Biomedical Engineering Reference
In-Depth Information
Disadvantages
1. Relative angular data are given, not absolute angles, thus severely lim-
iting the data's assessment value.
2. It may require an excessive length of time to fit and align, and the
alignment over fat and muscle tissue can vary over the time of the
movement.
3. If a large number are fitted, movement can be encumbered by the straps
and cables.
4. More complex goniometers are required for joints that do not move as
hinge joints.
3.2.2 Special Joint Angle Measuring Systems
More recently in the area of ergonomics, a special glove system has been
developed to measure the kinematics of the fingers and the thumb. Figure 3.4
shows the construction of the glove transducer, which comprises a lightweight
elastic glove with sensors on the proximal two joints of each finger and thumb
plus a thumb abductor sensor. Each transducer is a loop of fiber-optic cable
with a constant infrared source and is etched in the region of the joint of
interest. As the joint flexes, the fiber bends and light escapes; the greater
the bend, the more light that escapes. Thus, the flexion angular displacement
is detected as a reduction in light intensity received by the detector and is
precalibrated against the bending angle. A major use for such a system has
been in the study of repetitive strain injuries (cf. Moore et al., 1991).
3.2.3 Accelerometers
As indicated by its name, an accelerometer is a device that measures acceler-
ation. Most accelerometers are nothing more than force transducers designed
to measure the reaction forces associated with a given acceleration. If the
acceleration of a limb segment is a and the mass inside is m , then the force
exerted by the mass is F
ma . This force is measured by a force transducer,
usually a strain gauge or piezoresistive type. The mass is accelerated against
a force transducer that produces a signal voltage V , which is proportional
to the force, and since m is known and constant, V is also proportional to
the acceleration. The acceleration can be toward or away from the face of
the transducer; the latter is indicated by a reversal in sign of the signal. In
most movements, there is no guarantee that the acceleration vector will act
at right angles to the face of the force transducer. The more likely situation
is depicted in Figure 3.5, with the acceleration vector having a component
normal to the transducer and another component tangent to the transducer
face. Thus, the accelerometer measures the a n component. Nothing is known
=
Representative paper: Morris, 1973.
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