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PREPARED FOR THE U.S. DEPARTMENT OF ENERGY,UNDER CONTRACT DE-AC02-76CH03073PPPL-3894UC-70PPPL-3894Control System for the NSTX Lithium Pellet InjectorbyP. Sichta, J. Dong, R. Gernhardt,G. Gettelfinger, and H. KugelOctober 2003PRINCETON PLASMA PHYSICS LABORATORYPRINCETON UNIVERSITY, PRINCETON, NEW JERSEY
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Control System for the NSTX Lithium Pellet Injector *P. Sichta, J. Dong, R. Gernhardt, G. Gettelfinger, H. KugelPrinceton Plasma Physics Laboratory, P.O. Box 451, Princeton, New Jersey 08543Abstract-- The Lithium Pellet Injector (LPI) is being developedfor the National Spherical Torus Experiment (NSTX). The LPIwill inject ‘pellets’ of various composition into the plasma inorder to study wall conditioning, edge impurity transport, liquidlimiter simulations, and other areas of research. The controlsystem for the NSTX LPI has incorporated widely used advancedtechnologies, such as LabVIEW and PCI bus I/O boards, tocreate a low-cost control system which is fully integrated into theNSTX computing environment. This paper will present thehardware and software design of the computer control system forthe LPI.I. INTRODUCTIONThe National Spherical Torus Experiment (NSTX,) isevaluating the physics principles of spherical torus (ST)geometry [1]. An NSTX Lithium Pellet Injector (LPI) hasbeen developed to inject ‘pellets’ of various composition intothe plasma in order to study wall conditioning, edge impuritytransport, liquid limiter simulations and other areas of STresearch. The LPI project has presented an excitingopportunity to build a control system at NSTX that includedadvanced technologies. The vision for this system extendedbeyond designing an autonomous LPI control system. Aprimary goal of the design was to explore the recentadvancements made by the NSTX Central Instrumentation andControls group that address three key NSTX computinginterfaces: network, data, and synchronization. Designconsiderations for the NSTX LPI control system included thefollowing: Low-Costo Commercial Off The Shelf (COTS) partso Rapid development and testo Minimize special designs Maintainabilityo Use common componentso Avoid CAMAC Integration into the NSTX Computing and ControlsEnvironment Create an architecture that is suitable as a ‘template’for other systems and collaboratorsThe system is based on a rackmount PC using PCI input/outputboards. The application software uses National Instruments LabVIEW, running on the Microsoft Windows operatingsystem. LabVIEW was chosen because it offers direct supportfor a wide selection of PCI input/output boards. LabVIEWalso has software interfaces to the two primary components ofthe NSTX computing environment, MDSplus [3] and EPICS[4]. Windows and LabVIEW provide a variety of solutions forremote access to the LPI computer.II. HARDWARE DESIGNA block diagram of the system is shown in Fig. 2 (see nextpage). The system’s mechanical design includes eight launchbarrels, a revolving 400-pellet magazine, and multiple valvesfor controlling the launch sequence [2]. The controls havebeen partitioned into three subsystems, Magazine Control,Launch Control, and Pellet Flight Telemetry.A. Magazine ControlKey Features 400-pellet revolving magazine0.036 degrees radial accuracyComplete rotation in one minuteLocal controls to facilitate pellet loadingThe revolving magazine is a circular assembly that has 400tubes, 50 radial columns each with 8 axial rows. Each tubecan be hand-loaded with a cartridge and its payload, aprefabricated pellet. The magazine is rotated to align with afixed-position vertical array of 8 propellant barrels, asdepicted in Fig. 1, below. The magazine’s rotation iscontrolled using a 25:1 micro-step-controlled stepper motor,coupled to the magazine through a metal drive chain with a 6:1gear ratio. A 5000 pulse-per-revolution encoder is coupled tothe magazine assembly to keep track of the magazine’sposition.The stepper motor and encoder are wired to a NationalInstruments Stepper Motor Driver and PCI Stepper MotorController board. Coupled with National Instruments-suppliedFig. 1. Diagram of the 400-pellet magazine assembly and the launch barrelarray. Dark and light dots indicate an empty or full magazine tube.
