User Tools

Site Tools


accounting_for_inertia

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

Both sides previous revision Previous revision
Next revision
Previous revision
accounting_for_inertia [2018/06/13 13:49]
marijn.nijenhuis
accounting_for_inertia [2023/06/08 16:35] (current)
marijn.nijenhuis
Line 33: Line 33:
   - 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 position of the center of mass (note: this position 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 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 rigidly connected to node 2. At this 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. +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'' because the definition differs from the numbers that SolidWorks returns **. SolidWorks returns +
-$$ a $$+
  
-''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)  +** Check the full syntax list for the correct use of ''nprops(i).mominertia'' **. SPACAR Light requires the vector that contains the actual tensor componentsso ''[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 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. 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.
Line 71: Line 72:
 %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
Line 84: Line 85:
 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;
Line 94: Line 95:
  
 %% SIMULATION %% SIMULATION
-out1 = spacarlight(nodes, elements, nprops, eprops);+out = spacarlight(nodes, elements, nprops, eprops);
 </code> </code>
  
    
accounting_for_inertia.1528890568.txt.gz · Last modified: 2018/06/13 13:49 by marijn.nijenhuis