Meta-Controller: Few-Shot Imitation of Unseen Embodiments and Tasks in Continuous Control

Generalizing across robot embodiments and tasks is crucial for adaptive robotic systems. Modular policy learning approaches adapt to new embodiments but are limited to specific tasks, while few-shot imitation learning (IL) approaches often focus on a single embodiment. In this paper, we introduce a few-shot behavior cloning framework to simultaneously generalize to unseen embodiments and tasks using a few (\emph{e.g.,} five) reward-free demonstrations. Our framework leverages a joint-level input-output representation to unify the state and action spaces of heterogeneous embodiments and employs a novel structure-motion state encoder that is parameterized to capture both shared knowledge across all embodiments and embodiment-specific knowledge. A matching-based policy network then predicts actions from a few demonstrations, producing an adaptive policy that is robust to over-fitting. Evaluated in the DeepMind Control suite, our framework termed \modelname{} demonstrates superior few-shot generalization to unseen embodiments and tasks over modular policy learning and few-shot IL approaches. Codes are available at \href{https://github.com/SeongwoongCho/meta-controller}{https://github.com/SeongwoongCho/meta-controller}.

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Peer review

Reviewer dCFx6/10 · confidence 4/52024-06-22

Summary

This paper presents a method for few-shot imitation learning for new embodiment. Specifically, it presents a method that learns a state representation that decouples embodiment-specific and task-specific knowledge and a meta-learning framework that transfers between embodiments and tasks. Results suggest that the resulting policy improves over decision transformer-based or modular policy-based baselines.

Strengths

1. A design choice (separating the state encoder from the matching-based policy network) that decouples state learning from task-specific knowledge learning. 2. The proposed method demonstrates better generalization to unseen embodiments in DMC, outperforming existing baselines.

Weaknesses

Only demonstrates results in DMC. The reviewer is not certain if this scales to real-world control environments, as the different embodiments operate at various scales, and the model needs to be able to be robust to environment dynamics (here the environment is static). The reviewer acknowledges that this limitation is also mentioned in the limitation section.

Questions

1. Given that state representation learning is decoupled from policy representation learning, it would be interesting to visualize the embedding learned by the state encoder and see how distinct it is between different morphologies. 2. Does the performance of the model scale with model capacity and data? I.e. currently the model is pre-trained with “28 tasks from 9 embodiments … up to 2000 demonstration trajectories for each task and embodiment”. What if some of the tasks and embodiments are removed? Do some embodiments/tasks matter more than others?

Rating

6

Confidence

4

Soundness

3

Presentation

3

Contribution

3

Limitations

The limitation section is present.

Reviewer iQ1f5/10 · confidence 4/52024-07-10

Summary

This paper aims to generalize to unseen embodiments and tasks by few-shot behavior cloning. It proposes a modular framework to capture both shared knowledge across all embodiments and embodiment-specific knowledge. It utilizes a matching-based method to enhance the robustness to overfitting. It shows superior performance in DMC simulations.

Strengths

* The frameworks proposed by the paper are reasonable. In order to generalize to different embodiments and share common knowledge, it designs embodiment-specific parameters and shared parameters for networks. * The paper demonstrates strong performance compared with the baselines.

Weaknesses

* The models are complex and matching-based policies are time-consuming. The models consist of multiple different transformer blocks. The matching-based policies need to compute similarities with all pairs in demonstrations to predict one action. * The environments are too simple. The paper only focuses on the DeepMind Control suite and they are only state-based environments. What about the performance when the environments are image-based or manipulation tasks?

Questions

* See weakness * What is the learning curve for the models compared with PDT+PEFT? Are the models more efficient since they are modular networks? * The experiments only compare the results of at least 5-shot. What about the performance when given fewer demonstrations? * The episode sampling strategy is confusing. The claim "D and Q are obtained from adjacent training epochs" limits the adaption of the models. "a temporal segment size of 10 for episodic meta-learning". Does it mean that the horizon of each few-shot demonstration is 10 and is it enough to provide task information? It is better to provide more details about few-shot demonstrations.

Rating

5

Confidence

4

Soundness

3

Presentation

2

Contribution

2

Limitations

* The paper has provided several limitations including simulations, highly stochastic dynamics, long-term planning and computational complexity.

