43 accepted papers
Equation (9) in [1] was calculated from $\mathbf {J}=\mathbf {U}^\top \mathbf {U}$ and is thus inconsistent with (7) in [1], which defines the decomposition as $\mathbf {J}=\mathbf {U}\mathbf {U}^\top$. The correct parameter mapping $\boldsymbol{\pi }=\mathbf {f}(\boldsymbol{\theta })$ consistent wi
Recognizing and identifying human locomotion is a critical step to ensuring fluent control of wearable robots, such as transtibial prostheses. In particular, classifying the locomotion mode and estimating the gait phase are key. In this work, a novel, interpretable, and computationally efficient alg
As quadruped controllers approach greater maturity for locomotion on level ground, a next challenge relates to enabling these systems to carefully choose contacts in cluttered or discontinuous terrain. In pursuit of this goal, this paper proposes an approach to motion generation for dynamic hopping
For robotic transtibial prosthesis control, the global tibia kinematics can be used to monitor gait cycle progression and command smooth and continuous actuation. In this work, these global tibia kinematics define a phase variable impedance controller (PVIC), which is implemented as the nonvolitiona…
Powered lower-limb prostheses have shown promise in helping individuals with amputation regain functionality that passive prostheses cannot provide. However, the best method for controlling these devices in coordination with their users is still an open research topic. While powered devices can repl…
Differential Dynamic Programming (DDP) is an efficient computational tool for solving nonlinear optimal control problems. It was originally designed as a single shooting method and thus is sensitive to the initial guess supplied. This work considers the extension of DDP to multiple shooting (MS), im…
This letter presents a controller for robots that balance in a vertical plane on a rolling contact on a flat horizontal surface. It is an extension of Featherstone's balance controller to the case of robots that balance on rounded feet or wheels. Simulation results demonstrate the ability of the new
This letter presents a novel algorithm for the continuous control of dynamical systems that combines Trajectory Optimization (TO) and Reinforcement Learning (RL) in a single framework. The motivations behind this algorithm are the two main limitations of TO and RL when applied to continuous nonlinea
Phase variable control based on global tibia kinematics holds promise for predicting gait cycle progression to continuously control robotic transtibial prostheses. Calibration of the phase variable is critical to ensure its monotonic behavior, to approach a linear relationship with gait percentage,…
Specialized motions such as jumping are often achieved on quadruped robots by solving a trajectory optimization problem once and executing the trajectory using a tracking controller. This approach is in parallel with Model Predictive Control (MPC) strategies that commonly control regular gaits via o…
A fundamental problem in legged locomotion is to verify whether a desired trajectory satisfies all physical constraints, especially those for maintaining contacts. Although foot tipping can be avoided via the Zero Moment Point (ZMP) condition, preventing foot sliding and twisting leads to the more c
Optimization-based robot control strategies often rely on first-order dynamics approximation methods, as in iLQR. Using second-order approximations of the dynamics is expensive due to the costly second-order partial derivatives of the dynamics with respect to the state and control. Current approache…
An essential need for many model-based robot control algorithms is the ability to quickly and accurately compute partial derivatives of the equations of motion. State of the art approaches often use analytical methods based on the chain rule applied to existing dynamics algorithms. Although these me
This paper considers the problem of designing legged robots for traversing uneven terrain, wherein terrain characteristics represent uncertainty for the design process. When this process encompasses a wider variety of terrains, the likelihood of the designed robot falling in the real world should de…
Seemingly in defiance of basic physics, cats consistently land on their feet after falling. In this paper, we design a controller that lands the Mini Cheetah quadruped robot on its feet as well. Specifically, we explore how trajectory optimization and machine learning can work together to enable hig…
This letter presents a method for data-driven user-specific gait speed estimation for people with Spinal Cord Injuries (SCIs) walking in lower-limb exoskeletons. The scarcity of training data for this population is addressed by leveraging common patterns across users that relate gait changes to spee
In this letter, we propose a parameterization of the rigid-body inertia tensor that is singularity free, guarantees full physical consistency, and has a straightforward physical interpretation. Based on a version of log-Cholesky decomposition of the pseudo-inertia matrix, we construct a smooth isomo
Reinforcement Learning (RL) has witnessed great strides for quadruped locomotion, with continued progress in the reliable sim-to-real transfer of policies. However, it remains a challenge to reuse a policy on another robot, which could save time for retraining. In this work, we present a framework f…
This letter presents a method to reduce the computational demands of including second-order dynamics sensitivity information into the Differential Dynamic Programming (DDP) trajectory optimization algorithm. An approach to DDP is developed where all the necessary derivatives are computed with the sa
Reduced-order template models are widely used to control high degree-of-freedom legged robots, but existing methods for template-based whole-body control rely heavily on heuristics and often suffer from robustness issues. In this letter, we propose a template-based whole-body control method grounded
Realizing the potential of active lower-limb pros-theses to increase user mobility and efficiency requires safe, reliable, stable, and intuitive control strategies. The two prevailing classes of lower-limb prosthesis control can be categorized as volitional and non-volitional. Volitional control str…
