LRS Micro-g LaCoste FG5 User manual

Table of Figures
1. CONCEPT AND HISTORY OF THE FG5.................................................. 1-1
1.1. The FG5 Absolute Gravimeter................................................................................................... 1-1
1.2. HISTORY...................................................................................................................................... 1-1
1.3. FG5 Design Features .................................................................................................................... 1-2
2. DESIGN: COMPONENTS AND FUNCTION ............................................. 2-1
2.1. The Dropping Chamber .............................................................................................................. 2-2
2.1.1. CART/DRAG-FREE CHAMBER........................................................................................... 2-4
2.1.2. TEST MASS ............................................................................................................................ 2-5
2.1.3. DRIVE MECHANISM ............................................................................................................ 2-6
2.1.4. SERVICE RING ...................................................................................................................... 2-6
2.1.5. VIEWING PORT..................................................................................................................... 2-7
2.1.6. THE DROP .............................................................................................................................. 2-7
2.2. The Interferometer Base.............................................................................................................. 2-9
2.2.1. LASER..................................................................................................................................... 2-9
2.2.2. OPTICS AND BEAM PATH .................................................................................................. 2-9
2.3. The Superspring......................................................................................................................... 2-12
2.3.1. SUPERSPRING MASS ......................................................................................................... 2-12
2.3.2. SPHERE DETECTOR SYSTEM .......................................................................................... 2-12
2.4. The System Controller ............................................................................................................... 2-14
2.4.1. REQUIRED HARDWARE ................................................................................................... 2-15
2.4.2. OPTIONAL HARDWARE.................................................................................................... 2-15
2.4.3. SOFTWARE .......................................................................................................................... 2-15
2.5. Electronics................................................................................................................................... 2-15
2.5.1. TIMING SYSTEM ................................................................................................................ 2-15
2.5.2. DROPPER CONTROLLER .................................................................................................. 2-17
2.5.3. SUPERSPRING CONTROLLER.......................................................................................... 2-18
2.5.4. LASER CONTROLLER........................................................................................................ 2-18
2.5.5. POWER SUPPLIES............................................................................................................... 2-19
2.5.6. OPTIONAL SYSTEMS......................................................................................................... 2-19
2.5.6.1. ENVIRONMENTAL SENSORS....................................................................................... 2-19
2.5.6.2. ROTATION MONITOR.................................................................................................... 2-20
3. HOW TO SET UP AND RUN THE FG5..................................................... 3-1
3.1. Setting Up the FG5 ....................................................................................................................... 3-1
3.1.1. ELECTRONICS CASE ........................................................................................................... 3-1
3.1.1.1. Model ML-1 Laser:.............................................................................................................. 3-2
3.1.1.1.1. Warm Up........................................................................................................................ 3-2
3.1.1.1.2. Operation .......................................................................................................................3-2
3.1.1.2. WEO Model 100 Laser ........................................................................................................ 3-3
3.1.2. SUPERSPRING ....................................................................................................................... 3-5
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Table of Figures
3.1.3. INTERFEROMETER...............................................................................................................3-6
3.1.4. DROPPING CHAMBER TRIPOD ..........................................................................................3-6
3.1.5. ION PUMP ...............................................................................................................................3-7
3.1.6. LEVELING THE DROPPING CHAMBER ............................................................................3-8
3.1.7. CABLE CONNECTIONS ........................................................................................................3-9
3.1.8. THE SUPERSPRING.............................................................................................................3-11
3.1.8.1. TRAVEL LOCK.................................................................................................................3-12
3.1.8.2. SUPERSPRING ZERO-POSITIONING............................................................................3-12
3.1.9. BEAM VERTICALITY .........................................................................................................3-13
3.1.10. FRINGE OPTIMIZING..........................................................................................................3-14
3.2. Running the FG5.........................................................................................................................3-17
3.2.1. SYSTEM CONTROLLER .....................................................................................................3-17
3.2.2. DROPPER CONTROLLER...................................................................................................3-17
3.2.3. PROGRAM SETUP ...............................................................................................................3-17
3.3. Shutting Down the FG5 ..............................................................................................................3-18
