Air muscle:
Abstract
Air muscle is essentially a robotic actuator which is replacing the conventional pneumatic cylinders at a rapid pace. Due to their low production costs and very high power to weight ratio, as high as 400:1, the preference for Air Muscles is increasing. Air Muscles find huge applications in biorobotics and development of fully functional prosthetic limbs, having superior controlling as well as functional capabilities compared with the current models. This paper discusses Air Muscles in general, their construction, and principle of operation, operational characteristics and applications.Introduction
Robotic actuators conventionally are pneumatic or hydraulic devices. They have many inherent disadvantages like low operational flexibility, high safety requirements, and high cost operational as well as constructional etc. The search for an actuator which would satisfy all these requirements ended in Air Muscles. They are easy to manufacture, low cost and can be integrated with human operations without any large scale safety requirements. Further more they offer extremely high power to weight ratio of about 400:1. As a 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 realisd 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 biorobotic actuator as it operates almost similar to a biological muscle
Air Muscle Schematic- McKibben Model
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.
Construction
Air muscle construction [Shadow air muscle: 30mm]
The header at each end of the muscle consists of an Aluminium ring, and a
Delrin plastic bung, with a 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 also.
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 can be applied from one end. This pull 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.
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:
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 describes the lumped effects of friction arising from sources such as contact between the braid and the bladder and between the fibers of the braid itself. Neglecting the second and third terms on the right hand side of above equation and assuming the actuator maintains the form of a right circular cylinder with an infinitesimally thin bladder yields known solutions. The solution to the second term on the right side of the equation is based on a non-linear materials model developed by Mooney and Rivlin in the 1940’s and 1950’s proposed a relationship between stress (σ ) and strain (ε ) given by σ = dW dε where W is the strain
energy density function. Using the assumptions of initial isotropy and incompressibility, W can be described as a function of two strain invariants ( I1 and I2 ):
where Cij are empirical constants. Only two Mooney-Rivlin constants (C10 =118.4 kPa
and C01 =105.7 kPa) were necessary for accurate results with the natural latex rubber bladder, however, other materials may require additional constants. For the case of the McKibben actuator, the experimental methods required to determine these constants are dramatically simplified because the McKibben actuator’s strain invariants, constrained by braid kinematics, are nearly the same as the strain invariants for uniaxial tension .This fortuitous relationship eliminates the need for multi-axial testing that would otherwise be necessary. Solving equation a using the non-linear Mooney-Rivlin materials model results in a McKibben actuator model whose structure is allowed to deform as well as store elastic energy in a non-linear fashion. This model is given by:
where Fmr is the predicted force, and parameters N , Lo , B, and Ro are shown in figure 1 and figure 2. Bladder thickness is denoted by to and is used in the bladder volume calculation. λ1 refers to the actuator’s longitudinal stretch ratio and is given by
λ1 = Li/ Lo, where Li is the actuator’s instantaneous length and Lo is the original, resting state length.
Figure: 2
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: 1). As pressure increases, the actuator contracts proportionally until it reaches its maximally contracted state at maximum pressure (figure: 2). 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.
Estimation of Frictional Effects
The third term on the right of equation a represents these frictional losses which are a function of (1) braid material, (2) bladder material, (3) pressure, and (4) actuator length. In lieu of a model that incorporates a function for each of these, we have taken the intermediate step of lumping all of these effects into a single parameter ( Ff ) as a simple function of pressure. Analysis of the experimental data and theory predictions ( Fmr ) suggests a linear form given by:
Ff = mP + b
where m and b are empirically determined constants. The actuator model, which now includes the geometry of the braid and bladder, the material properties of the bladder, and a term for frictional effects (all three terms of equation a) is given by:
F = Fmr − Ff
A comparison of this model versus experimental results for the largest actuator (nominal braid diameter of 1-1/4 in.) is presented in figure 3.5. The figure shows a reasonably close fit for each of the four activation pressures tested. Similar results were obtained for the two smaller actuators (nominal braid diameters of ¾ and 1/2) but are not shown
Figure: 3
Dynamic Properties
To measure the force-velocity properties of the McKibben actuator, a series of experiments were conducted with the axial-torsional Bionix (MTS Systems Corp., Minnesota, U.S.A.) tensile testing instrument. Actuators of three sizes were constructed and tested. Each experiment measured the force output at a constant pressure over the contraction range at various velocities. One end of the actuator was rigidly attached to the load cell while the other end was moved in response to the instrument’s digital controller. Step velocity profiles were applied such that one end of the actuator was rapidly accelerated and held to a constant velocity until the end of the actuator’s working length was reached. Input step velocity profiles tested included 1, 10, 25, 50, 100, 150, 200, 250, and 300 mm/s for concentric contractions and 1, 10, 25, 50, 100, and 150 mm/s for eccentric contractions. Up to 500 mm/s is possible; however, instantaneous fluctuations in velocity of 15 percent were measured during trails at 500 mm/s. The magnitude of these fluctuations decreased at lower velocities, and was less than 9 percent at 300 mm/s and 6 percent at 200 mm/sec. This anomaly is thought to arise from the hydraulic pump.
