EXPERIENCE IN USING BASED EMBEDDED CONTROLLERS WITHEPICS ENVIRONMENT FOR BEAM TRANSPORT IN SPES OFF-LINE

TARGET PROTOTYPEM.Montis, M.Giacchini, LNL INFN, Legnaro, Italy

Abstract

EPICS[1]waschosenasgeneral framework todevelop thecontrol system of SPES facility under construction at LNL[2].We report some experience in using some commercial de-vices based on Debian Linux to control the electrostaticdeflectors installed on the beam line at the output of targetchamber. We discuss this solution and compare it to otherIOC implementations in use in the Target control system.

INTRODUCTION

SPES accelerator, under construction at the Legnaro Na-tional Laboratories (LNL), is on ISOL facility (Isotope Se-paration On-Line) dedicated to the production of radiac-tive ion beams with high energy and high degree of pu-rity. In a structure like this one, the core of the project arethe production target and the ionization ion system, whichwere carried out a study and design from scratch. Thisstructure is currently in production for execution of experi-ments from the beginning of 2010.

Analyzing the characteristics required for inspection ofthis apparatus, it is possible to observe that:

• an heterogeneity of hardware and software solutionsare required;

• we have to manage a distribuite control system;

• it is mandatory a real-time processing for every con-trol variable, in order to ensure a robust system.

According to specific requests for software control, op-timization of costs that derives from using an opensourcesoftware and favouring cooperation between laboratories,EPICS was chosen as control system software for the SPESfacility.

The control network architecture created to manage theoff-line target prototype is a multi-level structure composedby machines dedicated to network management and allEPICS hosts that implement the real control system envi-ronment. For managing all the devices used for the ionbeam production, we decided to use PLC solutions in everysub-system in which safety is mandatory and it is necessaryto implement fault-tolerant solutions, while particular em-bedded systems equipped with EPICS software manage allthe devices dedicated to the ion beam manipulation (ionbeam acceleration and ion beam focus)[3]. Appropriate so-lutions have been implemented for the integration of PLCsubsystem within the EPICS environment, which it will ex-plain in the next section.

Figure 1: SPES Off-Line Target Prototype: systems andsignals managed.

THE CONTROL SYSTEM

System Description

Managing the ion beam geometry, linearity and acce-leration is made through the adjustment of the intensity ofelectrostatic fields generated by appropriate medium vol-tage power supplies (0-4kV) and high voltage power sup-plies (0-65kV). Controlling these power supplies it is pos-sible to supervise the ion transport efficiency and optimizethe emittance index. For this reason it is necessary to adoptappropriate control strategies to ensure a sufficient level ofprecision in the voltage settings, and security to all the de-vices managed.

We decided to base control algorithms on the followingstrategies:

• voltage control: initially we created a closed-loop sys-tem with PID regulators for controlling all the powersupplies used in the deflection, focus and estractionsystems. After this we changed the control strategyand we implemented a new control based in bisectionalgorithm because during the runs users had to usuallychange control parameters;

• current monitoring: the presence of any current couldlead to the establishment of electrical discharges, re-sulting in the breakdown of components. For thisreason we chose to implement a current monitoringstrategy, with system shutting down when values ex-ceeded a preset alarm threshold. Because of currentvalues depends on power supplies work condition, wedecided to define two different kind of thresholds:

1. one current threshold for voltage’s transitory;

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Figure 2: EPICS application developed: it is possible to see the connections between all the Record Istance Files definedand the signals form the field.

2. one current threshold when power supplies workin regime (costant voltage).

Appropriate EPICS Databases these strategy, and all thedatabases created for the power supplies remote control de-fine the EPICS control system application used to managethe ion beam accelerator and focus systems for the SPESoff-line target prototype.

Hardware SolutionWhile for the others sub-systems that constitute the

SPES off-line target prototype we used computer general-purpose to control them, for the accelerator and opticsub-systems we decided to use particular embedded de-vices equipped with data acquisition cards and the softwareneeded to define an EPICS Input/Output Controller (IOC).

The hardware chosen is an embedded computer (mi-croIOC) based on the standard PC/104 and distributed byCosylab, which allows for a robust and flexible remote con-trol of devices connected to it, thanks to the absence ofmoving parts (such as hard drives and cooling fans) and avariety of communication interfaces available.

The Channel Access servers, used within the networkfor systems management of optical beam acceleration, aremicroIOC with the following characteristics:

• VGA connection;

• two ethernet connections, one in DHCP and one withstatic IP;

• two USB connestions;

• one RS232 connection;

• one AD I/O board (ADIO 104-AIO12-8) used for dataacquisition and signal processing. It is possible tomanage:

– 16 digital input;

– 16 digital output;

– 24 analog input (12bit resolution);

– 12 analog output (12bit resolution).

Using this kind of boards we can control all the si-gnals desired making an error of ±1V for the mediumvoltage power supplies and ±15V for the high vol-tage power supplies. While in the first case the erroris acceptable, for the high voltage power supplies itis mandatory to decrease this value. One possibilitysuggested is to replace the ADIO board with anotherboard having at least 16bit resolution for the analoginput/output.

