Communication networks for nodes on the run

March 1, 2000
Need to communicate with transfer cars or other equipment that moves from one end of a facility to another without laying cabling all over the place? Here’s a solution

Most communication networks transmit data between equipment that does not change its physical location. What do you do, though, if you have to communicate with an automatic guided vehicle or transfer car that delivers parts from one end of a factory to another, or a crane hoist that moves parts from a storage facility to a shipping dock many feet away?

The usual solutions require the installation of additional cable conductors, festoon cable-management systems, or dedicated conductor rails. Other solutions are based on radio or infrared control systems. All of these solutions, however, may not offer the necessary reliability and flexibility, and may add cost to the total solution.

A third alternative for communicating with moving equipment is to send signals over the same rails that power the moving equipment. Engineers at the continuous annealing line at LTV Steel Co., Cleveland, found that such a power-rail based communication system solved many of their equipment communication problems.

The previous communication system used radio frequency control to direct and monitor transfer cars that picked up steel coils at the delivery end of the annealing line, Figure 1, and delivered them to a storage saddle 60 ft away. The RF system would also direct an overhead crane to lift the 50,000-lb coils and transport them to the shipping department.

Radio frequency control, however, was not always reliable. “The Cleveland area has a crowded 400-MHz radio frequency band. Congested voice and control-signal traffic caused so much radio frequency interference that our system often sent false signals that would shut down the end-of-the-line coiling function,” said a senior project engineer at LTV.

Typically, a signal would be transmitted two or three times until it was either received or the system timed out and sent an alarm. If the fault was not corrected within 10 minutes, the entire annealing line would stop along with subsequent operations such as banding and labeling.

In their search for a better control solution, LTV engineers looked at carrierbased systems and cable reels, but selected the SmartRail power-rail based communication system from Universal Standard Safety Trolley Corp., Pittsburgh.

This solution combines electrical power and communication signals over existing ac or dc power supply lines, thus putting the signals on the same power bar that the customer’s equipment uses, Figure 2. The signals conform to the CEBus protocol EIA/IS-60, which uses spreadspectrum, power-line-carrier technology. This technology enables this digital, peerto- peer control system, to superimpose data and control information on a 60-Hz, 480-V power circuit, transmit the data to the communicating devices, then separate the communication signal from the power component, Figure 3.

With full-duplex signal transmission, data can travel among devices that are located up to 1,000 ft apart.

Using power rails to send data between the control PC and the shuttle car’s PC eliminated the false signals. Operating in the 9,600 Baud range (the previous radio control system operated at 2,400 baud), engineers now receive valid control messages in one or two seconds. The message fault timer is set at five seconds, (1% of the original duration).

Network components

The SmartRail system consists of two printed circuit boards. The controller board has on-board microprocessors and communication chips. The second board, known as the power-line coupler, safely connects the high-frequency data signal to the ac power lines of a three or four continuous-conductor bar, Figure 3. It puts the communication signal onto the power rail and filters it from the power signal, securing data communications.

A node can consist of the controller, power line coupler, one or more I/O modules, a pendant, and the hoist, transfer car, or other piece of moving equipment. The power-rail system can communicate with several nodes, each having a unique address. The nodes may be grouped logically for multiple, independently operating networks in a given plant.

All nodes continually broadcast their status onto the rail. Node status messages are received by other nodes on the network, which then take appropriate action.

Each printed-circuit control board can connect to a maximum of four digital I/O modules for a total of 64 I/O points in any combination of inputs and outputs.

The system is also compatible with RS- 232 and RS-422 protocols used with programmable controllers and distributed control systems and several common manufacturing networks such as Modbus and DataHighway.

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