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2.8  Display shapes : geometry declaration, pre-test

Before actually taking measurements, it is good practice to prepare a wire frame-display (section 2.8.1 and section 4.1.1 for other examples) and define the sensor configuration (section 2.8.2).

The information is typically saved in a specific .m file which should look like the d_mesh('TutoPre-s3') demo without the various plot commands. The d_pre demo also talks about test preparation.

2.8.1  Modal test geometry declaration

A wire-frame model is composed of node and connectivity declarations.

Figure 2.15: Test analysis : wire-frame model.

Starting from scratch (if you have not imported your geometry from universal files). You can declare nodes and wire frame lines using the fecom Add editors. Test wire frames are simply groups of beam1 elements with an EGID set to -1. For example in the two bay truss (see section 4.1.1)

 % fecom('AddNode') would open a dialog box
 fecom('AddNode',[0 1 0; 0 0 0]); % add nodes giving coordinates
 fecom('AddNode',[3  1 1 0;4  1 0 0]); % NodeId and xyz
 fecom('AddNode',[5      0 0 0    2 0 0;
                  6      0 0 0    2 1 0]);
 % fecom('AddLine') would add cursor to pick line (see below)
 fecom('AddLine',[1 3 2 4 3]);  % continuous line in first group
 fecom('AddLine',[3 6 0 6 5 0 4 5 0 4 6]); % 0 for discontinuities
 %fecom('save')  % will let you save the model to a mat file
 feutilb('write',cf.mdl) % generates a script

Note that

The feplot and fecom functions provide a number of tools that are designed to help in visualizing test results. You should take the time to go through the gartid, gartte and gartco demos to learn more about them.

2.8.2  Sensor/shaker configurations

The wireframe (experimental geometry) declaration needs a structure with fields

Alternatives to the 5 column .tdof are provided for convenience, readability or to ease specification

Once a sensor configuration defined and consistent with input/output pair declarations in measurements (using the .dof field described in  section 2.2.4), you can directly animate measured shapes (called Operational Deflection Shapes) as detailed in section 2.8.3. For interpolation of unmeasured DOFs see section 3.3.2. Except for roving hammer tests, the number of input locations is usually small and only used for MIMO identification (see section 2.9).

The following illustrates the first two forms


% simply give DOFs (as a column vector)
wire.tdof = [1011.03 1001.03 2012.07 1012.03 2005.07 1005.03 1008.03  ...
 1111.03 1101.03 2112.07 1112.03 2105.07 1105.03 1108.03 1201.07 ...
 2201.08 3201.03 1206.03 1205.08 1302.08 2301.07 1301.03 2303.07 1303.03]';

% Transform to 5 column format, which allow arbitrary orientation

cf=feplot(wire); % With a .tdof field, a SensDof,Test is defined automatically
sdth.urn('Tab(Cases,Test){Proview,on,DefLen,.1}',cf) % see sdtweb sdth#urn

% You can now display FRFs or modes using
% Display FRF
cf.def=ci.Stack{'Test'}; % automatically uses sensor definition 'Test'
% Local estimation, displayed before validation
idcom('e .05 6.5')
cf.def=ci.Stack{'IdAlt'}; % Same as right click->animate modes on left pole list

This other example deals with the definition of sensors from local bases.

wire=demosdt('DemoGartDataTest'); % Load test gemetry
wire=feutil('rmfield',wire,'tdof'); % tdof will be edefined later from bas
% In this example, each node is associated to a local basis
% They all correspond to the same basis doing this transformation
% xtest -> -yfem, ytest -> zfem, ztest -> -xfem,
wire.bas=[wire.Node(:,3) ... % see sdtweb basis for .bas format
 repmat([1 0  0.0 0.0 0.0   0.0 -1.0 0.0  0.0 0.0 1.0   -1.0 0.0 0.0],size(wire.Node,1),1)];
cf=feplot(wire);fecom(cf,'showbasDID'); % Show all three directions at each node location
% Define the orientation of the sensors in the local basis 
tdof = [1011.02 1001.02 2012.03 1012.02 2005.03 1005.02 1008.02  ...
 1111.02 1101.02 2112.03 1112.02 2105.03 1105.02 1108.02 1201.03 ...
 2201.01 3201.02 1206.02 1205.01 1302.01 2301.03 1301.02 2303.03 1303.02]';
% Build tdof in global coordinate using wire.bas.
% Display Sensors

It is also fairly common to glue sensors normal to a surface. The sensor array table (see section 4.6) is the easiest approach for this objective since it allows mixing global, normal, triax, laser, ... sensors. The following example shows how this can also be done by hand how to obtain normals to a volume and use them to define sensors.

% This is an advanced code sample
model=demosdt('demo ubeam');

MAP=feutil('getnormal node MAP',model.Node, ...
  feutil('selelt selface',model)); % select outer boundary for normal

i1=ismember(MAP.ID,[360 365 327 137]); % nodes where sensors are placed
model=fe_case(model,'sensdof','test', ...
  [(1:length(MAP.ID))' MAP.ID MAP.normal]);

% display the mesh and sensors

2.8.3  Animating test data, operational deflection shapes

Operational Deflection Shapes is a generic name used to designate the spatial relation of forced vibration measured at two or more sensors. Time responses of simultaneously acquired measurements, frequency responses to a possibly unknown input, transfer functions, transmissibilities, ... are example of ODS.

When the response is known at global DOFs no specific information is needed to relate node motion and measurements. Thus any deformation with DOFs will be acceptable. The two basic displays are a wire-frame defined as a FEM model or a wire-frame defined as a SensDof entry.

 % A wire frame and Identification results
 cf=feplot(wire);   % wire frame
 cf.def=IdMain;     % to fill .dof field see sdtweb('diiplot#xfread')
 % or the low level call : cf.def={IdMain.res.',IdMain.dof,IdMain.po}

 % Sensors in a model and identification results

 cf.sel='-outputs';  % Build a selection that displays the wire frame
 cf.def=IdMain;      % Display motion on sensors
 fecom('curtab Plot');

When the response is known at sensors that need to be combined (non global directions, non-orthogonal measurements, ...) a SensDof entry must really be defined.

When displaying responses with iiplot and a test geometry with feplot, iiplot supports an ODS cursor. Run demosdt('DemoGartDataId plot') then open the context menu associated with any iiplot axis and select ODS Cursor. The deflection show in the feplot figure will change as you move the cursor in the iiplot window.

More generally, you can use fecom InitDef commands to display any shape as soon as you have a defined geometry and a response at DOFs. The Deformations tab of the feplot properties figure then lets you select deformations within a set.

[ci,cf]=demosdt('DemoGartDataId dock');
 % or the low level call : 
 % cf.def={ci.Stack{'Test'}.xf,ci.Stack{'Test'}.dof,ci.Stack{'Test'}.w}
 fecom('CurTab Plot');

You can also display the actual measurements as arrows using

 cf.sens=ci.Stack{'Test'}.dof; fecom ShowArrow; fecom scc1;

For a tutorial on the use of feplot see section 4.4.

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