Controller, data highway

Whether you can see them or not the oscillations from control valves are everywhere in the process [Ref. 6.2 and 6.3). It is just a matter of how large and how important. When the amplitude is less than the exception reporting setting of controllers, data highways, and historians, the oscillations do a disappearing act on operator displays and trends [Ref. 6.1). All control valves have a resolution limit determined by the amount of stick-slip shown as the stair case in valve position as the valve first starts to open in Figure 6-1. In the best throttling valves with air actuators, the stick-slip is normally only about 0.1% and its effect is not observable in... 

Microprocessor technology permitted these technical issues to be addressed in a cost-effec tive manner. In the mid-1970s, a process control architecture referred to as a distributed control system (DCS) was introduced and almost instantly became a commercial success. A DCS consists of some number of microprocessor-based nodes that are interconnec ted by a digital communications network, often called a data highway. The key features of this architecture are as follows ... 

Distributed Control System (DCS) A system that divides process control functions into specific areas interconnected by communications (normally data highways) to form a single entity. It is characterized by digital controllers, typically administered by central operation interfaces and intermittent scanning of the data highway. 

The distributed control system (DCS) hardware areas are often referred to as "process computer rooms." I/O Rooms contain the incoming and outgoing wiring, cables and data highway links, and often small transformers and other related electrical equipment. Often, additional space is needed for a master process engineering computer terminal/work station for process control system changes and for critical safety instrumented systems (SIS) for interlocks and emergency shutdowns. 

A tremendous cost savings is realized by the multiplexing of signals due to the elimination of many multi-conductor cables and thousands of field terminations. Also, because the PCMs are remote from the central control room, they can be installed in the equipment modules. This means that each equipment module can be virtually stand-alone in terms of electrical and control systems, thus allowing almost complete prefabrication in the fabrication yard the only remaining field work is connecting to the data highways. 

An important feature of the distributed control system chosen is that the PCMs can communicate over a microwave link, thus allowing the well pad manifold modules to be included in the GC control system the hardwire data highway is limited to a distance of 5000 ft. (1524m) from the Host. The well chokes and manifold controls will remain on the SCAOA system, but the well pad will require continuous process control of the test separator, water injection flow control, and artificial lift gas flow control. 

Since that time, however, several of the more established control systems vendors have moved into the DCS area and indications are that they are prepared to develop the microwave radio data highway. 

If the DCCS is required to replace an existing control system (i.e. if the plant is being retrofitted) then the microcontrollers will normally be located in the central control room (CCR). In this case, the distribution features of the DCCS will not be fully utilised and the significance of the digital data highway (DH) will also be reduced. 

This consists principally of the host which is used to supervise several process areas and may well have six or more data highways linked to it. Hosts are powerful computers with their own operator stations and other peripherals. They are an integral element of modern control systems and only very small processes may be controlled and monitored effectively without a host. However, if not configured properly, a host computer can be overloaded quite easily and will then fail to operate satisfactorily. One serious loading problem for a host is the fast scanning rate required for DDC loops (usually once per second and up to three times per... 

FIG. 8-65 A DCS using a broadband (high-bandwidth) data highway and field bus connected to a single remote control unit that operates smart devices and single-loop controllers. 

Microprocessor-based equipment, such as smart instruments and single-loop controllers with digital communications capability, are now used extensively in process plants. A field bus, which is a low-cost protocol, is necessary to perform efficient communication between the DCS and devices that may be obtained from different vendors. Figure 8-65 illustrates a LAN-based DCS with field buses and smart devices connected to a data highway. 

Larger, commercial installations will also be equipped with a Supervisory Control And Data Acquisition system (SCADA). The SCADA system is connected via a data highway with the PLC(s) used in the system. 

Instrumentation philosophy (local/remote control, hardwired/data highway, failure mode(s), analog/digital, emergency alarms, etc.). 

System consoles attached to the data highway act as a local console for any of the local control units. In addition, system consoles can be used to change linking functions of the distributed elements. 

Another major development of modern control systems is the data highway. Field information going to and received from devices such as thermocouples, transmitters, controllers, and other instruments are converted into digital numbers. The reason for the digital highway, which conveys all of this information to and from the field instruments and the control room, is to reduce the amount of wiring required. For example, some of the most advanced computer control systems for process plants can convey 150,000 bits of information per second using a coaxial cable. 

The committee noted that the new TOCDF control system Ethernet backbone does not have a parallel or redundant pathway. Although the single Ethernet backbone may have electronic reliability as good as the former redundant data highway, it could be vulnerable to failure from physical damage. TOCDF representatives stated that the installed cost of the single Ethernet was approximately 60,000. Provision of a redundant Ethernet path at TOCDF is worthy of reconsideration. 

In the planned modifications to their PDARS configuration at TOCDF, plant devices are to be connected to PLCs via Allen Bradley 1700 series I/O modules, and PLC-3s and PLC-5s are networked by a redundant Data Highway II. This segment of the control system remains unchanged from the initial system. 

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