Research Overview

@INPROCEEDINGS{dennler2021combing,
  author={Dennler, Nathaniel and Shin, Eura and Matarić, Maja and Nikolaidis, Stefanos},
  booktitle={2021 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS)},
   title={Design and Evaluation of a Hair Combing System Using a General-Purpose Robotic Arm},
   year={2021},
  volume={},
  number={},
  pages={3739-3746},
  doi={10.1109/IROS51168.2021.9636768},
  abstract={This work introduces an approach for automatic hair combing by a lightweight robot. For people living with limited mobility, dexterity, or chronic fatigue, combing hair is often a difficult task that negatively impacts personal routines. We propose a modular system for enabling general robot manipulators to assist with a hair-combing task. The system consists of three main components. The first component is the segmentation module, which segments the location of hair in space. The second component is the path planning module that proposes automatically-generated paths through hair based on user input. The final component creates a trajectory for the robot to execute. We quantitatively evaluate the effectiveness of the paths planned by the system with 48 users and qualitatively evaluate the system with 30 users watching videos of the robot performing a hair-combing task in the physical world. The system is shown to effectively comb different hairstyles.},
}

@article{nemlekar2021twostage,
  title={Two-Stage Clustering of Human Preferences for Action Prediction in
         Assembly Tasks},
  author={Heramb Nemlekar and Jignesh Modi and Satyandra K. Gupta and Stefanos Nikolaidis},
  journal={2021 IEEE International Conference on Robotics and Automation (ICRA)},
  year={2021},
  month={May},
  url={https://arxiv.org/abs/2103.14994},
  abstract={To effectively assist human workers in assembly tasks a robot must proactively offer support by inferring their preferences in sequencing the task actions. Previous work has focused on learning the dominant preferences of human workers for simple tasks largely based on their intended goal. However, people may have preferences at different resolutions: they may share the same high-level preference for the order of the sub-tasks but differ in the sequence of individual actions. We propose a two-stage approach for learning and inferring the preferences of human operators based on the sequence of sub-tasks and actions. We conduct an IKEA assembly study and demonstrate how our approach is able to learn the dominant preferences in a complex task. We show that our approach improves the prediction of human actions through cross-validation. Lastly, we show that our two-stage approach improves the efficiency of task execution in an online experiment, and demonstrate its applicability in a real-world robot-assisted IKEA assembly.}
}

@inproceedings{dennler2021personalizing,
    author={Dennler, Nathaniel and Yunis, Catherine and Realmuto, Jonathan and Sanger, Terence and Nikolaidis, Stefanos and Matarić, Maja},
    booktitle={2021 30th IEEE International Conference on Robot Human Interactive Communication (RO-MAN)},
     title={Personalizing User Engagement Dynamics in a Non-Verbal Communication Game for Cerebral Palsy},
    year={2021},
    pages={873-879},
    doi={10.1109/RO-MAN50785.2021.9515466},
    abstract={Children and adults with cerebral palsy (CP) can have involuntary upper limb movements as a consequence of the symptoms that characterize their motor disability, leading to difficulties in communicating with caretakers and peers. We describe how a socially assistive robot may help individuals with CP to practice non-verbal communicative gestures using an active orthosis in a one-on-one number-guessing game. We performed a user study and data collection with participants with CP; we found that participants preferred an embodied robot over a screen-based agent, and we used the participant data to train personalized models of participant engagement dynamics that can be used to select personalized robot actions. Our work highlights the benefit of personalized models in the engagement of users with CP with a socially assistive robot and offers design insights for future work in this area.}
}

@InProceedings{puranic2020signal,
  title = {Learning from Demonstrations using Signal Temporal Logic},
  author = {Aniruddh Puranic and Jyotirmoy Deshmukh and Stefanos Nikolaidis},
  booktitle = {Conference on Robot Learning},
  year = {2020},
  month = {November},
  abstract = {We present a model-based reinforcement learning framework for robot locomotion that achieves walking based on only 4.5 minutes of data collected on a quadruped robot. To accurately model the robot’s dynamics over a long horizon, we introduce a loss function that tracks the model’s prediction over multiple timesteps. We adapt model predictive control to account for planning latency, which allows the learned model to be used for real time control. Additionally, to ensure safe exploration during model learning, we embed prior knowledge of leg trajectories into the action space. The resulting system achieves fast and robust locomotion. Unlike model-free methods, which optimize for a particular task, our planner can use the same learned dynamics for various tasks, simply by changing the reward function.1 To the best of our knowledge, our approach is more than an order of magnitude more sample efficient than current model-free methods.}
}

