15-02-2010, 07:05 PM
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ABSTRACT
ARTIFICIAL MUSCLES “ A STUDY
For many years, the idea of a human with bionic muscles immediately conjures up science fiction images of a TV series superhuman character that was implanted with bionic muscles and portrayed with strength and speed far superior to any normal human. As fantastic as this idea may seem, recent developments in electroactive polymers (EAP) may one day make such bionics possible. Polymers that exhibit large displacement in response to stimulation that is other than electrical signal were known for many years. Initially, EAP received relatively little attention due to their limited actuation capability. However, in the recent years, the view of the EAP materials has changed due to the introduction of effective new materials that significantly surpassed the capability of the widely used piezoelectric polymer, PVDF. As this technology continues to evolve, novel mechanisms that are biologically inspired are expected to emerge. EAP materials can potentially provide actuation with lifelike response and more flexible configurations. While further improvements in performance and robustness are still needed, there already have been several reported successes. In recognition of the need for cooperation in this multidisciplinary field, there is a series of international forums that are leading to a growing number of research and development projects and to great advances in the field.. In this paper, the field of EAP as artificial muscles will be reviewed covering the state of the art, the challenges and the vision for the progress in future years.
INTODUCTION
How do you a build the musculoskeletal system a humanoid robot? If you're Sony, you
use a system of servo motors, gears, metal rods and cables. But that's a poor imitation of
the human body where the movement of limbs is dictated by the smooth, coordinated
contraction of muscle fibers. Enter the breakthrough: electroactive polymers, also called
artificial muscles. Recently unveiled by a scientist in Albuquerque, these artificial
muscles contract when exposed to an electric current. Attach one end to the pelvis of a
humanoid robot and the other end to the back of its knee and you have a robot that can do
leg curls. Strap together enough such fibers, couple them with a smart contact feedback
system, and you can teach a robot to walk using its own artificial muscles.
Imagine it: no motors to wear out, no cables to snap, no rods to break: just muscle-like
fibers that contract in response to an electric current. It's nothing short of a revolution in
robotics. No doubt, the industry will rely heavily on this technology in the years ahead.
There's even hope that such fibers might somehow be used in human patients to aid those
who have, for one reason or another, lost the use of their limbs.
Hope for human patients! As humans live longer there is a growing need for availability
of organs for transplant however shortage in donations necessitates the development of
artificial alternatives. Advances in medicine have led to the availability of artificial blood,
replacement joints, heart valves, and heart-lung machines that are common implanted. In
the United States, nearly one in ten individuals is using some type of an implanted
medical device [Malchesky, 2001]. Muscle is a critically needed organ and its availability
in an artificial form for medical use can greatly contribute to the improvement of the
quality of life of many humans. The emergence of effective electroactive polymers (EAP)
that are also known as artificial muscles can potentially address this need. These
materials are human made actuators that have the closest operation similarity to
biological muscles. While these actuation materials are far from being ready for use as
implants enormous progress has been made in recent years turning them into a promising
technology for consideration in medical applications.
Nature as a Biologically-Inspiring Model
Evolution over millions of years made nature introduce solutions that are highly power-efficient and imitating them offers potential improvements of our life and the tools we use. Human desire and capability to imitate nature and particularly biology has continuously evolved and with the improvement in technology more difficult challenges are being considered. Imitation of biology may not be the most effective approach to engineering mechanisms using man-made capabilities. It is inconceivable to imaging flying with a machine that has feathers and flapping wings, where obviously a machine like that will not allow us to reach the distances and carry the loads that aircraft are doing today. The introduction of the wheel has been one of the most important inventions that human made allowing to travel great distances and perform tasks that would have been otherwise impossible within the life time of a single human being. While wheel based locomotion mechanisms allow reaching great distances and speeds, wheeled vehicles are subjected to great limitations with regards to traversing complex terrain with obstacles. Obviously, legged creatures can perform numerous functions that are far beyond the capability of an automobile. Producing legged robots is increasingly becoming an objective for robotic developers and considerations of using such robots for space applications are currently underway. Making miniature devices that can fly like a dragonfly; adhere to walls like gecko; adapt the texture, patterns, and shape of the surrounding as the octopus (can reconfigure its body to pass thru very narrow tubing); process complex 3D images in real time; recycle mobility power for highly efficient operation and locomotion; self-replicate; self-grow using surrounding resources; chemically generate and store energy; and many other capabilities are some of the areas that biology offers as a model for science and engineering inspiration. While many aspects of biology are still beyond our understanding and capability, significant progress has been made and the field of biomimetics is continuing to evolve.
