Copper's Not the Only Way to Network
You have to install a new manufacturing data network. Before you call for a truck-load of twisted-pair copper wire, better check out all the applications. There may be areas where copper won't work.There are other choices. Fiber-optic technology is not new, but is becoming easier to use. Radio frequency is moving from warehouse data collection to sensors and networking.
You have to install a new manufacturing data network. Before you call for a truck-load of twisted-pair copper wire, better check out all the applications. There may be areas where copper won't work.
There are other choices. Fiber-optic technology is not new, but is becoming easier to use. Radio frequency is moving from warehouse data collection to sensors and networking. Infrared can do more than just change channels on a TV.
Not your father's fiber
The drawbacks of using fiber-optics are well known. Splicing requires special tools and expertise. Glass fiber is not very flexible and was easily broken. The price of material as well as labor is expensive.
Recent technology trends are minimizing these drawbacks according to Jeff Meyers, product marketing manager at Omron Electronics Inc. (Schaumburg, Ill.). Prices have been falling for all the various system components. Crimp and cleave termination methods are easier and less expensive than epoxy polish methods.
Mr. Meyers notes, "Fiber-optic media offers many benefits. The cable media is both corrosion resistant and a nonconductor of electricity. Further, light is immune to all types of electrical noise and provides electrical isolation between devices."
Light carries digital data
Relcom Inc. (Forest Grove, Ore.) describes the components of a system: a transmitter; a connecting medium; and a receiver.
The transmitter is a light emitter—an LED or a laser—and the electronics that modulate the light. The receiver is a light detecting diode with electronics that return the light signal to electrical signals. In a typical industrial setting, the transmitter can be embedded in a sensor with the receiver in a controller input card.
Optical Cable Corp. (Roanoke, Va.) discusses the technical issues of fiber optic cables. There are two categories of fibers—multimode and single-mode. Other factors used to define fibers are size, optical performance, coatings, and strength.
Fiber cables consist of these components (see diagram):
Glass core, carries most of the light;
Cladding, confines the light to the core;
Substrate layer of glass (sometimes), adds to diameter and strength;
Primary buffer coating, first layer of mechanical protection;
Secondary buffer coating, outer layer of mechanical protection.
Fibers are identified by the size of the core and the overall size of the glass in micrometers. For instance, a popular size of cable in local area networks is 62.5/125. This means that the core is 62.5
Copper talks to fiber-optics
What if there is an existing twisted pair copper network that needs a fiber-optic section? AMP Inc. (Harrisburg, Pa.) has the answer in Raylan twisted pair to fiber media converters. Available in single- or 12-port configurations, Raylan converters allow organizations to protect current investment in twisted pair cabling while extending fiber optic wiring where needed.
Phoenix Contact (Harrisburg, Pa.) has a series of interface modules that convert serial data to optical fiber and back. PSM-EG-..FO family of converters are available for RS-422, -232, -485, Profibus, and Interbus. There is an RS-232 converter with a D-sub connector that connects to the serial port of a personal computer.
In cases where power as well as data signals must be provided for remote sensors, NT International Inc. (Minneapolis, Minn.) has Fiber Optic Interface system.
The Fiber Optic Interface System consists of a Transmitter Module and a Power Module connected with fiber-optic cable. The Power Module supplies light energy through the one optical fiber to the Transmitter Module which converts it to electrical power. A second optical fiber transmits the signal back to the Power Module where it is converted to 4-20 mA.
The sensor, which can be commercially available pressure, flow, level, or temperature, connects to the Module with standard wiring.
Sometimes it is just not possible or economical to use a physical cable to connect a device to a data network. Remote Terminal Units (RTUs) that relay information in certain applications like waste-water treatment plants may be miles from the controller. There may be physical barriers to running a cable. The path from one device to the network may cross an extremely hot area.
Grayhill (LaGrange, Ill.) is among the companies providing radio technology. Grayhill's Bob Hochreiter, vice president new product development, says that wireless local area networks (WLANs) increase flexibility, mobility, and ability to reconfigure.
He notes that there are two parts to a WLAN: the access point transceiver and the remote client transceiver. Transceivers are radios that can both send and receive. The access point is the stationary transceiver that attaches to the plant's main LAN. Remote client transceivers are connected to the remote sensors or controllers and communicate to the access point.
Gateways and bridges
Transceivers in a WLAN can perform an additional valuable service. They can not only be a communications channel like an invisible wire, but they can also serve as a "gateway" or "bridge." A gateway allows a network with one protocol to talk to a network with a different protocol, for instance, a device network in the automation cell talking to an Ethernet information network. A "bridge" links networks with similar protocols.
As a gateway, the remote client transceiver may be on a device network. It accepts the data, translates them to radio signals, and sends them to the access point transceiver. The access point transceiver attached to the factory Ethernet information system receives and forwards the data to that system.
Don't be afraid to use this technology because of interference or losing the data signal. Grayhill's wireless system utilizes technology developed by the United States military during World War II as a way to send radio signals that resisted jamming and were difficult for unauthorized radios receive and decode.
Spread Spectrum technology is characterized by use of wide frequency spectra. A true spread spectrum signal meets these two criteria:
The transmitted signal bandwidth is much wider than the information bandwidth (see figure) second page of article. Instead of a narrow frequency like FM radio, the actual data is modulated across the wide waveband and sounds like noise to unauthorized receivers.