EngineeringNetworkRACK MOUNT PCwith LabVIEWAnalog InputsPCI-7334MOTIONCONTROLL ERSLO-SYNSTEPPER CODERDIGITAL I/OGAS MANIFOLD RETRACTParker Valves ClosedinterlockOPTO22 SOLIDSTATE RELAYSSOLENOID VALVESFILL VALVESPlasma Perm.PPPL (PCI) TIMINGMODULESCRYDOM HVDCSOLID STATERELAYSPARKER IOTA-ONEVALVE POWERSUPPLY 5 SinterlockNSTXFacilityClock /-15 VDCPOWERSUPPLYTo Analog Inputsof PELLET TARGETPLATETarget Plate VibrationSignalConditioningLASER-BASEDVEL OCITYDETECTORSbeam interruptionManifold Vibration4PELLETTELEMETRYPARKER VALVES'SHOCK'DETECTORSFig. 2. Block Diagram of the LPI Control System HardwareLabVIEW stepper motor software functions, this tightlyintegrated stepper motor control solution was virtually ‘turnkey’.B. Launch ControlKey Features Independent timing and velocity for up to eightpellets per NSTX plasma-shotFast hardware interlock inhibits launch of pellet in theabsence of the plasmaHigh voltage solid state relay switching networkpermits sharing of a single impulse-driven valvepower supplyTiming is preset, but hardware can supportasynchronous launch triggersPellet Launch Control is the most complex of the three LPIsubsystems. The configuration for each of the eight barrelsinclude a fast-acting (200 microsecond) Parker-Hannifin Series 9 solenoid valve, a pressurized ‘fill plenum’, and asolenoid/pneumatic valve-set to pressurize the fill plenumprior to the launch. The Series 9 launch valves are solenoidoperated and can be affected by the magnetic fields near theNSTX vacuum vessel. The valves are encased in a mu-metalenclosure to prevent inadvertent valve operation.The eight launch valves are driven from a single, ParkerHannifin model Iota-One power supply.This supplyoverdrives the launch valve with a 180 microsecond, 280 volt‘spike’ at the rising edge of the control pulse, which isnominally 28 vdc. Before energizing the valve, however, thevoltage must pass through two high voltage (500 vdc) solidstate relays (SSR). The first SSR is enabled by a (externallyderived) plasma permissive interlock. The second SSR iscontrolled by the PPPL PCI NSTX Timing system [5].1) PCI NSTX Timing SystemThe PCI NSTX Timing system was designed at PPPL in orderto have a CAMAC-free timing and synchronization system forNSTX diagnostics and other control applications. Each twoboard set provides six multifunction timing channels. Oneboard resides in the control computer in a PCI bus slot. This isa commercially-produced board (CeSys model XC2S) thatcontains a Field Programmable Gate Array (FPGA). TheFPGA has been loaded with a configuration that can decodeNSTX Facility Clock events, such as T(0), provide timingdelay functions, and then generate TTL timing signals. TheTTL outputs are connected to an external (PPPL) circuit boardlocated in a rack-mount electronics chassis. This boardprovides electrical isolation of the timing signals. The LPI isthe first ‘field’ application of the new PCI Timing System.Two (6-channel) PCI timing boards were required to supportthe eight launch valves, transient digitizer triggers, and othertiming signals.
C. Pellet Flight TelemetryKey Features Vibration sensors:o Parker valve (solenoid) actuationo Cartridge impact at end of magazine tubeo In-vacuum Target Plate pellet impact, usedfor test and calibrationOptical sensor: Pellet exiting magazine tubeThe LPI has sensors to detect that the launch sequenceoperated normally and also to provide information such astime of injection and pellet velocity. Several piezoelectricvibration sensors and one laser/optical detector are used.These signals are digitized using a Datel model PCI-417J.This device has eight, 12-bit, simultaneous sampling ADCs, atrates up to 2.5 MHz. The digitizer streams the data across thePCI bus to system memory.III. SOFTWARE DESIGNThe software is based upon LabVIEW, running on theWindows operating system.LabVIEW Key Features Popular and powerful programming environment, ade-facto standard for small control systemsDriver and VI software support for PCI I/O boardssupplied by board vendorsSupport for remote (program) controlFree interface software for MDSplus and EPICSSoftware extensions for web browsers and databasesLike the hardware design, the software design was partitionedaccording to the LPI subsystem architecture, MagazineControl, Launch Control, and Launch Telemetry. SeparateLabVIEW programs were written to serve each LPIsubsystem, as depicted in Fig. 3. Each program can functionautonomously to serve a subset of LPI functions, but fornormal LPI operations all three programs are required.To coordinate operations of the LPI, the three programs usedLabVIEW’s Global Variables to pass information. There areabout fifteen global variables, such as watchdog counters,readiness permissives, launch-sequence state information,position information, NSTX shot number etc LabVIEW’s‘Tab’ feature was used to permit a ‘layered’ displaymechanism. The operator can use the mouse to poke the tab,and bring up a new display to the forefront. 10 displays (total)are used for the LPI.The Pellet Launch program is also known as the ‘main’program. For normal operations, the operator only needs toaccess this program’s front panel. The main program can passmagazine-rotation requests to the Stepper Motor controlprogram through the Global Variable space. Configurationdisplays to allow user to configure control software (override,MDSplus path, stepping rate, deadbands, etc lDigitizerControlFig. 