HYDROSTATIC LEVELLING SYSTEM GOING MOBILE

P. Bestmann, C. Barreto, C. Charrondiere, CERN, Geneva, Switzerland

Abstract

The LHC Collimator Survey Train has alreadyshown that automated survey measurements in theLHC are technically feasible [1]. Nevertheless manyconstraints apply when making automated measure-ments in an accelerator environment. The researchof adapted measurements techniques and strategiesis an essential part in the development process ofa new generation survey train. From the automa-tion point of view, the measurements in the verticalplane are particularly challenging and one solutionwould be the use of a Hydrostatic Levelling System.They are frequently used in high precision monitoringapplications but with a few compromises a mobileand very flexible version can be build. This paperdescribes the approach, development and tests of amobile HLS which is able to cope with the constraintsand boundary conditions given by the LHC.

INTRODUCTION

Hydrostatic Levelling Systems (HLS) are standardin high precision and long term monitoring applica-tions. Depending on the used system and technologya precision of a few microns can be obtained. This isvalid with stable conditions. Stabilized temperatureor temperature compensation, controlled atmosphericpressure and vibration decoupling are making suchprecisions achievable. A mobile application will notfulfil these conditions -or- will loose its advantageof being mobile and flexible. A first prototype ofa mobile HLS (mHLS) is showing that under thegiven conditions the precision of the mHLS is at leastas good as the optical levelling. In addition, sucha system is scalable with a minimum effect of theprecision. Doubling the measurement distance willnot double the errors. This makes a mHLS system aserious candidate for automated height measurementsin the LHC tunnel.

SYSTEM DESCRIPTION

Different HLS technologies are available with differ-ent precisions and all of them have their advantagesand drawbacks. Capacitive systems are sensitiveto condensation or water on the electrode surfaces,tactile systems to water on the needle and floaterbased systems to the mechanical manipulation duringa mobile use. Therefore the most promising systemwas an ultrasonic system with the transducer alreadyemerged in water. Such a System was developedat DESY and used for years with good experience

feedback [2]. DESY made one of their recent systemsavailable for some tests at CERN.

Sensor Configuration

The ultrasonic system from DESY has a mea-surement range of ± 10mm which can be extendedto a certain limit but taking into account a largerextrapolation from the reference piece. The heightdifference to be measured between two consecutiveLHC magnets can reach up to 250mm, dependingon the position in the ring. This is far beyondthe range of a possible extension of the UltrasonicHLS sensors. Therefore the sensors are supportedby a vertical translation stage equipped with a highprecision linear encoder. The sensors are kept withthe initial range which is limiting the extrapolationfactor and shifted vertically using the stages. Moreoverthis allows to position both pots using the theoreticalheight difference to have almost the same HLS readingswhich is reducing even more systematic errors. Themajority of the height difference is measured with theoptical scales and only the last fractions of a mm ismeasured using the HLS. The system is composed by:

• A vertical translation stage with build in motorcontroller.

• The composite measurement pot with referencepiece and transducer.

• A high precision absolute optical scale.• The measurement rack with electronics and power

supplies.

Figure 1: Section of measurement pot.

The translation table is a small and lightweightdovetail table with 250mmrange and build-in motorcontroller. The communication is based on a RS232protocol. The controller provides relative positioningdata using the stepper controller which is not preciseenough to measure the vertical displacements. ARensihaw RSLA encoder with an absolute optical scaleis used to measure the vertical position of the HLSpots. The accuracy is given with ± 1µm/m andthe communication is based on the BiSS -C Protocol.The scale is glued with a fixed point on the bottomof the sensor support while the read-head is directlyattached to the support of the HLS reference piece.The composite pot and the transducer itself cantherefore be dismounted without altering the systemconstants. A measurement is combined by a sensorconstant (Hardware related offset of the scale and thelower reference surface of the sensor body) plus thereading of the optical scale plus the HLS reading.In order to avoid useless water movements or waterspilling out during the system manipulation, a pinchvalve is installed in the centre of the tube. Thisvalve is normally closed and opened only for themeasurements.

Figure 2: mHLS system on a Quadrupole.

Electronics

The needed electronics are installed in a 19 inch rackon a mobile platform so that it can be towed by the

vehicles used in the LHC. The rack is housing:

• The two 12V 110Ah Batteries.• The 24V mains power supply, UPS and charging

circuit.• The measurement crate with main and rec boards.• The valve for the control of the water flow.• The PXI with touch-screen.

