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Drift safety goes digital

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For most workers at Myuna Colliery, a subsidiary of Centennial Coal located in the Newcastle Coalfield of NSW, the local train station is the top end of the dolly car track. Driven by a powered winding system, the dolly car transports men plus ancillary materials into the depths of the underground coal mine by means of a ‘drift’--or sloping tunnel. The winder then hauls rail-cars full of workers back to the surface once a shift is completed, some eight hours later.

The 600m drift, which has a gradient of one-in-four, is the primary means of ingress into the mine. The dolly car makes the trip many times a day, carrying entire shifts of 60 to 80 workers, or pods of equipment.

A 52mm diameter twisted steel rope connects the dolly car to a cylindrical drum located in the ‘winder house’ on the surface. Driven by a 750kW dc motor, the drum winds in or meters out the rope.

Reliable performance of the mine winder is critical in terms of both personnel safety and efficient mining operations; three shifts a day, 365 days per year. According to Greg Briggs, maintenance manager for Centennial Myuna, consideration of safety and compliance issues recently led to a full overhaul of the Myuna mine winder safety and control system. Not only was the existing system over 20 years old, making maintenance a challenge, but 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.

“Although it still operated safely, the winder was very outdated,” said Briggs. “In addition to safety compliance and reliability, we also needed better techniques to monitor performance. So we had three strong reasons to upgrade the system to modern digital technology.”

Digital revolution

Rockwell Automation Australia , which had designed and manufactured the existing system (as Reliance Automation), was selected to engineer the winder upgrade.

The primary focus was to replace existing analogue safety monitoring equipment with a digital safety and control system to meet Safety Integrity Level (SIL) 2 requirements. The project also incorporated a full digital upgrade of the power electronics driving the dc motor.

Undertaken during Christmas 2003, stage one involved the replacement of four analogue-regulated, parallel-configured silicon controlled rectifiers (SCR) with modern digital technology. “The new power converters combined forward and reverse functionality in each power box,” said Michael Graves, project engineer with Rockwell Automation Drive Systems. “So we effectively replaced four SCR power boxes with two boxes containing dual-function S6Rs.”

A critical path was the redesign and fabrication of the existing control cabinets to house the new power structure platform and digital regulators. Not only were the size tolerances for the new framework down to one millimetre, but the new power boxes each weighed 120kg. “We had a very tight timeframe, so everything had to be well thought out in advance,” said Graves. “All tasks were listed in schedules, down to each specific wire change.”

In conjunction with the power electronics upgrade, an Allen-Bradley ControlLogix programmable automation controller (PAC) was introduced as the first component of the new winder safety and control system. The new power structure was integrated with ControlLogix by means of fibre optic communications to new digital power module interfaces (PMI). This replaced the existing relay control system, and enables ControlLogix to send instructions directly to the winder drive system.

Check once, check twice

The second stage of the Myuna winder upgrade project took place during Easter 2004 and involved the full installation of the new safety and control system.

This expanded upon the role of ControlLogix as the main controller, plus introduced an additional dedicated safety control circuit, founded on an Allen-Bradley GuardPLC 1200 safety controller that monitors most of the critical emergency functions and fulfils the SIL 2 requirements.

“Basically, the winder has to run at prescribed speeds in designated zones, depending on what’s happening with the dolly car,” said Briggs, adding that typical speeds range from 0.5 to 2.2m/sec (2 to 8km/hr).

“The system also has to make sure that everything happening on the dolly car itself, such as hydraulic pressures for brakes and so forth, 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.

One of the most fundamental requirements is that both the speed and position of the winder be checked by different sources and at different locations.

For speed checking, two encoders have been installed, one at the motor and one at the drum (gear loss encoder). These are connected to the two high-speed counter inputs of the GuardPLC safety controller, where comparator functions monitor the integrity of the devices and the system.

The speed is also independently checked by the ControlLogix system using drive tacho readings.

For position checking, hunting tooth limit switches are used to monitor the position of the drum with reference to each of the required speed zones. The existing hunting tooth limit switches were retained, and integrated with the GuardPLC system. An absolute encoder, linked to ControlLogix via the DeviceNet communications fieldbus, independently monitors the position of the winder, retaining its reading at all times, even under loss of power.

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 Graves.

“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 analogue card packs that formerly performed the control and safety functions.

“The analogue electronics--such as potentiometers and metering equipment--required routine maintenance and calibration about every six months,” said Keech.

“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.”

Down in the dolly car

Operation of the Myuna winder is the responsibility of the ‘winder driver’, who sends commands to the winder from his location down in the dolly car.

This critical link is realised by wireless communications, which are also integrated with the Rockwell Automation ControlLogix system.

“It was important that the drivers felt little impact of the upgrade,” said Briggs. “Other than a smoother ride! So we asked for the dolly car to be driven using the same controls, the same commands, levers, keys, buttons and so forth.”

Keech says this added challenge to the system design. “It meant we had to understand how everything worked in absolute detail. There’s a whole list of procedures for things such as testing end-of-drift and brake functionality. This all had to be mimicked in the programming of the software.”

The feedback from the winder drivers has been very positive, acknowledges Briggs. “We’ve given them diagnostics capability, so they can check how the winder is operating from the dolly car.

“In the past, the winder used to trip out and they wouldn’t know why. Now they can reset simple faults from the dolly car itself,” he said.

“Since these types of faults could happen as many as three times a day, this saves a lot of time and angst. We’re probably saving 10 to 15 minutes each time,” Briggs said.

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