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Center of Gravity

D

dannerj

New member
I am trying to create a point on the assembly center of gravity and I can't quite seem to remember how to do it. I've searched the forums and the posts I have found have not helped.


I tried writing relations using PRO_MP_COGX and it said it was a bad expression. I also tried mp_cg_x.


I am running Wildfire 2.0 M220, and I do not have Behavorial Modeling.


Any help would be greatly appreciated.


Thank you!
 
I assume you have defined three datum planes in X, Y and Z direction (let's name them FRONT, SIDE and TOP) and assembly coordinate system created in the cross-section of those datum planes. To create point in the assembly centre of gravity, first create point with offsets from all three planes (FRONT, SIDE, TOP), then open Relations window (Tools > Relations) and write the following:


$d1 = mp_cg_x("","ACS0","")


$d2 = mp_cg_y("","ACS0","")


$d3 = mp_cg_y("","ACS0","")


where d1 is dimension of the previously created point in X-direction, d2 in Y-direction and d3 in Z-direction. "$" prevents Pro/Efrom relation error if the value of the "mp_cg_" function is negative. ACS0 is the name of the assembly coordinate system to which the centre of gravity refers. You MUST enclose ACS0 in double quotes or the relation will return an error. AND you MUST use commas to delimit three parameter for the mp_cg_ function.


In the help, there used to be exact syntax for calculating mass properties, but I can't find it anymore.
 
nice copy from the thing I attached ...


It works for parts, the "path" in that expression is the problem
 
When I try and use this method it gives me this error message, even though I have calculated the mass properties.


"error Mass properties for this model are not calculated yet."


Any further Ideas?
 
Do you have mass properties (density) assigned? Have you already calculated mass properties for the model?
 
Do you have BMX modul? if yes, the problem seems to be easy to be solved. Create an datum analysis featureto calculate COG and put a datum point there. Remember to assign material density.
 
Maybe I am trying to make this to simple or maybe not as fancy as you want but when you do your mass prop on your assembly you get your x y z cordinates for your assy. center of gravity right? So if you add a new cordinate system and place it at thoseexact cordinates then you can now placeyour point on that new cordinate system. You will have to manually adjust its location as you add and remove components and your center of gravity changes.


View attachment 2748
 
There is no need for any programming.


Insert an analysis feature. Select Model analysis-->compute-->close.


coming back to the window, unselect the default "volume" and say "YES" for XCOG, YCOG and ZCOG.


YOU WILL NOW HAVE A CO-ORDINATE SYSTEM OR YOU ALSO HAVE THE OPTION OF HAVING A POINT AT THE ASSEMBLY CG.
 
There is a "Suggested Technique for Creating a Datum Coordinate System at the Center of Gravity of a Part" in the knowledge base at ptc.com.

This describes how to automatically create a CSYS at the C of G.

One thing though, I can't actually get it to work! The option to create the C of G is a check box which is greyed out when I follow the technique!

I'd be interested to know if anyone uses this method.

By the way, using Wildfire 3.0
 
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