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FG5 Absolute Gravimeter User`s Manual - Micro

1. connector on the computer time interval card in the Magma PCI unit e Connect the BNC cable from the interferometer base ANALOG connector to ANALOG FRINGE IN on the rear of the SIM e For WEO Model 100 laser o Connect the BNC cable from the OUTPUT connector on the front panel of the laser controller to g Laser on the front of the SIM Make sure the meter select switch on the laser controller is set to the 17 position e For the Model ML 1 Laser o Connect the Laser LOCK and Laser MODE BNC cables from the ML 1 controller to the respective locations on the front of the SIM e Connect the BNC cable from the METER MONITOR connector on the ion pump power supply to g ION on the SIM Set the meter select switch to the 10 A scale e Make sure DC power on the FG5 Power Supply is OFF and attach the rest of the system cables as described below Both ends of all cables are labeled with the proper location for each connector Connect the rotary shaft encoder cable blue Lemo connector from the power supply panel to the blue Lemo connector on the motor drive assembly o Connect the DC motor power cable orange Lemo connector from the power supply panel to the orange Lemo connector on the motor drive assembly o Connect the APD power cable green Lemo connector from the power supply panel to the interferometer base power connector 3 1 7 The Superspring e Connect the Superspring control cab
2. RDI ACCINCI AE 51 4 2 5 POPPE A T TE T 51 4 3 E E 54 4 3 1 PASM O 55 4 5 2 Laser AU CHING 56 4 3 3 uud UH 0 56 4 3 4 JJ 57 5 VACUUM CHAMBER TURBO PUMP AND BAKING OUT 59 5 1 SF RING Ur TE LORBO TCM uibus is 59 5 2 BAKING OUT THE DROPPING 61 2224 TT CADIS the RI 61 5 3 Bl OR TING TEI On 62 0 ELECTRONICS CONNECTIONS 64 6 1 64 64 52 OPTEI 64 7 WIBAVELYOSDEESEERC 65 7 1 Goo RT 65 7 2 65 7 3 ENVIRONMENTAL NOD T 65 7 4 TEMPERATURE STABILITY 65 MES c 65 SYSIEMESPECIPBICATVIO NS 66 Micro g LaCoste FG5 User s Manual 8 1 66 8 2 WEIGHT AND DIMENSIONS X XXX ccccceccccecccecscececcececescececescecscscusescesescecacees 66 8 3 OPERATING 000000000 66 0 WARRANTY 67 10 E 68 68 1 TEAR D
3. the laser passes through an optical isolator This component allows the laser light to travel through it but does not allow reflected light to return back to the laser cell This 1s important because any errant light entering the laser cell referred to as feedback can interfere with the stability of the frequency lock The isolator is optimized at the factory to provide maximum feedback rejection and the user should not have to adjust it However if it 15 noticed that a piece of the isolator is loose contact Micro g immediately to receive information on reassembling the isolator or receiving a replacement If a piece is loose it is extremely likely that isolator 15 no longer functioning and that the laser will not lock reliably The only adjustment necessary regarding the isolator is this whole unit must be rotated so that its polarization matches that of the laser Simply place a power meter at the output of the isolator and rotate the isolator until the power 15 maximized Clamp the isolator in place 4 3 3 5 Axis Mount The fiber 15 coupled to the laser head a 5 1 mount The name refers to the fact that the mount allows lateral translation of the fiber relative to the beam in both the X and Y directions 2 axes the mount allows tilt of the fiber both pitch and yaw 2 axes and the mount allows longitudinal translation of the fiber so as to focus the laser beam into the fiber 1 axis Note the mount
4. Corner Test Mass 4 Bottom Drag Free Chamber Cover Figure 5 Front view of the cart drag free chamber The test mass contains a retro reflective corner cube surrounded by a support structure which 1s balanced at the optical center of the corner cube The corner cube 15 a three surface mirror which has the special optical property that the reflected beam 1s always parallel to the incident beam In addition the phase shift of the reflected beam 1 virtually constant with respect to any slight rotation or translation of the corner cube around its optical center When in contact with the cart the test mass 1s supported by three spherical feet or 5 that fit and orient it to vees in the cart The drive mechanism 1s a support structure inside the dropping chamber on which the cart drag free chamber travels up and down and is driven by a DC servo motor The cart 1s attached to a belt that 1s driven up and down by a shaft attached to the motor The motor 15 located outside of the chamber 15 connected to the shaft via a ferrofluidic feedthrough this device allows the vacuum to be maintained inside the chamber while a shaft rotates through the wall The motor also turns an optical shaft encoder that provides accurate information to the dropper controller on the position and velocity of the pulley At the beginning of a drop the cart accelerates downwards with an acceleration greater than g allowing the test mass to begin freefall
5. 00555 nnne eene 30 3 1 3 32 3 1 4 Dropping Chamber E RUE 33 3 1 5 Leveling the Dropping Chamber 33 3 1 6 IE ite HR MPa M 34 257 35 3 1 8 DON a EE E E E EEES 36 3 1 9 PONV 36 SAAD Fringe OPUMUZO ON e r ETE 37 a2 SOFTWARE SET eee eee eee 39 3 2 1 39 2 2 2 2 27 E 39 Micro g LaCoste FG5 User s Manual 3 2 3 s 40 3 2 4 T a 40 3 3 RUNNING THE E R 42 3 3 1 Starting the eene nnne essen ness 42 3 3 2 PDO D 43 3 3 3 deu SIV TR 43 3 4 PHONG DOWN 44 4 ADJUSTMENT AND 1 1 46 4 1 RE RS 46 4 1 1 Removing the Superspring sss 46 4 1 2 The Spring Position is off the 8 46 4 1 5 Superspring WIL not g0 10 47 4 1 4 Gravity data shows very large Scatter or large Residuals 46 4 1 5 LUIDIT E E EA 49 w DROPPING CHAMBER P cee E E E ee 49 4 2 1 TOM TIO 49 4 2 2 E COn L SUC S E E 50 4 2 3 Dropper Tuning 9 50 4 2 4 Drive
6. this not only changes the angle but the focus distance from the fiber entrance to the focusing lens as well turning each one in the direction of maximum power Now return to the X and Y screws and adjust them slightly to maximize the power Then return to the three front screws and repeat the procedure After many 10 or more iterations the laser power should be maximized 4 3 4 Fiber polarization When the power is maximized or at least about 100 uW for an ML 1 laser it 15 then necessary to rotate the fiber so as to match its polarization to that of the laser Note that unfortunately this most likely means a great 1f not complete loss of light 1n the fiber Finally note that this procedure requires not only a sensitive laser power meter but a high quality rotatable polarizer as well Shine the light from the output of the fiber through the polarizer and onto the laser power meter Rotate the polarizer until the laser power 15 maximized and note the value This is the transmitted power Now rotate the polarizer until the power is minimized this might require rescaling of the power meter Next form a coil of excess fiber in your hand and let the heat slightly change the length of the fiber This will most likely cause the power to increase Note the maximum value attained This is the rejected power Calculate the ratio of rejected to transmitted This ratio should be less than 1 100 o If the
7. 21 FIGURE 9 VERTICALITY CONFIGURATION THE ANGLE OF THE LASER LIGHT OUT THE FIBER IS ADJUSTED UNTIL THE REFERENCE AND TEST BEAMS REFLECTED OFF THE SURFACE OF THE ALCOHOL POOL APPEAR AT THE EXACT SAME LOCATION IN THE TELESCOPE e ese een 22 FIGURE 10 SUPERSPRING SCHEMATIC c scecscecccccccccccecccccccecececeeececsesesesesesesececcccececeeues 25 FIGURE 11 SUPERSPRING INTERFEROMETER SETUP 2 31 FIGURE 14 IF THE LED LIGHT IS OFF OF THE DETECTOR THE SPHERE POSITION WILL READ ABOUT ZERO VOLTS AS THE LIGHT IS BROUGHT ON THE DETECTOR THE VOLTAGE APPEARS TO GO THE WRONG WAY KEEP MOVING THE SPRING POSITION UNTIL IT IS ABOUT THE TRUE 0 40 4 40 00000 47 FIGURE 12 BENT SUPERSPRING FLEXURE THE SMALL WIRE AND THE BRASS PORTION OF THE FLEXURE SHOULD 1 48 FIGURE 13 USING A FEELER GAUGE TO MONITOR THE GAP SPACING ON THE SUPERSPRING CUIDE PINS RR __ 49 FIGURE 14 CHARACTERISTIC RESIDUALS SIGNAL WHEN THE VACUUM IS POOR NOTE THE VERTICAL SCALE IS ABOUT 10X LARGER THAN NORMAL cceccseccssccscccsccesscesseescencs 50 FIGURE 15 CHAMBER ALIGNMENT PLOT 1 SHOWS POOR ALIGNMENT PLOT 2 SHOWS THE DROPPING CHAMBER TILTED SUCH THAT THE CART POSITION AT THE BOTTOM OF THE DROP IS DIRECTLY BELOW THE TOP OF THE DROP NOTE THAT IN GENERAL THE TILT
8. OF THE CHAMBER IS NOT NECESSARILY VERTICAL ALSO THE TILT AND ROD BEND HAVE BEEN GREATLY 1 52 FIGURE 16 USING THE X Y DETECTOR TO SET DROPPING CHAMBER BUBBLE LEVELS IN GENERAL AS THE CART IS LIFTED UP AND DOWN IT WILL ALSO MOVE SLIGHTLY SIDEWAYS THUS THE REFLECTED LASER BEAM WILL APPEAR TO TRANSLATE TILT THE CHAMBER UNTIL THE POSITION OF THE REFLECTED BEAM AT THE TOP OF THE DROP IS THE SAME AS AT THE BOTTOM OF THE DROP THEN SET THE BUBBLE LEVELS TO READ ZERO AGAIN THE ROD BEND AND TRANSLATION ARE EXAGGERATED 53 f FIGURE 17 LOCATION OF THE DROPPER TRIPOD ADJUSTABLE FEET BY ADJUSTING THE HEIGHT OF THESE 3 FEET THE TILT OF THE DROPPING CHAMBER CAN BE MADE TO Micro g LaCoste FG5 User s Manual MATCH THAT OF THE SUPERSPRING ccccccccecececececececsceescsecccescccccececeseeeeeeeeeeueeecesseecs 54 FIGURE 18 TURBO PUMP CONNECTED TO DROPPING CHAMBER NOTE THE MINIMUM STRESS IN THE 6 0 000000000000000000 60 IN INTRODUCTION 1 1 1 1 2 Micro g LaCoste FG5 User s Manual History A ballistic absolute gravimeter works by dropping an object in a vacuum and measuring the time it takes to fall a specified distance Galileo recognized that the acceleration of a freely falling body is independent of its mass and legend has it that he demonstrated this by dropping objects of
9. fixed by stopping data collection and clicking the Switch button found in Setup tab of the System parameters This can be done in replay as well Laser WEO 15 the software automatically detecting right peak The difference in uGals between laser peak wavelengths is about 25 wGals 3 3 3 Reprocessing Data Once the measurement 1 finished or if it 1 stopped after the completion of at least the first set clicking Process Go will cause the system to replay the data The program will ask you if you would like to overwrite the previous output file project txt Clicking YES or choosing a different output filename will cause the program to read the data files from the disk and re process each drop If desired it 15 possible to change the input parameters common examples include a new nominal pressure more detailed location values etc and then replay the data The parameter settings in place at the actual time of measurement can always be recovered by clicking Reset All 43 IN 3 4 Shutting Down the FG5 If the FG5 is not to be used for a long period shut everything down with exception of the 10n pump power supply Even if the instrument is to be stored for a few months it is better to leave it under vacuum with the ion pump on Only if shipping regulations require it should the 10n pump be turned off e If desired back up any data and then close all the windows on the comp
10. interferometer s Analog output and measuring the peak to peak voltage of the fringe signal Again see Section 3 1 10 for detailed set up procedures 23 2 5 Superspring Theory The Superspring Figure 10 is a long period active vertical isolator used to compensate for small vertical motions of the first beam splitter The Superspring has a short 20 cm mainspring with a natural period of about 1 second The mainspring is contained in a support housing also supported by springs that 1s actively servo controlled to track the Superspring mass at the end of the mainspring The result is a long period 30 60 second or 16 30 mHz spring mass system that isolates against ground motions occurring at a higher frequency than its own enhanced natural frequency This insures that any change in the length of the test beam is due only to the acceleration of the dropped object Micro g LaCoste FG5 User s Manual 24 Micro g LaCoste FG5 User s Manual Window Top Flange Aneroid Temperature Compensating Lever Focus Lever Focus Motor Mass Corner Cube Sphere Emitter Service Ring Level WR Bottom Flange NE Linear Actuator Detector Figure 10 Superspring schematic The servo mechanism works as follows The Superspring sphere detector system senses motions of the Superspring mass relative to the inner support housing An infrared LED located on the support housing directs light th
11. it 1s possible to move the test and reference beams to the center of the telescope viewfinder by adjusting mirror 3 located in front of 36 IN Micro g LaCoste FG5 User s Manual the telescope objective Note this does not affect the interferometer alignment it is only for the user s convenience 3 1 10 Fringe Optimization To optimize the fringe signal the test and reference beams must be made perfectly coincident and parallel The two interfering beams should be perfectly overlapped and also have no angular deviation for the greatest signal Note though that at this point the beam is vertical no further adjustments can or should be made to the reference beam The test beam is adjusted to match the position of the reference beam The translation of the test beam is done by adjusting the translator plate usually called the twiddler The angular deviation is adjusted by adjusting mirror mount 2 located below the telescope see Figure 8 Note that for convenience it 1s possible to move the test and reference beams to the center of the telescope viewfinder by adjusting mirror 3 located in front of the telescope objective Note this does not affect the interferometer alignment it is only for the user s convenience Look the viewport on the right side of the IB not the telescope and adjust the twiddler until the test beam overlaps the reference beam Look in the telescope and adjust mirror 2 so that the
