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Hi, 

I’m working on a tethered ROV and need to study the influence of the cable on the ROV. I need to assess the forces vector and torques vector between the cable and the attachment point.

On previous exchanges with Vortex support I learned that I can access the control force and some equations in the constraints of the cable. 

I start to access all the constraints from universe :

constraints = self.universe.constraints

 Then I test if they are valid, if they're linked to the cable I want to study and I select the one that is attached to the ROV:  

    for c in constraints:
       if not len(dir(c)): 
       if c.enable and self.cable_droit_name in c.name:
          if c.type == cylindrical.type: 
             if "Attachment_FirstExtremity" in c.name:

Then, I access the forces and torques: 

c.coordinates[0].currentStateControlForce
c.equations[0].lastForce
c.equations[1].lastForce
c.coordinates[1].currentStateControlForce
c.equations[2].lastForce
c.equations[3].lastForce

 My problem is that I don't see along/around which axis those forces/torques are defined.

To try to understand how those forces are defined I designed a small example by hanging a mass to a cable attached to a fixed part. This fixed part is either vertical (configuration 1), turned of 45° around y axis (configuration 2) or of 90° around y axis (configuration 3). The cable fixation to the part is a flexible attachment with the attachment direction in the same direction as the part. 

I started to measure de forces and torques in those two configurations with Matlab Simulink to compare the results with Vortex. In Simulink, the forces are expressed in the 

part reference frame (xa,ya,za).

For the configuration 1:

In Simulink, I measure: Forces = [0;0;-1.095] Torques = [0;0;0], which makes sense.

In Vortex, I measure : 

c.coordinates[0].currentStateControlForce = 1,095

c.equations[0].lastForce = 0

c.equations[1].lastForce = 0

c.coordinates[1].currentStateControlForce = 0

c. equations[2].lastForce = 0

c.equations[3].lastForce = 0

Which would show that the c.coordinates[0].currentStateControlForce is along the z axis wich is the same for the global frame (z0) and the part frame (za) in this configuration. 

For the configuration 2:

In Simulink, I measure: Forces=[0.7745;0;-0.7746] Torques=[0;-0.01372;0]

In Vortex, I measure : 

c.coordinates[0].currentStateControlForce = 1,077

c.equations[0].lastForce = 0

c.equations[1].lastForce = 0,198

c.coordinates[1].currentStateControlForce = 0

c. equations[2].lastForce = 0

c.equations[3].lastForce = -0,014

There, the c.coordinates[0].currentStateControlForce does not seem to be along the z axis of the part nor of the global frame.  

The c.equations[3].lastForce seems to correspond to the torque around the y axis. 

For the configuration 3:

In Simulink, I measure: Forces=[1.095;0;0] Torques=[0;-0.02732;0]

In Vortex, I measure : 

c.coordinates[0].currentStateControlForce = 1,044

c.equations[0].lastForce = 0

c.equations[1].lastForce = 0,332

c.coordinates[1].currentStateControlForce = 0

c.equations[2].lastForce = 0

c.equations[3].lastForce = -0,027

This makes me think that the c.equations[0].lastForce would be along the y axis. But I don't know for the others. And it would confirm that the c.equations[3].lastForce may be around the y axis. 

To see if the forces depend on the global reference frame or not, I tried to turn the assemblies aroung the z axis. This is the result when I turn the configuration 2 of 90° around the z axis:

c.coordinates[0].currentStateControlForce = 1,077

c.equations[0].lastForce = 0

c.equations[1].lastForce = 0,198

c.coordinates[1].currentStateControlForce = 0

c. equations[2].lastForce = 0

c.equations[3].lastForce = 0,014

The forces and torques are the same as with the not turned configuration 2, which makes me think that they are not expressed in the global frame, except for the c.equations[3].lastForce = 0,014 whose sign is reversed. I struggle to understand why. 

If I activate the view of the constraints in the cable, I can see the orientation of the attachment constraint which is not in the same direction as the part. Maybe the forces are expressed in this frame ? But then is there a way to know the exact orientation of this frame according to the part's frame or of the universe frame ? This is the only way I could analyze those values.  

If I directly access the forces applied to the ROV, I would have all the forces (like thrusters, drag, buoyancy,...) and not only the forces from the cable.

Thanks in advance for your help. 
Regards,

Ornella

Just a little correction for the second par of the script, a line was missing : 

    for c in constraints:
       if not len(dir(c)): 
          continue
       if c.enable and self.cable_droit_name in c.name:
          if c.type == cylindrical.type:
             if "Attachment_FirstExtremity" in c.name: 

I thought I found the solution but it's not working when I turn the assemblies around the z axis. 

It seems that the forces are expressed in the "part0" of the constraint, which is the first element of the cable linked to the fixed part. 

