Spiking Neural Network Discovers Energy-Efficient Hexapod Motion in Deep Reinforcement Learning

In Deep Reinforcement Learning (DRL) for robotics application, it is important to find energy-efficient motions. For this purpose, a standard method is to set an action penalty in the reward to find the optimal motion considering the energy expenditure. This method is widely used for the simplicity of implementation. However, since the reward is a linear sum, if the penalty is too large, the system will fall into local minima and no moving solution can be obtained. In contrast, if the penalty is too small, the effect may not be sufficient. Therefore, it is necessary to adjust the amount of the penalty so that the agent always moves dynamically, and the energy-saving effect is sufficient. Nevertheless, since adjusting the hyperparameters is computationally expensive, we need a learning method that is robust to the penalty setting problem. We investigated on the Spiking Neural Network (SNN), which has been attracting attention for its computational efficiency and neuromorphic architecture. We conducted gait experiments using a hexapod agent while varying the energy penalty settings in the simulation environment. By applying SNN to the conventional state-of-the-art DRL algorithms, we examined whether the agent could explore for an optimal gait with a larger penalty variation and obtain an energy-efficient gait verified with Cost of Transport (CoT), a metric of energy efficiency for gait. Soft Actor-Critic (SAC)+SNN resulted in a CoT of 1.64, Twin Delayed Deep Deterministic policy gradient (TD3)+SNN resulted in a CoT of 2.21, and Deep Deterministic policy gradient (DDPG)+SNN resulted in a CoT of 2.08 (1.91 for normal SAC, 2.38 for TD3, and 2.40 for DDPG). DRL combined with SNN succeeded in learning more energy efficient gait with lower CoT.