Reviewer EmwS7/10 · confidence 4/52024-07-11

Summary

This paper introduces a framework called Meta-Controller for few-shot behavior cloning that can generalize to unseen robot embodiments and tasks in continuous control problems. The key contributions are: 1. A joint-level input/output representation to unify state and action spaces across heterogeneous robot embodiments. 2. A novel state encoder with two components: - A structure encoder to capture morphological knowledge - A motion encoder to capture dynamics knowledge Both use adaptive parameters to specialize to specific embodiments and tasks. 3. A matching-based policy network that leverages a few demonstrations to predict actions for new tasks. 4. A training protocol involving episodic meta-learning followed by few-shot fine-tuning. The authors evaluate Meta-Controller on various tasks from the DeepMind Control suite, demonstrating superior few-shot generalization performance compared to existing modular policy learning and few-shot imitation learning approaches. Key results show Meta-Controller outperforms baselines, especially on challenging tasks like the reacher-four embodiment. Ablation studies validate the importance of the structure encoder, motion encoder, and matching module.

Strengths

This paper has several strengths: - To the best of my knowledge, the proposed approach is novel. The Meta-Controller framework addresses an important challenge in robotics - simultaneous generalization to unseen embodiments and tasks with few-shot learning. This is a solid step beyond existing work that typically focuses on either embodiment generalization or task generalization, but not both. To solve the problem, this paper introduces a well-thought-out architecture that combines several innovative components: - Joint-level I/O representation for handling heterogeneous embodiments - Structure and motion encoders with adaptive parameters - Matching-based policy network for few-shot adaptation - The evaluation is thorough. The authors conduct extensive experiments using the DeepMind Control suite, comparing their approach against multiple baselines from both modular policy learning and few-shot imitation learning domains. The paper also includes comprehensive ablation studies that validate the importance of each component in the architecture. - The performance is strong. Meta-Controller consistently outperforms existing methods, especially on challenging tasks like the reacher-four embodiment, demonstrating its effectiveness. - The paper is well-structured and clearly written, making it accessible despite the complexity of the topic. And the figures are nice and neat.

Weaknesses

While the paper has many strengths, there are also some potential weaknesses or areas that could be improved: - The proposed method assumes the number of joints is equal to the action dimension. However, this assumption may not always hold, as many robots incorporate passive joints, such as in four-bar linkages. Please correct me if my understanding is wrong. - The proposed method appears to share conceptual connections to some retrieval-based techniques [1,2]. A comparative empirical evaluation with these related methods could provide helpful insights. - Limited real-world validation: As the experiments were conducted exclusively within simulated environments (the DeepMind Control suite), open questions remain regarding the approach's transferability to real-world robotic systems incorporating more complex dynamics and noise. - Limited failure case discussion: Analyzing scenarios in which the Meta-Controller performs less effectively could offer perspectives for future enhancements. A more in-depth failure case analysis may strengthen the presentation. [1] Pari, Jyothish, et al. "The surprising effectiveness of representation learning for visual imitation." arXiv preprint arXiv:2112.01511 (2021). [2] Sridhar, Kaustubh, et al. "Memory-consistent neural networks for imitation learning." arXiv preprint arXiv:2310.06171 (2023).

Questions

- In the case of robot arm manipulation, what are the benefits of utilizing the proposed structure-motion encoder over the direct application of end-effector-based control? - Could you clarify what is meant by the "Embodiment-specific positional embedding" as depicted in Figure 2? How do you implement it? - It appears that the action $m_j$ is conditioned solely on the feature of joint $j$ ($z_j$). Why not consider the state features from all joints to simultaneously generate motions for each joint? - What necessitates the use of an action encoder and decoder? Why not simply retrieve actions based on their similarity?

Rating

7

Confidence

4

Soundness

3

Presentation

4

Contribution

3

Limitations

See weaknesses.

Reviewer TTwU4/10 · confidence 4/52024-07-11

Summary

This paper introduces Meta-Controller, a few-shot behavior cloning framework designed for adaptation to various embodiment and control tasks. The framework includes a transformer-based structure-motion state encoder that captures knowledge across different embodiments, and a matching-based policy network that generates adaptive policies. Experimental results demonstrate that the proposed method surpasses other baseline methods, and ablation studies confirm the effectiveness of each module within the framework.

Strengths

1、The structure encoder captures knowledge across different embodiments, enhancing the model's generalization ability across various embodiment types. Additionally, the motion encoder enables the model to comprehend the temporal dynamics of states, which simultaneously improves performance in controlling joints and achieving goals. 2、The ablation study effectively demonstrates the contribution of each module within the framework. 3、The architecture of each module is clearly illustrated and easy to understand. 4、The appendix provides extensive details on training and evaluation, facilitating the reproduction of the experiment.

