### Modal Assurance Criteria MAC with Ansys for select Measurement Locations

Where should the accelerometers be placed? - Test Person
Test 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
!!! Inputs
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
fini
/filname,myse   ! name of the super element
/solu
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 ***

alls
solve
fini

!!! Compute Mode Shapes of SE
/filname, file
/prep7
*get,etmax,etyp,0,num,max
et,etmax+1,50              ! SuperElement Type

type,etmax+1
mat,1
se,myse                    ! Creates Super Element

/solu
! Selects only superelement
esel, s, type,,etmax+1
nsle

antype, modal
modopt, lanb, num_modes
solve

!!! Computes MAC matrix
/post1
rstmac, file, 1, all, file, 1, all, 0.01, 0.9,, 1
finish

!!! Computes Mode Shape of Full Model
! so that WB solution will be 'pretty'
/solu
alls
esel,u,type,,etmax+1
nsle

antype, modal
modopt, lanb, num_modes
solve

Results
For the cantilever beam model of the previous post, we could either pick semi-optimal points (e.g. named selection of MeasPoints_good):
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.
MAC Matrix

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.

1. Thank you for the nice post. Does the RSTMAC compute the MAC matrix of the SE with respect to itself or of the SE with respect to the full FE ?

1. In the example here, the DOF used are of the superelement (SE) and is computed with respect to itself.

2. Hello can we use these commands in ANSYS APDL to compute mac matrix
rstmac, file, 1, all, file, 1, all, 0.01, 0.9,, 1

should we define the two files ?

3. Yes, you need to define the two files. The two files in the command are the same file which is the default name used in Ansys workbench.

### ANSYS User Defined Results

There is an abundant of options in ANSYS classic when one wishes to post process results. ANSYS workbench default pull down menu post processing options are more limited but they can still be accessed via the User Defined Results. One way not commonly used but can come in handy is as follows: Zeroth: Under Analysis Settings, there is "Output Controls" where you can toggle to "Yes" what you would like to save before the solution starts. This is like OUTRES in APDL. Output Controls First: After solving the model, click on Solution in the tree to highlight it. Solution Second: Click on Worksheet in the toolbar. Worksheet Third: In the worksheet, you will see list of results that are saved. Right click on it to create the User Defined Results. Create User Defined Results So here we have it. You could of look up the different expressions in the help document but I find this method of accessing the results convenient.  Example: Aspec

### Export Stiffness Matrix from Ansys

It is sometimes useful to extract the mass and stiffness matrix from Ansys.     *SMAT, MatK, D, IMPORT, FULL, file.full, STIFF       *PRINT, matk, matk, txt Exporting mass matrix would be similar:       *SMAT, MatM, D, import, full, file.full, MASS The above script uses APDL Math to get the job done. (Please see previous post for another example). The ordering of the matrix is unfortunately not concurrently exported. To verify the sequencing is as expected, we will work to replicate a truss example in the  Finite Element Trusses course notes by Bob Greenlee. Figure 1: Truss Problem Setup Model Creation Script to create model: /prep7 !! Creates Model to reflect course notes ! Properties et ,1,1  mp , ex, 1, 29.5e6 r , 1, 1 ! Geometry n ,1 $n ,2, 40$  n ,3, 40, 30 $n ,4, 0, 30 e ,1,2$  e ,2,3 $e ,1,3$  e ,3,4 ! Boundary Conditions d ,1,ux,0 $d ,1,uy,0 d ,2,uy,0 d ,4,ux,0$  d ,4,uy,0 f ,2,fx,20e3 f ,3,fy,-25e3 ! solves /solu eqslv , sparse