The automated control system for transporting coal from deep underground mining operations requires a fast, reliable communications bus.
The vertical shaft at Jim Walter Resources (JWR) Brookwood, Alabama Blue Creek Mine
No. 7 is 533 meters (1,750 feet) deep. At the top is a 45 meter (150 foot) tower called a "headframe." Within this frame, a hoist transports coal in two containers called "trips." As one trip is being dumped at the top, the other is loading at the bottom of the shaft. Both trips are connected to a looped steel cable. They move up and down in a back-and-forth rocking motion, or "seesaw" fashion so that the trips continuously exchange places.
The seesaw motion is accomplished by a large rotating drum that constantly reverses direction. It is driven by two large load-sharing motors controlled by electronic drives interfaced to a PLC. The drums are braked at critical points in each rotation cycle to position the trips precisely at their respective docking stations.
Each trip hauls close to 25,000 kg (28 tons) of raw material and travels at speeds up to 12 m (40 feet) per second. That creates a tremendous amount of inertia that resists precise positioning; and thereby demands carefully controlled braking and communication of the exact location of each trip at all times. In this dynamic application not a lot of time is allowed for a control system to do calculations and to send messages back and forth. In fact, conventional bus systems operating at 56 kilobits/sec, cannot possibly handle such a requirement.
For the successful and safe operation of the system, the trips must not stop short or overtravel the docking stations. This is exactly why a fast, reliable bus for the control system is such a critical issue.
The system at JWR uses Siemens dc drives and the Simatic TI 555 programmable logic controller, which both are designed for PROFIBUS connectivity. PROFIBUS is very fast, able to transfer data at 1.5 megabits/sec for a distance of up to 200 meters without using repeaters. In this application, it is important for the bus cycle time to be shorter than the program cycle time of the PLC. PROFIBUS has a bus cycle time of only 6 miliseconds - well within the required range of the Simatic TI 555.
A common benefit of fieldbus systems is their simplified cabling. PROFIBUS DP systems come in either twisted-pair or fiber-optic configurations. Given the short distances between components, we opted for the twisted pair. Such simplified cabling saves time and money.
For control systems requiring critical communications transfer and response time such as the coal mining transportation hoist, the PROFIBUS system is ideal. Reliability, during almost two years of operation, has been impeccable.