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Hindawi Journal of Robotics Volume 2020, Article ID 8702962, 7 pages https://doi.org/10.1155/2020/8702962 Research Article Visual Navigation with Asynchronous Proximal Policy Optimization in Artificial Agents Fanyu Zeng and Chen Wang School of Computer Science and Engineering, Center for Robotics, University of Electronic Science and Technology of China, Chengdu 611731, China Correspondence should be addressed to Fanyu Zeng; zengfanyu_cs@163.com Received 12 February 2020; Revised 10 August 2020; Accepted 21 September 2020; Published 15 October 2020 Academic Editor: Weitian Wang Copyright © 2020 Fanyu Zeng and Chen Wang. )is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Vanilla policy gradient methods suﬀer from high variance, leading to unstable policies during training, where the policy’s performance ﬂuctuates drastically between iterations. To address this issue, we analyze the policy optimization process of the navigation method based on deep reinforcement learning (DRL) that uses asynchronous gradient descent for optimization. A variant navigation (asynchronous proximal policy optimization navigation, appoNav) is presented that can guarantee the policy monotonic improvement during the process of policy optimization. Our experiments are tested in DeepMind Lab, and the experimental results show that the artiﬁcial agents with appoNav perform better than the compared algorithm. asynchronous variants of AC algorithms, termed as asyn- 1. Introduction chronous advantage actor-critic (A3C), and showed that Navigation in an unstructured environment is one of the most parallel actor-learners have a stabilizing eﬀect on training important abilities for mobile robotics and artiﬁcial agents artiﬁcial agents. Researchers can construct navigation agents [1–3]. Traditional methods mainly divide navigation into based on these DRL algorithms. However, vanilla policy several parts [4]: simultaneous localization and mapping gradient methods have poor data eﬃciency [19], which leads (SLAM) [5–7], path planning [8], and semantic segmentation to navigation agents suﬀering from high variance and un- [9, 10]. )e methods mentioned are not an end-to-end al- stable policies. gorithm where each part is a challenging research subject, and In this work, we take A3C as an example to show how to the fusion of each part often leads to large computational guarantee the policy monotonic improvement. )e training errors. To reduce the fusion error, we focus on the end-to-end environment is DeepMind Lab [20], and it is a ﬁrst-person navigation based on deep reinforcement learning where 3D virtual environment designed for research and devel- navigational abilities could emerge as the byproduct of an opment of general artiﬁcial intelligence. DeepMind Lab can artiﬁcial agent learning policy with reward maximization. be used to study how autonomous artiﬁcial agents learn With the fast development of deep learning [11–14], a complex tasks in large, partially observed, and visually di- variety of DRL architectures have been proposed [2]. Mnih verse worlds. In addition, the worlds are rendered with rich et al. [15] presented the advances in training deep neural science ﬁction-style visuals. Actions are to look around and networks to develop the deep Q-network (DQN), which can move in the 3D virtual world, and example tasks include learn successful policies directly from high-dimensional navigation in diﬀerent mazes. Mirowski et al. [21] proposed image inputs using end-to-end reinforcement learning. On- a DRL navigation method based on A3C [18], augmented policy reinforcement learning methods such as actor-critic with auxiliary learning targets, to train artiﬁcial agents to (AC) [16, 17] were proposed such that the actor is the policy, navigate in DeepMind Lab. For ease of expression, we call and the critic is the baseline. Minh et al. [18] presented the DRL navigation using A3C as a3cNav. 2 Journal of Robotics In this paper, the issues on policy optimization for R � c r , which is a discounted sum of rewards. )e t k�0 t+k navigation based on the vanilla policy gradient are analyzed; action-value function Q � E[R | s � s, a] is the expected t t this type of navigation cannot control the change of expected return following action a from state s under policy π. )e advantage when an artiﬁcial agent learns to navigate in a value function V � E[R | s � s] is the expected return t t maze. Based on the navigation techniques presented in [21], from state s. we show how to reduce training variances and get higher In policy-based methods, let π(a | s; θ) be a policy with reward when an artiﬁcial agent interacts with an environ- parameters θ, which is updated by performing gradient ment. Inspired by [19, 22], we adjust the policy update ascent on E[R ]. Policy gradient algorithms adjust the policy process of the navigation in [21] to guarantee