Powered Exoskeleton


A powered exoskeleton, also known as powered armor, or exoframe, is a powered mobile machine consisting primarily of an exoskeleton-like framework worn by a person and a power supply that supplies at least part of the activation-energy for limb movement. Powered exoskeletons are designed to assist and protect the wearer. They may be designed, for example, to assist and protect soldiers and construction workers, or to aid the survival of people in other dangerous environments. A wide medical market exists in the future of prosthetics to provide mobility assistance for aged and infirm people.

Other possibilities include rescue work, such as in collapsed buildings, in which the device might allow a rescue worker to lift heavy debris, while simultaneously protecting the worker from falling rubble.  A fictional ‘mech’ is different from a powered exoskeleton in that the mecha is typically much larger than a normal human body, and does not directly enhance the motion or strength of the physical limbs. Instead the human operator occupies a cabin or pilot’s control seat inside a small portion of the larger system. Within this cabin the human may wear a small lightweight exoskeleton that serves as a haptic control interface for the much larger exterior appendages.

The earliest exoskeleton-like device was a set of walking, jumping, and running assisted apparatus developed in 1890 by a Russian named Nicholas Yagin. As a unit, the apparatus used compressed gas bags to store energy that would assist with movements, although it was passive in operation and required human power. In 1917, US inventor Leslie C. Kelley developed what he called a ‘pedomotor,’ which operated on steam power with artificial ligaments acting in parallel to the wearers movements. With the pedomotor, energy could be generated apart from the user. The first true exoskeleton in the sense of being a mobile machine integrated with human movements was co-developed by General Electric and the United States military in the 1960s. The suit was named Hardiman; powered by hydraulics and electricity, it allowed the wearer to amplify their strength by a factor of 25, so that lifting 25 pounds was as easy as lifting one pound without the suit. A feature dubbed ‘force feedback’ enabled the wearer to feel the forces and objects being manipulated.

While the general idea sounded promising, the actual Hardiman had major limitations. It was impractical due to its 1,500-pound (680 kg) weight. Another issue was the fact it is a slave-master system, where the operator is in a master suit which is in turn inside the slave suit which responds to the master and takes care of the work load. This multiple physical layer type of operation may work fine, but takes longer than a single physical layer. When the goal is physical enhancement, response time matters. Its slow walking speed of 2.5 ft/s further limited practical uses. The project was not successful. Any attempt to use the full exoskeleton resulted in a violent uncontrolled motion, and as a result it was never tested with a human inside. Further research concentrated on one arm. Although it could lift its specified load of 750 pounds (340 kg), it weighed three quarters of a ton, just over twice the liftable load. Without getting all the components to work together the practical uses for the Hardiman project were limited.

Los Alamos Laboratories worked on an exoskeleton project in the 1960s called ‘Project Pitman.’ In 1986, an exoskeleton prototype called the ‘LIFESUIT’ was created by Monty Reed, a US Army Ranger who had broken his back in a parachute accident. While recovering in the hospital, he read Robert Heinlein’s ‘Starship Troopers’ and from Heinlein’s description of ‘Mobile Infantry Power Suits,’ he designed the ‘LIFESUIT,’ and wrote letters to the military about his plans for the device. In 2001 ‘LIFESUIT One’ (LSI) was built. In 2003 ‘LS6’ was able to record and play back a human gait. In 2005 ‘LS12′ was worn in a foot race known as the Saint Patrick’s’ Day Dash in Seattle. Monty Reed and ‘LIFESUIT XII’ set the Land Speed Distance Record for walking in robot suits. ‘LS12’ completed the 3-mile race in 90 minutes. The current ‘LIFESUIT’ prototype ’14’ can walk one mile on a full charge and lift 92 kg (200 lb) for the wearer.

In 2007 it was reported that the Pentagon granted development funds to The University of Texas at Dallas’ nanotechnologist Ray Baughman to develop military-grade artificial myomer fibers. These electrically-contractive fibers are intended to increase the strength-to-weight ratio of movement systems in military powered armor. One of the proposed main uses for an exoskeleton would be enabling a soldier to carry heavy objects (180–700 lbs) while running or climbing stairs. Not only could a soldier potentially carry more weight, he could presumably wield heavier armor and weapons. Most models use a hydraulic system controlled by an on-board computer. They could be powered by an internal combustion engine, batteries, or potentially fuel cells. Another area of application could be medical care, nursing in particular. Faced with the impending shortage of medical professionals and the increasing number of people in elderly care, several teams of Japanese engineers have developed exoskeletons designed to help nurses lift and carry patients.

Exoskeletons could also be applied in the area of rehabilitation of stroke or Spinal cord injury patients. Such exoskeletons are sometimes also called ‘Step Rehabilitation Robots.’ An exo-skeleton could reduce the number of therapists needed by allowing even the most impaired patient to be trained by one therapist, whereas several are currently needed. Also training could be more uniform, easier to analyze retrospectively and can be specifically customized for each patient. At this time there are several projects designing training aids for rehabilitation centers (LOPES exoskeleton, Lokomat, ALTACRO and the gait trainer, Hal 5.)

