Preparare il file VRML per la simulazione

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Questo tutorial spiega come utilizzare FreeCAD e l'ambiente Simulazione Robot per simulare i movimenti dei robot a 6 assi.
Il tutorial si concentra sulla creazione del file VRML utilizzato per la visualizzazione.
La base del file VRML è un modello di FreeCAD.
La versione di FreeCAD utilizzata è la 0.11.4252ppa1 in Ubuntu 32 bit.

Aprire un file o crearne uno con FreeCAD

Il tutorial si basa su un file STEP di un robot Stäubli TX40 (TX40-HB.stp).
È possibile scaricare il file file TX40-HB.stp da Stäubli.
Anche se non ho ancora avuto tempo di controllare, il metodo dovrebbe valere anche per un modello realizzato completamente in FreeCAD.
Dopo aver aperto il file, si dovrebbe ottenere questo:

Notare che, nell'importazione, il robot è composto di 8 forme, direttamente sulla radice dell'albero del documento.
La struttura del file VRML esportato può cambiare se sono utilizzati i gruppi. Le forme sono ordinate a partire dalla base allo strumento.
L'ultima forma contiene gli assi di rotazione di tutti gli assi del robot.
I nomi delle forme sono attribuiti in modo correlativo da FreeCAD, (dato che per ora (marzo 2011) FreeCAD non importa i nomi inclusi nei file STEP):

nome FreeCAD nome STEP
TX40_HB HORIZONTAL BASE CABLE OUTLET
TX40_HB001 SHOULDER
TX40_HB002 ARM
TX40_HB003 ELBOW
TX40_HB004 FOREARM
TX40_HB005 WRIST
TX40_HB006 TOOL FLANGE
TX40_HB007 ?

In questa importazione, cambiare la modalità di visualizzazione, “Display Mode”, di ogni forma, ad eccezione di TX40_HB007, da "Flat Lines" a "Shaded" per dare un buon aspetto all'esportazione VRML. Ho anche cambiato i colori in [245, 196, 0] e [204, 204, 204] per farli corrispondere meglio al giallo di Stäubli.
Nascondere TX40_HB007 perché contiene gli assi di tutti i giunti e non può essere smontato.

Measure geometric characteristics

In order to build the Denavit-Hartenberg table (see 6-Axis Robot) and prepare the vrml file, you need to get characteristics of the robot. For now, the measurement tool of FreeCAD is not ready, you can use the axes included in TX40_HB007 (the co-ordinates are indicated on the bottom left when you point an object with the mouse) or you have to use the Python console to get some information about the geometry. Note that the DH-table is only required if you need to use the inverse kinematics, i.e. get the Cartesian coordinates or drive the robot with Cartesian coordinates. The DH-table for this robot is the following (mm, deg and deg/s):

i d θ r α θmin θmax Axis velocity
1 320 q1 0 -90 -180 180 555
2 35 q2 - 90 225 0 -125 125 475
3 0 q3 + 90 0 90 -138 138 585
4 225 q4 0 -90 -270 270 1035
5 0 q5 0 90 -120 133.5 1135
6 65 q6 0 0 -270 270 1575

The csv file is then:

a  , alpha, d  , theta, rotDir, maxAngle, minAngle, AxisVelocity
0  ,   -90, 320,     0,      1,      180,     -180, 555
225,     0,  35,   -90,      1,      125,     -125, 475
0  ,    90,   0,    90,      1,      138,     -138, 585
0  ,   -90, 225,     0,      1,      270,     -270, 1035
0  ,    90,   0,     0,      1,    133.5,     -120, 1135
0  ,     0,  65,     0,      1,      270,     -270, 1575

Export to vrml

Export the document to a vrml file. The structure of the vrml file is the following:

 #VRML V2.0 utf8
 
 
 Group {
   children 
     Group {
       children [ 
         Group {
         …
 
         },
           
         Group {
         …
 
         },
           
         Group {
         …
 
         },
           
         Group {
         …
 
         },
           
         Group {
         …
 
         },
           
         Group {
         …
 
         },
           
         Group {
         …
 
         },
           
         Group {
         …
 
         } ]
 
     }
 
 }

You can notice that we have 8 independent groups corresponding to the 8 shapes.

