[ODE] movement force conversion

Jon Watte hplus-ode at mindcontrol.org
Tue Mar 15 14:15:40 MST 2005

The physics system will ensure that the feet remain planted, because 
it will create contact joints between floor and foot, and you give it 
some amount of friction. If you have a sloppy animation, it will 
exceed foot friction, and it will slip. If you make friction low, it 
will "walk" as if on ice, and quite possibly fall over. The draw-backs 
of a physically modeled system!

You're right in that the entire leg adds inertia that makes the hip 
bending harder than it "should" be. Similarly, when bending the hip 
bone, you actually also counter-bend the pelvis it's attached to. 
However, if you accumulate error, and increase the torque applied for 
errors that remain after a few steps (a la a typical integrating 
controller system), it will all average out. To make the character not 
fall over, either be really good with your animation, or add a magic 
torque to the root bone (typically, your pelvis) that attempts to 
right it up (make it match the animation) every frame; again with an 
error integration to yield extra force as necessary.

I know it sounds somewhat crazy, but it really does work, assuming 
you have a reasonable physical skeleton and reasonable walk cycle 


			/ h+

-----Original Message-----
From: ode-bounces at q12.org [mailto:ode-bounces at q12.org]On Behalf Of
Matthias Baas
Sent: Tuesday, March 15, 2005 11:07 AM
To: ode at q12.org
Subject: Re: [ODE] movement force conversion

Jon Watte wrote:
> Now, calculate the torque necessary to move the body to the 
> desired angle in the next timestep 

How do you do that? A character consists of an entire hierarchy of rigid 
bodies that are connected with joints. How do I calculate, say, the 
torque I have to apply to the left hip joint? There is not just one 
single rigid body attached to the hip joint but an entire subhierarchy 
(aka the left leg). And if I later apply a torque to the knee it will 
also influence the thigh which I have to take into account when 
calculating the torque of the hip joint and so on...
How do I ensure that a foot with ground contact remains planted? What if 
both feet have ground contact?

- Matthias -
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