Abstract: This article analyses the characteristics and differences of the three control systems — PLC, DCS and FCS — pointing out their origins and directions. Keywords: PLC; DCS; FCS
Fieldbus control systems, practical since the 1990s, are developing rapidly as the newest control systems in the world — a hot spot of automation drawing intense attention from vendors and users alike. Their arrival will bring automation another revolution, deeper and broader than any before, opening a new era.
In some industries FCS grew from PLC; in others from DCS — so FCS is tied to PLC and DCS by a thousand threads yet differs from them in essence. This article analyses the three systems' features and differences, their origins and directions.
Today continuous process control (PA, or industrial process control) knows three major systems: PLC, DCS and FCS.
Notice one thing from the above: none of the three process-control systems was developed for power stations — none took power plants as its first object, and none names power stations first in its applicable scope (some never mention them). Strangely, all three — especially DCS and PLC — are now widely and successfully used in power plants.
As noted, FCS grew from DCS and PLC, bearing their features plus a revolutionary step forward. Meanwhile new DCS and new PLC are converging: new DCS has strong sequencing; new PLC handles closed loops well; both form large networks; their application scopes already overlap heavily. This section compares only DCS with FCS. Earlier chapters touched on their differences; here we cover architecture, investment, design and use.
The key to DCS is communication — the data highway is the backbone. Tasked with networking all system parts, the highway's design determines overall flexibility and security. Media: twisted pair, coaxial or optical fibre.
The highway's design parameters reveal a given DCS's strengths and weaknesses: (1) how much I/O it handles; (2) how much control-loop information; (3) how many users and devices (CRTs, stations) it adapts to; (4) how thoroughly data integrity is checked; (5) maximum highway length; (6) how many branches it supports; (7) whether it supports other makers' hardware (PLCs, computers, data loggers).
For communication integrity most DCS vendors offer redundant highways; for security, complex protocols and error detection. A protocol is a set of rules ensuring transmitted data arrives and is understood as sent. DCS uses two communication means: synchronous (a clock regulates transmission and reception) and asynchronous (report system without clock).
Three keys:
(1) The core of FCS is the bus protocol — the bus standard. As described earlier, once a bus protocol is fixed, its key technologies and devices are determined. In principle all buses are alike — both solving two-way serial digital communication — but for various reasons their protocols differ greatly.
For interoperability and true openness, the user layer of the IEC fieldbus communication model explicitly provides device description. Every field device is described by a Device Description (DD) — think of it as the device's driver, holding all necessary parameter descriptions and the master's operating steps. Since DD contains all information needed for communication and is master-independent, true interoperability follows.
Does reality match? No. The adopted fieldbus international standard contains 8 types, the original IEC type being merely one among eight, equal in standing to the others. Those seven, whatever their market share, each carry a full software/hardware support base — they form systems and products — whereas the original IEC standard is an empty frame with neither software nor hardware support. Mutual compatibility and interoperability among these buses is, at present, nearly impossible.
The picture then: an open FCS is open and interoperable within one specific fieldbus type — any vendor's product following that bus protocol can join the bus network, interoperable with the rest.
(2) The foundation of FCS is digital intelligent field devices. They are the hardware base of FCS. The reason is simple: FCS runs two-way digital fieldbus signalling between control devices and field devices. If field devices follow no common bus protocol and cannot communicate digitally, "two-way digital communication" is empty talk — not a fieldbus system. Moreover, a fieldbus hallmark is added field-level control: without multifunction intelligent field devices, FCS's features — simpler systems, easier design and maintenance — are hollow.
(3) The essence of FCS is field-level information processing. Whether DCS or fieldbus, a system must process at least the same information. In fact fieldbus draws more information from the field: information volume is undiminished, even increased, while cables shrink dramatically. That demands both far higher cable information capacity and extensive on-site processing, reducing field-to-control-room traffic. Fieldbus's essence is field-level information processing.
Reducing information traffic is a key principle of network design and configuration, often improving response time: put nodes exchanging heavily on the same branch.
Reduced traffic and reduced cable sometimes conflict; then investment rules: if the chosen system's response time allows, choose the cable-saving plan; if response is tight and slightly less traffic suffices, choose the traffic-saving plan.
Some fieldbus instruments now carry many function blocks; identical blocks on one network branch are a fact, and choosing which instrument's block to use is a configuration question. The principle: minimise bus traffic — generally choose the block on the instrument producing the most relevant outputs.
With fieldbus, users cut field wiring drastically, gain multivariable communication from single instruments, full interoperability across vendors, added field-level control, greatly simpler integration, and easy maintenance. Fig. 1 shows a typical fieldbus system: in traditional systems every field device needs a dedicated twisted pair to the control room for 4–20 mA; in the fieldbus system (Fig. 2) device-to-junction pairs remain, but one twisted pair from junction box to control room carries the digital communication.
