Page 308 - Handbook of Biomechatronics

300 Andres F. Ruiz-Olaya et al.

This especially applies to exoskeletons that are wearable and kinematically
equivalent to the human arm. Typical biomechanical effects that cannot eas-
ily be captured within a human arm model used for exoskeleton develop-
ment include (Schiele and van der Helm, 2006):
• The intersubject variability of human limb link parameters (Denavit-
Hartenberg parameters such as length of bones, distances between
rotation axes, orientations of rotation axes).
• The variability within an individual subject of joint centers of rotation
during movement. This can cause misalignments in the joints axes of
exoskeleton and human joints.
• The intersubject variability of body segment dimensions: mass, size,
volume, and so forth.
The unavoidable kinematic incompatibility between the robot and the
human limb can cause several problems, such as unwanted reaction forces
in the human joints, shear forces, and additional pressure at the attachment
points. A key aspect of human-exoskeleton interaction relies on an adequate
transmission of mechanical power generated by exoskeleton to the human
body. Transmitting power from the device to the human body is challenging
because biological tissues and interfaces deform and displace when forces are
applied, absorbing power. Thus, a part of the mechanical power generated
by the exoskeleton is not used for the enhancement of human motor per-
formance, but is absorbed in compression of soft tissues, or lost to unwanted
effects (i.e., skin/tissue stretch and slippage of the exoskeleton with respect
to the skin).
Effective ways for the exoskeletons to transmit mechanical power to the
body are essential. Exoskeletons interact with the human body by means of
multiple physical contact points, frequently using a wide physical interface
such as a cuff or an orthosis to smoothly transmit the loads to the user. The
human-robot physical interface should be designed to provide a safe and
comfortable interaction, while transmitting the torque/force to the human
body. Conventional “shell and strap” style attachments are found on most of
the developed exoskeletons in the literature. These systems consist of a rigid
(or semirigid) shell with one or more strap-style fasteners and padding for
subject comfort.

3.3 Technologies in Exoskeletons
Robotic exoskeletons involve sensors, actuators, mechanical structures,
algorithms, and control strategies capable of acquiring information to exe-
cute a motor function. A key feature of exoskeletons is the direct interaction