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Title: Maximum Likelihood Constraint Inference for Inverse Reinforcement Learning

Paper content:
While most approaches to the problem of Inverse Reinforcement Learning (IRL) focus on estimating a reward function that best explains an expert agent’s policy or demonstrated behavior on a control task, it is often the case that such behavior is more succinctly represented by a simple reward combined with a set of hard constraints.
In this setting, the agent is attempting to maximize cumulative rewards subject to these given constraints on their behavior.
We reformulate the problem of IRL on Markov Decision Processes (MDPs) such that, given a nominal model of the environment and a nominal reward function, we seek to estimate state, action, and feature constraints in the environment that motivate an agent’s behavior.
Our approach is based on the Maximum Entropy IRL framework, which allows us to reason about the likelihood of an expert agent’s demonstrations given our knowledge of an MDP.
Using our method, we can infer which constraints can be added to the MDP to most increase the likelihood of observing these demonstrations.
We present an algorithm which iteratively infers the Maximum Likelihood Constraint to best explain observed behavior, and we evaluate its efficacy using both simulated behavior and recorded data of humans navigating around an obstacle.
Advances in mechanical design and artificial intelligence continue to expand the horizons of robotic applications.
In these new domains, it can be difficult to design a specific robot behavior by hand.
Even manually specifying a task for a reinforcement-learning-enabled agent is notoriously difficult (Ho et al., 2015; Amodei et al., 2016) .
Inverse Reinforcement Learning (IRL) techniques can help alleviate this burden by automatically identifying the objectives driving certain behavior.
Since first being introduced as Inverse Optimal Control by Kalman (1964) , much of the work on IRL has focused on learning environmental rewards to represent the task of interest (Ng et al., 2000; Abbeel & Ng, 2004; Ratliff et al., 2006; Ziebart et al., 2008) .
While these types of IRL algorithms have proven useful in a variety of situations (Abbeel et al., 2007; Vasquez et al., 2014; Ziebart, 2010; , their basis in assuming that reward functions fully represent task specifications makes them ill suited to problem domains with hard constraints or non-Markovian objectives.
Recent work has attempted to address these pitfalls by using demonstrations to learn a rich class of possible specifications that can represent a task (Vazquez-Chanlatte et al., 2018) .
Others have focused specifically on learning constraints, that is, behaviors that are expressly forbidden or infeasible (Pardowitz et al., 2005; Pérez-D'Arpino & Shah, 2017; Subramani et al., 2018; McPherson et al., 2018; Chou et al., 2018) .
Such constraints arise in safety-critical systems, where requirements such as an autonomous vehicle avoiding collisions with pedestrians are more naturally expressed as hard constraints than as soft reward penalties.
It is towards the problem of inferring such constraints that we turn our attention.
In this work, we present a novel method for inferring constraints, drawing primarily from the Maximum Entropy approach to IRL described by Ziebart et al. (2008) .
We use this framework to reason about the likelihood of observing a set of demonstrations given a nominal task description, as well as about their likelihood if we imposed additional constraints on the task.
This knowledge allows us to select a constraint, or set of constraints, which maximizes the demonstrations' likelihood and best explains the differences between expected and demonstrated behavior.
Our method improves on prior work by being able to simultaneously consider constraints on states, actions and features in a Markov Decision Process (MDP) to provide a principled ranking of all options according to their effect on demonstration likelihood.
We have presented our novel technique for learning constraints from demonstrations.
We improve upon previous work in constraint-learning IRL by providing a principled framework for identifying the most likely constraint(s), and we do so in a way that explicitly makes state, action, and feature constraints all directly comparable to one another.
We believe that the numerical results presented in Section 4 are promising and highlight the usefulness of our approach.
Despite its benefits, one drawback of our approach is that the formulation is based on (3), which only exactly holds for deterministic MDPs.
As mentioned in Section 3.3, we plan to investigate the use of a maximum causal entropy approach to address this issue and fully handle stochastic MDPs.
Additionally, the methods presented here require all demonstrations to contain no violations of the constraints we will estimate.
We believe that softening this requirement, which would allow reasoning about the likelihood of constraints that are occasionally violated in the demonstration set, may be beneficial in cases where trajectory data is collected without explicit labels of success or failure.
Finally, the structure of Algorithm 1, which tracks the expected features accruals of trajectories over time, suggests that we may be able to reason about non-Markovian constraints by using this historical information to our advantage.
Overall, we believe that our formulation of maximum likelihood constraint inference for IRL shows promising results and presents attractive avenues for further investigation.

assistant: text: Our method infers constraints on task execution by leveraging the principle of maximum entropy to quantify how demonstrations differ from expected, un-constrained behavior.