components are connected through hinged joints which permit rotational
motion about the joint. The gyroscope is the only sensor that can
directly measure that rotation.
Traditional gyroscopes are rather large and contain
rapidly spinning masses which actually exert a gyroscopic force on the
object which they measuring. Both their size and interaction forces
make them unsuitable for mounting on the human body, particularly at
the extremities where they would greatly influence the motion of the
hand, foot, etc.
new and unique Motus gyroscope is a miniature sensor
that contains a micro machined quartz tuning fork and 6,400 solid state
electronic components. It operates on the Coriolis principle and
measures true inertial angular rate. Its influence on what is being
measured is negligible since it is small and contains no rotating parts.
not affect the Motus gyroscope, but directly affects
accelerometers which are typically designed to sense Earth's
gravitational acceleration. The gravity insensitive feature of the Motus gyroscope
greatly simplifies test protocols. For example, using it, one can
measure a hand tremor with the arm/hand held vertically or in a
horizontal plane, yielding the same clear tremor measurement. The Motus
most tests possible where they were previously thought to be impossible.
(sometimes termed an angular rate
sensor) is easy to use, requires
little training and can eliminate the need for multi-camera
photographic systems with their attendant need for complicated analyses
and coordinate transformations. Unlike the camera systems, the Motus system is
portable and can be easily taken to various other clinics, even into
the patient's home.
Movement at the very low frequencies typically found
in human motion can be measured by the Motus system which will measure down to DC
or zero hertz. It also measures frequencies up to 70 hertz, typically
far above those found in human movement. The Motus system then filters them so that the
response is flat to 30 hertz and displays the data from 0 to 20 hertz
on the monitor. Note that other sensors often do not have a DC
The location of the Motus gyroscope is uncritical in certain
types of tests on rotating limbs since there is no response to
"tilting" as seen in accelerometer use. Furthermore, since the
gyroscope measures true inertial angular rate it can, for example, be
placed at either end or in the middle of a lower leg and achieve
identically correct results.
Since the gyroscope measures angular rate in degrees
per second, a simple integration will yield angular position.
Accelerometers require two integrations to get position and can easily
introduce significant errors in the double integration computation due
not only to sensor drift but also to the mixing of Earth's gravity
signals with the desired human motion signals.
The Motus gyroscope uses a single crystal
quartz element as its key component, thus benefiting from the long term
stability of quartz. This design feature results in excellent long term
stability of calibration, eliminating the need for periodic
recalibration and yielding highly repeatable measurements of rotational