AIR MUSCLE

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Certificate

DEPARTMENT OF MECHANICAL ENGINEERING,

aBSTRACT

In this article the study is made regarding very fast developing pneumatic actuation system, Air muscle. The air muscle is also termed as Artificial Pneumatic Muscle (PAM). Air muscle is, basically a muscle like structure similar to muscle in a human body. It works on same principal that of human muscle i.e. applying force when contracted axially and expanding in radial direction. This radial expansion if achieved by means of high pressure compressed air then the device is called as Pneumatic Air Muscle. Due to radial expansion increase in volume is compensated by a reduction in axial length of muscle kipping volume theoretically constant.

The article contains information regarding construction, working, types, design considerations, features, operating characteristics, materials and applications mainly for robot actuation. These Pneumatic actuators made mainly of a flexible and inflatable membrane. The very basic need in any Robotic system is actuation of arms of robot. For this purpose Air muscles are ideal actuators over conventional linear pneumatic actuators i.e. cylinders. As Air muscles are very light in weight it reduces overall size and weight of system. Yet It give only one directional motion with one muscle but this drawback can be overcome by using two muscles instead of one, still investing less than conventional one. Biggest drawback is it provides very less force as compared to conventional system as it is limited by material used for flexible and inflatable membrane. The biggest use of air muscle is to replace human muscle by artificial muscle i.e. in orthotic and prosthetic devices because it shows force length properties same as human muscle. Second biggest use of artificial muscle is in Bio-Robotics i.e. making of humanoid robot. The most general application of air muscle is Industrial Robotics, as major attractions about air muscle are the low weight and the inherent compliant behavior. Compliance is due the compressibility of air and, as such, can be influenced by controlling the operating pressure. The force is not only dependent on Pressure but also on their state of inflation, which makes for a second source of spring-like behavior.

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Key words – Air muscle, Compliance, Bio-Robotics, Industrial Robotics

CONTENTS

INTRODUCTION & SCOPE ………………………………………….07

HISTORY……………………………………………………………….08

NATURAL VS. ARTIFICIAL MUSCLE……………………………...09

FEATURES OF ARTIFICIAL MUSCLE……………………………...09

CONSTRUCTION & WORKING……………………………………...10

MATERIALS AND PROPERTIES…………………………………….16

EXPRIMENTAL RESULTS……………………………………………17

STUDY OF MECHANICAL BEHAVIORS OF PAM………………...18

CHARACTRISTICS

DYNAMIC……………………………………………………..21

OPERATING …………………………………………………..22

PAM USING SHAPE MEMORY ALLOY……………………………23

ADVANTAGES AND DISADVANTAGES OF AIR MUSCLE……..25

APPLICATIONS :

HUMONIDE HAND ……………………………………………26

HUMONIDE EYE ……………………………………………...26

ARTIFICIAL LIMBS……………………………………………27

FUTURE DEVELOPMENTS :…………………………………………29

PLEATED PNEUMATIC ARTIFICIAL MUSCLES

COSTING & PRICES (SHADOW KIT)……………………………….29

CONCLUSION………………………………………………………….30

REFERENCES………………………………………………………….30

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INTRODUCTION AND SCOPE

In any Robotic application the most important part is the actuation of

Robot arms, the actuation can be done by using various methods such as

Mechanical Hydraulic and Pneumatic actuation. Mechanical systems have

their own restrictions such as high inertia of system. Hydraulic systems have

restrictions like fluid leakage and fluid cost, yet it gives highest power for

actuation due to its two way positive displacements. So it is necessary to

concentrate on pneumatic actuation because of certain advantages such as low

weight, use of air which is still free of cost, low pressure working so less

hazards and no leakage problem, low input power since air is compressible.

The very first pneumatic actuator is a pneumatic cylinder. In which high

pressure air acts on piston and push the piston in either way front or back.

The disadvantage with this actuator is size of cylinder and low power to

weight ratio. Also pneumatic has application in medical field i.e.

