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McKibben
Artificial Muscles - Construction
.
A
McKibben Artificial Muscle, if constructed from the materials
and dimensions specified below, can generate an isometric force
of about 200 N when pressurized to 5 bar and held to a length
of 14 cm. This actuator is relatively small. Here in the Biorobotics
Laboratory, we've constructed larger ones capable of isometric
forces over 1600 N at 5 bar. The image shown here places an uninflated
actuator above one inflated to 3 bar. The caliper jaws in this
image are set to 7.5 cm.
Design
and Fabrication
McKibben Artificial Muscles are available commercially, or can
be easily assembled from parts in less than 30 minutes. The parts
required (and some suggested sources), the necessary tools, and
each step in the assembly process are described below. If you'd
like to see more details of the parts shown on this page, try
clicking on an individual image.
Parts
List
The following list of parts describes the components used to assemble
the muscles for the Anthroform Biorobotic Arm Project. If you'd
like to make a few of your own, don't worry if some of the parts
aren't available in your locale as you can substitute any reasonable
fascimile.
- The expandable
inner bladder is essentially an elastic tube. The tube shown
here is Manosil ® Silicone Rubber with an inner diameter
of 5/32" and wall thickness of 1/32". Manosil ® Silicone
Rubber is available from:
Barnant Co.
28W092
Commercial Avenue
Barrington, IL 60010
800/637-3739 Cat. No. 75-300-350 (5/32"x1/32"x50')
-
The key parameter
of the bladder material is the capability to sustain repeated
strains of over 300%. A variety of rubber materials (both natural
and synthetic) can achieve this, however, the silicone rubber
listed above manages these large strains with little plastic
strain and minimal creep. If the application doesn't involve
exposure to UV light or petroleum products, you might achieve
longer fatigue lives with natural latex rubber.
The
original use of the braided shell shown here was as a flexible
conduit for electrical wiring. If your interests are in actuators
capable of much higher loads than would be possible with the
materials listed above, you could substitute a braided stainless
steel mesh for the polyster mesh shown here.
-
The
hose barb shown here can be found at any pneumatic or hydraulic
component supplier. This one came from:
-
In
addition to the hose barb, you'll need a few miscellaneous
components to attach the actuator to your air source (e.g.
appropriate fittings, valves, and regulators).
-
The
hose plug shown here is about 1/2" long and was made from
a piece of 1/8" diameter aluminum rod on a lathe. The ridges
cut into the rod provide a secure surface for the wire used
to wrap the assembly (see below for details). I haven't tried
it yet, but you could probably get away with using a 1/2"
10-24 screw.
-
The
wire used to wrap the ends of the assembly is 24 gage galvanized
wire. It is available at your local hardware store.
-
The
type of "artificial tendons" used to attach the muscle really
depends on your application. If you're making a musculo-skeletal
model, you'll want a material with appropriate visco-elastic
properties. For our purposes here, we'll just use some three
stand nylon string (with a diameter of approximately 1/16")
also available at your local hardware store.
-
Silicone
caulking is used to prevent the wire which wraps the ends
of the assembly from puncturing neighboring muscles. Again,
you'll find this at your local hardware store. If you live
near a coast, stop by your local marine store and buy some
elastic rigging tape. It does the same job and you don't have
to wait for the caulking to cure.
Assembly
Tools
To assemble a McKibben Artificial Muscle, you need only a few
basic tools:
- A pair
of pliers,
- A pair
of wire cutters,
- A pair
of scissors, and
- A lighter,
some matches, a candle, or a heat gun.
Assembly
Steps
McKibben Artificial Muscles can be built to different size specifications
depending on the performance required. If you use the dimensions
suggested below, the assembled actuator will have a resting length
of approximately 14 cm and an overall length of approximately
18 cm (not including tendons). The assembly process takes around
30 minutes and is outlined below.
-
Use
the scissors to cut a piece of silicone rubber tubing to form
the expandable inner bladder. The length of the cut piece
should measure 16.0 cm.
-
Use
the scissors to cut a piece of the polyester sleeving to form
the outer braided shell. The length of this piece should measure
18.0 cm.
-
Use
the lighter, matches, candle, or heat gun to singe the ends
of the polyester sleeving. The idea here is just to keep the
sleeving from unraveling. If you get carried away, you'll
end up with melted drops of burning polyester in your lap.
If that happens, quickly douse the flames with some HOT McDonald's
coffee and call your attorney.
-
Insert
the hose plug into one end of the expandable inner bladder
until is completely inside the tube. The end of the hose plug
and the expandable inner bladder should be flush with each
other.
-
Insert
the hose barb into the other end of the expandable inner bladder.
-
Using
the wire cutters, cut eight 20 cm lengths from the spool of
24 gage galvanized wire. Four pieces of the wire will be used
to make a pressure tight seal and four pieces will be used
to attach the "tendons" to the "muscle".
-
Now
insert the expandable inner bladder inside the outer braided
shell until the hose plug end is flush with the end of the
outer braided shell.
-
Wrap
a piece of the wire you just cut around the end of the shell-bladder-hose
plug assembly. Wrap the wire around three times and then twist
the ends togather. The objective here is a pressure tight
seal, so use some pliers to make it as tight as possible.
Cut off the excess wire.
-
Wrap
another piece of the wire about 0.5 cm away from the first
one. The hose plug end of the assembly should now look as
shown here.
-
Repeat
the above two steps at the hose barb end of the assembly.
-
To
verify you've got a pressure tight assembly, connect it to
your pressure source. If you're using Manosil ® Silicone
Rubber tubing, approximately 1 bar is required to initiate
inflation, so use a regulator to limit the pressure. Don't
exceed a maximum pressure of 5 bar (~75 psi). Be sure to wear
eye protection.
-
Depending
on your application, you'll need some way of attaching the
muscle. One method is to wire wrap a piece of nylon string
onto each end of the muscle. Cut two pieces of the three strand
nylon string and singe the ends to keep it from unraveling.
Use the four remaining pieces of wire (two pieces at each
end) to attach the string to the actuator as shown here. Tug
on each end to make sure the string is securely attached.
-
If
your application will have multiple muscles next to each other,
the sharp edges of the wire wrappings can puncture the neighboring
muscle. To prevent this situation, apply some silicone caulking
around the wire wrapped ends.
-
The
completed assembly, when connected to a pressure regulator
and pressurized to 3 bar (~45 psi), should look as shown here.
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