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the outer layer and the dermis, and united at the underlying level by a subcutaneous
body tissue (Nicholls, 1992). It also acts as a pain warning system and a temperature
controller of the human body. Skin covers the surface of our body which can be up to
1.5 to 2m 2 in an adult, with an average weight of 5 kg for a density of 1250 kg/m 3 .
There are two main types of skin: glabrous and hairy. Glabrous skin (hairless skin) is
found on the soles of feet and palmar surfaces of hands. The major part of other skin
surfaces is hairy.
All skin functions can be divided considering its permeability or imperviousness
The material: outer cover of the human body, a barrier to the intake of harmful
elements or pathogens, or to the loss of water from the body;
The energy: thermal energy, not only in the phenomena of thermoregulation but
also of mechanical energy since it is the contact surface, for example, between a
limb and the substrate;
The information: base of tactile sense and the crossing point of information
regarding temperature, luminosity, humidity or even the sound vibrations.
3.3.2 Classification of biological sensors
A criterion to classify cutaneous receptors is the reaction to a preferential stimulus.
This classification creates multiple types of receptors of which three are known as
nociceptors, thermoreceptors and mechanoreceptors (Peruchon, 1979). Nociceptors
Nociceptors take care of the sensations of pain and are thus the safety devices indis-
pensable for the protection of the manipulator system. It is difficult to differentiate
between this category of receptors and the other touch receptors. It has been proved
that the same nerve endings are in charge of variations in mechanical or thermal stimuli
and of reproducing pain sensations if the stimulus amplitude crosses a certain level.
However, there are some receptors with a higher trip point and which can thus be
considered as pure nociceptors. Nociceptors can be very active even when there is no
pain; conversely, a pain can be very intense without a significant activation of noci-
ceptors. We all know that a strong emotion, a state of acute stress or even an intense
concentration can ease a sensation of pain. Thermoreceptors
Temperature is a parameter that evidently comes into play during tactile exploration.
Man is a homeothermic animal; he carries out thermal exchanges through conduction,
convection and evaporation. The sensations of heat or cold appear when we deviate
from thermal neutrality. The ambient temperature and also the humidity and wind
have a role in creating these sensations. Thermal sensitivity is due to the thermore-
ceptors in the skin. They can be divided into two groups - cold receptors and heat
receptors, spatially separated on the skin surface (expanse) and in its layers. They
are characterised by a continuous and constant electrical activity in the absence of
thermal variations (Darian-Smith, 1984). The frequency of pulses that they generate
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