12. s Manual Figure 1 shows how gravity is measured with an FG5 A test body containing a corner cube retro reflector 1s dropped from the top of the dropping chamber The laser light 15 split to reflect off the falling corner cube and a fixed corner cube which serves as a reference The mass accelerates to the bottom of the chamber and the raw fringe signal is detected by the photodiode as the dropped object falls The optical fringes in the raw fringe signal are timed to create calibrated time and distance pairs The lower part of the figure demonstrates the increase in the fringe signal frequency as the test body accelerates Micro g LaCoste FG5 User s Manual Di Ti 2 4 23 D T2 572 Test Object Photo detector Fixed Reference Raw Fringe Signal Di Ti D T2 T3 Figure 1 Direct Measurement of Absolute g the optical fringes go through zero the precise time is recorded by an atomic clock A least squares fit to the time and distance pairs is used to determine g 1 3 Units in Gravitational Measurements 15 defined to be the magnitude of the acceleration experienced by a freely falling body at a specified point As such it 15 simply a scalar and 15 reported in units of distance per squared time interval In the 5 1 system of units gravity 15 nominally about 9 8 m s For historical reasons gravity 15 also commonly reported in the CGS syste
13. Once a certain separation typically about 3 mm between the cart and the test mass 1s reached the cart slows 16 IN down and then tracks the test mass maintaining a constant separation of a few millimeters Finally at the bottom of the drop the cart gently catches the test mass and decelerates to a stop Micro g LaCoste FG5 User s Manual The tracking during the drop is accomplished by a sophisticated servo system that works as follows By keeping track of the cart position using a shaft encoder on the belt drive pulley and using the interferometer fringes to establish the object position the distance between the cart and the mass can be derived During freefall this separation is maintained at a constant distance by using a servo motor drive system to control the cart inside the Dropping Chamber Because there is essentially no relative motion between the test mass and the drag free chamber the effects of residual air drag are eliminated 2 3 3 The Service Ring The Service Ring Figure 6 is at the base of the Dropping Chamber It provides connections and mounting for the following e A bellows type vacuum valve for the initial evacuation of the vacuum system e A Ferrofluidic rotary vacuum feedthrough which connects the motor shaft to the cart drive mechanism e servo motor rotary shaft encoder assembly which moves the cart and senses its position Anion pump mounted on a 274 Conflat flange which maintains t
14. Verticality Because a non vertical laser beam in the dropping chamber will always result in a gravity estimate that 1s too low an error of 9 arcsec in verticality corresponds to a gravity value error of approximately 1 Gal The error increases as 07 it is critical to align the beam with the local vertical as accurately as possible provide a local reference standard a pool of alcohol 15 inserted in the interferometer In this configuration the test beam reflects directly off the surface 21 N of the liquid and back up to the first beam splitter Only when the beam is truly vertical will the beam reflect back directly through itself reflect off the bottom of beam splitter 1 and be parallel to the reference beam The telescope is focused precisely to infinity and thus parallel rays are focused to the same point in the view finder By adjusting the input angle of the laser fiber left side of Figure 9 the reference and test beams can be made to coincide in the telescope and this can only happen when the beam is perfectly vertical Micro g LaCoste FG5 User s Manual Figure 9 Beam Verticality Configuration The angle of the laser light out of the fiber is adjusted until the reference and test beams reflected off the surface of the alcohol pool appear at the exact same location in the telescope 2 4 2 Beam Alignment In addition to making the test beam perfectly vertical the user must also optimize the beam alignme
15. also allows rotation of the fiber relative to the beam yet one more axis for an actual total of 6 subject of the next section Getting laser light through a fiber 1s somewhat tricky and requires patience and practice However the principles are quite simple one is trying to align the entrance of the fiber with a laser beam focused down to a few microns in diameter Both the location of the fiber entrance and the fiber s angle must coincide with that of the laser beam 56 Micro g LaCoste FG5 User s Manual e Attach the 5 axis mount to the laser head and translate it such that the laser light 1s traveling through the center Verify by holding a piece of paper up and making sure the beam 1 not clipped e Attach the fiber to the 5 axis mount and tighten firmly Use the X and Y screws on the side of the 5 axis mount to get some light through the fiber While you should never look directly into the fiber it should be possible to see the output end of the fiber glow with a small amount of light If no light is visible slowly translate the X and Y screws in a search pattern while looking for a glow at the output end of the fiber When a small glow 1 visible it is now best to attach the fiber to a laser power meter e Using the power meter carefully adjust the X and Y screws until the power is maximized Now use the three screws on the front to adjust the angle of the fiber Iterate through all three screws
16. changes and subsidence or rebound of the earth s crust 7 2 Site Stability It 15 always best to select a site the lowest level of a building to reduce vibrations as much as possible basement with a thick concrete floor 15 usually best Avoid floors with composition materials if possible and set up the instrument on a solid tile or concrete floor 7 3 Environmental Noise Heavy heating or cooling equipment as well as bipedal or vehicular traffic can cause vibrations which tend to increase the drop to drop scatter of the observations This can usually be seen as a large systematic change in the drop to drop scatter between day and night observations remote environmentally quiet site usually minimizes these changes 7 4 Temperature Stability Although the FG5 will operate properly over a wide temperature range it is important to have a site with good temperature stability to minimize possible problems with temperature sensitive components e g laser and Superspring 7 5 AC Power Problems with AC power are not uncommon especially in remote field environments Make sure that ground 15 available before plugging in system It 15 always best to use voltage stabilizers and or uninterruptible power supplies to minimize problems with unreliable line voltage Some system components e g the WEO Model 100 iodine laser are more sensitive to line voltage fluctuations than others so it 15 always best to stabilize line voltage well
17. different weight from the leaning tower of Pisa though this may be apocryphal Newton s theory of gravitation also required that the acceleration of a falling body in an external gravity field did not depend on its size shape or mass Thus measuring the acceleration of a freely falling object is equivalent to measuring gravity This freefall acceleration 1s given the special symbol g to remind us that gravity 1s responsible The FG represents the latest generation of ballistic absolute gravimeters and is based on technology developed over the last forty 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 were the 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 The FG5 is now a firmly established standard in the fields of geophysics and metrology As of 2006 over 45 instruments have been constructed and are em
18. enough to satisfy the requirements of the most sensitive component 65 8 System Specifications 8 1 Power 100 240V AC all components with the exception of the WEO laser sense the line voltage automatically Averag e load 350W 8 2 Weight and Dimensions XXXX Weight O O 0 O O O IB amp Laser Dropper Superspring Turbo Pump Dropper Tripod Superspring Tripod Electronics Rack Total 20 kg 25 kg 20 kg 15 kg 20 kg 12 kg 15 kg 127 Transit Case Dimensions OO 0 0 0 0 0 IB amp Laser Dropper Superspring Turbo Pump Dropper Tripod Superspring Tripod Electronics Rack Computer 64 x 56 x 38 64 x 38 x 80 31 x 30x 57 37 37 47 77 x 56 x 28 64 56 31 51 65 10 38 x 32 7 8 3 Operating Temperature 15 30 C 60 F 90 F internal temperature Micro g LaCoste FG5 User s Manual 66 IN 9 Warranty Micro g LaCoste FG5 User s Manual The warranty covering the FG5 Absolute Gravimeter is as follows Micro g LaCoste Inc hereby warrants to purchaser that the instrument delivered hereunder shall be free of defects in material and workmanship appearing within one year from the date of delivery Purchaser or any third party purchaser must give written notice of any defect covered by this warranty to seller within 13 months of the date of delivery of the instrument to purchaser For any defect covered by this warranty seller sha
19. levels are set at the factory to insure that the dropper is oriented correctly at each setup They are used to ensure that the cart precisely catches the test mass at the bottom of the drop even if they are not precisely vertical with respect to the outside of the chamber or even if they are not exactly straight However if the bubbles somehow become loose or if the chamber is opened for any reason they will need to be reset Note Unlike the Superspring the bubbles are not simply set to make the dropper level Rather the dropper needs to be tilted generally not level to an optimal position and then the bubbles are set so that this position can be repeated Make sure the chamber is well separated 5 or more turns of the feet from the interferometer and that the laser beam traveling into the chamber 15 vertical using the alcohol pool 51 Micro g LaCoste FG5 User s Manual Figure 16 Chamber alignment Plot 1 shows poor alignment Plot 2 shows the dropping chamber tilted such that the cart position at the bottom of the drop is directly below the top of the drop Note that in general the tilt of the chamber is not necessarily vertical Also the tilt and rod bend have been greatly exaggerated Place the XY detector part of the standard FG5 toolkit in the path of the vertical beam and attach the power cable to the Auxiliary output connector on the FG5 Power Supply Attach the X and Y outputs to Chl and Ch2 of an oscill
20. lock the laser prior to measurement e For WEO lasers The laser 15 locked on a peak 4 g and the Monitor output is connected to g LASER on the SIM e the Rubidium LOCK is on In g Select Process Go or hit the GO button on the toolbar or use F5 as a quick key Assuming the meter 15 functioning correctly the State display will show the value of gravity among many other things a graph of each drop relative to the current mean value and the residuals of the parabola fit See the g User s Manual for a complete discussion of all the g windows When first set is completed it is automatically saved to disk Note that at this point if the application 1s stopped the Project 1s no longer in real time mode That is if you enter Stop and then Process Go the program will replay the data rather than operate the FG5 When all the sets have finished it 1s safe to quit the application the data are already automatically saved at the end of each set If you notice a problem and stop the acquisition during the first set you can restart the project without creating a new project If you stop during any other set after Set 1 however g will automatically save all the completed sets and you will lose whatever data was in the incomplete set Note that it is important not to stop g until the data from the previous set s has been written to disk this can be seen on the bottom right of the screen Stopping the program du
21. position for the bellows valve on the dropping chamber has been determined turn the turbo pump power switch on The roughing sequence of the pump will start immediately and then the turbo pump will slowly increase its speed When the turbo pump reaches its nominal operating speed usually about 70 75 krpm The small green LED on the side of the turbo pump will blink until the turbine has come to full speed at which point it will be lit continuously Ideally while pumping down the system the AC power will not be interrupted However if the power is interrupted the system will not actually vent to atmospheric pressure the turbo pump has a valve that closes automatically in the event of a power outage Of course if possible it is best to use an Uninterruptable Power Supply UPS while turbo pumping 5 2 Baking Out the Dropping Chamber When the dropping chamber has been exposed to air or when the 10n pump has been off for more than one month it should be baked out while the turbo pump is operating Note while baking out it 15 extremely important that the power to ion pump remains OFF Disconnect the high voltage cable from the ion pump Bake out involves heating the dropping chamber and 1 pump to evaporate water and other heavy molecules from the interior surfaces while the system 15 being turbo pumped This decreases the pumping time by speeding the out gassing processes within the chamber In cases where the 10n pump h
