IEEE ROBOTICS AND AUTOMATION LETTERS, VOL. 3, NO. 4, OCTOBER 2018 3293 Dynamic Bilateral Teleoperation of the Cart-Pole: A Study Toward the Synchronization of Human Operator and Legged Robot Joao Ramos and Sangbae Kim Abstract—This letter presents the experimental evaluation of a bilateral feedback law for the teleoperation of an underactuated dynamic system: the Cart-Pole. This physical system illustrates another simple model, the Linear Inverted Pendulum (LIP); a popular template for legged robot control and, in this letter, the mapping channel between the operator and robot. We develop a scaling strategy based on geometric and kinematic similarity in order to generate dynamically feasible trajectories for the LIP with a natural frequency different than the human’s. Moreover, by modifying the classic equations for the Cart-Pole, we show how it can competently represent the proposed template quantitatively and visually. Experiments where a human operator dynamically controls slave systems with slower or faster natural frequencies illustrate the efficacy of the proposed method. This study is a step toward building a human–machine interface that dynamically synchronize operator and legged robot in order to eventually achieve complex motor behaviors. Index Terms—Humanoid and bipedal locomotion, telerobotics and teleoperation, haptics and haptic interfaces, human factors and human-in-the-loop, natural machine motion. I. INTRODUCTION T O BUILD a legged machine that can navigate unstructured terrains and robustly utilize tools originally made for hu- mans, is the dream of many roboticists. The applications for such capable robot are endless. They can be used for search and rescue, firefighting, space exploration or any other condition that endangers the life of the human responder. Although current technology has reached a state that we can build these sophisticated machines, their autonomy, adaptability, and robustness is yet far from what we evidence from its human counterparts. A short-term solution to bridge this gap is to utilize human motor control as a reference for robot motion in real-time. This strategy has been used widely in computer graphics and robotics [1], [2]. But we argue that these approaches miss the utilization of physical real-time feedback to the operator, such that s/he can adapt the motion to that particular human-machine configuration. Yet, very few studies have attempted to provide meaningful feedback to the operator in real-time [3], [4]. In Manuscript received February 24, 2018; accepted June 19, 2018. Date of publication July 4, 2018; date of current version July 19, 2018. This letter was recommended for publication by Associate Editor N. Tsagarakis and Editor K. Hashimoto upon evaluation of the reviewers’ comments. (Corresponding author: Joao Ramos.) The authors are with School of Engineering, Mechanical Engineering De- partment, Massachusetts Institute of Technology, Cambridge, MA 02139 USA (e-mail:, jlramos@mit.edu; sangbae@mit.edu). Digital Object Identifier 10.1109/LRA.2018.2852840 Fig. 1. Left: Linear inverted pendulum model. This template is defined by the total robot mass m R and its nominal height h R . The center of mass x R is controlled by regulating the position of the center of pressure p xR . Right: 3D representation of the LIP for Little HERMES [5], a bipedal robot capable of dynamic behaviors. addition, properly mapping the information between master and slave online is still an open problem in the literature, specially when both system have considerably different scales, dynamics or morphologies. To address these issues, we propose: (i) to utilize a simple template model as the mapping channel between both systems; and (ii) to impose geometric and kinematic similarity between them via bilateral feedback. The first item utilizes a simple model that captures the core dynamic behavior of each system in order to reduce the complexity and the amount of data that must be mapped from master to slave. Additionally, item (ii) provides means to perform dynamic state and force transfor- mations between human and robot, while still allowing them to respect their inherent characteristic time responses. The Prin- ciple of Dynamic Similarity has been extensively utilized for the study of animal motor behavior across different scales [6], [7], the modeling of fluid dynamics [8] and also for bilateral feedback teleoperation of micro-scale manipulators [9]. In this work we utilize a similar idea in order to dynamically couple operator and robot. The simple model utilized in this letter is the Linear Inverted Pendulum (LIP) (see Fig. 1), a popular template for legged robot control due to its simple but competent ability to capture core Center of Mass (CoM) behavior [10], [11]. To evaluate the proposed similarity mapping, we utilize a physical cart-pole system, illustrating the promising results from the teleoperation 2377-3766 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.