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accounting_for_inertia [2018/06/13 13:39]
marijn.nijenhuis
accounting_for_inertia [2023/06/08 16:35] (current)
marijn.nijenhuis
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 First, we need to apply a material to the SolidWorks part in order to account for the object's density. This can be done by right clicking on "Material" in the design tree and selecting the correct material.  First, we need to apply a material to the SolidWorks part in order to account for the object's density. This can be done by right clicking on "Material" in the design tree and selecting the correct material. 
  
-Next, a coordinate system has to be created which is aligned with the global coordinate system used in the Spacar simulation (the default coordinate system in SolidWorks is likely to be different from the one in your Spacar model). It is crucial that the orientation of the coordinate system in SolidWorks and Spacar is identical. While the position of the origins can be different (the inertia properties are calculated with respect to the center of mass anyway), it is recommended that the origin of the new coordinate system in SolidWorks coincides with at least a point that is also a node in the Spacar model (in order to deal with the location of the center of mass). If needed, a custom reference point can be created from a sketch node or with: SolidWorks => Features => Reference Geometry => Point. +Next, a coordinate system has to be created which is aligned with the global coordinate system used in the Spacar simulation (the default coordinate system in SolidWorks is likely to be different from the one in your Spacar model). It is crucial that the orientation of the coordinate system in SolidWorks and Spacar is identical. While the position of the origins can be different (the inertia properties are calculated with respect to the center of mass anyway), it is recommended that the origin of the new coordinate system in SolidWorks coincides with at least a point that is also a node in the Spacar model (in order to deal with the position of the center of mass). If needed, a custom reference point can be created from a sketch node or with: SolidWorks => Features => Reference Geometry => Point. 
  
 To define the orientation of this new coordinate system, two perpendicular edges/lines/axes are required which are aligned with the ''x''-, ''y''- or ''z''-axis of the Spacar model (the third one follows automatically).  To define the orientation of this new coordinate system, two perpendicular edges/lines/axes are required which are aligned with the ''x''-, ''y''- or ''z''-axis of the Spacar model (the third one follows automatically). 
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 {{ :wrench_reference_axis.jpg?direct |}} {{ :wrench_reference_axis.jpg?direct |}}
  
-Next, a new coordinate system is made (SolidWorks => Features => Reference Geometry => Coordinate system) and the axis generated in the previous step are selected for the ''x''- and ''y''-axis of this coordinate system. +Next, a new coordinate system is made (SolidWorks => Features => Reference Geometry => Coordinate system) and the axes generated in the previous step are selected for the ''x''- and ''y''-axis of this coordinate system. 
  
 Lastly, the inertia properties can be obtained from: SolidWorks => Evaluate => Mass Properties. Here, we have to specify the generated coordinate system from the previous step at "Report coordinate values relative to:". The units and number of decimals can be changed with the "option" button (for SPACAR Light, the values in kgm<sup>2</sup> are required). Take note of  Lastly, the inertia properties can be obtained from: SolidWorks => Evaluate => Mass Properties. Here, we have to specify the generated coordinate system from the previous step at "Report coordinate values relative to:". The units and number of decimals can be changed with the "option" button (for SPACAR Light, the values in kgm<sup>2</sup> are required). Take note of 
   - the total mass of the object    - the total mass of the object 
   - the moments of inertia taken at the center of mass and aligned with the output coordinate system.   - the moments of inertia taken at the center of mass and aligned with the output coordinate system.
-  - the location of the center of mass (note: this location is provided with respect to the selected coordinate system)+  - the position of the center of mass (note: these numbers are given with respect to the selected coordinate system) 
 + 
 +** Important: ** Depending on the version of SolidWorks, different conventions are used for the crossproducts (the off-diagonal components of the inertia tensor). With version 2021 and later, you can set the convention: we want the positive tensor notation. You can set this in the Options panel within the Mass Properties window. For older versions of SolidWorks where you don't have this option, be very wary of what SolidWorks returns. In that case, it's mostly likely that SolidWorks shows the inertia tensor with minus signs in front of the off-diagonal components: 
 +$$ \begin{bmatrix} I_{xx} & -I_{xy} & -I_{xz} \\ -I_{xy} & I_{yy} & -I_{yz} \\ -I_{xz} & -I_{yz} & I_{zz} \end{bmatrix} $$ 
 +If it's unclear, check the manual for your version to see which sign convention is being used for the off-diagonal terms.
  
