Review of NNs for solving manipulator inverse kinematics

Daniel Cagigas-Mu�iz, Artificial Neural Networks for inverse kinematics problem in articulated robots, Engineering Applications of Artificial Intelligence,
Volume 126, Part D, 2023 DOI: 10.1016/j.engappai.2023.107175.

The inverse kinematics problem in articulated robots implies to obtain joint rotation angles using the robot end effector position and orientation tool. Unlike the problem of direct kinematics, in inverse kinematics there are no systematic methods for solving the problem. Moreover, solving the inverse kinematics problem is particularly complicated for certain morphologies of articulated robots. Machine learning techniques and, more specifically, artificial neural networks (ANNs) have been proposed in the scientific literature to solve this problem. However, there are some limitations in the performance of ANNs. In this study, different techniques that involve ANNs are proposed and analyzed. The results show that the proposed original bootstrap sampling and hybrid methods can substantially improve the performance of approaches that use only one ANN. Although all of these improvements do not solve completely the inverse kinematics problem in articulated robots, they do lay the foundations for the design and development of future more effective and efficient controllers. Therefore, the source code and documentation of this research are also publicly available to practitioners interested in adapting and improving these methods to any industrial robot or articulated robot.

Hierarchical RL with continuous options

Zhigang Huang, Quan Liu, Fei Zhu, Hierarchical reinforcement learning with adaptive scheduling for robot control, Engineering Applications of Artificial Intelligence, Volume 126, Part D, 2023 DOI: 10.1016/j.engappai.2023.107130.

Conventional hierarchical reinforcement learning (HRL) relies on discrete options to represent explicitly distinguishable knowledge, which may lead to severe performance bottlenecks. It is possible to represent richer knowledge through continuous options, but reliable scheduling methods are lacking. To design an available scheduling method for continuous options, in this paper, the hierarchical reinforcement learning with adaptive scheduling (HAS) algorithm is proposed. Its low-level controller learns diverse options, while the high-level controller schedules options to learn solutions. It achieves an adaptive balance between exploration and exploitation during the frequent scheduling of continuous options, maximizing the representation potential of continuous options. It builds on multi-step static scheduling and makes switching decisions according to the relative advantages of the previous and the estimated continuous options, enabling the agent to focus on different behaviors at different phases of the task. The expected t-step distance is applied to demonstrate the superiority of adaptive scheduling in terms of exploration. Furthermore, an interruption incentive based on annealing is proposed to alleviate excessive exploration during the early training phase, accelerating the convergence rate. Finally, we apply HAS to robot control with sparse rewards in continuous spaces, and develop a comprehensive experimental analysis scheme. The experimental results not only demonstrate the high performance and robustness of HAS, but also provide evidence that the adaptive scheduling method has a positive effect both on the representation and option policies.

RL to learn not only manipulator skills but also safety skills

A. C. Ak, E. E. Aksoy and S. Sariel, Learning Failure Prevention Skills for Safe Robot Manipulation, IEEE Robotics and Automation Letters, vol. 8, no. 12, pp. 7994-8001, Dec. 2023 DOI: 10.1109/LRA.2023.3324587.

Robots are more capable of achieving manipulation tasks for everyday activities than before. However, the safety of manipulation skills that robots employ is still an open problem. Considering all possible failures during skill learning increases the complexity of the process and restrains learning an optimal policy. Nonetheless, safety-focused modularity in the acquisition of skills has not been adequately addressed in previous works. For that purpose, we reformulate skills as base and failure prevention skills, where base skills aim at completing tasks and failure prevention skills aim at reducing the risk of failures to occur. Then, we propose a modular and hierarchical method for safe robot manipulation by augmenting base skills by learning failure prevention skills with reinforcement learning and forming a skill library to address different safety risks. Furthermore, a skill selection policy that considers estimated risks is used for the robot to select the best control policy for safe manipulation. Our experiments show that the proposed method achieves the given goal while ensuring safety by preventing failures. We also show that with the proposed method, skill learning is feasible and our safe manipulation tools can be transferred to the real environment.