Rehabilitation of arms or muscles. So these all needs and disadvantages can be

overcome by Pneumatic actuator called as AIR MUSCLE. Air muscle is,

basically a muscle like structure similar to muscle in a human body. It works

on same principal that of human muscle i.e. applying force when contracted

axially and expanding in radial direction. This radial expansion if achieved by

means of high pressure compressed air then the device is called as Pneumatic

Air Muscle. Due to radial expansion increase in volume is compensated by a

reduction in axial length of muscle kipping volume theoretically constant.

Certainly an air muscle differs in many ways from human muscle. Very

important is, force –velocity properties of actuator are not like human muscle.

The important features of artificial muscle are Lightweight, Smooth, Flexible,

Powerful. They are easy to manufacture, low cost and can be integrated with

human operations without any large scale safety requirements. Furthermore

they offer extremely high power to weight ratio of about 400:1. As a

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comparison electric motors only offer a power ration of 16:1. Air Muscles are

also called McKibben actuators named after the researcher who developed it.

HISTORY

It was in 1958 that R.H.Gaylord invented a pneumatic actuator which’s original

applications included a door opening arrangement and an industrial hoist. Later in

1959 Joseph.L.McKibben developed Air Muscles. The source of inspiration was the

human muscle itself, which would swell when a force has to be applied. They were

developed for use as an orthotic appliance for polio patients. Clinical trials were

realized in 1960s. These muscles were actually made from pure rubber latex, covered

by a double helical weave (braid) which would contract when expanded radially. This

could actually be considered as a bio-robotic actuator as it operates almost similar to a

biological muscle.

The current form air muscles were developed by the Bridgestone Company,

famous for its tires. The primary material was rubber i.e. the inner tube was made

from rubber. Hence these actuators were called ‘Rubbertuators’. These developments

took place around 1980s.

Later in 1990s Shadow Robotic Company of the United Kingdom began

developing Air Muscles. These are the most commonly used air muscles now and are

associated with almost all humanoid robotic applications which were developed

recently. Apart from Shadow another company called ‘The Merlin Humaniform’

develops air muscles for the same applications, although their design is somewhat

different from the Shadow muscles.

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Fig no. 1 Air Muscle Schematic- McKibben Model

NATURAL VS. ARTIFICIAL MUSCLE

NATURAL MUSCLE

1] Natural muscles are made of cells and tissues.

2] These muscles are impossible to reproduce externally.

ARTIFICIAL MUSCLE

Artificial muscles are made of rubber or inflatable membrane.

These muscles can be reproduced externally.

3] Force applied is inversely proportional to velocity of actuation.

4] Natural muscles have natural compliance.

Force velocity relationship is not similar to natural muscle

It also has natural compliance if used with compressed air.

5] Force is directly proportional to length and size of muscle.

Force –length characteristics are similar to natural muscle.

CONCLUSION

For effective use of air muscle in hominid robot industry attention should be towards material of muscle, its compliance and size of muscle.

FEATURES OF ARTIFICIAL MUSCLE

Lightweight:Air Muscles weigh between 10g and 150g, depending on size –

Particularly useful for weight critical applications. The 30mm Air

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Muscle weighs 80g.

Smooth: Unlike pneumatic cylinders, Air Muscles have no 'stiction', and an

Immediate response. This results in a smooth natural movement.

Powerful:

Air muscles produce forces up to 700 N at pressures of only a few bar.

Reproducible:

Air muscles are reproducible and can be manufactured in identical size.

CONSTRUCTION

Fig no.2 Shadow 30mm Air muscle (Extended)

Extended

These measurements are taken when the muscle is fully stretched out, under a load of at least 50N, and a pressure of 0 bar.

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Hole – Hole Spacing (1) - 290mm

Total Muscle Length (2) - 250mm

Active Length (3) – 230mm

The Hole-Hole spacing is the distance between the holes in the fittings at either end of the muscle. This is adjustable, as the fittings can be screwed in or out.They can also be removed entirely, creating a more compact muscle. Use anM10 screw instead, and remember to use PTFE tape to ensure a good seal.

