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 the need for service personnel to visit each site for inspection, data collection/logging or making adjustments. Real-time monitoring, system modifications, troubleshooting, increased equipment life and automatic report generating are just a few of the benefits that come with today's SCADA systems.

As technology advances, such systems will be the 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 the decisions are based upon:

• Price – The quality of system components and workmanship may suffer when vendors low-bid to win the contract. The vendor may then indiscriminately cut costs to make a profit.
• Proprietary equipment – If proprietary, closed-protocol equipment is installed in the system, the customer can pay inflated prices and face possible future equipment integration problems due to obsolete or irreplaceable components.
• Excessively complex or customized equipment – If the SCADA system is too complex to understand, operate and support, the 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 many 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. 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. For some, answers can be found by examining Tables A and B (see bottom). Others, with little or no SCADA knowledge, will need to equip themselves with more background information.

A brief history

SCADA began in the early 1960s as an electronic system operating as input/output transmissions between a master and a remote station. The master station would receive data through a telemetry network and then store data on mainframe computers.

In the early 1970s, distributed control systems (DCS) were developed to control separate remote subsystems and in the 1980s, process control could be distributed among remote sites. Further development enabled DCS to use programmable logic controllers (PLC), which have the ability to control sites without taking direction from a master.

In the late 1990s, SCADA systems were built with DCS capabilities and systems were customized based on certain proprietary control features built in by the designer. Now, with the 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 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 the requirement is a new system or upgrading an older one, it is important to know the 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 the Master Terminal Unit (MTU), the Remote Terminal Unit (RTU), communication equipment and SCADA software. The MTU is located at the 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 the MTU initiates a send command. The central processing unit within the RTU receives a data stream via hardware equipment protocol. When the RTU sees its specific address embedded in the protocol, data is interpreted and the CPU directs the specified action to take. The protocol used can be open like Modbus, TCP/IP or a proprietary closed protocol. Some RTUs, called "smart PLC" or remote access PLCs, provide remote programmable functionality while retaining the 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 the MTU.

The way the MTU/RTU transmission network or topology is set up can vary, but the system must feature uninterrupted, bi-directional communication in order to properly function. Methods to accomplish this include private medium, where the end user owns, operates, licenses and services the medium, and/or public medium, where the customer pays for a monthly, per time or volume use.

Private media transmission includes wire lines or buried cable and modems, and is usually limited to low bandwidth. When it makes sense for a company to string or bury its own communication cable between sites, companies should consider staffing requirements necessary to support the technical/maintenance aspects of the system. The second method to consider is wireless transmission and includes spread spectrum, microwave or VHF/UHF radios.

Spread spectrum is license-free and available to the public in the 900 MHz and 5.8 GHz bands. Some spread spectrum radios have the ability to re-strengthen signals for the next radio in line. These repeater radios are used to span distances and generally have built-in error correction, encryption and other features, making them a reliable, secure and long-lasting solution for network communication.

Microwave radio transmits at high frequencies through parabolic dishes mounted on towers or on top of buildings. This media uses point-to-point, line-of-sight technology and communication may become interrupted at times due to misalignment and/or atmospheric conditions.

VHF/UHF radio (good for up to 30 miles) is an electromagnetic transmission with frequencies of 175 MHz-450 MGz-900 MHz received by special antennas. A license from the FCC must be obtained and coverage is limited to special geographical boundaries.

Public media transmission includes services offered by a local telephone or cable company, and in some systems and/or subsystems, it may provide a more suitable method for data transfer. The Public Switch Telephone Network, Generally Switched Telephone network, and the Cellular network are dial-up services suitable for occasional use. If a 24-hour permanent connection for analog data transmission between two or more locations is needed, the Private Leased Line should be considered. The Digital Data Service with DSL and ISDN should be considered for high speed/low error rate, computer-to-computer applications. WiFi equipment utilizes broadband as well, but on a time-share basis when it makes sense to use the infrastructure of another company. PCS/CDPD service, provided by cellular companies and Low Earth Orbit or geosynchronous satellites can also be used for continuous communication.

Finally, the employment of an easy-to-use SCADA software package, commonly known as the human machine interface (HMI), installed on PC hardware provides a reliable representation of the real system at work. An HMI allows the operator to view virtually all system alerts, warnings and functions as well as change set points and analyze, archive or present data trends.