Improving sample efficiency in actor-critic RL (A2C with NNs) through multimodal advantage function

Jonghyeok Park, Soohee Han, Reinforcement learning with multimodal advantage function for accurate advantage estimation in robot learning, Engineering Applications of Artificial Intelligence, Volume 126, Part C, 2023 DOI: 10.1016/j.engappai.2023.107019.

In this paper, we propose a reinforcement learning (RL) framework that uses a multimodal advantage function (MAF) to come close to the true advantage function, thereby achieving high returns. The MAF, which is constructed as a logarithm of a mixture of Gaussians policy (MoG-P) and trained by globally collected past experiences, directly assesses the complex true advantage function with its multi-modality and is expected to enhance the sample-efficiency of RL. To realize the expected enhanced learning performance with the proposed RL framework, two practical techniques are developed that include mode selection and rounding off of actions during the policy update process. Mode selection is conducted to sample the action around the most influential or weighted mode for efficient environment exploration. For fast policy updates, past actions are rounded off to discretized action values when calculating the multimodal advantage function. The proposed RL framework was validated using simulation environments and a real inverted pendulum system. The findings showed that the proposed framework can achieve a more sample-efficient performance or higher returns than other advantage-based RL benchmarks.

Learning options in RL and using rewards adequately in that context

Richard S. Sutton, Marlos C. Machado, G. Zacharias Holland, David Szepesvari, Finbarr Timbers, Brian Tanner, Adam White, Reward-respecting subtasks for model-based reinforcement learning, Artificial Intelligence, Volume 324, 2023, DOI: 10.1016/j.artint.2023.104001.

To achieve the ambitious goals of artificial intelligence, reinforcement learning must include planning with a model of the world that is abstract in state and time. Deep learning has made progress with state abstraction, but temporal abstraction has rarely been used, despite extensively developed theory based on the options framework. One reason for this is that the space of possible options is immense, and the methods previously proposed for option discovery do not take into account how the option models will be used in planning. Options are typically discovered by posing subsidiary tasks, such as reaching a bottleneck state or maximizing the cumulative sum of a sensory signal other than reward. Each subtask is solved to produce an option, and then a model of the option is learned and made available to the planning process. In most previous work, the subtasks ignore the reward on the original problem, whereas we propose subtasks that use the original reward plus a bonus based on a feature of the state at the time the option terminates. We show that option models obtained from such reward-respecting subtasks are much more likely to be useful in planning than eigenoptions, shortest path options based on bottleneck states, or reward-respecting options generated by the option-critic. Reward respecting subtasks strongly constrain the space of options and thereby also provide a partial solution to the problem of option discovery. Finally, we show how values, policies, options, and models can all be learned online and off-policy using standard algorithms and general value functions.

A survey on open hardware robotics

V. V. Patel, M. V. Liarokapis and A. M. Dollar, Open Robot Hardware: Progress, Benefits, Challenges, and Best Practices, IEEE Robotics & Automation Magazine, vol. 30, no. 3, pp. 123-148, Sept. 2023 DOI: 10.1109/MRA.2022.3225725.

Technologies from open source projects have seen widespread adoption in robotics in recent years. The rapid pace of progress in robotics is in part fueled by open source projects, providing researchers with resources, tools, and devices to implement novel ideas and approaches quickly. Open source hardware, in particular, lowers the barrier of entry to new technologies and can further accelerate innovation in robotics. But open hardware is also more difficult to propagate in comparison to open software because it involves replicating physical components, which requires users to have sufficient familiarity and access to fabrication equipment. In this work, we present a review on open robot hardware (ORH) by first highlighting the key benefits and challenges encountered by users and developers of ORH, and then relaying some best practices that can be adopted in developing successful ORH. To accomplish this, we surveyed more than 80 major ORH projects and initiatives across different domains within robotics. Finally, we identify strategies exemplified by the surveyed projects to further detail the development process, and guide developers through the design, documentation, and dissemination stages of an ORH project.

Dealing with affordances in robotics through RL

X. Yang, Z. Ji, J. Wu and Y. -K. Lai, Recent Advances of Deep Robotic Affordance Learning: A Reinforcement Learning Perspective, EEE Transactions on Cognitive and Developmental Systems, vol. 15, no. 3, pp. 1139-1149, Sept. 2023 DOI: 10.1109/TCDS.2023.3277288.

