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Tingnan Zhang

Tingnan Zhang

My current research focuses on deep reinforcement learning and its application in robotics.
Authored Publications
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    Style-Augmented Mutual Information for Practical Skill Discovery
    Ale Escontrela
    Jason Peng
    Ken Goldberg
    Pieter Abbeel
    Proceedings of NeurIPS (2022) (to appear)
    Preview abstract Exploration and skill discovery in many real-world settings is often inspired by the activities we see others perform. However, most unsupervised skill discovery methods tend to focus solely on the intrinsic component of motivation, often by maximizing the Mutual Information (MI) between the agent's skills and the observed trajectories. These skills, though diverse in the behaviors they elicit, leave much to be desired. Namely, skills learned by maximizing MI in a high-dimensional continuous control setting tend to be aesthetically unpleasing and challenging to utilize in a practical setting, as the violent behavior often exhibited by these skills would not transfer well to the real world. We argue that solely maximizing MI is insufficient if we wish to discover useful skills, and that a notion of "style" must be incorporated into the objective. To this end, we propose the Style-Augmented Mutual Information objective (SAMI), whereby - in addition to maximizing a lower-bound of the MI - the agent is encouraged to minimize the f-divergence between the policy-induced trajectory distribution and the trajectory distribution contained in the reference data (the style objective). We compare SAMI to other popular skill discovery objectives, and demonstrate that skill-conditioned policies optimized with SAMI achieve equal or greater performance when applied to downstream tasks. We also show that the data-driven motion prior specified by the style objective can be inferred from various modalities, including large motion capture datasets or even RGB videos. View details
    Learning Semantic-Aware Locomotion Skills from Human Demonstration
    Byron Boots
    Xiangyun Meng
    Yuxiang Yang
    Conference on Robot Learning (CoRL) 2022 (2022) (to appear)
    Preview abstract The semantics of the environment, such as the terrain type and property, reveals important information for legged robots to adjust their behaviors. In this work, we present a framework that learns semantics-adaptive gait controllers for quadrupedal robots. To facilitate learning, we separate the problem of gait planning and motor control using a hierarchical framework, which consists of a high-level image-conditioned gait policy and a low-level MPC-based motor controller. In addition, to ensure sample efficiency, we pre-train the perception model with an off-road driving dataset, and extract an embedding for downstream learning. To avoid policy evaluation in the noisy real world, we design a simple interface for human operation and learn from human demonstrations. Our framework learns to adjust the speed and gait of the robot based on terrain semantics, using 40 minutes of human demonstration data. We keep testing the performance of the controller on different trails. At the time of writing, the robot has walked 0.2 miles without failure. View details
    Preview abstract We propose an end-to-end framework to enablemultipurpose assistive mobile robots to autonomously wipetables and clean spills and crumbs. This problem is chal-lenging, as it requires planning wiping actions with uncertainlatent crumbs and spill dynamics over high-dimensional visualobservations, while simultaneously guaranteeing constraintssatisfaction to enable deployment in unstructured environments.To tackle this problem, we first propose a stochastic differentialequation (SDE) to model crumbs and spill dynamics and ab-sorption with the robot wiper. Then, we formulate a stochasticoptimal control for planning wiping actions over visual obser-vations, which we solve using reinforcement learning (RL). Wethen propose a whole-body trajectory optimization formulationto compute joint trajectories to execute wiping actions whileguaranteeing constraints satisfaction. We extensively validateour table wiping approach in simulation and on hardware. View details
    Preview abstract Evolution Strategy (ES) algorithms have shown promising results in training complex robotic control policies due to their massive parallelism capability, simple implementation, effective parameter-space exploration, and fast training time. However, a key limitation of ES is its scalability to large capacity models, including modern neural network architectures. In this work, we develop Predictive Information Augmented Random Search (PI-ARS) to mitigate this limitation by leveraging recent advancements in representation learning to reduce the parameter search space for ES. Namely, PI-ARS combines a gradient-based representation learning technique, Predictive Information (PI), with a gradient-free ES algorithm, Augmented Random Search (ARS), to train policies that can process complex robot sensory inputs and handle highly nonlinear robot dynamics. We evaluate PI-ARS on a set of challenging visual-locomotion tasks where a quadruped robot needs to walk on uneven stepping stones, quincuncial piles, and moving platforms, as well as to complete an indoor navigation task. Across all tasks, PI-ARS demonstrates significantly better learning efficiency and performance compared to the ARS baseline. We further validate our algorithm by demonstrating that the learned policies can successfully transfer to a real quadruped robot, for example, achieving a 100% success rate on the real-world stepping stone environment, dramatically improving prior results achieving 40% success. View details
    Real-time remodeling of granular terrain for robot locomotion
    Andras Karsai
    Daniel I. Goldman
    Daniel Soto
