The proposed digital reproduction task for a historical building orna- mental component is independently developed in a digital construction

J. Xu et al. / Automation in Construction 76 (2017)  85–96 87

3.1. Modeling module

This module is responsible for generating the component solid model, which consists of the  following:

a) 3D scanning instrument: The system utilizes a hand-held laser scan- ner with a high-speed projector to manually measure the compo- nent. The high-performance hardware and the efficient algorithm real-time produce the PCD file needed to create a 3D image of the component.

b) Modeling software: The PCD file undergoes removal of redundant and error points as well as noise reduction in a point cloud process- ing software to obtain a complete PCD model of the component; the system uses the mature commercial software Geomagic Studio to en- capsulate the PCD file and output an STL model, which can be iden- tified by the control module.

3.2. Control module

Fig. 1. The process flow chart.

system that actualizes 3D scanning and cement mortar-based 3D print- ing. The process is pided into the following four steps and includes al- gorithms that have been developed for executing this task (Fig.  1):

a) Model acquisition: the dimensional data of an intact historical build- ing component is directly measured using a hand-held structured light 3D scanner which is a triangulation laser scanner and has a high accuracy and mobility for short-range (b 1 m focal distance) scanning [25]; then the point cloud data (PCD) is inputted into a re- verse engineering software (e.g., Geomagic Studio), which automati- cally encapsulate the PCD into a 3D solid model.

b) Program generation: a STereo Lithography (STL) file is formatted from the solid model and inputted into the data processing module of the system. This is used to generate a machine control program in accordance with a hierarchical algorithm for model slicing and a modified scan line algorithm for nozzle path planning. The develop- ment of these algorithms is presented below in the next section of this paper.

c) Component printing: the control programs coordinate the machine movement and cement mortar extrusion so as to layer the construc- tion of the component.

d) Component installation: the printed component is post-processed (e.g., polishing) and installed to replace the damaged one in the his- torical building; this then finalizes the reproduction  process.

The digital construction system contains five modules (Fig. 2): (1) modeling, (2) control, (3) motion, (4) extrusion, and (5) data process- ing. Each is briefly introduced hereinafter.

This module numerically controls each of the others with the follow- ing functions:

a) Microprocessor: with powerful capability of information processing; managing man-machine interaction interface.

b) External input: with different data interfaces such as serial ports and Universal Serial Bus (USB) communications to receive external data input.

c) G code interpretation: identifying the numerical control code, G code, and translating  it as the control flows  of the motion     module.

d) Mechanical signal processing: dealing with machine motions such as moving, stopping, limiting, zeroing and  alarming.