Force estimation for a minimally invasive robotic surgery system

A minimally invasive medical system comprising a manipulator that has an effector unit equipped with a 6-degrees-of-freedom force/torque sensor, which has a sensor reference frame with three axes X, Y and Z that are orthogonal to each other and is configured for holding a minimally invasive instrument, said instrument, when held by said effector unit, having a first end mounted to said effector unit, a second end located beyond an external fulcrum that limits said instrument in motion and an instrument shaft with a longitudinal axis that is collinear with an instrument axis of said sensor reference frame, said system comprising a programmable computing device programmed for:

• determining a position of said instrument relative to the external fulcrum including determining an initial reference distance vector (DFulcrum 0) from the origin of said sensor reference frame to the external fulcrum;
• wherein determining the initial reference distance vector (DFulcrum 0) comprises:
• moving said minimally invasive instrument along both axes of said sensor reference frame, which are perpendicular to said instrument axis respectively, until reaction forces along both axes are below a given threshold;
• defining X and Y axis coordinates of the external fulcrum at a location where the reaction forces along said both axes are below the given threshold
• determining a Z position of the external fulcrum along said instrument axis using a lever principle, comprising:
• pivoting the instrument with respect to its tip in a first direction until a first contact force is reached;
• measuring the module of a first moment vector and the module of a first force vector corresponding to said first contact force;
• and calculating a first Z axis coordinate position of the external fulcrum along said instrument axis by dividing the module of said first moment vector by the module of said first force vector;
• pivoting the instrument with respect to its tip in a second direction, which is opposite direction to said first direction, until a second contact force is reached;
• measuring the module of a second moment vector and the module of a second force vector corresponding to said second contact force;
• calculating a second Z axis coordinate position of the external fulcrum along said instrument axis by dividing the module of said second moment vector by the module of said second force vector;
• and setting the initial reference distance vector (DFulcrum 0) using the mean value of said calculated first position and said calculated second position.