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 full-sized 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.

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