The 24V UPS system is giving an autonomy ofmore than 10h continuous battery operation. The PXIchassis has been especially chosen for 24V operationand the mains power supply of the HLS measurementcrate has been replaced by a custom build Versiondelivering the ±8V and 5V from the 24V UPS system.

Figure 3: Mobile Measurement Rack.

CONSIDERATIONS

Fully filled, single pipe systems are not often usedfor geodetic and precise measurement applicationsdue to their considerable drawbacks. Any differentialpressure and temperature gradient in the verticalwater column will lead to considerable errors on themeasurements [2]. A half filled system is not an optionfor a mobile system as it requires a horizontal pipeinstallation. A two hose system would be an optionto eliminate the differential pressure, but splash-wateror condensation in the air tube will make the mobileuse very difficult. In a first approach, the prototypeis build without air tube and the pressure is balancedthrough a 2mmhole on the top of the composite pots.

Pressure differences

The difference of atmospheric pressure between thetwo sensor positions has been measured in the LHCand with the used instruments so far not measurable.The constant airflow of 0.6m/s in the LHC tunnelmight however be a problem. First tests have shown

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Figure 4: Water density as function of the temperatureand corresponding height error for a water column of50mm.

that the orientation of the sensors with respect tothe airflow is changing the readings in the order of60µm. The figure 5 is showing the sensor readings fordifferent sensor orientations. Detailed measurementsof the differential pressure inside the pots still need tobe performed.

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Figure 5: Height error due to sensor orientation.

Water density and temperature effects

The temperature effects must be divided intodilatation effects on the hardware itself like the bodyof the sensor, the optical scale and the reference piece,but also the density of the used water [3]. The effectson the hardware can -assuming a stable ambienttemperature in the tunnel- be neglected in a firstapproach as they are the same for both sensors. Forthe ultrasonic measurement itself, the water densitydrops out of the formula due to the use of the referencepiece. But the vertical water column is much biggerthan in free water systems. A temperature differenceof 5◦C induces a height error of 1mm when using1.2m water column height which is the case for themobile system. A temperature difference at the potsand along the tube must be avoided or compensated.A temperature difference or gradient of the watercolumn can be reduced by permanently circulatingthe water within the system. In addition it can beconsidered that the ambient temperature in the LHCis stable to 2◦C along one day and the differencebetween the measurement position is smaller than 1◦C.

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Figure 6: Height error for different water columns

CONTROL APPLICATION

The control application for the system faces severalspecific challenges. On a first stage, developingthe individual drivers for the system main hardwareand modules. Afterwards, integrating them under ahigher level control under modular philosophy andincorporating data analytics and all types of auxiliarysystems into the process flow. In order to better facethese requirements, the development of the applicationwas done under LabVIEW, taking full advantage ofits core strengths: a rapid application developmentespecially regarding the integration of exotic hard-ware while still providing high level control and dataanalytics functionalities. The application is deployedon a PXI chassis (8180) able to accommodate all therequired communications protocols, namely USB forthe Optical Scale, RS232 for the translation stagestepper motors and TCP/IP for the HLS sensor. Thismodel also integrates a DC power supply model, arequirement for integration into the Survey train andfor a mobile application.

Architectural Overview

The development of the application needs to beaccomplished in a flexible, modular way, especiallyregarding the development of a novelty system whosecomponents can easily change in the near future. Forthis reason, basing the architecture under an ObjectOriented Programming (OOP) paradigm, we are ableto provide a great deal of agility and flexibility to arapidly evolving system. Synthetically, every majormodule represented can be replaced or expanded uponby simple inheritance, without any changes to thewhole application infrastructure, granting it a measureof durability in itself. The figure 7 is a representationof the main application modules, complete with a shortdescription of their functionalities.

Communication is carried out between the mod-ules by a queue based message system. By usingmessages that are classes themselves, as opposed tomore statically defined data types, we benefit fromthe same OOP derived advantages, leading to an agileand flexible communication network. Adding newmessages or replacing existing ones is again acted

Figure 7: Programm Modules and messaging betweenthem

out by simple inheritance from a parent ‘Message’class. Ultimately, this means that messaging – andconsequently system functionalities, can be expandedupon more readily and in a completely transparentway to the infrastructure mechanism that actuallycirculates the message objects. In this case, as theapplication follows a specific design pattern known asan Actor Framework based architecture, it concretelyimplements this mechanism via queues.

