[ODE] Flywheels

Martin C. Martin martin at metahuman.org
Tue Nov 18 09:18:32 MST 2003

ODE only uses a first order integrator, so quantities that are supposed 
to be conserved (like angular momentum) change value pretty quickly. 
ODE is designed mainly for impulsive physics, e.g. where things are 
colliding and the forces are big, but only for a little while.

What forces are acting on your flywheel?  How do you spin it, brake it 
and move it?  Just by applying forces, without specifying where those 
forces are coming from?  Do you want to use it to estimate what a real 
flywheel would do?  If so, you're probably better off writing your own 
special purpose simulation.

If you want to use ODE anyway, I'd thing you'd need a much smaller 
timestep, although I haven't played with it myself.  One way you can 
test: try setting your timestep to 1/100th of your current timestep 
(i.e. 0.00015), take 100 steps, and see if the results are similar to 
what you're getting now.  Make sure you, e.g., move the flywheel over 
that time.  I take it by "move the flywheel" you mean apply a force to 
the axle?  Remember, if you apply a torque to the axle that's not around 
it's axis, the flywheel won't move the "obvious" way.  If it does, 
there's something wrong with your simulation.  :)

- Martin

Brian Clarkson wrote:

> Flywheels can be driven by anything. In early navigation systems they were
> usually driven by
> compressed air or were designed as an elecric motor. I only want to spin,
> brake and move it and take readings of its state (angular velocities and
> position).
> Brian..
> -----Original Message-----
> From: DjArcas [mailto:djarcas at hotmail.com]
> Sent: 18 November 2003 10:40
> To: Brian Clarkson; ode at q12.org
> Subject: Re: [ODE] Flywheels
> I'm curious - is this a 'proper' flywheel, ie it's driven by an engine, and
> if the engine is turned off, it will drive the wheels for a moment? How are
> you intending to link the things together?
> ----- Original Message -----
> From: "Brian Clarkson" <brianclarkson at btconnect.com>
> To: <ode at q12.org>
> Sent: Tuesday, November 18, 2003 10:35 AM
> Subject: [ODE] Flywheels
>>Hi folks
>>Can anyone advise me. Can I use ode to simulate a flywheel. It will be
>>spinning at about 300 rads/s. Is this a
>>non starter or should it work. I need it principally for the rotational
>>inertia (startup, slow down ) but gyroscopic
>>precession would be a bonus. Is the 200 rads/s ok or should it be scaled.
>>How does the step time ( I like to use a fixed step time ) going to affect
>>the calcs. I am getting about 60 fps and using 0.015 steps with extra
> steps
>>for catch up. This means it will rotate just over 180 degs per step.
> (sorry
>>for the mixed units)
>>How about scaling the rotational speed. If so what else has to be scaled.
>>Your thoughts appreciated.
>>ODE mailing list
>>ODE at q12.org
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