the monotonic by updating parameters θ in the direction improvement of the navigation policy. Experimental results ∇ logπ(a | s ; θ)R that is an unbiased estimate of ∇ E[R ]. θ t t t θ t show that an artiﬁcial agent via appoNav learns better To reduce the variance of this estimate, Williams [29] navigation policy in DeepMind Lab and suﬀers from lower subtracted a learned function called baseline b (s ) for the t t standard deviation than a3cNav. return, so the improved gradient becomes ∇ logπ(a | s ; θ)(R − b (s )). )ere exists an equation θ t t t t t b (s ) ≈ V (s), and R − b (s ) can be seen as an estimate of t t t t t 2. Related Work the advantage of action at under state s . )e numerical value Traditional navigation, which is model-based, includes si- of Q (s, a) equals the value of R ; hence, the advantage multaneous localization and mapping (SLAM) [5, 7, 23], function can be rewritten as A(a , s ) � Q(a , s ) − V(s ). t t t t t path planning [8, 24], and semantic segmentation [9]. Each )is method is called actor-critic (AC) architecture where part of them is a challenge research area, and the fusion of the actor is the policy π and the critic is the baseline b them often leads to large computation error. Moreover, [16, 17]. Minh et al. [18] presented asynchronous variants of model-based navigation needs to model the environments AC algorithms, termed as asynchronous advantage actor- eﬀectively for some dynamic and complex scenes, which critic (A3C), and showed that parallel actor-learners have a severely aﬀect navigation performance. stabilizing eﬀect on training artiﬁcial agents. With recent advances in DRL, many navigation methods When a DRL agent interacts with its environment, the based on DRL have been proposed [2]. DRL navigation, state sequences of each interaction change a lot, leading to which is end to end, avoids the computation error caused by ﬂuctuations in rewards. )erefore, DRL algorithms (such as the fusion of traditional navigation. Mirowski et al. [21] DQN and A3C) have unstable ﬂuctuations during training. addressed navigation via auxiliary depth prediction and Researchers wonder whether they can ﬁnd a method to loop-closure classiﬁcation tasks. Jaderberg et al. [25] also reduce such ﬂuctuations while maintaining a steady im- used auxiliary tasks for navigation and incorporated A3C provement in the policy. Schulman et al. [22] proposed trust with control tasks and prediction tasks including pixel region policy optimization (TRPO) to make the monotonic control and reward prediction. By using features extracted improvement for the policy. Furthermore, Schulman et al. from the world model as inputs to an agent, Ha and [19] proposed proximal policy optimization (PPO) to Schmidhuber [26] used DRL to construct a world model and simplify the calculation of TRPO. In addition, Heess et al. used the model in a car navigation task. Bruce et al. [27] [30] proposed a distributed implementation of PPO, called leveraged an interactive world model based on DRL built distributed PPO. Besides the similar process of the gradient from a single traversal of the environment and utilized a update with A3C, distributed PPO includes various tricks, pretrained visual feature encoder to demonstrate successful such as normalizations (observation normalization, reward zero-shot transfer under real-world environmental varia- reshape normalization, and per-batch normalization of the tions without ﬁne-tuning. Banino et al. [28] proposed a advantages), sharing of algorithm parameters across local vector-based navigation method that fuses DRL with grid- workers, and additional trust region constraint. )ese tricks like representations in the artiﬁcial agent. When these DRL result in that the computation of distributed PPO is more navigation agents interact with environments, the state se- complex than appoNav. quences of each interaction change a lot, leading to large ﬂuctuations in rewards. )erefore, these DRL navigation 3.2. NavA3C + D D . In this work, we use the methods suﬀer from high variance and have unstable pol- 1 2 icies during training. NavA3C + D D architecture [21] as shown in Figure 1, 1 2 which includes 2 CNNs and 2 LSTMs. NavA3C + D D has 4 1 2 inputs: the current RGB image x , previous reward r , t t−1 3. Background previous action a , and the current velocity v . )e 2 CNNs t−1 t act as the encoder for RGB image x , and the ﬁrst LSTM 3.1. Reinforcement Learning. We consider the standard re- inforcement learning setting where an artiﬁcial agent in- makes associations between reward r and visual obser- t−1 vations x that are provided as context to the second LSTM teracts with an environment over a number of discrete time steps. At each time step t, the agent receives a state s from from which the policy π(a | s ; θ) and the value V(s ; θ ) are t t t v computed. Artiﬁcial agents based on this architecture try to the environment and outputs an