Re: Problem with "NA"

[Terry Duell]

Hello Stephen,

On Wed, 20 Feb 2013 14:56:03 +1100, Stephen Montgomery-Smith  
<address@hidden> wrote:


> I think that a much more likely source of creating NA is in evaluating
> the right hand side function.  That is, you are solving:
>
> dx/dt = f(t,x)
>
> I am guessing that f(t,x) is something rather horrible.
>
> Look for things like taking square roots of negative numbers when you
> compute f(t,x).  Or maybe you diagonalize a matrix, and it has repeated
> eigenvalues, and you use the eigenvectors (which will be non-unique or
> non-spanning).  Or some other operation of that type.

Nothing like any of that. Just plain old equations of motion f=ma, where  
the f's are forces due to tyre spring, suspension spring, damper velocity  
etc. Nothing out of the ordinary.

> I also find it useful to think about the situation physically.  For
> example, if the trailer is directly behind the vehicle rather than at an
> angle, maybe some force is trying to pull them apart, and since there is
> no angle you are essentially trying to break the hinge.  Or maybe the
> wheels on the axle are locked to each other, and at least one of the
> wheels has to skid.

Nothing like that to think about either, it is pitch plane ride dynamics  
simulation, wheels are point followers, so only looking at vertical  
excitation resulting in heave and pitch of sprung masses.

I am slowly getting there as a result of finding a couple of simple  
errors, now get every time step calculated sensibly for the trailer  
motions, and all but the last 6 time steps appear OK for the prime mover.

Thanks for your advice.

Cheers,
--
Regards,
Terry Duell