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Introduction
Exoskeletons are powered robotic devices that can be worn by humans to enhance their strength or restore movement. Developments are principally being driven by two applications: to increase the strength and load-handling capabilities of the able-bodied and to restore or enhance the movement and strength of individuals suffering from disabilities or injuries. Understandably, there has been much interest from the military, where the technology could enhance the performance of personnel in the battlefield and also assist in the rehabilitation of those injured in combat.
Efforts to construct practical exoskeletons can be traced back to 1965 when General Electric was commissioned by a joint US Army/Navy initiative to develop the "Hardiman", which was intended to allow the wearer to lift loads of 1,500 pounds (680 kg). This never reached production due to technological limitations of the day such as a lack of high-speed signal processing and in the intervening half century, interest in exoskeletons had waxed and waned. Today, the technology is in its ascendancy, in part due to technological developments and also with the realisation that exoskeletons could confer real benefits to the disabled and a growing ageing population.
Technological issues
A major difference between an exoskeleton and most other forms of robot is that its motion and actions must follow exactly those intended by its wearer. This requires advanced position/force sensors, precise actuator control and high-speed signal processing. Data are then fed to a series of actuators which create motion. While sensor and signal processing technologies are now sufficiently well-developed, the power source often remains problematic, as there are currently few practical sources with sufficient energy density to sustain the operation of an exoskeleton for more than a few hours. While this is usually adequate for lower-body, medical exoskeletons, it is insufficient for full-body/military applications. Available power sources include non-rechargeable and rechargeable batteries and small internal combustion engines; all having respective benefits and limitations. Electrochemical fuel cells are also being considered, and wireless energy transfer, an emerging technology, is another potential, longer-term solution. Many research designs are tethered to a separate power source, and for an exoskeleton that will not need to operate in a completely stand-alone mode, this may be acceptable.
Actuators are also problematic in some applications. Standard hydraulic cylinders are...