22. position of the signal on the oscilloscope it might be necessary to adjust the X and Y ranges though it 15 best if both channels have the same V div Now watch the motion of the signal as the cart is slowly lowered As an example say the signal moved down to the left by two divisions With the cart at the bottom use the appropriate trial and error combination of dropping chamber tripod feet to move the signal in the same direction and the same amount as the signal moved as the cart was lowered in our example down to the left by two divisions Here tripod feet refers to the large feet on the ground used to decouple the dropping chamber tripod from the interferometer Repeat the procedure raise the cart note the position watch the motion as it 1s lowered and adjust the appropriate tripod feet As the chamber angle is improved it will become necessary to go to finer and finer voltage scales to see changes the motion When the top and bottom position agree on the oscilloscope to 10 mV the chamber 15 aligned 23 Note that if the rods are slightly bent the signal might do a little loop or wiggle as the cart 1s raised and lowered This 1s normal It is only important that the top and the bottom of the travel are overlapped Micro g LaCoste FG5 User s Manual Once the chamber 15 aligned center and lock the bubble levels Then re couple the dropping chamber tripod to the interferometer Use the spa
23. rejection ratio is lt 1 100 then great Make sure the large black screws on the front of the 5 axis mount are tight fine tune the laser power and proceed to the Last Step o Ifthe rejection 15 gt 1 100 Note the orientation of the fiber relative to the 5 axis mount Slightly loosen so as not to drastically change the angle of the fiber the 3 large black screws on the front of the mount slowly rotate the whole fiber There are two optimal orientations of the fiber 37 Micro g LaCoste FG5 User s Manual 180 apart If the rejection was close to 1 100 rotate a few degrees If the rejection was basically 1 1 then rotate approximately 90 If the rejection was in between use the above information to estimate a reasonable amount of rotation o Once the orientation has been chosen use the 5 adjustment screws to get at least 100 uW of light through the fiber again Repeat the rejection measurement and calculation o Repeat whole procedure rotate regain the light measure the rejection until the rejection is at least 1 100 Once 1 100 is achieved use the 3 large black screws to clamp the fiber rotation into place and proceed to the Last Step Last Step Now that there is laser light through the fiber and the rejection is better than 1 100 we must finally optimize the laser power As above use all 5 screws to maximize the power Next carefully loosen the translation screws that attach 5 axis moun
24. sure to adjust the cross level first then the long level The cross level is opposite the Superspring travel lock knurled brass knob If the long level 15 adjusted first it will change when the cross level is adjusted When cross level is adjusted first it does not change when the long level 15 adjusted Only turn two of the feet while leveling this insures that the lower reference height does not change Again if using an FG5 with the Remote Operation option please consult the Tele g User s Manual for information on leveling the Superspring tripod Note While leveling the Superspring and dropping chamber tripods note that turning the tripod feet clockwise lowers the dropping chamber tripod and raises the Superspring tripod 3 1 3 Interferometer Remove the interferometer base from its shipping case taking care not to stress the fiber optic cable Remove the dust cap from the top Superspring window and place the interferometer base on the Superspring Orient the interferometer base so the fiber optic input 1s located directly above the Superspring travel lock The alignment pins on the top of the Superspring assure that the interferometer base 1s oriented correctly Lock the interferometer base in place by tightening the four 5 lobe knobs 32 IN Micro g LaCoste FG5 User s Manual 3 1 4 Dropping Chamber Tripod Remove the tripod tray from the Superspring case and place it carefully upside down the lid of the transi
25. the vacuum hose into the vacuum chamber should either be open or closed before turning on the turbo pump power e Ifthe dropping chamber is under partial vacuum the bellows valve should remain closed Do not open this valve until the turbo pump has evacuated the air inside the flexible hose and come to full speed Otherwise air in the hose can be sucked into the chamber Full speed is indicated by the blinking green LED near the power switch turning solid green Once the turbo pump has reached a normal speed and normal operating pressure slowly open the valve It 15 important to open this valve slowly because if there 1 actually air in the chamber a large amount of air can damage the turbo pump Slowly turn the valve until it is completely open all the while making sure that the turbo pump is still at full speed 60 IN Micro g LaCoste FG5 User s Manual e If dropping chamber is at full atmospheric pressure 1 the chamber was open for maintenance the vacuum valve must be opened before starting the turbo pump This will allow the roughing sequence of the turbo pump to remove the air from the hose and the chamber at the same time This is important because the turbo pump can be damaged if it is suddenly exposed to air when operating at its normal pumping speed In general the dropping chamber will also require a baking out procedure to remove water vapor from the system See Section 5 2 for details After the correct
26. this manual listed below are some common issues their explanation and their resolutions 4 1 Superspring Superspring problems usually manifest themselves as either an inability to zero properly or through high drop scatter and strange residuals In some cases it will be necessary to open the Superspring to ascertain the source of the problem 4 1 1 Removing the Superspring Cover If it is necessary to open the Superspring place it in its tripod level it make sure it is travel locked and loosen the 6 screws at the bottom of the main can and remove the whole can and lid straight up together Note that there are punch marks to indicate the orientation of the can relative to the Superspring base Note Do not open the cover by removing the bottom flange This will damage the internal electrical connections through the service ring 4 1 2 The Spring Position is off the Detector If for some reason the mass 15 extremely far from the center of its range e g due to an extreme change in either temperature or local gravity the spring controller may appear to be behaving incorrectly it may appear to go the wrong way e g more negative even though the motor is set to lift the mass See Figure 12 This 15 because all of the light from the LED is not actually on the detector As more light misses the detector either going off the top or bottom the absolute value of the voltage becomes less and less making it appear that the spr
27. 1s used to lock a second more powerful laser to the same frequency The result 15 a more robust output e The Micro g LaCoste Model ML 1 frequency intensity stabilized HeNe laser is characterized by a slow linear drift Unlike the WEO lasers it must be periodically calibrated to achieve the best accuracy but it is more rugged and powerful models of lasers need to be warmed up and reach thermal equilibrium before gravity data are acquired As a rule of thumb it 1s best to power on the laser the night before measurements if possible At a minimum the lasers should be one at least 1 4 hours for good results Again regardless of the laser model the light 15 always linearly polarized and reaches the interferometer via a single mode polarization maintaining fiber optic cable The fiber polarization 1s set to match that of the laser light at the factory under normal operations it is never necessary to adjust the rotation at the input of the fiber The light enters the fiber optic cable via a 5 axis mount that focuses the light down to a diameter of a few microns This adjustment is also done at the factory and 15 extremely sensitive The laser light exits the fiber and diffracts at a well defined angle to avoid back reflections A collimating assembly 15 attached to the output of the fiber and 15 adjusted such that the final beam 1 well collimated at approximately 8 mm Again note that this collimation adjustment is performed at the f
28. E ML 1 laser e Rear o ANALOG FRINGE IN o TRIG OUT o Ribbon Cable g o Ribbon Cable TELE g o AUTOLEVEL remote systems o 10 MHz Out 6 2 Magma PCI Unit e Rear o TIA Card Arm o TIA Card CLK o TIA Card CHA o 5 A2D Card Ribbon Cable o TELE g A2D Card Ribbon Cable 6 3 Power Supply e Front o IB Power Green o Superspring Yellow o Dropper Power Blue o Dropper Encoder Orange 1541 V 431 bod dd jd Ion Pump Controller METER MON WEO 100 Controller METER Output Controller LASER LOCK Controller LASER MODE FG5 Interferometer ANALOG Magma PCI Unit TIA Card ARM Magma PCI Unit g A2D Card Magma PCI Unit TELE g A2D Card FG5 Superspring Tripod Autolevel Computer CLOCK SIM Rear TRIG OUT SIM Rear 10 MHz OUT FG5 Interferometer TTL SIM Rear g SIM Rear TELE g FG5 Interferometer Power Superspring Chamber Dropper Chamber Power Dropper Chamber Encoder 64 IN 7 Gravity Site Selection Micro g LaCoste FG5 User s Manual 7 1 Geologic Stability To achieve best results a site should be located in a geologically stable area Generally it may be necessary to measure gravity at a particular site for a number of different reasons and geologic stability cannot be considered However when selecting a primary base station geological stability is important in minimizing long term variations in gravity resulting from groundwater
29. Micro s ACOSTE WWW MICROGLACOSTE COM FG5 Absolute Gravimeter User s Manual DECEMBER 2006 Micro g LaCoste FG5 User s Manual FG5 OPERATOR S MANUAL TABLE OF CONTENTS INBER OE C UR 5 INPRODUC HON M 7 1 1 7 1 2 THEFGS ABSOLUTE GRAVIMETER 7 1 3 UNITS IN GRAVITATIONAL 5 6 2 2 22 000000 030000000000004 9 ATOR 10 2 DETAILED THEORY OF 11 2 1 GATE TE 11 2 20 DESIGN HARDWARE COMPONENTS AND 12 29 DROPPING CHAMBER THEORY 14 Z9 The Dropping Chamber 14 PS NP The DFOSSIROD COT e pU 15 2 TIVO SSI 16 E EE gt 17 2 3 4 The Dropper CON 16 2 4 INTERFEROMETER HEORY ene eese Fosse oss eibi d sensns 19 2 4 1 _______ 21 242 BEOR A ______ 22 252 MINNIE 24 MEME cca cito o 21 SEEN 5 UI wA 29 3 1 INSI ROVTE NTAS EMDE RD 29 3 1 1 LECTO EE MID PM CM 29 3 1 2 Setting up the Superspring
30. OWN 70 Table of Figures FIGURE 1 DIRECT MEASUREMENT OF ABSOLUTE AS THE OPTICAL FRINGES GO THROUGH ZERO THE PRECISE TIME IS RECORDED BY AN ATOMIC CLOCK LEAST SQUARES FIT Micro g LaCoste FG5 User s Manual TO THE TIME AND DISTANCE PAIRS IS USED TO DETERMINE ccce eene 9 FIGURE 2 THE FG5 SYSTEM THE LASER SYSTEM CONTROLLER AND ELECTRONICS RACK ARE NOT SHOW d 13 FIGURE 3 FRONT VIEW OF THE DROPPING CHAMBER 0 0 12000 14 FIGURE 4 SIDE VIEW OF DROPPING 6219 15 FIGURE 5 FRONT VIEW OF THE CART DRAG FREE 2 0 000 1 3 16 FIGURE 6 SIDE VIEW OF THE SERVICE RING ccccssscesccssccscccscccsccesscessessescesccesceesseessees 17 FIGURE 7 BEAM PATH IN FG5 INTERFEROMETER THE REFERENCE BEAM PASSES STRAIGHT THROUGH THE FIRST SPLITTER AND IS THEN SPLIT AGAIN TO ENTER THE DETECTOR AND THE OPTICAL DEVICES FOR ALIGNMENT THE TEST BEAM LEAVES VERTICALLY FROM THE FIRST SPLITTER TRAVELS THROUGH THE DROPPER AND SUPERSPRING AND IS RECOMBINED WITH THE REFERENCE BEAM AT SPLITTER 2 NOTE THAT ONLY THE TELESCOPE AND NOT THE FRINGE WINDOW VIEWPORT IS SHOWN IN THE FIGURE 20 FIGURE 8 SCHEMATIC OF FG5
31. a analysis procedures and to vary environmental corrections An optional remote operation package is available that allows virtually complete remote control of the instrument and the software via an internet connection 10 N 2 DETAILED THEORY OF OPERATION Micro g LaCoste FG5 User s Manual 2 1 g Determination As mentioned in the Introduction FG5 determines the acceleration of gravity by using an interferometer to accurately track a test mass during freefall in a vacuum The photodetector circuit at the output of the interferometer produces a pulse each time this interference signal crosses zero and the times of these pulses are measured precisely using a rubidium clock In the simplest example these time and distance pairs could then used to find a least squares solution to the following equation 2 X Xo t Vo t5 Soli where xo vo and go are free parameters providing the best estimates for the initial position velocity and gravity respectively Of course g 1s the parameter of most interest In practice however the situation 1 a bit more complicated The finite value of the Earth s gravity gradient y approximately 3 uGal cm causes a measurable change in g over the small length of the drop and this complicates this standard equation Xot Wot Bot 2 6 24 In principle the gradient should be free parameter as well but the short length of the drop provides little sensitivity it
