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A shaker is a device that excites the object or structure according to its amplified input signal. Several input signals are available for modal testing, but the sine sweep and random frequency vibration profiles are by far the most commonly used signals.
Small objects or structures can be attached directly to the [[Simulation table|shaker table]]. With some types of shakers, an armature is often attached to the body to be tested by way of piano wire (pulling force) or stinger (Pushing force). When the signal is transmitted through the piano wire or the stinger, the object responds the same way as impact testing, by attenuating some and amplifying certain frequencies. These frequencies are measured as modal frequencies. Usually a load cell is placed between the shaker and the structure to obtain the excitation force.
For large civil engineering structures much larger shakers are used, which can weigh 100[[kg]] and above and apply a force of many hundreds of [[newtons]]. Several types of shakers are common: rotating mass shakers, electro-dynamic shakers, and electrohydraulic shakers. For rotating mass shakers the force can be calculated from knowing the mass and the speed of rotation; for the electro-dynamic shaker the force can be obtained through a load cell, or an accelerometer placed on the moving mass of the shaker. Shakers can have an advantage over the impact hammer as they can supply more energy to a structure over a longer period of time. However, problems can also be introduced; shakers can influence the dynamic properties of the structure and can also increase the complexity of analysis due to [[window function|windowing]] errors.
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