1. Actuator: Pneumatic Artificial Muscles
By David Torgesen
Actuator: Pneumatic Artificial Muscles

2. References
[1] Wikipedia contributors. "Pneumatic artificial muscles." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 3 Feb [Retrieved 11 March 2010].
[2] Daerden, Frank and Lefeber, Dirk, “Pneumatic Artificial Muscles: actuators for
robotics and automation,” [retrieved 11 March 2010].
[3] Wang, Che-Wei, “Soft Pneumatic Exoskeleton,” [retrieved 11 March 2010]

3. Outline What are pneumatic artificial muscles (PAMs)?
Types of Pneumatic Artificial Muscles
What are the advantages of PAMs?
What are the disadvantages of PAMs?
Modern Applications
Conclusion

4. What are Pneumatic Artificial Muscles?
PAMs are contractile or extensional devices operated by pressurized air.
Contracted
Extended

5. What are Pneumatic Artificial Muscles? (Cont.)
PAMs were first developed in the 1950s for use in artificial limbs.
They have also been known under the names McKibben Artificial Muscles and Rubbertuators.

6. What are Pneumatic Artificial Muscles? (Cont.)
The figure to the right shows the PAM in action under a constant load.
In (a), the pressure is zero. This provides for the maximum length.
In (b), the pressure is increased, which contracts the muscle and decreases the length.
In(c), the pressure is once again increased, and the muscle contracts even more.
This action can be characterized as an inverse bellows.

7. What are Pneumatic Artificial Muscles? (Cont.)
The figure to the right shows the PAM in action with a constant pressure.
In (a), the load is at its greatest, which extends the length.
In (b), the load is decreased, which decreases the length.
In(c), the load is removed which decreases the length of the muscle to its minimum length.

8. What are Pneumatic Artificial Muscles? (Cont.)
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

9. What are Pneumatic Artificial Muscles? (Cont.)
The chart to the right summarizes these findings.
At minimum contraction, as the pressure increases, the force also increases.
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 muscle contraction.

10. What are Pneumatic Artificial Muscles? (Cont.)
Antagonistic setup:
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.

11. What are Pneumatic Artificial Muscles? (Cont.)
PAMs VS. Skeletal Muscles
Differences
Skeletal muscles
Do not change volume during contraction
Have a modular structure (they are made up of parallel and series connected microscopic contractile systems
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
Similarities
Monotonically decreasing load-contraction relation
Both need to be setup antagonistically in order to allow bidirectional motion

12. Types of Pneumatic Artificial Muscles
The McKibben Muscle
The Pleated PAM
The Yarlott Muscle
The Robotic Muscle Actuator
The Morin Muscle
The Baldwin Muscle
The Paynter Hyperboloid Muscle

13. Types of Pneumatic Artificial Muscles (Cont.)
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

14. Types of Pneumatic Artificial Muscles (Cont.)
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.

15. Types of Pneumatic Artificial Muscles (Cont.)
Yarlott Muscle
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 1.7kPa.

16. Types of Pneumatic Artificial Muscles (Cont.)
Robotic Muscle Actuator (ROMAC)
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 membrane material.
Because of the absence of friction and membrane stretching, a much higher force is attained compared to muscles with stretching membranes.

17. Types of Pneumatic Artificial Muscles (Cont.)
Morin Muscle
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 McKibben’s design.
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

18. Types of Pneumatic Artificial Muscles (Cont.)
Baldwin Muscle
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 (10-100kPa).
Creates forces of up to 1600 N at these low pressure.

19. Types of Pneumatic Artificial Muscles (Cont.)
Paynter Hyperboloid Muscle
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.

20. What are the advantages of PAMs?
They are very lightweight.
They directly connect to the structure they power.
Replacement of a defective muscle is quick and easy.
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 environmentally friendly.

21. What are the disadvantages of PAMs?
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 muscle movement.
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 to rupture.

22. Modern Applications
Because pneumatic muscles have compliant behavior, they can be placed on robots that perform delicate operations.

23. Modern Applications (Cont.)
The Soft Pneumatic Exoskeleton 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.

24. Modern Applications (Cont.)
Systems are also being developed for upper body use.
This is a pneumatic muscle suit.

25. Conclusion
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.