[SOLVED] Cable-Driven Actuation

Hi SOFA forum,

I have been trying to implement a cable-driven actuation mechanism to my soft gripper. I followed roughly the same process as the CableGripper tutorial available for the SoftRobots plugin. I have debugged my code and every thing seems to be running okay (no obvious errors or warnings). However, when I try to interactively move my gripper down to grasp a body, the only thing that moves is my “pullPoint”, which is acting as the common origin to all my gripper’s fingers. I checked everything again but can’t seem to find where the problem might be.

Just a note:
– I have a U-shaped ‘channel’ running through the inner-face of my gripper finger.
This is where I will install pins around which the cable can slide, and so right
now, the cable is not actually penetrating any material in the model (Could
this be the problem?)

I am not sure how much of my code to post so I’ll organise it here below.

The file containing the bodies included in the scene as well as the controller:

# -*- coding: utf-8 -*-
# Script by: Sara Hijazi in fulfillment of the requirements of MSc Robotics @UoB, 2020
# Code extracts and Templates by SOFA used below are fully and correctly  referenced in the text accompanying these files

# Importing libraries
import Sofa
from stlib.physics.rigid import Floor
from stlib.scene import Node, get
# Utilising SOFA templates to model cable
from softrobots.actuators import PullingCable
from splib.loaders import loadPointListFromFile

# Some global variables
#trans_finger=[0, 30, 0]
#trans_hemi=[0, 0, 0]
#rot_finger=[-90, 0, 0]
#rot_hemi=[0, 0, 0]
scale3d_finger=[0.125, 0.125, 0.125]
scale3d_hemi=[1.0, 1.0, 1.0]

# A python class created by SOFA is inherited from, redefining interaction keys to implement a 'controller' script--> "PythonScriptController.h"
# SOFA Documentation is further used, supplying info on available functionality and options
class FingerController(Sofa.PythonScriptController):
    def __init__(self, node, cable):
        inputvalue =cable.getObject("CableConstraint").findData('value')
        self.name = "FingerController"

    # Inheriting onKeyPressed method to redefine interactive keys and create new behaviour
    # Function recovers user interaction via keyboard as input for body manipulation in scene
    def onKeyPressed(self,c):
        if (c == "+"):
            inputvalue.value =  inputvalue.value[0][0] + 1.

        elif (c == "-"):
            displacement = inputvalue.value[0][0] - 1.
            if(displacement < 0):
                displacement = 0
            inputvalue.value = displacement

# Defining a function for gripper finger
# Add fixing box
# Add in routing points to function attributes to implement tendon drive
def Finger(parentNode=None, name="Finger", rotation=[0.0, 0.0, 0.0],
    translation=[0.0, 0.0, 0.0], routingPoints=[0.0, 0.0, 0.0]):
    
    # Attach child to root node
    finger = parentNode.createChild(name)
    
    # Meshing and addition of mechanical model
    # Mesh is loaded using a MeshLoader GMSH component
    finger.createObject('MeshGmshLoader', name="mesh_loader_finger", filename='/Users/Sarah/Desktop/Thesis/Assemblies/Recents/finger_update_1_7_vol')
    # Mesh is then stored in a MeshContainer component linked to the loader through its source
    finger.createObject('Mesh', src='@mesh_loader_finger', name="finger_mesh_container")
    # MechanicalObject component created to store the DOFs and node positions of mesh
    finger.createObject('MechanicalObject', name='finger_tetras_obj', template='Vec3d',
        showObject='true', showObjectScale='1', translation=translation,
        rotation=rotation, scale3d=scale3d_finger)
    
    ## Implementing constitutive law of material and mass
    # Define material to be simulated by adding a ForceField component
        # This describes what internal forces are created when the object is deformed
        # Additionally, this will define how stiff or soft the material is as well as its behaviour
    finger.createObject('TetrahedronFEMForceField', template='Vec3d', name='FEM_finger',
        method='large', poissonRatio='0.48', youngModulus='20000000')
    # The vertexMass is defined with the uniformMass component which assumes a uniform and homogeneous distribution of node mass in the body
    finger.createObject('UniformMass', totalMass='0.03254')
    
    # Since dimensions are all in mm, rdefine gravity action on the scene in mm
    parentNode.findData('gravity').value = '0, -9810, 0';
    
    # Creating a tendon/cable object that will attach to the finger via the moveFinger function

    # This create a BarycentricMapping. A BarycentricMapping is a key element as it will create a bi-directional link
    # between the cable's DoFs and the finger's ones so that movements of the cable's DoFs will be mapped
    # to the finger and vice-versa;

    cable = PullingCable(finger,
            "PullingCable",
            pullPointLocation=routingPoints,
            rotation=rotation,
            translation=translation,
            cableGeometry=loadPointListFromFile("/Users/Sarah/Desktop/SOFA/SOFA_v19.06.99_custom_MacOS_v11/plugins/SoftRobots/my_projects/Testing/cableRouting.json"));

    cable.getObject("CableConstraint").findData('minForce').value = '0'
    
    # Instantiate function
    FingerController(finger, cable)
    
    return finger
        
        
# Defining a function for gripper finger
def Tissue(parentNode=None, name="breastTissue", rotation=[0.0, 0.0, 0.0],
    translation=[0.0, 0.0, 0.0], fixingBox=[0.0, 0.0, 0.0]):
    
