M.H. Lee, H.R. Nicholls
Mechatronics, No. 9, 1999
Summary
In
industrial applications, contact interaction are an important feature of
physical manipulation systems, but research in the field of tactile sensors has
undergone a drastic slowdown after the ’80, when is was supposed that it would
had been a fundamental sensor for the following decades. A tactile sensor is
therefore defined as a device that can measure a property of an object through
the contact with it. The other 4 sensing modalities are basically all advanced
today in technology and even computer vision has became cheaper.
The difficulties which
have somehow stopped research in tactile sensors is due to the fact that in
human being this sensing modality isn’t localized, it’s complicated to
transduce and it’s difficult to imitate. In industry some basic forms of
sensing, such as “spatial switches” are quite common and easily accessible,
therefore they are not in the matter of this stat of art survey. In the 90’ the
studies directed the transducing methods to the following technologies, not
basically available: Resistance and Conductance, Capacitance, Piezoelectric and
Pyroelectric, Magnetic, Magnetoelectric, Mechanical, Optical, Ultrasonic and
strain gauges. Interesting researched have been performed in cutaneous sensor,
which are basically divided in to extrinsic (mounted at or near the contact
interface) and intrinsic sensing (which consist in derivation of contact data
from force sensing within a mechanical structure), this study covers about the
former one, which doesn’t deal with force/torque sensors. An important method
for obtaining cutaneous sensor is by using array of integral sensing elements,
which have been demonstrated to be capable of having a spatial resolution of
2-4 mm (Beebe). Gray and Fearing reported an 8 ⨯ 8 capacitive
fabricated array of 1mm2 area.
One
of the major problems regard inverse analysis, which is the issue of computing
the changes on the surface from the sensed data gathered remotely through the
elastic medium, since there isn’t a unique solution.
Artificial
sensing fingers appear to be another interesting application for exploration
and grasping, in this field at least two types of tactile sensors are
considered: one for contact point localization and one for detecting more
spatially diffuse dynamic events, such as contact slip.
Soft
materials are becoming an interest matter for tactile sensing research and
gels, followed by powders appear to be the best material in terms of impact and
strain energy dissipation, conformability to surface and hysteresis effects.
Also the fact that human tissue is composed by electrolytic materials have
inspired researched such as Sawahata, Gong and Osada to use polyacrylamide,
which, with similar mechanical properties, can capture the electrical change
(piezoelectric effect). Tactile sensor can also reduce kinematic errors in
stiffness control by locating precise contact point and tracking changes, being
useful for dexterous multi-fingered hands (Howe).
Whiskers
have also objects under study, in fact they appear to be fast, accurate and
cheap, essentially being single point sensors. Son, Cutkosky and Howe
demonstrated how intrinsic and extrinsic tactile sensors can be effective with
less than 1 mm error contact location. Tactile sensors appear to have
application also in haptic perception (integration of cutaneous surface sensing
with information from position and movement variable of the manipulator),
teleoperation (remote human operating a robot) and virtual reality (for which
multi-sensor gloves or other actuators have been creator to provide tactile
sense to the operator). Processing of the data my be with fuzzy logic,
rule-based systems or model-based systems, but neural networks appear to be the
fastest.
Key
Concepts
Tactile Sensors
Key Results
Toyota ins an example of
organization pushing workers to have “safe partnership” with robots, in order
to achieve this either intrinsically safer equipment must be provided (Tobita
et al.) or there must be comprehensive collision avoidance (Suita et al.),
therefore tactile sensors would be a fundamental tool for human robot
interaction ensuring reliability and safety conditions.
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