Development of a Full-sized Bipedal Humanoid Robot Utilizing Spring Assisted Parallel Four-bar Linkages with Synchronized Actuation

Besides the difficulties in control and gait generation, designing a full-sized (taller than 1.3m) bipedal humanoid robot that can walk with two legs is a very challenging task, mainly due to the large torque requirements at the joints combined with the need for the actuators' size and weight to be small. Most of the handful of successful humanoid robots in this size class that exist today utilize harmonic drives for gear reduction to gain high torque in a compact package. However, this makes the cost of such a robot too high and thus puts it out of reach of most of those who want to use it for general research, education and outreach activities. Besides the cost, the heavy weight of the robot also causes difficulties in handling and raises concerns for safety. In this paper we present the design of a new class of fullsized bipedal humanoid robots that is lightweight and low cost. This is achieved by utilizing spring assisted parallel four-bar linkages with synchronized actuation in the lower body to reduce the torque requirements of the individual actuators which also enables the use of off the shelf components to further reduce the cost significantly. The resulting savings in weight not only makes the operation of the robot safer, but also allows it to forgo the expensive force/torque sensors at the ankles and achieve stable bipedal walking only using the feedback from the IMU(Inertial Measurement Unit.) CHARLI-L (Cognitive Humanoid Autonomous Robot with Learning Intelligence Lightweight) is developed using this approach and successfully demonstrated untethered bipedal locomotion using ZMP (Zero Moment Point) based control, stable omnidirectional gaits, and carrying out tasks autonomously using vision based localization. INTRODUCTION Since the debut of Honda's P2 humanoid robot in 1996 [1], efforts in developing full-sized humanoid robots around the world resulted in a number of successful designs. WABIAN of Waseda University [2], HRP series of JAIST [3], HUBO of KAIST [4] and JOHNNIE of Technical University of Munich [5] are some good examples. One of the biggest challenges in the development of such robots is to design the joints for the lower body that can produce enough torque to handle the static and dynamic loads while being compact enough to be packaged in the constrained space within the robot. To achieve this, most of these robots use DC servomotors with harmonic drives for actuation and gear reduction, but this approach makes them heavy and expensive. In this paper, we present a design of a new class of fullsized bipedal humanoid robots that is lightweight and low cost. This is achieved by utilizing spring assisted parallel four-bar linkages with synchronized actuation in the lower body to reduce the torque requirements of the individual actuators which also enables the use of off the shelf components to further reduce the cost significantly. The approach of using of a four-bar linkage as a mechanism for bipedal robots has been suggested before. Arnaud Hamon et. al. [6] presented a cross four-bar linkage design for the knee joint to take advantage of the spring force. The results from the simulation have shown that the spring could be used for making the system more energy efficient. J. McKendry et. al. [7] developed a new kinematic mechanism for a bipedal robot which consists of several four-bar linkages with a single actuator. However, it used three legs to stabilize the body. Hyeung-Sik Choi, et. al.[8] Proceedings of the ASME 2011 International Design Engineering Technical Conferences & Computers and Information in Engineering Conference IDETC/CIE 2011 August 28-31, 2011, Washington, DC, USA