  Digital Twin technology is being envisioned to be an integral part of the
industrial evolution in modern generation. With the rapid advancement in the
Internet-of-Things (IoT) technology and increasing trend of automation,
integration between the virtual and the physical world is now realizable to
produce practical digital twins. However, the existing definitions of digital
twin is incomplete and sometimes ambiguous. Herein, we conduct historical
review and analyze the modern generic view of digital twin to create its new
extended definition. We also review and discuss the existing work in digital
twin in safety-critical robotics applications. Especially, the usage of digital
twin in industrial applications necessitates autonomous and remote operations
due to environmental challenges. However, the uncertainties in the environment
may need close monitoring and quick adaptation of the robots which need to be
safety-proof and cost effective. We demonstrate a case study on developing a
framework for safety-critical robotic arm applications and present the system
performance to show its advantages, and discuss the challenges and scopes
ahead.