EAP actuated robotic mechanisms are enabling engineers to create devices that were previously only imaginable in science fiction. One such commercial product has already emerged in Dec. 2002 is a form of a Fish-Robot. It swims without batteries or a motor and it uses EAP materials that simply bend upon stimulation. For power it uses inductive coils that are energized from the top and bottom of the fish tank. This fish represents a major milestone for the field, as it is the first reported commercial product to use electroactive polymer actuators
The evolution of artificial muscles in robotics
The introduction of the wheel has been one of the most important inventions that human made allowing to travel great distances and perform tasks that would have been otherwise impossible within the life time of a single human being. While wheel based locomotion mechanisms allow reaching great distances and speeds, wheeled vehicles are subjected to great limitations with regards to traversing complex terrain with obstacles. Obviously, legged creatures can perform numerous functions that are far beyond the capability of an automobile. Producing legged robots is increasingly becoming an objective for robotic developers and considerations of using such robots for space applications are currently underway. Making miniature devices that can fly like a dragonfly; adhere to walls like gecko; adapt the texture, patterns, and shape of the surrounding as the octopus (can reconfigure its body to pass thru very narrow tubing); process complex 3D images in real time; recycle mobility power for highly efficient operation and locomotion; self-replicate; self-grow using surrounding resources; chemically generate and store energy; and many other capabilities are some of the areas that biology offers as a model for science and engineering inspiration. While many aspects of biology are still beyond our understanding and capability, significant progress has been made and the field of biomimetics is continuing to evolve [Bar-Cohen and Breazeal, 2003]. The evolution in the capabilities that are inspired by biology has increased to a level where more sophisticated and demanding fields, such as space science, are considering the use of such robots. At JPL, four and six legged robots are currently being developed for consideration in future missions to such planets as Mars. Such robots include the LEMUR (Limbed Excursion Mobile Utility Robot). This type of robot would potentially perform mobility in complex terrains, perform sample acquisition and analysis, and many other functions that are attributed to legged animals including grasping and object manipulation. This evolution may potentially lead to the use of life-like robots in future NASA missions that involve landing on various to planets. The details of such future missions will be designed as a plot, commonly used in entertainment shows rather than conventional mission plans of a rover moving in a terrain and performing simple autonomous tasks. Equipped with multi-functional tools and multiple cameras, the LEMUR robots are intended to inspect and maintain installations beyond humanity's easy reach in space. This spider looking robot has 6 legs, each of which has interchangeable end-effectors to perform the required mission. The axis-symmetric layout is a lot like a starfish or octopus, and it has a panning camera system that allows omni-directional movement and manipulation operations.
EAP as Artificial Muscles
One of the key aspects of driving mechanisms that emulate biology is the development of actuators that mimic the capability of biological muscles. The potential for such actuators is continuously growing as advances are being made leading to more effective electroactive polymers (EAP). These materials have functional similarities to biological muscles, including resilience, quiet operation, damage tolerance, and large actuation strains (stretching, contracting or bending). They can potentially provide more lifelike aesthetics, vibration and shock dampening, and more flexible actuator configurations. These materials can be used to make mechanical devices and robots with no traditional components like gears, and bearings, which are responsible to their high costs, weight and premature failures. Also, they could potentially be used as artificial organ to assist or operate the heart and/or its valve, the eye lid and/move the eyeball as well as control the focal length of its length, and allow mobility of the legs and/or hand as well as provide smart prosthetics (also known as cyborgs). As an example of a capability of EAP materials that is inspired by biology, a team developed a miniature robotic arm. This robotic arm illustrates some of the unique capabilities of EAP, where its gripper consists of four EAP fingers (made by Ionic polymer metal composite strips) with hooks at the bottom emulating fingernails. This arm was made to grab rocks similar to human hand. Generally, there are many polymers that exhibit volume or shape change in response to perturbation of the balance between repulsive intermolecular forces, which act to expand the polymer network, and attractive forces that act to shrink it. Repulsive forces are usually electrostatic or hydrophobic in nature, whereas attraction is mediated by hydrogen bonding or van der Waals interactions. The competition between these counteracting forces, and hence the volume or shape change, can be controlled by subtle changes in parameters such as solvent, gel composition, temperature, pH, light, etc
The type of polymers that can be activated by non-electrical means include: chemically activated, shape memory polymers, inflatable structures, including McKibben Muscle, light activated polymers, magnetically activated polymers, and thermally activated gels [Chapter 1 in Bar-Cohen, 2004]. Polymers that are chemically stimulated were discovered over half-a-century ago when collagen filaments were demonstrated to reversibly contract or expand when dipped in acid or alkali aqueous solutions, respectively [Katchalsky, 1949]. Even though relatively little has since been done to exploit such Ëœchemo-mechanicalâ„¢ actuators, this early work pioneered the development of synthetic polymers that mimic biological muscles. The convenience and practicality of electrical stimulation and technology progress led to a growing interest in EAP materials.
