Active Stiffness Control of Surgical Continuum Robots

Mohsen Mahvash

Instructor of Surgery

Children's Hospital, Boston

Harvard Medical School

Abstract:

Historically, the study of robot-environment interaction has focused on industrial robots interacting with stiff environments in manufacturing tasks. In these applications, robot position control is infeasible since uncertainties in both the position and stiffness of the environment lead to excessive contact forces as well as to the jamming and wedging of parts. A variety of techniques have been developed to address these problems. These include the addition of passive compliant components to the end effector as well as the active control of contact using hybrid force / motion control or stiffness control.

Recently, continuum robots including steerable catheters, multi-backbone snake-like robots and concentric tube robots have been introduced for surgical applications.  Despite the inherent passive compliance of these robots, interactions with soft biological environments can still lead to excessive and damaging contact forces. Consequently, hybrid force / motion control and stiffness control can offer the same benefits for continuum robots that these control approaches provide in industrial robot applications. Implementing such controllers on continuum robots is a challenging problem, however, since the kinematic and force mappings are coupled, complex and highly dependent on the type of continuum robot.

In this talk, I will explore our vision for creating real-time control laws that are applicable to broad classes of continuum robots. As an example, I will discuss our results developing active stiffness controllers. We have employed two unifying principles in developing these controllers. First, we use flexion of the robot itself to measure and control tip forces. Second, we utilize a unified kinematic model that can represent many types of continuum robots in contact with their environment. I will explain these ideas in the context of an experimental implementation of stiffness control on a concentric tube robot.

Selected References:

1. 1.M. Mahvash, and P. E. Dupont, Stiffness control of surgical continuum manipulators, IEEE Transactions on Robotics, under review.
   

2. 2.P. Dupont, J. Lock, B. Itkowitz, and E. Butler, “Design and control of concentric tube robots,” IEEE Transactions Robotics, in press.
   

3. 3.M. Mahvash and P. Dupont, "Bilateral Teleoperation of Flexible Surgical Robots." New Vistas and Challenges in Telerobotics Workshop, IEEE 2008 International Conference on Robotics & Automation, Pasadena, CA, 19-23 May, 2008.
   

Active stiffness control of a concentric tube robot: (a) experimental setup (b) controller can both increase and decrease natural tip stiffness of robot.

Relevant Web Links: http://biorobotics.bu.edu