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accounting_for_inertia [2023/06/08 16:34]
marijn.nijenhuis
accounting_for_inertia [2023/06/08 16:35] (current)
marijn.nijenhuis
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   - the position of the center of mass (note: these numbers are given 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. Mostly likely, SolidWorks shows the inertia tensor with minus signs in front of the off-diagonal components:+** 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 caseit's mostly likely that SolidWorks shows the inertia tensor with minus signs in front of the off-diagonal components:
 $$ \begin{bmatrix} I_{xx} & -I_{xy} & -I_{xz} \\ -I_{xy} & I_{yy} & -I_{yz} \\ -I_{xz} & -I_{yz} & I_{zz} \end{bmatrix} $$ $$ \begin{bmatrix} I_{xx} & -I_{xy} & -I_{xz} \\ -I_{xy} & I_{yy} & -I_{yz} \\ -I_{xz} & -I_{yz} & I_{zz} \end{bmatrix} $$
 +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 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.+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 meters, kilograms, and kgm<sup>2</sup>. Also, it is recommended that you use scientific notation with sufficient decimal places. ** 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 meters, kilograms, and kgm<sup>2</sup>. Also, it is recommended that you use scientific notation with sufficient decimal places.
accounting_for_inertia.txt · Last modified: 2023/06/08 16:35 by marijn.nijenhuis