We present a reduced-order approach for dynamic and efficient bipedal control, culminating in 3D balancing and walking with ATRIAS, a heavily underactuated bipedal robot. These results are a development toward solving a number of enduring challenges in bipedal locomotion: achieving robust 3D gaits at various speeds and transitioning between them, all while minimally draining on-board energy supplies. Our reduced-order control methodology works by extracting and exploiting general dynamical behaviors from the spring-mass model of bipedal walking. When implemented on a robot with spring-mass passive dynamics, e.g. ATRIAS, this controller is sufficiently robust to balance while subjected to pushes, kicks, and successive dodge-ball strikes. The controller further allowed smooth transitions between stepping in place and walking at a variety of speeds (up to 1.2 m/s). The resulting gait dynamics also match qualitatively to the reduced-order model, and additionally, measurements of human walking. We argue that the presented locomotion performance is compelling evidence of the effectiveness of the presented approach; both the control concepts and the practice of building robots with passive dynamics to accommodate them.

Although there has been recent progress in control of multi-joint prosthetic legs for rhythmic tasks such as walking, control of these systems for non-rhythmic motions and general real-world maneuvers is still an open problem. In this article, we develop a new controller that is capable of both rhythmic (constant-speed) walking, transitions between speeds and/or tasks, and some common volitional leg motions. We introduce a new piecewise holonomic phase variable, which, through a finite state machine, forms the basis of our controller. The phase variable is constructed by measuring the thigh angle, and the transitions in the finite state machine are formulated through sensing foot contact along with attributes of a nominal reference gait trajectory. The controller was implemented on a powered knee-ankle prosthesis and tested with a transfemoral amputee subject, who successfully performed a wide range of rhythmic and non-rhythmic tasks, including slow and fast walking, quick start and stop, backward walking, walking over obstacles, and kicking a soccer ball. Use of the powered leg resulted in clinically significant reductions in amputee compensations for rhythmic tasks (including vaulting and hip circumduction) when compared to use of the take-home passive leg. In addition, considerable improvements were also observed in the performance for nonrhythmic tasks. The proposed approach is expected to provide a better understanding of rhythmic and non-rhythmic motions in a unified framework, which in turn can lead to more reliable control of multi-joint prostheses for a wider range of real-world tasks.

In this article, we present the design of a powered knee-ankle prosthetic leg, which implements high-torque actuators with low-reduction transmissions. The transmission coupled with a high-torque and low-speed motor creates an actuator with low mechanical impedance and high backdrivability. This style of actuation presents several possible benefits over modern actuation styles in emerging robotic prosthetic legs, which include free-swinging knee motion, compliance with the ground, negligible unmodeled actuator dynamics, less acoustic noise, and power regeneration. Benchtop tests establish that both joints can be backdriven by small torques (∼1-3 N•m) and confirm the small reflected inertia. Impedance control tests prove that the intrinsic impedance and unmodeled dynamics of the actuator are sufficiently small to control joint impedance without torque feedback or lengthy tuning trials. Walking experiments validate performance under the designed loading conditions with minimal tuning. Finally, the regenerative abilities, low friction, and small reflected inertia of the presented actuators reduced power consumption and acoustic noise compared to state-of-the-art powered legs.

B iological bipeds have long been thought to take advantage of compliance and passive dynamics to walk and run, but realizing robotic locomotion in this fashion has been difficult in practice. Assume The Robot Is A Sphere (ATRIAS) is a bipedal robot designed to take advantage of the inherent stabilizing effects that emerge as a result of tuned mechanical com pliance (Table 1). In this article, we describe the mechanics of the biped and how our controller exploits the interplay bet ween passive dynamics and actuation to achieve robust locomotion. We outline our development process for the incremental design and testing of our controllers through rapid iteration. By show time at the Defense Advanced Research Pro jects Agency (DARPA) Robotics Challenge (Figure 1), ATRIAS was able to walk with robustness, locomote in ter rain from asphalt to grass to artificial turf, and traverse changes in surface height as large as 15 cm without planning or visual feedback. Furthermore, ATRIAS can accelerate from rest, transition smoothly to a running gait, and reach a top speed of 2.5 m/s (9 km/h). Reliably achieving such dynamic locomotion in an uncertain environment required rigorous development and testing of the hardware, software, and control algorithms. This endeavor culminated in seven live shows of ATRIAS walking and running, with disturbances and without falling, in front of a live audience at the DARPA Robotics Challenge. Approaches to Biped Control Walking and running on two legs is an enduring challenge in robotics. Avoiding falls becomes especially tricky when the terrain is uncertain in both its geometry and rigidity. A promising approach to achieving stable control is to relin quish some authority to purposeful passive dynamics, per haps by adding mechanical compliance [1] or removing actuators entirely [2]. If the machine's unactuated dynam ics are thoughtfully designed, they can passively attenuate disturbances and require smaller adjustments from the controller [33].

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