e talked about the number of points needed Someone told me ay not pl - PDF document

1 e talked about the number of points need
e talked about the number of points needed ... Someone told me ay not pletely defined.Co Experimental Techniques - FPage 1Pete Avitabiley Pete Avitabile We talked about the llustration by Mike Avitabileast discussion we had regarding the total number of pointsmental modal test was directed towards thee description for the mode shape. But in order to definenamic model, the requirement can be different. Let'sat was identified earlier regarding the modeo identify a good dynamic model. a simple structurhat we have discussed before.ple plate structure. MODE 1 MODE 2 MODE3 anyeasurement locationshat can be used to describe ic characteristics of theructure. If the experimental modal analysis is being conducted correlate with a finite element model, then it is necessary toe number of well distributed measurement dofsn order to have sufficient spatial distribution for comparison toe model. It is also important in order to visualize the shape.et's consider a finite element model that has 45 node pointsents. Let's entalodal test with 15 evenly distributed measurement locations. Aparison of the finite element node points and experimentaleasurement points to describe thodes of the plate. MODE 1 MODE 2 his comparison, there is a sufficient distribution of pointshat the modes can be uniquely defined for the correlationhe finite element model and experimental modal model. Ife model is to be correlated using the Modal Assurance Criterianformation to perform a valid analysis. However, a responseodel may not need the same distribution of points. e talked about the number of points needed ... Someone told me ay not pletely defined.Co Experimental Techniques - FPage 2Pete Avitabilewo separate points where. y three points on the structure where critical response needso be measured. This is pictured below with the input forces inue and response measurements in red. FORCE RESPONSE nt-to-point frequency reion describes theeristic output response due to a unit sinusoidal inputs is computed for a particular input-outputeasurement on the structure. A particular frequency responseeasurement has no relationshi

2 p to other measurements on theructure.
p to other measurements on theructure. If only one frequency response function is oft doesn't matter whether two, three or one hundredtional measurements are made - the individual frequencyeasurement for a particular input-outputeasurement does not get better or worse as additionaleasurements are added.s is not true of the finite element approach where addingtional nodes and elements generally has a direct effect one results.)he only requirement is to determine what the dynamiche system is due to the applied forces, then the onlynts that are required for the experimental modal model arehe three red points and two blue points pictured. These pointspletely describe the response of the system at these points.he points shown may not adequately describe theeformation sufficiently to determine exactly how the load isstributed through the system - but this limited set of points iscient to define the dynamic model.et's continue on and consider that the system may need toodified with structural ch. The figureow depicts the structure with a rib stiffener modification thatd possibly be proposed to modify the structure. In order to any of these structural dynamic modifications, a set ofo be included. gure. RESPONSE these two models show a completely different set ofeasurement dofs that are needodels described. For the response model and structuralnamic modification model, a significantly different set of dofsn order to develop the proper dynamic model to.o have an adequate dynamic model, there must bents to describeall dofs where forces arplied to the systemall dofs where response needs to be measuredall dofs where structural odifications are considered additional dofs included in the model are included for yourmental modal test for you to better visualize the (or for correlation of a modelat is the purpose of the test). This minimum set of dofs isl that is required to develop an appropriate dynamic model.ou have a better understanding of how manynts are needed for a modal test - it varies depending on theltimate use of the dynamic model. If you have any otherions about modal analysis, just ask me.