The Total Muscle Length is the length of the whole muscle, excluding the fittings.

The Active Length is the length of the part of the muscle which contracts under pressure, and does not include the headers.

Fig no.3 Shadow 30mm Air Muscle (Contracted)

Contracted

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These measurements are taken when

Pressure = 3bar

Load =50N.

Hole – Hole Spacing - 220mm

Total Muscle Length - 180mm

Active Length =160mm

Headers and Fittings:

The header at each end of the muscle consists of an Aluminum ring, and a Delrin plastic bung, with an M10 female thread. This thread can be used as a means of attachment, and to allow air into or out of the muscle. The muscle is supplied with two Delrin fittings, one of which comes with a 6mm push-fit connector.

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Figure 4: Muscle headers and fittings

WORKING

The inner rubber tube is inflated by entering air at a pressure, usually limited

to 3.5 bar. The movement of this tube is constrained by the braid. When the tube gets

inflated it experiences a longitudinal contraction. This would create a pull at both ends

of the tube. Usually one end of the tube will be attached to somewhere so that force

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can be applied from one end. This pulls when effectively utolised could provide the

necessary motion. The working of the Air Muscle closely resembles that of the natural

muscle and hence the name Muscle given to it along with Air. The figure below

shows the physical appearance of the muscle at different stages of its working.

Fig 5 Air Muscle at different stages

Theoretical Model

Using conservation of energy and assuming the actuator maintains dV dP equal to

zero, reasonable for actuators built with stiff braid fibers that are always in contact

with the inner bladder, the tensile force produced can be calculated from:

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Where,

P - The input actuation pressure,

dV - the change in the actuator’s interior volume

dL - the change in the actuator’s length

Vb - the volume occupied by the bladder

dW - the change in strain energy density

(Change in stored energy/unit volume).

Ff - Friction arising from sources such as contact between the

Braid and the bladder and between the fibers of the braid

Itself.

Fig 6 at ambient pressure, the actuator is at its resting length

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Fig7 State at maximum pressure

McKibben actuators are fabricated from two principle components: an inflatable inner

bladder made of a rubber material and an exterior braided shell wound in a double

helix. At ambient pressure, the actuator is at its resting length (figure: 6). As pressure

increases, the actuator contracts proportionally until it reaches its maximally

contracted state at maximum pressure (figure: 7). The amount of contraction is

described by the actuator’s longitudinal stretch ratio given by λ1 = Li / Lo where L is

the actuator’s length, and subscript i refers to the instantaneous dimension and the

subscript o refers to the original, resting state dimension.

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MATERIALS AND PROPERTIES

EXPRIMENTAL RESULTS:-

EXPRIMENT NO -1 In Figure 8 The mass M is constant and the pressure difference across the membrane, i.e. its gauge pressure, is increased from an initial value of zero. At zero gauge pressure the volume enclosed by the membrane is minimal, Vmin, and its length maximal, lmax. If the muscle is pressurized to some gauge pressure p1, it will start to bulge and at the same time develop a pulling force. The mass will thus be lifted until the generated force equals Mg. The membrane’s volume will then have grown to V1 and its length contracted to l1. Increasing the pressure further to p2 will continue this process. From this experiment two basic actuator behavior rules can be deduced: (1) a PAM shortens by increasing its enclosed volume, and (2) it will contract against a constant load if the pneumatic pressure is increased.

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Figure 8: PAM operation at constant load.

EXPRIMENT NO -2 The other rules can be derived from the second experiment, shown in Figure 9. The gauge pressure is now kept at a constant value p, while the mass isn diminished. In this case the muscle will inflate and shorten. If the load is completely removed, as depicted by Figure 2 (c), the swelling goes to its full extent, at which point the volume will reach its maximum value, Vmax, the length its minimal value, lmin, and the pulling force will drop to zero. The PAM cannot contract beyond this point, it will operate as a bellows at shorter lengths, generating a pushing instead of pulling force. This means that a PAM will shorten at a constant pressure if its load is decreased and its contraction has an upper limit at which it develops no force and its enclosed volume is maximal.

