  This paper presents a novel multi-axis force-sensing approach in robotic
minimally invasive surgery with no modification to the surgical instrument.
Thus, it is adaptable to different surgical instruments. A novel 6-axis optical
force sensor, with local signal conditioning and digital electronics, was
mounted onto the proximal shaft of a da Vinci EndoWrist instrument. A new
cannula design comprising an inner tube and an outer tube was proposed. The
inner tube is attached to the cannula interface to the robot base through a
compliant leaf spring with adjustable stiffness. It allows bending of the
instrument shaft due to the tip forces. The outer tube mechanically filters out
the body forces from affecting the instrument bending behavior. A mathematical
model of the sensing principle was developed and used for model-based
calibration. A data-driven calibration based on a shallow neural network
architecture comprising a single 5-nodes hidden layer and a 5x1 output layer is
discussed. Extensive testing was conducted to validate that the sensor can
successfully measure the lateral forces and moments and the axial torque
applied to the instruments distal end within the desired resolution, accuracy,
and range requirements.
