Parametric optimization with volume goal

[CharlesM]

Dear FreeCAD community,


I am trying to conduct volume driven parametric optimization using FreeCAD and Python. I am experiencing some problems, could you help me ?


Let's say we have a cube of side length = L , initially equal to 1. How could I create a python code that would :

1) Analyze the volume of the cube
2) Modify the driving parameter "L"
3) Stop when the goal volume is reached


I tried two things without getting the result I wanted :

1) Externally, with the following piece of code :

Code: Select all

import sys 
FREECADPATH = r'PATH\TO\FREECAD'
sys.path.append(FREECADPATH)

import FreeCAD, FreeCADGui as Gui, Part, PartDesign 
FreeCAD.openDocument(r'PATH\TO\DOCUMENT',True)

goal = FreeCAD.ActiveDocument.getObjectsByLabel("Body")[0]
vol = goal.Shape.Volume

obj = 100    #Goal volume

print(vol)
print(obj)

a = 1
if vol < obj : 
    while vol - obj < 5: 
        doc.getObject('Sketch').setDatum(9,App.Units.Quantity(a))
        a = a+1
        print(vol)      
else:  
    while vol - obj > 5:
        doc.getObject('Sketch').setDatum(9,App.Units.Quantity(a))
        a = a-1
        print(vol)

( I am quite new to FreeCAD, and I am unsure about wheter or not this is correct way (setDatum) to do what I want)



2) Inside the python console :

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body=FreeCAD.ActiveDocument.getObjectsByLabel("Body")[0]
>>> vol = body.Shape.Volume
>>> vol
0.9999999999999998
>>> # Gui.Selection.addSelection('learning','Body','Pad.Sketch.')
>>> # Gui.Selection.clearSelection()
>>> vol
0.9999999999999998
>>> body.recompute
<built-in method recompute of PartDesign.Body object at 0x00000265CF6F3C18>
>>> vol
0.9999999999999998

In both case, I was not able to change the volume, even though the dimensions of the cube changed in the user interface.

1mmvol.png (46.53 KiB) Viewed 542 times

100mmvol.png (52.02 KiB) Viewed 542 times

Code: Select all

OS: Windows 10 Version 2009
Word size of FreeCAD: 64-bit
Version: 0.20.2.29177 +426 (Git)
Build type: Release
Branch: (HEAD detached from 0.20.2)
Hash: 930dd9a76203a3260b1e6256c70c1c3cad8c5cb8
Python 3.8.10, Qt 5.15.2, Coin 4.0.1, Vtk 8.2.0, OCC 7.6.3
Locale: French/France (fr_FR)

[openBrain]

2) Inside the python console :

Code: Select all

body=FreeCAD.ActiveDocument.getObjectsByLabel("Body")[0]
>>> vol = body.Shape.Volume
>>> vol
0.9999999999999998
>>> # Gui.Selection.addSelection('learning','Body','Pad.Sketch.')
>>> # Gui.Selection.clearSelection()
>>> vol
0.9999999999999998
>>> body.recompute
<built-in method recompute of PartDesign.Body object at 0x00000265CF6F3C18>
>>> vol
0.9999999999999998

2 mistakes here.

* to recompute, call

Code: Select all

body.recompute

with the parentheses

* you can't use an intermediate variable as it will store the value but loose the link. You have to evaluate

Code: Select all

body.Shape.Volume

after each modification

[CharlesM]

Dear openBrain,


Thank you for your answer.


The following code works :

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goal = 1000000
FreeCAD.ActiveDocument.Sketch.setDatum('L',1)
cont = 1 
while True : 
    FreeCAD.ActiveDocument.Sketch.setDatum('L',cont)
    FreeCAD.ActiveDocument.recompute()
    print(FreeCAD.ActiveDocument.Body.Shape.Volume)
    cont = cont+1
    if goal - FreeCAD.ActiveDocument.Body.Shape.Volume  <= 1: 
        break
print(FreeCAD.ActiveDocument.Body.Shape.Volume)

[edwilliams16]

You method is very slow and not robust. One simple root finder you could implement is the bisection method. https://en.wikipedia.org/wiki/Bisection_method
You start with two values of your datum, one too large, one too small and successively reduce the interval in which the correct value lies. Stop when you have reached the desired accuracy.