IoT Blockchains for Digital Twins

Introduction

Digital Twins (DTs) are digital representations of living or non-living physical objects. DTs have been widely considered in the context of manufacturing as a conceptual model in the product lifecycle management process (Grieves, 2019). Digital modelling in manufacturing combines computer aided design techniques (e.g., 3D models) with additive or subtractive manufacturing processes. ISO defines a DT in manufacturing as a fit for purpose digital representation of an observable manufacturing element with a means to enable convergence between the element and its digital representation at an appropriate rate of synchronization (ISO 2021). A DT can also be considered as a projection of physical objects into digital spaces e.g., virtual reality. DTs can also be used to optimize asset performance through monitoring, diagnostics, or prognostics (Tao et al., 2018). In the context of the built environments, DTs have been used to capture spatial data capturing the building, smart city, etc. (Deng et al., 2021). DT technologies can also model living organisms, including humans. Human DTs (HDTs) are emerging for healthcare (Croatti et al., 2020) and social interaction. A broad range of applications for DTs and related technologies has led to a broad range of definitions for DTs (See, e.g., Minerva et al., 2020 and Voas et al., 2021). All DTs rely, to a greater or lesser extent on sensing operations (typically based on IoT devices) for the creation and operation of the DT. Blockchain Digital Twins (BDTs) are a subset of the DTs that incorporate blockchains to provide additional trust-based features.

The connectivity between the physical object and its DT is one of the main characteristics of DTs. A static digital twin only requires connectivity with the physical world (1) when the digital twin is created as a digital model of the physical object, or (2) when the DT is used to drive some physical process (e.g., manufacturing replicas of a scanned physical object- model based manufacturing). A dynamic DT maintains a digital representation of the current state of the physical object. The current state of the physical object is typically characterized using IoT sensors. This DT - Physical object connectivity, whether for model creation or state maintenance, is a form of machine- machine communication. 5G and 6G networks provide additional capabilities to support machine-machine communication. The ITU-T recognized DTs as a use case driving additional requirements for 6G features (ITU-T, 2020). Maintaining the linkage between the DT and physical world requires continuous connectivity between the DT and the Sensors monitoring the physical object's status. For a movable physical object, these sensors either need to be attached to the object or the range of motion needs to be constrained. The integrity and trustworthiness of the DTs as representation of the physical objects' current state relies on the authenticity of the data as well as the modelling process.