Balancing is a fundamental need for legged robots due to their unstable floating-base nature. Balance control has been thoroughly studied for simple models such as the linear inverted pendulum thanks to the concept of the instantaneous capture point (ICP), yet the constant center of mass height assu
This letter presents a new predictive control architecture for high-dimensional robotic systems. As opposed to a conventional Model Predictive Control (MPC) approach to locomotion that formulates a hierarchical sequence of optimization problems, the proposed work formulates a single optimization pro
This paper presents a control strategy for quadruped robots to hop on their rear legs in three-dimensional space. The proposed approach generates nominal center of mass (CoM) trajectories based on a template spring-loaded inverted pendulum (SLIP) model. Tracking this reference remains a challenge du…
This paper studies push recovery for humanoid robots based on a variable-height inverted pendulum (VHIP) model. We first develop an approach for treating zero-step capturability of the VHIP with a novel methodology based on Hamilton-Jacobi (HJ) reachability analysis. Such an approach uses the sub-ze…
This letter studies jumping for wheeled-bipedal robots, a motion that takes full advantage of the benefits from the hybrid wheeled and legged design features. A comprehensive hierarchical scheme for motion planning and control of jumping with wheeled-bipedal robots is developed. Underactuation of th
This letter outlines an estimation framework to detect changes in the intended gait speed of an assistive exoskeleton user during walking. A twice-per-step estimation strategy, termed a Buttressed Kalman Filter, is presented to leverage changes in foot placement to infer changes in desired speed. Th
This paper outlines steps toward a framework for model-based user intent detection to enable fluent human-robot interaction in assistive exoskeletons. An interacting multi-model (IMM) estimation scheme is presented to address state estimation for lower-extremity exoskeletons and to handle their hybr…
This letter presents a Differential Dynamic Programming (DDP) framework for trajectory optimization (TO) of hybrid systems with state-based switching. The proposed Hybrid Systems DDP (HS-DDP) approach is considered for application to whole-body motion planning with legged robots. Specifically, HS-DD
We present a novel control strategy for dynamic legged locomotion in complex scenarios that considers information about the morphology of the terrain in contexts when only on-board mapping and computation are available. The strategy is built on top of two main elements: first a contact sequence task…
Versatility is one of the main factors driving the adoption of robots on the assembly line and in other applications. Compared to fixed-automation solutions, a single industrial robot can perform a wide range of tasks (e.g., welding, lifting). In other platforms, such as legged robots, versatility i
This letter considers the optimal control problem of an extended spring-loaded inverted pendulum (SLIP) model with two additional actuators for active leg length and hip torque modulation. These additional features arise naturally in practice, allowing for consideration of swing leg kinematics durin
This paper shows how CasADi’s state-of-the-art implementation of algorithmic differentiation can be leveraged to formulate and efficiently solve gait optimization problems, enabling rapid gait design for high-dimensional biped robots. Comparative studies on a 7-DOF planar biped show that CasADi gene…
Gaits in legged robots are often hand tuned and time based, either explicitly or through an internal clock, for instance, in the form of central pattern generators. This strategy requires trial and error to identify leg timings, which may not be suitable in challenging terrains. In this letter, we i
As legged robots are sent into unstructured environments, the ability to robustly manage contact transitions will be a critical skill. This paper introduces an approach to probabilistically fuse contact models, managing uncertainty in terrain geometry, dynamic modeling, and kinematics to improve the…
This paper studies collaboration through the cloud in the context of cooperative adaptive control for robot manipulators. We first consider the case of multiple robots manipulating a common object through synchronous centralized update laws to identify unknown inertial parameters. Through this devel…
This paper presents an implementation of model predictive control (MPC) to determine ground reaction forces for a torque-controlled quadruped robot. The robot dynamics are simplified to formulate the problem as convex optimization while still capturing the full 3D nature of the system. With the simp…
With the increased application of model-based whole-body control in legged robots, there has been a resurgence of research interest into methods for accurate system identification. An important class of methods focuses on the inertialparameters of rigid-body systems. These parameters consist of the
This paper introduces a new robust, dynamic quadruped, the MIT Cheetah 3. Like its predecessor, the Cheetah 3 exploits tailored mechanical design to enable simple control strategies for dynamic locomotion and features high-bandwidth proprioceptive actuators to manage physical interaction with the en…
This paper introduces a new policy-regularized model-predictive control (PR-MPC) approach to automatically generate and stabilize a diverse set of quadrupedal gaits. Model-predictive methods offer great promise to address balance in dynamic robots, yet require the solution of challenging nonlinear o…
While a number of controllers exist for dynamic humanoid walking over known uneven terrain, the ability to negotiate moderate changes in ground height without environment perception is still lacking. Such capability would mitigate problems caused by inaccurate sensing and reduce online terrain-depen
This paper presents a method for the generation of dynamic walking gaits with a 3D Dual-SLIP model and its application to a simulated Hubo+ based humanoid. Previous approaches with the Dual-SLIP model have only focused on the planar case, wherein self-stable gaits can be found. When extended to 3D h…
The Dual-SLIP model has been proposed as a walking template that inherently encodes a rich set of human-like features. Previous work has used the 3D Dual-SLIP with bio-inspired leg actuation to generate a human-like dynamic walking gait over a wide range of speeds. The work presented in this paper e…