3.3.1. COMPUTER ..........................................................................................................................3-18
3.3.2. SUPERSPRING......................................................................................................................3-18
3.3.3. INTERFEROMETER.............................................................................................................3-19
3.3.3.1. OPTION 1: Model ML-1 Laser. ........................................................................................ 3-19
3.3.3.2. OPTION 2: WEO Model 100 Laser ...................................................................................3-19
3.3.4. DROPPING CHAMBER........................................................................................................3-19
3.3.5. POWER ..................................................................................................................................3-19
3.4. Disassembling and Packing the FG5.........................................................................................3-20
3.4.1. ELECTRONICS .....................................................................................................................3-20
3.4.2. SYSTEM CONTROLLER .....................................................................................................3-20
3.4.3. ROTATION MONITOR (IF INCLUDED)...........................................................................3-21
3.4.4. DROPPING CHAMBER........................................................................................................3-22
3.4.5. DROPPING CHAMBER TRIPOD ........................................................................................3-22
3.4.6. INTERFEROMETER BASE.................................................................................................. 3-22
3.4.7. SUPERSPRING......................................................................................................................3-23
3.4.8. TURBO PUMP (IF USED) ....................................................................................................3-23
4. ADJUSTMENT AND MAINTENANCE........................................................4-1
4.1. The Dropping Chamber...............................................................................................................4-1
4.1.1. REPLACEMENTS AND ADJUSTMENTS ............................................................................4-1
4.1.1.1. Removing The Dropping Chamber Cover............................................................................4-1
4.1.1.2. Replacing the Dropping Chamber Cover..............................................................................4-2
4.1.1.3. Replacing the Drive Belt ......................................................................................................4-4
4.1.1.4. Adjusting the Drive Belt Tension .........................................................................................4-5
4.1.1.5. Replacing the Ferrofluidic Vacuum Feedthrough.................................................................4-5
4.1.1.6. Replacing the V-Plate ...........................................................................................................4-6
4.1.1.7. Replacing the Linear Bearings..............................................................................................4-7
4.1.1.8. Replacing the Shaft Bearings—Drive Pulley........................................................................4-8
4.1.1.9. Replacing the Shaft Bearings—Top Pulley ..........................................................................4-9
4.1.1.10. Replacing the Rotary Shaft Encoder...................................................................................4-9
4.1.1.11. Pumping Down the Dropping Chamber ...........................................................................4-10
4.2. The Interferometer .....................................................................................................................4-10
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Table of Figures
4.2.1. ALIGNMENT........................................................................................................................ 4-10
4.3. The Superspring......................................................................................................................... 4-13
4.3.1. REPLACEMENTS AND ADJUSTMENTS.......................................................................... 4-13
4.3.1.1. Removing the Superspring Cover ...................................................................................... 4-13
4.3.1.2. Removing the Service Ring................................................................................................ 4-13
4.3.1.3. Replacing the Coil (Linear Actuator)................................................................................. 4-14
4.3.1.4. The Mass Mainspring/Hanger............................................................................................ 4-15
4.3.1.5. Replacing the Flexures....................................................................................................... 4-15
4.3.1.6. Assembling the Superspring............................................................................................... 4-18
4.3.1.7. Replacing the Focus Lever Motor...................................................................................... 4-19
4.3.1.8. Adjusting the Micro-Switches............................................................................................ 4-19
4.3.1.9. The Aneroid Wafer Assembly............................................................................................ 4-19
4.3.1.10. The Delta Rods................................................................................................................. 4-20
4.4. Timing System and Data Acquisition....................................................................................... 4-21
4.4.1. TIMING ................................................................................................................................. 4-21
4.4.1.1. Avalanche Photo Diode Printed Circuit Board .................................................................. 4-23
4.4.1.2. APD Board and Photo Diode Supply Module.................................................................... 4-23
4.4.1.3. Rubidium Oscillator ........................................................................................................... 4-24
4.4.2. DATA ACQUISITION.......................................................................................................... 4-25
4.4.2.1. Computer Interface Cards .................................................................................................. 4-25
4.5. DROPPER CONTROLLER ..................................................................................................... 4-25
4.5.1. DROPPER CONTROL MODES ........................................................................................... 4-27
4.5.1.1. STANDBY.........................................................................................................................4-27
4.5.1.1.1. To Select ...................................................................................................................... 4-27
