Biomedical Engineering Reference
In-Depth Information
Fig. 12.5
Example footstep
normal force and synthesized
waveform using the simple
normal force texture algo-
rithm described in the text.
The respective signals were
captured through force and
acceleration sensors inte-
grated in the vibrotactile
display device [
108
]
7.0
0.0
-7.0
0.8
0.4
0.0
1 sec
governed by:
F
(
t
)
=
m
x
¨
+
b
x
˙
+
K
(
x
−
x
0
),
x
0
=
k
2
ξ(
t
)/
K
(12.3)
In the stuck state, virtual surface admittance
Y
(
s
)
=˙
x
(
s
)/
F
(
s
)
is given, in the
Laplace-transformed (
s
) domain, by:
ms
2
)
−
1
Y
(
s
)
=
s
(
+
bs
+
K
,
K
=
k
1
k
2
ξ/(
k
1
+
k
2
)
(12.4)
where
represents the net plastic displacement up to time
t
. A Mohr-Coulomb
yield criterion is applied to determine slip onset: when the force on the plastic unit
exceeds a threshold value (which may be constant or noise-dependent), a slip event
generates an incremental displacement
ξ(
t
)
Δξ(
t
)
, along with an energy loss of
Δ
W
representing the inelastic work of fracture growth.
Slip displacements are rendered as discrete transient signals, using an event-based
approach [
55
]. High frequency components of such transient mechanical events are
known to depend on the materials and forces of interaction, and we model some of
these dependencies when synthesizing the transients [
110
]. An example of normal
force texture resulting from a footstep load during walking is shown in Fig.
12.5
.
12.3.3 Multimodal Displays
Several issues arise in the rendering of multimodal walking interactions, including
combinations of visual, auditory and haptic rendering to enable truly multimodal
experiences. A model of the global rendering loop for interactive multimodal expe-
riences is summarized in Fig.
12.6
. Walking over virtual grounds requires the use of
specific hardware devices that can coherently present visual, vibrotactile and acoustic
signals. Several devices dedicated to multimodal rendering of walking over virtual
grounds are described below, in Sect.
12.3.4
, while examples of multimodal sce-
narios are discussed in Sect.
12.3.5
. Multimodal rendering is often complicated due
to hardware and software constraints. In some cases, crossmodal perceptual effects
