Guralp Systems CMG-1T - Manual (Page 23)

Operator’s Guide CMG-1T Ocean Bottom
Seismometer
September, 1999, Issue C Section 5,
Page 1
5. LEVELLING PLATFORM
The large three-component CMG-1T seismometer base is fitted inside a
levelling platform which is operated with 2 high torque dc motors.
The operating principle and the philosophy of the levelling platform is based on
the natural stability provided by the bell and socket type construction joint. The
three-component sensors are contained in a machined cavity inside a structure
that resembles an inverted dome whose exterior is machined to a spherical form.
The unit rests in a ring shaped bearing so that the dome can move freely in
azimuth and tilt directions.
A metal post, mounted centrally under the dome, carries a machined ball which
articulates with a cylindrical cavity in a driving block below it. The driving
block, moving on a plane surface below the dome effectively translates the
position of the dome expressed in spherical co-ordinates into positions on a
plane expressed in polar co-ordinates. The described position mechanism
consists of a turntable and a lead screw to drive the central block.
The described mechanism is more stable and more compact than the more usual
gimbal arrangement and also has fewer moving parts. For broadband OBS
seismometer applications the gimbal arrangement would not be sufficiently
stable.
The rotary motion of the bowl is provided by a worm-drive driven by a dc
motor. The tilt drive is provided by a 1 mm pitched lead screw which linearly
positions the bearing block on a pair of parallel slides. The lead screw is also
driven by a dc motor.