Figure 9: PAM operation at constant pressure.

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EXPRIMENT CONCLUSION :- Concluding from both experiments a fifth rule can be added: for each pair of pressure and load a PAM has an equilibrium length. This behavior is in absolute contrast to that of a pneumatic cylinder: a cylinder develops a force which depends only on the pressure and the piston surface area so that at a constant pressure, it will be constant regardless of the displacement.

MECHANICAL BEHAVIORS:-

Fig 10.1 Initial and final length of the PAM at the operating air pressure without pulling force;

Fig 10.2 Variation of unstretched length against the air pressure within PAM

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Fig 10.3 Length definitions when PAM exerted by pulling force.

The unstretched length of the PAM as mentioned was measured by increasing the pressure 0–5 bar (gauge Pressure) without applying the pulling force. The plots given in Fig 11 show the behavior of the unstretched length of the three PAMs according to

the air pressure within the PAM. The lines indicate the simulation results of the second order Polynomial function. It can be seen that the unstretched length decreases

as the air pressure within the PAM increases,

Fig. 11 Plot of unstretched length against air pressure.

Pressure dependent unstretched length

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Experimental results of contraction and elongation of the PAM

Fig 12.1 Three-dimensional plots

Fig 12.2 Two dimensional plots.

DYNAMIC CHARACTERISTICS

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Fig 13 1 Graphs showing dynamic characteristics of 30mm Air Muscles

Fig 13 2 Graphs showing dynamic characteristics of 30mm Air Muscles

OPERATING CHARACTERISTICS

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Figure:14 Operating Characteristics

Mc-KIBBEN ARTIFICIAL MUSCLE USING SHAPE-MEMORY POLYMER (SMP)

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When these actuators are applied to robotic joints, the joints are typically driven by pairs of actuators located antagonistically to increase the joint stiffness. The actuator can be considered as a simple spring-like elastic element, or a “gas spring,” whose stiffness is proportional to the inner pressure [8]. Since the actuator is effectively a spring, the shape fixity of the actuator is low. Here we define shape fixity as the ability of an actuator to maintain its actuated state against external forces without energy consumption. Moreover, even with active control the ability of the McKibben actuator to maintain a fixed state under varying external forces is non-trivial because of its nonlinear characteristics and hysteresis. In this study, we propose anew pneumatic actuator based on the McKibben actuator which has enhanced shape fixity properties. This is achieved by the use of a shape-memory polymer to modulate the stiffness, and hence control the deformations, of the braided mesh shell.

Shape-memory polymers (SMP)

Shape-memory polymers are often described as two-phase structures comprised of a hard (fixing) phase and a soft (reversible) phase. The hard and soft phases represent two elastic modules: one in the lower-temperature, higher-stiffness “glassy” plateau and the other in the higher temperature, lower-stiffness “rubbery” plateau. The reversible change in the elastic modulus between the glassy and rubberyStates of SMPs can be as high as 500 times.

Fig15 Relationship between the elastic modulus and temperature of the SMP.

A conventional McKibben actuator can be considered as a device with only two states; un actuated and actuated. Transition between these states is controlled by air pressure. If we now modify the device by the introduction of SMPs, we introduce a second control mechanism, namely temperature control. The two control parametersof pressure and temperature mean that the actuator can exhibit more states.

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Typical operation of the SMP McKibben actuator involves the following control sequence

Fig 16 McKibben artificial muscle that uses SMP (PH: high pressure, PL: low pressure).

Starting in state S1, the actuator is warmed above Tg. The actuator now enters S2.

In S2 the SMP is soft and can be deformed. When the internal bladder is pressurized, the actuator shortens and/or produces a force if it is coupled to a mechanical load. After the actuator attains its desirable length, it is cooled below Tg and the actuator enters state S3.

In S3 the SMP is fixed in its rigid state. If the internal pressureWithin the bladder is released the actuator moved to state S4.

In S4 the actuator maintains its length indefinitely without the need for an air supply. When the actuator is next heated above Tg, the SMP enters state S5.

