Navigating a mobile robot in complex, cluttered, and dynamic environments has a wide variety of important applications. For instance, assistive robots working in malls, cafeterias, and hospitals can benefit from a robust navigation policy that allows for efficient and safe movement in unstructured environments with dense crowds. Such navigation algorithm is also desperately needed by social devices such as Alexia 111Alexa: https://developer.amazon.com/alexa and Jibo 222Jibo: https://www.jibo.com/. Due to the lack of mobility, they rely on far-field speech recognition and speech synthesis to communicate with users at a very low information rate. If being mounted on a mobile base with sophisticated navigation skills, they could move into the user’s close-proximity and interact with users via visual interfaces at a much higher information rate. In addition, the high mobility is beneficial for an automated warehouse, where a large number of robots need to coordinate with each other for efficient transportation. For accomplishing a high delivery throughput, every robot needs to continuously make progress toward its goal by passing through the cluttered and dynamic environment made by its fellow robots.
Unfortunately, classical algorithms for navigation in dynamic environments suffer from two major impediments: the robot freezing problem and the navigation lost problem. The robot freezing problem arises when the environment surpasses a certain level of dynamic complexity. Specifically, the motion planner decides that all forward paths are unsafe and thus forces the robot to come to a complete stop or to perform unnecessary maneuvers like oscillating between two directions to avoid collisions. Previous works attempt to unfreeze the robot in dense crowds by increasing the prediction accuracy of the moving agents, which unfortunately has shown to be insufficient due to the lack of coordination among agents [2, 3, 4, 5]. This conclusion is also supported by the investigation of human behaviors in dense crowds [6, 7]. The navigation lost problem arises when the robot fails to accurately localize itself in a given map due to the large localization uncertainty or error . Most previous solutions to the navigation lost problem are passive methods. They assume that the robot motion and the pointing direction of the sensors cannot be controlled, and focus on selectively utilizing the sensor stream to minimize the localization uncertainty or error [9, 10, 11]. However, in highly dynamic scenarios with dense human crowds, the salient features necessary for localization may all be occluded and thus the robot must actively determine “where to move” to resolve occlusion and “where to look” to recover from the localization lost [12, 13].
In addition, the robot freezing problem and the navigation lost problem are tightly coupled in a dense crowd, which usually is not considered in prior works. On the one hand, given a plan about “where to move” and “where to look” for resolving the navigation lost problem, the robot needs to accomplish the actual movement in the physical world. To make such movement safe and collision-free, the robot must have high mobility in the dense crowds, which requires solving the robot freezing problem as a prerequisite. On the other hand, before executing the actions toward the goal for “moving” or “looking”, the navigation algorithm needs to understand the routes between the robot’s current position and the target location, which requires an accurate localization of the robot in the map, i.e., we need to solve the navigation lost problem first.
In this paper, we solve the challenging problem of robotic global navigation in crowd scenarios. In particular, given a map and a goal, the robot needs to navigate through the dense crowds and complex static obstacles and eventually reach the goal safely, accurately, and efficiently without getting frozen or lost. This task can be applied in many real-world scenarios. We present a novel framework to handle the robot freezing and the navigation lost problems simultaneously. As illustrated in fig:overview, our framework consists of two modes: the normal mode and the recovery mode. In the normal mode, our robot is driven by a LiDAR-based localization algorithm and a reinforcement learning based local planner, which work together to endow the robot with the goal-approach ability as well as excellent collision-avoidance mobility. However, the normal navigation policy can hardly tackle the navigation lost problem that is ubiquitous in situations with extremely high-density. More specifically, as the robot navigates with the dense crowds, the LiDAR-based localization will easily fail and then the robot will lose the knowledge about where it is. In this situation, the robot will switch into the localization recovery mode. In particular, we sample a set of locations with rich landmark features, called recovery points, from a given 2D map. The robot will dynamically select one of these discriminative points and then approach the selected point to re-localize itself in the map. The optimal recovery point is determined using a reinforcement learning-based optimization that maximizes the accessibility of the recovery point, in order to optimally balance the benefit and overhead of the recovery action. During the navigation, the robot will actively switch between the normal mode and the recovery mode, according to the instantaneous situation in the scenario. To assess the performance of our proposed algorithm in such a challenging navigation task, we set up a set of complex simulated environments as the testing benchmark, which will be released upon the publication of this paper. We also design a set of metrics for performance evaluation. The experimental results verify the excellent mobility and reliability of our method in challenging navigation scenarios.