4.5.1.1.2. Function ....................................................................................................................... 4-27
4.5.1.1.3. To Deselect .................................................................................................................. 4-27
4.5.1.2. MANUAL ..........................................................................................................................4-27
4.5.1.2.1. To Select: ..................................................................................................................... 4-27
4.5.1.2.2. Function: ...................................................................................................................... 4-27
4.5.1.3. DROP................................................................................................................................. 4-27
4.5.1.3.1. To Select: ..................................................................................................................... 4-27
4.5.1.3.2. Function: ...................................................................................................................... 4-28
4.5.1.4. OSCILLATE...................................................................................................................... 4-28
4.5.1.4.1. To Select: ..................................................................................................................... 4-28
4.5.1.4.2. Function: ...................................................................................................................... 4-28
4.5.2. Analog Servo.......................................................................................................................... 4-28
4.5.2.1. Cart-Position ...................................................................................................................... 4-29
4.5.2.2. Cart Velocity ...................................................................................................................... 4-29
4.5.2.3. Sphere Servo ...................................................................................................................... 4-30
4.5.2.4. Active Sphere Servo........................................................................................................... 4-30
4.5.2.5. Feedforward Sphere Servo................................................................................................. 4-31
4.6. Superspring Controller .............................................................................................................. 4-31
4.6.1.1. SUPERSPRING CONNECTIONS .................................................................................... 4-31
4.6.1.2. MOTOR DRIVE SELECTION ......................................................................................... 4-33
4.6.1.2.1. OFF .............................................................................................................................. 4-33
4.6.1.2.2. AUTO .......................................................................................................................... 4-33
4.6.1.2.3. WINDOW.................................................................................................................... 4-33
4.6.1.2.4. REMOTE ..................................................................................................................... 4-33
4.6.1.2.5. MANUAL.................................................................................................................... 4-33
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Table of Figures
4.6.1.3. Active Servo .......................................................................................................................4-33
4.6.1.4. Coil Drive (Current Driver) ................................................................................................4-34
4.6.2. Maintenance Schedule............................................................................................................4-34
4.6.2.1. Annually .............................................................................................................................4-34
4.6.2.2. Semi-annually..................................................................................................................... 4-34
5. TROUBLESHOOTING ...............................................................................5-1
5.1. The FG5 System ............................................................................................................................5-1
5.2. System Problems ...........................................................................................................................5-2
5.2.1. DROPPING CHAMBER..........................................................................................................5-2
5.2.2. INTERFEROMETER AND LASER........................................................................................5-5
5.2.3. SUPERSPRING........................................................................................................................5-8
5.2.4. SYSTEM CONTROLLER .....................................................................................................5-10
5.2.5. ELECTRONICS .....................................................................................................................5-11
5.2.6. ENVIRONMENTAL SENSORS PACKAGE .......................................................................5-12
5.2.7. ROTATION MONITOR ........................................................................................................5-13
5.3. Gravity Site Selection..................................................................................................................5-14
5.3.1. GEOLOGIC STABILITY ......................................................................................................5-14
5.3.2. SITE STABILITY ..................................................................................................................5-14
5.3.3. ENVIRONMENTAL NOISE.................................................................................................5-14
5.3.4. TEMPERATURE STABILITY..............................................................................................5-14
5.4. AC POWER.................................................................................................................................5-15
6. SWITCHING THE AC POWER ..................................................................6-1
6.1. LASER ...........................................................................................................................................6-1
6.1.1. WEO Model 100 Laser.............................................................................................................6-1
6.1.2. ML-1 Laser...............................................................................................................................6-2
6.2. Portable Ion Pump Power Supply ...............................................................................................6-2
6.3. Computer.......................................................................................................................................6-2
