  Digital twins, which are a new concept in industrial control systems (ICS),
play a key role in realizing the vision of a smart factory, and they can have
different effective use cases. With digital twins, we have virtual replicas of
physical systems so that they precisely mirror the internal behavior of the
physical systems. Hence, synchronization is necessary to keep the states of
digital twins in sync with those of their physical counterparts. Otherwise,
their behavior may be different from each other, and it can lead to wrong
decisions about the system that can have catastrophic consequences. In this
paper, we propose three different architectures for digital twins, and then by
investigating their ability to follow the physical system's behavior, we will
determine the best architecture, whose output has the lowest error compared
with the physical system's output.
