8 Replies Latest reply on Jul 29, 2013 3:44 PM by Jared Conway

    Coupled Constraints

    Shawn Mahaney

      I often simulate assemblies with hydraulic cylinders in them.  I can simulate the cylinder, once fluid is closed off in it, as a rigid link.

       

      But often there are pairs of cylinders, spaced apart, which remain in fluid communication with each other after the control valve is closed.  So, the total fluid in them remains constant but one could send fluid into the other if the assembly they are in bends or twists.  (Our frames are necessarily open C-sections, which we know twist quite a bit - sample pic attached)

       

      In this case I want to set up a constraint like a coupled pair of rigids, where total length (A+B) remains constant.  Is there a way to set this up?

        • Re: Coupled Constraints
          Jared Conway

          What do you want to couple exactly in the sim? Pressures in the system or the amount the cylinder is pushed out?

            • Re: Coupled Constraints
              Shawn Mahaney

              The sum of the lengths of the two links should be kept constant.

               

              Or - the resultant force in each link could be made equal (the cylinders will have the same hydraulic pressure), but I think this would be an iterative process in the solver.  It will naturally be solving for displacements and strains, with force and stress a secondary computation.

                • Re: Coupled Constraints
                  Bill McEachern

                  you could model it with a rocker arm and a slider mechanism or something similar - a mechanism....

                    • Re: Coupled Constraints
                      Shawn Mahaney

                      I've done that before, and it's not too bad of an approximation if it's a boundary condition to the assembly of interest. 

                       

                      But at the moment I'm looking at cylinders which are inside the system (fork lift mast main cylinders; the ones in the picture above are the tilt cylinders which go to the truck (or my test stand)), so I want to see the implications of the load on both ends.    I could put a phantom assembly of rigids in the middle, with a pivot point, but this gets ungainly and I could see having trouble getting it stable.

                        • Re: Coupled Constraints
                          Bill McEachern

                          I have not tried this but it might be worth taking a look at: Using cyclic symmetry. This is essentially a matching of displacements. It might be possible to devise a scheme that inverts this condition. I have not thought about it very long but might be worth more consideration.

                            • Re: Coupled Constraints
                              Bill McEachern

                              While I can see why the following approach could get tricky and you have probably tried it already,  but why not replace the cylinders with a force (constant hydraulic pressure) and a pin and a slider... does that get close?

                                • Re: Coupled Constraints
                                  Shawn Mahaney

                                  Because the force is unknown.  The face it would apply to is the final translational constraint for the moving portion of the the assembly.  I could compute the force from the weight and applied load (through the chains, etc), but the simulation would still be unstable.

                                  Also of keen interest here is distortion in the moving portion and bending of the entire assembly, so I don't want to fix anything in space where it isn't really fixed.

                                   

                                  Here's what I have been doing, but it's manually iterative.

                                  [picture a two-stage fork lift mast, the inner portion sliding vertically in the outer, the bottom of the outer is fixed]

                                  - apply slider BC (horizontal plane) to one side at the top end of the cylinder

                                  - apply approximate expected force to other side

                                  - apply same expected force to the static portion on both sides

                                  - Run.

                                  - check that resultant force from the boundary condition is the same as the force applied to the other side.

                                  - correct if different

                                  - repeat until forces balance

                                    • Re: Coupled Constraints
                                      Jared Conway

                                      sounds like a good fit for something like abaqus with multiphysics where it could solve the pressure problem at the same time.

                                       

                                      have you seen other FEA tools that have an option like what you're looking for? I haven't really seen anything like that before. I'm sure with something like abaqus you could build your own special elements to do what you describe. with simulation, you might be able to automate your process which sounds pretty reasonable.

                                       

                                      the only other things i could think of are playing with springs or probably the approach that i would take is look at the worst case scenario from both sides. a few more studies but it would be a lot less work to get setup in a way that is reasonable and that you can trust the results.