By David Torgesen
 Wikipedia contributors. "Pneumatic artificial muscles."
Wikipedia, The Free Encyclopedia
Wikipedia, The Free Encyclopedia, 3 Feb. 2010. [Retrieved 11 March 2010].
 Daerden, Frank and Lefeber, Dirk, “Pneumatic Artificial Muscles: actuators for
robotics and automation,”
[retrieved 11 March 2010].
 Wang, Che
Wei, “Soft Pneumatic Exoskeleton,”. 2008.
[retrieved 11 March 2010]
What are pneumatic artificial muscles
Types of Pneumatic Artificial Muscles
What are the advantages of PAMs?
What are the disadvantages of PAMs?
PAMs are contractile or extensional devices
operated by pressurized air.
PAMs were first developed in the 1950s for
use in artificial limbs.
They have also been known under the names
McKibben Artificial Muscles and
The figure to the right shows
the PAM in action under a
In (a), the pressure is zero.
This provides for the
In (b), the pressure is
increased, which contracts
the muscle and decreases the
In(c), the pressure is once
again increased, and the
muscle contracts even more.
This action can be
characterized as an inverse
The figure to the right shows
the PAM in action with a
In (a), the load is at its
greatest, which extends
In (b), the load is
decreases the length.
In(c), the load is removed
which decreases the length
of the muscle to its
From these two experiments, the PAM at a constant load
and the PAM at a constant pressure, 5 basic actuator
behavior rules can be deduced:
1. a PAM shortens by increasing its enclosed volume
2. a PAM will contract against a constant load if the pneumatic
pressure is increased
3. a PAM will shorten at a constant pressure if its load is decreased
4. a PAM’s contraction has an upper limit at which it develops no force
and its enclosed volume is maximal
5. for each pair of pressure and load a PAM has an equilibrium length
The chart to the right
summarizes these findings.
At minimum contraction, as the
pressure increases, the force also
As the percent contraction increases,
the force decreases at each pressure.
This is similar to the human muscle.
The force drops from its highest value
at full muscle length to zero at full
Fluidic actuators are contractile device and can, therefore, only
generate motion in one direction. Just like skeletal muscles, two
actuators need to be coupled in order to generate bidirectional
motion. The figure below illustrates this concept.
PAMs VS. Skeletal Muscles
Do not change volume during contraction
Have a modular structure (they are made up of parallel and series connected microscopic
Are organized in units whose activation depends on the level of external load
Come in fast and slow types, depending on the need of sustained action and speed
Have integrated multiple force and strain sensors
Have energy stored in them and running through them
Can serve as energy source or even building material for muscles of other biological systems
Monotonically decreasing load
Both need to be setup antagonistically in order to allow bidirectional motion
The McKibben Muscle
The Pleated PAM
The Yarlott Muscle
The Robotic Muscle Actuator
The Morin Muscle
The Baldwin Muscle
The Paynter Hyperboloid Muscle
The McKibben Muscle
This is the most frequently used PAM.
It is a cylindrical braided muscle that has both its tube and its
sleeving connected at both ends to fittings
The Pleated PAM
This PAM is of the membrane rearranging kind. This means no
material strain is involved when it is inflated.
The muscle membrane has a number of pleats that unfold as it
inflates. No friction is involved in this process. As a result, practically
no energy is required to expand the membrane.
Comprises an elastomeric bladder of a prolate spheroidal shape netted
by a series of cords that run axially from end to end.
Upon inflation, the shell’s surface area remains more or less constant,
and a surface rearranging occurs. This reduces shell stretching, and
more pneumatic energy can be transformed into mechanical energy.
This muscle was designed to operate at low gauge pressures, as low as
Robotic Muscle Actuator
Consists of an articulating polylobe
bladder harnessed by a wire netting
and closed at either ends by fittings.
The total surface area remains
constant during inflation. This is due
to the tensile stiffness of the
Because of the absence of friction
and membrane stretching, a much
higher force is attained compared to
muscles with stretching membranes.
The purpose of the Morin Muscle was to
detect change in the pressure of a fluid
and transmit that change to a controller.
Therefore, the Morin Muscle cannot really
be called an artificial muscle in the current
context, but it was the inspiration of
In the figure at the right, there are three
different designs of the muscle.
(a) an overpressure design
(b) an underpressure design
(c) a concentric membranes design
Consists of a very thin surgical
rubber membrane embedded by
glass filaments in the axial
direction. This results in a modulus
of elasticity that is much higher in
the direction parallel to the fibers
than in the direction perpendicular
to the fibers.
Gauge pressures have to be limited
to low values (10
Creates forces of up to 1600 N at
these low pressure.
In its fully elongated state, the
membrane has the shape of a
hyperboloid of revolution.
Inextensible, flexible threads are
anchored to the end fittings and are
embedded in the membrane.
The membrane bulges into a nearly
spherical shape at full contraction.
They are very lightweight.
They directly connect to the structure they
Replacement of a defective muscle is quick and
Compliant Behavior: when a force is exerted on
a PAM it does not increase the force of actuation
(it “gives in”). This is important with human
interaction and delicate procedures.
They use air to operate and are therefore
The force of the PAM is dependant on both the
pressure and the state of inflation.
PAMs are non
linear systems and therefore
more difficult to control.
Because gas is compressible, PAMs that use long
tubes must have a control system that can deal
with a delay between the control signal and the
If the shell of the PAM becomes misshapen by
some external force, non
uniform swelling of the
bladder will occur, which may cause the bladder
Because pneumatic muscles have compliant
behavior, they can be placed on robots that
perform delicate operations.
The Soft Pneumatic
is a pneumatic
muscle suit for the lower
extremities. It can assist in
heavy lifts, muscle
reinforcement and walking.
The suit is lightweight,
portable, and comfortable.
The system sustains an idle
power state until muscle
assistance is needed.
Systems are also being
developed for upper
This is a pneumatic
Pneumatic muscles seem to have a bright
future in technology, especially in the design
of pneumatic suits. These suits could not only
assist those who need help in lifting things
but also those who are unable to walk. It
would be a special day for a person unable to
walk to be able to take those first steps.