    # Attach child to root node
    tissue = Node(parentNode, "breastTissue")
    
    # Meshing and addition of mechanical model
    # Mesh is loaded using a MeshLoader GMSH component
    tissue.createObject('MeshGmshLoader', name="mesh_loader_tissue", filename='/Users/Sarah/Desktop/Thesis/Assemblies/Recents/hemi_vol')
    # Mesh is then stored in a MeshContainer component linked to the loader through its source
    tissue.createObject('Mesh', src='@mesh_loader_tissue', name="tissue_mesh_container")
    # MechanicalObject component created to store the DOFs and node positions of mesh
    tissue.createObject('MechanicalObject', name='tissue_tetras_obj', template='Vec3d',
        showObject='true', showObjectScale='1', translation=translation,
        rotation=rotation, scale=scale3d_hemi)
    
    ## Implementing constitutive law of material and mass
    # Define material to be simulated by adding a ForceField component
        # This describes what internal forces are created when the object is deformed
        # Additionally, this will define how stiff or soft the material is as well as its behaviour
    tissue.createObject('TetrahedronFEMForceField', template='Vec3d', name='FEM_tissue',
        method='large', poissonRatio='0.495', youngModulus='27500')
    # The vertexMass is defined with the uniformMass component which assumes a uniform and homogeneous distribution of node mass in the body
    tissue.createObject('UniformMass', totalMass='0.4')
    
    # Since dimensions are all in mm, rdefine gravity action on the scene in mm
    parentNode.findData('gravity').value = '0, -9810, 0';
         
    return tissue

The file containing the gripper controller:

# -*- coding: utf-8 -*-
import Sofa

def getTranslated(points, vec):
    r=[]
    for v in points:
        r.append( [v[0]+vec[0], v[1]+vec[1], v[2]+vec[2]] )
    return r

class GripperController(Sofa.PythonScriptController):
    def __init__(self, node, fingers):
        self.fingers = fingers
        self.name = "FingerController"

    def onKeyPressed(self,c):
        dir = None
        # UP key :
        if ord(c)==19:
            dir = [0.0,1.0,0.0]
        # DOWN key : rear
        elif ord(c)==21:
            dir = [0.0,-1.0,0.0]
        # LEFT key : left
        elif ord(c)==18:
            dir = [1.0,0.0,0.0]
        elif ord(c)==20:
            dir = [-1.0,0.0,0.0]

        if dir != None:
            for finger in self.fingers:
                mechanicalObject = finger.getObject("finger_tetras_obj")
                mechanicalObject.findData('rest_position').value = getTranslated(mechanicalObject.rest_position, dir)

                cable = finger.getChild("PullingCable").getObject("CableConstraint")
                p = cable.pullPoint[0]
                cable.findData("pullPoint").value = [p[0]+dir[0], p[1]+dir[1], p[2]+dir[2]]

The file containing the assembled gripper and scene initiation:

# -*- coding: utf-8 -*-
# Script by Sara Hijazi in fulfillment of the requirements of MSc Robotics @UoB, 2020

import Sofa
import os
import sys
from stlib.scene import ContactHeader
from stlib.scene import Node, get
#from stlib.visuals import ShowGrid
from stlib.physics.rigid import Floor
from gripperComponents import Finger
from gripperComponents import Tissue
from moveGripper import GripperController

def Gripper(parentNode=None):
    # Create gripper common origin from which fingers attach
    gripper = Node(parentNode, "Gripper")
    
    # Attach fingers; orient, position and fix
    G_f1 = Finger(gripper, "Finger 1",
                    rotation=[-75, 60, 0],
                    translation=[-5.169, 30.0, -3.0],
                    routingPoints=[0, 45, 0])
    
    G_f2 = Finger(gripper, "Finger 2",
                    rotation=[-75, 180, 0],
                    translation=[0.0, 30.0, 6.0],
                    routingPoints=[0, 45, 0])
    
    G_f3 = Finger(gripper, "Finger 3",
                    rotation=[-75, -60, 0],
                    translation=[5.169, 30.0, -3.0],
                    routingPoints=[0, 45, 0])
    
    # Structure and set controller
    GripperController(gripper, fingers=[G_f1, G_f2, G_f3])
    
    return gripper
    
# Creating scene, initialising rootNode
def createScene(rootNode):
    ### Set-up ###
    # Required plugins
    rootNode.createObject('RequiredPlugin', name='soft_robots', pluginName='SoftRobots')
    rootNode.createObject('RequiredPlugin', name='sparse', pluginName='SofaSparseSolver')
    rootNode.createObject('RequiredPlugin', name='python', pluginName='SofaPython')
    # Toggle display options
    rootNode.createObject('VisualStyle', displayFlags='showForceFields showWireframe showBehaviorModels')
    
    # Add in contact header when collision models have been created
    
    # Attach children from root
    Tissue(parentNode=rootNode, name="breastTissue", rotation=[0.0, 0.0, 0.0],
    translation=[0.0, 0.0, 0.0]) # Add fixing box
    
    # Adding prefab floor to root
    floor = Floor(rootNode, name="Floor", translation=[0.0, 0.0, 0.0], uniformScale=0.25, isAStaticObject=True, color=[1.0, 0.3, 0.6])
    
    # Instantiate gripper and fingers
    Gripper(parentNode=rootNode)
    
    return rootNode

I am not sure as to what I could be missing or what has gone wrong so any pointers are helpful!

Thanks in advance,

Sara