IPPAM Intelligent Prosthesis actuated by pleated Pneumatic Artificial ...

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known type is that of the so-called Pleated PneumaticArtificial Muscle (PPAM) [21], [22] shown in Figure 9.characteristic properties that can be inferred from thementioned experiment are: (1) a loaded PPAM contracts whenthe internal pressure p is increased, and (2) a loaded PPAMshortens by increasing its enclosed volume V.A second experiment is shown in Figure 11. The mass M isgradually reduced whilst the internal pressure p is kept at aconstant value. In this case the muscle will swell up andthereby shorten. If the mass is completely removed, asdepicted in configuration (c), the muscle reaches its minimumlength, l min , and maximum volume, V min . The pulling force hasnow dropped to zero. This implicates that: (3) a PPAM willshorten when its load is decreased, and (4) there exists anupper limit at which the PPAM no longer generates a pullingforce and the enclosed volume is maximal.Figure 9: 3 contraction levels of a PPAM [25]The core element is a flexible reinforced membrane, at bothends attached to fittings along which mechanical power istransferred to a load. As the membrane is inflated or gas issucked out of it, it is bulged out or squeezed, respectively.Related to this radial expansion or compression, the membranecontracts or stretches axially and exerts a pulling force on itsload. Like already cited the PPAM’s energy source is air. Thatway the actuator is powered by the pressure differencebetween the internal air and the ambient air. To see andunderstand how a mechanism – in casu a PPAM plus externalload – works, two setups will be considered.Figure 11: PPAM at constant pressureInterpretation of the PPAM behaviour within the twomentioned experiments uncovers a contrast to the behaviour ofa pneumatic cylinder: the force that can be generated by apneumatic cylinder only depends on pressure and pistonsection implying that at a constant pressure, the force remainsconstant independent of the piston displacement.The pulling force, exerted by the PPAM, can be derivedmathematically and set as:F2L= p. L0 . f ( ε,, a)(1)RFigure 10: PPAM at constant loadFigure 10 depicts the working mechanism of a PPAM atconstant load. Within this experiment the increase of internalair pressure from zero to p 2 results in a changing musclelength l and enclosed volume V. At zero gauge pressure(configuration a) the muscle has its minimum volume, V min ,and its maximum length, l max . If the muscle is pressurized top 1 , it will start to swell up and generate a pulling forcesimultaneously. The mass M will thus be lifted just until thegenerated force equals the gravitative force Mg. Now themuscle length and muscle volume have changed to l 1 and V 1respectively (configuration b). Further increase of the internalpressure will continue this process (configuration c). Thein which p represents the internal air pressure in the PPAM, L 0its full length, ε its momentary contraction, R its minimumradius and a a dimensionless factor which takes account of themembrane’s elasticity. If this membrane consists of hightensile strength and stiffness material, the influence ofelasticity can be dropped and set to zero. Now, one can seedirectly that f is a dimensionless function, depending only onthe contraction and slimness of the muscle. Using L 0 = 110mm and R = 11.5 mm, results in the theoretical forcecharacteristics visualized in Figure 12.4
∂M ∂M1 ∂M2 ⎛ ∂F1∂r1⎞⎛ ∂F2 ∂r2⎞= − = ⎜ . r1+ F1.⎟ − ⎜ . r2+ F2.(5)⎟∂θ∂θ∂θ⎝ ∂θ∂θ⎠ ⎝ ∂θ∂θ⎠Figure 12: Theoretical muscle force vs. contraction ratio [27]Figure 13: Antagonistic set-up [27]For small contractions, the forces are extremely high, while forlarge contractions, the forces drop to zero.Summarized, the following benefits of the PPAM can be put:• high force-weight ratio;• natural compliance;• no need for complex gearing mechanisms;• shock absorbance during impact;• intrinsic safety.B. The actuators: antagonistic set-upAs earlier mentioned, a PPAM can only generate a pullingforce. Just like in human anatomy, two actuators need to becoupled in order to generate a bidirectional motion, one foreach direction. This specific connection of the muscles to theload is generally named as an antagonistic set-up. Such anantagonistic coupling is achieved with a pull bar and levermechanism, which is depicted in Figure 13. While one musclecontracts along its axial centre-line and rotates the joint in itsdirection, the other muscle will elongate. The internal pressuredifference between the two muscles determines generatedtorque Т and consequently also angular position θ. Bothpressures p 1 and p 2 can either be increased or decreased in thatway that the joint stiffness K raises or lowers respectively,without influencing the angular position. Thus, angularposition as well as stiffness can be set and controlledindependently.The generated torque of one muscle is given by:2M i(θ ) = pi.L0 . fi(θ ). ri(θ ) = pi.ti(θ )(2)And the total joint torque is then defined as:M ( θ ) M ( θ ) − M 2(θ ) = p1.t1(θ ) − p2.t 2(= , (3)1 θ)C. Dimensioning the actuatorsThe generated pulling force of a PPAM depends – see formula(1) – on the internal pressure, the initial length, the contractionratio and the slimness of the muscle. Consequently, the pullingforce characteristics can be manipulated by varying theseparameters within their allowed range, limited by themechanical properties of the used muscle materials.Considering the earlier quoted torque values, the requiredplantar flexion torque varies from 120 [Nm] to 160 [Nm] forpatients weighing 80 [kg]. The major design restriction fromthe aesthetic point of view is the dimension of the prosthesis,in order to respect the enclosed volume of muscles, tendonsand bones in human foot and calf. In this way, the upcomingbottleneck is the available lever arm, which is quite short,implying the need of larger pneumatic muscles.Compactness and pneumatic power generation won’t beaddressed extensively, as the first prototype will be a proof-ofconceptfor the implementation of compliant actuation inprosthetics. After preliminary testing and evaluation, the firstprototype will be manipulated and fine-tuned in view offurther test campaigns. At the end of 2006, a first prostheticfoot should be available for tests with patients in lab settings.For the dimensioning of the PPAM, the maximum plantarflexion torque – for patients weighing 80 [kg] – of 160 [Nm]is taken into account. This high torque has thus to begenerated by the pneumatic muscles just before toe-off. Infact, it should be gradually increased from zero in earlymidstance to 160 [Nm] in toe-off [20], due to the form of thetorque curves shown in Figure 14. The upper characteristicrepresents low step speed while the middle and the lowercharacteristic represent moderate and high step speed,respectively. Notify that step speed, as such, doesn’t influencethe torque value since it remains quite the same. Only thecurve shape from zero to 50% gait cycle scarcely differs.While the total joint stiffness is set as:∂= − 1 MK C = −∂θwith(4)5
Page 6: Figure 14: Ankle torque vs. percent
Page 9: III. CONCLUSION AND FUTURE WORKThe

IPPAM Intelligent Prosthesis actuated by pleated Pneumatic Artificial ...

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