Page 347 - Handbook of Electronic Assistive Technology

336 HANDBOOK OF ELECTRONIC ASSISTIVE TECHNOLOGY

injuries. It should be customisable to an individual’s own contours and anatomical needs
(Chen et al., 2016). The contact method, contact intensity and contact areas on the body
should be considered in the design. The interface should also ideally tap into the user’s
residual capabilities.
Control strategies: The strategies that have been employed for controlling robotic
exoskeletons are conventional control, intuitive control and biosignal integration control.
Typically, exoskeletons are controlled using conventional systems such as joysticks, but-
tons, steering wheels and touchscreens. To develop the exoskeleton to function with more
accuracy to user intent, intuitive control becomes important – this is where the user is able
to control the device with use of motion, gesture, eye movement and force.
Actuation mechanisms: Ideally, the robotic exoskeleton should generate natural move-
ments of the limb with the wearer not subjected to any vibration, jerk or sudden motion
change. The choice of actuator has a significant effect on the performance of robots in
terms of the generated output force/torque, efficiency and portability (Huo et al., 2016).
For the active joints of lower limb exoskeletons, actuators with a small volume, a high
power-to-weight ratio, high efficiency and compliance are needed (Chen et al., 2016).
Current actuator mechanisms used in robotic exoskeletons are:

1� Electric motors – these are the most widely used due to their relatively high power
output; they are easy to power through portable rechargeable batteries and can be
controlled by analog or digital input signals from a control circuitry system (Maciejasz
et al., 2014).
2� Pneumatic actuators – these are powered by compressed air. They are lighter than
electric actuators and have lower inherent impedance but are harder to control due
to their nonlinear nature. Since they require pressurised air, the overall size of the
system is increased by the size of the compressor. Pneumatic actuators are suitable in
applications where the system remains stationary (Weightman et al., 2014).
3� Hydraulic actuators – these are powered by hydraulic pressure. They have a high
torque output and are very sensitive and responsive (Maciejasz et al., 2014).
However, their weight, predisposition to fluid leakages and larger size makes
them less favourable choices for robotic rehabilitating applications (Gopura et al.,
2011). Due to their high power output-to-weight ratio, hydraulic and pneumatic
actuators are generally suitable for exoskeletons for human performance
augmentation (Huo et al., 2016).
4� Pneumatic muscle actuators (PMAs) – these actuators are commonly used and
consist of a rubber inner tube surrounded by a braided mesh shell with flexible, but
nonextensible, threads. When the inner tube is pressurised it expands in a balloon-
like manner but the expansion is constrained by the braided shell (Tsagarakis and
Caldwell, 2003). As the volume of the inner tube increases with the increase in
pressure, the pneumatic muscle shortens and/or produces tension if it is coupled to
a mechanical load. Due to such physical configuration, PMAs have generally lower
weight compared to other actuators, but also have slow and nonlinear dynamic