@InProceedings{chen2020experiments,
  title = {Fair Contextual Multi-Armed Bandits: Theory and Experiments},
  author = {Chen, Yifang and Cuellar, Alex and Luo, Haipeng and Modi, Jignesh and Nemlekar, Heramb and Nikolaidis, Stefanos},
  booktitle = {Proceedings of the 36th Conference on Uncertainty in Artificial Intelligence (UAI)},
  pages = {181--190},
  year = {2020},
  editor = {Jonas Peters and David Sontag},
  volume = {124},
  series = {Proceedings of Machine Learning Research},
  month = {03--06 Aug},
  publisher = {PMLR},
  pdf = {http://proceedings.mlr.press/v124/chen20a/chen20a.pdf},
  url = { http://proceedings.mlr.press/v124/chen20a.html },
  abstract = {When an AI system interacts with multiple users, it frequently needs to make allocation decisions. For instance, a virtual agent decides whom to pay attention to in a group, or a factory robot selects a worker to deliver a part.Demonstrating fairness in decision making is essential for such systems to be broadly accepted. We introduce a Multi-Armed Bandit algorithm with fairness constraints, where fairness is defined as a minimum rate at which a task or a resource is assigned to a user. The proposed algorithm uses contextual information about the users and the task and makes no assumptions on how the losses capturing the performance of different users are generated. We provide theoretical guarantees of performance and empirical results from simulation and an online user study. The results highlight the benefit of accounting for contexts in fair decision making, especially when users perform better at some contexts and worse at others.},
}

@inproceedings{claure2020bandits,
  author = {Claure, Houston and Chen, Yifang and Modi, Jignesh and Jung, Malte and Nikolaidis, Stefanos},
  title = {Multi-Armed Bandits with Fairness Constraints for Distributing Resources to Human Teammates},
  year = {2020},
  month = {March},
  isbn = {9781450367462},
  publisher = {Association for Computing Machinery},
  address = {New York, NY, USA},
  url = {https://doi.org/10.1145/3319502.3374806},
  doi = {10.1145/3319502.3374806},
  abstract = {How should a robot that collaborates with multiple people decide upon the distribution of resources (e.g. social attention, or parts needed for an assembly)? People are uniquely attuned to how resources are distributed. A decision to distribute more resources to one team member than another might be perceived as unfair with potentially detrimental effects for trust. We introduce a multi-armed bandit algorithm with fairness constraints, where a robot distributes resources to human teammates of different skill levels. In this problem, the robot does not know the skill level of each human teammate, but learns it by observing their performance over time. We define fairness as a constraint on the minimum rate that each human teammate is selected throughout the task. We provide theoretical guarantees on performance and perform a large-scale user study, where we adjust the level of fairness in our algorithm. Results show that fairness in resource distribution has a significant effect on users' trust in the system.},
  booktitle = {Proceedings of the 2020 ACM/IEEE International Conference on Human-Robot Interaction},
  pages = {299–308},
  numpages = {10},
  keywords = {reinforcement learning, multi-armed bandits, trust, fairness},
  location = {Cambridge, United Kingdom},
  series = {HRI '20}
}

@inproceedings {zhang2019youtube,
  author={Hejia Zhang and Po-Jen Lai and Sayan Paul and Suraj Kothawade and Stefanos Nikolaidis},
  title={Learning Collaborative Action Plans from Unlabeled YouTube Videos},
  booktitle={Robotics Research, The 19th International Symposium, {ISRR} 2019},
  location={Hanoi, Vietnam},
  year={2019},
  month={October},
  abstract={Videos from the World Wide Web provide a rich source of information that robots could use to acquire knowledge about manipulation tasks. Previous work has focused on generating action sequences from unconstrained videos for a single robot performing manipulation tasks by itself. However, robots operating in the same physical space with people need to not only perform actions autonomously, but also coordinate seamlessly with their human counterparts. This often requires representing and executing collaborative manipulation actions, such as handing over a tool or holding an object for the other agent. We present a system for knowledge acquisition of collaborative manipulation action plans that outputs commands to the robot in the form of visual sentence. We show the performance of the system in 12 unlabeled action clips taken from collaborative cooking videos on YouTube. We view this as the first step towards extracting collaborative manipulation action sequences from unconstrained, unlabeled online videos.},
}

