Biomedical Engineering Reference
In-Depth Information
6.2.4.4 Inertial Systems
Inertial systems include
accelerometers
and
gyroscopes
and are based on the principle
of measurement of inertia. Accelerometers operate on a mass-spring principle. Two
charged plates are separated and a capacitance or resistance between them is given
as a function of their separation. One plate is suspended over the other on flexible
mounting and acceleration causes this mounting to flex giving a change in plate
separation. The change in capacitance or resistance is measured and the change in
separation calculated. The second derivative of the change in separation with respect
to time gives the acceleration at the attachment point [
57
].
Gyroscopes are devices used for measuring orientation and can be used in gait
analysis to give segment orientation. In order to obtain the limb orientation the angular
acceleration must be integrated twice with respect to time and this will amplify any
initial errors. The sensors themselves are small and lightweight, and can detect a large
range of angular velocities. If gyroscopes are combined with accelerometers then the
data can be used to easily obtain the kinematics of the subject's movement [
57
,
60
].
6.3 Simulating Virtual Anatomical Humans
6.3.1 Rigid Body Physics and Muscular Actuation
Physical simulation offers the possibility of truly responsive and realistic animation.
We have observed in the last decade a renewed interest in the use of physical simula-
tion for interactive character animation and simulation, and many recent publications
demonstrate tremendous improvements in robustness, visual quality and usability.
For a detailed review about physics-based character animation, we refer the reader
to the survey paper [
61
]. In a physics-based system, virtual humans need forces and
torques to actively move around. In order for such forces or torques to be realistic,
they must originate from within the character. We use the term
actuators
to describe
the mechanisms that generate the forces and torques that make a character move.
Common frameworks make use of a combination of joint torques (straightforward
DOF actuation model), external forces (e.g. to control the global translation and ro-
tation) and virtual forces (joint torque emulation of external force effect). In addition
to these actuation schemes, a fair amount of recent works are dedicated to actively
actuate virtual anatomical humans through simple muscle actuators.
Biological systems are actuated through contraction of muscles, which are at-
tached to bones through tendons. When muscles contract, they generate torques at
the joint(s) over which they operate, causing the attached limbs to rotate in oppo-
site directions. In addition to contracting when activated, muscles (and tendons, in
a lesser degree) have the ability to stretch. This makes them behave like unilateral
springs and allows for an efficient mechanism for energy storage and release. Since
muscles can only pull, at least two muscles are required to control a single DOF
