TGPO: Temporal Grounded Policy Optimization for Signal Temporal Logic
  Tasks
Authors: name: Yue Meng, name: Fei Chen, name: Chuchu Fan
Organization: Massachusetts Institute of Technology

Learning control policies for complex, long-horizon tasks is a central
challenge in robotics and autonomous systems. Signal Temporal Logic (STL)
offers a powerful and expressive language for specifying such tasks, but its
non-Markovian nature and inherent sparse reward make it difficult to be solved
via standard Reinforcement Learning (RL) algorithms. Prior RL approaches focus
only on limited STL fragments or use STL robustness scores as sparse terminal
rewards. In this paper, we propose TGPO, Temporal Grounded Policy Optimization,
to solve general STL tasks. TGPO decomposes STL into timed subgoals and
invariant constraints and provides a hierarchical framework to tackle the
problem. The high-level component of TGPO proposes concrete time allocations
for these subgoals, and the low-level time-conditioned policy learns to achieve
the sequenced subgoals using a dense, stage-wise reward signal. During
inference, we sample various time allocations and select the most promising
assignment for the policy network to rollout the solution trajectory. To foster
efficient policy learning for complex STL with multiple subgoals, we leverage
the learned critic to guide the high-level temporal search via
Metropolis-Hastings sampling, focusing exploration on temporally feasible
solutions. We conduct experiments on five environments, ranging from
low-dimensional navigation to manipulation, drone, and quadrupedal locomotion.
Under a wide range of STL tasks, TGPO significantly outperforms
state-of-the-art baselines (especially for high-dimensional and long-horizon
cases), with an average of 31.6% improvement in task success rate compared to
the best baseline. The code will be available at
https://github.com/mengyuest/TGPO