Hallo,

Your question is unclear. That is the reason of no reply from others. Your Intension of using this posiiton model is not clear from description.  You need to describe more about your model if you still want help.

Regards
Joel Jossy

Hello, thanks for taking the time to reply. I apologize for not being clear in the first place. I also apologize in advance for the "ms paint" skills that are about to come.

I made a drawing that perhaps would make the situation clear. The simplest model for the study of vehicular suspensions is a single degree of freedom model which is excited by the movement of the base. I made a simple schematic with OMEdit shown below. Please disregard the fact that as it is, the body can move just about everywhere, in a real model there would be constraints that would allow it to move only vertically.



The problem is to compute the displacement of the body given a known displacement (not force, the resultant contact force is one of the things I'd like to compute) of the ground. This has to be something very obvious, so I apologize again

Thanks for the attention,

Hi,

Still there are contradictions. You are telling that ".....model which is excited by the movement of the base" and still wand to fix it in space? Because if you use Modelica library model 'fixed' at the base then that flange will have:
d(fixed.flange_a.s)/dt=0. I think you are aware of that. Then still why you want to use this model at base?

It should be not complex unless if you want to use visualization. Also i have no expertise in animation. If you want a simple model and want to find the curve of y(t)/dt. Then just input a force exertion model (where force flows out intermittently at varying amplitudes) to the flange of spring-damper model. You can attach fixed model to base of Force model.This only works if you want to simulate how mass behaves vertically if the base of the Tyre is given with shocks (like gutter in road).

Best wishes,
Joel Jossy

Hi Joel,

Thanks again for you time,

Of course I don't want it fixed in space. I just used the fixed block to have something I could "move" up and down to show what I needed, I apologize (again
for the confusion.

Animation would not be a problem, but using a force as input is a problem, because the contact force between the tires and the ground is unknown, it depends on the dynamics of the system. It has to be computed by the system.

I *have* the curve "y(t)", and I want to find the displacement of the sprung mass, "x(t)", in the figure. This is certainly possible, because there are vehicle models and road models in OpenModelica, but they are all very complex and I need a simple model for demonstration purposes.

Anyways, thank you the attention,

Ramiro.

Hi Ramiro,

Got full picture now. sorry that i asked you to make force model at base.  But you have the answer now and it is easy if you model it yourself. Four models should define the system

(1)Mass--->(2)Spring&damper---->(3)Model for giving shocks (y(t) and variable force)----->(4)Fixed

Modelling 3rd one is challenging (i am not aware of its existence in library) I usually make my own models with required variables and other features needed. So i suggest you to make one. Its not that difficult, its flanges will have relative motion. Flange_b.s connected to fixed model and flange_a.s connected to 2nd model. Equation may be written like

flange_a.s=y(t);
You can leave force (flange.f) as variable. Solver will compute these variables while simulation. But you must give some initial values like position, for successful simulation. Hope i could help you.

Best Regards,
Joel Jossy

Hello Joel, thanks for the information, I'll surely do as you suggest, but as this was a short term  project for a student of mine, I built a model on another system that I was more familiar with.

I have no problems with programming part of the solution myself, I just wanted to make sure I was not missing something obvious and reinventing the wheel, since this model, with excitation coming from the movement of the base, is really very common in mechanical engineering for suspension simulation and vibration isolation, also for mechanisms normally we would input the velocity of a link and compute the velocity of other links, in civil engineering it's used to compute the response to earthquakes and so on.

Please understand that I'm not complaining, I fully understand that no one has any obligation to do for free the things that are useful to me, I'm just commenting on a design choice that was very different from what I would have made.

Thanks again for the time you spent on this,

Ramiro.

Hi Ramiro,

Sorry if i confused you. i suggested inclusion of model-3 (as in previous message) thinking you need a simple model only. so that:

_It gives a known displacement input to spring-damper mass system upwards through flange_a
_at base it can be fixed (flange_b.s=0) and movable top flange (flange_a.s) through which displacement transfers to spring-damper model

But sorry for that, if we exclude that suspension in tire then tire would be rigid (like tire of bull cart). In reality it need

(1)Mass--->(2)car suspension(Spring&damper)---->(3)Tire Model(Spring&damper)----> (4) Model for giving displacement (y(t) ----->(4)Fixed model

Reactionary forces in the model are automatically computed. So it should normally give you result. Hope this helps you.

Best wishes,
Joel Jossy