Most of these software packages use standard data manipulation/presentation tools for reporting and archiving and integrate well with Microsoft Excel, Access and Word.

Collected data can also be sent to Web servers that dynamically generate HTML pages to be viewed on the operator's LAN or published to the Internet.

The microprocessor option

With this basic understanding of SCADA system components, a facility may want to consider utilizing a microprocessor and/or PLC-based SCADA system over a basic RTU or a proprietary system for the following reasons:

Microprocessors, like MTUs, can continuously collect, process and store data, operating independently from the MTU through "intelligent" programming. By utilizing a microprocessor-based level meter, a SCADA system provides both a master and local display that automatically gathers, processes and reports data necessary to comply with local, state and federal regulations.

Microprocessors can provide security and monitoring of door switches, heat and motion detectors. Managers/operators can be informed 24 hours a day through automatic e-mail, paging and dial-up call features. Multiple users can easily be added and, if open architecture protocol is used, future equipment can easily be integrated.

Microprocessor-based SCADA systems can reduce the number of man-hours needed for on-site visual inspections, adjustments, data collection and logging. Continually monitoring and troubleshooting potential problems increases equipment life, reduces service calls, reduces customer complaints and increases system efficiency. Simply put, open-architecture, microprocessor-based SCADA systems are an excellent means for process control facilities to save time and money.

Review

The return on investment and benefits produced by a properly engineered microprocessor-based SCADA system far outweigh the initial investment if the right equipment is chosen and installed. From the hundreds of SCADA system providers to choose from, one poor decision may lead down the path to countless frustrations, inefficiencies and unnecessary expenses.

Hopefully, by conducting a pre-SCADA system assessment, facilities will be better equipped to avoid such problems. PE

Table A – Questions to Consider for:

SCADA systems in general

1. What features/benefits will SCADA perform at the master/remote site?
2. What type of SCADA system/hardware is presently installed (proprietary, outdated, etc.)?
3. How many sites, stations and dependant users does the present/future system require?
4. Will a remote station collect data independently from a master station?
5. Will a master station control local input/output and back-up operations?
6. Will the present/future system communicate with additional (LAN, Internet, etc.) points?
7. Considering the present/future software used, is it fully documented, easy to learn/use and likely to have future support?
8. Does the present/future budget need to change for higher return on investment?
9. What aspect of SCADA requires further investigation?

The field equipment

1. Rate the performance standards of each site and all field equipment. Are they reliable and cost-effective?
2. What present equipment needs replacing?
3. What present/future process needs expansion, monitoring and control?
4. What are the future equipment/expansion needs?
5. Can new equipment integrate well with the present system?
6. What is the present/future maintenance and service arrangement? Will it change with new equipment? Will the facility use its own maintenance personnel or contract out?
7. What are the present costs for operation, personnel, field inspection, maintenance, repair, travel time, gas, report generating, etc? Is it cost-effective? How will it change with a new SCADA system?

The communication path

1. What telemetry network or communication path is presently installed?
   a. Topology: (point-to-point, point-to-multipoint, multipoint-to-multipoint)
   b. Transmission mode: (hardwire, telephone, fiber optics, radio/microwave)
2. What are the transmission modes available and if changes are made, how will that affect vendor service charges – at each site?
3. What topology and transmission mode is best suited and cost-effective for present/future application?
4. What are the distances and obstacles to span between the control center and each present/future site?

The protocol (encoding/decoding)

1. What protocol (open, closed) is used (present/future) and will it integrate well with future equipment?
2. Is there complete documentation?
3. Is multi-vendor software to communicate with a variety of manufacturer's equipment needed?
4. What protection and safeguards are needed/used to keep out hacking, tampering, sabotage and other unauthorized use? Consider all security issues.

Table B – SCADA equipment purchasing principles

1. Must be supplied by a reputable, established, and customer-focused manufacturer.
2. Must use open architecture, (Modbus) protocol to keep future integration options open.
3. Must be non-proprietary, proven technology that integrates well with existing/future equipment.
4. Must be cost-effective, easy to learn/use, reliable and not cause disruptions to everyday business operation.
5. Must be supportable, well documented and designed to use locally supplied electrical components.