Some pattern or code other than the data transmitted determines the actual transmitted bandwidth. These codes allow the authorized receiver to filter the needed information from the signal.
Cellular data transfer
How about cellular technology to send data over great distances? Wireless Systems Inc. (WSI, Lakewood, Co.) uses a new technology called CDPD (Cellular Digital Packet Data) to send data from an RTU. The WSI RTU monitors data from up to 64 analog and digital I/O points.
The RTU has an IP address so that it is possible to expose the data to the Internet. With password protection built in the application server, privacy is achievable. The appropriate user can use a web browser over the cellular phone line to access data.
According to Jeff Cook, technology development vice president, the unit is capable of sending messages to as many as 64 e-mail clients when an exception report is generated as well as to an alphanumeric pager.
Devices for wireless networks are appearing. "Traceable" from Control Co. (Friendswood, Tex.) is a radio-signal remote thermometer that is traceable to NIST for accuracy. The remote sensor sends data to the digital display from as far as 100 feet. One display is capable of receiving data every 30 seconds from as many as three sensors. An audible alarm sounds if the reading from a sensor falls outside of the minimum or maximum set points, and the display unit retains minimum and maximum values.
Ellison Sensors International Ltd. (Wrexham, Wales, U.K.) has a pressure sensor with a built-in radio transmitter that will send data signals at distances of up to 500 metersline of sight.
PR9500 allows monitoring of pressures at safe distances from hazardous or inaccessible environments. Output interface cards produce a calibrated analog signal which can be 4-20 mA, 0-5 V, or 0-10 V. There is an optional RS-232 port.
Like clicking the TV remote
Infrared data transmission is a new wireless technology. A TV remote has a simpler version of infrared communications. Infrared industrial communications is more complex, but products are beginning to appear exploiting this technology.
The Infrared Data Association (IrDA, www.irda.org ) states its purpose "to create an interoperable, low cost, low power, half-duplex serial data interconnection standard that supports a walk-up, point-to-point user model that is adaptable to a wide range of appliances and devices".
IrDA, aimed primarily at the computer industry, has developed standards for infrared data ports to support high-speed data communications.
Phoenix Contact (Harrisburg, Pa.) has released an infrared communications product. Mike Nager, product marketing manager, says that PSM-optical-data-link has proved its worth in such applications as robotic cells, conveyors, and cranes.
The network is configured just as if it were wire. PSM will communicate up to 200 meters and operates either indoors or outside. Eletromagnetic interferences do not affect infrared transmission. Frequency shift keying and a daylight filter improve system and transmission reliability.
Common RS-485 bus interfaces are supported including Interbus, Profibus, and Modbus. RS-232 and -422 are also supported. PSM transmits data at two Mb/sec.
The converters need direct line-of-sight with each other to communicate, although the divergence in the infrared signal means that there is a large tolerance in alignment. The protocols include error checking so that missing data in the string triggers a retransmit command. A low signal alarm informs operators when the head is dirty enough to inhibit transmission.
For the many reasons that make copper a difficult medium for running data networks in factories and process plants, there are workable alternatives. Check the applications, environment, and physical obstacles when installing a data network. Pick from the array of technologies to get the data moving.
Flexible fibers put Matsushita on line
Constructing CRTs (Cathode Ray Tubes) is the most complex part of manufacturing a television. Controlling such diverse processes as screening (arranging phosphor on the panel in precise arrangements to deliver the required colors), masking (coating the inside of the tube with resistive materials), the bulb process (putting the panel and mask together and running them through a frit oven to join them), and others requires reliable, high-speed manufacturing practices.
Engineers at American Matsushita Electronics Company (AMEC, Troy, O.) were given the task of adding a new production line—very quickly. All design, construction, and implementation had to be completed within a year.
"This was an extremely fast-paced project," says Equipment Engineering Team manager Raul Villarreal. "Specifications were very important. Since we had been thinking about alternatives to our existing networking, we knew right away what we wanted to do. We just didn't know exactly how we were going to implement the networks because we didn't know the layout of the equipment."
System design started with the Omron (Schaumburg, Ill.) CVM1 programmable controller. "We specified these PLCs primarily because of their networking capabilities," remarks Mr. Villarreal. "Our plan called for using the PLCs to gather a variety of production control data. The memory capabilities, transparent networking, and CPU speed were all attractive to us."
Several levels of networks
Several dozen CVM1 PLCs are networked on several levels. Some are masters linked with Omron's Sysmac Net fiber-optic token ring network to other PLCs and PCs. Data collection and HMI (human-machine interface) is handled by a number of PCs running IBM's PlantWorks connected with IBM's token ring network. An IBM AS400 sits atop the network connected to the factory system with another IBM token ring network.
"Production count data and machine status data are kept in individual PLCs," notes Mr. Villarreal. "PCs on the token ring network grab needed data on command and send it to a database. The database node is a port to the MIS AS400 which looks at it as just another read-only node in the system."
AMEC's previous lines, built in 1989, employed glass fiber cables with the Sysnet fiber-optic network. Improvements in fiber technology since has given AMEC engineers the options of using the new flexible plastic fibers. These new fiber cables are easier to connect and install. Changes in machine layout can be accommodated more readily, and the startup was facilitated with this ease of installation.
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