Toward hybrid force/position control for the Cerberus epicardial robot

Cable Tension Optimization for an Epicardial Parallel Wire Robot

HeartPrinter is a novel under-constrained 3-cable parallel wire robot designed for minimally invasive epicardial interventions. The robot adheres to the beating heart using vacuum suction at its anchor points, with a central injector head that operates within the triangular workspace formed by the anchors, and is actuated by cables for multipoint direct gene therapy injections. Minimizing cable tensions can reduce forces on the heart at the anchor points while supporting rapid delivery of accurate injections and minimizing procedure time, risk of damage to the robot, and strain to the heart. However, cable tensions must be sufficient to hold the injector head's position as the heart moves and to prevent excessive cable slack. We pose a linear optimization problem to minimize the sum of cable tension magnitudes for HeartPrinter while ensuring the injector head is held in static equilibrium and the tensions are constrained within a feasible range. We use Karush-Kuhn-Tucker optimality conditions to derive conditional algebraic expressions for optimal cable tensions as a function of injector head position and workspace geometry, and we identify regions of injector head positions where particular combinations of cable tensions are optimally at minimum allowable tensions. The approach can rapidly solve for the minimum set of cable tensions for any robot workspace geometry and injector head position and determine whether an injection site is attainable.

Introducer Design Concepts for an Epicardial Parallel Wire Robot

Background

Cardiac gene therapies lack effective delivery methods to the myocardium. While direct injection has demonstrated success over a small region, homogenous gene expression requires many injections over a large area. To address this need, we developed a minimally invasive flexible parallel wire robot for epicardial interventions. To accurately deploy it onto the beating heart, an introducer mechanism is required.

Methods

Two mechanisms are presented. Assessment of the robot's positioning, procedure time, and pericardium insertion forces are performed on an artificial beating heart.

Results

Successful positioning was demonstrated. The mean procedure time was 230 ± 7 seconds for mechanism I and 259 ± 4 seconds for mechanism II. The mean pericardium insertion force was 2.2 ± 0.4 N anteriorly and 3.1 ± 0.4 N posteriorly.

Conclusion

Introducer mechanisms demonstrate feasibility in facilitating the robot's deployment on the epicardium. Pericardium insertion forces and procedure times are consistent and reasonable.

Parallel Force/Position Control of an Epicardial Parallel Wire Robot

Gene therapies for heart failure have emerged in recent years, yet they lack an effective method for minimally invasive, uniform delivery. To address this need we developed a minimally invasive parallel wire robot for epicardial interventions. Accurate and safe interventions using this device require control of force in addition to injector position. Accounting for the nonidealities of the device design, however, yields nonlinear and underconstrained statics. This work solves these equations and demonstrates the efficacy of using this information in a parallel control scheme, which is shown to provide superior positioning compared to a position-only controller.