action a according to its learned policy π. In return, the environment gives the agent a maximize the cumulative reward R during their interaction with the maze and minimize the auxiliary depth losses next sate s and a reward r . )e goal of reinforcement t+1 t learning is to maximize the accumulated reward L and L . Finally, the agent can learn how to Depth1 Depth2 Journal of Robotics 3 CNN CNN LSTM LSTM t–1 {v , a } t t–1 Figure 1: a3cNav architecture. In the architecture, image x is the input of a3cNav, and following the full connection layer is a two-layer CNN which outputs depth D as well as a two-layer stacked LSTM which outputs depth D , policy π, and value V. In addition, auxiliary task 1 2 used in this architecture in which the ﬁrst LSTM only receives the reward and the velocity and previously selected action are fed into the second LSTM. navigate in DeepMind Lab. For ease of expression, we the global network of a3cNav, leading to the unstable training of the agent. In this section, we improve the pa- rename NavA3C + D D as a3cNav. 1 2 a3cNav is based on the A3C framework into which rameter updates of a3cNav to guarantee its policy mono- unsupervised auxiliary tasks are incorporated. )erefore, its tonic improvement. loss function includes the loss of A3C L and the loss of In [22], a policy can be rewritten as A3C auxiliary tasks. a3cNav can be optimized as follows: η(π ) � η(π) + ρ (s) π (a | s)A (s, a), π (3) s a L (θ) � L + λ L + λ L , (1) a3cNav A3C Depth1 Depth1 Depth2 Depth2 where π denotes a stochastic policy and π is another policy. where λ and λ are weighting terms on the indi- Depth1 Depth2 η(π) and η(π) are the expected discounted cost for π and π, vidual loss components. respectively. Here, ρ (s) is the distribution of the state s π )e global parameters θ of a3cNav are updated in according to π, and A is the advantage function following π. multithread environments, and θ are copied to the local Equation (3) implies that if we want to reduce η or leave it as worker parameters θ . )e local worker of a3cNav interacts constant, we should keep the expected advantage π(a | s) ′ a with the maze, and the policy gradients wrt θ and the value A (s, a)≤ 0 at every state s when a policy update π ⟶ π. )is ′ π gradients wrt θ are computed from the policy loss and value demonstrates that if we want to reduce the training variance of loss. )e gradient for the parameter update is proportional a3cNav and keep its policy monotonic improvement, we must to the product of advantage function A . Equation (2) shows guarantee π (a | s)A (s, a)≤ 0. However, a3cNav cannot a π the calculation of gradients: control the change of the expected advantage when the artiﬁcial ′ ′ dθ⟵ dθ + ∇ logπa s ; θ R − V s ; θ agent learns to navigate in the maze. ′ t t t v To make the policy monotonic improvement, Schulman + β∇ H π s ; θ et al. [22] proposed a trust region constraint, as shown in θ t , (2) equation (4), over policy update to make π(a | s)A a π (s, a)≤ 0: z R − V s ; θ t v dθ ⟵ dθ + v v zθ π a |s θ t t max E A , t t θ π a |s θ t t old where H(π(s ; θ )) is the entropy of the policy π, which (4) improves exploration by discouraging premature conver- E KLπ ·|s , π ·|s ≤ δ. gence to suboptimal deterministic policies. )en, asyn- t θ t θ t old chronous update of θ using dθ and of θ using dθ are v v Equation (4) is relatively complex and is not compatible with applied into the global network for parameter update. the architectures which include parameter sharing between the policy function and the value function, or with auxiliary tasks 4. Approach [19]. )e policy and the value network of a3cNav both share the same network, and a3cNav has the auxiliary depth prediction. 4.1. Monotonic Policy Improvement. )e artiﬁcial agent in- )erefore, TRPO cannot be used into a3cNav. teracts randomly with the environment which in turn gives high-dimensional images to the agent. Hence, a3cNav has π a s π a s θ t t θ t t ⎢ ⎥ ⎡ ⎢ ⎝ ⎝ ⎠ ⎠⎤ ⎥ ⎣ ⎛ ⎛ ⎞ ⎞⎦ E min A , clip , 1 − ε, 1 + ε A . t t t poor data eﬃciency and robustness. In addition, complex π a s π a s θ t t θ t t old old navigation environment that sends changing images to the (5) artiﬁcial agent aggravates the variance and instability of PPO [19] improves TRPO with only ﬁrst-order opti- training. In detail, each local worker of a3cNav interacts with the maze, and the gradients with big variance are applied to mization and replaces the constraint with the clipped 4 Journal of Robotics Table 1: )e states that the artiﬁcial agent sees in stairway_to_melon. Time 600 700 800 900 1000 1100 1200 1300 1400 1500 Episode )e ﬁrst episode )e second episode )e third episode Time 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 Episode )e ﬁrst episode )e second episode )e third episode surrogate objective as equation (5). Hence, PPO is a ﬁrst- policy generated by appoNav has lower variance and more order optimization method and is compatible with pa- stable training performance. rameter sharing and auxiliary tasks. 