Articles and Technical Papers
New Techniques Provide Solution to Data Communication Dilemmas in Water Districts
WATER/Engineering and Management (July 1996)
The need for reliable, wireless, wire and fiber optic data communications in the water industry is an ever growing concern. In a typical city wide water district, with an extensive surrounding source field, water consumption demand estimates are made daily. The source fields, including streams, lakes and wells, are then selected on a gallon per watt basis by individually monitoring pump efficiencies. This process achieves the lowest possible electricity consumption. In addition, for example, monitoring of up-stream systems and clear wells may be required in order to immediately shut down the field, adding to the importance of reliable data communications.
The increasing utilization of Programmable Logic Controllers (PLCs), Remote Telemetry Units (RTUs) and Distributed Control Systems ( DCSs), in extensive SCADA (Supervisorial Control and Data Acquisition) installations is making the transmission of data as important as its collection, processing and distribution.
The major manufacturers of PLC processors provide as close to bullet-proof data integrity as possible. Nevertheless, upon installation in real-world situations (e.g., in water production applications) data communications must often take place under " adverse environment" conditions. Electromagnetic interference, shifting ground planes, and long transmission distances all created problems that compromise data reliability. IN data acquisition, control and SCADA applications, the individual PLCs, RTUs, PCs and other stand-alone devices achieve a very high standard of data reliability and accuracy. Unfortunately, the conditions typical for industrial applications have often resulted in data transmission "hassle problems" producing compromised data integrity.
In a typical DCS, the need to monitor or control digital discrete or analog I/O in a timely manner and at considerable distances between PCs, PLCs, RTUs, and other devices presents many challengers to instrumentation engineers and technicians in their effort to select the appropriate combination of equipment for wireless and wire, as well as fiber optic data transmission. Connectivity challenges, including the increasing expense of wire installation, are becoming more troublesome as advanced data acquisition and control requirements present technical staff with technology overload headaches. The complexity of today's digital technology has reached a point where it has become difficult to identify and diagnose a problem, let alone get operations back on-line following a shutdown.
Fortunately, and alternative is now available in which advanced Carrier Modulation (CM) techniques, including Frequency Hopping Spread Spectrum Radio and Frequency Shift Key (FSK) wire technology, can be employed for applications requiring the highest level. of data reliability and integrity, also providing "user friendly" installation and reconfiguration. The Data-Linc Group has focused its efforts on the design, development and manufacture of industrial grade plug-and-play data communications equipment providing the desired connectivity, interoperability and compatibility with industrial automation equipment. Modems and cables are pre-configured by DATA- LINC for each application, thereby assuring true plug-and-play connectivity with the PLCs, PCs, and RTUs(no modem dialed settings, programming or adapters required). Slot mount versions of the modems can also be supplied, housed in the enclosures of the PLCs and powered off the back plane, for rack mount convenience.
Wireless Spread Spectrum
A wide variety of radio modems already exist of wireless data communications. Traditionally, however a site license had required and in many industrial applications the technology remained inadequate. For example, in multi-drop installations, response times of 500 milliseconds were unacceptable for typical SCADA networks.
Spread spectrum radio technology is now gaining increasing acceptance. Implementation of the technology in the license-free 902-928 Mz and 2400-2484.5 MHz bands eliminates a need for the end user to obtain a site license.
The two most common techniques employed for industrial spread spectrum data transmission are referred to as Frequency Hopping and Direct Sequence. For the industrial data acquisition and control market, including SCADA applications, the frequency hopping alternative is particularly useful where reliability and data integrity are paramount. Full duplex uncompressed data rates at 115.2 Kbaud are now available with a range capability to 90 miles and a turnaround delay (response time) between the Master and successive Remote Units of 5-15 milliseconds.
For mulipoint communications between a Master radio and multiple remotes, the majority of installations will achieve a net data throughput approaching the 115.2 Kbaud point-to-point capability. Even where severe interference problems are encountered, are encountered, a significant percentage of multidrop control and monitoring functions will require data throughput of no more than 19.2 Kbuad. Since they are narrow band transmitters, frequency hopping can utilize more than 100 distinct frequencies within the license free band in accordance with a pre-set hopping algorithm designed to maintain synchronization of the Remote Units with the Master. Error checking technology is used to identify incorrect data packets which are then re-sent at the newt frequency hop. Such techniques decreased data transmission rates accordingly, making the 115.2 Kbuad full duplex uncompressed data rate and the rapid response time important to comfortably maintain required net throughput rates where problems such as interference, contention, multi-path selective fading and collision are encountered. In addition, the hopping technology facilitates the use of multiple pairs in close proximity by setting distinct algorithms to preclude mutual interference.
