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"A Whole Lot Less Shakin' Going On..." HEPI Installation Nears Completion

"A Whole Lot Less Shakin' Going On..." HEPI Installation Nears Completion

- Contributed by Ken Mason

The hydraulic external pre-isolator (HEPI) prototype has been installed and is in its final phase of development at the LIGO Advanced System Test Interferometer (LASTI) facilities here at MIT. A need has developed at the two LIGO Observatories to reduce the seismic noise created by lumberjacks, wind, trucks, and other outside disturbances. These vibrations can cause the interferometers to lose locking which reduces the "up-time" of the instruments. HEPI is an active isolation system. The controller for the external pre-isolators monitors the ground and payload then sends a signal to each of the actuators. The actuators offset the ground motion and hold the optics table motionless.

HEPI Pre-isolator. MEPI Pre-isolator.

The HEPI system differs from the previously installed Magnetic External Pre-Isolator (MEPI) system only by the actuator being used to push on the stacks. Where the MEPI system used an electromagnetic actuator, the HEPI system uses hydraulics. All other components at the chambers (offload springs, housing, sensors, controller) remain the same, allowing a comparison of the results. The photo at left above shows an HEPI pre-isolator mounted to the Beam Splitter Chamber pier at the LASTI facilities, the photo at right shows a MEPI pre-isolator mounted on one of the Horizontal Access Module piers.

The power to move the six-ton stack is supplied by a pump station, which was designed and built at Caltech. Running through the pump is a clear, food grade, and water-soluble hydraulic fluid. Tubing and manifolding deliver the fluid to eight actuators where a control valve on each actuator creates the pressure differential required to move the payload in six degrees of freedom. The hydraulic actuator and much of the technology were developed at Stanford University.

Hydraulic Pump Station. Tubing and Manifold.
Valve Panel. Hydraulic Actuator.

Each of the four piers has two actuators, two inertial sensors, and two displacement sensors oriented in the vertical and tangential directions. These feedback sensors are combined with an STS-2 triaxial seismometer positioned on the ground to accurately position and thus isolate the platform from ground vibrations.

Richard Mittleman.

Results of the isolation data gathered for both the HEPI and MEPI system should be available shortly. Watch this column for further developments!

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