Inertia Calculator

Course Support URDF for Robot Modeling
1

To make my life simpler, I tweaked the inertia calculator code to output the full string needed for the URDF file. Hope it’s useful to anyone who stumbles across it.
/K

urdf_inertial_calculator.py

#! /usr/bin/env python

import math

class InertialCalculator(object):

    def __init__(self):
          print "URDF Inertial Calculator Initialised..."
  
    def start_ask_loop(self):
  
        selection = "START"

        while selection.upper() != "Q":     # accept 'q' as well
            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 = raw_input(">>")
            self.select_action(selection)

        print "URDF Inertial Calculator Quit...Thank you"

    def select_action(self, selection):
        if selection == "1":
            mass = float(raw_input("mass>>"))
            width = float(raw_input("width>>"))
            depth = float(raw_input("depth>>"))
            height = float(raw_input("height>>"))
            self.calculate_box_inertia(m=mass, w=width, d=depth, h=height)
        elif selection == "2":
            mass = float(raw_input("mass>>"))
            radius = float(raw_input("radius>>"))
            self.calculate_sphere_inertia(m=mass, r=radius)
        elif selection == "3":
            mass = float(raw_input("mass>>"))
            radius = float(raw_input("radius>>"))
            height = float(raw_input("height>>"))
            self.calculate_cylinder_inertia(m=mass, r=radius, h=height)
        elif selection.upper() == "Q": # accept 'q' as well
            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, need to define which axes are which for ixx, iyy, and izz'
        print ('Iw="' + str(Iw) + '" Id="'+ str(Id) + '" Ih="' +
            str(Ih) + '" ixy="0.0" ixz="0.0" iyz="0.0"')

    def calculate_sphere_inertia(self, m, r):
        I = (2*m*pow(r,2))/5.0
        print ('SPHERE ixx="' + str(I) + '" ixy="0.0" ixz="0.0" iyy="' 
                + str(I) + '" iyz="0.0" izz="' + 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 ixx="' + str(Ix) + '" ixy="0.0" ixz="0.0" iyy="' 
                + str(Iy) + '" iyz="0.0" izz="' + str(Iz) + '"')

if __name__ == "__main__":
    inertial_object = InertialCalculator()
    inertial_object.start_ask_loop()
1 Like
2

Hi,

Thanks a lot for the contribution!
Love any contribution the community makes :wink:

3

That’s great. Happy to help.
I have tweaked it further to keep the string formatting consistent by clarifying the axes for a box that need to be entered. The user will have to figure this out eventually, and doing it up front pays dividends. This keeps it easy to just copy and paste.
/K

urdf_inertial_calculator.py

#! /usr/bin/env python

import math

class InertialCalculator(object):

    def __init__(self):
          print "URDF Inertial Calculator Initialised..."

    def start_ask_loop(self):

        selection = "START"

        while selection.upper() != "Q":     # accept 'q' as well
            print "#############################"
            print "Select Geometry to Calculate:"
            print "[1]Box width(x-axis)*depth(y-axis)*height(z-axis)"
            print "[2]Sphere radius(r)"
            print "[3]Cylinder radius(r)*height(h)"
            print "[Q]END program"
            selection = raw_input(">>")
            self.select_action(selection)

        print "URDF Inertial Calculator Quit...Thank you"

    def select_action(self, selection):
        if selection == "1":
            mass = float(raw_input("mass>>"))
            width = float(raw_input("x-axis length>>"))
            depth = float(raw_input("y-axis length>>"))
            height = float(raw_input("z-axis length>>"))
            self.calculate_box_inertia(m=mass, w=width, d=depth, h=height)
        elif selection == "2":
            mass = float(raw_input("mass>>"))
            radius = float(raw_input("radius>>"))
            self.calculate_sphere_inertia(m=mass, r=radius)
        elif selection == "3":
            mass = float(raw_input("mass>>"))
            radius = float(raw_input("radius>>"))
            height = float(raw_input("height>>"))
            self.calculate_cylinder_inertia(m=mass, r=radius, h=height)
        elif selection.upper() == "Q": # accept 'q' as well
            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 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 ixx="' + str(I) + '" ixy="0.0" ixz="0.0" iyy="' 
                + str(I) + '" iyz="0.0" izz="' + 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 ixx="' + str(Ix) + '" ixy="0.0" ixz="0.0" iyy="' 
                + str(Iy) + '" iyz="0.0" izz="' + str(Iz) + '"')

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