32. able and turn computer power OFF Travel lock Dropper Turn DC Power Off Turn off laser HV and AC power Turn AC Power off Travel lock Superspring Disconnect all cables but get power to ion pump and generally leave fiber optic connected between laser and IB Unless shipping overseas keep 10n pump powered ON Remove dropper double check it 15 travel locked Remove dropper tripod and disassemble Remove IB from Superspring and place it and laser in shipping box Remove Superspring double check it 1s travel locked Pack Superspring tripod 70
33. about 20 mV from zero enable the ZERO switch on the SIM This brings the spring to the center of its range within a few minutes When the position 15 within about 20 mV of zero or stops moving disable SS ZERO It is normal for the SS POS value to fluctuate as the reference mass bounces on its spring but eventually it should damp out and the fluctuations should be lt 20 mV At this point enable SERVO After five minutes or so the scatter of the spring position should be 1 2 mV a voltmeter If the spring is not totally at thermal equilibrium the position will slowly drift in one direction This is normal and should not affect the measured gravity value the spring is moving at approximately a constant velocity and will not affect the gravity determination 3 1 9 Beam Verticality Before optimizing the alignment of the interferometer make sure the test beam is perfectly vertical Place the alcohol pool in the interferometer taking care to roughly center the laser beam in the pool In the telescope you should see a solid light the reference beam and a vibrating light test beam that is reflecting from the alcohol pool Adjust the angle of fiber optic using the mirror mount left side of IB until the test beam is exactly centered over the reference beam in the telescope Use the beam blockers as necessary to help determine the positions of both the reference and test beams Then remove the alcohol pool Note that for convenience
34. actory 15 extremely sensitive critical for the gravity measurement and should not need to be adjusted Finally because the laser light 1s polarized it 1 necessary to rotate the output of the fiber about its axis so as to evenly split the laser power between the test and reference beams horizontally polarized light 1s transmitted more efficiently into the reference 2 beam while more vertically polarized light 1s transmitted in the test beam the nominal position 1s thus at about 45 Again this adjustment 15 performed at the factory and should not need to be changed by the user Micro g LaCoste FG5 User s Manual 28 IN Micro g LaCoste FG5 User s Manual 3 FG5 SETUP NOTE These instructions are based on the assumption that all subsystems of the FG5 are aligned correctly and operating properly If adjustment or alignment 15 necessary consult Section 4 Adjustment and Maintenance for instructions before proceeding with set up When setting up the FGS it is helpful to use the FG5 Setup Checklist in Section 10 3 1 Instrument Assembly Locate and mark a reference point on the floor where gravity will be measured The floor should be as clean smooth and level as possible It is best to set up the FG5 on a concrete pier or hard tile floor at ground level Note that these instructions assume that the dropping chamber 15 already at a high vacuum level See Section 5 for instructions on pumping out the dr
35. as been off for several weeks it may be helpful to bake out the chamber even though it has not been opened 5 2 1 Heating the Chamber The magnets on the ion pump can be damaged by high temperatures and should be removed prior to baking out the Dropping Chamber Remove the screws that hold the magnets in place and then use a screwdriver to pry the magnets apart Take care as these magnets are quite strong Once removed keep the magnets away from each other 61 IN To heat the chamber wrap the heat tape around ion pump and the chamber Wrap tightly so that good thermal contact is maintained between the tape and the chamber Make sure that the tape never wraps over itself this will get extremely warm and can damage the tape Also keep the heat tape away from the chamber bubble levels they will be damaged if overheated Finally make sure the travel lock 15 engaged this allows thermal contact between the test mass and the chamber Turn on the power to the heat tape and use a voltmeter with temperature mode to monitor the temperature place the temperature sensor between the tape and the chamber to make sure that you are monitoring the hottest part For the first 15 minutes or so carefully monitor the temperature and adjust the heat tape current to maintain the temperature at about 60 70 The temperature of the chamber should never exceed 80 as this can damage the ferrofluidic feedthrough Micro g LaCoste FG5 U
36. atter in the measurements 3 2 4 Control Setup The first section in Control Setup is General Terms showing the gravity corrections that can be applied For the initial setup select all of these terms Tidal Terms is next Select ETGTAB for the first test run For each new location it 15 necessary to enter Setup and Run Ocean Load Make sure that the ocean load files created have unique filenames and that you are using the correct ocean load files for your current location 40 For the laser section select the Auto Peak Detect the software will then automatically select the mode and lock the laser prior to each set Micro g LaCoste FG5 User s Manual For the Data section of this page enter the start and stop fringe times typically 35 ms to 200 ms Again a complete discussion of all of these options 1s discussed in the g User s Manual 4 IN 3 3 Running the Gravimeter Micro g LaCoste FG5 User s Manual 3 3 1 Starting the Measurement Before starting the meter make sure of the following Superspring and Dropper have been unlocked Superspring servo is and the units have bee separated e The dropper controller is set to the drop mode e ML 1 lasers The laser is in REMOTE mode with the left LED illuminated and should therefore be UNLOCKED at this point Whether the mode is RED or BLUE at this point 15 irrelevant The software will automatically select the mode and
37. ber issues can be broken into the following categories 10n pump feedthrough tuning drive belt and alignment 4 2 1 lon pump Issues Once the vacuum 15 established and the 10n pump 15 functioning typically there 15 very little that can go wrong with an ion pump However if the vacuum is somehow lost and the vacuum level is compromised while gravity data are being acquired a characteristic sine wave will start to appear in the residual plot see Figure 15 At this point the data are un recoverable and the system will need to have the vacuum restored see Section 5 for more details 49 Micro g LaCoste FG5 User s Manual Residuals Wee NIENT Figure 15 Characteristic Residuals signal when the vacuum is poor Note the vertical scale is about 10X larger than normal 4 2 2 Feedthrough Issues The ferro fluidic feedthrough 15 what allows the external motor to drive the dropper cart through the vacuum wall When the 5 has been sitting idle for a while 10s of minutes it is normal to see a momentary increase in 10n pump current for the first few drops This is due to the release of molecules in the feedthrough mechanism The ion pump then catches up and the vacuum returns to normal The feedthroughs typically have a lifetime of about 2 3 years When they begin to fail you will notice that the ion pump increases are larger and take many many drops to return to normal Wh
38. dow to the interferometer Belt Tension Adjustment Top Flange Top Pulley Assembly Viewing Port Top Drag Free Chamber Cover Cart Drag Free Chamber Test Mass Bottom Drag Free Chamber Cover Guide Rods Drive Belt Helical Shaft Coupler Ferrofluidic Feedthrough Helical Shaft Coupler Bottom Pulley Servo Motor Assembly Travel Lock lon Pump Shaft Encoder Bottom Flange Figure 3 Front view of the dropping chamber 14 Micro g LaCoste FG5 User s Manual Top Flange Viewing Port Belt Tension Adjustment Top Pulley Assembly Top Drag Free Chamber Cover Cart Drag Free Chamber Linear Bearings Bottom Drag Free Chamber Cover Drive Belt Guide Rods Service Ring Bottom Pulley Assembly Bottom Flange Figure 4 Side view of dropping chamber 2 3 2 The Drag free cart The drag free cart 15 used to lift drop and catch the test mass The term drag free refers to the fact that though the chamber 15 evacuated there are still some residual air molecules The cart effectively pushes these molecules out of the way of the test mass which 15 falling inside the cart In addition to reducing drag the cart also reduces magnetic and electrostatic forces on the test mass I5 Micro g LaCoste FG5 User s Manual Glass Sphere Linear Detector Guide Rods Top Drag Free Chamber Cover EA i Cube
39. een off for more than a few hours it will be necessary to use the turbo pump to regain the vacuum If the vacuum is very poor ion pump off for many weeks or the chamber has been opened to atmosphere it will be necessary to bake heat the chamber while turbo pumping 5 1 Setting up the Turbo Pump Make sure ion pump power is OFF Remove the turbo pump from its case and place it near the dropping chamber vacuum flange Connect the turbo pump to the dropping chamber using the flexible vacuum hose which has a vacuum flange on both ends The vacuum hose is normally stored in the turbo pump case underneath the turbo pump Make sure not to stress the bellow tube See Figure 19 59 Micro g LaCoste FG5 User s Manual d A Wy E Wy NS 1111 Figure 19 Turbo pump connected to dropping chamber Note the minimum stress in the hose Attach the vacuum hose to the turbo pump The quick flange has a clamp which mates the two vacuum flanges with an o ring seal It is important to keep the o ring seal and vacuum flanges free of dirt or scratches to avoid leaks Plug the turbo pump into the proper AC power Make sure the small relief valve on the turbo pump vacuum flange 15 closed Note before turning on the power it is very important to determine the status of the vacuum in the chamber Depending on the vacuum level the bellows valve the opens from
40. en the feedthrough finally fails completely the increase s permanent through all the drops and the vacuum does not recover until the dropper is idle At this point it is necessary to replace the feedthrough Though the replacement procedure is beyond the scope of this manual it 1s something an advanced user can perform Please contact Micro g LaCoste for instructions on the replacement 4 2 3 Dropper Tuning Adjustments Again a full discussion of dropper tuning circuit 15 too large for this manual but there are a few common tricks that can be useful With time and or temperature changes it 15 possible that the servo circuit needs to be adjusted from the original factory settings The common symptom 15 that mode will time out and the dropper controller will go into RESET mode DROP mode normally still works well To correct this open the lid of the SIM unit and locate the dropper controller circuit board on the left In the middle of this board is a 50 IN potentiometer labeled POS GAIN short for position gain Simply use a small regular screwdriver and turn this clockwise approximately 1 4 turn Micro g LaCoste FG5 User s Manual 4 2 4 Drive Belt Replacement If the drive belt breaks please contact Micro g LaCoste for instructions Spare drive belts are standard equipment in the FG5 toolkit and this is a repair that advanced users can make themselves 4 2 5 Dropper Alignment The bubble
41. er base The upper set up height should be approximately 6 cm Record this value in the system check log The sum of the upper and lower set up heights approximately 13 cm will be entered as the Setup Height term in Process Setup Information See g Software Manual for details e Release the dropping chamber travel lock by turning the motor shaft slightly counterclockwise with a 4 mm hex wrench to release the pressure on the travel lock mechanism While holding this position pull up on the brass knob rotate it 90 in either direction and gently release it so the pin 15 not engaged on the motor shaft 3 1 6 Cable Connections Note for details on the electronics setup please refer to the SIM or Tele g User s Manual Connect the computer power cable to the power supply panel or AC outlet 34 IN e Connect the PCMCIA cable from the Magma PCI unit to the PCMCIA slot on the computer Micro g LaCoste FG5 User s Manual e Check that the grey ribbon cable from the rear of the SIM is connected to the Magma PCI unit e Connect the BNC cable from the interferometer base TTL connector to the CHAN A connector on the computer time interval card in the Magma PCI unit e Connect the BNC cable from the 10 MHz connector on the power supply to the CLK connector on the computer time interval card in the Magma PCI unit e Connect the BNC cable from the TRIG OUT connector on the SIM to the EXT
