31 research outputs found
A Terradynamics of Legged Locomotion on Granular Media
The theories of aero- and hydrodynamics predict animal movement and device
design in air and water through the computation of lift, drag, and thrust
forces. Although models of terrestrial legged locomotion have focused on
interactions with solid ground, many animals move on substrates that flow in
response to intrusion. However, locomotor-ground interaction models on such
flowable ground are often unavailable. We developed a force model for
arbitrarily-shaped legs and bodies moving freely in granular media, and used
this "terradynamics" to predict a small legged robot's locomotion on granular
media using various leg shapes and stride frequencies. Our study reveals a
complex but generic dependence of stresses in granular media on intruder depth,
orientation, and movement direction and gives insight into the effects of leg
morphology and kinematics on movement
A Unified Perspective on Multiple Shooting In Differential Dynamic Programming
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), improving
its robustness to initial guesses. A novel derivation is proposed that accounts
for the defect between shooting segments during the DDP backward pass, while
still maintaining quadratic convergence locally. The derivation enables
unifying multiple previous MS algorithms, and opens the door to many smaller
algorithmic improvements. A penalty method is introduced to strategically
control the step size, further improving the convergence performance. An
adaptive merit function and a more reliable acceptance condition are employed
for globalization. The effects of these improvements are benchmarked for
trajectory optimization with a quadrotor, an acrobot, and a manipulator. MS-DDP
is also demonstrated for use in Model Predictive Control (MPC) for dynamic
jumping with a quadruped robot, showing its benefits over a single shooting
approach
Modeling of the interaction of rigid wheels with dry granular media
We analyze the capabilities of various recently developed techniques, namely
Resistive Force Theory (RFT) and continuum plasticity implemented with the
Material Point Method (MPM), in capturing dynamics of wheel--dry granular media
interactions. We compare results to more conventionally accepted methods of
modeling wheel locomotion. While RFT is an empirical force model for
arbitrarily-shaped bodies moving through granular media, MPM-based continuum
modeling allows the simulation of full granular flow and stress fields. RFT
allows for rapid evaluation of interaction forces on arbitrary shaped intruders
based on a local surface stress formulation depending on depth, orientation,
and movement of surface elements. We perform forced-slip experiments for three
different wheel types and three different granular materials, and results are
compared with RFT, continuum modeling, and a traditional terramechanics
semi-empirical method. Results show that for the range of inputs considered,
RFT can be reliably used to predict rigid wheel granular media interactions
with accuracy exceeding that of traditional terramechanics methodology in
several circumstances. Results also indicate that plasticity-based continuum
modeling provides an accurate tool for wheel-soil interaction while providing
more information to study the physical processes giving rise to resistive
stresses in granular media
Continuous Versatile Jumping Using Learned Action Residuals
Jumping is essential for legged robots to traverse through difficult
terrains. In this work, we propose a hierarchical framework that combines
optimal control and reinforcement learning to learn continuous jumping motions
for quadrupedal robots. The core of our framework is a stance controller, which
combines a manually designed acceleration controller with a learned residual
policy. As the acceleration controller warm starts policy for efficient
training, the trained policy overcomes the limitation of the acceleration
controller and improves the jumping stability. In addition, a low-level
whole-body controller converts the body pose command from the stance controller
to motor commands. After training in simulation, our framework can be deployed
directly to the real robot, and perform versatile, continuous jumping motions,
including omni-directional jumps at up to 50cm high, 60cm forward, and
jump-turning at up to 90 degrees. Please visit our website for more results:
https://sites.google.com/view/learning-to-jump.Comment: To be presented at L4DC 202
CAJun: Continuous Adaptive Jumping using a Learned Centroidal Controller
We present CAJun, a novel hierarchical learning and control framework that
enables legged robots to jump continuously with adaptive jumping distances.
CAJun consists of a high-level centroidal policy and a low-level leg
controller. In particular, we use reinforcement learning (RL) to train the
centroidal policy, which specifies the gait timing, base velocity, and swing
foot position for the leg controller. The leg controller optimizes motor
commands for the swing and stance legs according to the gait timing to track
the swing foot target and base velocity commands using optimal control.
Additionally, we reformulate the stance leg optimizer in the leg controller to
speed up policy training by an order of magnitude. Our system combines the
versatility of learning with the robustness of optimal control. By combining RL
with optimal control methods, our system achieves the versatility of learning
while enjoys the robustness from control methods, making it easily transferable
to real robots. We show that after 20 minutes of training on a single GPU,
CAJun can achieve continuous, long jumps with adaptive distances on a Go1 robot
with small sim-to-real gaps. Moreover, the robot can jump across gaps with a
maximum width of 70cm, which is over 40% wider than existing methods.Comment: Please visit https://yxyang.github.io/cajun/ for additional result