Preparation of the vrml file

All shapes in the vrml file are expressed in the base frame, independently from each other. For the Robot Simulation Workbench, we need to create a structure where a movement of a shape induces a movement of all shapes situated afterwards in the structure. The placement of the shapes will be relative to the preceding shape, so we need to include some translations from the absolute reference system to the relative one. The translations are described in the following picture:

With

A=(0, 0, 168)
B=(0, 107.8, 320)
C=(0, 104.15, 545)
D=(0, 35, 601)
E=(0, 35, 770)
F=(0, 35, 835).

Let's take the example of axis 4 between ELBOW and FOREARM, situated at D=(xd, yd, zd). The anchor for the FreeCAD axis is

 "DEF FREECAD_AXIS4 Transform { rotation 0 1 0 0 children ["

This corresponds to a rotation about the y-axis. In the CAD model, the rotation is about the z-axis. Thus, we need to a rotation about the x-axis of before the FreeCAD axis definition and of after it. Also, a translation of (-xd, -yd, -zd) is needed just before the Group corresponding to the definition of FOREARM to express it in the relative reference frame centered at D. This means that a translation of (xd, yd, zd) must be inserted before the first rotation. At the end, the vrml-file from the definition of ELBOW to the definition of FOREARM looks like this:

       # ELBOW
       Group {
         … here comes the unmodified definition of ELBOW
   
       },
         
       Transform {
         translation 0 35 601
         rotation 1 0 0 1.5707963
         children
           DEF FREECAD_AXIS4 Transform { rotation 0 1 0 0 children
             Transform {
               rotation 1 0 0 -1.5707963
               children
                 Transform {
                   translation 0 -35 -601
                   children [
       # FOREARM  
       Group {
         ... here comes the unmodified definition of FOREARM
   
       },

At the end of the document, the appropriate closing brackets must be inserted: “ ]}}}},” for each of the 6 axes. Eventually, the document looks like this (I don't know if I can link the file here because of copyrights):

 #VRML V2.0 utf8
   
   
 Group {
   children
   Group {
     children [ 
       # HORIZONTAL BASE CABLE OUTLET 
       Group {
           ... here comes the unmodified definition of HORIZONTAL BASE CABLE OUTLET
    
       },
         
       Transform {
         translation 0 0 168
         rotation 1 0 0 1.5707963
         children
           DEF FREECAD_AXIS1 Transform { rotation 0 1 0 0 children
             Transform {
               rotation 1 0 0 -1.5707963
               children
                 Transform {
                   translation 0 0 -168
                   children [
       # SHOULDER
       Group {
           ... here comes the unmodified definition of SHOULDER 
   
       },
         
       Transform {
         translation 0 107.8 320
         #rotation 0 0 1 0
         children
           DEF FREECAD_AXIS2 Transform { rotation 0 1 0 0 children
             Transform {
               #rotation 0 0 1 0
               children
                 Transform {
                   translation 0 -107.8 -320
                   children [
       # ARM  
       Group {
           ... here comes the unmodified definition of ARM 
   
       },
         
       Transform {
         translation 0 104.15 545
         #rotation 0 0 1 0
         children
           DEF FREECAD_AXIS3 Transform { rotation 0 1 0 0 children
             Transform {
               #rotation 0 0 1 0
               children
                 Transform {
                   translation 0 -104.15 -545
                   children [
       # ELBOW
       Group {
           ... here comes the unmodified definition of ELBOW
   
       },
         
       Transform {
         translation 0 35 601
         rotation 1 0 0 1.5707963
         children
           DEF FREECAD_AXIS4 Transform { rotation 0 1 0 0 children
             Transform {
               rotation 1 0 0 -1.5707963
               children
                 Transform {
                   translation 0 -35 -601
                   children [
       # FOREARM  
       Group {
           ... here comes the unmodified definition of FOREARM
   