    Deniz Kerimoglu
    Sehoon Ha
    Space Robotics (2022)
    Preview abstract Recent studies of robot movement in flowable granular media inspired by difficulties faced by extraterrestrial rovers reveal a coupled locomotor/substrate effect where the robot spontaneously remodels its environment. Such coupling occurs in certain limb/wheel movement patterns that results in a localized granular flow allowing the robot to effectively “swim” up highly flowable slopes. However, these gaits were discovered via trial and error by human operators. as the highly hysteretic nature of easily flowable terrain also creates tractability and predictability challenges in locomotion planning and gait policies. To overcome this, additional anchoring structures on intruding appendages can dynamically stabilize slopes to prevent undesired flows and slipping during locomotion. Granular media’s multiphase properties make it amenable to creative manipulations dependent on the physics of the intruding structure. A pair of robot studies showcase both selective solidification and fluidization strategies in flowable slopes to locomote successfully. To accelerate gait discovery in both studies, a machine learning approach for real-time characterization of the terrain flow could allow robots to control the flowable substrate for effective locomotion. A future neural network trained with sufficient spatiotemporal terrain data could predict granular flow with high accuracy and generality, augmenting gait learning with knowledge of the environment’s evolution during movement. View details
    Learning Model Predictive Controllers with Real-Time Attention for Real-World Navigation
    Anthony G. Francis
    Dmitry Kalashnikov
    Edward Lee
    Jake Varley
    Leila Takayama
    Mikael Persson
    Peng Xu
    Stephen Tu
    Xuesu Xiao
    Conference on Robot Learning (2022) (to appear)
    Preview abstract Despite decades of research, existing navigation systems still face real-world challenges when being deployed in the wild, e.g., in cluttered home environments or in human-occupied public spaces. To address this, we present a new class of implicit control policies combining the benefits of imitation learning with the robust handling of system constraints of Model Predictive Control (MPC). Our approach, called Performer-MPC, uses a learned cost function parameterized by vision context embeddings provided by Performers---a low-rank implicit-attention Transformer. We jointly train the cost function and construct the controller relying on it, effectively solving end-to-end the corresponding bi-level optimization problem. We show that the resulting policy improves standard MPC performance by leveraging a few expert demonstrations of the desired navigation behavior in different challenging real-world scenarios. Compared with a standard MPC policy, Performer-MPC achieves 40% better goal reached in cluttered environments and 65% better sociability when navigating around humans. View details
    Safe Reinforcement Learning for Legged Locomotion
    Jimmy Yang
    Peter J. Ramadge
    Sehoon Ha
    International Conference on Robotics and Automation (2022) (to appear)
    Preview abstract Designing control policies for legged locomotion is complex due to underactuation and discrete contact dynamics. To deal with this complexity, applying reinforcement learning to learn a control policy in the real world is a promising approach. However, safety is a bottleneck when robots need to learn in the real world. In this paper, we propose a safe reinforcement learning framework that switches between a safe recovery policy and a learner policy. The safe recovery policy takes over the control when the learner policy violates safety constraints, and hands over the control back when there are no future safety violations. We design the safe recovery policy so that it ensures safety of legged locomotion while minimally interfering with the learning process. Furthermore, we theoretically analyze the proposed framework and provide an upper bound on the task performance. We verify the proposed framework in three locomotion tasks on a simulated quadrupedal robot: catwalk, two-leg balance, and pacing. On average, our method achieves 48.6% fewer falls and comparable or better rewards than the baseline methods. View details
    Adversarial Motion Priors Make Good Substitutes for Complex Reward Functions
    Ale Escontrela
    Jason Peng
    Ken Goldberg
    Pieter Abbeel
    2022 IEEE/RSJ International Conference on Intelligent Robots and Systems, IROS (2022) (to appear)
    Preview abstract Training high-dimensional simulated agents with under-specified reward functions often leads to jerky and unnatural behaviors, which results in physically infeasible strategies that are generally ineffective when deployed in the real world. To mitigate these unnatural behaviors, reinforcement learning (RL) practitioners often utilize complex reward functions that encourage more physically plausible behaviors, in conjunction with tricks such as domain randomization to train policies that satisfy the user's style criteria and can be successfully deployed on real robots. Such an approach has been successful in the realm of legged locomotion, leading to state-of-the-art results. However, designing effective reward functions can be a labour-intensive and tedious tuning process, and these hand-designed rewards do not easily generalize across platforms and tasks. We propose substituting complex reward functions with "style rewards" learned from a dataset of motion capture demonstrations. This learned style reward can be combined with a simple task reward to train policies that perform tasks using naturalistic strategies. These more natural strategies can also facilitate transfer to the real world. We build upon prior work in computer graphics and demonstrate that an adversarial approach to training control policies can produce behaviors that transfer to a real quadrupedal robot without requiring complex reward functions. We also demonstrate that an effective style reward can be learned from a few seconds of motion capture data gathered from a German Shepherd and leads to energy-efficient locomotion strategies with natural gait transitions. View details