Editor's note: Figs. 1–3 were not preserved in the web version.
How much cable fieldbus saves, the editor has not computed — but the cabling of DCS power plants shows its share of capital cost. A 2×300 MW coal-fired plant, unit-based thermal system, one combined control building per unit (boiler-turbine-generator centralised control) with the control room at +12.6 m matching the operating floor: DCS was WDPF-II, 4,500 I/O per unit. Cable routing used EC software — 8 people, 1.5 months; per 300 MW unit in the main building, 4,038 automation cables totalling 350 km (excluding fire-alarm and auxiliary-plant cables); galvanised steel supports, trays and troughs ~95 t per unit, plus aluminium-alloy fittings ~55 t.
A 4×MW oil-and-gas station, unit-based, DCS TELEPERM-XP, 5,804 I/O per unit: routing by EC software — 12 people, 2.5 months; per 325 MW unit, 4,413 automation cables totalling 360 km; galvanised steel trays ~200 t per unit. Station cables fall into six classes: HV power, LV power, control, thermal-control, weak-current (computer) and others. Laying two 300 MW units together means ~8,500 automation cables, of which thermal-control and weak-current exceed 5,000 — about 60% by count.
The comparison so far is purely technical; now economics. The premise: DCS versus a typical, ideal FCS. Why this assumption? DCS has long satisfied its founding requirements and now merely improves — no "typical ideal" applies. FCS, practical only since the 1990s, still falls short of its founding requirements — openness, two-way digital communication, intelligent field devices, high-speed buses — partly due to the international standard wars: for a decade bus consortia raced to write standards, build products and seize markets, seeking international-standard status and legitimate market share. That war has subsided; the majors now realise that true market capture requires perfecting systems and products. We predict that soon, perfected fieldbus systems and products will become the world mainstream.
Specifically:
Some FCS grew from PLC, some from DCS — now that FCS is practical, what future for PLC and DCS?
PLC appeared in the USA in the late 1960s to replace relays in logic, timing and counting sequence control, building flexible program control. Named and defined in 1976: a digital electronic computer dedicated to control, using programmable memory for instructions executing logic, sequencing, timing, counting and arithmetic, controlling machines and processes via analogue and digital I/O. Thirty years on, the PLC is mature and complete, with analogue closed-loop functions added. Its place in FCS seems settled without much argument (see Fig. 3, the IEC-recommended FCS architecture): PLCs hang on the high-speed bus as stations, exploiting their switching strengths. Moreover, thermal plant auxiliaries — makeup-water treatment, circulating water, ash handling, coal handling — are mostly sequential processes where PLC excels; in the editor's view, auxiliary-plant control should prefer bus-compliant PLCs, or PLCs able to exchange information with FCS.
Since the first microprocessor-based controller in 1973, DCS matured into a complete, safe, reliable digital distributed system, outperforming all predecessors, meeting every demand of power-plant DAS, MCS, SCS and APS — and now building management networks over industrial Ethernet for the growing management calls. DCS monitoring can cover the whole process of a large thermal unit.
But since FCS went practical in the 1990s, published views keep recurring: "from now on, new FCS will gradually replace traditional DCS"; "once control moves to the field, traditional DCS becomes unnecessary and will vanish"; "within ten years the traditional 4–20 mA analogue signalling will yield to two-way digital fieldbus, and analogue-digital DCS will be superseded by all-digital FCS"... In one sentence: FCS replaces DCS; DCS dies.
Spoken by authorities, these views are not groundless. Digital communication is a trend — technological progress no one can stop. Two-way fieldbus signalling and its enormous impetus accelerate the transformation of field devices and control instruments, producing ever more capable digital intelligent devices — absent from DCS — whose advantages in power-plant design, configuration, commissioning, operation, maintenance and management DCS cannot offer. Moreover, FCS grew from DCS and PLC, retaining their features — absorbing DCS's years of development and field experience, lessons included — so "FCS will replace DCS" seems natural.
Yet DCS has developed for nearly 30 years and pervades thermal plants; its design philosophy, configuration and function matching are highly perfected (though further development continues, e.g. advanced software for information integration), penetrating every corner of power-plant control and reflected even in FCS. From that angle, DCS can hardly be said to die. And as noted, in fields where FCS cannot fully exercise its strengths, DCS still has its place.
We need not argue over words about who replaces whom. Just as today's DCS and new PLC, after years of development, complement each other into new systems while keeping their own characters — today's DCS is not the DCS of old, nor the new PLC the PLC of its origins — saying DCS replaced PLC, or PLC replaced DCS, is plainly inappropriate.
From the analysis: with FCS's arrival, digital distributed control DCS will not die — it merely moves from the centre of the control system to a station on the fieldbus. Put another way, DCS's central position is broken from now on. The future power-plant control system will be a new system with FCS at the centre, carrying DCS philosophy.
