Actin  Version 5.5.5
Software for Robotics Simulation and Control
Mass Properties

To edit the mass properties of a manipulator or link, we use the mass properties plugin. Each link on a manipulator (EcManipulatorLink) stores a map of EcRigidBodyMassProperties, which can be thought of as "point" masses which store a set of 10 unique values: mass, first moment of inertia, and second moment of inertia. To show the editor first stop the simulation (if it is running), and then click on "Mass Properties" from the Edit -> Advanced menu.


The editor is shown below on the right side of the main render widget.


Selecting a Manipulator and Link

To begin editing, select a manipulator and link from the top of the widget. This will select which link which will contain the mass components. The last link in the list is the "base" link of the manipulator.

Select a Manipulator and Link

Selecting a Link Mass Component

After selecting a link to edit, you can select a link mass component to edit from the drop down list. By default each link has a base mass, which is selected by default when you select a link. When a link mass component is selected, you can use the Link Component Mass Properties editor to edit the mass, center of mass on the link, and the first and second moments.

Link Component Mass Properties Editor

Editing a Link Mass Component

When you have a link mass component selected, you can select to show the ellipsoid for the mass component, which will also show a dragger dialog. The dragger allows you to move the center of the ellipsoid by dragging the rings and arrows.

Using the Dragger

The last entry in the link mass component drop down is the "Total" mass, and can be selected to show ellipsoid of the sum of all the link mass components on the link. For example, if we take the base mass,


then add another point mass above the vehicle,


the total mass ellipsoid in this case might look like this:

The total mass ellipsoid cannot be edited, it is read only. If you try and move the dragger, it will not move.

If your link consists of a primitive shape (EcShape), then you can use the geometry of the shape to estimate and create the proper mass property ellipsoid (assuming constant density of your shape). Below is an example where the geometry of EcTriangularLozenge that was edited and its mass properties no longer properly reflect its shape.


To adress this, select the manipulator and its base link, and select "Set from shape". The resulting mass property ellipsoid is shown

Before selecting Set from shape
After selecting Set from shape