7. PUMPING/BAKING THE CHAMBER ........................................................7-1
7.1. Choosing the Pumping Method....................................................................................................7-1
7.1.1. Cold Pumping the Chamber...................................................................................................... 7-1
7.1.2. Baking Out the Chamber ..........................................................................................................7-1
7.2. Connecting the Turbo Pump to the Dropping Chamber...........................................................7-2
7.3. Heating the Chamber....................................................................................................................7-3
7.4. Starting the Ion Pump ..................................................................................................................7-4
8. THE ROTATION MONITOR.......................................................................8-1
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Table of Figures
8.1. INSTALLATION ......................................................................................................................... 8-1
9. CHECKLISTS AND LOGS ........................................................................ 9-1
9.1. Switching the AC Power .............................................................................................................. 9-1
9.2. FG5 Setup...................................................................................................................................... 9-2
9.3. Dropping Chamber Pump Down and Bake-Out ....................................................................... 9-5
9.4. Dropping Chamber Maintenance ............................................................................................... 9-7
9.4.1. Removing Dropping Chamber Cover....................................................................................... 9-7
9.4.2. Replacing Dropping Chamber Cover....................................................................................... 9-7
9.4.3. Replacing Drive Belt................................................................................................................ 9-8
9.4.4. Adjusting Belt Tension ............................................................................................................ 9-9
9.4.5. Replacing Ferrofluidic Vacuum Feedthrough.......................................................................... 9-9
9.4.6. Replacing V-plate .................................................................................................................. 9-10
9.4.7. Replacing the Linear Bearings ............................................................................................... 9-11
9.4.8. Replacing Shaft Bearings (Drive Pulley) ............................................................................... 9-13
9.4.9. Replacing Shaft Bearings (Top Pulley).................................................................................. 9-14
9.4.10. Replacing Rotary Shaft Encoder ............................................................................................ 9-15
9.5. Interferometer Alignment.......................................................................................................... 9-16
9.6. Superspring Maintenance.......................................................................................................... 9-18
9.6.1. Removing Superspring Cover ................................................................................................ 9-18
9.6.2. Replacing Superspring Cover................................................................................................. 9-18
9.6.3. Removing Superspring Service Ring ..................................................................................... 9-19
9.6.4. Replacing Superspring Coil ................................................................................................... 9-19
9.6.5. Replacing Superspring Flexures............................................................................................. 9-20
9.6.6. Assembling Superspring ........................................................................................................ 9-22
9.6.7. Replacing Superspring Focus Lever Motor............................................................................ 9-25
9.6.8. Adjusting Superspring Micro-Switches ................................................................................. 9-26
9.7. Packing the FG5 ......................................................................................................................... 9-26
10. INDEX ........................................................................................................... 10-1
Figure 2-1 The FG5 System......................................................................................... 2-1
Figure 2-2 Front view of the dropping chamber ...................................................... 2-2
Figure 2-3 Side view of the dropping chamber ........................................................2-3
Figure 2-4 Front view of the cart/drag-free chamber..............................................2-4
Figure 2-5 Side view of the cart/drag-free chamber................................................2-5
Figure 2-6. Side view of the service ring................................................................... 2-6
Figure 2-7 Top view of the service ring. .................................................................... 2-7
Figure 2-8 Side view of interferometer optics and beam path .............................2-11
Figure 2-9 The Superspring. ...................................................................................... 2-13
Figure 2-10 The Superspring sphere detection system..........................................2-14
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Table of Figures
Figure 2-11 Timing diagram...................................................................................... 2-16
Figure 2-12 Rotation Monitor....................................................................................2-21
Figure 3-1: Superspring / Interferometer Setup...................................................... 3-4
Figure 4-1 Drive Pulley Assembly............................................................................. 4-7
Figure 4-2: Interferometer Top Lid.......................................................................... 4-11
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Table of Figures
This Page is Intentionally Blank
vii

Concept and History of the FG5 1
1. Concept and History of the FG5
1.1. The FG5 Absolute Gravimeter
The FG5 absolute gravimeter is a high precision, high accuracy, transportable
instrument that measures the vertical acceleration of gravity (g). The operation
of the FG5 is simple in concept. A test mass is dropped vertically by a
mechanical device inside a vacuum chamber, and then allowed to fall a distance
of about 20cm. The FG5 uses a laser interferometer to accurately determine the
position of the free-falling test mass as it accelerates due to gravity. The
acceleration of the test mass is calculated directly from the measured trajectory.
