Unsupervised Cross-Domain Transfer in Policy Gradient Reinforcement Learning via Manifold Alignment

The success of applying policy gradient reinforcement learning (RL) to difficult control tasks hinges crucially on the ability to determine a sensible initialization for the policy. Transfer learning methods tackle this problem by reusing knowledge gleaned from solving other related tasks. In the case of multiple task domains, these algorithms require an inter-task mapping to facilitate knowledge transfer across domains. However, there are currently no general methods to learn an inter-task mapping without requiring either background knowledge that is not typically present in RL settings, or an expensive analysis of an exponential number of inter-task mappings in the size of the state and action spaces. This paper introduces an autonomous framework that uses unsupervised manifold alignment to learn intertask mappings and effectively transfer samples between different task domains. Empirical results on diverse dynamical systems, including an application to quadrotor control, demonstrate its effectiveness for cross-domain transfer in the context of policy gradient RL.  Policy gradient reinforcement learning (RL) algorithms have been applied with considerable success to solve highdimensional control problems, such as those arising in robotic control and coordination (Peters & Schaal 2008). These algorithms use gradient ascent to tune the parameters of a policy to maximize its expected performance. Unfortunately, this gradient ascent procedure is prone to becoming trapped in local maxima, and thus it has been widely recognized that initializing the policy in a sensible manner is crucial for achieving optimal performance. For instance, one typical strategy is to initialize the policy using human demonstrations (Peters & Schaal 2006), which may be infeasible when the task cannot be easily solved by a human. This paper explores a different approach: instead of initializing the policy at random (i.e., tabula rasa) or via human demonstrations, we instead use transfer learning (TL) to initialize the policy for a new target domain based on knowledge from one or more source tasks. In RL transfer, the source and target tasks may differ in their formulations (Taylor & Stone 2009). In particular,  2007a) that describes the relationship between the two tasks is typically needed. This paper introduces a framework for autonomously learning an inter-task mapping for cross-domain transfer in policy gradient RL. First, we learn an inter-state mapping (i.e., a mapping between states in two tasks) using unsupervised manifold alignment. Manifold alignment provides a powerful and general framework that can discover a shared latent representation to capture intrinsic relations between different tasks, irrespective of their dimensionality. The alignment also yields an implicit inter-action mapping that is generated by mapping tracking states from the source to the target. Given the mapping between task domains, source task trajectories are then used to initialize a policy in the target task, significantly improving the speed of subsequent learning over an uninformed initialization. This paper provides the following contributions. First, we introduce a novel unsupervised method for learning interstate mappings using manifold alignment. Second, we show that the discovered subspace can be used to initialize the target policy. Third, our empirical validation conducted on four dissimilar and dynamically chaotic task domains (e.g., controlling a three-link cart-pole and a quadrotor aerial vehicle) shows that our approach can a) automatically learn an inter-state mapping across MDPs from the same domain, b) automatically learn an inter-state mapping across MDPs from very different domains, and c) transfer informative initial policies to achieve higher initial performance and reduce the time needed for convergence to near-optimal behavior.