The active alignment system is capable of positioning accelerator components of CLIC (Compact Linear Collider) with a few micron precision. An electronic processing and command system connects the micro-movers and sensors of this system to the CERN-PS complex control system

Bouché, J M

Coosemans, Williame

Pittin, R

CERN Document Server

OPEN-98-00202 Feb 1998CLIC Note 350Active Alignment System for CLIC 30 GHz Modules in CTF2Jean-Marc Bouché, Williame Coosemans, Romain PittinCERN, European Laboratory for Particle Physics, Geneva, Switzerland1. INTRODUCTIONCERN is studying a linear collider (CLIC) to obtain electron-positron collisions with center-of-mass energies in the TeV range. The CLIC scheme is based on beam acceleration at highgradient (100 MV/m) and high frequency (30 GHz) with RF power generation by the Two BeamAcceleration (TBA) method. Pulsed microwave power is extracted from the drive linac by meansof power generating transfer structures and fed into the main linac through waveguide feeders.To demonstrate the feasibility of CLIC a test facility (CTF 2) is being constructed with the30 GHz two-beam section consisting of four identical modules resembling as closely as possiblethe real CLIC design . The accelerating, correction and beam detection components for the 30 GHz modules mustbe aligned to an accuracy of d10 µm.  The alignment system used, consisting of supports, positionsensors and actuators, was developed  in the CLIC alignment test facility.A 30 GHz module for CTF2 consists of two girders, one per linac, supporting accelerating orpower generatig transfer structures, and beam position monitors.  The quadrupoles sit above eachgirder on independent supports.  For phase 1 of CTF2 two modules are installed.2. SUPPORT AND DISPLACEMENT SYSTEMThe accelerating and transfer cavities, and the beam position monitors sit on girders on pre-aligned vees (Fig. 1a).  The girders are supported by inter-girder articulated supports. Thesesupports fixed at the end of a girder have two micromovers with link rods for the verticalmovement, one micromover with one link rod and a screw stop for the horizontal movement,and two link rods to support and adjust the front of the next girder.  The distance between twoarticulation points is 1.41 m. The quadrupoles sit on a ground metal plate supported by threemicromovers with link rods for the vertical movemnet and two micromovers with link rods and ascrew stop for the horizontal movement (Fig. 1b and 1c)The micromovers of an inter-girder articulation and the micromovers of the nearestquadrupoles sit precisely on a metal plate. This plate is aligned and fixed on a concrete blockintegrated into the concrete floor. There is one concrete block for two modules (Fig. 1b).   1a    1b     1c    1dFig. 1  Different views of CTF23. ALIGNMENT : METHOD AND SENSORSThe alignment system has two main fonctions.  The first is to pre-align the elements to makesure that the beam can pass through the aperture and produce signals in beam position monitors.These signals are then used to make the final alignment.  The second fonction is, once aligned, tomaintain the elements in this position.The Wire Positioning System (WPS) is used to position the girders and the quadrupolesupports.  The reference for each linac is a wire under tension.  The spatial position of the wiresis fixed by four reference systems, one at each end of the two modules.  Each reference systemconsists of a Hydrostatic Leveling System (HLS) and a WPS put in the theoretical position bygeometrical measurement from the local geodetic network (Fig. 1d).  The inter-girderarticulations and the extremities of the quadrupole supports are each fitted with a WPS which isitself precisely located with respect to the axes of the accelerator components (Fig. 1b).  TheWPS sensors measure the distance between the axis of the sensor and the wire in vertical andhorizontal directions perpendicular to the accelerator axis. An accelerometer/tiltmeter (TMS) isused on each girder and each quadrupole support to measure transverse tilt and vibrations.4. MAIN CHARACTERISTICS OF THE INSTRUMENTSGirderx Silicon-carbidex Length including extremity plates : 1.40 mx Cross section : 0.6 x 0.12 mx Wall thickness : 7.5 mmx Module of elasticity : 2100 daN/mm²x Density : 2.65 g/cm³x Thermal expansion : 4.8 u 10-6x Very good thermal conductivityx Weight : a 12 kgx Tolerance for adjustment of the vees and     the end support plates  < 5 µmMicromoverx Stepwise motorisationx Length at mid course : 155 mmx Diameter : 60 mmx Travel : ± 4 mmx Resolution : 0.2 µmx Repeatability : 1 µmx Maximum load along thrust axis : 400 NWire Positioning System (WPS)x Two axesx Measurement range : ± 5 mmx Resolution : 0.1 µmx Repeatability : 1 µmx Bandwidth : 0-10 HzWirex Carbon + aramid fiber (Kevelar)x Apparent diameter : 0.50 mmx Weight of 100 m : 20 g + 16 gx Elastic limit : a 300 Nx Mass of counterweight used : 6 kgTilt Meter System (TMS)x The instrument measures the inclinationand the acceleration in two axesx Resolution : 10-7 radx Repeatability : 10-6 radx Bandwith : 0 to 100 Hzx Measurement range : ± 3u10-3 radHydrostatic Leveling System  (HLS)x Measurement range : 5 mmx Resolution : 0.2 µmx Repeatability : 1.2  µmx Separate external electronics5. ELECTRONICSTwo distinct versions of the electronic processing and command system have beendeveloped, the first using standard components and the second using specially developed higherperformance hardware.5.1 System installed in the CTF2The system now installed in the CTF2 consists of :x 38 stepping motor driven linear actuators and 10 double Europe 4 axis motor drivercards.  The cards provide complete motor control and are connected to the controlsystem using an RS232 bus.x 78 measurement axes including four HLS, ten TMS (three axes), eighteen WPS (twoaxes) and eight thermometers.  Analogue signal processing units are installed near eachof the sensors in crates located in niches in the concrete support block. The concretesupport block also provides radiation protection.  The processed analogue signals arebrought together in the patch panel and are connected to the control systems.5.2 New DevelopmentThis system, which is now being produced, will provide increased alignment precision, higherprocessing speed, simultaneous and synchronised displacements, decreased equipment volume,decreased cost, and easier and more flexible use.  