In S5 the actuator has returned to its pre-actuation state, and has exhibited shape recovery.

ADVATAGES OF SMP OVER CONVENTIONAL Mc-KIBBEN MUSCLE

The actuator can be fixed more rigidly than conventional pneumatic actuators using the phase change of the SMP material.

The actuators can achieve relatively large deformation between two rigid states.

The actuators can maintain a continuous desirable length. If only part of the actuator is heated, only that portion of the SMP will

transition to the rubbery state and hence, when internal air pressure is increased only that portion of the structure will actuate.

ADVANTAGES OF AIR MUSCLES

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A varying force-displacement relation at constant gas pressure, contrary to pneumatic cylinders, which results in a muscle-like behavior; an adjustable compliance, due to gas compressibility and the dropping force-displacement characteristics

A maximum displacement or stroke of up to 50% of initial length

The absence of friction and hysteresis, as opposed to other types of PAMs

The ability to operate at a wide range of gas pressures, and thus to develop both very low and very high pulling forces

Lightweight - Air Muscles weigh as little as 10 gm - particularly useful for weight-critical applications

Lower Cost - Air Muscles are cheaper to buy and install than other actuators and pneumatic cylinders

Smooth - Air Muscles have no 'stiction' and have an immediate response. This results in smooth and natural movement.

Flexible - Air Muscles can be operated when twisted axially, bent round a corner, and need no precise aligning.

Powerful - Air Muscles produce an incredible force especially when fully stretched.

Damped - Air Muscles are self-dampening when contracting (speed of motion tends to zero), and their flexible material makes them inherently cushioned when extending.

Compliant - Being a soft actuator, Air Muscles systems are inherently compliant.

DISA DVANTAGES OF AIR MUSCLES

The force which can be applied is only tensile in nature. For both kinds of forces additional mechanisms are required.

The efficiency of Air Muscles is not as good as electric motors Its total displacement is only about 20% to 30% of its initial length Friction between the netting and the tube leads to a substantial hysteresis in

the force-length characteristics; this obviously has an adverse effect on actuator behavior and necessitates using complex models and control algorithms

Rubber is often needed to avoid the tube from bursting, this comes at the cost of a high threshold pressure—typically about 90 kPa —that has to be overcome in order to start deforming the rubber material and below which the actuator will simply not operate

APPLICATIONS

HUMONIDE HAND

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Fig 17 HUMONIDE HAND

The entire physical system is illustrated in Fig. 1. The physical hand and arm comprises 4 fingers, a thumb and elbow, all actuated via air muscles. Each air muscle requires a precisely controlled source of air pressure to accurately position the stroke-length of each air muscle, positioning the hand fingers and forearm in the required positions. This controlled source of air is supplied by the Valve Board

HUMONIDE ROBOT EYE

Fig 18 Humanoid robot eye.

As shown in Fig. 18, the mechanism of the humanoid robot eye is imitation of the human eye, which is modified slightly with the assumptions mentioned above. There

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is no contact between the pulley base (back base) and the eyeball. With cooperative stretching or shrinking of six EOMs, the eyeball can rotate with 3 DOF.

ARTIFICIAL LIMBS

Fig 19.1 Artificial limb developed at Fig 19.2 the Dexterous hands

The bio robotics Lab, University of

Washington.

Merits of artificial limbs1. Continuous and extended operation for about 8-10 hours.2. Low weight3. Quieter operation4. User satisfaction5. No maintenance or low levels of maintenance.

The Dexterous hands

The dexterous hand was developed by the Shadow robotic company. The hands operate just like human hands with five fingers. It is powered by 28 Air Muscles. The size is almost same as human hands as they closely fit into a human hand.

FUTURE DEVELOPMENTS

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PLEATED PNEUMATIC ARTIFICIAL MUSCLES

Fig 20 Pleated PAM photograph.