Our contributions can be summarized as follows:
We address for the first time the challenging problem of the robot navigation in complex environments through dense crowds without suffering from getting frozen or getting lost.
we formulate a novel framework to handle the navigation lost and the robot freezing problem simultaneously, by switching between the normal mode and the recovery mode during navigation.
We propose a reinforcement learning based recovery algorithm that enables the robot to regain the localization by approaching recovery points adaptively.
We provide a benchmark including simulated and real-world scenarios to evaluate navigation algorithms and to demonstrate our proposed method’s superior performance.
Ii Related Work
The robot freezing problem has been widely studied for mobile robot navigation. One ad-hoc solution to resolve the suboptimal frozen state is to follow an essentially arbitrary path through the crowd, but such highly evasive paths often are dangerous and thus are not desirable for service robot applications. One culprit behind the freezing robot problem is the uncertainty explosion, i.e., the union of the overly conservative predictions about the trajectories of nearby moving obstacles blocks all the possible movements of the robot and makes the robot fail to find a clear path passing through the dense crowd. Some previous research thus focused on controlling the predictive covariance, for instance, by repetitive re-planning [14, 15], belief space feedback planning , or developing high-fidelity independent human movement models [17, 18, 19, 20]. However, as argued in [3, 4, 2], even perfect individual prediction (i.e., when the robot is aware of all other agents’ accurate trajectories) cannot get rid of the freezing robot problem when the navigation algorithm is lack of the mathematical models of cooperation between the robot and humans. This is because when the robot is not anticipating cooperation, it will still choose a highly evasive maneuver rather than adapting its trajectory to the humans to make room for navigation. As a result, it is concluded that a model for joint collision avoidance among nearby agents is a prerequisite for effective navigation in the dense crowd, which unfortunately is not available in most of the previous navigation algorithms.
The navigation lost problem arises when the robot’s localization uncertainty accumulated during the navigation becomes so large that the robot cannot accurately locate itself in a given map . The robot may also get lost due to the localization error, e.g., in a dynamic environment, the robot may mix up known static obstacles with unknown dynamic obstacles . Most previous solutions to the navigation lost problem are passive methods. They assume that the robot motion and the pointing direction of the sensors cannot be controlled, and focus on selectively utilizing the sensor stream to minimize the localization uncertainty or error, e.g., by using different filters  or more sophisticated modeling about the dynamic scenes [10, 11]. However, for highly dynamic scenarios with dense human crowds, the pedestrians may block all the landmark features necessary for localization, and thus the robot must choose sophisticated policies for determining the robot’s motion direction and the camera’s pointing direction. Some methods recover the localization by asking the robot to look at places with special properties, e.g., with high saliency , road tracks . Some other approaches try find optimal actions that can minimize metrics with respect to the localization quality, including the entropy  or number of hypotheses about the robot’s current location . The localization recovery problem can also be formulated and solved under the more general POMDP (Partial Observable Markov Decision Making) framework, which can be solved in a tractable manner by using Gaussian belief space approximations [16, 23].
Iii Proposed Framework
To tackle the robot freezing problem and the navigation lost problem simultaneously, as illustrated in fig:overview, we present a novel navigation framework that controls the robot’s behavior in two modes: the normal navigation mode and the localization recovery mode. During navigation in challenging scenarios, the robot will actively switch between these two modes in an online manner to accomplish the navigation task.
Iii-a Normal navigation policy
In the normal mode, the robot utilizes the SLAM algorithm to accomplish the normal navigation mission, e.g., approaching its goal in (un)structured static environments. In particular, we use the state-of-the-art LiDAR SLAM algorithm, Cartographer , as our basic localization module. In general, Cartographer can handle the low-density dynamic obstacles based on the map information updated in real-time. However, as the density of dynamic obstacles (such as human crowds) increases, its performance can be degraded significantly. Therefore we first incorporate a reinforcement learning based collision avoidance method, which can significantly improve the robot’s mobility to approach the goals compared with the traditional local planner when avoiding collisions in the dense crowds. In particular, as in our previous work , we used an Actor-Critic based PPO algorithm  to train a local planner for crowd avoidance. The Actor-Critic framework  has been widely used in the reinforcement learning scheme. Commonly, the Actor module serves as the controller, while the Critic module is used to guide the gradient update of the Actor module by minimizing the surrogate loss
is an estimation to the advantage function,is the reward at timestep ,
denotes the probability ratio, and
is a hyperparameter.is computed as
where , with as the discounting factor and as the state value function for the state , and
In this paper, we train the Actor and Critic in a way similar to our previous paper . The Actor and Critic’s inputs are the robot’s laser scan , the velocity of robot itself , and goal information , while the output is a collision-free velocity command fed to the robot. It is worth noting that here the RL-based collision avoidance policy plays a different but more important role than the one as a local motion planner in our previous work . Inspired by the work , here the RL-based collision avoidance is used to accomplish a global planner by combining with traditional grid-based global planners. In particular, the goal information now is the sub-goal assigned by the grid-based global planner rather than the final goal of the agent.