Processing, in the version 1 system, made by theCPU of the VME controller, is made by local units in the version 2 system.  These local unitsconsist of VME Main Control Card (MCC) modules operating in slave mode. The MCC modulescontain two Digital Signal Processors (DSP), one dedicated to motor movement control and theother to numerical signal processing, local RAM, control and communication with the VME bus.Logical circuits and volatile memory are contained in two Logic Cell Arrays (LCA).  The DSPclock rate is 40 MHz.The motor driver circuits are arranged in-groups of six, with five circuits on each three unithigh Europe crates.  Three cascaded fibre optic cables connect the crates to the MCC.Asynchronous communication is used in order to avoid problems caused by delays in fibre opticstransmission.Analogue signals are captured using 14 and 16 bit (resolution and precision respectively) A/Dconverters that are capable respectively of 20 and 14 channel differential multiplexing.  Extra carehas been taken to ensure that the 16 bit A/D’s precision is not degraded by cross-talk with themultiplexing (MPX).  The A/D converters communicate with the MCC via two fibre opticscables.Each MCC can control 80 actuators and 160 sensors with an average response time of 1 ms.the decentralised processing of this system allows the number of elements to be increased evenfurther without degrading the response time.  Communication by fibre optics cables ensures highprecision and immunity to electronic noise.6. CONTROL SYSTEMThe active alignment of the 30 GHz modules of the CLIC Test Facility is done in Real Time(RT) through the standard Control System which is used for the operation of all the acceleratorsin the CERN-PS complex.6.1 Hardware Architecturex UNIX Workstations (IBM RS6000) or X-terminals running under AIXx UNIX  IBM systems used as files servers, database servers, development and backupserversx VME Crates, 68000 processor running LYNX OSx Ethernet Network and FDDI6.2 Software ArchitectureThe PS Control System is strongly based on an object oriented approach with the “ControlModules”, equipment classes with the same functionality, distributed over the Control Network,to the VME crates.  Class and instance variables are encapsulated in the VME “data tables” andare handled by RT tasks and the class functions only.  Configurations are maintained by means ofan Oracle database.  C and C++ languages are used to write Drivers, RT tasks and applicationprograms where MOTIF is used for the graphic applications. Communications are based on aCERN UDP Remote Procedure Call.6.3 Front End LevelTwo main classes of equipment are connected to the VME crate :x for the sensors, three analogiue input boards with 32 differential analogue inputchannels and 12 bit resolution are used for acquisition of temperature, HLS, TMSvalues and 4 height-differential-channel digitisers with 16 bit resolution for acquisitionof the TMS values.x A VME card providing 8 RS232 communication channels per sub-slot to the motorinterfaces drives the micromovers.The CPU board has 32 Mbytes memory.  Two home made modules are required fordiagnostics, remote reboot, timing reception and generation of interupts.The analogue value levels provided by the captors  and the motor parameters are scanned andtreated by means of two specific RT tasks that finally store the resulting values in the “ControlModule” data-table.  The alignment process itself (in reality one per beam) only requires access todata-table values and position correction algorithms.Its function can be quickly described as follows :x obtain wire reference position valuesx obtain quadrupole, girder position and tilt valuesx correct captor values according to wire displacement, wire sag and operator inter-actionx check positions and tilts against tolerancesx process data and calculate correction values for actuatorsx check complete motor system status: power on, not movingx send corrections to quadrupole and girder actuatorsx record all sensor values and micromovers positions in backup files for later historicanalysis.These different tasks are synchronized with the normal PS accelerator timing systemproviding a basic period time of 1.2 s and different sub-timings in this period. Sensor scanning isperformed every basic period and alignment is active only every 2.4 s.  It allows the processing ofstable acquisition values, not perturbed by the actuator movements.6.4 Interface LevelThe application program, which is available in the console manager of any operationworkstation, is built around a full screen graphic representation of the two 30 GHz modules andtheir components (Fig. 2).Fig. 2  Copy of the active control screenDirect interaction on this picture allows individual control of active equipment by knobwidgets. This is completed by normal operation facilities like starting or stopping the alignmentprocess for a beam, or introducing manual offset position corrections for each girder orquadrupole.7. CONCLUSIONThe system is now successfully running with a particle beam.  In closed loop, the elements arecontinuously maintained, with respect to the wire, in a window less than 5P and steeringadjustments are carried out by controlling quadrupole displacements. Next year the experiments will be upgraded by installation of two new modules with the newelectronic and control system described above. With the characteristics of this new system, incombination with the performances of the mechanical supports and the sensors, we will be veryclose to a good solution for an alignment system for the CLIC machine.BIBLIOGRAPHYW.Coosemans, H. Mainaud. Pre-Alignement of CLIC using the Double-Wire method, CERN Janvier 1997.CLIC Study Group, CTF2 Design Report, CERN, CLIC Note 304, June 1996.CLIC Study Group, The CERN Linear Collider, CERN, CLIC Note 195, May 1993.R.Rausch, Ch.Serre (editors), Common Control System for the CERN Accelerators, ICALEPCS 1991, p. 54, Tsukuba,Japan, November 1995.

Active Alignment System for CLIC 30 GHz Modules in CTF2

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