Learning Forum

[yanivVK]

Hello,

I am trying to do a Rigid Body Dynamic analysis of a manipulator arm with two triangular shaped plates, a spherical joint connector and springs in place. I want to find the forces experienced by the spherical joint, springs as well as the rotation of the spherical joint wrt the x and z axis.
I am applying a force of 30lbf on the top plate. There is gravity effect and the lower cylindrical segment is fixed. The materials used are ABS+PC plastic. I have not used any contacts. Just the spherical joint, fixed joint (Body to ground) and two other fixed joints(Body-Body) which connect the spherical joint connectors' cylindrical part to the top plate. 

When I apply the force on one of the 3 smaller hemispherical faces (where springs are connected; Ref Fig 1), I get a solution without any interference of the springs/spherical joint into the body. Although there is quite a bit of rotation about the Y axis which I want to completely restrict. 

Case -1 



                                                                         Fig-1

Final position:




When I apply the same force on the center of the top plate (near the spherical joint support location; Ref Fig 3), then the rotation about Y gets out of hand and the springs as well as the spherical joint go through the surfaces of the lower plate. 




Final position case-2





I want to :
1) Restrict the rotation about Y axis completely. Spherical joints don't provide any option for restricting one DOF or of applying stops. I don't know how else to apply a constrain to prevent rotation about Y.

2) Restrict the springs and spherical joint from going through surfaces. Adding contacts always gives me an error and prevents problem from solving. I have been told to use only joints in Rigid body Dynamics. 

Any advice would be highly appreciated. I have searched extensively online and haven't found anything yet. I have just recently started working on Rigid Body Dynamics in ANSYS (1 month), so I am not sure if I am making any silly mistakes that is causing all this. Would love to know your inputs on this. 

Thanks in advance. 

[peteroznewman]

1) replace the spherical joints with Universal joints. You have to orient the U-joint so pay attention to the Reference Coordinate System under the joint.

2) Add a frictional contact and make sure the Pinball Radius is large enough so that the Contact Tool shows the Initial Status as Near Open. What kind of error do you get from using contacts?  ANSYS Rigid Dynamics is not as robust as other RBD software I have used for contact modeling.

 

[yanivVK]

Hello Mr. Newman,

Thank you for your reply. 

1) Replacing the spherical joints with the Universal joint prevented the Y axis rotation. Thank you for the suggestion. The Reference coordinate system worked on its own to match the coordinate axes perfectly. 

2) Adding a frictional contact still results in the solution being unsolved. We can add only a forced frictional contact in the RBD solver. Also, I couldn't add a contact tool in the RBD. So, I couldn't check the initial status. 

I tried with a friction coefficient of 0.1 and a Pinball radius of 0.2. There was no solution in this case and from the Solution info I got the following warning 

*** Warning:

    None of the LCP solvers succeeded, Abort computation.

    No set of active contact stops can satisfy all the constraint equations. Solve cannot proceed

 Solve failed 

    Error found at time 1.377963e-04

    Solution terminated before normal completion


I got the same error with a Pinball radius of 0.5 as well. 

When I increase the friction coefficient slightly to 0.15, keeping the pinball radius the same, I get multiple warning messages and the solver fails.

No node exists for the spring mesh reference: Left Spring.

No element exists for the spring mesh reference: Left Spring. Results cannot be evaluated for this spring.

No node exists for the spring mesh reference: Far Spring.

 

No element exists for the spring mesh reference: Far Spring. Results cannot be evaluated for this spring.

etc


At this point I still don't know how to prevent the springs and spherical joint from penetrating into other surfaces during the dynamic analysis' last steps. 


Also, I wanted to ask you which RBD software do you use or recommend if ANSYS RBD is not the way to go? 

[yanivVK]

I think I figured it out. It sounds really silly now that I see it. It was a problem with the low stiffness of the springs. When the spring force couldn't resist the applied force any more, the system failed and springs plus spherical joint went through the base plate in a snap. I increased the stiffness and it gives me a proper behavior. 
It was right in front of me and I didn't see it. 

Thank you again Mr Newman for taking the time out to reply to my question. 

Regards.