       },
         
       Transform {
         translation 0 35 770
         #rotation 0 0 1 0
         children
           DEF FREECAD_AXIS5 Transform { rotation 0 1 0 0 children
             Transform {
               #rotation 0 0 1 0
               children
                 Transform {
                   translation 0 -35 -770
                   children [
       # WRIST
       Group {
           ... here comes the unmodified definition of WRIST
   
       },
         
       Transform {
         translation 0 35 835
         rotation 1 0 0 1.5707963
         children
           DEF FREECAD_AXIS6 Transform { rotation 0 1 0 0 children
             Transform {
               rotation 1 0 0 -1.5707963
               children
                 Transform {
                   translation 0 -35 -835
                   children [
       # TOOL FLANGE
       Group {
           ... here comes the unmodified definition of TOOL FRAME
   
       },
         
       Group {
           ... here comes the unmodified definition of TX40_HB007
   
       } # "]" was deleted from this line
     ]}}}},
     ]}}}},
     ]}}}},
     ]}}}},
     ]}}}},
     ]}}}},
     ] # this is the "]" deleted from the line above
   }
 }

Here is a patch to obtain the vrml file suitable for robot simulation:

 7a8
 >         # HORIZONTAL BASE CABLE OUTLET 
 95968a95970,95981
 >         Transform {
 >           translation 0 0 168
 >           rotation 1 0 0 1.5707963
 >           children
 >             DEF FREECAD_AXIS1 Transform { rotation 0 1 0 0 children
 >               Transform {
 >                 rotation 1 0 0 -1.5707963
 >                 children
 >                   Transform {
 >                     translation 0 0 -168
 >                     children [
 >         # SHOULDER
 128428a128442,128453
 >         Transform {
 >           translation 0 107.8 320
 >           #rotation 0 0 1 0
 >           children
 >             DEF FREECAD_AXIS2 Transform { rotation 0 1 0 0 children
 >               Transform {
 >                 #rotation 0 0 1 0
 >                 children
 >                   Transform {
 >                     translation 0 -107.8 -320
 >                     children [
 >         # ARM  
 206503a206529,206540
 >         Transform {
 >           translation 0 104.15 545
 >           #rotation 0 0 1 0
 >           children
 >             DEF FREECAD_AXIS3 Transform { rotation 0 1 0 0 children
 >               Transform {
 >                 #rotation 0 0 1 0
 >                 children
 >                   Transform {
 >                     translation 0 -104.15 -545
 >                     children [
 >         # ELBOW
 267111a267149,267160
 >         Transform {
 >           translation 0 35 601
 >           rotation 1 0 0 1.5707963
 >           children
 >             DEF FREECAD_AXIS4 Transform { rotation 0 1 0 0 children
 >               Transform {
 >                 rotation 1 0 0 -1.5707963
 >                 children
 >                   Transform {
 >                     translation 0 -35 -601
 >                     children [
 >         # FOREARM  
 417854a417904,417915
 >         Transform {
 >           translation 0 35 770
 >           #rotation 0 0 1 0
 >           children
 >             DEF FREECAD_AXIS5 Transform { rotation 0 1 0 0 children
 >               Transform {
 >                 #rotation 0 0 1 0
 >                 children
 >                   Transform {
 >                     translation 0 -35 -770
 >                     children [
 >         # WRIST
 422053a422115,422126
 >         Transform {
 >           translation 0 35 835
 >           rotation 1 0 0 1.5707963
 >           children
 >             DEF FREECAD_AXIS6 Transform { rotation 0 1 0 0 children
 >               Transform {
 >                 rotation 1 0 0 -1.5707963
 >                 children
 >                   Transform {
 >                     translation 0 -35 -835
 >                     children [
 >         # TOOL FLANGE
 435627c435700,435707
 <         } ]
 ---
 >         } 
 >       ]}}}},
 >       ]}}}},
 >       ]}}}},
 >       ]}}}},
 >       ]}}}},
 >       ]}}}},
 >       ]