DCS - Distributed Control System

A typical DCS configuration

DCS stands for 'distributed control system'. The term 'distributed' means that several processors are operating together. This is usually achieved by dedicating tasks to different machines. It does not necessarily mean that the separate computers are physically located in different areas of the plant.

Figure shows how a typical system may be arranged. The following notes relate to individual parts of that system. In practice, each manufacturer will usually offer some variant of the system shown in this diagram, and the relevant description should be consulted, but the comments made here are general ones which may help to identify points which should be considered and discussed when a new or refurbished system is being considered.

1.1 The central system cabinets

Located near the centre of Figure are the cabinets which house the processorsthat executes the control functions. These cubicles also contain the attendant interface and input/output (I/O) cards and the necessary power supply units (PSUs). The latter will usually be duplicated or triplicate, with automatic changeover from one to another in the event of the first failing. (This automatic changeover is often referred to as 'diode auctioneering’ because silicon diodes are used to feed power from each unit onto a common bus-main. In the event of the operational power-supply unit failing, its diode prevents a power reversal while the back-up power unit takes over.) At this time it is important that the system should raise an alarm to warn that a PSU failure has occurred. Otherwise the DCS will continue to operate with a diminished power-supply reserve and any further failure could have serious consequences.
Clearly, the DCS cannot operate continuously from batteries alone. Are liable and stable source of power will therefore need to be available (usually ll0V or 240V AC). If the DCS includes internal back-up batteries it will continue to operate if the AC feed is lost, but such batteries are normally sized to retain essential data in the memory and to provide a limited amount of functionality. They may also allow limited control to be performed, but all this will function for only a short period (typically 30 min) and it is therefore usual to provide an external uninterruptable power supply (UPS) system which can allow the plant to be operated for a longer time. The duration of this period warrants very careful consideration. Long periods require large and expensive batteries and charger systems, and this expense can rarely be justified (especially since such a major power loss will probably have disabled all pumps, motors etc.). Instead, it is common to provide a battery capacity that will allow the plant to be safely shut down in the event of power failure. The determination of the time required for such an operation is a matter of discussion with the process design engineers and the plant management.

In addition to supplying the computer system, the power-supply system will usually also have to provide DC supplies for 4-20 mA transmitters and for limit-switch contacts. (The voltage connected to a contact and thence to the DCS input channel is often referred to as the 'wetting voltage'.) Transmitters operating on the 4-20mA range which are powered from the DCS are sometimes called 'passive'. In comparison, those that operate from local power supplies are called 'active'.

The I/O cards consist of analogue and digital input and output channels. Analogue inputs convert the incoming 4-20mA signals to a form which can be read by the system. The printed-circuit cards for analogue inputs may or may not provide 'galvanic isolation'. With a galvanically isolated device the signal circuit is electrically isolated from others, from the system earth and from the power-supply common rail. Galvanic isolation simplifies circuit design since it prevents inadvertent shortcircuiting, but consideration should be given to the possible build-up of static charges on completely ungrounded circuits, which could cause damage to input devices (which are usually rated for not more than a few tens or hundreds of volts). This is normally an important consideration only in areas of very low humidity or where there is a strong presence of charged particles.
The commissioning processes, and the task of identifying and correcting faults, are operations which are considerably assisted by the provision of light-emitting diode status indicators (LEDs) on the digital output cards.

Some systems provide switches on the digital input cards, which can be of assistance with commissioning and fault-finding. However, inadvertent or deliberate mal operation of such switches can have serious consequences, since the DCS is then provided with incorrect plant-status information and it may take inappropriate action. (The use of logic probes, which inject signals into a system to check its operation, is also to be deprecated, for similar reasons.)

Analogue and digital I/O channels are normally grouped into 8, 16, 32or 64 channels per printed-circuit card. 8 or 16 analogue input (AI)channels are commonly accommodated on a card, but analogue output(AO) channels consist of current generators and so occupy more space and are more expensive than AI channels, which are based on small operational-amplifier devices (op-amps). Digital input (DI) channels are very simple and cheap and may be grouped into 16 or even 32 inputs to a single card. Digital output (DO) channels driving lower-power devices are also simple and cheap, and may also comprise 16 or 32 inputs to a single card, but DOs for higher-power devices (such as solenoid valves) usually require the provision of relays. These may be included on the card or they may be separate.

When considering the provision of spare I/O channels, careful thought must be given to the grouping of channels. If a system has 256 analogue input channels available, of which only 230 are actively used, it may be said to have 11% spare capacity in this area. However, the grouping of functional areas into cards will inevitably result in the occurrence of more spare channels in one area than in another. It is possible, therefore, to have the required amount of spare I/O capacity available in terms of the overall system, but to be unable to modify or extend a particular part of the system safely, because no spare channels have been provided in the required area. Spare capacity should be provided both in the form of 'populated’ channels (i.e. spare inputs and outputs on individual cards) and 'unpopulated’ space (i.e. spaces for additional cards). To avoid a spaghetti-like tangle of cross-connections, the spare spaces should be sensibly distributed through the system.

1.2 Termination and marshalling

It is important to understand that the grouping of inputs and outputs on the I/O cards does not always correspond with the grouping of signals into multipair cables, which is dictated by the physical arrangement of equipment on the plant. While it is sensible to avoid mixing different control systems (e.g. feed water control and combustion control) onto a single card, the signals associated with a single system will not necessarily all be carried in the same cable. The result is that a certain degree of cross connection or 'marshalling' is always required.

Well-designed systems will provide adequate facilities for neatly marshalling the signal connections, but this inevitably requires that the identification of signal connections and their location in the cable system is known at an early stage of the contract. The later this problem is resolved, the more complex and untidy the system will become. Complexity and untidiness can be dangerous because it can lead to mistakes occurring during commissioning or afterwards.

1.3 Operator workstations

The operator workstations consist of screens, on which plant information can be observed, plus keyboards, trackballs or 'mouse' devices allowing the operator to send commands to the system. They also comprise printer’s for operational records, logging of events (such as start-up of a pump), or alarms. Some systems also provide plotters (one use of plotters is to detect the possible stalling of an axial-flow fan)

The screens can be ordinary cathode-ray tube types as used with personal computers, or they may be large-screen plasma displays or projection systems. The selection of the type of screen depends on the operational requirements, but will ultimately be determined by the available budget. Critical ergonomic factors affect the optimum design of the workstations, and great care must be exercised to ensure that the plant can be operated safely under all conceivable modes of failure, and that no computer-assisted errors can occur due to the operator being confused by the information presented to him or her.

An important consideration is the screen update time. This is the time between the occurrence of an event and its appearance on the screen. As system loading is increased, this time can become extended, but the operator will need to be made aware of each event as soon as possible after it occurs, so that corrective action can be taken. An update time of 1 s is barely adequate to deal with fast-moving events, but it can be quite difficult to achieve.