Learning Forum

[Tom Ber]

Hi,

I would like to read the values of centroid and area moment of inertia Ixx and Iyy for selected faces. In version 22 this is possible using measureHelper, but I can't find these values in version 19.2. Is this possible also in older versions?

?

[mjmiddle]

This will not work for the face of a solid. Your face must be a surface body:

 

body1 = GetRootPart().Bodies[0]
#body1 = GetRootPart().Components[0].Content.Bodies[0].Master    # If body in component
mass = body1.MassProperties.Mass
volume = body1.Shape.Volume
centroid = body1.MassProperties.PrincipleAxes.Origin
totalSurfaceArea = body1.Shape.SurfaceArea
principalXAxis = body1.MassProperties.PrincipleAxes.DirX
principalYAxis = body1.MassProperties.PrincipleAxes.DirY
principalZAxis = body1.MassProperties.PrincipleAxes.DirZ
principalMomentXAxis = body1.MassProperties.GetMoment(body1.MassProperties.PrincipleAxes.AxisX)   # only matches when no material is assigned so that mass=0
principalMomentYAxis = body1.MassProperties.GetMoment(body1.MassProperties.PrincipleAxes.AxisY)
principalMomentZAxis = body1.MassProperties.GetMoment(body1.MassProperties.PrincipleAxes.AxisZ)

If no material and width is assigned, you get area properties, not mass properties. 

[Tom Ber]

Perfect Thanks, this is exactly what I need.

I also need to read the Ixy.

I tried to use GetInertiaTensor(), but didn't get the right value.

Could you help me with that too? Should I create new topic?  

 

[mjmiddle]

inertia = body1.MassProperties.GetInertiaTensor(body1.MassProperties.PrincipleAxes)
inertia[0,0]    # 3x3 matrix. Since frame used principal axes, all entries are zero except the diagonal: [0,0], [1,1], [2,2]

[Tom Ber]

Hi mjmiddle,

Thanks a lot for the answer. As i understand, If the body/face is not point-symmetric, then Ixy should be other than 0.

Below I am sending you the code with the printed() values and the picture where you can see the values from "Mass Properties" tool. I wonder why the values read from Mass Properties are different from those read from GetInertiaTensor().

 

 

selected_bodies = Selection.GetActive().Filter[IDesignBody]()

Body1 = selected_bodies.Items[0]

Centroid = Body1.MassProperties.PrincipleAxes.Origin

 

Inertia_Tensor = Body1.MassProperties.GetInertiaTensor(Body1.MassProperties.PrincipleAxes)

 

print("Centroid:{} mm".format(Centroid*1e3))

Centroid:Point: (-3,07067122925524, 2,867257943362, 0) mm

print("Ixx:{} mm^4".format(Inertia_Tensor[0,0]*1e12))

Ixx:235733.0278 mm^4

print("Iyy:{} mm^4".format(Inertia_Tensor[1,1]*1e12))

Iyy:692513.5556 mm^4

print("Izz:{} mm^4".format(Inertia_Tensor[2,2]*1e12))

Izz:928246.5834 mm^4

 

print("Ixy:{} mm^4".format(Inertia_Tensor[0,1]*1e12))

Ixy:0.0 mm^4

print("Ixz:{} mm^4".format(Inertia_Tensor[0,2]*1e12))

Ixz:0.0 mm^4

 

 

[mjmiddle]

Principal axes through the centroid should have zero terms for the product of inertia terms. The argument in the command is a Frame:

Inertia_Tensor = Body1.MassProperties.GetInertiaTensor(Body1.MassProperties.PrincipleAxes)

The Body1.MassProperties.PrincipleAxes is a Frame with principal axes all going through the body origin. If you create a different frame not going through origin, the product of inertia terms will be non-zero.

For the non-matching value to the GUI printout, see the comment in the code I sent:

# only matches when no material is assigned so that mass=0

So the values given by code are volume and area moments of interia, not mass moment of inertia. So you'll have to compute the mass moments of inertia.