Investigation of Vibration Related Signals for Monitoring of Large Open‐Pit Rotary Electric Blasthole Drills

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Branscombe, Edward A.
Electric Blasthole Drills , Signals for Monitoring , Vibration
This thesis investigated the monitoring and analysis of signals related to the vibration of large rotary blasthole drills. The research focused on a machine with electric drilling actuators: such machines are used as primary production equipment in the drill‐and‐blast cycle of the surface (open‐pit) mining process. The performance of such machines is limited by the onset of severe vibrations, which can arise due to the interaction of geology, bit, drill string, machine structure, and control settings. Experimental data for the thesis were obtained during field periods at an iron ore mine in Minnesota’s Mesabi Iron Range. The data acquisition and signal analysis techniques which were utilized are presented, including smoothing of signals and calculated variables such as specific energy. Ambient vibration sources and vibration aliasing issues are investigated. Results from analyzing structural response tests indicate that, as expected, the natural frequency of the drill mast decreases with increasing bit depth – although the mounting position of accelerometers distorts this trend. The pull down force (weight on bit) is shown to have no appreciable impact on the mast’s natural frequency, nor on the mast’s damping ratio. A strong relationship between rotary speed and the dominant vibration frequency peaks at 3x and 6x rotary speed is demonstrated, and a physical explanation of the 6x vibration peak is postulated. The rotary motor current is shown to consistently exhibit frequency peaks at 3x and 6x rotation speed, indicating that this variable is a good candidate for use either as a substitute for accelerometer feedback, or as an auxiliary signal to detect down‐the‐hole vibration when it is not manifested by the mast mounted accelerometers. System identification is used to demonstrate that the dynamic relationship between vibration and rotary current, while it can be modeled locally, varies with depth and geology and hence is essentially a time‐varying process. This results in the amplitude of rotary current not being usable as a proxy for vibration amplitude. Nonetheless, it is demonstrated that the root‐mean‐square (RMS) of the low frequency current oscillations, in a nonlinear combination with the RMS of the current signal as a whole, may be able to serve as a proxy for the RMS of the vibration signal.
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