tailieunhanh - Sensors and Methods for Mobile Robot Positioning P2

Gyroscopic precession is a key factor involved in the concept of operation for the north-seeking gyrocompass, as will be discussed later. Friction in the support bearings, external influences, and small imbalances inherent in the construction of the rotor cause even the best mechanical gyros to drift with time. Typical systems employed in inertial navigation packages by the commercial airline industry may drift about during a 6-hour flight [Martin, 1986]. Space-Stable Gyroscopes The earth’s rotational velocity at any given point on the globe can be broken into two components: one that acts around an imaginary vertical axis normal to. | Chapter 2 Heading Sensors 31 T I cd where T applied input torque I rotational inertia of rotor cd rotor spin rate Q rate of precession. Gyroscopic precession is a key factor involved in the concept of operation for the north-seeking gyrocompass as will be discussed later. Friction in the support bearings external influences and small imbalances inherent in the construction of the rotor cause even the best mechanical gyros to drift with time. Typical systems employed in inertial navigation packages by the commercial airline industry may drift about during a 6-hour flight Martin 1986 . Space-Stable Gyroscopes The earth s rotational velocity at any given point on the globe can be broken into two components one that acts around an imaginary vertical axis normal to the surface and another that acts around an imaginary horizontal axis tangent to the surface. These two components are known as the vertical earth rate and the horizontal earth rate respectively. At the North Pole for example the component acting around the local vertical axis vertical earth rate would be precisely equal to the rotation rate of the earth or 15 hr. The horizontal earth rate at the pole would be zero. As the point of interest moves down a meridian toward the equator the vertical earth rate at that particular location decreases proportionally to a value of zero at the equator. Meanwhile the horizontal earth rate . that component acting around a horizontal axis tangent to the earth s surface increases from zero at the pole to a maximum value of 15 hr at the equator. There are two basic classes of rotational sensing gyros 1 rate gyros which provide a voltage or frequency output signal proportional to the turning rate and 2 rate integrating gyros which indicate the actual turn angle Udd 1991 . Unlike the magnetic compass however rate integrating gyros can only measure relative as opposed to absolute angular position and must be initially referenced to a known orientation by some .