[ODE] Help?? Actuating a ball-and-socket joint with AMotor in User mode

[Ivan Velickovic]

Hi,
My problem may seem trivial to expirienced ODE users but for me it's a  
large headache.

I'm trying to convert a robot model given as DH matrix to ODE model.  
Primary DH model is built with several sphere joinints with odd axis  
orinentation. So I need to use a Ball-And-Socket joint to represent it  
with ODE. The problem is actuation of such joints. If I use AMotor, it  
need to be engaged in AMotorUser mode. In AMotorUser mode I can't get data  
about current motor angle, so seting relative position of actuated bodies  
could be pretty comlicated.

Is there an easier way to acomplish this (setting relative position  
between bodies connected with Ball joint and AMotor in User mode) but  
calculating relative position in each time step from known position in  
previous time step and current angular velocities and then reseting new  
velocities according to the position error?

A sample code would be nice.

Thanks