Modal analysis of a robot arm using the finite element analysis and modal testing

The objective of the present work was to study the technique of modal analysis using theoretical and experimental methods. A PUMA UNIMATION five axis robot was used as a model. The finite element analysis was performed using MacNeai Schwendler Corporation\u27s NASTRAN code. Intergraph\u27s Rand Micas pre- and post-processor was used to discretize the model and subsequently to display the animated mode shapes after the analysis was done with NASTRAN. The modal testing was performed using Structural Measurement Systems\u27 STAR software and Bruel \u26 Kjaer\u27s instrumentation including a spectrum analyzer, accelerometer and hammer. Due to the size and weight of the structure it was not possible to test it in its free-free condition. Hence it was tested in its supported condition. The results from the two analyses were then compared. The first thirteen modes from both the analyses showed a one-to-one correspondence. The natural frequencies showed a deviation from 1% to 33%. Although the difference of 33% seems large, it was only for one mode which could reflect a computational error. For most of the modes, the variation was less than 10%. The comparison of mode shapes was done by observation of the displayed animated mode shapes. Also, a technique called Modal Assurance Criterion for numerical correlation of mode shapes is discussed. The experimental mode shapes showed an erratic behavior. This was probably due to the nonlinearities in the structure. The causes of the discrepancies between the finite element method and modal testing are discussed in the text. Similar patterns of deflection could be seen in the two analyses