In this frame, it seems that :

c.coordinates[0].currentStateControlForce     is along z axis as on conf1 its value was the weight of the system

c.equations[0].lastForce                                     is along y axis as its value was always 0

c.equations[1].lastForce                                     is along x axis

c.coordinates[1].currentStateControlForce     is along z axis

c. equations[2].lastForce = 0                             is along x axis

c.equations[3].lastForce = 0                              is along y axis as its value was not 0 for conf 2 and conf 3

I could access the transform matrix of part0 and apply its rotation matrix on those vectors, which gave me the forces and torques in the world frame. 

I then applied the inverse of the rotation matrix of the fixed part to those vectors to obtain the forces and torques in the fixed part reference frame. 

It seemed to work except for an inverted sign on z forces and y torques. 

I then tried to turn my assemblies around the z axis to see if this solution was robust, but it wasn't at all and the obtained values didn't make sense at all.

This is the code I did for those transforms:

                part0 = c.attachments[0].part
                part1 = c.attachments[1].part 

                transform_constraint = part0.transform
                rotation_matrix_constraint = np.zeros((3,3))
                for i in range(3):
                    for j in range (3):
                        rotation_matrix_constraint[i,j] = (transform_constraint[i])[j]

                transform_attache = part1.transform
                rotation_matrix_attache = np.zeros((3,3))
                for i in range(3):
                    for j in range (3):
                        rotation_matrix_attache[i,j] = (transform_attache[i])[j]
                rotation_matrix_attache_inv = np.linalg.inv(rotation_matrix_attache)

                Forces = VxSim.VxVector3(0,0,0)
                Forces[2] = c.coordinates[0].currentStateControlForce
                Forces[1] = c.equations[0].lastForce
                Forces[0] = c.equations[1].lastForce
                # Transform to world frame:
                Forces = Array_to_vector3((np.dot(rotation_matrix_constraint,Vector3_to_array(Forces))).T)
                # Transform to fixed part frame:
                Forces = Array_to_vector3((np.dot(rotation_matrix_attache_inv,Vector3_to_array(Forces))).T)
                self.outputs.Forces.value = VxVector3(Forces)

                Moments = VxSim.VxVector3(0,0,0)
                Moments[2] = c.coordinates[1].currentStateControlForce
                Moments[0] = c.equations[2].lastForce
                Moments[1] = c.equations[3].lastForce
                # Transform to world frame:
                Moments = Array_to_vector3((np.dot(rotation_matrix_constraint,Vector3_to_array(Moments))).T)
                # Transform to fixed part frame:
                Moments = Array_to_vector3((np.dot(rotation_matrix_attache_inv,Vector3_to_array(Moments))).T)
                self.outputs.Moments.value = VxVector3(Moments)

Hi Rémi, 

Thanks for your answer. 
So, if I understand correctly, the forces have their own reference frame which is attached to the reference frame of the part0 of the constraint but they don't necessarily have the same orientation ? 

And there's no way to access the orientation of the forces reference frame according to the world during simulation ?

But if I start the simulation in a specific configuration (cable vertical or horizontal) I can know the initial orientation of the forces reference frame, as long as the transform matrix of the part so I can determine the transform between the forces and the part, which should then stay the same during the whole simulation ? 

If not all the cable is horizontal but the portion I'm interested in is horizontal at the begining of the simulation (as in the image below), can I consider that the secondary axis will start along the z+ world axis  as you stated ? 

The only thing I can be sure of is that the currentStateControlForce is along the axis of the cable ? I thought it was the same if I use a grabbing tool to measure the tension in the cable at a specific point. But I compared the tension of a grabbing tool placed at the attachment point with the currentStateControlForce for all my configurations and their values are slightly different. Plus, it seems that the grabbing tool works only in traction and not compression (the fourth configuration is with the system reversed : mass on top, fixed point below) as its value is 0 for the last configuration. I also notice that force in the reversed configuration is opposed to the force in the first configuration. Even if they should both be in -Z direction. Is it because the second one is in compression or just because the cable direction (from mass to fixed part) is reversed in the last configuration ?  

Regards, 

Ornella

Hi Daniel, 

Thanks for your answer. 
So, if I understand you correctly, the forces of a constraint are defined along the axis of the part0 of this constraint ? Meaning that for example, the constraint.coordinates[0].currentStateControlForce would be along the u0 axis shown in the drawing of your first link ? Because I can access the transform matrix of the part0 and I tried to apply it on the forces values but the result didn't seem correct, as stated in my 3rd message here. 

Yes, exactly, I'm interested in the forces applied by the cable to the ROV at attachment point. 

I guess it would be a viable solution. So I would add a part, fix it with and RPRO constraint to the ROV on one side and attach the cable to this part with a flexible attachment point and then observe the outputs of the RPRO constraint ? 

Cheers,

Ornella

Hi Daniel, 

I tried with the RPRO constraint, it seems to work perfectly, and this solution is much easier than to access the cable internal properties. 

Thank you very much. 

Cheers, 
Ornella