3 Replies Latest reply on Feb 14, 2008 11:58 AM by Ian Hogg

    Control to uncontrol motion

    Valentin Leung
      Dear All,
      I have a pretty simple problem but I am not sure I can simulate it with Motion (at least, I still did not find the way to).

      It's pretty simple, I have a cap that is articulated with a box, the box is fixed to the ground, I have a revolute joint between both. I have a kind of geometry that I have simplified has a spring between the box and the cap.
      Basically, when a user push the cap up, he will feel some resistance, after a certain angle, the spring will pass an equilibrum point and will flip the cap over.

      Basically, you may find this kind of motion on a shampoo cap. (head & shou***, that's the first brand that comes into my mind)

      Now, my problem is too simulate this motion. if I control a rotation to a certain angle, let say that I push the lid untill it is vertical, nothing happens afterwards... which is logical as I don't control it beyond.

      I tried to simulate a tab that will push the cap untill the equilibrum point and hope that the spring will force the flip over, but I cannot get a proper contact.

      So I finally come to ask help here..

      First, can I simulate this kind of motion in Motion?

      Thanks
        • Control to uncontrol motion
          Ian Hogg
          Hi Valentin,

          In terms of modeling a cap like a shampoo bottle, are you trying to just show the movement or actually model the real situation. Animating the cap to show how it works is really trivial in just animation/assembly motion. Modeling the true function is a little more advanced.

          Plastic caps rely on a press fit somewhere that snaps to a locked position to keep the lid closed (so it requires a certain amount of force to open the cap). Then when you open the lid, you have to overcome the same press fit to pop the cap. To model that behaviour really requires COSMOSWorks to accound for the deformation that occurs when the lid is in the pressed fit stage and flexure due to where your applying force to open the lid. In many cases this may be a nonlinear analysis as relative deformation is large, and if using plastics, they exhibit nonlinear stiffness characteristics.

          You cannot really use COSMOSMotion to model the engagement of the lid as it's fundamental assumption is that all bodies are rigid (do not deform).

          Let me know if you can discuss more what your goal is and I can comment further.

          Cheers,

          Ian

          • Control to uncontrol motion
            Valentin Leung
            Thanks Ian,

            I tried nonlinear under Cosmosworks but did not get anything good for the moment. For example, I draw my mechanism as it is molded, tried to fix a line around which the system should rotate, but each time, the analysis fails... I think I need some courses on the material properties on plastic deformation.
            Anyway, I gave up and now, I wanted just to try to simulate the response of my system when I open it. On some caps, you have a living hinge that acts as a hinge and then, you have a kind of spring made from a deformable zone hinged with the cap and the body. When you open the cap, there is some kind of resistance and when you get pass a point, the cap will flip back due to the spring. So I modelize my cap with the living hinge (just cut it in two bodies and made a revolute join). Then, I tried to replace my spring geometry by a translational spring. I replace it by a linear model (just to see), I computed a stiffness using cosmosworks by doing linear deformation and looking at the resultant force.

            But after I input all these data, I cannot find a way to proper control the cap correctly, what I want to recreate is the fact that I push the lid back and pass a point, it will flip back on its own because of the spring tension.

            So for now, just a approximated kinetic model simulation, not a nonlinear static one.

            Thanks for your help

            Valentin
              • Control to uncontrol motion
                Ian Hogg
                Hi Valentin,

                Having a picture of the living hinge would help, but the way I would do this is with an over center spring (a spring attached to the cap that is mounted on the opposite side of the hinge some distance away). In other words, have a spring that will close the cap to a certain rotation angle, but beyond that it will go over the center of the hinge and fully open it. It should then just be a matter of tweaking the preload and the stiffness (or free length and stiffness).

                Cheers,

                Ian