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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Table of Contents
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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Table of Contents
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
vi

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

Design: Components and Function 2
Figure 2-3 Side view of the dropping chamber
CART/DRAG-FREE CHAMBER
2-3
The cart /drag-free chamber (Figure 2-4 and Figure 2-5) houses the test mass.
The purpose of the drag-free chamber is to reduce the residual air drag inside
the evacuated dropping chamber. The chamber also reduces magnetic and
electrostatic forces on the test mass, and provides a convenient method for

Design: Components and Function 2
dropping and catching the test mass, as well as returning it to the top of the
chamber for the next drop. A Light Emitting Diode (LED) , located on the
cart, directs light through an optical glass sphere attached to the test mass.
The sphere focuses the light onto a linear detector, also mounted on the cart.
This system senses the position of the cart with respect to the test mass. A
servo-motor/drive belt system (Figure 2-2) moves the cart inside the
Dropping Chamber, using active feedback from the position sensor to
maintain the cart in a constant position relative to the test mass during free-
fall. Since there is essentially no relative motion between the test mass and
the drag-free chamber, the effects of residual air drag are eliminated.
Figure 2-4 Front view of the cart/drag-free chamber
2-4

Design: Components and Function 2
Figure 2-5 Side view of the cart/drag-free chamber
2.1.1.TEST MASS
The Test Mass (Figure 2-4 and Figure 2-5) is a retroreflective corner cube
surrounded by a support structure and balanced at the optical center of the
corner cube. The corner cube is a three-surface mirror which has the special
optical property that the reflected beam is always parallel to the incident
beam. In addition, the phase shift of the reflected beam is virtually constant
with respect to any slight rotation or translation of the corner cube around its
optical center1.
2-5
1Peck, Edson, J. Opt. Soc. Amer., 38, (1948)

Design: Components and Function 2
2.1.2.DRIVE MECHANISM
The drive mechanism (Figure 2-2) is a support structure inside the dropping
chamber on which the cart/drag-free chamber travels up and down, driven
by a DC servo motor.
2.1.3.SERVICE RING
The Service Ring (Figure 2-6 and Figure 2-7) is the base of the Dropping
Chamber. It provides connection and mounting for the following:
• A bellows-type vacuum valve for the initial evacuation of the vacuum
system
• A Ferrofluidic rotary vacuum feedthrough which connects the motor
shaft to the cart drive mechanism
• A servo motor/rotary shaft encoder assembly which moves the cart
and senses its position
• An electrical vacuum feedthrough which allows connection of the test
mass tracking electronics to the controller
• An ion pump, mounted on a 2¾” Conflat flange, which maintains the
vacuum once the chamber has been evacuated by the roughing pump
• Spare 2¾” Conflat and Mini-Conflat flanges are blanked off, and can
be used for additional vacuum accessories
2-6
Figure 2-6. Side view of the service ring.

Design: Components and Function 2
Figure 2-7 Top view of the service ring.
2.1.4.VIEWING PORT
The viewing port (Figure 2-2 and Figure 2-3) is located in the top flange of the
dropping chamber. It allows visual observation of the dropping chamber
interior when the rotation monitor is not fitted to the system. The rotation
monitor (when fitted to the system) is mounted to the top flange of the
dropping chamber, directly above the viewing port. When the rotation
monitor is not mounted, a cover for the port is used to exclude ambient light
from the interior of the dropping chamber during measurements.
2.1.5.THE DROP
2-7
In drop mode, a signal from the computer to the dropper controller initiates
the drop sequence. The cart drag-free chamber is driven slowly from its
bottom position to the “hold” position at the top of the drop. A second pulse
initiates the drop, and the cart accelerates downward at more than 1 g,
leaving the test mass in free-fall.