Weaknesses

1、The paper uses only four different embodiments (12 in total) to demonstrate generalization ability, which is a relatively small sample size. 2、The proposed method operates only in a 2D coordinate space, limiting its application potential in realistic 3D environments. 3、The multiple transformer-based architecture constrains the possibility of real-time inference.

Questions

1、The ablation study in Table 2 is incomplete. Results should be included for scenarios where only the fs module is removed and where all three modules are removed. 2、Since the author only provides results based on 4 different embodiment types. It raises the question of whether the policy would still be effective with variations in embodiment configurations, such as random changes in joint length within the same embodiment. 3、Since the policy is learned from a fixed trajectory dataset, its sensitivity to noise perturbations during transitions is unclear. A robustness analysis would further validate the generalization ability of the proposed method.

Rating

4

Confidence

4

Soundness

2

Presentation

3

Contribution

2

Limitations

The author thoroughly addresses the paper's limitations, which is commendable. However, some issues remain unresolved, such as the efficiency problems caused by the transformer-based architecture. This inefficiency contradicts the motivation of enabling adaptation to various tasks and embodiments in the real world. Similarly, the constraint of operating in a 2D space limits the method's practical applicability.

Authorsrebuttal2024-08-13

We respectfully disagree with the reviewer. There have been extensive community efforts in building foundation models for continuous control [11, 12, 13, 14], given their significant potential to create versatile robotic agents that can be utilized in various real-world applications. Since generalization capability inherently stems from the scale of the model and training data, most (if not all) approaches are built upon large-scale Transformer backbones. Although these models may not be advantageous for real-time operation compared to lightweight models, we believe that the value of these efforts should not be underestimated simply due to computational cost, especially when considering their capabilities and significance for general-purpose robots. Additionally, there have always been parallel efforts in the machine learning community to develop more capable models and to accelerate their computation. As discussed in our rebuttal, there is an extensive body of work aimed at reducing the computational demands of Transformers, including techniques such as quantization[15], pruning[16], distillation[17], and linear attention[18], as well as methods to enhance hardware utilization, such as FlashAttention[19, 20, 21]. These approaches are orthogonal to our method and generally applicable to any Transformer backbone. Moreover, considering ongoing developments in hardware that rapidly reduce the cost of computation and enhance the applicability of large-scale models, we believe it is important to refrain from judging the real-time applicability based solely on current computational limitations. [11] Janner et al., “Offline Reinforcement Learning as One Big Sequence Modeling Problem”, NeurIPS, 2021 [12] Sun et al., “SMART: SELF-SUPERVISED MULTI-TASK PRETRAINING WITH CONTROL TRANSFORMERS”, ICLR, 2023 [13] Chen et al., “Decision Transformer: Reinforcement Learning via Sequence Modeling”, NeurIPS, 2021 [14] Liu et al., “Masked Autoencoding for Scalable and Generalizable Decision Making”, NeurIPS, 2022 [15] Liu et al., “Post-Training Quantization for Vision Transformer”, NeurIPS, 2021 [16] Kim, Sehoon, et al. "Learned token pruning for transformers." Proceedings of the 28th ACM SIGKDD Conference on Knowledge Discovery and Data Mining. 2022. [17] Sanh, V. "DistilBERT, a distilled version of BERT: smaller, faster, cheaper and lighter." Proceedings of Thirty-third Conference on Neural Information Processing Systems (NIPS2019). 2019. [18] Choromanski, Krzysztof Marcin, et al. "Rethinking Attention with Performers." International Conference on Learning Representations. 2020. [19] Dao, Tri, et al. "Flashattention: Fast and memory-efficient exact attention with io-awareness." Advances in Neural Information Processing Systems 35 (2022): 16344-16359. [20] Dao, Tri. "FlashAttention-2: Faster Attention with Better Parallelism and Work Partitioning." The Twelfth International Conference on Learning Representations. [21] Shah, Jay, et al. "Flashattention-3: Fast and accurate attention with asynchrony and low-precision." arXiv preprint arXiv:2407.08608 (2024).