@article{fontaine2022evaluating,
  author = {Fontaine, Matthew C. and Nikolaidis, Stefanos},
  title = {Evaluating Human–Robot Interaction Algorithms in Shared Autonomy via Quality Diversity Scenario Generation},
  year = {2022},
  issue_date = {September 2022},
  publisher = {Association for Computing Machinery},
  address = {New York, NY, USA},
  volume = {11},
  number = {3},
  url = {https://doi.org/10.1145/3476412},
  doi = {10.1145/3476412},
  abstract = {The growth of scale and complexity of interactions between humans and robots highlights the need for new computational methods to automatically evaluate novel algorithms and applications. Exploring diverse scenarios of humans and robots interacting in simulation can improve understanding of the robotic system and avoid potentially costly failures in real-world settings. We formulate this problem as a quality diversity (QD) problem, of which the goal is to discover diverse failure scenarios by simultaneously exploring both environments and human actions. We focus on the shared autonomy domain, in which the robot attempts to infer the goal of a human operator, and adopt the QD algorithms CMA-ME and MAP-Elites to generate scenarios for two published algorithms in this domain: shared autonomy via hindsight optimization and linear policy blending. Some of the generated scenarios confirm previous theoretical findings, while others are surprising and bring about a new understanding of state-of-the-art implementations. Our experiments show that the QD algorithms CMA-ME and MAP-Elites outperform Monte-Carlo simulation and optimization-based methods in effectively searching the scenario space, highlighting their promise for automatic evaluation of algorithms in human–robot interaction.},
  journal = {J. Hum.-Robot Interact.},
  month = {sep},
  articleno = {25},
  numpages = {30},
  keywords = {automatic scenario generation, human-robot interaction, Quality diversity optimization}
}

@inproceedings{fontaine2021importance,
    title={On the Importance of Environments in Human-Robot Coordination},
    author={Matthew C. Fontaine and Ya-Chuan Hsu and Yulun Zhang and Bryon Tjanaka and Stefanos Nikolaidis},
    year={2021},
    month={July},
    url={https://arxiv.org/abs/2106.10853},
    booktitle={Proceedings of Robotics: Science and Systems},
    doi={10.15607/RSS.2021.XVII.038},
    abstract={When studying robots collaborating with humans, much of the focus has been on robot policies that coordinate fluently with human teammates in collaborative tasks. However, less emphasis has been placed on the effect of the environment on coordination behaviors. To thoroughly explore environments that result in diverse behaviors, we propose a framework for procedural generation of environments that are (1) stylistically similar to human-authored environments, (2) guaranteed to be solvable by the human-robot team, and (3) diverse with respect to coordination measures. We analyze the procedurally generated environments in the Overcooked benchmark domain via simulation and an online user study. Results show that the environments result in qualitatively different emerging behaviors and statistically significant differences in collaborative fluency metrics, even when the robot runs the same planning algorithm.}
}

@inproceedings{fontaine2021shared,
    title={A Quality Diversity Approach to Automatically Generating Human-Robot
           Interaction Scenarios in Shared Autonomy},
    author={Matthew C. Fontaine and Stefanos Nikolaidis},
    year={2021},
    month={July},
    url={https://arxiv.org/abs/2012.04283},
    booktitle={Proceedings of Robotics: Science and Systems},
    doi={10.15607/RSS.2021.XVII.036},
    abstract={The growth of scale and complexity of interactions between humans and robots highlights the need for new computational methods to automatically evaluate novel algorithms and applications. Exploring diverse scenarios of humans and robots interacting in simulation can improve understanding of the robotic system and avoid potentially costly failures in real-world settings. We formulate this problem as a quality diversity (QD) problem, where the goal is to discover diverse failure scenarios by simultaneously exploring both environments and human actions. We focus on the shared autonomy domain, where the robot attempts to infer the goal of a human operator, and adopt the QD algorithm MAP-Elites to generate scenarios for two published algorithms in this domain: shared autonomy via hindsight optimization and linear policy blending. Some of the generated scenarios confirm previous theoretical findings, while others are surprising and bring about a new understanding of state-of-the-art implementations. Our experiments show that MAP-Elites outperforms Monte-Carlo simulation and optimization based methods in effectively searching the scenario space, highlighting its promise for automatic evaluation of algorithms in human-robot interaction.}
}

