Behavior Trees (BTs) were first conceived in the computer games industry as a tool to model agent behavior, but they received interest also in the robotics community as an alternative policy design to Finite State Machines (FSMs). The advantages of BTs over FSMs had been highlighted in many works, but there is no thorough practical comparison of the two designs. Such a comparison is particularly relevant in the robotic industry, where FSMs have been the state-of-the-art policy representation for robot control for many years. In this work we shed light on this matter by comparing how BTs and FSMs behave when controlling a robot in a mobile manipulation task. The comparison is made in terms of reactivity, modularity, readability, and design. We propose metrics for each of these properties, being aware that while some are tangible and objective, others are more subjective and implementation dependent. The practical comparison is performed in a simulation environment with validation on a real robot. We find that although the robot’s behavior during task solving is independent on the policy representation, maintaining a BT rather than an FSM becomes easier as the task increases in complexity.

Task and motion planning (TAMP) is a promising approach for efficient long-horizon manipulation planning, which is a prerequisite for being able to deploy manipulation systems in human-centered environments at scale. TAMP systems often rely on so-called predicates to abstractly describe the world. Today, predicates and their groundings are often hand-engineered. Furthermore, robot action parameterizations required to fulfill desired predicates are typically discovered by sampling naively or using oracles (again hand-engineered). We aim to automate predicate discovery and grounding with a system that learns to classify the state of predicates in a set of scenes while concurrently learning to generate scene configurations that fulfill the desired predicates. Our results show that high classification accuracies and generation success rates can be achieved with architectures based on multi-layer perceptrons (MLPs) and graph neural networks (GNNs) that are trained on bounding box as well as point cloud-based features in a Generative Adversarial Network (GAN)-inspired fashion, decisively outperforming both decision tree and uniform sampler baselines. The integration of our framework into a TAMP system demonstrates its positive impact on solving mobile manipulation tasks. A reference implementation of our method and data are available at https://github.com/ethzasl/predicate_learning.

This paper presents an Active Search policy that balances between moving the camera and removing occluding objects to search for and retrieve a target object in clutter. While both types of action can reveal unobserved parts of a scene, they typically vary in execution complexity and time. Our proposed method explicitly reasons about the occluded spaces in the scene where the target object may be hidden, and uses reinforcement learning to compute the value of each action with the ultimate goal of finding and retrieving the target object in minimal time. Results in simulation and real-world experiments demonstrate a significant improvement in both task execution speed and success rate compared to baseline grasping strategies.

In this paper we provide a practical demonstration of how the modularity in a Behavior Tree (BT) decreases the effort in programming a robot task when compared to a Finite State Machine (FSM). In recent years the way to represent a task plan to control an autonomous agent has been shifting from the standard FSM towards BTs. Many works in the literature have highlighted and proven the benefits of such design compared to standard approaches, especially in terms of modularity, reactivity and human readability. However, these works have often failed in providing a tangible comparison in the implementation of those policies and the programming effort required to modify them. This is a relevant aspect in many robotic applications, where the design choice is dictated both by the robustness of the policy and by the time required to program it. In this work, we compare backward chained BTs with a fault-tolerant design of FSMs by evaluating the cost to modify them. We validate the analysis with a set of experiments in a simulation environment where a mobile manipulator solves an item fetching task.

We present a novel hierarchical POMDP framework to solve an object search and delivery task where the agent is given a prior belief about the possible item locations. Solving POMDPs is computationally demanding and, as such, applications have typically been limited to small environments. The proposed hierarchical POMDP framework performs reasoning on multiple spatial scales in order to reduce computation time. The problem is first solved in the top layer of the hierarchy with a coarsely discretized state space. Its solution is refined in the lower layers with increasing resolution. Three different methods for propagating information down the spatial hierarchy are discussed and validated in simulation. We show that a two-layer multi-scale POMDP decreases computation time by an order of magnitude allowing for real-time applications while maintaining high solution quality. For large problems that require three layers to reach the desired resolution, computation time speedups by two orders of magnitude are achieved.