    Zero-Shot Retargeting of Learned Quadruped Locomotion Policy Using A Hybrid Kinodynamic Model and Predictive Control
    He Li
    Patrick Wensing
    2022 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2022) (2022) (to appear)
    Preview abstract As a rivaling control technique, Reinforcement Learning (RL) has demonstrated great performance in quadruped locomotion. However, it remains a challenge to reuse a policy on another robot, i.e., policy transferability, which saves time for retraining. In this work, we reduce the gap by devloping a planning-and-control framework that systematically integrates RL and Model Predictive Control (MPC). The planning stage employs RL to generate a dynamically-plausible trajectory as well as the contact schedule. These information are then used to seed the MPC in the low level to stabilize and robustify the motion. In addition, our MPC controller employs a novel Hybrid Kino-Dynamics (HKD) model which implicitly optimizes the foothold locations. The results are surprisingly good since the policy trained for the Unitree A1 robot could be transferred to the MIT Mini Cheetah with the proposed pipeline. View details
    Fast and Efficient Locomotion via Learned Gait Transitions
    Yuxiang Yang
    Erwin Coumans
    Byron Boots
    Conference on Robot Learning (2021)
    Preview abstract We focus on the problem of developing energy efficient controllers for quadrupedal robots. Animals can actively switch gaits at different speeds to lower their energy consumption. In this paper, we devise a hierarchical learning framework, in which distinctive locomotion gaits and natural gait transitions emerge automatically with a simple reward of energy minimization. We use evolutionary strategies (ES) to train a high-level gait policy that specifies gait patterns of each foot, while the low-level convex MPC controller optimizes the motor commands so that the robot can walk at a desired velocity using that gait pattern. We test our learning framework on a quadruped robot and demonstrate automatic gait transitions, from walking to trotting and to fly-trotting, as the robot increases its speed. We show that the learned hierarchical controller consumes much less energy across a wide range of locomotion speed than baseline controllers. View details
    Preview abstract Reinforcement learning provides an effective tool for robots to acquire diverse skills in an automated fashion.For safety and data generation purposes, control policies are often trained in a simulator and later deployed to the target environment, such as a real robot. However, transferring policies across domains is often a manual and tedious process. In order to bridge the gap between domains, it is often necessary to carefully tune and identify the simulator parameters or select the aspects of the simulation environment to randomize. In this paper, we design a novel, adversarial learning algorithm to tackle the transfer problem. We combine a classic, analytical simulator with a differentiable, state-action dependent system identification module that outputs the desired simulator parameters. We then train this hybrid simulator such that the output trajectory distributions are indistinguishable from a target domain collection. The optimized hybrid simulator can refine a sub-optimal policy without any additional target domain data. We show that our approach outperforms the domain-randomization and target-domain refinement baselines on two robots and six difficult dynamic tasks. View details
    Learning to walk on complex terrains with vision
    Ale Escontrela
    Erwin Johan Coumans
    Peng Xu
    Sehoon Ha
    Conference on Robotic Learning (2021)
    Preview abstract Visual feedback is crucial for legged robots to safely and efficiently handle uneven terrains such as stairs. However, effectively training robots to effectively consume high dimensional visual input for locomotion is challenging. In this work, we propose a framework to train a vision-based locomotion controller for quadruped robots to traverse a variety of uneven environments. Our key idea is to model the locomotion controller as a hierarchical structure with a high-level vision policy and a low-level motion controller. The high-level vision policy takes as input the perceived vision inputs as well as robot states and outputs desired foothold placement and base movement of the robot, which is realized by low level motion controller composed of a position controller for swing legs and a MPC-based torque controller for stance legs. We train the vision policy using Deep Reinforcement Learning and demonstrate our approach on a variety of uneven environments such as step-stones, stairs, pillars, and moving platforms. We also deploy our policy on a real quadruped robot to walk over a series of random step-stones. View details
    Learning Agile Robotic Locomotion Skills by Imitating Animals
    Edward Lee
    Erwin Johan Coumans
    Jason Peng
    Sergey Levine
    Robotics: Science and Systems 2020, RSS Foundation (2020)
    Preview abstract Reproducing the diverse and agile locomotion skills of animals has been a longstanding challenge in robotics. While manually designed controllers have been able to emulate many complex behaviors, building such controllers often involves a tedious engineering process, and requires substantial expertise of the nuances of each skill. In this work, we present an imitation learning system that enables legged robots to learn agile locomotion skills by imitating real-world animals. We show that by leveraging reference motion data, a common framework is able to automatically synthesize controllers for a diverse repertoire behaviors. By incorporating sample efficient domain adaptation techniques into the training process, our system is able to train adaptive policies in simulation, which can then be quickly finetuned and deployed in the real world. Our system enables an 18-DoF quadruped robot to perform a variety of agile behaviors ranging from different locomotion gaits to dynamic hops and turns. View details
