How to output mass of solid in SimScape multibody

I created a solid by revolving a cross-section, and have given it a density and want the mass calculated from the geometry. How do I output the mass number? I want to scale the density to obtain a certain mass value. I have tried h=find_system('modelname','ClassName','Solid') and then get_param(h,'Mass'), and all I get is the value 1, no matter what density I use.

Answer 1

[Sebastian Castro]

You unfortunately cannot get this information in the latest release so far (R2016a). MathWorks has received quite a few requests for this already.

When you query the block's "Mass" parameter as you did, it's actually looking at a hidden parameter in the block where, instead of calculating mass/inertia from geometry, you specify your own mass and moments of inertia. This is why you get 1, which is the default value for that parameter.

- Sebastian

[Steven Wassom]

Thank you. Could I apply gravity to the solid and connect it to another solid with a joint constraint, then ground the other solid and output the weight force inside the joint? That may be a workaround.

[Sebastian Castro]

That sounds like it should work. You could probably just use a Weld joint to keep a body fixed against gravity and see what the constraint/reaction force is.

[Steven Wassom]

I did that just before I saw your answer--and it worked great. In the configuration block my units of gravity acceleration are m/s2, and I did a brick test mass with density units of lbm/in3, and I notice that the constraint force units are N. I'm still pretty new to Multibody; how do I force the units to be what I want?

[Sebastian Castro]

In the Simulink-PS Converter block, you can enter units (e.g., lbf).

[Steven Wassom]

Thanks for all your help on this; I'm good for now, and will start a new thread with any other questions.

[Ben Rancici]

Is it now possible to get the derived mass from a body block in R2017b?

[Sebastian Castro]

Yes. According to the release notes, you can view these derived quantities in the Solid block starting with R2017b: https://www.mathworks.com/help/physmod/sm/release-notes.html

"The Solid block now displays the calculated values of the solid mass properties—mass, center of mass, moments of inertia, and products of inertia. Set the Inertia > Type parameter to Calculate from Geometry in order to expose a new node named Derived Values. Click the Update button under this node to calculate the solid mass properties and display their values in the field provided below the button."

- Sebastian

[Ben Rancici]

Thank you for your quick answer. I indeed have seen the values under the new node "Derived Values", and I was wondering if there were any mean to use them directly under Simulink. Is it possible?

[Sebastian Pinzello]

Hi Sebastian,

I'm just wondering if there has been any update to Ben Rancici's question. It would be super useful to be able to quickly sum up all the masses of a mechanical system.

Cheers,

Sebastian