42. er switch on the FG5 Power Supply rear of electronics case e Connect the applicable cables to the laser and turn on the laser power first AC power then the HV Consult the manual appropriate to the laser you are using Is it important to turn on the laser as early as possible in the Setup procedures to let the laser reach thermal equilibrium before the measurements begin 3 1 2 Setting up the Superspring Tripod 30 Micro g LaCoste FG5 User s Manual Interferometer Base 9 Beam Blockers jl d s Collimating N CS X Telescope e i Y Interferometer Side Access Door 2 8 Fringe 2 Viewer J l 7 2 gt Interferometer Superspring Tripod Locking Screws Clamps Bull s Eye SZ Tripod Tripod Leveling Feet Figure 11 Superspring Interferometer Setup Note that the bubble level on the Superspring tripod is on operator s left when the FG5 is set up Also the line from the center of the tripod to the bull s eye level should be oriented along the North South axis Figure 11 illustrates the location of the Superspring and interferometer base for setup Remove the Superspring tripod from the interferometer case and place on the floor over the reference mark Orient the tripod so the bull s eye level mounted near one of the leveling feet is facing the electronics case if possible line from the bull s eye level to the center of the trip
43. he operating voltage within five minutes turn off the power and continue pumping with the turbo pump for at least one hour before trying the 1 pump again Leaving the ion pump with low voltage and excessive current significantly shortens the lifetime of the pump NOTE If 10n pump voltage immediately goes to 4 kV without ramping up this could indicate a possible open in 10n pump circuit This means the ion pump is at full voltage but is not actually ionizing any molecules 1 e it has not 62 IN started and the current it draws will be very near zero this situation the ion pump is possibly not ready Leave the turbo pump on with the valve open at this point Next take a hard object 1 a screwdriver and gently but firmly tap the 1 pump not the controller This can release molecules in the ion pump and start the ionizing process If successful you should see the ion pump voltage drop and then slowly ramp back up to near 4kV The current should now also be non zero This indicates that the ion pump is functioning Then move on to the next step Micro g LaCoste FG5 User s Manual Once the ion pump has reached its operating voltage monitor the ion pump current This value should be approximately 0 1 mA and if the vacuum 15 good enough slowly falling as the ion pump ionizes less and less molecules drawing less and less current This means the 10n pump 1 operating
44. he vacuum once the chamber has been evacuated by the roughing pump e Spare 274 Conflat and Mini Conflat flanges are blanked off and can be used for additional vacuum accessories Electrical Vacuum Valve Control lon Pump Feed Thru Knob 4 N amp TOT o ud aC EE OE ue Lock Ring Thermocouple Gauge Figure 6 Side view of the service ring 17 IN 2 3 4 The Dropper Controller The dropper controller is the servo circuit that controls the motor that drives the cart Located in the SIM please the SIM User s Manual for more information it is also the interface between the user and the dropping chamber Micro g LaCoste FG5 User s Manual The controller can be operated two modes OSC and DROP The operator controls the status of these modes and the dropper triggering with the RESET switch and the TRIGGER switch In DROP mode a single trigger causes the controller to drive the motor to lift the cart and test mass to a specified height An additional trigger then initiates the drop sequence the cart pulls away with an acceleration greater than g and then tracks the test mass maintaining a specified separation distance during free fall and finally softly catches the test mass at the bottom If a TTL pulse is entered into EXT TRIG this will also cause a lift and a separate pulse will cause the drop These are normally supplied by the computer during data acquisition To exit DROP mode press RESET The control
45. ilibrium again a minimum of about 1 4 hours before measuring the 1f voltage If the software 1s using the wrong wavelength for the gravity calculation obviously the reported gravity value will be incorrect If it 1s using the adjacent peak value off by one peak e g instead of the gravity value will be wrong by approximately 25 uGal Therefore if looking at gravity data that appears to have 20 30 uGal jumps between sets double check that the 11 voltages and autopeak detection setup in the software are configured correctly Finally note that if the laser hops between peaks during a set acquisition the average value of the set can be off by any fraction of 20 30 uGal i e if the laser hops halfway through a set the gravity value would be off by about 12 u Gal 22 IN Micro g LaCoste FG5 User s Manual 4 3 2 Laser Alignment For both the 1 and WEO lasers aligning the fiber to the laser head correctly is extremely important not only does a proper alignment insure the maximum interference fringe amplitude it also governs the power stability of the laser light Both the fiber optic cable and the laser light itself are polarized and it is crucial that the fiber be aligned with the direction of the laser beam and also rotated about its axis so that its polarization matches that of the laser beam Between the laser head and the entrance to the fiber optic coupler or fiber
46. ing is nearer zero than it actually is Note that this problem is handled automatically in the newer Superspring controllers 46 Micro g LaCoste FG5 User s Manual Figure 12 If the LED light is off of the detector the SPHERE position will read about zero volts As the light is brought on the detector the voltage appears to 20 the wrong way at first Keep moving the spring position until it is about the true zero value Note that the spring position will only go positive and negative about the true zero value This phenomenon is known as the Superspring S Curve For example if the if the light 1s shining off the bottom of the detector raising motor will cause more light to shine on the detector As the mass 15 raised further more and more light comes back on to the lower negative side The spring thus appears to be at a more negative position even though it 1s rising The converse holds true if the spring 1s being lowered down from above the detector The Zero function of the SIM see SIM Reference Manual automatically accounts for this when zeroing the spring position As a check to make sure everything 15 functioning normally the spring should oscillate between positive and negative voltage when it 1 around the true zero center of its range 4 1 3 Superspring will not go to Zero In some cases the automatic zero function will not bring the system to the center of its range Normally this happens when the s
47. le yellow Lemo connector from the power supply panel to the connector at the base of the Superspring The DC Power 35 IN Micro g LaCoste FG5 User s Manual should still be off at this point but double check do not attach the Superspring control cable while Servo is on Release the Superspring travel lock by pulling out the brass travel lock knob until it engages the shaft and slowly rotating it counterclockwise until 1t reaches the stop approximately 180 Slowly release the lock knob The arrow on the lock knob points down when the spring 1s locked and up when it is unlocked Use a voltmeter to monitor the spring position SS POS and wait for the spring to settle down so that the scatter 1 about 50mV or less 3 1 8 DC Power On Turn on DC Power Near the DC Power switch note that the Rubidium clock light will flash briefly and then stay dark until the clock locks at 10 MHz typically within 5 or so minutes On the SIM verify that the dropper is in reset mode and the spring has both ZERO and SERVO off Depending on conditions it is possible that one or more of these switches will be ON when the power is enabled Make sure to turn them off as quickly as possible For more information on the SIM please refer to the SIM User s Manual Next determine the spring position After un travel locking the spring will take a few minutes to settle After it has determine its approximate mean position If it is farther than
48. ler is now insensitive to triggers OSC oscillation mode is used to slowly raise and lower the cart the object 1s never in freefall to create slow and constant interference fringes The magnitude of this fringe signal is used for system alignment purposes see Setup information in Section 3 1 10 To initiate OSC mode first make sure the dropper 15 un travel locked and the press OSC You should see the position LEDs on the front of the SIM indicate a slow movement of the cart To stop OSC mode press the TRIGGER button at any time The cart will automatically stop at the bottom of the next oscillation cycle Alternatively you can press DROP and this will place the controller in DROP mode at the bottom of the next oscillation cycle Take care not to hit the RESET button directly as this will immediately and roughly drop the test mass and cause excessive wear on the balls and vees 18 IN 2 4 Interferometer Theory Micro g LaCoste FG5 User s Manual Refer to Figure 7 and Figure 8 for the following descriptions of the beam path The optical fiber directs the laser light from the laser head to the interferometer base At the input of the interferometer a lens collimates the light from the optical fiber It is then directed to Beamsplitter 1 where it 1s 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 and the fringe vie
49. ll repair or replace defective components of the instrument on a timely basis at its sole expense provided that such warranty service shall be performed by seller at its facility in Lafayette Colorado U S A and all cost of returning the instrument to seller shall be borne by purchaser This warranty does not cover labor costs and other contingent expenses incurred by purchaser or a third party for the diagnosis of defects and does not extend to the instrument if it has been a subject to misuse neglect accidents acts of God or causes of a similar nature or b altered by anyone other than seller without seller s prior approval This warranty is in lieu of all other warranties except seller shall pass through any warranty issued by a manufacturer of any component part of the instrument and subrogate purchaser with respect to any claims thereunder This warranty is expressly conditioned on the following performance by the purchaser during the warranty period I In repairing or replacing component parts in the instrument purchaser shall use only the following parts A parts supplied by seller B parts obtained from third parties to the extent such parts were made by the same manufacturer as the part being replaced and C similar parts upon prior written approval of seller which approval shall not be unreasonably withheld II During the warranty period set forth above purchaser shall give seller prompt notice of any and all problems a
50. m of units This CGS unit of Icm s is given the name Gal after the famous father of gravity Galileo Using these units the nominal gravity is 980 Gal Precision gravity measurements are often given in units of micro gals 1 uGal 10 Gal measurement resolution of 1 micro Gal pu Gal therefore requires a measurement of earth s field with a precision of 1 part in 10 1 part billion Another IN Micro g LaCoste FG5 User s Manual common gravity unit used in field measurements of gravity is the mGal 1 mGal 1000 uGal 1 uGal 10 Gal The conversion between uGal and SI units is 1 uGal 10 m s 1 4 FG5 Design Features The 5 incorporates a number of significant advancements over previous designs which reduce or eliminate systematic errors identified in the earlier versions These improvements are Enhanced electronics reflect newer technology and make the instrument smaller and easier to use This absolute gravimeter is designed to work with iodine stabilized laser systems WEO models 100 and 200 traceable to the BIPM primary standards The system controller has been updated to a standard Intel based laptop computer a user friendly Windows based full featured software program is used for data acquisition 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 dat
51. meter base Remove the legs from the tripod Place the legs in the dropping chamber case along with the tripod tray and feet o Close dropping chamber case and secure all latches e Interferometer Micro g LaCoste FG5 User s Manual OO O O 0O 44 Micro g LaCoste FG5 User s Manual Insert the dust plug into the top of the interferometer base Loosen the four 5 lobe knobs which attach the interferometer base to the Superspring Remove the interferometer base from the Superspring and gently place it in the shipping case along with the laser Take care not to stress the fiber optic Close the interferometer case and secure all latches e Superspring cont O O O The Superspring should already be travel locked Insert the dust plug the top of the Superspring Loosen the three 5 lobe knobs which attach the Superspring to the Superspring tripod Remove the Superspring from the tripod and place in its shipping case Close the Superspring shipping case and secure all latches 45 IN 4 Adjustment and Maintenance With proper and careful setup and operation the FG5 should provide accurate and reliable results However if you suspect that there is a problem before attempting repairs or adjustments yourself it is usually a good idea to first contact Micro g LaCoste at Micro g LaCoste FG5 User s Manual www microglacoste com contact htm So while a full list of all adjustments 1s beyond the scope of