Iii-B Localization recovery policy
The localization could get lost during navigation in a set of challenging situations, such as in featureless places, when sensor views being blocked, or when the robot is getting stuck in the middle of a dynamic crowd. To deal with these cases, we present a recovery mechanism so that the robot can adaptively switch to the recovery mode and regain the localization certainty. We first precompute a set of candidate recovery points in the global map, which can facilitate the localization recovery. Then, we use a reinforcement learning based recovery policy to determine to approach which recovery point for localization recovery. Note that this policy will continuously select a suitable recovery point during the navigation and it may switch to another recovery point before reaching the recovery point chosen last time, to optimally balance navigation efficiency and localization uncertainty.
Iii-B1 Candidate recovery points
Given the 2D grid map obtained offline from Cartographer , we need to recognize locations that are more helpful for re-localization. One straightforward solution is to step back to the starting point and then find another way to achieve the destination. This policy, even though sometimes adopted by humans, however, is not efficient and will suffer from the freezing problem when the scenario is crowded. Here, similar to the visual tracking and visual SLAM, we choose the recovery points from regions with sharp corners or fine structures, whose spatial invariance and stability of matching are beneficial for re-localizing the robot in the global map. In particular, we use the Harris corner detector  to extract corners from the 2D map. Next, we use -Means to cluster the extracted corners. To refine the clustering, if any corner is found far from its cluster centroid, a new cluster will be created, which naturally prevents corners far away from being clustered together. Since the cluster centroids may not locate in the passage-ways of the map (e.g., it may be close to the boundaries of the environment), we offset the centroids towards the map center into the passage-ways and the corresponding final positions are treated as candidate recovery points, as shown in the bottom-right in fig:overview. To facilitate the recovery point selection, we assign each of them a preference weight :
where is the number of corners in the -th cluster. Thus is proportional to the number of detected corners belonging to the same cluster. In this way, the recover cluster with more corner points will be preferred. This is desirable for the robustness of the recovery. In particular, the robot’s localization uncertainty will continue increasing when approaching the recovery region and thus it cannot reach the recovery point exactly. If the recovery region has only a few corners, the robot may miss all of them due to the localization error and the localization recovery will fail. If the region has many corners, the robot’s recovery task has a higher probability to succeed.
Iii-B2 Actor-Critic based recovery
After we compute the candidate recovery points, we hope that the recovery algorithm can automatically choose a near-optimal recovery point by combining the knowledge about the number of features near the recovery points, the flow of surrounding dynamic obstacles (e.g., pedestrians), and the distance between the recovery points and the eventual destination.
In order to estimate the chance of the robot reaching one candidate recovery point under the surrounding dynamic environment, we adopt the Actor-Critic framework. Different with the Actor-Critic for normal navigation in sec:normal, here the Critic no longer guides the gradient update of the Actor, but guides the Actor to the most accessible recovery point under current dynamic situation. In other words, we utilize the Critic module as a high-level guidance for recovery point selection, while the Actor module is implemented using our collision avoidance method for the low-level planning policy. Although the Critic is used for different purposes compared to that in sec:normal, we do not retrain a Critic for the recovery policy, but instead using the value function in eqn:value_fun as a Critic. This is a reasonable solution because when we used reinforcement learning algorithm to train Actor and Critic, the reward function is designed in such a way that the robot will obtain a high reward when it reaches the goal assigned by the user. Thus, according to eqn:value_fun, Critic will learn to give higher scores to those world states in which the robot is more likely to reach a given target. In this way, the Critic can use the laser scan data to evaluate the chance of passing through the surrounding dynamic obstacles to reach each destination. Similarly, in the recovery policy, we can use Critic to evaluate the accessibility of a recovery point according to the current observation of the pedestrian flow, formally implemented as
where is the value function of the Critic, and is the position of the -th recovery point. In other words, the higher the value the Critic given, the easier for the robot to reach the recovery point, and vice versa.
In addition, we also prefer a recovery point that is on the robot’s way toward the final destination, because the recovery points that are off the way would suffer the navigation efficiency. Thus, we define another evaluation function for the recovery points as:
where is the position of the final goal, and again is the position of the -th recovery point.
Finally, to select the most suitable point for recovery, we consider a set of different factors, including the preference weight of a candidate recovery points, the distance from each candidate to the goal, and the information about the dynamic environment, i.e., the pedestrian flow in the scenario. Formally, the optimal -th recovery point is determined as
where , , and are hyperparameters.
