7Prefabrication has the advantages of simplicity, speed and economy but has been inflexible to 8 changes in design which is a primary reason behind its limited market share in the construction 9 industry. To tackle this drawback, this study presents a Robotic Prefabrication System (RPS) 10 which employs a new concept called "re-fabrication": the automatic disassembly of a 11 prefabricated structure and its reconstruction according to a new design. The RPS consists of a 12 software module and a hardware module. First, the software employs the 3D model of a 13 prefabricated structure as input, and returns motor control command output to the hardware. 14 There are two underlying algorithms developed in the software module. First, a novel algorithm 15 automatically compares the old and new models and identifies the components which the two 16 models do not have in common in order to enable disassembly of the original structure and its 17 refabrication into the new design. In addition, an additional novel algorithm computes the 18 optimal refabrication sequence to transform one model into another according to the 19 differences identified. Meanwhile, the hardware module takes the motor control commands as 20 input and executes the appropriate assembly/disassembly operations, and returns the desired 21 refabricated structure in real-time. Validation tests on two lab-scaled prefabricated structures 22 Corresponding author. 23 performed all assembly operations with acceptable placement precision. 24 Refabrication 26 1. Introduction 27 In theory, most common construction components can be decomposed to a combination of 28 parts and connectors, such as bricks and cement, wooden slabs and mating joints, or girders 29 and bolts. It follows that most construction activities can be broken down into a series of 30 assembly operations to form larger and larger assemblies from individual parts. Over the last 31 few decades, individual elements, also called prefabricated components, have become popular 32 in the construction industry. Prefabrication is a construction practice which manufactures the 33 majority of building´s sub-assemblies ranging from wall panels to complete rooms in a 34 controlled factory environment, before transporting the sub-assemblies to the construction site 35 for assembly [1]. Modular buildings and modular homes, which are recently getting more 36 popular in the construction industry, are a representative example of adopting the concept of 37 prefabrication [2]. Compared to site-cast (or in-situ) construction, precast concrete elements 38 offer faster production, lower cost, and more efficient assembly of elements [3]. For example, 39it has been reported that replacing in-situ concrete casting panels with prefabricated elements 40 has resulted in a 70% reduction in construction time and a 43% reduction in labour cost [4].
41Moreover, the use of precast concrete elements leads to a cleaner and safer construction 42 environment [4-5]. 43 Despite these benefits, off-site construction methods are estima...
