GB1240830A - Improvements in or relating to indicating instruments - Google Patents

Abstract

1,240,830. Inclinometers. SCIENTIFIC DRILLING CONTROLS. Oct. 7, 1968 [Oct. 5, 1967], No.45355/67. Heading G1N. An inclinometer for monitoring the axis of a borehole comprises a number of mutually perpendicular transducers for measuring rotation about the vertical and deviation from the magnetic North. As shown, Fig.2, the transducers are located along the axis AA' of a cylindrical aluminium casing la and consist of two electrolytic inclinometers 8 and two fluxgates 9 measuring along parallel axes perpendicular to the main axis AA'. Each inclinometer 8 comprises a curved, sealed tube 11, Fig. 4 partially filled with electrolyte 12, with end electrodes 14 and a central electrode 15. The two halves of the transducer, forming variable resistors, are connected in an A.C. bridge circuit, the unbalance indicating the angle of twist about the appropriate horizontal axis. Each fluxgate 9 comprises two coils 20, Fig. 5 wound on parallel cores 19 and connected in an A.C. bridge circuit. The normal bridge current saturates the cores for half of each half-cycle, and the axial component of the earth's magnetic field varies this saturation time to produce a second harmonic output from the bridge, which is demodulated to give a D. C. signal voltage proportional to the axial component of the earth's field. The signals from the two inclinometers and the two fluxgates are combined to determine the inclination of the main axis AA' with respect to the vertical and the magnetic North. In another embodiment, Fig.6, deviation of the main axis from the vertical is sensed by a pendulum 67 with a weight 69 at one end, attached to the casing by a rod with a reduced neck portion 68 of square cross-section, each face of which carrying a resistance strain gauge 66. Opposite pairs of strain gauges are connected in bridge circuits to determine angular displacement about two mutually perpendicular horizontal axes. The direction of the main axis relative to the magnetic North is sensed by three mutually perpendicular fluxgates 72, each producing a D.C. feedback substantially equal and opposite to the external field component being measured. For use in a deep borehole the transducer assembly is mounted in a cylindrical casing 57 in a pressure casing 58, and is provided with a slot 60 and pin 62 for accurate angular location. The D. C. signals developed by the transducers are converted to sequential pulses separated by intervals proportional to the magnitude of the signals for transmission to surface equipment, where the pulse intervals are measured by a counter for digital display, and also converted to analogue values for storing or memory devices, e.g. capacitors, and fed into computers where the attitude of the borehole axis is calculated.