After deciding which recovery point to approach, the chosen recover point is treated as an intermediate goal and is fed to the global planner as a waypoint. Then the global planners passes the generated sub-goal to the reinforcement learning based local planner that we developed in  for local navigation. The entire localization recovery process is shown in fig:rl_recovery_behavior.
Iii-C Switch strategy
In previous sections, we have described two modes of our navigation policy in detail. Here we will explain how to switch between these two modes automatically. We propose two simple yet efficient trigger conditions for switching, specifically, the deviation between odometry and SLAM localization and the covariance for SLAM localization. We believe that the odometry position system may be not accurate enough due to the slipping and cumulative error, but it will not drift a large distance in an instant. To the contrary, the SLAM positioning system is generally accurate but may have a significant drift if the mistake happens during the feature matching between the laser data and the global map. Thus the difference between the outputs of these two localization systems can be used as the signal for the mode switch. We also the covariance for SLAM localization output to track the uncertainty of SLAM system. If any of these two values is larger than a given threshold, the system will step into the recovery mode automatically. As shown in fig:overview, while entering the recovery mode, the robot will first start recovering from the last odometry output. Next, it will move to the selected recovery point in an adaptive manner. The robot will get back to the normal mode when the recovery procedure succeeds, i.e., when the covariance of the robotic localization is smaller than a given threshold.
Iv Experiments and Results
In this section, we first briefly introduce our simulation platform and define the evaluation metrics. Then we introduce the details of the entire simulation experiment. Finally, we analyze our experimental results and run the algorithm successfully in a real robot.
Iv-a Experiment setup and metrics
To build a simulation environment for quantitatively measuring the navigation effectiveness in a complex dynamic environment, we combine two simulators, Gazebo555http://gazebosim.org/ and Menge . The Gazebo simulator is responsible for the simulation of the robot system, while Menge serves as the driver for simulating the crowd movement within the Gazebo environment. Then, we build three typical scenarios including the corridor, the supermarket, and the airport, all with crowds as shown in fig:sim_env. We use the Turtlebot2 as our mobile robot platform. According to the actual hardware parameters of the Rplidar A2 sensor mounted on the real robot, for the 2D LiDAR sensor in the simulation we set its sweep range to , the maximum sweep radius to , and the angular resolution to
As described in fig:fig_detail, we can estimate the position of the robot in two ways, either from odometer or from the Cartographer localization algorithm. And from the simulator, we have access to the ground truth of the robot position, also shown in fig:fig_detail. Given the mesh model of a simulation scenario, we can also compute the set of candidate recovery points, which are colorized according to the preference weights calculated by eqn:map_values, where the lighter the color, the greater the weight.
To verify that our algorithm can effectively reduce the probability of robots getting lost or frozen in the crowd, we propose three metrics to evaluate the navigation performance, i.e., the lost rate, the frozen rate, and the success rate. We define a robot to be lost if the distance between the SLAM estimation of the robot position and the ground truth position of the robot from the simulator is always greater than a given lost threshold in a period of . More formally, the robot is
Because a robot with the frozen issue will stop moving forward and just turn around in place, we determine whether a robot is frozen or not based on whether its linear speed along the robot’s forward direction is always no less than a given threshold speed during the time period . More formally, the robot is
We determine whether a robot succeeds in the navigation task by checking the robot’s arrival at the target position, i.e., whether the robot has
where is the goal position and is the threshold for arriving.
Based on eqn:lost_rate, eqn:freezing_rate, and eqn:success_rate, we can compute the lost rate, the frozen rate, and the success rate of the navigation algorithm.
In addition, we also want to investigate whether our algorithm will have an impact on the navigation efficiency of the robot in terms of the time cost reaching the target point. Thus, we further evaluate the mean robot speed for the navigation trials in which the robot successfully reaches the destination.
Iv-B Implementation details
In this part, we first summarize in tab:hyperparameters the hyperparameters used in our algorithm.
|in eqn:fusion_fun||0.5||in eqn:lost_rate||3.0|
|in eqn:fusion_fun||0.2||in eqn:lost_rate||10|
|in eqn:fusion_fun||1.0||in eqn:freezing_rate||0.2|
|in fig:overview||3.0||in eqn:freezing_rate||10|
|in fig:overview||0.08||in eqn:success_rate||0.5|
Then we make a comprehensive comparison among three different approaches (baseline, RL, and (RL)) in all the three testing scenarios as shown in fig:sim_env. The baseline method combines the ROS movebase navigator with the Cartographer localization , where the movebase navigator uses the dynamic window approach  for local planning and Dijkstra algorithm for global planning. The RL method replaces the local planner in the baseline method with the deep reinforcement learning based local planner . The (RL) method is the approach that we proposed in this work, which uses the reinforcement learning for both the local collision avoidance and localization recovery.
