Safety, Risk Awareness and Exploration in Reinforcement Learning
Technical Report Identifier: EECS-2016-20
May 1, 2016
Abstract: Replicating the human ability to solve complex planning problems based on minimal prior knowledge has been extensively studied in the field of reinforcement learning. Algorithms for discrete or approximate models are supported by theoretical guarantees but the necessary assumptions are often constraining. We aim to extend these results in the direction of practical applicability to more realistic settings. Our contributions are restricted to three specific aspects of practical problems that we believe to be important when applying reinforcement learning techniques: risk awareness, safe exploration and data efficient exploration.
Risk awareness is important in planning situations where restarts are not available and performance depends on one-off returns rather than average returns. The expected return is no longer an appropriate objective because the law of large numbers does not apply. In Chapter 2 we propose a new optimization objective for risk-aware planning and show that it has desirable theoretical properties, relating it to previously proposed risk-aware objectives: minmax, exponential utility, percentile and mean minus variance. In environments with uncertain dynamics, exploration is often necessary to improve performance. Existing reinforcement learning algorithms provide theoretical exploration guarantees, but they tend to rely on the assumption that any state is eventually reachable from any other state by following a suitable policy. For most physical systems this assumption is impractical as the systems would break before any reasonable exploration has taken place. In Chapter 3 we address the need for a safe exploration method. In Chapter 4 we address the specific challenges presented by extending model-based reinforcement learning methods from discrete to continuous dynamical systems. System representations based on explicitly enumerated states are not longer applicable. To address this challenge we use a Dirichlet process mixture of linear models to represent dynamics. The proposed model strikes a good balance between compact representation and flexibility. To address the challenge of efficient exploration-exploitation trade-off we apply the principle of Optimism in the Face of Uncertainty that underlies numerous other provably efficient algorithms in simpler settings. Our algorithm reduces the exploration problem to a sequence of classical optimal control problems. Synthetic experiments illustrate the effectiveness of our methods.