Profibus pulls draglines into the twenty-first century
In the coal mining industry, the dragline is the workhorse of the open-cut mine's operation, moving massive amounts of overburden 24 hours a day throughout the year. Draglines scoop up overburden and typically swing it around a 120° arc before dumping the load and returning to the original position for the next load. The dragline's swing cycle is on average 60 seconds and during this cycle its power profile can swing from 30 MW peak while powering to 9 MW while regenerating.
Traditional management of this process requires large and complex DC relay logic, and at most sites the electrical set-up has remained unchanged over decades. "This makes it difficult for dragline operations to truly benefit from a variety of modern communication and automated systems that are becoming available," says Andrew Guilbert, Manager Electrical Engineer, Inete Pty Ltd. However, since the mid nineties, many of the draglines in Australia have been undergoing major overhauls and electrical upgrades including complete machine rewiring. This presented operators and maintenance personnel with the opportunity to push for the implementation of modern-day automation, protection and control systems and to draw on advances in PLC control and fully enunciated graphical user interfaces for the machine control systems.
This 'electrical evolution' resulted in unprecedented amounts of real-time information flowing through the many controlling subsystems and becoming available to operators during dragline operation. An example of this is the operator's controlling master-switch reference signals. Previously, this was an analog input to the PLC where it was converted, scaled, shaped, filtered, clamped and sent as an analog output to the respective drive for further processing. A further advance in this process has been to communicate the processed master switch reference from the main controlling PLC to the digital drives via a Profibus DP network.
There was a pressing need for a high-speed, robust communications mechanism to communicate process-critical information between core controlling devices (the overall controlling PLCs and interfacing motion controller drive systems). Recognising this, a coal mining company in central Queensland, BMACO, undertook a trial in 2002 to implement a Profibus network on a machine to assess the viability of this communication protocol in the harsh dragline working environment. "Since this trial was the first known such installation in the world, we built in full hard-wired redundancy for the Profibus network," explains Guilbert.
This network was implemented using a GE Fanuc IC693CPU363 PLC CPU with five Siemens DC Simoreg drives and a Siemens AC Masterdrive. The network was initially commissioned to communicate at 500 Kbps and, once proven, was set to run at the maximum Profibus bit rate of 12 Mbps. "The trial was a huge success and Profibus has now been adopted as a standard for communications between the controlling PLCs and the motion drives," says Guilbert. "The redundancy has been stripped out of the machine, effectively making the Profibus network the cornerstone of the draglines operation."
The upgrade has freed up valuable real estate in the electrical switch rooms. It also provides simplicity in modifying control and enhancing automation and enunciation of the dragline without any additional hardware controls to be installed. The robustness and speed of the Profibus network on this dragline has resulted in it being implemented on seven more draglines. "If a dragline is undergoing a major electrical upgrade, this would be the most appropriate time to consider implementing Profibus, especially where large amounts of data need to be moved around reliably and fast," concludes Guilbert.
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