Open Cut » Overburden Removal
Field tests on the hoist drive of draglines have shown "overshoot" and oscillations of the drive gearbox torque during the lifting stage of the cycle. These dynamics, superimposed on the mean torque requirement, contribute to additional damage in the mechanical components of the dragline particularly in the hoist drive. The project investigated the implementation of feedback control on the hoist drive to reduce the torque overshoot and dynamics with a goal of increasing component life, or alternatively increasing suspended load for the same life.
The peak torque applied to the hoist drive occurs while accelerating the bucket. This torque is a sum of two effects, one accelerating the bucket and the other associated with a dynamic ringing of the hoist drive. A model of the hoist system was developed to simulate and trial various control strategies and determine their effect. The model was validated with field experimental measurements of electrical and physical parameters. The results of the simulations indicate that the peak torque during hoist can be reduced by 10 - 20% below present levels.
An economic analysis of the cost reductions through the implementation of improved hoist control indicates a potentially significant saving in maintenance costs. The reduction in peak torque should result in a tripling of gear life. The increase in the life of the gearing and other components has the potential to save dragline owners $250,000 annually through reduced maintenance. A full feedback control strategy would require a significant research and development effort in order to minimise the risk to large powerful machinery. A less risky alternative would be a system using improved precompensation of the operator input. The precompensation would reduce most torque overshoot by 20 - 40% and utilises components which are already a part of the hoist control network. Further research to develop and implement an improved hoist control system is recommended.
The economic analysis model was also used to review productivity advantages through increases in the suspended load. Hoist control was found to reduce the damage inflicted on machines through increases in suspended loads above the rated suspended load. The model showed that it was cost effective to improve the control system and increase the suspended load despite large increases in maintenance cost.
The extensive documentation of the modelling work carried out by WBM makes this report a vital key in further development of dragline control systems, and economic productivity analyses.