Micro-g LaCoste FG5 User manual

Table of Contents
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. TEST MASS ........................................................................................................... 2-5
2.1.2. DRIVE MECHANISM ........................................................................................... 2-6
2.1.3. SERVICE RING ..................................................................................................... 2-6
2.1.4. VIEWING PORT.................................................................................................... 2-7
2.1.5. THE DROP ............................................................................................................. 2-7
2.2. The Interferometer ....................................................................................................... 2-9
2.2.1. LASER....................................................................................................................2-9
2.2.2. OPTICS AND BEAM PATH ................................................................................. 2-9
2.3. The Superspring.......................................................................................................... 2-13
2.3.1. SUPERSPRING MASS ........................................................................................ 2-13
2.3.2. SPHERE DETECTOR SYSTEM ......................................................................... 2-13
2.4. The System Controller................................................................................................ 2-15
2.4.1. REQUIRED HARDWARE .................................................................................. 2-16
2.4.2. OPTIONAL HARDWARE................................................................................... 2-16
2.4.3. SOFTWARE ......................................................................................................... 2-16
2.5. Electronics ................................................................................................................... 2-17
2.5.1. TIMING SYSTEM ............................................................................................... 2-17
2.5.2. DROPPER CONTROLLER ................................................................................. 2-19
2.5.3. SUPERSPRING CONTROLLER......................................................................... 2-19
2.5.4. LASER CONTROLLER....................................................................................... 2-20
2.5.5. POWER SUPPLIES.............................................................................................. 2-20
2.5.6. OPTIONAL SYSTEMS........................................................................................ 2-21
2.5.6.1. ENVIRONMENTAL SENSORS.................................................................. 2-21
2.5.6.2. ROTATION MONITOR............................................................................... 2-21
3. HOW TO SET UP AND RUN THE FG5 ........................................ 3-1
3.1. Setting Up the FG5 ....................................................................................................... 3-1
3.1.1. INTERFEROMETER ............................................................................................. 3-1
3.1.2. ELECTRONICS CASE .......................................................................................... 3-2
3.1.2.1. Model AL-1 Laser:.......................................................................................... 3-3
3.1.2.1.1. Warm Up ................................................................................................. 3-3
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3.1.2.1.2. Operation ................................................................................................. 3-3
3.1.2.2. WEO Model 100 Laser.................................................................................... 3-4
3.1.3. DROPPING CHAMBER TRIPOD......................................................................... 3-4
3.1.4. ION PUMP.............................................................................................................. 3-6
3.1.5. ROUGH LEVELING INTERFEROMETER.......................................................... 3-7
3.1.6. DROPPER VERTICALITY.................................................................................... 3-8
3.1.7. BEAM VERTICALITY .......................................................................................... 3-8
3.1.8. VERTICAL ADJUSTMENT OF THE TEST BEAM ............................................ 3-9
3.1.9. REFERENCE HEIGHT ........................................................................................ 3-11
3.1.10. THE SUPERSPRING.......................................................................................... 3-13
3.1.11. TRAVEL LOCK ................................................................................................. 3-14
3.1.12. CONNECT REMAINING SYSTEM CABLES ................................................. 3-14
3.1.13. SUPERSPRING ZERO-POSITIONING ............................................................ 3-16
3.1.14. FRINGE OPTIMIZING ...................................................................................... 3-17
3.1.15. Dropper Controller .............................................................................................. 3-19
3.2. Running the FG5 ......................................................................................................... 3-19
3.2.1. SYSTEM CONTROLLER SETUP....................................................................... 3-19
3.2.2. PROGRAM SETUP .............................................................................................. 3-20
3.3. Shutting Down the FG5 .............................................................................................. 3-20
3.3.1. COMPUTER ......................................................................................................... 3-20
3.3.2. SUPERSPRING .................................................................................................... 3-22
3.3.3. INTERFEROMETER............................................................................................ 3-22
3.3.3.1. OPTION 1: Model AL-1 Laser..................................................................... 3-22
3.3.3.1. ........................................................................................................................ 3-22
3.3.3.2. OPTION 2: WEO Model 100 Laser .............................................................. 3-22
3.3.3.2. ........................................................................................................................ 3-22
3.3.4. DROPPING CHAMBER ...................................................................................... 3-23