52. n for data acquisition it is NOT necessary to enter values for h The correct Guide Card Parameters must be set in the same manner using the Setup button beneath the Fringe Card box The recommended settings for a standard FG5 dropper are Input Multiplexor 4 Prescale 250 Fringes Acquired 700 39 A2D card settings must also be entered For the National Instruments card the recommended settings are as follows Micro g LaCoste FG5 User s Manual Table 1 DAQ Card Settings Multiplier Channel 0 0 100 Channel 1 0 Channel 2 0 Channel 3 0 1 Channel 4 537 5 125 Single Ended The final change on the system page is to ensure that the Serial Baro box is unchecked 3 2 3 Acquisition Setup Next select the appropriate start time option and then enter the drop interval a minimum of 5 seconds should be used with an FG5 The set interval should set to your choosing Finally look at Pulse Delay the time between the lift and drop of the test mass This should be about 1 3 of the drop interval or set to a minimum of 3 seconds If the intervals are inconsistent the software will warn you Typical intervals for an FG5 10 seconds per drop There is a whole possible philosophical discussion of how much data 15 enough The short answer 15 to take enough data so that statistically you have the precision you desire This might require a short test run to determine the drop to drop sc
53. nner wrenches provided in the FG5 toolkit to tilt the dropper tripod so that the dropping chamber bubble levels are centered to agree with those of the Superspring See Figure 18 Location of the Dropper Tripod Adjustable Feet By adjusting the height of these 3 feet the tilt of the dropping chamber can be made to match that of the superspring for the location of the feet used to tilt the dropping chamber tripod relative to interferometer base Dropper Tripod Adjustable Feet Figure 18 Location of the Dropper Tripod Adjustable Feet By adjusting the height of these 3 feet the tilt of the dropping chamber can be made to match that of the Superspring 4 3 The Laser Laser issues can usually be grouped into two categories stability issues and alignment issues Stability the laser wavelength is crucial this provides the 54 IN Micro g LaCoste FG5 User s Manual length standard with which the gravity value is calculated It must be stable to at least few parts in 10 Alignment problems in the laser usually result in a loss of power which will ultimately cause a decrease in the interference fringe amplitude If the amplitude is too small the data acquisition system will fail With a bit of careful analysis it can actually be quite easy to at least diagnose most laser problems 4 3 1 Laser Stability ML 1 lasers For a complete description of the ML 1 operation please consult the ML 1 User s Manual Temperatu
54. normally though the turbo pump is still helping at this point If the voltage 1s stable but the current continues to increase after 5 or so minutes turn the ion pump off and wait approximately one hour before trying the ion pump again Once the ion pump has started and the current seems to be steadily decreasing close the vacuum valve fully but leave the turbo pump It 1 normal for the current to increase after the valve 15 closed but after a few minutes it should begin decreasing again as the 10n pump continues to pump After the current has begun to decrease reliably for 5 or so minutes the turbo pump can finally be turned off After the turbo pump has come to a stop use the relief valve on the turbo pump to re fill the vacuum hose with air and then close the valve so as to be ready for a future pump down Remove the hose from the chamber and turbo pump Replace the blank flanges on the vacuum valve and turbo pump intake and remove the bellows tube Replace the turbo pump in the shipping case it will no longer be needed for operation 63 6 Electronics Connections Micro g LaCoste FG5 User s Manual Below 1s a list of the basic connections needed to measure gravity with the FG5 Note that when shipped from the factory the cables that connect the various FG5 electronic components are labeled 6 1 SIM e Front o g ION Ion Pump Current o Voltage WEO 100 laser o LASER LOCK 1 laser o LASER MOD
55. nt Because this procedure maximizes the interference fringe amplitude this is often referred to as fringe optimization The configuration of the Mach Zender interferometer is such that it gives the user complete freedom to make the test and reference beams perfectly coincident and collinear at the detector Please see Section 3 1 10 for detail on the setup steps The tipping plate is adjusted to translate the test beam so as to be coincident with the reference beam This 15 best viewed the viewport at the end of the interferometer 22 IN not the telescope By tilting the tipping plate about its horizontal and or vertical axis the test beam is simply translated relative to the fixed reference beam Note that the angle of the test beam 1s not changed This should be adjusted so that the reference beam 15 perfectly coincident with the reference beam in the viewport Micro g LaCoste FG5 User s Manual In addition the angle of the test beam can be changed using mirror 2 below the telescope This should be adjusted so that the test beam is perfectly overlapped with the reference beam in the telescope Again when two points meet at the same point in the telescope they are parallel When the test and reference beams are truly collinear after beamsplitter 2 concentric bull s eye fringe patterns will be visible the viewport window Good overlap should also be quantitatively verified using an oscilloscope to view the
56. od should be aligned either north or south to minimize Coriolis accelerations on the dropped object The tripod can 3l IN Micro g LaCoste FG5 User s Manual be centered over the mark by viewing the mark through the hole the center of the tripod Rough level the Superspring tripod using bull s eye level If using an FG5 with the Remote Operation option please consult the Tele g User s Manual for information on leveling the Superspring tripod Measure the lower reference height using the depth gauge provided The lower reference height is the distance between the Superspring tray ring and the reference mark approximately 5 15 cm Place the depth gauge parallel surface on the machined inside ring of the Superspring tripod Pass the gauge rod through the hole in the center of the Superspring tripod and extend it until it hits the reference mark on the floor Tighten the locking screw and measure gauge length using the ruler Record this value in the system check log This is referred to as the lower set up height Place the Superspring in the tripod Orient the Superspring so that the travel lock brass knurled knob on the service ring is pointing toward the bull s eye level Clamp the Superspring to the tripod by turning the three 5 lobe knobs fully clockwise This rotates the clamps in place over the base of the Superspring Level the tripod noting the two precision level vials on the base of the Superspring Be
57. oosen the Allan screws and remove the broken flexure Take a replacement flexure standard pieces in an FG5 toolkit and attach the end on the outside of the Superspring first The outside is the portion that connects to the main rings and vertical posts that are attached to the Superspring base Next attach the inner side of the flexure Un travel lock the Superspring and let it settle Locate the 6 guide pins along the Mainspring tube as shown in Figure 14 and use a feeler gauge or eyeball in worst case to verify that that all 6 gaps are equal If one or more is too small loosen and reset the inner portion of the corresponding flexure Iterate this procedure for all flexures until all 6 gaps at the guide pins are equal 48 Note Never loosen the guide pins These are set at the factory and are your only reference for aligning the flexures that allows the servo mechanism to have the necessary clearance to function Micro g LaCoste FG5 User s Manual Figure 14 Using a feeler gauge to monitor the gap spacing on the Superspring guide pins 4 1 5 Alignment Completely aligning the Superspring 15 beyond the scope of this manual but the rare case that the Superspring bubble levels become misaligned resetting them 1s simple Just place the Superspring on a perfectly level table and set the bubbles to the center using a 2 5 mm Allan wrench and an 8 mm open end wrench 4 2 Dropping Chamber Dropping cham
58. opping chamber 3 1 1 Electronics Case For a full discussion of the FG5 electronics please refer to either the Tele g or SIM instruction manual e Place the electronics case in a convenient location about 1 meter from the reference mark Check the input voltage settings and make sure they are set to the proper AC line voltage All components in the FG5 system with the exception of the WEO laser automatically switch to accommodate any line voltage from 100 240 VAC 50 60 Hz Please refer to the WEO laser manual for instructions on setting the proper line voltage If ion pump has been maintaining the dropper vacuum on battery power apply AC power to the ion pump power supply See Section 5 for more details on the ion pump controller e Make sure the following switches are off o Power Supply Main AC power rear o Power Supply Main DC power rear o Laser power both main AC power and high voltage key switch e Open Interferometer case and place the laser on the floor about 1 m from the reference mark Take care not to stress the fiber optic cable Connect the main AC power cable from FG5 power supply rear of electronics case to the AC power receptacle If an uninterruptible power supply is to be used with the FG5 it should be connected to the main AC 29 Micro g LaCoste FG5 User s Manual power source and the FG5 AC power supply should be connected to the UPS output e Turn on the main AC pow
59. opping chamber into dropping chamber tripod with the ion pump oriented towards the beam blocker side of the IB Place the dropping chamber tripod feet under the tripod and adjust them until they are just touching the legs of the dropping chamber tripod Turning each foot one turn counter clockwise one turn at a time lift the dropping chamber two total turns Fine tune the level of the dropping chamber If necessary fine tune the level of the Superspring Measure and record the upper reference height Un travel lock the dropping chamber Verify DC power is OFF Connect the cables from the electronics to Superspring IB and dropping chamber Un travel lock the Superspring Turn on DC power 68 gt 00060000 OODD Micro g LaCoste FG5 User s Manual If necessary Zero the Superspring When spring motion has calmed down enable Superspring Servo Check and adjust 1f necessary beam verticality Use Twiddler and lower mirror to maximize fringe signal check with oscilloscope and OSC mode Check and adjust 1f necessary beam verticality again Place dropper in DROP mode Verify that Rubidium clock has stabilized POWER light 15 steady ON Turn on Magma Box if necessary Turn on laptop PC power Turn computer power ON and set up software Enter the total reference height and check all other parameters Take data 69 IN 000000 00900090999 Micro g LaCoste FG5 User s Manual Tear Down Back up data 1f applic
60. oscope and place the oscilloscope in mode 500 mV div on both channels and 250 ms time Use the oscilloscope s position knobs to center the two grounded channels in the middle of the oscilloscope Finally attach the or sum channel to a digital voltmeter With the oscilloscope set to DC coupling on both channels use the adjustment knobs on the XY detector to center the signal in the oscilloscope Manually run the cart up and down using a 4 mm Allen wrench Watch the value on the voltmeter and make sure that it doesn t change by more than 10 throughout the travel of the cart thus indicating that laser light 15 staying on the detector the whole time If the spot moves visibly as the cart 1s raised and lowered use the tripod feet to tilt the chamber so that the spot appears stationary Then repeat the above center the signal and make sure the spot stays on the detector throughout the cart s travel 52 Micro g LaCoste FG5 User s Manual Figure 17 Using the X Y detector to set dropping chamber bubble levels In general as the cart is lifted up and down it will also move slightly sideways Thus the reflected laser beam will appear to translate Tilt the chamber until the position of the reflected beam at the top of the drop is the same as at the bottom of the drop Then set the bubble levels to read zero Again the rod bend and translation are exaggerated Manually lift the cart to the top of its travel Note the
