IEEE TRANSACTIONS ON ROBOTICS, VOL. 28, NO. 2, APRIL 2012 291 Kinematics-Based Detection and Localization of Contacts Along Multisegment Continuum Robots Andrea Bajo, Student Member, IEEE, and Nabil Simaan, Member, IEEE Abstract—In this paper, we present a novel kinematic-based framework for collision detection and estimation of contact loca- tion along multisegment continuum robots. Screw theory is used to define a screw motion deviation (SMD) as the distance between the expected and the actual instantaneous screw axis (ISA) of motion. The expected ISA is computed based on the unconstrained kine- matics model of the robot, while the actual ISA is computed based on sensory information. Collisions with rigid environments at any point along the robot are detected by monitoring the SMD. Contact locations are estimated by the minimization of the SMD between the ISA that is obtained from a constrained kinematic model of the continuum robot and the one that is obtained from sensor data. The proposed contact detection and localization methods only re- quire the relative motion of each continuum segment with respect to its own base. This strategy allows the straightforward general- ization of these algorithms for an n-segment continuum robot. The framework is evaluated via simulations and experimentally on a three-segment multibackbone continuum robot. Results show that the collision-detection algorithm is capable of detecting a single collision at any segment, multiple collisions occurring at multiple segments, and total-arm constraint. It is also shown that the es- timation of contact location is possible at any location along the continuum robot with an accuracy better than 20% of the segment nominal length. We believe this study will enhance manipulation safety in unstructured environments and confined spaces. Index Terms—Collision detection, continuum robots, estimation of contact, screw theory. I. INTRODUCTION C URRENT robotic systems are incapable of fully charac- terizing their interaction with the environment. Full char- acterization of the interaction includes the following: discerning collisions, localizing contact constraints, and estimating inter- action forces. Although there are mature algorithms for compli- ant hybrid motion/force control [1]–[5], there exists no unified framework for the impact and postimpact phases. These algo- rithms require a priori knowledge of the environmental con- Manuscript received April 18, 2011; revised October 23, 2011; accepted October 31, 2011. Date of publication December 8, 2011; date of current ver- sion April 9, 2012. This paper was recommended for publication by Associate Editor Y. Choi and Editor B. J. Nelson upon evaluation of the reviewers’ com- ments. The work was supported by the National Science Foundation under Career Grant IIS-1063750. The authors are with the Department of Mechanical Engineering, Vanderbilt University, Nashville, TN 37212 USA (e-mail: andrea.bajo@vanderbilt.edu; nabil.simaan@vanderbilt.edu). This paper has supplementary downloadable material available at http://ieeexplore.ieee.org. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TRO.2011.2175761 straint geometry via the formulation of natural and artificial constraints [6] or motion and constraint screws [7]–[9]. Previous works individually focused on collision detection [10], [11] and estimation of constraint locations [12]–[14]. In [10] and [11], the generalized momentum of serial robots was used to identify contact incidence and the link at which contact occurs. In [12], a least-squares method using the esti- mate of contact location from tactile sensors and joint torque measurements to estimate the magnitude and the location of contact force was proposed. In [13] and [14], two different probabilistic approaches for contact estimation were presented. Other researchers tried to overcome the limitations of rigid-link robots by developing sensitive robotic skins [15]. Continuum robots are continuously bending, infinite-degree- of-freedom elastic structures [16] that offer an opportunity to overcome the limitations of rigid-link robots. This opportunity stems from the ability of continuum robots to change their nat- ural shape, when interacting with the environment. The goal of this paper is to capture this information and provide a general framework for collision detection and contact-location estima- tion along multisegment continuum robots. The motivation behind this study originates in the field of medical robotics. The onus of safeguarding against sur- gical trauma currently lies on the surgeon. New surgical paradigms, such as natural orifice transluminal endoscopic surgery (NOTES), demand deeper anatomical reach along in- creasingly tortuous paths and often require multiple tools to safely coexist in confined surgical site. This added complexity requires intelligent surgical slaves that are able to detect colli- sions with other surgical effectors and the surrounding anatomy to prevent inadvertent trauma to the patient and to the end ef- fectors. These robots will need to support automated or semiau- tomated insertion into the anatomy, estimate contact locations along their structure, regulate their contact forces, and use their multipoint interactions to enhance end-effector precision and safety. With this goal in mind, researchers have relied on passive compliance of continuum robots [17]–[20] and wire-actuated robots [21]. However, reliance on passive compliance of sur- gical robots comes with a price of performance degradation in terms of payload carrying capability and position accuracy. This paper complements previous works that provide contin- uum robots with the ability to act as sensors, as well as surgical intervention platforms. In [22], the authors proposed and val- idated a method for the estimation of wrenches at the tip of multisegment continuum robots, such as the one that is shown in Fig. 1. In [23], the problem of contact detectability using joint- level force measurements and the fixed centrodes of instanta- neous planar motion for a single-segment continuum robot was 1552-3098/$26.00 © 2011 IEEE