Reviewer TTwU2024-08-13

Your response shows your unprofessionalism and lack of seriousness. - The third and fourth paragraphs in the response appear to be **rephrasings** of the first two paragraphs. - The mentioned works [11, 12, 13, 14] primarily aim to address general decision-making problems. In contrast, the focus of this submission is on the generalization of robotic control systems across different embodiments and tasks. The starting point of different problems implies distinct requirements. While solving general decision-making problems may prioritize model effectiveness on wide-range tasks over computational cost and real-time performance, the same leniency cannot be applied to robotic control systems. Ignoring the real-time demands and limited computational resources during system deployment is unprofessional for real-world applications that require precision and reliability. Related works[a, b, c, d] focusing on the similar problem settings have verified the transferability of the model on real robots. Why can't your work do it? - For robotic control systems, the need for real-time performance in real-world robots is paramount. The paper's oversight in addressing the computational requirements and real-time capabilities necessary for practical deployment is a significant shortcoming. This is especially critical given the diverse hardware systems that entail varying control frequencies and dynamic responses, which should be integral to model design considerations. The techniques[15~21] are potential solutions to reduce the computational demands of Transformers but have not been verified on similar embodied control tasks yet. In summary, the methodology presented in this paper appears to be under-considered and lacks depth in addressing the complexities of the problem at hand. **It does not seem to have delved into the nuances of robotic control systems and their requirements for continuous control.** Given the above concerns, I believe that the paper does not meet the rigorous standards required for publication in NeurIPS. It is essential for robot learning research to not only present new ideas but also to thoroughly consider and address the practical implications and challenges associated with real-world applications. Ref: [a] Mirage: Cross-Embodiment Zero-Shot Policy Transfer with Cross-Painting [b] Cross-Embodiment Robot Manipulation Skill Transfer using Latent Space Alignment [c] Polybot: Training One Policy Across Robots While Embracing Variability [d] RoboDuet: A Framework Affording Mobile-Manipulation and Cross-Embodiment

Reviewer iQ1f2024-08-10

Thanks for the response. I agree that currently no simulation or benchmark considers diverse embodiments and tasks. * Most simulations are related to other controlling patterns. Do you think it's still important to consider diverse embodiments since most robot arms can share similar end-effector control? * Can this work provide some insights for other settings like end-effector control or mobile robots?

Authorsrebuttal2024-08-12

We appreciate the comment. We address each of questions as follows: > **Q1.** Most simulations are related to other controlling patterns. Do you think it's still important to consider diverse embodiments since most robot arms can share similar end-effector control? **A1.** We think that for simple robots, such as robot arms with serial manipulator and similar end-effectors, the end-effector-based control can be a reasonable alternative to handle diverse morphologies. However, we would like to emphasize that our research is designed to address a wider range of multi-joint robots, such as the ones including snakebots [8], crawler robots [9], and quadrupeds [10]. These have diverse control patterns and complex morphologies (e.g., non-serial manipulators) that computing inverse kinematics for end-effector-based control is often challenging. We believe that considering a variety of embodiments is important because it allows us to tackle challenges across different types of robots, not just those with similar end-effectors. > **Q2.** Can this work provide some insights for other settings like end-effector control or mobile robots? **A2.** Our approach may also provide valuable insights for settings like end-effector control or mobile robots, particularly in scenarios where robots are customized for specific applications. For example, our method could facilitate data-efficient learning of controllers for new custom robots with unique joint configurations. Additionally, developing and deploying control solutions for new robots with complex kinematics, including end-effector-based control or mobile robots, can be costly and time-consuming. Our approach, which learns controllers from a limited number of expert trajectories, could offer a more efficient and cost-effective alternative in these settings. [8] Pettersen et al., “Snake robots”, Annual Reviews in Control, Volume 44, 2017 [9] Orita et al., “Robust Human Tracking of a Crawler Robot”, Journal of Robotics and Mechatronics Vol.31 No.2, 2019 [10] Fan et al., “A Review of Quadruped Robots: Structure, Control, and Autonomous Motion”, Advanced Intelligent Systems, Vol. 6, 2024

Reviewer iQ1f2024-08-13

Thanks for the detailed response. I will increase my rating.

Reviewer EmwS2024-08-10

Thanks to the authors for the detailed response. I maintain my positive rating.

Reviewer dCFx2024-08-12

Thanks to the authors for the detailed response. I maintain my positive rating.

Reviewer TTwU2024-08-13

Thanks for the detailed response. However, I believe that **the capability of real-time inference is the prerequisite to achieving the embodied generalization for continuous control**. The large model can be deployed on cloud computing for high-level reasoning but is not applicable for continuous motion control, considering the latency in communication and the requirements of high-frequency control. So I will maintain my original rating unless the authors successfully demonstrate a feasible solution to deploy the model in real-world robotic platforms.

Program Chairsdecision2024-09-25

Decision

Accept (poster)

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