Experimental Results
The experimentally measured output force of a single McKibben actuator, plotted as a function of both length and velocity, is shown in figure 4. The results shown are from an actuator whose nominal braid diameter was ¾ inch and constructed with a natural latex bladder. The actuator pressure was 5 bar and the original, resting state length of the actuator was 180 mm. The output force is clearly a function of length, but not of velocity. Similar results were obtained at lower pressures and with the other two sized actuators, but are not shown
Operating Characteristics
Specifications for a typical Air Muscle (the Shadow company)
| Diameter | Weight | Pull (3.5 bar) | Maximum pull | Length |
| 30 mm | 80 g | 35 kg | 70 kg | 290 mm (stretched) |
stretched form
These measurements are taken when the muscle is fully stretched out, under a load of at least 50N, and a pressure of 0 bar.
Hole – Hole Spacing 290mm
Total Muscle Length 250mm
Active Length 230mm
1: 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 an M10 screw instead, and remember to use
PTFE tape to ensure a good seal.
2: The Total Muscle Length is the length of the whole muscle, excluding the fittings.
3: The Active Length is the length of the part of the muscle which contracts under pressure, and does not include the headers.
These measurements are taken when the muscle is pressurised to 3bar, with a load of 50N.
Hole – Hole Spacing 210mm
Total Muscle Length 170mm
Active Length 150mm
Differences from pneumatic cylinders
The Air Muscle is a low pressure actuator with a set of operational features unique in the field of robotics and automationa) - Smooth jerk free motion from start to finish due to the complete lack of stiction, the feature of standard actuators which produces the characteristic jerk so well known in air operated devices.
b) - Compliance - Although they can produce the force needed to move a function the Air Muscle will also yield when an obstacle is encountered, thus preventing damage to the object and the "Arm" - a distinct advantage where robots mix with humans.
c) - Light weight - The materials from which the Air Muscle is made are non-metallic and give it a relatively high power-to-weight ratio - a critical feature in choosing an actuator for a mobile robot
Advantages of Air Muscles
Power to weight ratios in excess of 1 kW/kg, by way of comparison, electric drives typically has some 100 W/kg
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
The possibility of direct connection to a robotic joint, i. e. without having to use any gears, because of their high output forces at all speeds.
Some of the advantages spelt out by the shadow company typical to their products are:-
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.
Efficient - a muscle length can be maintained with minimal energy input.
Fast -full contraction can be achieved in less than one second from rest.
Disadvantages
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
Rubber deformation, like any material deformation, needs energy, this will lower the force output of this type of muscle up to 60%.
Applications
Humanoid robots
The major application of Air Muscles is in the field of humanoid robots. As these actuators nearly resemble the characteristics of actual skeletal muscles, they can perform a verity of functions as is performed by the human hand. Coupled with the implementation of neural networks and powerful, precise sensors they are capable of high end applications such as assembling of very minute components etc.
Humanoid robot manufactured by Shadow robotic company
Artificial limb developed at the bio robotics Lab, University of Washington.
At the bio robotic lab of university of Washington the limb as shown figure was developed. The major requirements of their research team were:
1. Continuous and extended operation for about 8-10 hours.
2. Low weight
3. Quieter operation
4. User satisfaction
5. No maintenance or low levels of maintenance.
Merlin Humaniform Air Muscles attached to human hand
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. The figure shown illustrates this fact.
The muscle can perform any function the human hand performs. Besides it is equipped to swivel its fingers. It makes use of 28 Air muscles for these movements. The human hand has 24 muscles. The additional four in case Dexterous hands due to the swiveling motion.
Further developments
The Pleated Pneumatic Air Muscles [PPAMs]: As a result growing research in the field of Air Muscles, another variant called pleated pneumatic air muscles were developed. Pleated pneumatic artificial muscles are strong and lightweight actuators that perform very well in position control and other automation and robotic tasks. They are easy to use, require no gearing and are easy to connect and replace. A high degree of positioning accuracy is accomplished with them and this just by using off-the-shelf pressure regulating servo-valves together with simple PI control techniques. Furthermore, they can easily be made to have a soft touch so as not to damage fragile objects or to effect a safe man-machine interaction. Because of their inherent characteristics PPAMs are suitable for powering walking and running machines. Autonomous machine operation can then be guaranteed in a number of ways, e. g.by using on-board small size internal combustion engines.
Some of the stated advantages of PPAMs are:-
A maximum displacement or stroke of up to 50% of
initial length;
The absence of friction and hysteresis, as opposed to other types of PAM.
The pleated pneumatic Air Muscles
Conclusion
Even though Air Muscles are not capable of offering an extremely wide range of operations, but in the case of artificial legs, humanoid robots etc they offer a wide range of possibilities. With further developments in neural networks and sensor equipments, it might be possible replace an entire limb for an amputee and function normally like a natural limb would do. The only draw back lies in developing a complete theoretical model for calculating the characteristics such as fatigue etc. Research is also directed towards substituting for Air with nitrogen or other gases for maximum efficiency and better damping