Center of Gravity

[Sachiel7]

I have a few questions about the center of gravity in Inventor 8. First off, is it supposed to shift, perhaps when a mechanism is driven?
I'm not even sure how accurate it is. It's based on the weight of all the parts, right? How can I check to make sure everything weighs the right amount?
Thanks!

[Cory]

"File"--"iProperties"--"Physical"

You can adjust the material in this menu, to get an accurate weight.

You need to do this for every part file in an assembly though.

[Arefin Bari]

Originally Posted by Cory

"File"--"iProperties"--"Physical"

You can adjust the material in this menu, to get an accurate weight.

You need to do this for every part file in an assembly though.

I am not sure about Inventor 8 but in Inventor 7, on the left side menu (where they list all the parts) if you right click on the assembly and go to properties, and then click on the tab physical, it shows you weight. Inventor 7 and Inventor 8 shouldnt be THAT different from each other.

-Arefin.

<edit>

I just talked to Cory, and came to a conclusion that you will need to specify on each file the type of material you are using in order to get the right mass from the assembly.

</edit>

[Squirrelrock]

If yor weights are correct, then if you move a part when center of gravity is on, it should move with the changes. Otherwise, just go to file--properties and click on update in the physical properties tab.

Squirrel

[rmmlg]

I've tried it out and gotten moving a part to change the cg. You must go under tools and then go to update mass properties. If the cg marker is gray, then you need to do update mass properties in order to get an accurate position.

I've also had a lot of luck with getting a weight estimate for our bot with it. For our chassis and drive train, Inventor estimated 55 pounds, and it was actually 53 pounds. I had to place in dead weights for things such as compressor and motors since the cad models aren't weight accurate. I presume that the cg is jsut as accurate. Just remember to add in your battery weight and the weight of any tetras, etc that you are picking up.

[Greg McCoy]

The center of gravity calculation is not accurate for parts/assemblies which have user-defined mass. It only works for parts which Inventor has calculated the mass from the volume of the object and density of the material.

[jdiwnab]

Originally Posted by Greg McCoy

The center of gravity calculation is not accurate for parts/assemblies which have user-defined mass. It only works for parts which Inventor has calculated the mass from the volume of the object and density of the material.

One work around is to make the part to be the right size and shape, weigh it on an accurate scale, then compute density from the Volume provided. You can then make a new material for that part. I did it for the battery becuase all of the models I could find used the default material. I looked up the volume (file>iproperties>physical>volume) and wieghted the battery. Using my calculator I used d=m/v to find the density. I made a new material (format>material>new) and entered in the density I found (its in g/cm^3). I called it the battery material but kept the coloring that it already had on it.

PM if confused.

[JasJ002]

Just a little heads up this is a very tedius tool that few teams use, although it would be very impressive to the judges.

[jdiwnab]

Originally Posted by JasJ002

Just a little heads up this is a very tedius tool that few teams use, although it would be very impressive to the judges.

Which tool would you be refering to? the center of gravity or the materials?

If you are talking about the material, yes if you wait untill the last. If you tell inventor you are making it out of aluminum when you make it, then it is much less tedious. the most tedious part is when I was applying materials to the KOP gear box. the model looked good, but lacked a lot of material/constraints. When they include the ball bearings and all of them need materials, you will get tired of materials and wonder why you want them . But when you find that you might tip, and you can fix it all before your robot is together, it is very useful.