Global Positioning System Reference
In-Depth Information
The devices are worn at the hip and when it rises and falls during walking, the
pendulum (which by inertia tends to stay motionless) touches a switch contact that
in turn feeds an electronic processor. Averaged footfall count, speed, and other
parameters are calculated (see Figure 7.4). The system needs careful calibration
since it works on the basis of average length of footstep. Turning corners,
shuffling, or climbing all lead to errors so the devices are quite inaccurate (5% is
typical). A more sophisticated approach uses an accelerometer chip in place of the
pendulum.
Figure 7.4 Mechanical pedometer.
As the hip rises and falls, acceleration is greatest as it changes direction, and
the accelerometer will therefore give a peak reading. It is normal to filter the
pulses to avoid any anomalies. Accelerometer chips have the advantage of no
moving parts that are subject to friction and wear and are very small. They can
also be used for other useful purposes, for example, to detect the orientation of the
wearer by sensing the force of gravity on up to three axes. An injured person who
is on the ground, for example, would be easy to detect since the footsteps would
have stopped and the orientation of the device would be different. We now
consider how these inertial chips function.
 
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