Exoskeletons could also be regarded as wearable robots: A wearable robot is a mechatronic system that is designed around the shape and function of the human body, with segments and joints corresponding to those of the person it is externally coupled with. Teleoperation and power amplification were said to be the first applications, but after recent technological advances the range of application fields is said to have widened. Increasing recognition from the scientific community means that this technology is now employed in telemanipulation, man-amplification, neuromotor control research and rehabilitation, and to assist with impaired human motor control.

Engineers of powered exoskeletons face a number of large technological challenges to build a suit that is capable of quick and agile movements, yet is also safe to operate without extensive training. One of the largest problems facing designers of powered exoskeletons is the power supply. There are currently few power sources of sufficient energy density to sustain a full-body powered exoskeleton for more than a few hours. Non-rechargeable primary cells tend to have more energy density and store it longer than rechargeable secondary cells, but then replacement cells must be transported into the field for use when the primary cells are depleted, of which may be a special and uncommon type. Rechargeable cells can be reused but may require transporting a charging system into the field, which either must recharge rapidly or the depleted cells need to be able to be swapped out in the field, to be replaced with cells that have been slowly charging.

Internal combustion engine power supplies offer high energy output, but they also typically idle, or continue to operate at a low power level sufficient to keep the engine running, when not actively in use which continuously consumes fuel. Battery based power sources are better at providing instantaneous and modulated power; stored chemical energy is conserved when load requirements cease. Engines which do not idle are possible, but require energy storage for a starting system capable of rapidly accelerating the engine to full operating speed, and the engine must be extremely reliable and never fail to begin running immediately. Engines which are small and lightweight typically must operate at high speed to extract sufficient energy from a small engine cylinder volume, which both can be difficult to silence and induces vibrations into the overall system. Internal combustion engines can also get extremely hot, which may require additional weight from cooling systems or heat shielding.

Electrochemical fuel cells such as solid oxide fuel cells (SOFC) are also being considered as a power source since they can produce instantaneous energy like batteries and conserve the fuel source when not needed. They can also easily be refueled in the field with liquid fuels such as methanol. However they require high temperatures to function; 600 °C is considered a low operating temperature for SOFCs. Most research designs are tethered to a much larger separate power source. For a powered exoskeleton that will not need to be used in completely standalone situations such as a battlefield soldier, this limitation may be acceptable, and the suit may be designed to be used with a permanent power umbilical.

Initial exoskeleton experiments are commonly done using inexpensive and easy to mold materials such as steel and aluminum. However steel is heavy and the powered exoskeleton must work harder to overcome its own weight in order to assist the wearer, reducing efficiency. The aluminium alloys used are lightweight, but fail through fatigue quickly; it would be unacceptable for the exoskeleton to fail catastrophically in a high-load condition by ‘folding up’ on itself and injuring the wearer. As the design moves past the initial exploratory steps, the engineers move to progressively more expensive and strong but lightweight materials such as titanium, and use more complex component construction methods, such as molded carbon-fiber plates. Carbon nano tubes are light weight, ten times stronger, and more heat resistant than titanium.

The powerful but lightweight design issues are also true of the joint actuators. Standard hydraulic cylinders are powerful and capable of being precise, but they are also heavy due to the fluid-filled hoses and actuator cylinders, and the fluid has the potential to leak onto the user. Pneumatics are generally too unpredictable for precise movement since the compressed gas is springy, and the length of travel will vary with the gas compression and the reactive forces pushing against the actuator. Generally electronic servomotors are more efficient and power-dense, utilizing high-gauss permanent magnets and step-down gearing to provide high torque and responsive movement in a small package. Geared servomotors can also utilize electronic braking to hold in a steady position while consuming minimal power.

Flexibility is another design issue, and which also affects the design of unpowered hard shell space suits. Several human joints such as the hips and shoulders are ball and socket joints, with the center of rotation inside the body. It is difficult for an exoskeleton to exactly match the motions of this ball joint using a series of external single-axis hinge points, limiting flexibility of the wearer. A separate exterior ball joint can be used alongside the shoulder or hip, but this then forms a series of parallel rods in combination with the wearer’s bones. As the external ball joint is rotated through its range of motion, the positional length of the knee/elbow joint will lengthen and shorten, causing joint misalignment with the wearer’s body. This slip in suit alignment with the wearer can be permitted, or the suit limbs can be designed to lengthen and shorten under power assist as the wearer moves, to keep the knee/elbow joints in alignment. A partial solution for more accurate free-axis movement is a hollow spherical ball joint that encloses the human joint, with the human joint as the center of rotation for the hollow sphere. Rotation around this joint may still be limited unless the spherical joint is composed of several plates that can either fan out or stack up onto themselves as the human ball joint moves through its full range of motion.

