Try to realise the practical utilities of the standardisations—the user’s project cycle is shorter using standardised software components and their cost is lower. These benefits are real and tangible. Because the OPC standards are based, in turn, upon computer industry standards, technical reliability is assured.
The original specification standardised the acquisition of process data. It was quickly realised that communicating other types of data could benefit from standardisation. So standards for alarms and events, historical data and batch data were launched.
Additionally, you need to know the current and emerging OPC specifications and their applicability. The original OPC Data Access used to move real-time data from PLCs, distributed control systems and other control devices to human machine interface and other display clients. The Data Access 3 specification is now a Release Candidate. It leverages earlier versions while improving the browsing capabilities and incorporating XML-DA Schema.
OPC Alarms & Events provides alarm and event notifications on demand (in contrast to the continuous data flow of Data Access). These include process alarms, operator actions, informational messages and tracking/ auditing messages.
OPC Batch specification carries the OPC philosophy to the specialised needs of batch processes. It provides interfaces for the exchange of equipment capabilities (corresponding to the S88.01 physical model) and current operating conditions.
Client-to-server and server-toserver communication across Ethernet fieldbus networks is facilitated by OPC Data exchange. It provides multi-vendor interoperability. It also adds remote configuration, diagnostic and monitoring/management services.
OPC Historical Data Access provides access to data already stored. From a simple serial data logging system to a complex SCADA system, historical archives can be retrieved in a uniform manner.
Moreover, all standardisations ensure secured mode of operation—OPC Security specifies how to control client access to servers in order to protect this sensitive information and to guard against unauthorised modification of process parameters. All these commands allow the users to identify, send and monitor control commands which execute on a device.
How to communicate for better control?
Although computers, PLCs and remote terminal units communicate with each other digitally, most end devices (valves, pressure transducers, switches, etc) still use analogue signals. For example, an analogue value of 4 mA might correspond to a pressure of no flow, while a value of 20 mA might correspond to a 1000GPM flow value. With discrete devices, the presence of a signal might represent a ‘closed’ or ‘alarm’ condition, while the absence of a signal might represent ‘open’ or ‘normal.’
But keep in mind, in the future, the 4-20mA standard will be replaced with a digital, two-way, multidrop commu-nication—FieldBus. You need to know the reason behind that. In two-way communications, a value can not only be read from the end device but also be written to the device. For example, the calibration constants associated with a particular sensor can now be stored directly in the device itself and changed as needed. The multi-drop capability of a FieldBus results in the most immediate cost savings for users.
With analogue devices, a separate cable needs to be run between the end device and the control system because only a single analogue signal can be represented on the circuit. Modern distributed systems partially solve this problem by locating remote multiplexing devices out in the field. The ultimate solution, however, is to be able to connect a reasonable number of sensors all located in the same area to the same cable. Although this will not happen overnight, you should be prepared to accept this tectonic shift in technology.
Know advanced processcontrol
To get an edge over your competitors, it is always advisable to learn advanced process control with respect to the underlying theory, implementation studies, the benefits that its applications will bring and projections of future trends.
Initially, advanced process control meant any algorithm or strategy that deviated from the classical three-term PID controller. The advent of computers offered more convenient alternatives—feed forward control, multivariable control and optimal process control. Indeed, the proliferation of so-called advanced control methodologies can only be attributed to the advances made in the electronics industry, especially in the development of low-cost digital computational devices (circa 1970). Nowadays, advanced control is synonymous with the implementation of computer-based technologies.
Also, try to understand the impact of advanced process control on product yield, energy consumption, product quality, process safety, environmental emissions, etc. Usually, cost savings ranging from 2 to 6 per cent of the operating cost are observed with the implementation of advanced controls. These benefits are clearly significant and achieved by increasing process efficiency, hence allowing plants to be operated closer to their designed capacity.