Iv-C Result analysis
tab:benchmark shows the comparison results on three different scenarios in our benchmark. When using the baseline navigation policy, the robot is almost impossible to reach the goal due to the high density and few features in the dense crowd scenario. When using the RL navigation policy, the frozen rate declines significantly and the robot gets some chance to reach the goal, thanks to the high mobility of the RL-based local planner. However, this method does not deal with the lost problem and thus the robot can get lost in the scenario. The (RL) policy can significantly increase the success rate of the navigation task because it considers the lost and frozen issues simultaneously. One interesting phenomenon is that the frozen rate of (RL) is higher than that of the RL in the supermarket and the airport scenarios. This is because the RL policy focuses on the local collision avoidance and does not pay attention to the re-localization. Thus its risk of getting stuck is lower. However, it has a much higher risk in getting lost in the large scenarios like the supermarket and the airport, which is supported by RL’s much larger lost rate than that of (RL).
|Methods||Corridor Scenario||SuperMarket Scenario||Airport Scenario|
The superiority of (RL) in performance is due to our novel recovery policy. In particular, when the robot realizes that it may get lost, the response of the baseline policy is to turn around in place for safety and look for nearby features to rescue the robot from the lost, which can be an efficient solution in a static or moderately dynamic scene. However, in a highly dynamic scenario, even though its lost level will not get worse, the robot will get frozen and thus still cannot reach the goal successfully. While using our recovery policy, rather than simply turning around in place, the robot will first find a path to break free of the crowded neighborhood and then move toward an accessible recovery point to re-localize itself. Once the robot re-localizes successfully, it will switch back to the normal navigation mode. fig:recovery provides a step-by-step illustration of this procedure.
Note that rather than choosing one recovery point and then moving toward it until it is reached, (RL) will determine the best recovery point in a dynamic and adaptive manner during its navigation. In particular, it may switch between recovery points according to its current observation about the pedestrian flow. For instance, in fig:adaptive_a and fig:adaptive_b, we can see that the bottom-right region has more free space to pass, and thus the robot will choose the recovery point in the bottom-right as the temporary goal. However, during the movement toward the goal, the bottom-right region is filling with more pedestrians, and thus the robot will adaptively switch to another recovery point that is more accessible, as shown in fig:adaptive_c and fig:adaptive_d.
In fig:density_exp, we show how the lost rate, the frozen rate, the success rate, and the mean velocity change when the pedestrian density in the scenario varies. In these three simulation scenarios, we gradually increase the number of pedestrians from 100 to 250. Then we can observe that, with the increase of pedestrian density, the mean velocity and the success rate of (RL) navigation policy decrease. The frozen rate is always very small, thanks to our RL-based collision avoidance. The lost rate increases due to the increasing difficulty to have access to the recovery point. It is worth noting that narrow aisles we set in the airport environment make the robot’s workspace too congested, which leads to the frozen rate higher than the lost rate .
Iv-D Real-world experiment
In this part, we verify that our (RL) framework can enable a physical mobile robot to pass through heavy crowds and arrive at the goal accurately in the real-world crowded environment. Consistent with previous simulations, we use Turtlebot2 as the robot platform and Rplidar A2 as the sensor. We choose the canteen as our real world experimental scenario, as shown in fig:realscene. The robot is given a pre-built map as shown in fig:realscene_recovery and goal position in the map. The (RL) navigation algorithm is then executed to test whether the robot can maintain unlost and unfrozen, and can eventually arrive at the goal successfully.
The experimental results in fig:real_world show that the Actor from our Actor-Critic based navigation framework can accomplish a powerful local planner responding quickly to the dynamic crowd, so that the robot does not get frozen in the crowded scene. In addition, thanks to the excellent mobility of the robot, the Critic of Actor-Critic framework is used as a high-level strategy to guide the robot toward the recovery points for active re-localization, so that the robot can achieve robust localization in the crowded environment. Please refer to https://sites.google.com/view/rlslam for more results.
In this paper, we propose a novel reinforcement learning based navigation framework to get a robot unlost and unfrozen in dense pedestrian crowds. In addition, we provide a benchmark including three typical scenarios with dense pedestrians. For future work, we plan to include camera resources including the depth, semantic labels, optical flows into our system to resolve our current limitation of only using 2D LiDAR information.
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