# Design and Control of a Open-Source, Low Cost, 3D Printed Dynamic Quadruped Robot
## 1 Introduction  
As we can see from people and animals, legged locomotion has the potential to handle rough terrain better than any other locomotion types. However, legged locomotion requires complex leg mechanism, high power density actuators and advanced control methods, which have prevented the widespread use of legged robots for a long time. Introduction of the Big Dog quadruped [1] has proved that a quadruped robot with a true rough terrain traversability is indeed possible, but the high power density required for the robot could be only realized by hydraulic actuators powered by a two-stroke gasoline engine, which are not preferable for smaller robots and indoor applications. Although there have been approaches to scale down the hydraulically actuated quadruped [2], hydraulic actuation has not been widely adopted for legged robots yet due to its complexity, weight, cost and maintenance issues. Electric motors, although simpler, lighter and and more convenient to use compared to the hydraulic actuators, have lower power density and torque and is not suitable for directly driving joints of legged robots in general. For bipedal robots powered by electric motors, the conventional actuation method uses high-speed, low-torque motors paired with high ratio gear reducters such as the Harmonic drive to generate the high output torque, which are controlled by a high-gain PD control method to minimize the positional tracking error. This actuation method provides high torque density and low positional tracking error, both of which are crucial for bipedal robots to achieve static stability with small footprint, but has limited backdrivability, high inertia and low maximum joint velocity due to the high reduction ratio and not ideal for walking over uneven terrain without a precise model of the surface where compliance, backdrivability and torque controllability of the joints are needed. Some approaches, such as in [3], uses force torque sensors to provide high bandwidth close loop torque control and active electrical compliance, but such setup is still limited by the high inertia and low transparency due to the high ratio gear reducerOne popular method to provide backdrivability and compliance for the joint is the series elastic actuators (SEAs), where an elastic element is connected in series with the energy source to provide the system a passive compliance and impact robustness. Additional torque and deflection sensors can be used for a close-loop control of the output torque and position. ANYbotics ANYmal [4] uses modular SEA actuators for its joints, and has showed the capability to operate over a wide range of unstructured environments. However SEA has several disadvantages as well: they are relatively expensive, heavy and bulky due to the complex design, has fixed stiffness determined by the elastic element and limited torque control bandwidth. A more direct method, called the quasi-direct drive (QDD) [5], uses a high-torque motor with a large stator radius, paired with gear reducer with relative low gear ratio, typically planetary gear or a multi bar linkage [6] without an elastic component. Such a system is highly backdrivable due to the low gear ratio, and has a high torque control bandwidth that can be used to replicate a user-designed nonlinear stiffness and damping profiles, which is beyond the capability of SEAs. QDD has been successfully used for several medium sized quadrupeds such as MIT Cheetah series [7] and ALPHRED [8], showing excellent maneuverability comparable to hydraulic powered quadrupeds, and terrain-blind walking over uneven terrain and even stairs using proprioceptive feedback alone. Recently, the big commercial success of quadrotors have made high performance brushless motors, micro motor controllers and advanced Lithium batteries commercially available as affordable off-the-shelf components.