3.3.5. POWER ................................................................................................................. 3-23
3.4. Disassembling and Packing the FG5 ......................................................................... 3-23
3.4.1. ELECTRONICS:................................................................................................... 3-23
3.4.2. SYSTEM CONTROLLER.................................................................................... 3-24
3.4.3. ROTATION MONITOR (IF INCLUDED) ......................................................... 3-24
3.4.4. DROPPING CHAMBER ...................................................................................... 3-25
3.4.5. TRIPOD................................................................................................................. 3-25
3.4.6. SUPERSPRING .................................................................................................... 3-25
3.4.7. INTERFEROMETER............................................................................................ 3-26
3.4.8. TURBO PUMP (IF USED) ................................................................................... 3-26
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 ................................................................................. 4-2
4.1.1.4. Adjusting the Drive Belt Tension .................................................................... 4-3
4.1.1.5. Replacing the Ferrofluidic Vacuum Feedthrough............................................4-3
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Table of Contents
4.1.1.6. Replacing the V-Plate...................................................................................... 4-4
4.1.1.7. Replacing the Linear Bearings ........................................................................ 4-5
4.1.1.8. Replacing the Shaft Bearings—Drive Pulley .................................................. 4-6
4.1.1.9. Replacing the Shaft Bearings—Top Pulley..................................................... 4-7
4.1.1.10. Replacing the Rotary Shaft Encoder ............................................................. 4-7
4.1.1.11. Pumping Down the Dropping Chamber........................................................ 4-8
4.2. The Interferometer ....................................................................................................... 4-9
4.2.1. TROUBLESHOOTING THE BEAM PATH ......................................................... 4-9
4.2.2. ALIGNMENT......................................................................................................... 4-9
4.2.2.1. Laser................................................................................................................ 4-9
4.2.2.2. Standard Optical Isolator................................................................................. 4-9
4.2.2.3. /2-Faraday Isolator combination ................................................................... 4-10
4.2.2.4. Optical Isolator Replacement ........................................................................ 4-10
4.2.2.5. Adjusting mirrors 1 and 2-The first two mirrors........................................... 4-10
4.2.2.6. Beam Expander ............................................................................................. 4-11
4.2.2.6.1. Focusing lens:........................................................................................ 4-11
4.2.2.6.2. Collimating lens:.................................................................................... 4-12
4.2.2.7. Leveling the Tripod Tray .............................................................................. 4-12
4.2.2.8. Making the test beam vertical using the alcohol reference surface............... 4-12
4.2.2.9. Leveling the Dropper .................................................................................... 4-13
4.2.2.10. Setting the Bubble Levels on the Tripod..................................................... 4-14
4.2.2.11. Adjusting Mirrors 3 & 4 to steer beam through dropper and make beam
vertical. ..................................................................................................................................... 4-14
4.2.2.12. Adjusting the Superspring position and final adjustment mirror ................ 4-15
4.2.2.13. Final Mirror Translation-Getting Beams Overlapped In Fringe Viewer..... 4-16
4.2.2.14. Adjusting the APD signal detector.............................................................. 4-16
4.2.2.15. Adjusting The Fringe Amplitude ................................................................ 4-17
4.2.2.16. Replace all covers and tighten all screws.................................................... 4-17
4.3. The Superspring.......................................................................................................... 4-18
4.3.1. REPLACEMENTS AND ADJUSTMENTS ........................................................ 4-18
4.3.1.1. Removing the Superspring Cover ................................................................. 4-18
4.3.1.2. Removing the Service Ring........................................................................... 4-18
4.3.1.3. Replacing the Coil (Linear Actuator)............................................................ 4-19
4.3.1.4. The Mass Mainspring/Hanger....................................................................... 4-20
4.3.1.5. Replacing the Flexures .................................................................................. 4-20
4.3.1.6. Assembling the Superspring.......................................................................... 4-22
4.3.1.7. Replacing the Focus Lever Motor ................................................................. 4-24
4.3.1.8. Adjusting the Micro-Switches....................................................................... 4-24
4.3.1.9. The Aneroid Wafer Assembly....................................................................... 4-24
4.3.1.10. The Delta Rods............................................................................................ 4-24
4.4. Timing System and Data Acquisition........................................................................ 4-25
4.4.1. TIMING ................................................................................................................ 4-25
4.4.1.1. Avalanche Photo Diode Printed Circuit Board ............................................. 4-26
4.4.1.2. APD Board and Photo Diode Supply Module............................................... 4-26
4.4.1.3. Scaler-Counter Printed Circuit Board ........................................................... 4-28
4.4.1.4. Rubidium Oscillator ...................................................................................... 4-32
4.4.1.5. Universal Time Interval Counter................................................................... 4-32