The Pleated PAM was developed because of several weak points of the braided muscles. Firstly, due to dry friction between the braid and the tube these actuators have substantial hysteresis. For this material deformation should be avoided.This can be done by using membrane rearranging in order to allow for inflation.The principle of rearranging is to have a membrane that in some way unfurls as it is inflated. When such PAMs contract their membranes’ surface area do not change contrary to the increasing surface area of deforming membranes. The basic idea of the PPAM was to do this by using a cylindrical membrane of a high tensile stiffness and high flexibility and folding it together along its central axis, like accordion bellows.

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The membrane material needed to expand is thus stashed away inside the folds. At both ends the membrane is tightly locked to fittings, which also carry the gas inlet and outlet ducts. When such a PAM is pressurized it shortens and bulges. As the membrane has a high tensile stiffness, the expansion is highest in the middle of the membrane and gradually goes down toward both ends where no expansion at all can occur. Ideally, Fig 21 such a PAM would have an infinite amount of infinitely narrow pleats, leading to an ax symmetrical membrane surface that would thus only be loaded by meridional stresses (i.e. along fold lines) and not by parallel stresses (i.e. along parallels, which are sections of the surface and any plane perpendicular to the axisof symmetry). This can be seen as follows. If any parallel stress would exist in such a membrane at some equilibrium contraction, it would unfold the membrane further, since folds cannot withstand tensile stress. As a result of this, the membrane diameter would have to increase, which, at a fixed contraction, can only happen by stretching in the meridional direction. The high tensile stiffness of the material makes it nearly unstretchable though and, therefore, unfolding can only happen if the membrane contracts further.

Fig 21Pleated PAM, (a) Uninflated, at rest and(b) Inflated.

COSTING & PRICES (SHADOW KIT)

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CONCLUSION

The air muscles do not provide us wide range of operational states, also it not that much cost effective as that of conventional pneumatic systems. The physical and operational characteristics are not properly defined yet. So it is difficult to predict its behavior in actual working conditions. But in the case of artificial legs, humanoid robots etc they offer a wide range of possibilities. The developments in the field can lead us, to use air muscle in various new applications with new operating states including or excluding conventional one. The air muscle will replace natural muscle of human in nearer future with same characteristics and control.

REFERENCES

Shadow 30mm Air Muscle © The Shadow Robot Company Ltd. 2011

Design and Kinematic Analysis of a Novel Humanoid Robot EyeUsing Pneumatic Artificial Muscles. Xuan-yin Wang, Yang Zhang, Xiao-jie Fu, Gui-shan Xiang .Journal of Bionic Engineering 5 (2008) 264–270

Study on mechanical behaviors of pneumatic artificial muscleKanchana Crishan Wickramatunge, Thananchai Leephakpreeda *School of Manufacturing Systems and Mechanical Engineering, Sirindhorn International Institute of Technology, Thammasat University, P.O. Box 22,

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Thammasat-Rangsit Post Office, Pathum-Thanni 12121, Thailand 19 August 2009 Available online 15 September 2009

McKibben artificial muscle using shape-memory polymerKazuto Takashimaa,b, , Jonathan Rossitera,c, Toshiharu Mukaia 15 September ∗2010Available online 22 September 2010

Fabrication and control of miniature McKibben actuatorsM. De Volder , A.J.M. Moers, D. Reynaerts∗Katholieke Universiteit Leuven, Dept. Mech. Eng., Celestijnenlaan 300B, 3001 Leuven, Belgium. 1 January 2011, Available online 4 January 2011

The Concept and design of Pleated Pneumatic Artificial MusclesFrank Daerden, Dirk LefeberVrije Universiteit Brussel, Dept. of Mechanical Engineering, Multibody Mechanics Research Group, Pleinlaan 2, 1050 Brussel, Belgiumfrank.daerden@vub.ac.be

Fatigue Characteristics of McKibben Artificial Muscle ActuatorsGlenn K. Klute, Department of Bioengineering University of Washington Seattle, WA 98195-7962 gklute@u.washington.eduBlake Hannaford ,Department of Electrical Engineering University of Washington Seattle, WA 98195-2500 blake@ee.washington.edu

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