# Lumped parameters model: inverse dynamic

1. Mar 1, 2013

### serbring

Hi all, I have a ground excited single degree of freedom model, like that:

I know the excitation function, i.e. s(t), but I need to know the force that imposes a specific s(t). I need it, because I want to simulate a model (little bit more complex) with a multibody tool, that it doesn't allow to excite systems with a ground displacement. So I need to convert the ground excitation with an equivalent force excitation. How could I do that for the single degree of freedom model? Any suggestion is appreciated.
Hopefully the problem is well stated.

thanks

Cheers

Pietro

2. Mar 1, 2013

### AlephZero

You can model this be adding a large mass (say 1000 times the rest of the model) to the base, and then calculate the force to move the mass according to s(t) ignoring the rest of the model.

Or, you can connect the base to the ground with a large stiffness $K$, and apply a force $Ks(t)$ to it.

These modelling methods are usually (and unimaginatively!) called the "large mass" and "large stiffness" methods.

3. Mar 1, 2013

### serbring

Hi,

thanks for your reply, I tried with the large mass method and it works fine. Which approach do multibody sw use?

4. May 7, 2013

### serbring

Hi alphazero,

I tried the large mass method, but it doesn't work, the ground mass motion is always affected by the rest of the model. Any other suggestion is appreciated.

thanks

regards