Modern Era of Experimental Modal Analysis: One Historical Perspective, The

Sound and Vibration, Jan 2007 by Brown, David L, Allemang, Randall J

With the celebration of the 25th anniversary of the International Modal Analysis Conference (IMAC) in February 2007 by the Society of Experimental Mechanics (SEM), Sound and Vibration asked the University of Cincinnati to chronicle how the modern era of experimental modal analysis developed to the jumping off point of the first IMAC conference back in 1982 in Orlando, FL. The University of Cincinnati was involved in one path of technology advances that contributed to Dick DeMichele and Pete Juhl deciding to work with Union College to put on the first IMAC. In fact, as is the case in many technology areas, the University did not start out with a mission of developing experimental modal analysis technology but instead was working on a machining problem for the U.S. Air Force. The application problem of machine tool vibration and machine surface roughness led to the need to become involved in the emerging analog and digital measurement technology and ultimately to the central role in the development of experimental modal analysis technology. This is our story.

In order to discuss the path to the first IMAC. the concept of the modern era of experimental modal analysis must bo defined. There are a number of possible starting points for experimental measureents and modal analysis dating back to at least the Wheatstone Bridge (1843) and certainly an argument can be made to go back even further to the developments by Fourier (1822) or Prony (1793) or even further. However, the modern era really can be restricted to more recent history when measurements of force and motion could be accurately recorded, the theory of experimental modal analysis had been developed in the literature and commercial implementations of the research technology began to make experimental modal analysis available to more than the research community. With this in mind, this gives the 1960s as the start of the modern era of experimental modal analysis. If a specific year must be chosen, 1967 will be our choice for a number of self-serving reasons that will be discussed later.

The rationale for this involves the confluence of many technologies that were developed earlier in the 190Os and began to become mature and somewhat integrated by the 1960s. To begin with, the modern era of experimental modal analysis could not begin until the theoretical background of modern test methods was formulated. This background was developed in the 1930s, 1940s and 1950s and was well established in the literature by 1963. Two competing methodologies were developed during this period and became known as phase resonance and phase separation methods. Phase resonance methods were being pioneered by researchers in the aircraft area and involved using multiple sine forces to excite the aircraft into a normal mode of vibration (resonance) by adjusting the location, signed magnitude (O or 180� phase) and frequency of a set of multiple shakers. These methods continue to today as forced normal mode methods that are still used by some aerospace testing groups. The literature that first documents this approach somewhat rigorously was authored by Lewis and Wrisley in 1950' and De Veubeke in 19562 but many other authors contributed to this during the 1950s and early 1960s.1 '" A good state of the art review was published by Bishop and Gladwell in 1963."