As a popular concept proposed in the field of psychology, affordance has been regarded as one of the important abilities that enable humans to understand and interact with the environment. Briefly, it captures the possibilities and effects of the actions of an agent applied to a specific object or, more generally, a part of the environment. This article provides a short review of the recent developments of deep robotic affordance learning (DRAL), which aims to develop data-driven methods that use the concept of affordance to aid in robotic tasks. We first classify these papers from a reinforcement learning (RL) perspective and draw connections between RL and affordances. The technical details of each category are discussed and their limitations are identified. We further summarize them and identify future challenges from the aspects of observations, actions, affordance representation, data-collection, and real-world deployment. A final remark is given at the end to propose a promising future direction of the RL-based affordance definition to include the predictions of arbitrary action consequences.

Review of algorithms available in ROS-2

Steve Macenski, Tom Moore, David V. Lu, Alexey Merzlyakov, Michael Ferguson, From the desks of ROS maintainers: A survey of modern & capable mobile robotics algorithms in the robot operating system 2, Robotics and Autonomous Systems, Volume 168, 2023, DOI: 10.1016/j.robot.2023.104493.

The Robot Operating System�2 (ROS�2) is rapidly impacting the intelligent machines sector \u2014 on space missions, large agriculture equipment, multi-robot fleets, and more. Its success derives from its focused design and improved capabilities targeting product-grade and modern robotic systems. Following ROS�2\u2019s example, the mobile robotics ecosystem has been fully redesigned based on the transformed needs of modern robots and is experiencing active development not seen since its inception. This paper comes from the desks of the key ROS Navigation maintainers to review and analyze the state of the art of robotics navigation in ROS�2. This includes new systems without parallel in ROS�1 or other similar mobile robotics frameworks. We discuss current research products and historically robust methods that provide differing behaviors and support for most every robot type. This survey consists of overviews, comparisons, and expert insights organized by the fundamental problems in the field. Some of these implementations have yet to be described in literature and many have not been benchmarked relative to others. We end by providing a glimpse into the future of the ROS�2 mobile robotics ecosystem.

Reward machines as reward specification method for RL and their automated learning

Rodrigo Toro Icarte, Toryn Q. Klassen, Richard Valenzano, Margarita P. Castro, Ethan Waldie, Sheila A. McIlraith, Learning reward machines: A study in partially observable reinforcement learning, Artificial Intelligence, Volume 323, 2023 DOI: 10.1016/j.artint.2023.103989.

Reinforcement Learning (RL) is a machine learning paradigm wherein an artificial agent interacts with an environment with the purpose of learning behaviour that maximizes the expected cumulative reward it receives from the environment. Reward machines (RMs) provide a structured, automata-based representation of a reward function that enables an RL agent to decompose an RL problem into structured subproblems that can be efficiently learned via off-policy learning. Here we show that RMs can be learned from experience, instead of being specified by the user, and that the resulting problem decomposition can be used to effectively solve partially observable RL problems. We pose the task of learning RMs as a discrete optimization problem where the objective is to find an RM that decomposes the problem into a set of subproblems such that the combination of their optimal memoryless policies is an optimal policy for the original problem. We show the effectiveness of this approach on three partially observable domains, where it significantly outperforms A3C, PPO, and ACER, and discuss its advantages, limitations, and broader potential.

A PID-based global optimization algorithm

Yuansheng Gao, PID-based search algorithm: A novel metaheuristic algorithm based on PID algorithm, Expert Systems with Applications, Volume 232, 2023, DOI: 10.1016/j.eswa.2023.120886.

In this paper, a metaheuristic algorithm called PID-based search algorithm (PSA) is proposed for global optimization. The algorithm is based on an incremental PID algorithm that converges the entire population to an optimal state by continuously adjusting the system deviations. PSA is mathematically modeled and implemented to achieve optimization in a wide range of search spaces. PSA is used to solve CEC2017 benchmark test functions and six constrained problems. The optimization performance of PSA is verified by comparing it with seven metaheuristics proposed in recent years. The Kruskal-Wallis, Holm and Friedman tests verified the superiority of PSA in terms of statistical significance. The results show that PSA can be better balanced exploration and exploitation with strong optimization capability. Source codes�of PSA are publicly available at