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The Centre for Environmental Research in Minerals, Metals, and Materials
The University of British Columbia
Department of Mining Engineering
6350 Stores Road, Vancouver,
V6T 1Z4, BC, Canada
Tel: (604) 822-6217 Fax: (604) 822-5599
Email: cerm3@mining.ubc.ca

May the Force be with You:
Magnetic Levitation Hoisting Systems in Underground Mines

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Collaborations

UBC-CERM3 mounts an uplifting experience - An Innovative Linear Motor for Underground Hoisting Systems

Ryan Ulansky,

Graduate Student in CERM3

and

John A. Meech,

Professor of Mining Engineering and

Director of CERM3

The Vancouver Sky Train is not the only linear motor application to exploit the principles of magnetic levitation. Ryan Ulansky, a Master's student in UBC's Department of Mining Engineering is building a prototype system to replace conventional wire-rope hoists in underground mines and integrate underground haulage with delivery of ore to surface. Using virtual reality software together with a small model testbed, Ulansky is discovering that hoisting is an uplifting experience. (To see the actual presentation click here - note: this is very high band width. It may take considerable time to load and may require more resources than are available on your computer. For a low bandwidth version without animations, click here)

Background

The conceptual design of a magnetically-levitated skip for use in an underground mine is presented. A skip is used to move rock from an underground mine to a processing facility located on surface. Conventionally, a skip is raised and lowered by cables (or wire ropes), in a fashion similar to an elevator. In a magnetically-levitated skip, the cable is replaced with a linear induction or synchronous motor to provide propulsion and levitation.

Removal of cables from a magnetically-levitated skip provides many advantages over a conventional system including the ability:

• to negotiate corners, 

• to travel vertically in the shaft as well as horizontally to the mining face, and 

• to move a very large volume of material using multiple skips in a small shaft.

A magnetically-levitated skip is a safer, more environmentally-friendly, productive alternative to conventional skipping. It has the additional economic benefit of decreasing underground haulage requirements.

Introduction

The concept presented here describes a skip propelled by a linear induction motor. A conventional skip is propelled by being attached to cables that wind around a drive wheel attached to a counterbalance. Using cables reduces flexibility and limits system productivity. A magnetically-levitated skipping system (MPS) can have multiple containers on a special track inside a tubular-shaped shaft between an underground loading facility and a surface dump point. As no cables are used, the skips are free to negotiate corners, to travel to any depth in the mine, and to go directly to the mining face with advancement of the tubes as the face moves forward through the orebody. With multiple skips in the same shaft, an MPS may be competitive with conventional underground haulage systems.

Components

The MPS system consists of skips, track, loading and dump points, and a maintenance/storage facility, each of which is described below:

            Skip

The skip comprises a cylindrical tube with a lid to contain the payload. It is made of steel that interacts with a magnetic field around the tube to provide propulsion.  The ore is loaded into the skip and the lid is closed to prevent spillage. Guidance of the skip is provided by three sets of wheels that run on three sets of rails located equilaterally around the tube inner surface. The wheels and rails are distributed at angles of 120° around the inner circumference of the skip and tube. These rails are designed so that the lid will close and open automatically as the skip passes through either the dumping or loading stations respectively.

    

            Track

The track is cylindrical with three rails located on its interior to guide the skips. The exterior of the tube is wrapped with alternating rings of electrical windings and iron bands which compose the linear motor and generate the magnetic field which lift the skips to surface and control their  location and velocity.

 

            Loading Point

The ability of the MPS system to turn corners enables it to run vertically down the shaft and then horizontally along a drift to a location near the face. By approaching the face, a separate material handling system is not required to move material long distances from the face to the shaft. The face is advancing continually, so the loading point is moveable, allowing it to follow the face. The loading system consists of a dump pocket for a scoop-tram to dump its load, a crusher to reduce the muck to top-size suitable for loading, a surge bin, and a loading system for the skip.

