1 April 2002
Factory Floor: Wireless upgrade boosts steelmaker’s efficiency
By Mike Denholm and Eric P. Marske
Update time to transport vehicles and overhead cranes improved 900%.
USS-Posco, a Pittsburg, Calif.–based steel finishing plant significantly increased production efficiency by updating its wireless control system.
The plant ships more than 1.5 million tons of steel each year to 13 western states, Mexico, Canada, and Pacific Rim countries. Its roughly 1,000 employees produce 4,400 tons of finished steel products daily to 250 trucks and 12 railcars through an elaborate system of conveyors, automatic coil transport vehicles (CTVs), and overhead cranes.
Programmable logic controllers (PLCs) linked through a wireless network monitor control the steel plant’s entire transport operation. Adding new wireless modems to the radio control system resulted in a 900% improvement in update time for the 10 CTVs and two overhead cranes, from 18 to 2 seconds. Other benefits included improved system reliability, with no radio failures in one year of operation since the upgrade.
All key steel finishing processes run at continuous high speeds without interruption. The process starts with 30-ton coils of hot band steel welded together. As the steel processes at speeds up to 7,000 feet per minute, computers adjust for individual order requirements. What starts as a 4,000-foot-long coil of thick, hot band steel comes out as a 40,000-foot coil of much thinner steel.
The PLC-based traffic control system monitors the operation of the CTVs and gantry cranes. It controls movement of two gantry cranes and 10 CTVs from a central control room. All equipment in the traffic control system uses a common PLC model. The PLCs communicate with one another through a wireless control network using Electronic Systems Technologies’ ESTeem Model 192 series of wireless modems.
Users monitor all CTV locations from the control room to avoid a collision. Updates to the wireless system provided new features such as a systemwide slowdown command and a global system emergency stop. The original wireless control system was slow and unreliable. It had a maximum rate of 4,800 bits per second and used a half-duplex radio link that required two UHF frequencies. The average time to poll all 10 CTVs and both gantry cranes was 18 seconds.
The UHF radio modem can operate on a single existing licensed frequency. The new modem can operate at 19.2 kilobits per second in the same bandwidth as the original radio system that reduced the overall polling speed of the network to 2 seconds.
The control room uses Rockwell Software’s RSView32 to provide the human-machine interface (HMI) for system operators. The HMI software provides information on order scheduling, positioning
information, system status, and manual control. The control room has redundant PLC 5/20 processors for continuous system operation. Each PLC is connected to a separate radio system for complete system redundancy.
The modem has to communicate to CTVs that move between two buildings. A single antenna location could not provide reliable communication to both. To overcome this problem, the company installed and connected two antennas to the radio with a radio frequency splitter, which provides equal amounts of output power for each antenna through different coax cable lengths. A directional antenna provides communication down the narrow building. An omnidirectional antenna installed in the west-end loader area enables communications through the remainder of the plant.
The gantry crane and CTVs have very similar equipment layouts. Each PLC 5/20 is connected directly to the modem’s serial port. Both systems use mobile UHF antennas tuned for the operating frequency. The CTVs provide a very challenging communication platform, with 11,000 pounds of vehicle and a 30-ton capacity load moving throughout the plant at 500 feet per minute. There is very high vibration as the vehicle moves along the 2 miles of indoor track.IC
About the Authors
Mike Denholm is a systems engineer at Rockwell Automation–USS Posco. Eric P. Marske is a customer service engineer at
Electronic Systems Technology. He can be reached at firstname.lastname@example.org.
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