[ODE] holding a chain at a certain level for a marionette

[nick knouf]

Hello list members,

I'm having a problem with something that should be fairly simple...

I have a rectangular geom (mostly xz) with four vertical (+y) chains  
at each of the corners.  The chains are 20 units long.  Each chain is  
connected to the rectangular geom by a ball joint.  The end goal is  
the simulation of a very basic marionette-like creature.  I know the  
desired position of the last element in the chain.  I can start out  
the simulation just fine, with a regular applied force to the last  
element of each of the four chains--the marionette floats in space.   
However, if I apply additional work (above a holding force) to one of  
the chains, that part of the creature rises, as expected (creating a  
new holding position for that one chain only), but when I stop  
applying work, the chain does not remain where expected: the x and z  
components are within +- 0.01 of the desired position, but the y  
component varies by over -4.0.

Pseudo-code for the holding algorithm is as follows:

currentVelocity = finalChainElement.getLinearVel()
currentPosition = finalChainElement.getPosition()
desiredPosition = holdPosition

desiredVelocity = desiredPosition - currentPosition
desiredAccel = desiredVeclocity - currentVelocity
finalChainElement.addForce(holdingMass * desiredAcel + holdingForce)

I've tried a variety of additions to this method, including lookahead  
and PID controllers.  In every case the x and z components remain  
within +-0.01 of the desired position, while the y component is often  
-4.0 or larger.

I'm using ODE 0.8 with the PyODE bindings.

What could I do to make the y component of the chain be closer to its  
desired position?

Thanks,

nick knouf