 

 

To enable the skip to turnaround inside a narrow drift, a Skip Handler is used. The Skip Handler receives each vehicle from the return tube, rotates it 90° to be loaded with ore, rotates it another 90° to align with the delivery tube, then returns to receive the next vehicle from the return tube. The payload to each skip is weighed to prevent overloading and spillage. The cleanliness of an MPS system will be a prerequisite previously unheard of in conventional mining.

 

 

            Dump Point

The dump station involves rotating the track to invert the skip above the dump point. To dump a skip, the lid opens automatically as the vehicle rotates allowing the payload to fall into the bin. By controlling when the lid opens, the material can be sent to different locations if required, enabling a mine to sort high grade, low grade, waste, or neighboring mine materials that are all being hoisted within the same system.

 

 

            Maintenance and Vehicle Storage

A skip can be removed from the system for maintenance and repairs without having to  down the entire system. The maintenance facility is located on surface next to the dump point. In operation, a skip destined for maintenance is dumped as shown above, but instead of returning underground, it is diverted by an attached tube to the maintenance facility. For temporary storage, the skips are queued in front of the loading station underground to eliminate the need for continuous storage and recovery of skips.

 

 

Sub-Systems

To travel from a loading point to a dumping point, a skip must negotiate the track, be propelled up the shaft, be braked on descent, and have its speed controlled. Systems to perform these tasks are as follows:

            Guidance

Guidance is achieved through the use of rails in the track. To enable loading for several locations in a mine at the same time, the skips can be directed to appropriate exits connected to the main return track. Loaded skips are then merged back into the main delivery tube to surface using switching stations much like a railway system.

 

            Propulsion

A linear induction motor located around the circumference of the tube propels the skip along the tube. Only a small portion of the track surrounding the skip is energized at any one time as each skip passes. As the magnetic field in the energized portion of the track moves up the tube, the skips are essentially "pushed" to surface by this magnetic field. The frequency by which adjacent windings are energized will control the speed of travel of the skips up and down the delivery and return tubes.

 

            Braking

Braking is required to control the speed of the skip on its decent back into the mine on the vertical return trip down the shaft. To brake the skip, the same electric and iron core coils are used, but in the reverse sense as a generator. As the skip passes each coil, it induces a current in the coil. The induced current generates a magnetic field resisting the passage of the skip and slowing it down. To control the skip's descent, the electricity generated in these coils can be captured and modified using variable resistors. The current induced in these coils is connected to the main power grid. This energy is run through an inverter converting it from DC to AC current and a variable transformer is used to stabilize and step-up the voltage. In this way this energy is recovered to the main power grid. To provide a fail-safe emergency system, mechanical friction brakes are located along the length of the track. The brake consists of spring-loaded dogs which flip open and prevent skips from falling down the tube should a power failure occur.

 

       Speed Control

Controlling of the speed of a skip is important to avoid collisions and prevent damage should a skip become jammed or go out of control. The speed of each skip while traveling upward or horizontally is limited by the rate that the magnetic wave moves along the shaft. This is controlled directly by the frequency that power is supplied to adjacent coils. For normal operations, the system operates at 60Hz or at the requirements of the local power supply. When moving down the return shaft, sensors monitor the speed and braking power is adjusted accordingly. The control system is designed to maintain a particular spacing between skips. Control of several skips moving together will be achieved using a "convoy" control strategy.

 

 

Design Parameters

Table 1 presents some preliminary design ideas of the size and productivity levels of an MLS system. Preliminary calculations on energy utilization and copper wire requirements suggest that an MLS system could compete with conventional hoisting because of lower overall capital and operating costs. Capital costs are reduced because of reduced shaft-sinking requirements – a raise-boring machine can be used to create a hole size of about 1-2 m in diameter. Reduction in underground haulage equipment is also significant. Depending on the ability to recover energy from the return journey underground, operating costs are also reduced due to lower maintenance and safety requirements and the automation of the entire system. In particular a reduction in underground haulage requirements is likely.  Computer control is integral to the overall operation of this complex system. Conservatively speaking, over  14,000 tonnes per day is projected as a possible production rate from a 25 cm (~10 inches) diameter system. This is based on 5 skips passing a particular point every second. The limit to this rate is likely to be the dumping and/or loading cycle times.

