  In recent years, reinforcement learning (RL) has gained increasing attention
in control engineering. Especially, policy gradient methods are widely used. In
this work, we improve the tracking performance of proximal policy optimization
(PPO) for arbitrary reference signals by incorporating information about future
reference values. Two variants of extending the argument of the actor and the
critic taking future reference values into account are presented. In the first
variant, global future reference values are added to the argument. For the
second variant, a novel kind of residual space with future reference values
applicable to model-free reinforcement learning is introduced. Our approach is
evaluated against a PI controller on a simple drive train model. We expect our
method to generalize to arbitrary references better than previous approaches,
pointing towards the applicability of RL to control real systems.
