Adaptive Control of Sclera Force and Insertion Depth for Safe Robot-Assisted Retinal Surgery

Abstract

One of the significant challenges of moving from manual to robot-assisted retinal surgery is the loss of perception of forces applied to the sclera (sclera forces) by the surgical tools. This damping of force feedback is primarily due to the stiffness and inertia of the robot. The diminished perception of tool-to-eye interactions might put the eye tissue at high risk of injury due to excessive sclera forces or extreme insertion of the tool into the eye. In the present study therefore a 1-dimensional adaptive control method is customized for 3-dimensional control of sclera force components and tool insertion depth and then implemented on the velocity-controlled Johns Hopkins Steady-Hand Eye Robot. The control method enables the robot to perform autonomous motions to make the sclera force and/or insertion depth of the tool tip to follow pre-defined desired and safe trajectories when they exceed safe bounds. A robotic light pipe holding application in retinal surgery is also investigated using the adaptive control method. The implementation results indicate that the adaptive control is able to achieve the imposed safety margins and prevent sclera forces and insertion depth from exceeding safe boundaries.

Fig. 1.

Johns Hopkins Steady-Hand Eye Robot with its five degrees of freedom shown with red marks, spatial coordinate frame at the robot base and the body coordinate frame at the RCM point (a) representation of the x and y components of the sclera force in the body frame (f_sx and f_sy) and the tool insertion depth (b)

Fig. 2.

Schematic diagram for the adaptive force control of a 1-Dof velocity-controlled robot with mass m interacting with an environment with linear and unknown compliance γ.

Fig. 3.

Experimental setup including the SHER, dual force-sensing tool, interrogator, microscope and eye phantom (a), Close-up view of the eye phantom and the painted vessels on the retina (b)

Fig. 4.

Implementation of the adaptive sclera force control for sinusoidal reference trajectories, f_dx = 40sin(2t) mN and f_dy = −40sin(2t) mN

Fig. 5.

Variations of f_sx (top) and f_sy (bottom) for vessel following task, the discontinuous curves in each plot indicate the desired exponential f_dx and f_dy for the interval when the corresponding adaptive controller is activated (when |f_sx| or |f_sy| are between 100 and 70 mN). The horizontal lines of 120 mN and −120 mN are plotted in each figure.

Fig. 6.

Illustration of insertion depth adaptive control when the insertion depth exceeds 20 mm. Adaptive control is active when 17 < D < 20 mm.

Fig. 7.

Robot-assisted light pipe holding - plots for sclera force components (top) with fixed desired trajectories and insertion depth (bottom) with exponential desired trajectory.