Table 1: Proposed Design Parameters.

Current Research on Magnetic Levitation

There are several groups performing R&D on magnetic-levitation technology around the world. These applications are focused on creating high-speed trains to move people. Several countries have built full-scale prototypes, with a German group appearing to be in the lead. The German company, Transrapid International, has designed a train that has exceeded 450km/h (125 m/s) on a working test track. It has already carried over 200,000 paying passengers [1].

Vancouver’s SkyTrain [2] is the closest application of this technology to the system proposed here. The trains are propelled by a linear induction motor and are guided by wheels on rails. The major difference with our concept is that the windings are located on the car, not the track, and the motor is flat instead of being rolled around a tube.

NASA is also doing considerable work with linear induction motors [3]. They are developing a launch pad to employ a linear induction motor to accelerate the next generation of the Space Shuttle to a lift-off velocity of 600 mph to assist in launching vehicles into space.

In mining, there are several papers written on the potential applications of linear induction motors for hoisting, but these are strictly conceptual. In the early 1990s, the United Stated Bureau of Mines investigated a magnetically-levitated material handling system for underground coal mines [4]. It operated in a horizontal mode within a sloped shaft. The project was shelved when the USBM closed in 1994.

Current Status of the UBC Project

Todate much time and effort has been spent working on conceptual details and preliminary design features of the overall system. In 1998, a small working model was built to demonstrate the feasibility of the technique [5 ]. The model consisted of a single shaft 90 cm in height and 2.5 cm in diameter and was able to demonstrate controlled vertical movement (both up and down) of a 100g payload at speeds equivalent to a production rate of about 5 tpd. In the summer of 2002, the first prototype of an 8 cm model was constructed. Testing and modifications continue to be made to this model.

First and Second Prototype Models

We are using virtual reality software to design different types of loading and dumping stations, as well as different shape configurations for the shaft. This approach is particularly useful to identify critical issues regarding the geometry and limitations of each design. A 10 cm diameter prototype testbed with a continuous loop in both a vertical and horizontal mode of operation is being constructed to demonstrate productive capabilities and further enhance the "proof-of-concept" of such a system.

Diagram of the Prototype Test Loop

Conclusions

The development of a magnetically levitated skip can have significant benefits to the mining industry. Some of these include:

· Safety

- reduced diesel emissions

- reduced dust as the skip contains all of the dust

- automation keeps personnel out of hazardous locations

· Environment

- zero emission technology

- braking is recaptured as electricity

· Productivity

- multiple skips in the same shaft will increase the capacity of the system

- ability to turn corners allows the skip to be loaded at the face

- system is unaffected by the depth of the mining face

· Economics

- higher capital costs of the skips are offset by

- smaller shaft size and better stability, i.e., no shaft reinforcement

- lower operating costs

- lower maintenance costs

The significant advantages of this system make it worthy of more detailed investigation. We are currently constructing a 10-cm diameter prototype test track capable of a payload of several kilograms in the new UBC Centre for Environmental Research in Minerals, Metals and Materials (CERM3) within the Mine Automation and Environmental Simulation Laboratory.

References

1.  http://www.transrapid.de/en/information/technik_txt.html

2.  Vancouver's SkyTrain. Rapid Transit Project 2000, SkyTrain Millennium Line, Rapid Transit 2002 (Information Packet).

3.  http://liftoff.msfc.nasa.gov/News/2000/News-MagLev.asp

4.  J.J. Geraghty, W.E. Wright and J.A. Lombardi, 1995, Magnetic Levitation Transport of Mining Products, United States Bureau of Mines, United States Department of the Interior.

5.  Ryan Ulansky, 1998, Magnetically Levitated Skipping, UBC-MMPE Undergraduate Thesis.