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Mobile Robotic System 3D Prints Single-Piece Concrete Structures

The scientists at Nanyang Technological University (NTU) in Singapore have spent a lot of time and energy over the last few years researching construction 3D printing with concrete materials. Two years ago, the NTU Singapore Centre for 3D Printing (SC3DP) team, led by Assistant Professor Pham Quang Cuong with NTU’s School of Mechanical and Aerospace Engineering, published a paper about their work developing concurrent mobile 3D printing construction robots. The idea was that multiple robots working together to build a concrete structure wouldn’t be held back by common issues like volume constraints and long lead times.

Adoption of concrete 3D printing is limited because of problems like lack of mobility and small size, and the use of synchronized, mobile robots is an excellent place to start working on the issue of scalability. But now, Professor Cuong and his team are taking things to the next level. They’re still using mobile robots for a print-while-moving approach, but instead of a pair systems, they’ve developed a single-robot industrial AM platform that can complete large-scale construction printing all by itself.

“Our system is mounted on a mobile robot. The ability to move the robot base in space allows our robot to print structures that are larger than itself,” Professor Cuong explained. “Also, having a mobile base makes it easier to bring the robot into the construction site and move it around inside.”

The NTU team—comprised of Mehmet Efe Tiryaki, Xu Zhang, and Professor Cuong—published a paper about their new system, titled “Printing-while-moving: a new paradigm for large-scale robotic 3D Printing.”

The abstract reads, “Building and Construction have recently become an exciting application ground for robotics. In particular, rapid progress in material formulation and in robotics technology has made robotic 3D Printing of concrete a promising technique for in-situ construction. Yet, scalability remains an important hurdle to widespread adoption: the printing systems (gantry-based or arm-based) are often much larger than the structure be printed, hence cumbersome. Recently, a mobile printing system – a manipulator mounted on a mobile base – was proposed to alleviate this issue: such a system, by moving its base, can potentially print a structure larger than itself. However, the proposed system could only print while being stationary, imposing thereby a limit on the size of structures that can be printed in a single take. Here, we develop a system that implements the printing-while-moving paradigm, which enables printing single-piece structures of arbitrary sizes with a single robot. This development requires solving motion planning, localization, and motion control problems that are specific to mobile 3D Printing.”

This system only needs one robot to print differently sized single-piece structures, which also helps to ensure better structural properties.

The mobile robotic 3D printing system

Typically, construction materials wider than the construction 3D printing system’s gantry foothold distance can’t be printed. That’s because a printed structure’s dimensions are constrained by one of three things: the robot arm’s reach, the gantry’s restricted volume, or the framework which enables the printhead to move along a particular axis. But the NTU researchers have enabled their system to move in any direction, so long as it’s on a flat surface, by mounting an industrial robot manipulator to a wheeled base. Then, a hose is used to connect the platform’s manipulator flange nozzle to a pump.

The robot manipulator’s motions, and those of the mobile platform, are painstakingly planned out in this new system in order to achieve a coordinated effort. It uses feedback motion control, and highly accurate robot localization, to make sure that the nozzle deposits the concrete material at the right pace in the correct location. By placing a camera on the back of the mobile base, its “localization system” works better over a larger surface area.

Model of NTU’s 3D printing system setup and printing process pipeline

The NTU research team claims that their printing-while-moving system can increase the size of structures that one robot can fabricate. To prove it, they used the platform to 3D print a single-piece 210 x 45 x 10 cm concrete structure, which is definitely larger than the robotic arm’s 87 cm reach. This system could significantly increase the effectiveness of 3D construction printing. But, their work is not yet done, as the system does still have some limitations, particularly in terms of uneven work areas.

Professor Cuong explained, “We’re planning to add collaborative features to our robot. The idea is to have a human operator take the robot by hand and move it around the construction site, towards the desired location, guiding it to achieve high-precision assembly.”

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(Source: IEEE)

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Thesis Focuses on Using Cooperative 3D Printing with Robots to Improve the Technology’s Scalability

Illustration of the slicing strategy for cooperative 3D printing.