 {{ :wrench_mass_properties.jpg?direct |}} {{ :wrench_mass_properties.jpg?direct |}}
  
 ===== Adding mass and inertia properties to Spacar ===== ===== Adding mass and inertia properties to Spacar =====
-For this example a single leafsprings is simulated between node 1 ([0,0,0]) and node 2 ([0 0.1 0]), with one end of the wrench attached to node 2 (which coincides with the location of the coordinate system in solidworks). Furthermore, a third node at the center of mass of the wrench ([0.055 0.1 0.003]) is rigidly connected to node 2. At this node in the center of mass of the wrench, both the mass and inertia properites are added with the ''nprops(i).mass'' and ''nprops(i).mominertia'' fields.** Check the full syntax list for the correct use of ''nprops(i).mominertia'' because the definition differs from SolidWorks**. ''nprops(i).mominertia'' consists of a vector with length six, [Ixx Ixy Ixz Iyy Iyz Izz] in kgm<sup>2</sup>Note the tensor components are neededso IxyIxz and Iyz represent the negative of the products of inertia (which are provided by SolidWorks)  +For this examplea single leafsprings is simulated between node 1 ([0,0,0]) and node 2 ([0 0.1 0]), with one end of the wrench attached to node 2 (which coincides with the position of the coordinate system in SolidWorks). Furthermore, a third node at the center of mass of the wrench ([0.055 0.1 0.003]) is connected to node 2 with a rigid massless element. At the node in the center of mass of the wrench, both the mass and moment of mass properties are added with the ''nprops(i).mass'' and ''nprops(i).mominertia'' fields. 
 + 
 +** Check the full syntax list for the correct use of ''nprops(i).mominertia'' **. SPACAR Light requires the vector that contains the actual tensor components, so ''[Ixx Ixy Ixz Iyy Iyz Izz]'' in kgm<sup>2</sup>SolidWorks (from versions 2021 and later) refers to this as the positive tensor convention. Make sure that the units in the SolidWorks window are displayed in meterskilograms, and kgm<sup>2</sup>. Also, it is recommended that you use scientific notation with sufficient decimal places.
  
-A visualization of the first two vibration modes of this system is provided bellow. In order to provide a clearer image of this system, the CAD file of the wrench is included in the visualization. The visualization of CAD files in Spacar is out of the scope of this example and therefore not discussed. The matlab code of this example is provided at the end of this page.+A visualization of the first two vibration modes of this system is provided below. In order to provide a clearer image of this system, the CAD file of the wrench is included in the visualization. The visualization of CAD files in Spacar is outside of the scope of this example and therefore not detailed. This example only treats the dynamic behavior of the wrench. The MATLAB code of this example is provided at the end of this page.
  
 {{:wrench_1.gif|}} {{:wrench_1.gif|}}
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 %node 1 %node 1
 nprops(1).fix                = true;         %Fix node 1 nprops(1).fix                = true;         %Fix node 1
-nprops(3).mass               = 0.074;        %0.74Kg at the center of mass +nprops(3).mass               = 0.074;        %0.074 kg at the center of mass 
-nprops(3).mominertia         = [ 0.00000398 0.00000252 0.0000000 0.00013376 0.00000000 0.00013739]; %Inertia at the center of mass+nprops(3).mominertia         = [0.00000398 0.00000252 0.0000000 0.00013376 0.00000000 0.00013739]; %Inertia at the center of mass
  
 %% ELEMENT PROPERTIES %% ELEMENT PROPERTIES
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 eprops(1).orien    = [0 0 1];          %Orientation of the cross-section as a vector pointing along "width-direction" eprops(1).orien    = [0 0 1];          %Orientation of the cross-section as a vector pointing along "width-direction"
 eprops(1).nbeams   = 1;                %4 beam elements for simulating these elements eprops(1).nbeams   = 1;                %4 beam elements for simulating these elements
-eprops(1).flex     = 1:6;            %Model out-of-plane bending (modes 3 and 4) as flexible+eprops(1).flex     = 1:6;              %Consider all deformation modes (1 to 6) as flexible
 eprops(1).color    = 'grey'; eprops(1).color    = 'grey';
 eprops(1).opacity  = 0.7; eprops(1).opacity  = 0.7;
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 %% SIMULATION %% SIMULATION
-out1 = spacarlight(nodes, elements, nprops, eprops);+out = spacarlight(nodes, elements, nprops, eprops);
 </code> </code>
  
    
accounting_for_inertia.1528889989.txt.gz · Last modified: 2018/06/13 13:39 by marijn.nijenhuis