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linear actuator with freely rotating joints at each end

Hi All,

I am wanting to create a URDF model that models a linear actuator (a hydraulic ram) that is free to rotate at each end (i.e. the only aspect you control is ram length). The closest match appears to be a prismatic joint, but this joint requires an axis parameter as well as a length. The scenario I have only requires a length as the axis is constrained by the kinematics of the joint and changes continually with linear actuator length.

I can already get an angle measurement by other means, but I would also like to be able to calculate the angle from linear actuator length alone, and conversely calculate the required actuator length from angle targets using the standard transform libraries in ROS (or MoveIt!).

Thanks, Bart