Using the Actor framework model in LabVIEW alsoaccelerates OOP development by making use of theprovided custom scripting. However, due to its veryparticular and distributed design characteristics, itrequires explicit documentation regarding its function-alities. With a certain level of familiarity, one can takefull advantage of the power of the framework in havinga uniform, coherent and asynchronous messaging sys-tem for a dynamically set number of control modules.

Main UI

The root of the application represents the main UIthe operator will interact with. It also launches theother modules as asynchronous, independent processesand issues their commands. It is entirely devoid ofany processing or execution functionality that wouldoverlap with the hierarchy of the other modules. Thiscommand loop also doubles as the display update one,by taking advantage of dynamic user events issuedfrom the Display Module. The GUI was designed tobe as intuitive as possible, moving and opening otherwindows as to better make use of the limited space inthe touch-screen utilized by the operator.

Display Module

To outsource the processing of the data that getsdisplayed to the user away from the module thatactually generates the data, the data is routed through

Figure 8: Main User Interface

the Display module for processing before being pre-sented in the Main GUI. Therefore every module canproduce its own data, regardless of the type, and sendit under a “Display Data” type object queue to theDisplay Module, who will be in charge of forwardingand processing the data, if needed.

A particular functionality is the routing of dataoriginating from the Optical Scales. Since there isno deterministic way of knowing which of the twoactors will be initialized first – seeing as it dependson the USB port they are physically connected to.This makes them obviously vulnerable to eventualswapping. To make the interpretation of their datatransparent, a function of the Display module isprecisely aimed at switching the two appropriatelybefore presenting it to the rest of the application.

OPERATION

The mHLS sensors are mounted on a standardfiducial interface (figure 2) and can be installed on themajority of the magnets and reference points. Bothsensors are identical and can take the role of the loweror upper sensor. The rack is positioned roughly in themiddle between the two points to be measured andthe mHLS sensors are installed on the fiducials. Theoperator selects the corresponding fiducials from theDatabase using the interface and starts the measure-ments process. The pots are automatically moved tothe theoretical height difference between the fiducials.The water valve is opened and the first measurementis initiated to control the communication between thesensors. A set of 20 individual acquisitions is doneand if an inverse correlation of the measurementscan be detected the test is validated and a waterstabilisation message is given to the operator. Thestabilisation of the system takes a maximum of 40seconds and is followed by three measurements with20 acquisitions each. For detailed tests, one sensoris then moved by 1mm and the measurements arerepeated. The Operator can see and assess the curveand the associated statistics. An automatic statisticaltreatment is helping to validate the measurements.

Once approved, the valve is closed, the sensors areswapped, the motors adjust again the height differenceand the second configuration is measured.One sensor stays and the second one is put on therack and moved forward to the next fiducial. This ischanging the first orientation of the system at everymeasurement which helps reducing systematic errorsfurther once the measurements will be done in oneorientation only. For evaluation and test reasons thesystem is still semi-automatic. The measurement andvalidation process will be fully automated in the futureand the operator will only be notified of the results andin case of problems.

TESTS

A series of laboratory tests have been made in orderto validate the setup in terms of stability and precision.Stabilisation time, temperature influence and also thehandling of the system were parts of the tests. Theonly test able to demonstrate if this system is reallypracticable for measurements in accelerators is the realuse in the LHC. The winter shut-down 2015/2016 wasused to make detailed levelling measurements of thesector 7-8 in the LHC along with the first tests of themHLS system.

Labtests

The sensors have been installed in a network to-gether with 3 Fogale Capacitive HLS sensors in thegeodetic base laboratory at CERN. The tests con-ducted by Xiaoye He have shown a very good perfor-mance of the mHLS system [4]. We can conclude thatthe resolution is below 1µm and they agree within 5µmwith the Fogale HLS sensors. The different mobileplatform has therefore no influence on the performanceof the sensors under stable conditions of a laboratory.

LHC Tests

The tests in the LHC have been made in order toaddress two different aspects. The first is the controlof the system performance of the mHLS system underreal conditions. The real tunnel environment with allaccelerator components as well as real temperatureand ventilation conditions. The second aspect is thehandling of the system. The test should show if it ispracticable, how much time a measurement campaignwill take, the lifetime of the cables and componentsand also the installation in the rack which is towed bya tractor along the tunnel.

Figure 9: Measurement Sequence

The Tests have been made in the sector 7-8 of theLHC and at the same time as the standard opticallevelling campaign in order to have recent data forcomparisons. The measurement sequence is shownin figure 9 and was adapted to be coherent with theoptical levelling measurements. The measurementsystems have been switched on each point so thatproblems could be detected immediately. Eachheight difference was measured three times with 20individual acquisitions in both configurations thusgiving enough redundancy to evaluate the system. Anumber of verifications are done on the raw-data inorder to control the measurements.

