Nuclear Instruments and Methods in Physics Research A 389 (1997) 114-l 16
ELSEXIER
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SectionA
Using WWW technology in a control system* Fulvio Bilk, Roberto Pugliese” Beamline
Control Group. Scientijc
Division, Sinuotrone
Trieste, Padriciano
99. Trieste 34012, Ita!,
Abstract The World Wide Web (WWW) technology, based on the hyper-text principles and highly focused on the man-machine interface, is an easy and intuitive way of accessing data. By integrating Web and distributed control system technologies, operators and specialists may supervise a process without having to install any specific software and without bothering about the type of computer to use. Nearly any computer, loaded with a little piece of world-wide available software, may have access to the status variables, archive files and documentation of the controlled process. The enhanced inter-active features of the new Java technology make this scenario even more appealing. The paper describes a WWW-based control system: the Beamline Access Control System (BACS) of ELETTRA, a third generation synchrotron radiation facility. Keyrr&s:
Computer
science; Software;
Distributed
control
1. introduction ELETTRA is a 2 GeV state-of-the-art storage ring. The synchrotron light it produces can be used for chemical, physical and biological experiments. Due to the characteristics of the experiments the light has to be collimated, monochromatized and focused on the sample under test; this is carried out by an high sophisticated beamline working in ultra vacuum. The presence of people in potentially dangerous zones, which are surrounded with shielding walls, has to be avoided under particular conditions. The Beamline Access Control System (BACS) regulates the access to such restricted areas depending on safety conditions. BACS is a distributed control system. Each beamline is controlled by a Programmable Logic Controller (PLC) equipped with an Ethernet network card. Due to the intrinsic features of the system it is necessary to devise a supervision mechanism which enables a graphical visualization of the status variables of the distributed sub-systems. 2. Talking about the WWW The success of the Web is at the same time cause and effect of the spreading of Internet. The WWW merges
’ AlHENP96 abstract: SE-1 14 * Corresponding author. Tel.: +39 40 3758024; 3758575; e-mail:
[email protected]. 016%9002/97/$17.00 Copyright PII SO168-9002(97)00058-2
c
fax: f3940
system: WWW; PLC
techniques developed in fields such as data transfer, information retrieval, hypertext, multi-media and user interfaces to make a simple but powerful global information system [l]. The WWW world consist mainly of documents and links (i.e. references to other documents). All documents, apart from their format, look similar to the user and can be retrieved through a uniform addressing scheme. A simple protocol, called HyperText Markup Language (HTML), is used to allow a browser program to access the documents provided by the information servers. The WWW model gets over the frustrating incompatibilities of data format between information suppliers and users. It is this, the winning formula which leads to an effective full use of the Internet. As a matter of fact, there is an exponential growth in the number of Web servers and more and more powerful Network Information Retrieval (NIR) products get available. In this scenario, the introduction of the Web technology in the field of distributed control systems presents interesting advantages: No development has to be done at a user interface level. The Web server software is available in the public domain. Browsers run on the most common platforms and are system independent. The multi-media capabilities of the browsers can increase dramatically the expressive power of the presented information.
1997 Elsevier Science B.V. All rights reserved
F BillG. R. Pugliese f Nucl. Instr. and Mrth. in Phvs. Res. .A 3X9 (I9971 114. II6
- The user interface is standard, graphical and intuitive and the learning time is minimum. -- The user does not have to worry about the information localization. The server takes care of location transparence. Among the restrictions of this technology, evident ones can be considered:
the most
_ The format of the data directly accessible is limited by the built-in capabilities of the browsers to ASCII or HTML for the text and JPEG or GIF for the graphics. Other formats are available through external helpers. _~ Transactions between clients and servers are atomic. The user has to re-load the document to get updated information. These and other limits could be overcome by the advent of JAVA, the latest news in the field of Web technology and Internet applications [2].
3. The architecture
of the BACS
supervisor
The supervision system presents a distributed architecture At the user level it is possible to use any personal computer or workstation of the company internet,
II5
loaded with a WWW browser. A HP9000/712 workstation with HP-UX 9.05 and the NCSA http server plays the role of bridge towards the Beamline Access Control System [3]. Each beamline is controlled by a Siemens SIMATIC-SS PLC equipped with the CPIJ943 control unit and the CP143 communication processor [4]. Most of the HTTP servers offer the possibility to execute an external program. These programs are called gateways and use the Common Gateway Interface (CGI). A plain HTML document that the Web demon retrieves is static: a text file that does not change. On the other hand. when executed, a CGI program can output dynamic information. Gateways are bound to an URL address and they are executed whenever the URL is requested by a browser. Query parameters can be sent with the URL too. We have used this functionality to generate on the fly the document containing the requested process control information. These documents are formatted using the HTML standard before they are sent back to the user. The gateway we have developed acquires data from the plant PLCs. The system uses a database of descriptions to decide at first. how to collect and then. how to present the required data. A communication using the Sinec, H 1 protocol is established with the corresponding PLC whenever real-time data are required (see Fig. 1). The gateway creates the
WWW browser
htmd
0
t
Gateway
WWW server
PLC
Fig. I. Software
and hardware
architecture
of the BACS supervisor.
Ile. CONTROL!DISTRIBUTED
SYSTEMS
116
F. Bill& R. Pugliese 1 Nucl. Insb: and Meth. in Phw. Rex A 389 (1997) 114-l 16
HTML pages and includes graphics. The result is then sent back to the user. Some additional information such as a time-stamp is appended to the result, which is particularly important in the Web environment where transactions are atomic. The features implemented in the WWW server and the system architecture helped us to satisfy the security and confidentiality requirements: access is granted only to the users who call from inside the company intranet and the interaction with the plant PLCs is restricted by the gateway to avoid any potentially dangerous interference with the beamline access control.
4. Conclusion The BACS supervision system, operational since June 1995 presents some advantages which can be quantified from an economical point of view. In fact, we did not have to develop any software for the graphical user interface and the system is intrinsically open. A friendly,
standardized interface is presented to the user. The solution we have adopted integrates with the company communication network, where low level PLC oriented protocols coexist with high level protocols such as the Web HTTP. Further development work will concentrate on the implementation of a JAVA powered version of the supervisor aimed to get over the limits of the actual technology and enhance the interactivity level of the system.
References (W3C), http:j/www.w3. Cl1 The World Wide Web Consortium org/hypertext/WWW/. 131 HyperText Markup Language (HTML), http://www.w3.org/ hypertext/WWW/MarkUp/MarkUp.html. c31 The Java home page http://java.sun.com/. HTTPd Home Page http://hoohoo.ncsa. M The NCSA uiuc.edu/. with the SIMATICS5 115U, c51 H. Berger. Automating Siemens Berlin. 1989.