@article{fontaine2021illuminating,
  title={Illuminating Mario Scenes in the Latent Space of a Generative Adversarial Network},
  volume={35},
  url={https://ojs.aaai.org/index.php/AAAI/article/view/16740},
  journal={AAAI Conference on Artificial Intelligence},
  author={Matthew C. Fontaine and Ruilin Liu and Ahmed Khalifa and Jignesh Modi and Julian Togelius and Amy K. Hoover and Stefanos Nikolaidis},
  year={2021},
  month={February},
  abstract={Generative adversarial networks (GANs) are quickly becoming a ubiquitous approach to procedurally generating video game levels. While GAN generated levels are stylistically similar to human-authored examples, human designers often want to explore the generative design space of GANs to extract interesting levels. However, human designers find latent vectors opaque and would rather explore along dimensions the designer specifies, such as number of enemies or obstacles. We propose using state-of-the-art quality diversity algorithms designed to optimize continuous spaces, i.e. MAP-Elites with a directional variation operator and Covariance Matrix Adaptation MAP-Elites, to efficiently explore the latent space of a GAN to extract levels that vary across a set of specified gameplay measures. In the benchmark domain of Super Mario Bros, we demonstrate how designers may specify gameplay measures to our system and extract high-quality (playable) levels with a diverse range of level mechanics, while still maintaining stylistic similarity to human authored examples. An online user study shows how the different mechanics of the automatically generated levels affect subjective ratings of their perceived difficulty and appearance.}
}

@article{zhang2020repair,
  title={Video Game Level Repair via Mixed Integer Linear Programming},
  volume={16},
  url={https://ojs.aaai.org/index.php/AIIDE/article/view/7424},
  abstract={Recent advancements in procedural content generation via machine learning enable the generation of video-game levels that are aesthetically similar to human-authored examples. However, the generated levels are often unplayable without additional editing. We propose a “generate-then-repair” framework for automatic generation of playable levels adhering to specific styles. The framework constructs levels using a generative adversarial network (GAN) trained with human-authored examples and repairs them using a mixed-integer linear program (MIP) with playability constraints. A key component of the framework is computing minimum cost edits between the GAN generated level and the solution of the MIP solver, which we cast as a minimum cost network flow problem. Results show that the proposed framework generates a diverse range of playable levels, that capture the spatial relationships between objects exhibited in the human-authored levels.},
  number={1},
  journal={Proceedings of the AAAI Conference on Artificial Intelligence and Interactive Digital Entertainment},
  author={Zhang, Hejia and Fontaine, Matthew and Hoover, Amy and Togelius, Julian and Dilkina, Bistra and Nikolaidis, Stefanos},
  year={2020},
  month={October},
  pages={151-158}
}

@inproceedings{10.1145/3512290.3528705,
  author = {Tjanaka, Bryon and Fontaine, Matthew C. and Togelius, Julian and Nikolaidis, Stefanos},
  title = {Approximating Gradients for Differentiable Quality Diversity in Reinforcement Learning},
  year = {2022},
  isbn = {9781450392372},
  publisher = {Association for Computing Machinery},
  address = {New York, NY, USA},
  url = {https://doi.org/10.1145/3512290.3528705},
  doi = {10.1145/3512290.3528705},
  abstract = {Consider the problem of training robustly capable agents. One approach is to generate a diverse collection of agent polices. Training can then be viewed as a quality diversity (QD) optimization problem, where we search for a collection of performant policies that are diverse with respect to quantified behavior. Recent work shows that differentiable quality diversity (DQD) algorithms greatly accelerate QD optimization when exact gradients are available. However, agent policies typically assume that the environment is not differentiable. To apply DQD algorithms to training agent policies, we must approximate gradients for performance and behavior. We propose two variants of the current state-of-the-art DQD algorithm that compute gradients via approximation methods common in reinforcement learning (RL). We evaluate our approach on four simulated locomotion tasks. One variant achieves results comparable to the current state-of-the-art in combining QD and RL, while the other performs comparably in two locomotion tasks. These results provide insight into the limitations of current DQD algorithms in domains where gradients must be approximated. Source code is available at https://github.com/icaros-usc/dqd-rl},
  booktitle = {Proceedings of the Genetic and Evolutionary Computation Conference},
  pages = {1102–1111},
  numpages = {10},
  keywords = {neuroevolution, reinforcement learning, quality diversity},
  location = {Boston, Massachusetts},
  series = {GECCO '22}
}

