[ODE] spring and damping surface constants

[Manohar B.S]

How are you coupling the forces from the sphere to the
haptic device. Sphere in your case is the haptic proxy
(HIP). What's the haptic device that you are using?

Manohar.


--- Tanguy Fautre
<tanguy.fautre at spaceapplications.com> wrote:

> 
> Hello,
> 
> 
> We are currently using ODE for a running a physic
> simulation connected 
> to an experimental haptic device. Not so long ago,
> we started 
> investigating how (well) ODE was simulating the
> spring and damping constant.
> 
> ODE doc explicitly states that ERP and CFM can be
> used to simulate 
> springness and damping, using the following
> formulas:
> 
> ERP = h.kp / (h.kp + kd)
> CFM = 1 / (h.kp + kd)
> 
> 
> h = stepsize
> kp = spring constant
> kd = damping constant
> 
> 
> Or the other way around:
> 
> kp = ERP / (h.CFM)
> kd = (1 - ERP)
> 
> 
> However, when we start experimenting a simple
> scenario (a sphere falling 
> on a plane because of gravity), we've noticed that
> the forces applied on 
> the sphere in contact situations (using
> dJointFeedback) were not exactly 
> respecting the above formulas. While the spring
> constant seemed to be 
> correctly interpreted, the force generated because
> of the damping 
> constant was always wrong.
> 
> Empirically, we came up with the following
> relationship:
> 
> K = kp
> D = kd + h.kp
> 
> 
> K = observed spring constant
> D = observed damping constant
> 
> 
> Conclusion, it seems to us that it's impossible to
> modify the K spring 
> constant without affecting the D damping constant.
> Thus it is impossible 
> to have a contac with a zero damping constant. This
> situation only gets 
> worse as the stepsize is increased.
> 
> Is this an intended behaviour?
> Is it possible to have a perfect system where K = kp
> and D = kd ?
> 
> 
> Note: we only investigated the effect of ERP and CFM
> on contact joints, 
> not on other joint types. So I don't know whether
> the results are the 
> same with other joint types.
> 
> 
> Yours,
> 
> Tanguy
> 
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> 


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