  This paper concerns the forward kinematics and tension distribution of
sinking winches mechanism, which is a type of four-cable-driven partly
constrained parallel robot. Conventional studies on forward kinematics of
cable-driven parallel robot assumed that all cables are taut. Actually, given
the lengths of four cables, some cables may be slack when the platform is in
static equilibrium. Therefore, in this paper, the tension state (tautness or
slackness) of cables is considered in the forward kinematics model. We propose
Traversal-Solving-Algorithm, which can indicate the tension state of cables,
and further determine the pose of the platform, if the lengths of four cables
are given. The effectiveness of the algorithm is verified by four examples. The
results of this paper can be used to control sinking winches mechanism to
achieve the level and stable motion of the platform, and to make the tension
distribution of cables as uniform as possible.