3. Diagram depicts three LabVIEW programs on the same PC usingGlobal Variables communicate.A. NSTX Computing EnvironmentThe NSTX Computing Environment [6] is built around twoopen-source, global-community software packages, MDSplusand EPICS. MDSplus is a set of software tools for dataacquisition and storage and a methodology for management ofcomplex scientific data. EPICS is a set of software tools,libraries and applications to create distributed soft real-timecontrol systems. Both of these packages run on most popularoperating systems and CPU architectures. The EPICS andMDSplus communities have provided interfaces for LabVIEWprograms. The LPI software has included functions to readand write MDSplus and EPICS. These basic functions can beused to form the core of a template that can be used by (future)NSTX collaborator’s who also use LabVIEW.The MDSplus interface is used for saving shot-related dataafter each NSTX shot: Launch configuration: pressure, time, magazinelocation, etc Injected-pellet characteristics: size, pellet ID, etc . Transient digitizer waveforms Program and hardware status information (word)The EPICS interface is useful for automated operation: Shot time [e.g. T(-10)] to arm/disarm subsystems,run pre-shot checks, increase polling rates, etc Detecting aborted shot cycles Post ‘live’ system status to EPICS database forcentralized alarmingB. Pellet DatabaseA (LPI) Pellet Database is being planned. This database willcontain records that track the life-cycle of each pellet and thecurrent status of the 400-pellet cartridge tubes. Pelletcharacteristics will include items such as pellet ID, pellet mass,composition, manufacturing date, etc The database will alsorecord which magazine tube a pellet was loaded into, when itwas injected or removed, shot number/date of injection, etc The database will be accessed by a variety of staff such as thepellet fabricator, the cartridge loader, the LabVIEW programs,the physicist running the NSTX experimental plan, and otherresearchers interested in pellet history. Note that the MDSplusshot data will include some of the items associated with the
pellets (ID, composition, etc) that were launched into theplasma. The detailed design and access methods for thedatabase will be deferred until some LPI operations experiencehas been gained.IV. OPERATIONSThe operating scenario for the LPI is to have the NSTXVacuum Operator, under direction from the NSTX Physicistin-Charge, control and configure the LPI equipment from theNSTX Control Room. The LPI Operator will remotely accessthe LPI computer’s ‘desktop’ using VNC [7]. The operatoronly needs to access one program window to pressurize the fillplenums, set timing, enable barrels, and rotate the magazine tothe desired position. The planned Pellet Database will enhancethe Physicist’s ability to locate the desired pellets andstreamline communication with the LPI/Vacuum Operator.At the present time, the LPI software is started up manuallyusing a procedure. In the future this process will be fullyautomated, so that when the PC is powered up all programswill start and the PCI Timing board’s FPGA will automaticallydownload its ‘code’.V. CONCLUSIONThe LPI control system design and fabrication has been asuccess. The system’s installation and commissioning in theNSTX Test Cell is expected to commence shortly. To reducecosts, component selection was made using free/open software,commercially-produced components, and lastly, PPPLengineered components. The partitioning of the control systeminto subsystems has encouraged modularization, such that thedesign of hardware and software was performed efficiently in ateam environment.The LPI software development proceeded very smoothlybecause of LabVIEW’s rich and intuitive programming andtesting environment. LabVIEW included high-level functionssuch as closed-loop motor position control, and the NI MAXPCI module initialization and configuration utility. LabVIEWalso provided a simple way to simulate values from non-present hardware and to-be-written software modules. Thishelped to accelerate software testing and will enhanceintegrated system testingThe basic architecture of the LPI control system can be used asa ‘template’ for other LabVIEW-based systems. Componentssuch as the MDSplus interface, EPICS interface, and PCINSTX Timing System can be used by other control systems totightly integrate them into the NSTX computing and controlenvironment.ACKNOWLEDGMENTThis is work supported by U.S. DOE Contract DE-AC0276CH03073.REFERENCES[1] J. E. Menard, et al., "Beta-limiting MHD Instabilities in ImprovedPerformance NSTX Spherical Torus Plasmas", Nucl. Fusion, 43 (2003)330.[2] G. Gettelfinger, et al., “Lithium Pellet Injector for NSTX,” (this conf.)20th IEEE/NPSS Symposium on Fusion Engineering, San Diego, USA(2003).[3] W. Davis, et al., “The use of MDSplus on NSTX at PPPL,” 3rd IAEATCM on Control, Data Acquisition, and Remote Participation for FusionResearch, Padova, Italy (2001).[4] P. Sichta, et al., “Status of the Experimental Physics and IndustrialControl System at NSTX,” 19th IEEE/NPSS Symposium on FusionEngineering, Atlantic City, USA (2002).[5] P. Sichta, et al., “Developments to Supplant CAMAC Technology withIndustry Standard Technology at NSTX,” 4th IAEA TCM on Control,Data Acquisition, and Remote Participation for Fusion Research, SanDiego, USA (2003).[6] P. Sichta, et al., “Overview of the NSTX Control System,” 8thInternational Conference on Accelerator and Large Experimental PhysicsControl Systems, San Jose, USA (2001).[7] RealVNC Ltd., “Virtual Network Computing (VNC) SoftwareProduct,” URL: http://www.realvnc.com [AccessedSeptember, 2003].
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The stepper motor and encoder are wired to a National Instruments Stepper Motor Driver and PCI Stepper Motor Controller board. Coupled with National Instruments-supplied F ig.1 D a rmof t h e4 0-p lz ns by d u c array. Dark and light dots indicate an empty or full magazine tube. Control System for the NSTX Lithium Pellet Injector *
