A Pre-SCADA System Assessment
By Randy Dennison
Introduction Supervisory Control and Data Acquisition (SCADA) is a process control system that enables a site operator to monitor and control processes distributed among various remote sites. Such systems can be used to monitor and control land, air or water pollution control equipment, or just about any manufacturing process.
A properly designed SCADA system saves time and money by eliminating need for service personnel to visit each site for inspection, data collection/logging or make adjustments. Real-time monitoring, system modifications, troubleshooting, increased equipment life and automatic report generating are just a few of benefits that come with today’s SCADA systems.
As technology continues to advance, such systems will be operating standard for process control. But from hundreds of system providers available today, which one will a facility choose to partner with and why?
Choosing a system provider that will design a system applicable to an operator’s needs can be an overwhelming, confusing task. With little or no knowledge of SCADA and telemetry systems and an incomplete pre-system assessment, decisions can include costly mistakes. Too often decisions are based upon:
Price: The quality of system components and workmanship may suffer when vendors low-bid to win contract. The vendor may then indiscriminately cut costs to make a profit. Proprietary equipment – If proprietary, closed-protocol equipment is installed in system, customer can be forced to pay inflated prices and face possibility of future equipment integration problems due to obsolete or irreplaceable components, company shutdowns and a lack of support when it comes time for repair.
Excessively complex or customized equipment: If SCADA system is too complex to understand, operate and support, only recourse is to purchase expensive training and/or service contracts, which do not always guarantee prompt and professional service.
Years of experience or knowledgeable expertise: There are a host of reputable SCADA providers, with years of experience and knowledgeable expertise, who have designed systems that are too broad or expensive. Experience and knowledge are important but only as a starting point when selecting a vendor.
Sales people and/or flashy marketing: Effective sales and marketing strategies are meant to produce top-of-mind results. Avoid being lured or pressured into a purchase. Be equipped and make a sound decision based on all factors that affect optimum system performance.
These and other costly mistakes can be avoided through knowledge, understanding and careful assessment. Some will only need to consider Tables A and B of this article. Others, with little or no SCADA knowledge, will need to familiarize themselves with more background information.
A Brief History SCADA began in early 1960s as an electronic system operating as input/output transmissions between a master station and a remote station. The master station would receive data through a telemetry network and then store data on mainframe computers.
In early 1970s, distributed control systems (DCS) were developed to control separate remote subsystems and in 1980s, with development of microcomputer, process control could be distributed among remote sites. Further development enabled DCS to use programmable logic controllers (PLC), which have ability to control sites without taking direction from a master.
In late 1990s, SCADA systems were built with DCS capabilities and systems were customized based on certain proprietary control features built in by designer. Now, with Internet being utilized more as a communication tool, SCADA and telemetry systems are using automated software with certain portals to download information or control a process.
Engineered SCADA systems today not only control processes but are also used for measuring, forecasting, billing, analyzing and planning. Today’s SCADA system must meet a whole new level of control automation while interfacing with yesterday’s obsolete equipment yet remain flexible enough to adapt to tomorrow’s developments.
Whether requirement is a new system or upgrading an older one, it is important to know system components before deciding on who to talk with and what equipment is needed for a particular application.
System Components The four major SCADA system components include Master Terminal Unit (MTU), Remote Terminal Unit (RTU), communication equipment and SCADA software.
The MTU is located at operator’s central control facility and provides a man-machine software interface, two-way data communication and monitoring/control of remote field devices.
The RTU, located at a remote site, gathers data from field devices (pumps, valves, alarms, etc.) into memory until MTU initiates a send command. The central processing unit within RTU receives a data stream via hardware equipment protocol. When RTU sees its specific address embedded in protocol, data is interpreted and CPU directs specified action to take. The protocol used can be open like Modbus, TCP/IP or a proprietary closed protocol. Some RTUs, called “smart PLCs” or remote access PLCs, provide remote programmable functionality while retaining communications capability of an RTU. These devices are designed to perform control functions, check site conditions, re-program anytime from anywhere, and have any alarm or event trigger a call to a personal computer without any direction from MTU.
The way MTU/RTU transmission network or topology is set up can vary, but system must feature uninterrupted, bi-directional communication in order to properly function. Methods to accomplish this include private medium, where end user owns, operates, licenses and services medium, and/or public medium, where customer pays for a monthly, per time or volume use.