SCADA systems are widely used in industry forSupervisory Control and Data Acquisition of industrialprocesses. Companies that are members ofstandardisation committees (e.g. OPC, OLE for ProcessControl) and are thus setting the trends in matters of ITtechnologies generally develop these systems. As amatter of fact, they are now also penetrating theexperimental physics laboratories for the controls ofancillary systems such as cooling, ventilation, powerdistribution, etc. More recently they were also appliedfor the controls of smaller size particle detectors suchas the L3 muon detector and the NA48 experiment, toname just two examples at CERN.SCADA systems have made substantial progressover the recent years in terms of functionality,scalability, performance and openness such that theyare an alternative to in house development even forvery demanding and complex control systems as thoseof physics experiments. This paper describes SCADAsystems in terms of their architecture, their interface tothe process hardware, the functionality and applicationdevelopment facilities they provide. Some attention ispaid to the industrial standards to which they abide,their planned evolution as well as the potential benefitsof their use.
1 WHAT DOES SCADA MEAN?
SCADA stands for Supervisory Control And DataAcquisition. As the name indicates, it is not a fullcontrol system, but rather focuses on the supervisorylevel. As such, it is a purely software package that ispositioned on top of hardware to which it is interfaced,in general via Programmable Logic Controllers (PLCs),or other commercial hardware modules.SCADA systems are used not only in most industrialprocesses: e.g. steel making, power generation(conventional and nuclear) and distribution, chemistry,but also in some experimental facilities such as nuclearfusion. The size of such plants range from a few 1000to several 10 thousands input/output (I/O) channels.However, SCADA systems evolve rapidly and are nowpenetrating the market of plants with a number of I/Ochannels of several 100 K: we know of two cases ofnear to 1 M I/O channels currently under development.SCADA systems used to run on DOS, VMS andUNIX; in recent years all SCADA vendors have movedto NT. One product was found that also runs underLinux.
This section describes the common features of theSCADA products that have been evaluated at CERN inview of their possible application to the control systemsof the LHC detectors , .2.1 Hardware ArchitectureOne distinguishes two basic layers in a SCADAsystem: the "client layer" which caters for the manmachine interaction and the "data server layer" whichhandles most of the process data control activities. Thedata servers communicate with devices in the fieldthrough process controllers. Process controllers, e.g.PLCs, are connected to the data servers either directlyor via networks or fieldbuses that are proprietary (e.g.Siemens H1), or non-proprietary (e.g. Profibus). Dataservers are connected to each other and to clientstations via an Ethernet LAN. The data servers andclient stations are NT platforms but for many productsthe client stations may also be W95 machines. Fig.1.shows typical hardware architecture.
Download full report