4.4.2. DATA ACQUISITION......................................................................................... 4-34
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Table of Contents
4.4.2.1. Computer Interface Cards.............................................................................. 4-34
4.5. DROPPER CONTROLLER ...................................................................................... 4-35
4.5.1. DROPPER CONTROL MODES .......................................................................... 4-36
4.5.1................................................................................................................................. 4-36
4.5.1................................................................................................................................. 4-36
4.5.1.1. STANDBY .................................................................................................... 4-36
4.5.1.1.1. To Select ................................................................................................ 4-36
4.5.1.1.2. Function ................................................................................................. 4-36
4.5.1.1.3. To Deselect ............................................................................................4-36
4.5.1.2. MANUAL...................................................................................................... 4-36
4.5.1.2.1. To Select: ............................................................................................... 4-36
4.5.1.2.2. Function: ................................................................................................ 4-37
4.5.1.3. DROP............................................................................................................. 4-37
4.5.1.3.1. To Select: ............................................................................................... 4-37
4.5.1.3.2. Function: ................................................................................................ 4-37
4.5.1.4. OSCILLATE ................................................................................................. 4-37
4.5.1.4.1. To Select: ............................................................................................... 4-37
4.5.1.4.2. Function: ................................................................................................ 4-37
4.5.2. Analog Servo......................................................................................................... 4-38
4.5.2.1. Cart-Position.................................................................................................. 4-38
4.5.2.2. Cart Velocity.................................................................................................. 4-39
4.5.2.3. Sphere Servo.................................................................................................. 4-39
4.5.2.4. Active Sphere Servo ...................................................................................... 4-40
4.5.2.5. Feedforward Sphere Servo ............................................................................ 4-40
4.5.3. Superspring Controller ..........................................................................................4-40
4.5.3.1. SUPERSPRING CONNECTIONS ............................................................... 4-41
4.5.3.2. MOTOR DRIVE SELECTION..................................................................... 4-42
4.5.3.2.1. OFF ........................................................................................................ 4-42
4.5.3.2.2. AUTO .................................................................................................... 4-42
4.5.3.2.3. WINDOW.............................................................................................. 4-42
4.5.3.2.4. REMOTE ............................................................................................... 4-43
4.5.3.2.5. MANUAL.............................................................................................. 4-43
4.5.3.3. Active Servo .................................................................................................. 4-43
4.5.3.4. Coil Drive (Current Driver) ........................................................................... 4-43
4.5.4. Maintenance Schedule........................................................................................... 4-44
4.5.4.1. Annually ........................................................................................................ 4-44
4.5.4.2. Semi-annually................................................................................................ 4-44
5. TROUBLESHOOTING .................................................................. 5-1
5.1. The FG5 System ............................................................................................................ 5-1
5.2. System Problems ........................................................................................................... 5-1
5.2.1. DROPPING CHAMBER ........................................................................................ 5-1
5.2.2. INTERFEROMETER AND LASER ...................................................................... 5-4
5.2.3. SUPERSPRING ...................................................................................................... 5-7
5.2.4. SYSTEM CONTROLLER...................................................................................... 5-9
5.2.5. ELECTRONICS.................................................................................................... 5-10
5.2.6. ENVIRONMENTAL SENSORS PACKAGE ...................................................... 5-12
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Table of Contents
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 ................................................................................................................ 5-16
6. SWITCHING THE AC POWER ..................................................... 6-1
6.1. LASER ........................................................................................................................... 6-1
6.1.1. WEO Model 100 Laser ........................................................................................... 6-1
6.1.2. AL-1 Laser .............................................................................................................. 6-1
6.2. UTIC .............................................................................................................................. 6-1
6.3. MAIN POWER SUPPLY............................................................................................. 6-2
6.4. Portable Ion Pump Power Supply............................................................................... 6-2
6.5. Computer....................................................................................................................... 6-3
