Biomedical Engineering Reference
In-Depth Information
developing identification algorithms may be circumvented by the use of dual hybrid
teleoperation, or newly developed variants that are not based on orthogonal decompo-
sition of the task space into position- and force-controlled spaces. Another interesting
research area is the automatic selection of the position and force-controlled subspaces.
10.6 Basic Issues and Limitations of Teletaction Systems
Teletaction has been defined as being the transmission of cutaneous information from a
remote tactile sensor to an operator's skin through a tactile filter and display. The main
goal of teletaction is to obtain a realistic sense of touch about the shape, hardness, or
texture of an object as if touching it directly. As discussed before, teletaction is a branch
of haptic feedback with another branch consisting of force or kinesthetic feedback. Most
force feedback apparatuses do not provide information about texture, shape, and local
compliance. On the other hand, this information is very important in applications such as
surgery, where the feel of the environment provides knowledge that cannot be obtained by
purely visual means. But, for such critical applications, a teletaction system needs to be
supplied with exacting prerequisites which can be only provided by studying each element
of a system in order to establish that it meets the exacting limits and boundaries required.
Most of research work has, thus far, been dedicated to tactile sensors. Higher resolution,
low-power, and robust tactile sensors capable of detecting the environment through touch
are of profound interest in robotic and prosthetic applications. The latest advancement in
this field, called artificial skin, which is made of nanowires, was developed and introduced
by engineers at the University of California [103]. It is the first such sensor made out
of inorganic single crystalline semiconductors, which are integrated at large scale into
an active matrix backplane on a thin plastic support substrate. This kind of sensor which
works as an artificial skin, would help to overcome a key challenge in robotics by adapting
the amount of force needed to hold and manipulate a wide range of objects [103].
Due to highly demanding mechanical requirements, progress in teletaction display has
been slow. An ideal stimulator requires 50 N cm
−
2
peak pressure, 4 mm stroke and 25 Hz
bandwidth, power density of 5 W cm
−
2
with 1 mm
−
2
actuator density [104]. Two types
of teletaction systems are required in order to generate identical stress and strain patterns
on the fingertips which can be defined, respectively, as being 'stress matching' and 'strain
matching.' An alternative method for teletaction is to produce the object's contour so that
it contacts the human hand [18]. This method has some limitations, since the surface
defects on the tactile sensor are not the same as the object shape [105]. Additionally the
shape display makes it difficult to present shear stresses or tensile forces, which may be
possible with the strain matching approach. It is hard to build a stiff display which feels
like a rigid object, as the elastic layer is still necessary for anti-aliasing purposes that
occur in both the sensor and display parts of a teletaction system.
The physical distance between sensor and display, which causes delays in data transfer,
is the other main issue of teletaction systems. Time delay is an inherent problem within
most network communication and poses serious problems in haptic interaction when a
multiplicity of networks is involved. Sheridan and Ferrell [106] started investigating this
problem as early as 1963, when they found that time delay results in a loss of the sense of
causality between the operators' hands in team operations and action/reaction mismatch
in highly dynamic operations. In the presence of transmission delays, force feedback has
a strongly destabilizing effect.
