and
angle sensors and calculates the
next move for the actuators. The
other directly controls valves
which manage the air supply to
the body and hands. The valves
are fast-acting devices that are
separate from the muscles they
control. Festo says its next-generation Fluidic muscles will combine the valves and actuators
into single units.
The Fluidic muscles in the Humanoid are driven by an air compressor generating 8 bar. The
amount of air the Humanoid consumes depends on the intensity
of its movements. When demonstrated at the recent Hannover
Fair, the apparatus used air at
about 60 lpm.
The robot can either follow
preprogrammed motions or be
controlled in real time by a human operator wearing a data suit
and data glove. A PC relays the
movement instructions to the
microprocessors in the Humanoid. The robot can follow the
data suit movements with about
a half-second delay. Festo envisions future applications for
bionic stand-ins based on Fluid
muscles in places which are either inaccessible or too dangerous for humans.
Another example of how
McKibben-type muscles can
come in handy is the Airacuda, a
demonstration device also devised by Festo. It is a remote-controlled, pneumatically driven
fish. Its mode of movement is
analogous to that of fish classified as ray fins. These animals
contain thin, long rays of endoskeletal bone and swim by using muscles inside their trunk to
move their tail. The Airacuda’s
tail muscles consist of two Festo
Fluidic devices operating in antagonism. When inflated with
compressed air at 6 bar the muscles contract by about 20% to
make the tail flap. (The Airacuda
Fluidic muscles are super-small 5mm-diameter devices that aren’t
yet commercially available.)
Two additional muscles facilitate steering. The hull contains
a cavity which floods with water
or fills with air to make the
Airacuda sink or rise. A pressure sensor evaluates depth
and sends a corresponding signal to the electronic controls,
which then regulate valves
sending compressed air into the
chamber.
The fin itself contains a flank
connected to a rib structure. Diagonals in the structure are
what the Fluidic muscles alternately shorten and release to
move the tail back and forth. A
human operator controls the
tail and steers the fish via wireless link using a simple joystick
controller.
The air accumulator in the
fish was adapted from a paint
ball gun. It provides about 400 L
of compressed air at 300 bar,
enough to operate the fish for
about 35 min.
Festo says the Airacuda isn’t
just a demonstration platform
for Fluidic muscles: Fin drives
can have numerous advantages
over ordinary propellers. Perhaps the most compelling benefit is that a greater proportion of
the motion generated gets converted into thrust. MD
Copyright © 2006 by Penton Media, Inc.
Festo Corporation
Call: 1.800.99.FESTO
Fax: 1.800.96.FESTO
E-mail: [email protected]
Web: www.festo.com/usa