Experimental Procedures for Operational Modal Analysis of a Power Pack on a Drill Rig

All structures have modal properties such as natural frequencies and damping. In engineering it is often of interest to estimate these modal properties for certain structures, to be used when modelling for example fatigue. This is done by computing them from nite element models, by using experimental measurements or both. In the case of doing both, a nite element model is usually established rst and adjusted to t measurements from experiments. Atlas Copco Rock Drills AB is the company where this thesis has been performed and the subject is experimental procedures related to estimating modal properties of the so called power pack, which essentially is a modularised engine and hydraulic power source of an Atlas Copco drill rig. Their current method for estimating these properties is a classical procedure which makes use of an impact hammer that an operator strikes the power pack with to induce excitation. Due to concealment of behind other parts the power pack when mounted inside the drill rig, the number of places where the operator is able to strike the power pack in is limited. Another problem with the current procedure is that it can be di cult to strike the power pack with a strong enough impulse to generate reliable results. In this thesis a new experimental procedure for Atlas Copco to use is suggested. It is based on operational modal analysis (OMA), which uses the machinery's excitation from its operational conditions to compute modal estimates. A comparison between di erent experimental procedures have been done and the suggested procedure is the following: excitation by engine sweep; modal identi cation by the PolyMAX method and mode shape scaling by the harmonic scaling method. An experiment was performed to compare two OMA procedures. The suggested procedure is the one that generated the better results of the two.