Obviously, the size of your 3D print is limited to the size of your 3D printer…you wouldn’t try and 3D print a building, no matter how small, using a desktop system, right? Jace J. McPherson from the University of Arkansas put it more exactly in the honor’s thesis he wrote and submitted for his Bachelor’s degree in Computer Science and Computer Engineering:

“More specifically, an object cannot be printed if it is wider than the full horizontal movement range of an extrusion nozzle or if it is taller than the maximum height of the extrusion nozzle above the printing surface (i.e., the “print bed”).”

Chunker results with a cylinder and a car model.

According to McPherson’s thesis, titled “A Scalable, Chunk-based Slicer for Cooperative 3D Printing,” print jobs’ size limitations can hinder the technology’s goal of being “fully dynamic.” In the thesis, he focused on the issue of 3D printer scalability – limited by print bed size and use of a single printhead – and lack of manufacturing automation, and the idea of cooperative 3D printing, and a new slicing strategy for this technology, as a combined solution.

The abstract states, “Cooperative 3D printing is an emerging technology that aims to increase the 3D printing speed and to overcome the size limit of the printable object by having multiple mobile 3D printers (printhead-carrying mobile robots) work together on a single print job on a factory floor. It differs from traditional layer-by-layer 3D printing due to requiring multiple mobile printers to work simultaneously without interfering with each other. Therefore, a new approach for slicing a digital model and generating commands for the mobile printers is needed, which has not been discussed in literature before. We propose a chunk-by-chunk based slicer that divides an object into chunks so that different mobile printers can print different chunks simultaneously without interfering with each other. In this paper, we first developed a slicer for cooperative 3D printing with two mobile fused deposition modeling (FDM) printers. To enable many more mobile printers working together, we then developed a framework for scaling to many mobile printers with high parallel efficiency. To validate our slicer for the cooperative 3D printing process, we have also developed a simulator environment, which can be a valuable tool in visualizing and optimizing a cooperative 3D printing strategy. This simulation environment was also developed to export the visualization in a generic format for use elsewhere.”

Large-scale cooperative 3D printing. Many robots cooperate to produce a single object that does not require assembly upon completion. The final product in this figure is a topographical map of the state of Arkansas.

Cooperative 3D printing is made up of multiple independent, free-roaming robot 3D printers that receive instructions on how to print one part, or chunk, of a whole object. The mechanism makes it possible to autonomously complete large print jobs, with no interruptions, in a single piece, without human interaction. The parts are actually 3D printed on top of each other so they’re joined during the process and not after.

(a) Illustration of the chunk’s dimensions and printing limitations on the slope, and (b)a comparison of chunk width with robot width.

“Cooperative 3D printing solves physical scalability with the premise that multiple independent 3D printers can be used to produce a single object. These printers need to “cooperate” to produce objects that would normally exceed the size limitation of a traditional 3D printer. They must have the freedom to navigate a large area, such that their print range is limited only by the size of the print surface, as opposed to a fixed range imposed by the extrusion nozzle’s mechanism. To summarize, assuming the print surface is easy to scale, the potential print size will also be highly scalable,” McPherson wrote.

“This new mechanism also solves time scalability assuming new 3D printers that enter the fray can decrease the overall print time. Given that the number of printers is dynamic, we can quantify the time scalability as a function of the parallel efficiency from using any number of robots.”

The chunker design subdivides 3D models into chunks, which are then split up between the robots for 3D printing. The slicer converts these chunks into print commands for the robots, and the simulator creates a visual, using the slicer commands, that shows how real robots would complete their tasks. It’s important for the simulator to be properly designed, as it’s used to validate the chunker and slicer algorithms – if the simulator is not accurate, the rest of the process isn’t either.

In the rest of his thesis, McPherson describes how the slicer makes it possible to subdivide models so that chunks can be 3D printed in parallel, as well as demonstrating how to scale the slicer for more than two robots for additional degrees of spatial freedom.

“Results show that the developed slicer and simulator are working effectively,” McPherson wrote.

McPherson hopes that this project can help “lay the foundation for scalable Cooperative 3D printing,” which could open up a whole new direction of research for scaling 3D printing, and potentially even “revolutionize the way manufacturing processes are structured.”

“This thesis has presented, in detail, a feasible process for managing ?? 3D printing robots operating in parallel on a single print job, taking into account the geometric constraints, the communication requirements between robots, and the necessary pre-processing needed to properly subdivide a model for chunk-based printing,” McPherson concluded.

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