Underground coal mining

In order to comply with new safety guidelines issued by the NSW government, Centennial Coal's Myuna Colliery overhauled its mine winder safety and control system in 2004. Engineered by Rockwell Automation, the upgrade involved the installation of a digital safety and control system to meet Safety Integrity Level (SIL) 2 requirements, plus a full digital upgrade of the drive power electronics.

Reliable performance of the mine winder is critical in terms of both personnel safety and efficient mining operations. The primary means of ingress into the mine is via a 600 m sloping drift, traversed by a dolly car that carries shifts of 60 to 80 workers and ancillary materials. A 52 millimetre diameter twisted steel rope connects the dolly car to a cylindrical drum located in the 'winder house' on the surface. Driven by a 750 kilowatt DC motor, the powered winding system dictates the required direction and speed of the dolly car's travel in the drift.

According to Greg Briggs, maintenance manager for Centennial Myuna, the company was compelled to comply with the new Electrical Technical Reference for the Approval of Power Winding Systems (MDG 2005) guidelines, issued by the NSW Department of Mineral Resources in February 2003. Improved maintenance of the 20-year-old mine winder system was also an objective. "Although it still operated safely, the winder was very outdated," Briggs said. "In addition, for safety compliance and reliability, we needed better techniques to monitor performance. So we had three strong reasons to upgrade the system to modern digital technology."

"Basically, the winder has to run at prescribed speeds in designated zones-depending on what's happening with the dolly car," says Briggs, adding that typical speeds range from 0.5 to 2.2 metres per second (two to eight km per hour). "The system also has to make sure that everything happening on the dolly car itself-such as hydraulic pressures for brakes-are operating correctly. There are a whole host of parameters that have to be constantly monitored."

According to Tim Keech, applications engineer with Rockwell Automation Drive Systems, a key design philosophy of the new safety and control system was to enable cross-checking of critical parameters to ensure redundancy and safe operation of the winder. "The new regulations require the primary and secondary safety circuits to operate independently of each other," he said.

Both the speed and position of the winder are double-checked by different sources and at different locations in the new system. Speed checking is achieved using encoders connected to the two high-speed counter inputs of the GuardPLC safety controller, plus drive tacho readings. An absolute encoder, linked to ControlLogix via a DeviceNet communications fieldbus, monitors the position of the winder in the drift to confirm readings by hunting tooth limit switches.

In addition to speed and position monitoring, the safety and control system routinely performs a host of other critical checks-including torque proving. "Before you tell the dolly car brakes to lift, you want to make sure the inverters are working," said Keech. "So the control system will inject torque into the motor via the drive electronics and make sure it's available when the command comes for the brakes to lift."

The digital system replaced a panel of around 80 relays, plus electromagnetic devices and analog card packs that formerly performed the control and safety functions. According to Keech, the analog electronics-such as potentiometers and metering equipment-required routine maintenance and calibration about every six months. "Just one of the benefits of a digital design is that everything is set and fixed. Nothing can change or drift. This is obviously going to save a lot of time."

In addition, an Allen-Bradley PanelView electronic operator interface (EOI), linked to ControlLogix via EtherNet/IP and located in the winder house with the electronics panels, displays all alarms and a graphical representation of where the dolly car is in the drift.