How to control cable length in cable-driven manipulator

[Diogo]

Hello everyone

I’m trying to simulate a flexible manipulator that consists of a cilindrical tube actuated by 3 equidistant cables, passing through channels from bottom to top. So far, I’ve used the SofaPythonController setup that is very similar the CableGripper tutorial included in the plugin documentation. I’ve successfully implemented the 3 cables, but I cannot seem to be able to control the cable length directly. The tutorial is setup for “displacement”, but if I add one unit to each of the cables, the vertical coordinate of a point on top of the manipulator does not change by one centimeter, it changes by around 0.9 cm (the unit I’m using).

My question is how is the displacement measured, and if there is a way to just work with cable length, since that would make validating the results from theoritical models a little easier.

As I said, my setup is essentially the same as one finger in the CableGripper, I just added two other cables, but if my code is required then I’d be more than happy to share.

Thank you in advance for the amazing software and plugin, and I’m fully available for any further questions!

Windows 10/Sofa v20.12 (binary with SoftRobots pre-installed)

[Arya]

Hello @c4rs200, I have worked on a very similar bot with 3 cable actuators. I would love to see your code to solve your problem better, I assume you are manipulating the length of the cable actuator by changing the CableConstraint value. You can consider CableConstraint as the value of your Cable length since the value of CableConstraint is basically derived from it being a mechanical object with indices=range(len(cableGeometry)).

Note that all changes to be made to your CableConstraint.value need to be made in a function within the Sofa.PythonScriptController for them to reflect in your simulation dynamically.

Cheers!

[Diogo]

Hello @aryakarani

Thank you so much for your answer!! This is my current code:

# -*- coding: utf-8 -*-
from stlib.physics.deformable import ElasticMaterialObject
from stlib.physics.constraints import FixedBox
from stlib.scene import Node
from softrobots.actuators import PullingCable
from splib.loaders import loadPointListFromFile
import Sofa
import os

#Opening the file
file1 = open("myfile.txt","w")

#Defining the python controller
class CilinderController(Sofa.PythonScriptController):
    def __init__(self, node, cable1, cable2, cable3):
        self.node=node
        self.cableconstraintvalue1 = cable1.getObject("CableConstraint").findData('value')
        self.cableconstraintvalue2 = cable2.getObject("CableConstraint").findData('value')
        self.cableconstraintvalue3 = cable3.getObject("CableConstraint").findData('value')
        self.cableconstraintdisp1 = cable1.getObject("CableConstraint").findData('displacement')
        self.cableconstraintdisp2 = cable2.getObject("CableConstraint").findData('displacement')
        self.cableconstraintdisp3 = cable3.getObject("CableConstraint").findData('displacement')
        self.name = "CilinderController"
        self.n=1
        self.deformableObject = self.node.ElasticMaterialObject
        
      
    #Control actions for each cable (extend and retract)    
    def onKeyPressed(self,c):
        if (c == "4"):
            self.cableconstraintvalue1.value =  self.cableconstraintvalue1.value[0][0] + 1.
        if (c == "1"):
            self.cableconstraintvalue1.value =  self.cableconstraintvalue1.value[0][0] - 1.
        if (c == "5"):
            self.cableconstraintvalue2.value =  self.cableconstraintvalue2.value[0][0] + 1.
        if (c == "2"):
            self.cableconstraintvalue2.value =  self.cableconstraintvalue2.value[0][0] - 1.
        if (c == "6"):
            self.cableconstraintvalue3.value =  self.cableconstraintvalue3.value[0][0] + 1.
        if (c == "3"):
            self.cableconstraintvalue3.value =  self.cableconstraintvalue3.value[0][0] - 1.    
        if (c == "9"):
            #file1.write(str(Output1))
            self.node.getRootContext().animate = False
            file1.close() 

    #Writing the displacements and point position to a file        
    def onEndAnimationStep(self,dt):
        # displacement1  = self.cableconstraintdisp1.value
        # displacement2  = self.cableconstraintdisp2.value
        # displacement3  = self.cableconstraintdisp3.value
        displacement1  = self.cableconstraintvalue1.value
        displacement2  = self.cableconstraintvalue2.value
        displacement3  = self.cableconstraintvalue3.value
        # dispform1 = "{:.2f}".format(displacement1)
        # dispform2 = "{:.2f}".format(displacement2)
        # dispform3 = "{:.2f}".format(displacement3)
        dispform1 = displacement1
        dispform2 = displacement2
        dispform3 = displacement3
        position=self.deformableObject.dofs.getData("position").value
        positionform1="{:.2f}".format(float(position[493][0]))
        positionform2="{:.2f}".format(float(position[493][1]))
        positionform3="{:.2f}".format(float(position[493][2]))
        file1.write(str(self.n) + ',' + str(dispform1) + ","+ str(dispform2) + "," + str(dispform3) + ", ["+ str(positionform1) + ";" + str(positionform2) + ";" + str(positionform3)+"]"+'\n')
        self.n +=1         
            

def Cilinder(parentNode=None, name="Cilinder",
           rotation=[0.0, 0.0, 0.0], translation=[0.0, 0.0, 0.0],
           fixingBox=[0.0,0.0,0.0], pullPointLocation=[0.0,0.0,0.0]):

    #Defining the elastic material
    cilinder = Node(parentNode, "Cilinder")
    eobject = ElasticMaterialObject(cilinder,
                                   volumeMeshFileName="data/mesh/Elo3D.vtk",
                                   poissonRatio=0.49,
                                   youngModulus=48000000,
                                   totalMass=1.5,
                                   surfaceColor=[0.0, 0.8, 0.7],
                                   surfaceMeshFileName="data/mesh/EloSurface.stl")
    FixedBox(eobject,
             doVisualization=True,
             atPositions=[-30, 0 ,-30 ,30 ,5,30])

    #Creating the cables
    cable1 = PullingCable(eobject, cableGeometry=loadPointListFromFile("data/mesh/cable1S.json"))
    cable2 = PullingCable(eobject, cableGeometry=loadPointListFromFile("data/mesh/cable2S.json"))
    cable3 = PullingCable(eobject, cableGeometry=loadPointListFromFile("data/mesh/cable3S.json"))
    
    #Calling the controller
    CilinderController(cilinder,cable1,cable2,cable3) 

This is the relevant part, the rest is just defining the scene. As you can see, my goal is to get the cable lengths and the position of a point to a file. What happens right now is that the displacements show up as zero when the robot is at rest (the cable length at rest is meant to be 84.63 cm) and when I give inputs, it changes to “1” or “-1” etc, according to the given input.

There are other ways to define the cables, maybe that’s the issue? The cable geometry files are just a list of points through where the cables pass in the channels inside the robot

[Hugo]

Any success @c4rs200, following @aryakarani advices?

I must say, these constraints being part of the SoftRobot plugin, I am not super familiar with it. But keep us udpated 😉

Hugo

[Diogo]

Hi @hugo

I had to put the work on hold because of exams, but I’ve now returned and I’m still trying to figure things. I think me and @aryakarani have different implementations of the CableConstraint, since I think I used an old one that came with an older version of the SoftRobots plugin that is not the same as newer documentation shows, so I’m working around that, but thank you for checking in! I will update the thread if I am sucessful in order to help other people with the same problem

Best wishes,

Diogo

[Hugo]

Alright @c4rs200

Any update here? Let us know about your progress!
Best wishes,

Hugo

[Author]

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