21-06-2012, 02:32 PM
Measurement and Modeling of McKibben Pneumatic Artificial Muscles
Measurement and Modeling.pdf (Size: 73.08 KB / Downloads: 37)
Introduction
The McKibben pneumatic artificial muscle was developed in artificial limb research in the
1950s and 1960s (Schulte 1961, Gavrilovic and Maric 1969). They have recently been
commercialized by the Bridgestone Rubber Company of Japan for robotic applications (Inoue
1988), and re-engineered by Prof. Jack Winters for construction of biomechanically realistic
skeletal models. McKibben muscles consist of an internal bladder surrounded by a braided mesh
shell (with flexible yet non-extensible threads) that is attached at either end to fittings or to some
tendon-like structure (Fig. 1a). When the internal bladder is pressurized, the high pressure gas
pushes against its inner surface and against the external shell, and tends to increase its volume. Due
to the non-extensibility (or very high longitudinal stiffness) of the threads in the braided mesh shell,
the actuator shortens according to its volume increase and/or produces tension if it is coupled to a
mechanical load. This physical configuration causes McKibben muscles to have variable-stiffness
spring-like characteristics, non-linear passive elasticity, physical flexibility, and very light weight
compare to other kinds of artificial actuators (Hannaford and Winters 1990).
Static and dynamic tension-length relationships
McKibben muscle is an actuator which convert pneumatic (or hydraulic) energy into
mechanical form by transferring the pressure applied on the inner surface of its bladder into the
shortening tension. To find the relationship of the tension, length, and pressure, a theoretical
approach and several experiments will be analyzed with simplified modeling.
1) Static physical model of McKibben muscles
In order to find the tension as a function of pressure and actuator length without considering
the detailed geometric structure, a theoretical approach based on energy conservation is introduced
first.
Dynamic testing machine
For the following experiments, a testing system capable of producing and recording desired
patterns of the tension, length, and pressure of the actuator was built (Fig. 2). The system consists
of an IBM compatible personal computer (PC, with 16 MHz 386sx, real time updating rate up to 5
kHz), PC extension bus interface and timer circuit, A/D, D/A converters, analog filters and
amplifiers, pressure sensors (6.8 bar max.), strain gauge force sensors (100 N max.), filters,
amplifiers, a 1/4 horse power DC motor with PWM power amplifier (± 24 A max. current), an
optical angular position incremental encoder (1600 steps per revolution) and a decoder, a pressure
regulator (10 bar max.), an electro-valve (Festo proportional pressure regulator MPP-3-1/8, 10 bar
max.), two gas accumulators (3 in3 and 10 in3), and flexible tubing (f 1/8”)
By utilizing different combinations of the I/O channels and software settings, the system can
performs a variety of testing conditions, such as constant pressure testing, isometric testing,
isotonic testing, and pneumatic circuit testing, etc.
Quasi-static and dynamic characteristics
It has been shown that there is hysteresis in the tension-length cycle, and that the dominant
friction is the frequency insensitive Coulomb friction, which is caused by the contact between the
bladder and the shell, between the braided threads and each other, and the shape changing of the
bladder. The history dependence makes the friction difficult to predict precisely, especially after
the actuator is attached to artificial bones with curving shape.