[ODE] Vehicle Speed (Generic Units to Real World Units)

[DjArcas]

Surely the *easiest* way is to measure the angular velocity of the vehicle,
and to make it accelerate or brake accordingly?

----- Original Message ----- 
From: "Jeff Weber" <jeffreyweber@hotmail.com>
To: <ode@q12.org>
Sent: Wednesday, July 30, 2003 1:49 PM
Subject: [ODE] Vehicle Speed (Generic Units to Real World Units)


> I understand units in ODE are generic and i can choose whatever units I
want
> as long as I am consistent.
>
> I currently have things set up so that 8 ode units = 1 meter.  This is
> arbitrary and was chosen for reasons related to the graphics engine we are
> using.
>
> I have conversions that allow me to go back and forth between ode units
and
> meters and everything works well.  I can make a vehicle wheel with a
radius
> read from a spec sheet, then create a geom and body of the correct size to
> simulate it in my world.
>
> My problem now, is figuring out vehicle speed in real world units. (m/s
for
> instance).  I need to be able to relate my vehicle wheel's angular speed
to
> my vehicles speed so that if I want my vehicle to go 50 m/s I know what I
> need to set for my wheel joint's dParamVel ("Desired Velocity")
>
> I think my confusion is with the "time" unit. It's a pretty easy
calculation
> to go from angular velocity to linear velocity (ignoring slip) if you know
> the radius of the wheel.  However, the value returned from
> GetHinge2Angle2Rate() does not jive with the real world.  The vehicles
> linear speed calculated from the angular speed returned from this function
> is wrong.
>
> I'm sure it is because I assumed "seconds" as a unit of time.  My question
> is how can I figure out a unit of time that will give my correct results?
> Do I need to physically measure it within the context of my world?  For
> instance, knowing the radius of my wheel, and setting the "Desired
Velocity"
> of my wheel to 1 should I measure how long it takes to go a certain
distance
> and figure the time unit conversion from their?
>
> Is there a better/easier way?
>
> I apologize for the long post, but this is important for our project.
>
> -jeff
>
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