each with an axis parallel to the X, Y, and Z reference axes.
This would only work if the vehicle never turns or climbs an
incline, which would instantly reorient the accelerometer
sensor’s sensing axes. We need another component
to keep the orientation of the accelerometers constant
regardless of the motion and orientation of our vehicle.
The gyroscope (a.k.a., gyro) incorporates a rapidly
spinning mass that has the property that it maintains
a fixed orientation in space. Figure 15 shows a basic
gyroscope mounted in a gimbal frame that allows the gyro
to maintain a constant orientation during 3-D motions
of our vehicle. It is now obvious that if we mount three
accelerometers on three gyroscopes with sensing axes and
rotation axes oriented along the X, Y, and Z axes, we will
be able to determine the distance our vehicle has traveled.
Figure 16 shows a practical implementation of the 3-D
INS in which any motion that affects the orientation of the
INS causes error signals to be generated by the gyroscope
which, in turn, drives servo motors that reorient the INS
housing to maintain the accelerometers in their original
Using actual rotating gyros requires a lot of mechanical
fabrication and precision which lends itself to numerous
potential for errors and their ensuing inaccuracies. As
electronics people, we would like a more electronic and
less mechanical INS (less failure prone). The strapdown INS
shown in Figure 17 is attached to the vehicle and the error
signals from the gyros are fed to the computer with the
accelerometer signals to determine the vehicle’s position
regardless of orientation changes.
Earlier, we looked at the MEMS accelerometer which
limited the mechanical elements, improving its accuracy
and reliability, plus reducing its size which allowed it to
be fabricated in IC chip form. We can fabricate a MEMS
gyroscope using a cylindrical resonator, wine glass
resonator, vibrating wheel, or tuning fork arrangement
etched into a silicon substrate. The MEMS gyro uses a
piezoelectric element to generate an electrical signal which
represents the rotation of the gyro. Figure 18 shows the
n FIGURE 15. A Gimbal Frame Allows a Gyroscope to
Maintain Orientation during 3-D Motion.
n FIGURE 17. Strapdown Inertial Navigation System uses
Gyro Error Signals to Correct for 3-D Vehicle Motion.
n FIGURE 18. Tuning Fork Type MEMS Gyroscope.
n FIGURE 16. Gyroscope Based 3-D Inertial Navigation
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