5. Experiments 4.2. appoNav. To make the monotonic improvement for the navigation policy, we seek to incorporate the features of PPO 5.1. Experimental Settings. We implement our algorithm in into the local worker of a3cNav. In each thread, the im- TensorFlow and train it on Nvidia GeForce GTX Titan X proved local policy tends to improve monotonically. And the GPU and Intel Xeon E5-2687W v2@3.4GHz 17 CPU. new local gradients are applied to the global network, )e proposed method is evaluated in DeepMind Lab leading to the whole network with monotonic improvement. environments [20]. )e action space in DeepMind Lab has 8 As the navigation method is based on the monotonic policy actions: the agent can rotate in small increments, accelerate improvement of PPO, we call this navigation as appoNav. forward or backward or sideways, or induce rotational ac- Assume that the global network shared parameter vector celeration while moving. Reward encourages the agent to θ and local worker parameter vector θ . Equation (6) is the learn navigation; a reward is achieved when the artiﬁcial policy optimization loss of A3C [18]: agent reaches a goal from a random start location and orientation. If the agent reaches the goal, a new episode L � logπa s ; θ + βH π s ; θ . (6) A3C t t t starts, and the same interaction restarts. Fruit represents the reward in DeepMind Lab: apples are worth 1 point, When added to the local worker of a3cNav, the loss strawberries 2 points, and goals 10 points. function becomes the form of equation (5) with entropy of appoNav is evaluated by training the agent in stair- the policy, and it is rewritten for the local workers as way_to_melon and nav_maze_static_01 of DeepMind Lab. For ease of expression, we name stairway_to_melon as the π a ∣ s π a ∣ s ′ ′ ⎢ θ t t θ t t ⎥ ⎡ ⎢ ⎝ ⎝ ⎠ ⎠⎤ ⎥ ⎣ ⎛ ⎛ ⎞ ⎞⎦ stairway maze and nav_maze_static_01 as the static01 maze. E min A , clip , 1 − ε, 1 + ε A t t t π a ∣ s π a ∣ s ′ ′ θ t t θ t t old old In each case, blue curve stands for a3cNav and orange for appoNav. For experimental analysis, we run 2500 episodes + βH π s ; θ . t for the stairway maze and 7800 episodes for the maze01 maze. (7) Equation (7) is the policy update of the local worker of a3cNav, that is, appoNav. Each local worker has a low 5.2. Experimental Results and Analysis. Table 1 shows the variance than before and applies the new gradient to the images that the artiﬁcial agent sees in the stairway maze; we global network for the policy update. Finally, the whole stochastically select 3 episodes from time 600 to 2500 with Journal of Robotics 5 0 500 1000 1500 2000 2500 a3cNav appoNav Figure 2: Reward achieved by the artiﬁcial agent in stairway_to_melon. Table 2: Standard deviation of the reward in stairway_to_melon. Algorithm Standard deviation a3cNav 30.16 appoNav 27.24 Table 3: )e states that the artiﬁcial agent sees in stairway_to_melon. Time 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 Episode )e ﬁrst episode )e second episode )e third episode Time 3000 3200 3400 3600 3800 4000 4200 4400 4600 4800 Episode )e ﬁrst episode )e second episode )e third episode interval 100, which demonstrate three diﬀerent states at the std of appoNav and a3cNav is 27.24 and 30.16, respectively; same time with diﬀerent episodes. )e artiﬁcial agents can this shows that the learning process of the former is more receive diﬀerent images and be not stuck in one place, which stable than the latter one. demonstrates the agents learning to navigation in stairway )e reason why our method converges faster is that the maze. local worker of appoNav can generate a more stable policy Figure 2 shows the reward achieved by the artiﬁcial agent with the monotonic improvement when it interacts with the in stairway_to_melon; it shows that appoNav gets higher stairway. During the training iterations, improved accu- reward than a3cNav. In addition, we calculate the standard mulated gradients are applied for the parameter update of deviation (std) of the reward curve. From Table 2, the reward appoNav, which make appoNav more stable than a3cNav. 6 Journal of Robotics Conflicts of Interest )e authors declare that there are no conﬂicts of interest regarding the publication of this paper. Acknowledgments )is work was supported by the National Natural Science Foundation of China (U1813202, 61773093, and 62003381), National Key R&D Program of China (2018YFC0831800), Research Programs of Sichuan Science and Technology Department (17ZDYF3184), and Important Science and Technology Innovation Projects in Chengdu (2018-YF08- 00039-GX). 0 1000 2000 3000 4000 5000 6000 7000 8000 a3cNav References appoNav [1] G. N. DeSouza and A. C. 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Journal of Robotics – Hindawi Publishing Corporation
Published: Oct 15, 2020
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