7/29/2019 The Engineering of Distributed Control Systems

1/2

The Engineering of Distributed Control Systemsby Ren Simon

The classical measurement, control and actuator devices were based on simple

physical principles (mechanical, hydraulic, pneumatic, electrical).

Often they were used as stand-alone devices for relatively closed automationsolutions.

With the introduction of microprocessor technology and its fast spreading, the focusshifted from stand-alone devices to much more complex device systems.

These systems ofautomation devices including their necessarycommunication systems are called Process Control Systems (PCS).

Process Control Systems provide control and supervision of production processes.

They connect people (e.g. the operator) and machines. They consists of input /output devices, data processing units, human machine interfaces and communicationsystems.

1st generation PCSs are characterised by a centralised structure. A centraldevice scans all relevant process data and computes the actuator values.

There are two basic types of PCSs, one for process control and one formanufacturing. Their internal structures are similar, the market (user) will decideabout a possible fusion.

This fusion and the transition from centralised to de-centralised systems basedon serial communication systems (e.g. fieldbus) are milestones of the developmenttowards 2nd generation PCSs. This development hasn't been finished yet, but lotsof solutions are emerging.

A non-interrupted engineering process on the basis of common information modelsand data exchange technologies is the primary requirement for the design of 3rd

generation PCSs. These 3rd generation PCSs are called Distributed ControlSystems (DCS) here. Why becomes the engineering process so important?

Traditionally, PCS are seen from a run-time point of view (function, device, system).The engineering aspect becomes more important due to its increasing complexity

and costs involved.

The complexity results from several factors influencing the engineering process suchas different device components (input / output, data processing, HMI,communication), process physics (mechanical, electrical, ...), live cycle phases anddevice vendors.

Today, the integration of these different worlds usually is done by building hardwareand software interfaces and expensive commissioning processes based on trial-and-

7/29/2019 The Engineering of Distributed Control Systems

2/2

error approaches. This will become impossible under a true cost of ownershipprinciple.

The whole engineering process must be supported by integrated tools in order toavoid data losses and inconsistencies. The use of paper and even of electronic meansdon't provide a solution as long as common transfer syntax and standardised data

models are not used.

The standard IEC 1131 defines hardware and software models as well asprogramming languages for Programmable Controllers. Unfortunately, nomechanism for data exchange between engineering (programming) tools wasdefined.

To fill this gap, a neutral file exchange format (FXF) was specified by PLCopen. TheFXF is based on a STEP (ISO 10303) model and allows to transport PLC programsfrom the programming environment of one vendor to another. FXF is onerequirement for the Portability Level Certification by PLCopen and certainly a majorstep forward to open systems.

IEC 1131 languages are suitable not only for PLC programming, but also as basis forthe functional description of a Distributed Control System (cf. IEC 1499).

The fieldbus community developed some other approaches to exchange data byelectronic means. This includes Device Descriptive Languages and Device Data Base(HART, FF, PROFIBUS). As part of the Function Block activities (IEC TC65 (IEC1499), IEC SC65C WG7) the topic is also treated. These different solutions begin tooverlap.

The international standard for the exchange of product data ISO 10303 (STEP) offersthe only comprehensive solution (data description languages (EXPRESS), mapping to

implementation technologies, tools, Application Protocols). Although developed forproduct data it is quite suitable to solve engineering problems of DCS as severalprojects show. The integration of all relevant data (functional, material, powersupply, ...) becomes possible. However, some problems are still open (e.g. how tointegrate modern implementation technologies such as OLE, CORBA, Internet?).

Fieldbus wars prevented true interoperability between devices of different vendors,even today. Who wins, who loses? In PLC programming, only one standard emergedfrom many propriety solutions: IEC 1131. This is a real success of PLCopen andgenerates profits for all: technology providers, device vendors and end users. Whynot continue this successful story? PLCopen has a good chance to become a leader ofthis new progressive development: know-how, technology and market penetration. Itis necessary to act now, or other are doing it.

Users, ask for it!