Design: Components and Function 2
When the cart has traveled about 5 mm downward from the hold position (as
measured by the shaft encoder) a separation of about 3 mm between the cart
and test mass has been achieved. The dropper controller then uses feedback
from the linear detector to maintain this separation for the remainder of the
drop.
The free-falling test mass generates an interference fringe for each half-
wavelength (λ/2) of its movement. As the mass accelerates downward, the
fringes occur more and more closely in time. The resulting signal from the
avalanche photo diode (APD) is a “chirped” sine wave (Figure 2-12) whose
frequency is proportional to the free-falling test mass’s velocity.
Approximately a million fringes are generated during a single drop. A zero-
crossing discriminator (comparator) transforms the sinusoidal fringe signals
from the APD into a series of square Transistor-Transistor Logic (TTL)
pulses. The pulses are scaled (i.e., divided) by a user-defined factor (typically
4000) which is set in the scaler-counter. A universal time interval counter
(UTIC) measures the time interval between each scaled pulse. The g-program
fits each time and distance pair to a parabolic trajectory to determine the
value of g.
When the cart and test mass have descended past the catch point, the
controller signals the cart to reduce acceleration and then come to a stop. The
falling mass catches up to the descending cart and is brought gently to rest.
The system resets for the next drop. The entire sequence takes about 2
seconds and can repeated up to thirty times per minute.
2-8

Design: Components and Function 2
2.2. The Interferometer
The interferometer is a massive, rigid cast aluminum housing which supports
a laser and optics for splitting, directing, and recombining the laser beams.
2.2.1.LASER
The FG5 employs a stabilized helium-neon laser to provide an accurate and
stable wavelength used in the interferometric measurement system. There
are two lasers which are currently available for the FG5.
The Winters Electro-Optics Model 100 iodine stabilized laser. This laser is a
primary standard for the definition of the meter at the Bureau International
des Poids et Measures (BIPM) in Sevres, France. It is a highly stabilized
distance standard having an absolute frequency accuracy of 1 part in 1010 (50
kHz).
The Micro-g Solutions Model AL-1 frequency/intensity stabilized HeNe laser
is characterized by a slow, linear drift. Unlike the WEO Model 100 Iodine
Laser, it must be periodically calibrated to achieve the best accuracy.
However, it is more rugged than the iodine laser.
2.2.2.OPTICS AND BEAM PATH
The laser beam (Figure 2-8 and Figure 2-9) is directed by mirror #1 through
the isolator plate to mirror #2. From there it passes through the focusing
optics (microscope objective), and the collimating lens, where the beam is
expanded. It is then directed to beamsplitter #1 by mirror #3 and mirror #4,
where it is split into the test beam and the reference beam. The reference
beam is split again at beamsplitter #2 and travels to the Avalanche Photo
Diode (APD). The path length of the reference beam remains constant. The
test beam is reflected vertically at beamsplitter #1, and passes through a
compensator plate and a window in the bottom of the Dropping Chamber. It
is then reflected back down by the corner cube in the test mass. The test
beam returns through the window, the compensator plate, and beamsplitter
#1 and passes down through the interferometer base to the Superspring.
2-9

Design: Components and Function 2
The test beam passes through the top window of the Superspring chamber to
a corner cube in the Superspring mass. The test beam is then reflected back
through the window to the interferometer base, where it hits mirror #5,
passes through the translator plate, hits mirror #6, and is recombined with
the reference beam at beamsplitter #2.
This interferometer is a Mach-Zender interferometer with a fixed arm and a
variable (test) arm. During a drop, the motion of the test mass/corner cube
affects the path length of the test beam. The interference fringes which result
from the recombination of the test beam and the reference beam provide an
accurate measure of the motion of the test mass relative to the mass
suspended on the Superspring.
Two separate complementary, recombined beams are produced at
beamsplitter 2. The vertical recombined beam is reflected by mirror #7, and
is focused by a lens. The focused beam strikes the detector (APD), and the
interference fringes are converted to continuous wave (CW) signals and
transmitted to the scaler counter.
The other recombined beam travels horizontally until it reaches the
attenuator (or rattler) plate. This beam is split and reflects "rattles" between
the beamsplitter coating and the uncoated side of the attenuator plate. Three
beams of decreasing intensity emerge from the coated side. The first and
brightest of these beams is deflected vertically by a mirror into the fringe
viewer. The second and third beams exit the interferometer where a flag in
front of mirror mount #7 blocks the second beam, allowing the third
(dimmest) beam to enter the collimating telescope. The collimating telescope
is used to compare this weak reference beam with another beam reflected off
of an alcohol pool to allow alignment of the laser beam with the local vertical.
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Design: Components and Function 2
Figure 2-8 Top view of interferometer optics and beam path
2-11

Design: Components and Function 2
Figure 2-9 Side view of interferometer optics and beam path
2-12