Spinal flexibility is another challenge since the spine is effectively a stack of limited-motion ball joints. There is no simple combination of external single-axis hinges that can easily match the full range of motion of the human spine. A chain of external ball joints behind the spine can perform a close approximation, though it is again the parallel-bar length problem. Leaning forward from the waist, the suit shoulder joints would press down into the wearer’s body. Leaning back from the waist, the suit shoulder joints would lift off the wearer’s body. Again, this alignment slop with the wearer’s body can be permitted, or the suit can be designed to rapidly lengthen or shorten the exoskeleton spine under power assist as the wearer moves.

Control and modulation of excessive and unwanted movement is a third large problem. It is not enough to build a simple single-speed assist motor, with forward/hold/reverse position controls and no on-board computer control. Such a mechanism can be too fast for the user’s desired motion, with the assisted motion overshooting the desired position. If the wearer’s body is enclosed with simple contact surfaces that trigger suit motion, the overshoot can result the wearer’s body lagging behind the suit limb position, resulting in contact with a position sensor to move the exoskeleton in the opposite direction. This lagging of the wearer’s body can lead to an uncontrolled high-speed oscillatory motion, and a powerful assist mechanism can batter or injure the operator unless shut down remotely. A single-speed assist mechanism which is slowed down to prevent oscillation is then restrictive on the agility of the wearer. Sudden unexpected movements such as tripping or being pushed over requires fast precise movements to recover and prevent falling over, but a slow assist mechanism may simply collapse and injure the user inside.

Fast and accurate assistive positioning is typically done using a range of speeds controlled using computer position sensing of both the exoskeleton and the wearer, so that the assistive motion only moves as fast or as far as the motion of the wearer and does not overshoot or undershoot. This may involve rapidly accelerating and decelerating the motion of the suit to match the wearer, so that their limbs slightly press against the interior of the suit and then it moves out of the way to match the wearer’s motion. The computer control also needs to be able to detect unwanted oscillatory motions and shut down in a safe manner if damage to the overall system occurs.

A fourth issue is detection and prevention of invalid or unsafe motions. It would be unacceptable for an exoskeleton to be able to move in a manner that exceeds the range of motion of the human body and tear muscle ligaments. This problem can be partially solved using designed limits on hinge motion, such as not allowing the knee or elbow joints to flex backwards onto themselves. However, the wearer of a powered exoskeleton can additionally damage themselves or the suit by moving the hinge joints through a series of combined and otherwise valid movements which together cause the suit to collide with itself or the wearer. A powered exoskeleton would need to be able to computationally track limb positions and limit movement so that the wearer does not casually injure themselves through unintended assistive motions, such as when coughing, sneezing, when startled, or if experiencing a sudden uncontrolled seizure or muscle spasm.

An exoskeleton is typically constructed of very strong and hard materials, while the human body is much softer than the alloys and hard plastics used in the exoskeleton. An exoskeleton typically cannot be worn directly in contact with bare skin due to the potential for skin pinching where the exoskeleton plates and servos slide across each other. Instead the wearer may be enclosed in a heavy fabric suit to protect them from joint pinch hazards. The exoskeleton joints themselves are also prone to environmental fouling from sand and grit, and may need protection from the elements to keep operating effectively. A traditional way of handling this is with seals and gaskets around rotating parts, but can also be accomplished by enclosing the exoskeleton mechanics in a tough fabric suit separate from the user, which functions as a protective ‘skin’ for the exoskeleton. This enclosing suit around the exoskeleton can also protect the wearer from pinch hazards.

Powered armor has appeared in a wide variety of fiction, beginning with E. E. Smith’s ‘Lensman’ series in 1937. One of the most famous early versions was Robert A. Heinlein’s 1959 novel ‘Starship Troopers,’ which can be seen as spawning the entire sub-genre concept of military ‘powered armor.’ In addition to heightened strength and protection provided by the exoskeleton, other popular features include internal life support for hostile environments, protection from environmental hazards such as radiation and vacuum, weapons targeting systems, firearms affixed directly to the suit itself, and transportation mechanisms that allow the wearer to fly, make giant leaps, or speed by on ground.

In some portrayals of powered armor, the suit is not much larger than a human. These depictions can be described as a ‘battlesuit’ with mechanical and electronic mechanisms designed to augment the wearer’s abilities. Other power armors are portrayed as being much larger, more like a bipedal vehicle the size of a tank or much larger. These latter are frequently termed ‘Mecha,’ a Japanese adaptation of the English word ‘mechanical.’ The line between mecha and power armor is necessarily vague. The usual distinction is that powered armor is form-fitting and worn; mecha have cockpits and are driven, or that powered exoskeletons augment the user’s natural abilities, whilst mechas replace them entirely. However, the line between the two can be difficult to determine at times, especially considering that force feedback systems are often included for delicate maneuvers. Even in a larger mecha meant to be driven like a walking tank rather than worn, a realistic control system would have to be either cybernetic or form-fitting.

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