Urdf, exercise 3.0.inertia difference in values

[artem.nkfv]

i’am calculating inertia for sphere with mass: 0.01 and radius: 0.008 (using script that course provide)
Here is the python script :

#!/usr/bin/env python
import math


class InertialCalculator(object):

    def __init__(self):
        print("InertialCaluclator Initialised...")

    def start_ask_loop(self):

        selection = "START"

        while selection != "Q":
            print("#############################")
            print("Select Geometry to Calculate:")
            print("[1]Box width(w)*depth(d)*height(h)")
            print("[2]Sphere radius(r)")
            print("[3]Cylinder radius(r)*height(h)")
            print("[Q]END program")
            selection = input(">>")
            self.select_action(selection)

        print("InertialCaluclator Quit...Thank you")

    def select_action(self, selection):
        if selection == "1":
            mass = float(input("mass>>"))
            width = float(input("width>>"))
            depth = float(input("depth>>"))
            height = float(input("height>>"))
            self.calculate_box_inertia(m=mass, w=width, d=depth, h=height)
        elif selection == "2":
            mass = float(input("mass>>"))
            radius = float(input("radius>>"))
            self.calculate_sphere_inertia(m=mass, r=radius)
        elif selection == "3":
            mass = float(input("mass>>"))
            radius = float(input("radius>>"))
            height = float(input("height>>"))
            self.calculate_cylinder_inertia(m=mass, r=radius, h=height)
        elif selection == "Q":
            print("Selected Quit")
        else:
            print("Usage: Select one of the give options")


    # def calculate_box_inertia(self, m, w, d, h):
    #     Iw = (m/12.0)*(pow(d,2)+pow(h,2))
    #     Id = (m / 12.0) * (pow(w, 2) + pow(h, 2))
    #     Ih = (m / 12.0) * (pow(w, 2) + pow(d, 2))
    #     print("BOX w*d*h, Iw = "+str(Iw)+",Id = "+str(Id)+",Ih = "+str(Ih))

    def calculate_box_inertia(self, m, w, d, h):
        Iw = (1.0 / 12.0) * (d**2 + h**2) * m
        Id = (1.0 / 12.0) * (w**2 + h**2) * m
        Ih = (1.0 / 12.0) * (w**2 + d**2) * m
        print ('BOX ixx="' + str(Iw) + '" ixy="0.0" ixz="0.0" iyy="' 
                + str(Id) + '" iyz="0.0" izz="' + str(Ih) + '"')
            

    def calculate_sphere_inertia(self, m, r):
        I = (2*m*pow(r,2))/5.0
        print("SPHERE Ix,y,z = "+str(I))

    def calculate_cylinder_inertia(self, m, r, h):
        Ix = (m/12.0)*(3*pow(r,2)+pow(h,2))
        Iy = Ix
        Iz = (m*pow(r,2))/2.0
        print("Cylinder Ix,y = "+str(Ix)+",Iz = "+str(Iz))

if __name__ == "__main__":
    inertial_object = InertialCalculator()
    inertial_object.start_ask_loop()

This the output from the script for sphere:
InertialCaluclator Initialised…
#############################
Select Geometry to Calculate:
[1]Box width(w)*depth(d)*height(h)
[2]Sphere radius(r)
[3]Cylinder radius(r)*height(h)
[Q]END program

2
mass>>0.01
radius>>0.008
SPHERE Ix,y,z = 2.56e-07
#############################

But in solution of the course inertia value is different (1.28e-06):

<link name="footM1_link">
 	    <inertial>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <mass value="0.01" />
            <inertia ixx="1.28e-06" ixy="0.0" ixz="0.0" iyy="1.28e-06" iyz="0.0" izz="1.28e-06"/>
        </inertial>
        <collision>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <geometry>
                <sphere radius="0.008"/>
            </geometry>
        </collision>
        <visual>
            <origin rpy="0.0 0 0" xyz="0 0 0"/>
            <geometry>
                <sphere radius="0.008"/>
            </geometry>
        </visual>
	</link>

I’am misunderstood something or this is mistake in urdf file of course?

[duckfrost2]

Yes there is an error in the solution sorry about that.

You can check that the sam result is given inside any webpage that calculate moments of innertia, like this one: Moment of Inertia of a Solid Sphere Calculator | Online Moment of Inertia of a Solid Sphere Calculator App/Software Converter – CalcTown

[artem.nkfv]

thank you for response.