• The water communication of the pots is alwayschecked before a measurement.

• A still stabilizing water column is identified usingthe correlation coefficient between the two sen-sors.

• A measurement is validated only if not more than2 acquisitions are identified as blunders.

• The total amount of water needs to be constant.• The three subsequent measurements must be in

sufficient agreement.

The first value from interest is the zero offset of thesystem which has been determined with 1.18mm inthe laboratory prior to the measurements. During themeasurements the Zero offset was also 1.18mm butwith a RMS of 44µm as shown in figure 10. There isno trend or drift visible so far but it is looking like asystematic and periodic influence. All measurementsare done 3 times and the average is retained as finalvalue. The RMS of the repeated acquisitions is ingeneral well below 5µm with a few values reaching upto 10µm.

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Figure 10: Variation of Zero Offset

The calculation of the levelling as a whole using theLGC software is showing a well centered and gaussiandistribution of the residuals with an RMS of 41µm.

The determined height differences can now be com-pared with the ones determined by the optical levellingusing a LEICA NA2. The figure 11 is showing thedouble differences along with the measured height

differences. One can not see any influence of the actualheight difference but the double differences are shiftedby -60µm. Separating now the differences for the NA2outward and return measurements is showing thatthe outward measurements are shifted by -90µm andthe return measurements by -40µm. It was alreadysuspected earlier that the direct levelling measurementare affected by a systematic error due to the tunnelventilation and detailed tests are ongoing. With thepresent tests and data it is unclear if the mHLS ismore or less affected than the optical levelling. Butthe direction of the differences and their magnitudeare very promising for a potential correction.

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Figure 11: Double Differences and Height Differences

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Figure 12: Differences between outward and returnmeasurements compared with mHLS

CONCLUSION

The laboratory Tests have shown that the systemin behaving as known from the previous tests andexperience by DESY [2]. Even when used as singletube and open system with a 1.2m high water column.The use of the single silicon tube with 10mm innerdiameter is attenuating the water movements and astable state is reached in less than 40 seconds. Thereal conditions in the LHC tunnel have shown a nonrandom variation of the zero offset which is mostprobably be linked to the pressure differences in thepots due to the ventilation or residual temperatureeffects. Further laboratory tests will clarify this.The comparison with the optical levelling has shown

that there is a difference between the outward andreturn measurements of the optical levelling campaignwhich was already suspected before. More measure-ments in different conditions and configurations willbe needed to confirm this effect.

OUTLOOK

No show stoppers have been found so far and thedevelopment will continue with an improved prototypeversion. Points of improvements are the cables andtube protection. A further automation of the mea-surement and control process. Some detailed testswill be made on the differential pressure inside thepots due to the airflow in the tunnel. And thesource of the systematic zero offset variation mustbe confirmed and mitigated. The integration of acombined gyro and accelerometer module MPU-6050has already started and will allow the automatedmanipulation and verticalisation of the system usinga robot. In parallel first tests are running covering thepossible integration of this system on a train.

ACKNOWLEDGMENT

The authors would like to thank DESY for provid-ing one of their HLS systems and in particular M.Schlösser and Mathias Reinecke for the help during thedevelopment and modifications of the mHLS system.Mr. Xiaoye HE for the laboratory tests and perfor-mance evaluations. The CERN EN-STI-ECE Sectionand the CERN SU electrical and mechanical workshopfor their help and support during the prototype con-struction.

REFERENCES

[1] P. Bestmann, et al., ”The LHC Collimator SurveyTrain”, Proc. of IWAA 2010, DESY, Hamburg, (2010).

[2] M. Schlösser, et al., “High Precision Survey and Align-ment of Large Linear Colliders – Vertical Alignment”,Proc. of IWAA 2002, p. 343, Spring-8, Japan, (2002).

[3] Frank E. Jones, et al., “ITS90 Density of WaterFormulation for Volumetric Standards Calibration”,Journal of Research of the National Institute ofStandards and Technology, Volume 97, Number 3,May-June 1192

[4] Xiaoye HE, Activity Report of Project of SSSTCfollow up grant for EG 53-032010 ”Intercompari-son between HLS Sensors: Short Term Results”https://edms5.cern.ch/document/1554745/1

[5] P. Bestmann, Measurement report”Mobile HLS Test results in the LHC”https://edms5.cern.ch/document/1720017/1