Modal analysis is the study of the dynamic response and properties of structures when they experience external excitations such as noise and vibration. It is a very useful tool for determining how a system will respond to external excitations and is commonly applied in the design of buildings and vehicle frames.
Automotive manufacturers perform modal analysis on a vehicle’s structure by applying an external excitation to the frame and taking vibration readings at selected locations. Through specialized analysis techniques, they can determine how the body of the car will respond to vibrations at different frequencies, and can optimize the structural design to prevent them. Computer simulations are also used in many cases to simplify the analysis process.
Modal analysis has also been a useful tool in the design of buildings and bridges. Many structures have been known to collapse in the past due to excitation by forces such as earthquakes, tremors and winds. Engineers can use modal analysis to design structures so that they do not resonate when exposed to external forces with known frequencies.
External excitations can be applied using shakers or impact hammers for experimental testing. A shaker will vibrate at a desired frequency, or it can sweep through a frequency range, and it can also vibrate at random frequencies. Vibration measurements can be taken along the structure using accelerometers to determine the structure’s dynamic response. Several types of shakers are available, including: electromagnetic, rotating mass and electro-hydraulic shakers. Their size and shape can vary depending on the structure being tested.
Impact hammers are struck on a structure to apply an impulse force; ideally, if the impulse time is infinitely small, it causes a constant amplitude vibration over the frequency domain and all modes of vibration of the structure will be excited with an equal magnitude. The frequency range of the impact hammer can determined by the length of the impulse, and a shorter impulse will result in a larger range. A load cell on the hammer can measure the impact force and time, which can be used determine the useful frequency range. Impact hammers are suitable for smaller, lightweight structures where the signal-to-noise ratio is better for taking measurements.
The measured data can be analyzed using transfer functions that rely on Fourier analysis. The transfer function will reveal any resonances in the structure, including their magnitude and frequency. Further analysis can can also be used to determine the coherence of the data. Coherence validates the input and output data; this is useful for determining the accuracy of the results over the frequency range being tested.
With the advancements in computer technologies, many engineers and consultants are now resorting to using modeling and simulation to accomplish modal analysis. This method can be more cost effective than experimental methods depending on the size, shape ,design and complexity of a particular structure.
Modal analysis is one of many tools or methods available for designing and analyzing structures. It has proved to be useful in the the automotive and architectural design and construction industries. As with any method, it has its limitations, but it can be extremely useful it it is understood and applied correctly.