61. ployed throughout the world The FG5 Absolute Gravimeter The FG5 absolute gravimeter is a high precision high accuracy transportable instrument that measures the vertical acceleration of gravity The operation of the FGS is simple in concept test mass is dropped vertically by a mechanical device inside a vacuum chamber and then allowed to fall a distance of about 20cm A laser interferometer 15 used to determine the position of the test mass as a function of time during its freefall The acceleration of the test mass 15 calculated directly from the measured trajectory IN The interferometer generates an optical interference fringe each time the test mass falls the wavelength of the laser light These fringes are counted and timed with an atomic clock to obtain precise time and distance pairs A least squares fit to these data are used to determine the value of g This method of measuring gravity is absolute because the determination is purely metrological and relies solely on standards of length and time The distance scale 15 given by a frequency stabilized helium neon HeNe laser used in the interferometer A rubidium atomic time base provides the accurate time scale The value of gravity obtained with the FG5 can be used without the loop reductions and drift corrections normally required when using relative instrumentation With the FG5 the absolute gravity value is determined and reported immediately Micro g LaCoste FG5 User
62. pring is so far out of range that the circuit thinks the spring really 1s at zero or it 1s so close to zero that the motor moves extremely slowly The trick is to enable ZERO on the SIM and then travel lock and un travel lock the Superspring perhaps repeatedly This will cause the mainspring to bounce and the end will at some point cross the detector This allows the circuit to see the true position and adjust the position accordingly 47 IN If that does not work open the Superspring as described above and check to make sure that the limit switches have not be engaged Micro g LaCoste FG5 User s Manual 4 1 4 Gravity data shows very large Scatter or large Residuals If the gravity meter shows larger than normal scatter or the residual size is larger than about 10 nm this can indicate bent or broken Superspring flexures This is typically caused by rough handling during transport Remove the Superspring cover as described above and check each flexure for straightness Figure 13 shows a photograph of a bent Superspring Flexure Figure 13 Bent Superspring Flexure The small wire and the brass portion of the flexure should be parallel If the flexure s 15 bent the repair is usually as simple as taking a standard pencil and using the eraser end to push the flexure back straight If the small wire portion of the flexure has completely broken off it will be necessary to replace the flexure Simply l
63. quisition This brings the cart safely down the bottom at the end of next OSC cycle Note that you can press the DROP button at any point in cart s motion Alternatively pressing TRIG will place the dropper in REST mode at the end of the next OSC cycle Pressing RESET will cease all power to the motor causing it to drop from wherever it 1s If the fringe amplitude is too small less than 250 mV or has dropped noticeably from earlier measurements it 15 most likely an alignment problem Make sure that there 15 enough range in the Twiddler to overlap the beams well Make sure that the beams are both circular and not clipped Laser power 15 also directly related to fringe amplitude and it 15 possible that the laser may have lost power to a bump that has changed either the internal or fiber optic cable alignment While a complete discussion of laser power 15 beyond the scope of this manual see WEO 100 User s Manual double check the DC voltage laser power on the WEO controller to make sure it is what you expect typically 5 10 V 15 adequate Finally a double check against any bumps or misalignments it best to check verticality one last time and then do not touch the instrument until the end of the measurement cycle 38 IN Micro g LaCoste FG5 User s Manual 3 2 Software Set Up Note See the g User s Manual for a complete discussion of the software and setup procedures Listed below are some FGS specific set
64. re changes are the biggest cause of laser wavelength instability in the ML 1 The laser housing 15 designed to be temperature stabilized but even minute changes in environment temperature fractions of a C can cause the laser wavelength to change which in turn causes a noticeable change in the measured gravity value even 10s of wGals As the temperature changes the RED or BLUE gravity values will change However the mean value is usually quite stable to a few uGals 1 if the RED drifts up the BLUE will drift down a roughly equal and opposite amount Always let the laser come to thermal equilibrium before acquiring gravity data typically 1 4 hours is necessary WEO lasers Again please consult the WEO User s Manual for a complete discussion of the laser operation Unlike the ML 1 if the controller indicates the laser is locked the wavelength of the laser output will be correct and stable However if the laser has not yet reached thermal equilibrium it will frequently lose lock status and then take a few seconds to re lock As described earlier the wavelength is labeled by a peak name e g e f or Each peak has a unique 1f voltage and this is what is entered into the software so that the correct wavelength can be used in the gravity value calculation When the laser is not in thermal equilibrium i e when it is first powered on these 1f voltages will change Therefore it is important to wait until equ
65. ring this time can cause g to crash and data to be lost This process can take several seconds 42 IN Micro g LaCoste FG5 User s Manual 3 3 2 Data Quality While a complete discussion of data analysis and interpretation 1s beyond the scope of this manual a basic understanding will help ensure that your data is of high quality Drop Residuals The residuals are the difference between the actual measured fringe location and the final best fit parabola Make sure that the residuals are relatively flat lt 1 nm The State Window the Drop Gravity Value Verify it is reasonable Note the values of the analog signals For example are the barometric pressure and spring position reasonable and stable o Note the value in uGals of the gravity corrections Are they reasonable Drop Gravity Is the drop to drop scatter reasonable given your location In a quiet stable laboratory this should be approximately 5 30 uGal In the field of course this might be higher Is the mean stable There should be no noticeable drift in the mean value throughout the set Laser ML 1 Between sets verify that the system 1 unlocking the laser and then relocking to the alternate laser peak before the next set starts From set to set are the RED and BLUE gravity values self consistent for some reason the RED and BLUE locks have become switched in the software a 1 4 milli Gal difference will be observed This can be easily
66. rough an optical glass sphere attached to the Superspring mass The sphere focuses this light onto a split photodiode detector mounted on the opposite side of the support housing This signal from the split detector 15 fed back to a servo circuit which drives the support housing vertically canceling any relative motion between the test mass and the inner housing The drive mechanism 1s a linear coil actuator mounted between the support housing and the Superspring base So as vertical ground motion occurs the linear actuator moves the support housing up or down in such 22 IN a way as to keep the main spring length constant This active servo effectively weakens the main spring synthesizing a long period isolation device The apparatus is constrained to move only vertically by a linear system constructed of five flexures delta rods arranged in an upper V shaped array and a lower triangular array Micro g LaCoste FG5 User s Manual The servo circuit is activated by turning SS SERVO on with FG5 System Interface Module SIM please see the separate SIM User s Manual for more information Note that if SS SERVO is off and the spring 15 un travel locked the spring is just hanging freely and bouncing with the system s natural frequency about 1 Hz The Superspring system also contains a DC motor that is used to center the test mass vertically in its range zeroing the spring as described in the Setup instructions Sec
67. s determination Therefore in practice the gradient at the measurement location 15 either estimated or measured separately and entered into the software see the g Software User s Manual for details For reference the free air value of the gradient is 3 086 uGal cm X Xo Vo 5 gol Finally because the arm of the interferometer that reflects off the test mass gets shorter during the drop and because the speed of light is not infinite the fringes at the bottom of the drop will arrive sooner than they should The equation thus has to be further modified as At n Wot p WZ yt 2 6 24 2 Xo Vg 5280 where f is the retarded time given by 11 IN 2 2 Micro g LaCoste FG5 User s Manual The gravity value determined from a least squares fit to the above equation 15 thus the best estimate for the absolute value of g at the beginning top of this particular measurement or Usually this number is recalculated at a more convenient height perhaps for future comparison and corrections are made for things like earth tide ocean loading polar motion current changes in barometric pressure These corrections and their applications are discussed in the g Software User s Manual Design Hardware Components and Function The FG5 System Figure 2 consists of a Dropping or Vacuum Chamber Interferometer Base Superspring Laser System Controller and Electronics The test ma
68. ser s Manual Once the heat tape has reached equilibrium at about 60 and with the turbo pump on and evacuating the chamber leave the heat on for at least 4 8 hours Then turn the heat off but leave the turbo pump on letting the chamber cool for approximately 12 hours routine that works well 1s to start the chamber heat in the morning monitor it throughout the day turn the heat off at the end of the day and let the turbo run and the chamber cool throughout the night It should be ready for the ion pump the next day 5 3 Starting the lon Pump At this point the system should have cooled to room temperature The dropping chamber should be under a good vacuum the turbo pump should be operating at normal speed and the 1 pump should still be turned off The next step 1 to migrate the system to the ion pump so that the turbo pump can be removed If necessary remove the heat tape from the system and replace ion pump magnets Attach ion pump HV and safety ground cables to the chamber and the ion pump controller Turn on the ion pump controller AC power DC power this charges the backup battery and HV Monitor the 10n pump voltage on the main display by selecting kV Leave the turbo pump running and connected to the dropping chamber Check that the voltage 1 increasing to approximately 4 kV reading 4 0 on the kV scale within five minutes after turning on the ion pump If the 10n pump voltage has not reached t
69. ss is allowed to free fall inside the evacuated Dropping Chamber The Interferometer Base or 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 Each component is discussed detail below 12 Micro g LaCoste FG5 User s Manual Vacuum Chamber Free Falling Drag Free Corner Cube Chamber Drive lon Pump Motor Photo Detector Laser Light Interferometer Support Springs Mai Internal Reference Hr Corner Cube Spring Servo Coil Figure 2 The System The laser system controller and electronics rack are not shown 13 2 3 Dropping Chamber Theory Micro g LaCoste FG5 User s Manual 2 3 1 The Dropping Chamber The Dropping Chamber is an evacuated chamber which contains a drag free cart which in turn houses the test mass corner cube Figure 3 shows a schematic drive mechanism 1s used to drop track and catch the test mass inside the drag free cart Laser light passes through a window in the bottom of the Dropping Chamber to the corner cube inside the test mass and 1 then reflected back down through the win
70. ssociated with the operation or integrity of the instrument 67 IN Micro g LaCoste FG5 User s Manual 10 Checklists Setup For details on each step please consult Section 3 0000 OOO 0000090 D DD 00000 DDD O Place electronics box approximately 1 m from measurement location Check line voltage settings especially for WEO laser Check that 1 pump is powered on and operating correctly Connect to reliable AC power source Make sure the FG5 Power Supply AC amp DC power 1 off Make sure the WEO Controller switches AC and HV are set to OFF Turn FG5 Power Supply AC power Place laser on stable surface connect cables to WEO controller turn on WEO controller and enable HV The laser should emit laser light within a minute or SO Place Superspring tripod at measurement location Orient the line from the center of the tripod to the bull s eye level along a North South axis Use tripod feet to center the bull s eye level Measure and record the lower reference height Place and lock Superspring in tripod with travel lock oriented towards bull s eye level Level the Superspring tripod to the Superspring bubble levels Remove Superspring dust cap Place IB on top of Superspring with fiber optic oriented towards bull s eye level Assemble dropping chamber tripod Place dropping chamber tripod onto IB with the small hole in the tripod oriented towards the bull s eye level Remove dust cap from IB Place dr
