Page | 56
Points to Ponder
For many years there was an assumption that a gyro survey must be better than MWD. But from this section, you can see that that will not necessarily be the case. Older gyros were film read and drifted badly. Ruggedised gyros must sacrifice accuracy because they are made less sensitive to shock and vibration. Some gyros have fanciful claims of accuracy that defeat the laws of physics. Always use reputable companies and be sure to und s nd h l m ons of h ools p o d d. E n f you c n’ und s nd , ask for the calibration sheet to minimise the risk of receiving a bad gyro.
10.
s c Gy o h o y
The purpose of this chapter is to explain the principles of gyroscopes and how such sensors are used specifically in Gyrodata gyro survey tools to determine borehole azimuth. This section was donated to the book by John Weston of Gyrodata , but the operating principles are largely applicable to any commercial rate gyros.
10.1 Fundamental principles
Gyroscopes are used in various applications to sense either the angle turned through by a body (displacement gyroscopes) or, more commonly, its angular rate of turn about some defined axis (rate gyroscopes).
The most basic and the original form of gyroscopes make use of the inertial properties of a wheel or rotor spinning at hghsp d.M nyp opl f ml w h h chld’s oywhchh s h ym l o o suppo dby p ofgmb ls. When the rotor is spun at high speed, the rotor axis continues to point in the same direction despite the gimbals being rotated. This is a crude example of a mechanical, or conventional, displacement gyroscope.
The operation of a conventional spinning mass gyroscopes depend the following phenomena: • gyroscopic inertia • angular momentum • precession.
10.1.1 Gyroscopic inertia This is fundamental to the operation of all spinning mass gyroscopes, as it defines a direction in space that remains fixed. The establishment of a fixed direction enables rotation to be detected, by making reference to this fixed direction. The rotation of an inertial element generates an angular momentum vector which is coincident w h h xsofspnof h oo o ‘wh l’. s the direction of this vector which remains fixed in space, given perfection in the construction of the gyroscope. A practical reference instrument may be designed by having the rotor supported in a set of frames or gimbals which are free to rotate with respect to one another about orthogonal axes as shown in Figure 49. The orientation of the case of the
Figure 49: Schematic representation of a two axis gyroscope.
Made with FlippingBook flipbook maker