[ODE] Reaction forces with slider joint

[Gib Bogle]

Hi,

I'm simulating a chain-like object, made up of a number of sections 
(about 30) each 6 m long and with mass of about 1000 kg.  The sections 
are connected using ball-and-socket joints.  The last link in this chain 
is light (like a rod) and I want this one to have variable length.  The 
only way I've found to accomplish this is to make the last connection a 
slider joint, and to set the limits like this:

dJointSetSliderParam(joint, dParamLoStop, d-delta);
dJointSetSliderParam(joint, dParamHiStop, d+delta);

where where d is the extension that I want, and delta = 0.001 m (for 
example).

When I run the model, there is a startup period over which I ramp d from 
0 up to a certain values (e.g. 10 m), then I leave the joint as it is.  
The motion that the chain undergoes creates some substantial tensions in 
it, i.e. large reaction forces at the joints, up to about 3.3 MN.  For 
debugging purposes, I'm running to a steady state condition in which the 
tension forces are unchanging (in case you are interested, this is 
achieved by whirling the chain around at a fixed rotation speed of about 
1.5 rad/sec). 

The problem I'm trying to understand is that I see a large and 
apparently random variation in the tension at the end where the slider 
joint is (this is near the axis of rotation).  If I replace the slider 
joint by a ball-and-socket joint (like the others) the forces all settle 
down to steady state values.  With the slider joint the random 
fluctuation in tension actually increases with time.  After 100 sec I'm 
seeing values near the axis in the range 2.5 - 4.0 MN, i.e. randomly 
fluctuating about the correct mean value.  The model is not useable.

If someone knows something about the slider joint that could explain 
this problem, or lead to a way to eliminate it, I'd like to hear about 
it.  Otherwise, maybe someone can suggest a different, more stable way 
to achieve the effect that I want, i.e. the ability to vary the length 
of a link (a body).

Thanks
Gib