Stepper motors keep seismometers on the level in Davy Jones’ locker

July 19, 2012
Geophysicists like to keep tabs on the motion of earth’s tectonic plates and the earthquakes they generate using a device known as a seismometer


Nanometrics Inc.

Geophysicists like to keep tabs on the motion of earth’s tectonic plates and the earthquakes they generate using a device known as a seismometer. But that’s easier said than done when the plates are on the ocean floor.

Most dynamic motion seismometers use an inverted pendulum design that requires leveling prior to use. On dry land, leveling the instrument is easy. On the ocean floor thousands of feet below the surface, the leveling process gets a bit more challenging.

Researchers toss the seismometers into the ocean attached to weighted sleds. The sleds can land on the sea floor in just about any position, often on a muddy surface with an unknown topography. As a result, the mechanical leveling system must be able to right the sensing elements even when the instrument comes to rest upside-down. The seismometers made by Nanometrics Inc. in Canada use three inertial masses aligned on orthogonal axes to let the instruments measure in three dimensions. The three measurement axes within each of the Trillium Compact OBS (seafloor) and Compact All-Terrain (dry land) seismometers rigidly attach to each other. This lets the system level the platform as a whole.

Nanometrics mounts the seismometer in a motorized gimbal. The inner frame rotates the instrument around its own axis. The outer frame then rotates the instrument with respect to the case. Accelerometers on the seismometer and case determine the degree of tilt and, thus, the amount of correction needed. A microprocessor adjusts the motors’ position as required, fully leveling the system in less than 20 min.

The positioning mechanisms needed a high degree of torque to level the instrument. Normally, the easiest way to boost torque is to use a larger motor or add a speed-reduction ratio via a gearmotor. However, the design was space constrained, so a larger motor or gear reducer were not workable options. Increasing the diameter by only a couple of centimeters would force the use of a larger sled. This could make the difference between deploying 10 or 15 instruments on a given cruise, possibly boosting costs by hundreds of thousands of dollars.

Nanometrics worked with MICROMO in Clearwater, Fla., a member of the Faulhaber Group, to develop efficient motors that were compact, had high torque, and the reliability necessary for the seismometers. The design uses two stepper motors controlled by a microprocessor. The leveling algorithm reads the accelerometers to calculate how to make the platform nearly level. Final leveling is then checked using the seismometers.

The use of stepper motors makes the instrument dependable and simple to control. The motors are merely commanded to rotate the instrument to a specific position. The design transfers motion from the motor to the gimbaled seismometer using a worm gear, which promotes a compact and sturdy design. The gear also offers stability, even under exposure to shock and vibration. For example, worm gears are not easily back driven, which protects the gearbox load.

Next, the design team needed to integrate the worm gear with the gearhead. The obvious method was to tie the two together with a setscrew coupling, but the motor-shaft diameter is just 2 mm. In addition, setscrews could loosen over time, potentially preventing the motor from leveling the instrument. MICROMO developed a way to weld the drive gear directly onto the gearbox output shaft. The prewelded gear greatly speeds and simplifies the assembly process.

Though designed for underwater deployment, the motorized levelers also perform their duties on dry land, but with a slightly different twist. Rather than positioning the platform as a whole, the motors adjust each pendulum individually.

Using bubble levels, the installer manually levels the seismometer to within a few tenths of a degree. The additional leveling accuracy provided by the motors means the electronics aren’t working as hard to center the masses. This lets the instrument measure signals at extremely low frequencies, such as the natural resonance of the whole earth.

Seismometer installations on dry land are not without their own punishing conditions. For example, temperature variations deep underwater are fairly minimal. Such is definitely not the case on dry land, where a seismometer deployed at the South Pole faced operating temperatures that varied from 0 to 50°C below zero.

The MICROMO motors were designed to operate at temperatures as low as –58°C (–72°F). The system temperature of the South Pole unit stabilized around –50°C (–58°F), with the leveling motors performing exactly as planned.

© 2012 Penton Media, Inc.

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