@inproceedings{fontaine2021dqd,
  author = {Fontaine, Matthew and Nikolaidis, Stefanos},
  booktitle = {Advances in Neural Information Processing Systems},
  editor = {M. Ranzato and A. Beygelzimer and Y. Dauphin and P.S. Liang and J. Wortman Vaughan},
  pages = {10040--10052},
  publisher = {Curran Associates, Inc.},
  title = {Differentiable Quality Diversity},
  url = {https://proceedings.neurips.cc/paper/2021/file/532923f11ac97d3e7cb0130315b067dc-Paper.pdf},
  volume = {34},
  year = {2021},
  abstract={Quality diversity (QD) is a growing branch of stochastic optimization research that studies the problem of generating an archive of solutions that maximize a given objective function but are also diverse with respect to a set of specified measure functions. However, even when these functions are differentiable, QD algorithms treat them as "black boxes", ignoring gradient information. We present the differentiable quality diversity (DQD) problem, a special case of QD, where both the objective and measure functions are first order differentiable. We then present MAP-Elites via Gradient Arborescence (MEGA), a DQD algorithm that leverages gradient information to efficiently explore the joint range of the objective and measure functions. Results in two QD benchmark domains and in searching the latent space of a StyleGAN show that MEGA significantly outperforms state-of-the-art QD algorithms, highlighting DQD's promise for efficient quality diversity optimization when gradient information is available.}
}

@inproceedings{fontaine2020covariance,
  author = {Fontaine, Matthew C. and Togelius, Julian and Nikolaidis, Stefanos and Hoover, Amy K.},
  title = {Covariance Matrix Adaptation for the Rapid Illumination of Behavior Space},
  year = {2020},
  month = {June},
  isbn = {9781450371285},
  publisher = {Association for Computing Machinery},
  address = {New York, NY, USA},
  url = {https://doi.org/10.1145/3377930.3390232},
  doi = {10.1145/3377930.3390232},
  abstract = {We focus on the challenge of finding a diverse collection of quality solutions on complex continuous domains. While quality diversity (QD) algorithms like Novelty Search with Local Competition (NSLC) and MAP-Elites are designed to generate a diverse range of solutions, these algorithms require a large number of evaluations for exploration of continuous spaces. Meanwhile, variants of the Covariance Matrix Adaptation Evolution Strategy (CMA-ES) are among the best-performing derivative-free optimizers in single-objective continuous domains. This paper proposes a new QD algorithm called Covariance Matrix Adaptation MAP-Elites (CMA-ME). Our new algorithm combines the self-adaptation techniques of CMA-ES with archiving and mapping techniques for maintaining diversity in QD. Results from experiments based on standard continuous optimization benchmarks show that CMA-ME finds better-quality solutions than MAP-Elites; similarly, results on the strategic game Hearthstone show that CMA-ME finds both a higher overall quality and broader diversity of strategies than both CMA-ES and MAP-Elites. Overall, CMA-ME more than doubles the performance of MAP-Elites using standard QD performance metrics. These results suggest that QD algorithms augmented by operators from state-of-the-art optimization algorithms can yield high-performing methods for simultaneously exploring and optimizing continuous search spaces, with significant applications to design, testing, and reinforcement learning among other domains.},
  booktitle = {Proceedings of the 2020 Genetic and Evolutionary Computation Conference},
  pages = {94–102},
  numpages = {9},
  keywords = {hearthstone, evolutionary algorithms, MAP-Elites, optimization, quality diversity, illumination algorithms},
  location = {Canc'{u}n, Mexico},
  series = {GECCO '20}
}