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**

**!!! 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):

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.
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 ?

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

Delete