Modal Analysis - Determine The Dynamic Characteristics Of The Structure

Dynamic properties of structures are very important. For some products, such as large rotating machines, the life of the bearings and the whole machine are dependent on a smooth running system. In cases such as a paper machine, product uniformity is dependent upon a smooth running machine. In other cases such as automobiles and handheld power tools, such as chainsaws, the user’s perception of quality is highly dependent upon how smooth the item operates. Finally, in some cases, catastrophic failure can occur if a resonant frequency coincides with a major excitation frequency.

In any critical structure the dynamic properties are calculated with finite element analysis. The product design is iterated until it is acceptable. The design then needs to be validated with a modal test. This test will confirm that the analytical finite element analysis was appropriate, or give specific information as to how the analyst can improve the model.

Modal testing is performed by inputting a measured excitation force into the structure, and measuring its vibration. The excitation source can be an instrumented hammer, a shaker or a number of shakers. The excitation type for the shaker can be random, burst random, or stepped sine signals. The random signal type is optimum for speed of testing, the burst random gives a better coherence, while the stepped sine input is used to maximize the energy input. The data collected is then fitted with a modal model. This will give the modal properties of the structure, including the mode shape and the damping.