7 Replies Latest reply on Mar 30, 2018 1:40 PM by Kevin Chandler

    Can you reference mass & center of gravity at sketch level?

    Jordan Ballantyne

      This is my first post on this forum, please excuse me if this should be placed elsewhere.

       

      I am trying to find a value for the mass moment of my component (Mass*CoG) where the centre of gravity is the distance in the Y direction from the origin. I want to use that value to drive the length of a sketch so that I can use it as a sensor in a Design Study. Essentially, I am trying to optimize the part for rotational balance, where the mass moment must be within a certain range.

       

      I have tried driving the this from a design table however it will not update the sketch during the study (the value for mass moment remains the same, whilst the mass and center of gravity are changing).

        • Re: Can you reference mass & center of gravity at sketch level?
          Alex Lachance

          You can make a 3D sketch, make a nod and then put it coincident to the center mass. Then you just need to put your dims and you should be good to go

           

          Edit: You need to force rebuild it for the sketch's dimension to update itself

            • Re: Can you reference mass & center of gravity at sketch level?
              Jordan Ballantyne

              Not sure i understand, what do you mean by put your dims? Is it possible to get solidworks to rebuild between each scenario in a design study? The length of the sketch (mass*centre of gravity) is what I want to optimize for as it represents if the component is balanced, i can use the mass & centre of gravity points as goals or constraints within the study, but not a value which is a result from the multiplication of the two.

               

              To simplify, I want to draw a line and set its length to whatever Mass * CoG is.. for example if the mass is 3kg and the distance to the CoG is 5mm, then the line should be 3*5 (15mm).

                • Re: Can you reference mass & center of gravity at sketch level?
                  Alex Lachance

                  I don't do any of what you do so I can't answer over the technical side of your studies, but I know you can make dimensions in a 3D sketch relative to the center of mass.

                   

                   

                  And if you add something on your part, force rebuild it, the sketch rebuilds it's dimensions accordingly to the new location of the center of mass.

                   

                  The only thing that is a little harder is having the dimensions always keep their directions. As you can see on the second screenshot, since it's not symetrical anymore, the center of mass is not centered therefor the 14 1/16'' dimension is not a Y dimension anymore but a point to point.

                   

                  Edit: Forgot to mention, to make the nod coincident to the center of mass, you need to drag it over the center of mass.  You cannot select both and have them be coincident.

              • Re: Can you reference mass & center of gravity at sketch level?
                Vladimir Urazhdin

                I had a similar experience with design table and came up with conclusion: design table can be used to input variables only.

                Any results are not available for design table (or, just I can't figure out how to make them available)

                • Re: Can you reference mass & center of gravity at sketch level?
                  Kevin Chandler

                  Hello,

                   

                  I'm not in front of SW at the moment, so please bear with me.

                   

                  How about adding a reference CG, then create a 3D line from the origin to this point.

                  Then create a global variable using the measure option to add the dimension you're looking for.

                  Then this gv can be used in the sketch's equation(s).

                   

                  Alternatively, is the ref CG position reference in the equations dialog? If, so you dim can be referenced directly.

                   

                  Another possible method:

                  On the plane for your Y dimension, start a 2D sketch and add a sketch coincident to the ref CG.

                  Dim from the origin to the sketch point and then this dim will be exposed in the equations dialog for use in the other sketch.

                   

                  I hope these help.

                   

                  Cheers,

                   

                  Kevin

                  EDIT: convert entities on the 3D line onto a sketch plane and adf the dim (it will be driven but that is okay.

                   

                  Also, will be a circular loop?

                  Sketch uses cg and updates. Model changes and gets a new cg. New cg dim alters sketch...alters model...alters cg...alters sketch...

                  • Re: Can you reference mass & center of gravity at sketch level?
                    Kevin Chandler

                    Hello,

                     

                    To fix the dimension to maintain its value on one plane, I created a sketch on the desired plane with a construction line and a dim from the origin to a reference CG point.

                    You can see here the CG is beyond the dim:

                    The dim in the equation dialog ready for use in other equations or gv's:

                    I add a tab to the part to shift the CG and the dim updates:

                    Here's the dim's new value:

                     

                    The downside is that there's no indication that the CG crossed the origin.

                    Is this what you were looking for?

                     

                    Cheers,

                     

                    Kevin

                     

                    EDIT: To get the mass is to create a custom property and point a gv at it. Now it's available for the sketch's equations.

                     

                    EDIT II: To get around the whether the CG crossed the origin, you can add another fixed dim and then do a subtraction test to see where it's at.

                    I haven't test it yet, maybe later.

                     

                    EDIT III: Per EDIT II, here's the subtraction test to determine the moment's direction as to which side of the origin the CG is on.

                    1. Create two constructions, each starting at the origin and each the same length. Dim from the outboard endpoint of each line to the CG (with the lines fixated, these will be driven, which is we want)
                      Here's the dims with the tab added above suppressed:
                    2. I renamed these two dims for easier recognition
                    3. Using these dims, I computed their difference and then tested for positive or negative and assigned +/- accordingly (you don't need to bother checking for zero, the CG on the origin, because the moment arm dim will be zero, so the moment will be  mass X moment arm X direction = 0 at the origin)
                    4. Unsuppressing the added tab shifts the CG to the other side of the origin, so the direction changes from "-1" to "1":

                     

                    I hope this helps.

                    Cheers,

                    Kevin

                    • Re: Can you reference mass & center of gravity at sketch level?
                      Kevin Chandler

                      Just a bump to let you know that I edited my first post to add the "which side of the origin is the CG on" test.

                       

                      Cheers,

                       

                      Kevin