6.6. Docking Station............................................................................................................. 6-3
6.7. IOtech ADC16...............................................................................................................6-3
6.8. Turbo pump .................................................................................................................. 6-3
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
8.1. INSTALLATION.......................................................................................................... 8-1
8.2. THE DATA LOGGING SYSTEM.............................................................................. 8-2
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8.3. TAKING DATA ............................................................................................................ 8-2
9. CHECKLISTS AND LOGS............................................................ 9-1
9.1. Switching the AC Power............................................................................................... 9-2
9.1........................................................................................................................................... 9-2
9.2. FG5 Setup ...................................................................................................................... 9-4
9.3. Dropping Chamber Pump Down and Bake-Out ........................................................ 9-8
9.4. Dropping Chamber Maintenance ................................................................................ 9-9
9.4.1. Removing Chamber Cover ...................................................................................... 9-9
9.4.2. Replacing Dropping Chamber Cover .................................................................... 9-10
9.4.3. Replacing Drive Belt ............................................................................................. 9-11
9.4.4. Adjust Belt Tension............................................................................................... 9-13
9.4.5. Replace Ferrofluidic Vacuum Feedthrough .......................................................... 9-13
9.4.6. Replacing V-plate.................................................................................................. 9-14
9.4.7. Replacing the Linear Bearings .............................................................................. 9-16
9.4.7................................................................................................................................. 9-16
9.4.8. Replacing Shaft Bearings (Drive Pulley) .............................................................. 9-18
9.4.9. Replacing Shaft Bearings (Top Pulley) ................................................................. 9-19
9.4.10. Replacing Rotary Shaft Encoder ......................................................................... 9-20
9.5. Interferometer Alignment .......................................................................................... 9-22
9.5......................................................................................................................................... 9-22
9.6. Superspring Maintenance........................................................................................... 9-23
9.6.1. Removing Superspring Cover ...............................................................................9-23
9.6.2. Replacing Superspring Cover................................................................................ 9-23
9.6.3. Removing Service Ring......................................................................................... 9-24
9.6.4. Replacing Coil....................................................................................................... 9-24
9.6.5. Replacing Flexures ................................................................................................ 9-26
9.6.6. Assembling Superspring........................................................................................ 9-28
9.6.7. Replacing Focus Lever Motor ............................................................................... 9-30
9.6.8. Adjusting Micro-Switches..................................................................................... 9-31
9.7. Packing the FG5 .......................................................................................................... 9-31
10. INDEX........................................................................................ 10-1
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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
1-1
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

Concept and History of the FG5 1
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.
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.
• The system controller has been updated to a 486-type 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.
• Improvements to the electronics reflect new technology and make the
instrument smaller and easier to use.
• A user-friendly full-featured real-time software program takes
interferometer data and environmental data. This software provides an
immediate value for the local gravity in real-time.
• A user-friendly full-featured post-processing software program that
allows complete ability to vary data analysis procedures and to vary
environmental corrections.
• This absolute gravimeter is designed to work with a new rugged iodine-
stabilized laser system (WEO model 100) traceable to the BIPM.
1-2

Design: Components and Function 2
2. Design: Components and Function
The FG5 System (Figure 2-1) consists of a: Dropping Chamber,
Interferometer, Superspring, System Controller, and Electronics. A test
mass is allowed to free-fall inside the evacuated Dropping Chamber. The
Interferometer is used to monitor the position of the freely-falling test mass.
The Superspring is an active long-period isolation device used to provide an
inertial reference for the gravity measurement. The System Controller
(computer) allows a flexible user interface, controls the system, acquires data,
analyzes data, and stores the results. The Electronics provides high accuracy
timing necessary for the measurement and provides system servo control.
Figure 2-1 The FG5 System
2-1

Design: Components and Function 2
2.1. The Dropping Chamber
The Dropping Chamber (Figure 2-2 and Figure 2-3) is an evacuated
chamber which contains the Cart/Drag-Free Chamber which houses the Test
Mass. A Drive Mechanism is used to drop, track, and catch the test mass
inside the drag-free chamber. Laser light (Figure 2-1) passes through a
window in the bottom of the Dropping Chamber to the corner cube (inside
the test mass), then is reflected back down through the window to the
interferometer.
2-2
Figure 2-2 Front view of the dropping chamber