In summary, frequency hopping technology will sacrifice the highest possible data rates to assure the maintenance of communications, even in the face of the severest interference, although at a lower net throughput. Wide band transceivers, in contrast, utilizing direct sequence spread spectrum technology, will fail to maintain communications when the level if interference at the receiver exceeds the received signal power by even a small margin. Although its net throughput will always equal or exceed the frequency hopping transceiver, it is at the expense of data reliability in conditions where hostile environment problems are encountered. Wireless data transmission continues to benefit from rapid advancements in the technology. The commercial market is now enjoying the results of a decades long research program conducted by the military to ensure reliable data communications even under the most challenging battlefield conditions. Additional efforts by the private sector have added features and benefits appropriate for the industrial automation customer.
FSK Data Transmission Over Wire
Traditionally, data is transmitted between computers and/or peripheral devices such as PLCs and RTUs, either by direct multi-wire cable or through a "modem." Direct multi-wire cable is large (9-25 wires), bulky, cumbersome, and frequently difficult and expensive to install. It has a range limited to about 100 feet without line drivers. A modem is a device that converts digital signals into a form that can be transmitted much longer distances over single of double wire pairs, or telephone lines.
The superiority of FSK technology depends on its ability to convert ASCII data streams, or any other digitally encoded information, into frequency equivalents, thereby reproducing space and mark ( the 1's and 0's of the digital format) as discrete frequencies prior to transmission from point-to-point. FSK is suited to conditions where the transmission of digitally encoded data streams compromised data integrity as a result of the sensitivity of digital data to hostile environments (EMI, noise, etc.).
FSK is a communications technology largely immune to data transmission problems that can also provide exceptional versatility in the selection of communication paths. Data travels between electronic devices, such as computers of PLCs, as a flow of logic signals that shift back and forth between two voltage levels. If electrical noise, changing ground voltage or signal attenuation alters the difference between the two voltage excessively, data errors can result. FSK technology converts the data stream from changing voltage levels to changing frequencies.
The technology provides a family of data communications devices available for a broad range of industrial applications and providing "insurance" against data transmission problems in the water industry:
Typical Water District Installation
The City of Enid, Oklahoma Water Production Department has 150 water wells, two treatment plants, three raw water booster plants and 30 million gallons of storage capacity for treated water. The City maintains 200 miles of raw water gathering lines located in three counties. The average daily use of treated water is 10 million gallons. City water wells can produce 25 million gallons of untreated water per day, although peak usage in the summer rarely exceeds 17 million gallons. The treatment plants can treat up to 40 million gallons per day.
In 1991, electric usage exceeded $500,000. Ways to reduce power cost were sought. Each well and plant was surveyed to find out which wells, plants, pumps, etc. were most efficient. A well rehabilitation program was then instituted and a SCADA System with VFDs (Variable Frequency Drives) was selected consistent with particular attention to managing a sensitive aquifer.
The first leg of the project was to automate the newest well fields to be operated from the Enid plant. Cleo Springs (35 miles from Enid plat) and Ringwood (30 miles from Enid plant) each consist of approximately 30 wells and a pumping station. They are setup so the operator can start/stop each well from each plant over an in-place cable system.
Cleo springs, with the longest distance (7 miles) between plant and well, was targeted first. Following the selection of PLC communications from plant to well on the existing cable system had to be worked out. The particular Modicon 984 Series PLC chosen did not support half duplex communications with the programming software, so a communications medium had to be found to support two wire 9600 baud full duplex data communication at a distance of 7 miles on 19 gauge solid wire. This was a rather daunting task. Data-Linc MDL500 FSK wire modems were the only equipment located capable of meeting this challenge.
After installation of a Variable Frequency Drive, PLC and a computer with automation and control software at the Cleo Springs and Enid plant, both plants could be operated from Enid via phone line.
At Plant 2 in Enid, approximately four miles from the main plant, a PLC and VFD was installed to maintain minimal tower level at 60 psi. A continual decrease in the tower level will increase effluent pressure. If VFD rpm reaches 100 percent for a set time period, the backup pump will engage. When rpm drops below the set point for a specific time period, the backup pump will shut down, and when the tower level increases, pressure will return to 60 psi.
The savings to date over a four year period as a result of installation of the new SCADA system has amounted to well over $400,000. Work is now underway on an upgrade to a 32 bit operating system as well as improvements to the SCADA software. All remote plants will be linked together with a Data-Linc SRM6000 902-928 MHz spread spectrum radio system to avoid the long distance calling currently required.
This article has presented a brief overview of the specific conditions encountered in a typical water district data acquisition and control application. Many other factors can also come into play that further accentuate the need for top performance, industrial grade plug-and -play modems. Selection of the appropriate wireless and wire data communications equipment is particularly essential toward ensuring a "hassle free" installation as well as providing insurance against potential data transmission interruptions or compromised data integrity. The same care taken in the selection of computer hardware, software and peripheral equipment, such as PLCs or RTUs, should also be applied to the recognition that data communication between devices is equally as critical toward controlling installation costs and ensuring efficient, reliable on-going operations.
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