An Agent is in an Environment. a) Agent reads Input (State) from Environment. b) Agent produces Output (Action) that affects its State relative to Environment c) Agent receives reward (or feedback) for the Output produced. With the reward/feedback it receives it learns to produce better Output for given Input.
Where do neural networks come in ?
Optimal control theory considers control of a dynamical system such that an objective function is optimized (with applications including stability of rockets, helicopters). In optimal control theory, Pontryagin’s principle says: a necessary condition for solving the optimal control problem is that the control input should be chosen to minimize the control Hamiltonian. This “control Hamiltonian” is inspired by the classical Hamiltonian and the principle of least action. The goal is to find an optimal control policy function u∗(t) and, with it, an optimal trajectory of the state variable x∗(t) which by Pontryagin’s maximum principle are the arguments that maximize the Hamiltonian.
Derivatives are needed for the continuous optimizations. Deep learning models are capable of performing continuous linear and non-linear transformations, which in turn can compute derivatives and integrals. They can be trained automatically using real-world inputs, outputs and feedback. So a neural network can provide a system for sophisticated feedback-based non-linear optimization of the map from Input space to Output space.
The above could be accomplished by a feedforward neural network that is trained with a feedback (reward). Additionally a recurrent neural network could encode a memory into the system by making reference to previous states (likely with higher training and convergence costs).
Model-free reinforcement learning does not explicitly learn a model of the environment.
Manifestations of RL: Udacity self-driving course – lane detection. Karpathy’s RL blog post has an explanation of a network structure that can produce policies in a malleable manner, called policy gradients.
Practical issues in Reinforcement Learning –
Raw inputs vs model inputs: There is the problem of mapping inputs from real-world to the actual inputs to a computer algorithm. Volume/quality of information – high vs low requirement.
Exploitation vs exploration dilemma: https://en.wikipedia.org/wiki/Multi-armed_bandit. Simple exploration methods are the most practical. With probability ε, exploration is chosen, and the action is chosen uniformly at random. With probability 1 − ε, exploitation is chosen, and the agent chooses the action that it believes has the best long-term effect (ties between actions are broken uniformly at random). ε is usually a fixed parameter but can be adjusted either according to a schedule (making the agent explore progressively less), or adaptively based on heuristics.
AWS DeepRacer. Allows exploration of RL. Simplifies the mapping of camera input to computer input, so one can focus more on the reward function and deep learning aspects. The car has a set of possible actions (change heading, change speed). The RL task is to predict the actions based on the inputs.
What are some of the strategies applied to winning DeepRacer ?
- Implementation of the pure pursuit tracking problem, used by Scott Pletcher.
- Explicit reward based on proximity, distance and speed by Daniel Gonzalez and team in https://towardsdatascience.com/an-advanced-guide-to-aws-deepracer-2b462c37eea
- https://medium.com/dbs-tech-blog/an-introduction-to-aws-deepracer-from-a-2020-world-championship-finalist-3a63b5c8d8aa Fully autonomous vs Semi-autonomous. Input parameters for the reward function. Log analysis for optimizing the models.
- Faster training vs slower training – https://falktan.medium.com/aws-deepracer-how-to-train-a-model-in-15-minutes-a07ab77fb793 (PPO takes full lap to learn, line of sight learns in sub-lap distances).
- Soft-actor-critic algorith. SAC demystified – https://towardsdatascience.com/soft-actor-critic-demystified-b8427df61665 . SAC works to increase entropy (to encourage exploration) and not just maximize rewards.
Reward function input parameters – https://docs.aws.amazon.com/deepracer/latest/developerguide/deepracer-reward-function-input.html
“DeepRacer: Educational Autonomous Racing Platform for Experimentation with Sim2Real Reinforcement Learning” – https://arxiv.org/pdf/1911.01562.pdf
RL is not a fit for every problem. Alternative approaches with better explainability and determinism include behavior trees, vectorization/VectorNet, …
Richard Sutton and Andrew Barto’s book on RL: An introduction.
This paper explores incorporating Attention mechanism with Reinforcement learning – Reinforcement Learning with Attention that Works: A Self-Supervised Approach. A video review of the ‘Attention is all you need’ is here, the idea being to replace an RNN with a mechanism to selectivity track a few relevant things.
Multi agent Deep Deterministic Policy Gradients – cooperation between agents. https://www.youtube.com/watch?v=tZTQ6S9PfkE. Agents learn a centralized critic based on the observations and actions of all agents. https://arxiv.org/pdf/1706.02275.pdf .
Multi-vehicle RL for multi-lane driving. https://arxiv.org/pdf/1911.11699v1.pdf