The laser interferometer generates optical interference fringes as the test mass
falls. The fringes are counted and timed with an atomic clock to obtain precise
time and distance pairs. These data are fit to a parabolic trajectory to give a
measured value for g. This method of measuring gravity is absolute because the
determination is purely metrological and relies on standards of length and time.
The distance scale is given by a frequency stabilized helium neon (HeNe) laser
used in the interferometer. A rubidium atomic time-base provides the time scale
used for the accurate timing. The value of gravity obtained with the FG5 can be
used without the loop reductions and drift corrections normally required when
using relative instrumentation.
1.2. HISTORY
The FG5 is a new generation of absolute gravimeter based on technology
developed over the last thirty years by Dr. James Faller of the National Institute
of Standards and Technology (NIST), and his colleagues. Beginning with a
white-light-fringe interferometric system built in 1962, Faller and coworkers have
continuously improved the designs of the instruments. The most recent
predecessors of the FG5 was the series of six JILAg gravimeters, built in 1985 at
the Joint Institute of Laboratory Astrophysics (JILA), with support from NIST,
the Defense Mapping Agency (DMA), the National Oceanographic and
Atmospheric Administration (NOAA), the Canadian Geophysical Survey (GSC),
the University of Hanover Institute for Earth Measurement, Germany, the
Finnish Geodetic Institute, Finland, and the University of Vienna Institute for
Metrology and Geophysics, Austria.
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Concept and History of the FG5 1
1.3. FG5 Design Features
The FG5 incorporates a number of significant advancements in design which
reduce or eliminate systematic errors identified in the earlier versions, and which
make the FG5 easier to use. These improvements are:
• An inline interferometer beam path which eliminates systematic errors
from tilt-induced path length changes.
• Complete redesign of the Superspring, a device for providing an inertial
mass that contains a retroreflective corner cube. The new Superspring has
improved performance, and at the same time greatly reduced size. The drift
problems of earlier designs have been reduced substantially.
• Completely new tripod design, which supports the test chamber, for extra
stability. The tripod is now built symmetrically with respect to the drop line.
• Improvements to the electronics reflect new technology and make the
instrument smaller and easier to use.
• This absolute gravimeter is designed to work with a new rugged iodine-
stabilized laser system (WEO model 100) traceable to the BIPM.
• The system controller has been updated to an Intel-based personal
computer with a standard language interface. The decision to use standard
PC technology has allowed the FG5 to offer more computing power while
reducing the size of the instrument.
• “g”, a user-friendly, Windows©-based, full-featured software program
takes interferometer data and environmental data. This software provides an
immediate value for the local gravity in real-time. The program is also a full-
featured post-processing software program that allows complete ability to
vary data analysis procedures and to vary environmental corrections.
1-2