71. st mass 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 The two beams are recombined at Beamsplitter 2 and then split again One set 15 focused by a lens to strike the detector APD The interference fringes are converted to an Analog signal and a Transistor Transistor Logic TTL signal which 15 transmitted to the time interval analyzer card in the system controller 20 The other recombined beam set travels horizontally until it reaches the attenuator plate rattler This beam is split yet again and reflects between 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 travels horizontally into the fringe viewer The second and third beams are deflected vertically by a mirror A flag in front of the mirror blocks the second beam allowing the third dimmest beam to exit the interferometer where it 1s reflected off Mirror 3 and enters the collimating telescope Micro g LaCoste FG5 User s Manual APD Mirror 3 Fiber Collimating Assembly Beam Splitter 1 Beamsplitter 2 Rattler Mirror 2 1 Twiddler Figure 8 Schematic of FG5 interferometer 2 4 1 Beam
72. t case Remove the three tripod legs from the dropping chamber case and attach them to the tray If necessary tighten the legs by using the 30 cm cheater bar At this point the interferometer will be used to support the dropping chamber tripod First remove the dust cap from the top of the interferometer base Carefully place the dropping chamber tripod on the interferometer Orient the tripod so that the small hole 15 above the Superspring travel lock and laser fiber Be careful not to damage the mirror on the right side near the telescope of the interferometer base Carefully remove the dropping chamber from its case by the handles and gently place it into the pocket in the top of the tripod tray allowing the two vertical alignment pins in the tray to engage the sockets the dropping chamber base Orient the dropping chamber so the 1 pump is directly above the beam blocker controls on the interferometer Lock the dropping chamber in place with the three clamps by turning the 5 lobe know fully clockwise This rotates the dropping chamber clamps in place over the base of the chamber 3 1 5 Leveling the Dropping Chamber Note While leveling the Superspring and dropping chamber tripods note that turning the tripod feet clockwise lowers the dropping chamber tripod and raises the Superspring tripod Check the Superspring levels and adjust 1f necessary by leveling the Superspring tripod When the Superspring levels are centered
73. t to the laser head Loosen as little as possible so that the angle is not significantly changed and yet the 5 axis mount can still translate While monitoring the output power move the whole 5 axis mount relative to the laser beam until the power is maximized It is often possible to get an additional 15 uW of power using this trick When the power is maximized tighten the 5 axis mount back in place and fine tune with the 5 adjustment screws if necessary Goals o The isolator will transmit roughly 60 of the laser power o The fiber will transmit roughly 60 70 of the power With an ML llaser producing about 1 2 mW it should be possible to achieve 400 uW of power at the output end of the fiber 58 IN Micro g LaCoste FG5 User s Manual 5 VACUUM CHAMBER Turbo Pump and Baking Out Under normal operations the vacuum in the dropping chamber is maintained by an ion pump Any residual molecules in the chamber that enter the 10n pump are ionized by the 4 kV potential and plated out and thus the pressure the chamber 15 directly proportional to the number of ionizations per second 1 e the current drawn by ion pump Normal operation of ion pump is indicated by 4 kV and a stable current of less than about 3 10 A though it is normal for the current drawn to fluctuate with temperature This procedure only works at relatively high vacuum levels 10 mbar If the vacuum has been degraded i e ion pump has b
74. test beam overlaps the reference beam Use the two knobs that are diagonally opposite of each other The two beams are now at least roughly coincident and parallel Connect the ANALOG FRINGE output on the SIM to an oscilloscope with the following settings o Vertical scale 50 mV div o Horizontal scale 2 usec div o AC coupled input o Automatic trigger at OV Make sure the laser is locked Set the dropper to OSC mode by pressing OSC on the SIM This moves the cart slowly up and down at a constant velocity and produces a constant frequency fringe signal which is useful for adjusting the Twiddler and Mirror 2 Maximize the fringe signal on the oscilloscope by adjusting Mirror 2 Use the two adjustment knobs that are located diagonally from each other Optimize the mirror in both directions horizontal and vertical Further maximize the fringe signal on the oscilloscope by adjusting the twiddler 37 IN Micro g LaCoste FG5 User s Manual Return to Mirror 2 and fine tune in both directions to perfectly maximize the fringe amplitude Nominal fringe signal is 300 400 mV peak to peak Record this value in the system check log In general do not operate the FG5 if the fringes are below 300 mV or above 400 mV This can adversely affect the measured gravity value See Section 4 for troubleshooting details Terminate OSC mode by pressing DROP on the SIM this places the dropper controller in DROP mode ready for data ac
75. the dropping chamber tripod levels should be within two divisions or so of the center position If the levels do not agree this may indicate a problem Consult Section 4 on adjustment and maintenance for instructions Remove the blue pads and brass tripod feet from the Superspring case Make sure the pads and tripod feet are clean Place a tripod foot under each leg of the tripod Raise each foot and slide a blue pad under the foot Center the cone in each foot under the nylon ball on the end of each tripod leg Turn the leveling adjustment screws on the feet counterclockwise raising them until they just barely contact the balls It is important that the foot and the nylon foot of the tripod leg are perfectly centered with each other Otherwise the dropping chamber will shift sideways when it 15 lifted It is helpful to rotate or wiggle the foot slightly while it is in contact with the nylon ball to release any horizontal tension After each foot is in contact with the tripod leg rotate each tripod foot leveling screw one revolution counterclockwise using the mark on the top of the 33 IN adjustment screw as a reference Then rotate each tripod foot leveling foot one additional revolution as described previously It 15 important to do one turn at time this keeps the dropping chamber centered over the interferometer The two total turns raise the tripod off of interferometer so there 15 no contact between the two componen
76. tion 3 1 7 This motor is activated by turning SS ZERO ON with the SIM controller By monitoring the SS POS connector with a voltmeter one can see the position approach 0 V indicating that the mainspring is in the center of its range Note that if the gravimeter has moved to a location with a drastically different local gravity value than the previous measurement location it might take a few minutes for the spring to get to this zero position This 15 normal Finally though it will not cause permanent damage to the system it 1s best not to zero the Superspring with the servo activated 20 IN 2 6 Laser Theory The FG5 employs a stabilized helium neon laser to provide an accurate and stable wavelength used in the interferometric measurement system There are three lasers which are currently available for the FG5 Please see the user s manual appropriate to the laser type that shipped with your FG5 system Micro g LaCoste FG5 User s Manual e 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 1s a highly stabilized frequency standard trivially converted to a distance standard in a vacuum having an absolute frequency accuracy of 1 part in 10 50 kHz e The Winters Electro Optics Model 200 iodine stabilized laser This laser is based on the 100 model but 1odine stabilized laser
77. ts Each counterclockwise revolution of the leveling screw raises the tripod about 0 75 mm e Fine tune the level of the tripod tray by adjusting the dropping chamber tripod feet not the Superspring tripod legs Again be sure to adjust the cross level first then the long level The cross level 15 parallel to the telescope and the long level is perpendicular to the telescope e t is best to adjust the levels by raising the appropriate adjustment foot If the foot 15 used to lower that side of the tripod there 15 a risk that the tripod will contact the interferometer Micro g LaCoste FG5 User s Manual Note that there must be no contact between the tripod dropping chamber assembly and their cables and the interferometer Superspring assembly and their cables during the measurement Check the Superspring levels and adjust if necessary using the leveling feet on the Superspring tripod At this point all Superspring and Dropping Chamber bubbles should be centered e Measure the upper set up height using the ruler and fixture provided The upper reference height 1s the distance between the top of the interferometer base and the bottom of the dropping chamber Loosen the clamp on the ruler and pass the ruler up through the access hole in the dropping chamber tripod while pulling it slightly towards yourself the hole 15 located directly above fiber optic input on the interferometer base until it contacts the top of the interferomet
78. up notes Power on the computer system as follows 3 2 1 e Power on the AC to the Magma PCI unit Turn on the power switch on the front of the Magma e Verify that the PCMCIA cable is attached from the Magma PCI unit to the PCMCIA slot on the laptop computer e Power on the laptop computer e Note do not let the laptop sleep or hibernate this will cause problems with the laptop Magma connection and it will then be necessary to reboot everything in the above order Information Setup Reference Height enter the total reference height the sum of the lower and upper reference heights measured in Section 3 1 3 2 2 System Setup FGS5s that ship with L Series Micro g ML 1 lasers The laser frequencies are calibrated at Micro g see Section 2 6 for details on the ML 1 laser It should not be necessary to change these values Set the Pre run lock time to be approximately 30 60 This is the time the laser is allowed to lock prior to the beginning of each set In the Acquisition section it 1 necessary to make sure that there 1s enough time to take all the data and allow for laser lock between sets FG5s that ship with WEO 100 or 200 lasers These wavelengths are determined at BIPM and should never be changed Refer to the WEO manual for the exact value of the laser modulation frequency Use a voltmeter to measure the 1f voltages for peaks d g and enter these values here Note that if a peak d g 1 chose
79. uter and power the computer down Turn DC POWER off Turn off the laser high voltage and then turn off laser power Back up data if applicable and shut down the laptop computer Turn AC POWER off Dropping Chamber and Superspring o Travel lock the dropping chamber Lock the cart by turning the locking hub motor shaft counterclockwise using 4 mm Allen wrench or ball driver until the cart stops moving Pull and rotate travel lock knob 90 allowing the pin to drop onto the hub then rotate the locking hub clockwise until the pin engages the hub o Travellock the Superspring Pull out the travel lock brass knob until it engages the locking mechanism and rotate the lock 180 clockwise to lock it in place The arrow on the lock knob points down when it 15 locked and up when it 15 unlocked o Open the three dropping chamber clamps by turning the 5 lobe knobs fully counterclockwise so the clamps are outside the bottom flange of the dropping chamber e Gently lift the chamber off the tripod and set it in its case If the HV and safety ground cables are still connected take care not to stress them e Disconnect the following cables o APD power cable green Superspring control able yellow Dropper motor orange and encoder blue Interferometer TTL BNC cable Interferometer Analog BNC cable Ion pump monitor BNC cable o Any cable connected to the system computer e Dropping Chamber Tripod o Carefully remove the tripod from the interfero
80. wer Note that 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 passes down through the interferometer base to the Superspring The test beam passes through the top window of the Superspring chamber to the corner cube in the Superspring mass From here the test beam is reflected back up out of the Superspring and into the interferometer base where it reflects off Mirror 1 or the pick off mirror passes through the translator plate twiddler reflects off Mirror 2 and 15 recombined with the reference beam at Beamsplitter 2 19 Micro g LaCoste FG5 User s Manual Figure 7 Beam path in FG5 interferometer The reference beam passes straight through the first splitter and is then split again to enter the detector and the optical devices for alignment The test beam leaves vertically from the first splitter travels through the dropper and Superspring and is recombined with the reference beam at splitter 2 Note that only the telescope and not the fringe window viewport is shown in the figure This interferometer 1 of the Mach Zender type with a fixed reference arm and variable test arm During a drop the motion of the te

FG5 Absolute Gravimeter User`s Manual - Micro

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