[ODE] Flywheels

Brian Clarkson brianclarkson at btconnect.com
Tue Nov 18 15:11:13 MST 2003


Thank you for your comments.

I haven't done any coding yet. I was expecting you experts to tell me I
should do it some other way. But I thought I'd ask anyway. I was planning to
spin it with a local torque around its spinning axis. Then any movement
forces  would be applied as world angular and linear forces. This should
produce the gyroscopic processional behaviour
that you refer to as the "won't move the obvious way". Its this behaviour
that would be useful to have handled by ODE.

You say that ode does not conserve angular momentum. This is not my
observation. When my rigid bodies are in free fall and are spinning they
don't appear to slow down. i.e. when they fall of a shelf or a ledge. Are
you referring to something else here.


-----Original Message-----
From: Martin C. Martin [mailto:martin at metahuman.org]
Sent: 18 November 2003 14:19
To: Brian Clarkson
Cc: ode at q12.org
Subject: Re: [ODE] Flywheels

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,
> if the engine is turned off, it will drive the wheels for a moment? How
> 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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