Contact Force Control During Soft Tissue Interaction Using Handheld Robot

This paper presents a control scheme to maintain consistent contact force using a handheld robot during soft tissue interaction with imaging instruments such as probe-based confocal laser endomicroscopy (pCLE). The handheld robot incorporates a 6-DOF miniature micromanipulator and a force sensor for the control. Given force set for clearly visualizing tissue on contact, the handheld robot can maintain such a tiny force by actively controlling its tool tip using the micromanipulator. It allows keeping locating the tool tip within the depth-of-focus region of the imaging instrument regardless of actual force on the soft tissue engaged by hand motion delivered to the handpiece of the robot. The contact force is regulated primarily by controlling the depth of the tool tip from the tissue surface. This procedure is accomplished by controlling the position of the tool tip via closed-loop force control. To validate the control scheme, we evaluated force tracking performance at the tool tip given a sinusoidal force input with an amplitude of 0.1 N for various frequencies, while firmly fixing the robot. As a result, the handheld robot could maintain the target force with an RMS error of less than 7.8 mN. The handheld task holding the robot on top of a single contact point was also evaluated, which yields an RMS error of 22.6 mN along the Z-axis of the sensor. Finally, the robot-aided scanning over the surface of the soft tissue was performed using the handheld robot, and the error was significantly reduced by 31.6% on average compared to unaided operation.