Biomedical Engineering Reference
In-Depth Information
operated switches, used for lamps, dental equipment, or other machines, is often
accompanied by transient clicks accompanying the engagement solid mechanical
elements. The discrete quality of these mechanical signals contrasts with the more
continuous nature of those generated by a step onto natural ground coverings, such
as gravel, dry sand, or branches. Here, discrete impacts may not be as apparent,
and can be accompanied by both viscoelastic deformation and complex transient,
oscillatory, or noise-like vibrations generated through the inelastic displacement of
heterogeneous materials [
45
]. A few of the processes that can be involved include
brittle fracture and the production of in-solid acoustic bursts during rapid micro-
fracture growth [
1
,
2
,
45
], stress fluctuations during shear sliding on granular media
[
5
,
19
,
72
,
73
], and the collapse of air pockets in soil or sand.
A series of mechanical events can be said to accompany the contact of a shoed
foot with the ground. There may be an impact, or merely a soft landing, according
to the type of shoe, the type of ground, and the stride of the walker. Once the initial
transitory effects have vanished and until the foot lifts off the ground, there may
be crushing, fracturing, or little movement at all if the ground is stiff. There may
also be slipping if the ground is solid, or soil displacement if the ground is granular.
There may be other mechanical effects, such as the compacting of a compressible
ground material (e.g., soil, sod, snow). The question of what form of haptic signal
to reproduce in virtual reality applications is therefore not so simple to answer. The
sense of touch is nearly as refined in the foot as it is in the hand. It has, in fact,
great discriminative acumen, even through a shoe sole [
39
]. However, like vision or
audition, in accordance to the perceptual task, it may be satisfied by relatively little
input. In the case of foot, our habit to wear shoes plays in our favor since shoes filter
out most of the distributed aspects of the haptic interaction with the ground, save
perhaps for a distinction between the front and back of the foot at the moment of the
impact. In that sense, wearing a shoe is a bit like interacting with an object through
a hand-tool. The later case, as is well known, is immeasurably easier to simulate in
virtual reality than direct interaction with the hand. When it comes to stimulating the
foot, the options are intrinsically limited by the environmental circumstances. While
it is tempting to think of simulating the foot by the same methods as those used to
stimulate the hand [
43
,
44
], this option must be discarded in favor of approaches that
are specific to the foot. In particular, options involving treadmills, robot arms and
other heavy equipment will remain confined to applications where the motor aspects
dominate over the perceptual aspects of interacting with a ground surface [
10
,
20
,
48
,
79
].
Broadly speaking, then, foot-ground interactions can be said to be commonly
accompanied by mechanical vibrations with energy distributed over a broad range
of frequencies (see Fig.
12.2
). High-frequency vibrations can originate with a few
different categories of physical interaction, including impacts, fracture, and sliding
friction. The physics involved is relatively easy to characterize in restricted settings,
such as those involving homogeneous solids, but becomes more complex to describe
when disordered, heterogeneous materials are involved.