    Model-based Reinforcement Learning for Decentralized Multiagent Rendezvous
    Rose E. Wang
    Dennis Lee
    Edward Lee
    Brian Andrew Ichter
    Conference on Robot Learning (CoRL) (2020)
    Preview abstract Collaboration requires agents to align their goals on the fly. Underlying the human ability to align goals with other agents is their ability to predict the intentions of others and actively update their own plans. We propose hierarchical predictive planning (HPP), a model-based reinforcement learning method for decentralized multiagent rendezvous. Starting with pretrained, single-agent point to point navigation policies and using noisy, high-dimensional sensor inputs like lidar, we first learn via self-supervision motion predictions of all agents on the team. Next, HPP uses the prediction models to propose and evaluate navigation subgoals for completing the rendezvous task without explicit communication among agents. We evaluate HPP in a suite of unseen environments, with increasing complexity and numbers of obstacles. We show that HPP outperforms alternative reinforcement learning, path planning, and heuristic-based baselines on challenging, unseen environments. Experiments in the real world demonstrate successful transfer of the prediction models from sim to real world without any additional fine-tuning. Altogether, HPP removes the need for a centralized operator in multiagent systems by combining model-based RL and inference methods, enabling agents to dynamically align plans. View details
    Preview abstract Imitation learning is a popular approach for training effective visual navigation policies. However, collecting expert demonstrations for a legged robot is less practical because the robot is hard to control, and it walks slowly and cannot run continuously for a long time. In this work, we propose a zero-shot imitation learning framework for training a visual navigation policy on a legged robot from human demonstration (third-person perspective) only, allowing for more cost-effective data collection with better navigation capability. However, imitation learning from third-person perspective demonstrations raises unique challenges. Human demonstrations are captured with different camera perspectives, therefore, we design a feature disentanglement network~(FDN) that extracts perspective-agnostic state features. We reconstruct missing action labels by either building an inverse model of the robot's dynamics in the feature space and applying it to the demonstrations or developing efficient GUI to label human demonstrations. We take a model-based imitation learning approach for training a visual navigation policy from the perspective-agnostic, action-labeled demonstrations. We show that our framework can learn an effective visual navigation policy for a legged robot, Laikago, from expert demonstrations in both simulated and real-world environments. Our approach is zero-shot as the robot never tries to navigate a certain navigation path in the testing environment before the testing phase. We also justify our framework by performing an ablation study and comparing it with baseline algorithms. View details
    Preview abstract Designing agile locomotion for quadruped robots often requires extensive expertise and tedious manual tuning. In this paper, we present a system to automate this process by leveraging deep reinforcement learning techniques. Our system can learn quadruped locomotion from scratch with simple reward signals. In addition, users can provide an open loop reference to guide the learning process if more control over the learned gait is needed. The control policies are learned in a physical simulator and then deployed to real robots. In robotics, policies trained in simulation often does not transfer to the real world. We narrow this reality gap by improving the physical simulator and learning robust policies. We improve the simulation using system identification, developing an accurate actuator model and simulating latency. We learn robust controllers by randomizing the physical environments, adding perturbations and designing a compact observation space. We evaluate our system on two agile locomotion gaits: trotting and galloping. After learning in simulation, a quadruped robot can successfully perform both gaits in real world. View details
    Preview abstract We propose an architecture for learning complex controllable behaviors by having simple Policies Modulate Trajectory Generators (PMTG), a powerful combination that can provide both memory and prior knowledge to the controller. The result is a flexible architecture that is applicable to a class of problems with periodic motion for which one has an insight into the class of trajectories that might lead to a desired behavior. We illustrate the basics of our architecture using a synthetic control problem, then go on to learn speed-controlled locomotion for a quadrupedal robot by using Deep Reinforcement Learning and Evolutionary Strategies. We demonstrate that a simple linear policy, when paired with a parametric Trajectory Generator for quadrupedal gaits, can induce walking behaviors with controllable speed from 4-dimensional IMU observations alone, and can be learned in under 1000 rollouts. We also transfer these policies to a real robot and show locomotion with controllable forward velocity. View details
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