Where should the accelerometers be placed? - Test PersonTest People are tough to work with! I was one before so I would know. Have a bit of sympathy for them though as they could be frantically troubleshooting equipment in the middle of no-where or spend countless hours rolling-and-unrolling cables.
Goal of Measurement Points Selection
OK, we have a model with natural frequencies and mode shapes. If your company is well endowed, this is a simple question with pretest software like FEMtools or some other software that optimizes the MAC matrix. This would also help with correlation later. Generally speaking, the software will maximize the diagonal matrix while minimize the off diagonal matrix values. Loosely defined, this means the modes are relatively unique (mathematically speaking) for the chosen measurement locations.
Poor Man's Method for Picking Measurement Locations
Alternately, we can pick measurement locations by hand based on past experience and liberal use of engineering judgement. Here's a rough rule of thumb process in picking locations:
1. Print a drawing of the part
2. For all the computed modes of interest, mark the absolute maximum/minimum displacements with the mode number (e.g. red pen)
3. Also for each mode, mark the node point with the mode number (e.g. blue pen)
4. Group the locations that are close to each other
5. If too many measurement points remain:
5a. Review the points that aren't grouped
5b. Eliminate those mode numbers already represented by the other grouped points
6. Keep withering it down until an 'acceptable number' is achieved
7. Narrow down to a few sets of measurement locations to simulate
Computing Expected MAC Matrix
OK, now that we have a few candidate points for accelerometers, we could test them out with our model. The test MAC should match the analytical MAC if we did a good job in setting up the model. The following procedure will help us determine if the picked measurement locations will provide us with a good MAC.
Overall Procedure of the Command Snippet
In general, the script does the following:
1. Creates the Super-Element using measurement points as master nodes
2. Compute modal analysis results for the Super-Element
3. Compute MAC Matrix of the Super-Element using RSTMAC
4. Computes Modal Analysis of earlier model as if nothing happened
Command Snippet Code
num_modes = 3
! masternodes = 'MeasPoints_bad' ! Toggle which Named Selection Here
masternodes = 'MeasPoints_good' ! Toggle which Named Selection Here
!!! Creates Super-Element with Measurement Nodes
/filname,myse ! name of the super element
antype,substr ! analysis type: substructure
seopt, myse, 2 ! saves mass and stiffness matrix
cmsopt, fix, num_modes ! Craig-Bampton
cmsel, s, %masternodes%
m, all, uy ! creates master nodes *** IMPORTANT ***
!!! Compute Mode Shapes of SE
et,etmax+1,50 ! SuperElement Type
se,myse ! Creates Super Element
! Selects only superelement
esel, s, type,,etmax+1
modopt, lanb, num_modes
!!! Computes MAC matrix
rstmac, file, 1, all, file, 1, all, 0.01, 0.9,, 1
!!! Computes Mode Shape of Full Model
! so that WB solution will be 'pretty'
modopt, lanb, num_modes
For the cantilever beam model of the previous post, we could either pick semi-optimal points (e.g. named selection of MeasPoints_good):
or... a mediocre ones (e.g. named selection of MeasPoints_bad):
If you scroll through the long solver output file, you will find the MAC matrix listed when you solve the two cases individually for the two different set of measurement points.
The MAC matrix for "MeasPoints_good" had off diagonal matrix of 5%, 4% and 30.4%. It could be improved but is a whole lot better than "MeasPoints_bad" locations with MAC of 90% in the off diagonal matrix.
Workbench Archive File
Here's the v18.2 Archive File Link as reference.