The operating system (a Debian based GNU/Linux cu-stom distribution) and the EPICS software are stored in aFlash Card memory card. The EPICS environment is con-figured to initialize an IOC application at boot time. Inthis way the microIOC, when connected to the network,automatically creates the Channel Access Servers withoutfurther configuration by the user. The ADIO board uses aspecial EPICS Driver Support to interact with the controlsystem environment and manage all the digital and analogsignals.

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During studies made and through the EPICS Commu-nity, we analized that the most used way to interface PLCsystems with the EPICS environment is using particularhardware called OPC-Servers, that define a layer where thePLC signals are mapped in EPICS Process Variables. Tooptimize costs and minimize hardware complexity, we de-cided to implement it with a differet communication inter-face using the ADIO cards provided by microIOC.

For monitoring the embedded systems and the EPICSenvironment, we used an appropriate machine equippedwith Nagios[4] software; in this way it is possible to controlboth the hardware and the software status.

Software Solution and Application DevelopedDuring code development we tried to find a database

structure which would allow to minimaze the number ofProcess Variables we have to manage and simplify the soft-ware maintenance. In this way it is possible to have asynthetic and readable code for anyone who have to mo-nitor the SPES control system infrastructure. Following theproject directives, the control strategy adopted and studiesmade with other EPICS applications we decided to developa program structured in five different Record Instance file.Analizing Figure 2 shows:

• one Database, using the EPICS Driver Support, de-fines the interface between the EPICS envoronmentand the AD I/O board mounted on the microIOC. ThisRecord Istance File allows to manage all the inputscoming from the field and the outputs used to controlthe devices desired, such as indicated in Figure 3;

• one Database manages the Records used into theGraphical User Interface for setting all the power sup-plies’s control parameters;

• one Database monitors the currents provided by thepower supplies when they work on regime;

• one Database monitors power supplies’s currents du-ring voltage transitories;

• one Database realizes the voltage control strategy(PID regulator and bisection algorithm).

Realizing a modular structure for the software allows usto modify it in funcion of the experiment’s requests (con-cerning the control strategies) and the technical requests(for example hardware maintenance) without changing theentire application: for example we modified the controlstrategy switching between two different databases (as rap-presented in Figure 4) keeping unaltered all the other recordistance files, except for fews links used to forward the pro-cess chain.

From an informatic point of view, Databases are builtup with couples of template/substituion files: throught thisEPICS environment feature, it is possible to modify all theProcess Variables desired (number, type, attributes) and the

Figure 3: EPICS Control System Structure.

Figure 4: EPICS Application Database - Layers.

database architecture in a fast way editing a little set oftext files; at the same time, if someone prefers use gra-phical editors (such as VisualDCT) to edit template files,the database structure will result easier to read and modify,because all the Records needed by the appllication will beinstanced only at the IOC boot time. In this way both thedebug phase and the maintenance are simplified; in partic-ular this second feature is really important in a project likeSPES, where all the works are developed and managed bydifferent people.

For defining Process Variable names, we created a spe-cial Naming Convention that provides a descriptive namefor every Record and, through it, every user can easy findand modify all the control parameters desired. This Nam-ing Convention will be adopted for the entire facility’s con-trol system.

For every EPICS application developed, we created aspecific Graphical User Interface (GUI) with Medm, anEPICS tool that provides an complete editor for creatingand managing GUIs; an example of Medm GUI is visiblein Figure 5. These interfaces are only used by software de-velopers for managing the EPICS control system software,while specific GUIs developed in CSS are used by generalusers to supervise the SPES off-line target prototype and,in the future, all the SPES facility.

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Figure 5: EPICS GUI developed for system maintenance.

CONCLUSIONParticular hardware and software solutions are adopted

to design and develop the ion beam accelerator and focussystems in SPES off-line target prototype. On the hardwareside, we decided to use specific embedded devices basedon the PC/104 standard equipped with all the EPICS toolsneeded to connect them into the off-line target prototype’scontrol system. During this production period we observedthat this hardware solution guatantees good performances.This hardaware allows to create a trasparent layer betweenPLC systems and EPICS software too, minimizing possibleincompatibilities between these two environments.

On the software side, the EPICS application structureadopted permits an easy management by users and a easyreuse of most of the code wrote for others applications andexperiments, allowing a minimization of setting times. Ingeneral a modular approach for developping EPICS appli-cations is desirable, especially in large and distribuited en-vironment like the SPES facility.

REFERENCES[1] EPICS – Experimental Physics and Industrial Control System

www.aps.anl.gov/epics/.

[2] EPICS at Laboratori Nazionali di Legnaro http://www.lnl.infn.it/~epics

[3] M. Montis, Master’s degree thesis, Università degli Studidi Padova, a.a. 2009-2010 http://www.lnl.infn.it/~epics/THESIS/TesiMaurizioMontis.pdf.

[4] NAL (Nagios Alarm Handler)(Not Alarm Handler) http://www.lnl.infn.it/~epics/NAL.html

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