Proc. of the 4 th National Conference on Emerging Trends in Mechanical Engineering, March 18 ‐19, 2011 G. H. Patel College of Engineering & Technology, V. V. Nagar‐ 388120, Gujarat, India ISBN: 978‐81‐909376‐3‐4 Methodology for Estimation of Modal Parameters using Experimental Modal Analysis in light of Curve Fitting Technique N.V.Oza 1* , R.I.Patel 2 , S.S.Pathan 3 1. Pursuing M.E. Mechanical Engg. at L.D.College of Engineering , Ahmedabad. 2. Pursuing M.E. Mechanical Engg. at L.D.College of Engineering , Ahmedabad. 3. Sr. Lecturer in Mechanical Engg., at L.D.College of Engineering , Ahmedabad. * Corresponding author (e-mail: nigam91@yahoo.com) In this paper, concept of experimental modal analysis is discussed to derive dynamic properties of mechanical structures and equipments. Dynamic properties (mode shape, damping, and resonant frequencies) are calculated using MATLAB program. Amongst present curve fitting method, Rational Fraction Polynomials (RFP) method is used in the derivation of modal parameters. Results obtained from this method are compared with those obtained form experiment and shown in form of standard deviation. This standard deviation is computed from different experimental FRF values and analytically obtained FRF values. Keywords: Experimental modal analysis; Modal parameter extraction; Damping identification; Polynomial curve fitting 1. Introduction Vibration has many undesirable and harmful effects on life and performance of mechanical equipments and other structures. The effects of vibration are due to dynamic interaction between vehicles and bridges, structural motions due to earthquakes, noise generated by construction equipment , vibration transmitted from machinery to its supporting structures thereby interfering with their performance , damage as well as malfunction and failure due to dynamic loading, fatigue failure, oscillation of transmission lines[1]. The objective of this paper is to emphasis on dynamic analysis of such equipments and structures by capturing their actual dynamic behaviour during experimentation such that the adversity arising from vibration effects can be minimised to improve their life and performance. Dynamic analysis consists of experimental and operational modal analysis. In experimental modal analysis (EMA), structures are artificially excited by exciters (Impact hammers and shakers). In operational modal analysis (OMA), structure is analysed while it is operated upon. For large and heavy structures (civil structures such as bridge and dams), modal analysis is used to detect damage by ambient (traffic) condition [2] .Recent trends in dynamic analysis are extremely focused on better performance and life of structures. Self excited vibrations of tool result in unstable cutting process, poor surface finish, reduced productivity and damage on the machine itself. By considering spindle geometry (its diameter and length), bearing stiffness, tool holder geometry and selection of combination of depth of cut and spindle speed from stability lobe diagrams, machining operation can be made chatter free[3,4]. In vibration of rotating equipments (such as pump, turbine etc.), dynamic analysis is used to check their health as excessive noise of these equipments is experienced by personnel in large power plants and refineries due to damage or failure of seals [5]. 2. Methodology In this paper, EMA is focused upon. EMA is used to characterize resonant vibration in machinery and structures. In EMA, a mode of vibration is defined by three parameters; modal frequency, modal damping and mode shape. Modal parameter estimation is the process of determining these parameters from experimental data. Furthermore, a set of modal parameters can completely characterize the dynamic properties of a structure. This set of parameters is also called a modal model for the structure. Modes (or resonances) are inherent properties of a structure. 199