2015 IEEE International Conference on Robotics and Automation (ICRA) Washington State Convention Center Seattle, Washington, May 26-30, 2015 Mobile Manufacturing of Large Structures David Bourne, Howie Choset, Humphrey Hu, George Kantor, Chris Niessl, Zachary Rubinstein, Reid Simmons, Stephen Smith Robotics Institute, Carnegie Mellon University Abstract— Assembly of large structures requires large fix- dynamic scheduling. In this paper, we present each of these tures, often referred to as monuments. Their cost and massive techniques, describe how they contribute to the precision and size limit flexibility and scalability of the manufacturing process. efficiency of assembly, and discuss how the techniques can Numerous small mobile robots can replace these large struc- tures and, therefore, replicate the efficiency of the assembly be sequenced to achieve precise, efficient assembly of large- line with far more flexibility. An assembly line made up of scale structures. mobile manipulators can easily and rapidly be reconfigured to The long-term goal of this work is to create a rigorous support scalability and a varied product mix, while allowing for framework for rationally sequencing these techniques to near optimal resource assignment. The challenge to using small achieve the necessary precision for assembly in the most robots in place of monuments is making their joint behavior precise enough to accomplish the task and efficient enough to efficient sequence (see Figure ??) . After working through execute subtasks in a reasonable period of time. In this paper, the details of our current multi-robot assembly example, we we describe a set of techniques that we combine to achieve propose a generalized framework to accomplish similar tasks the necessary precision and overall efficiency to build a large that employs a “funnel” [12] or sequential composition [1] structure. We describe and demonstrate these techniques in concept in which multiple controllers are sequenced together the context of a testbed we implemented for assembling a wing ladder. to mitigate uncertainty. I. INTRODUCTION II. RELATED WORK A popular approach in industry for assembling large- scale structures is to establish assembly lines that use large, Assembly planners create a graph [26], [27] that encodes mounted fixtures to provide precise positioning of parts, the order of motion operations and the geometric constraints relative both to other parts and to the tools used in the ensuring that each motion can be executed. Typically, the part assembly. While these lines are efficient, the permanence must move in contact with other parts in the subassembly, of these fixtures requires costly infrastructure and results so an assembly can be viewed as a sequence of compliant in an assembly space that is not adaptable to changes in motions [13] where a transition from one compliant motion demand for different product lines. In addition, the lines are to another occurs at a change in contact state. Often, a vulnerable to issues that arise during assembly and frequently planner uses a graph, called a contact state graph, as well result in the entire line being delayed until they can be as knowledge of the parts’ geometries to determine the resolved. To address these drawbacks, we are investigating a sequence. [11], [14], [6]. Much effort has been dedicated new approach to assembly where the fixtures and the tooling to automatically generating the contact state graph [7], [15], are all onboard coordinated mobile robots. The mobility of [28]. the robots allows for rapid reconfiguration of the assembly Regardless of the choice of sequence planner, the overall space, which means better adaptability to meet changing system must be able to accurately align the part for final product demand and the ability to work around line issues. assembly, which is often quite difficult when the part’s pose The primary challenge to replacing mounted fixtures with has uncertainty. Here, compliant motions can be used to tooling on small mobile robots is achieving the precision minimize this uncertainty through a sequence of compliant required to reliably assemble the products. Positioning the motions [12], [17]. In our work, we use compliance (i.e., mobile robots and the parts to millimeter accuracy is difficult springs) to achieve compliant and force-controlled execution due to the uncertainty in both the environment and the and assist in part localization. Previous work has used passive controllers of the robots. In addition, complex coordination compliance to account for misalignment between a robot and is needed to get the robots, the parts, and the tools at the task constraints [24]. Series elastic actuators [18] introduced right place at the right time for each step in the assembly. compliance to reduce shock loads and enhance force control We have built a demonstration facility to explore the accuracy and stability. We have found from our previous efficacy of assembling large structures with a number of research that passively compliant mechanisms can improve heterogeneous mobile robots. We have developed a sequence precision without losing performance. If a part contacts a of coordinated behaviors that accomplishes an example structure prematurely, it will comply to the shape of the assembly task by combining four component techniques: contact area without causing damage. By taking advantage i) mobile manipulator hardware design, ii) fixture-free po- of known features in a given structure, passive compliance sitioning, iii) multi-robot coordination, and iv) real-time is both a precise and fast operation to position a part. 978-1-4799-6922-7/15/$31.00 ©2015 IEEE 1565 Our work to date has assumed the existence of a hierar- chical assembly planning graph, and this process description, which dictates both the order in which parts should arrive for assembly and the coordinated sequence of actions that must be carried out to successfully join specific parts, has driven the design characteristics of the robots required to execute assembly subsequences. The robots that we created are sometimes referred to as mobile manipulators because they combine aspects of both mobile robots and manipulator mechanisms [22]. While some systems [9] use multiple mobile manipulators to manipulate a single object, we are not aware of a multi-mobile manipulator system that was designed for assembly. As such, prior multi-robot mobile manipulators typically assume that all poses of the robot is Fig. 1. The yellow spars support the black ribs forming the ladder. Here, known at all times, at least with respect the mobile bases. a rib-carrying robot is bringing a rib into its final position. We believe new mobile manipulators must be created because existing systems either contain more degrees of freedom than necessary or lack the payload to carry out the assembly task. This is because mobile manipulators were not originally conceived to carry out coordinated assembly. To design these new systems, our work is inspired by i) the tool-design for assembly work [25], which developed a tool representation that includes the tool’s use-volume, i.e., the minimum space that must be free in an assembly to apply the tool, and ii) [5], which presented a detailed approach for modeling tools, selecting tools, and generating tool movements. Their work allowed tools to be modeled as articulated devices. Fig. 2. Omnibases III. MODULAR MOBILE MANIPULATOR ROBOT SYSTEM the current system include accelerometers, cameras, lasers, The work described in this paper uses assembly of airplane potentiometers and limit switches. wings as a motivating application, but we believe the ideas The behaviors, algorithms, and framework developed to are general to assembly of many large structures such as support the rib-insertion task is the focus of the rest of ships, large construction vehicles, and trains. To date, we this paper. The task begins by having the rib-carrier robot have focused on two tasks: coordination of two robots to approach the end of the wing ladder assembly, carrying a rib insert ribs into a partially-assembled wing ladder and sensor- length-wise (Figure 1). The robot rotates to enable it to move based coordinated motion of two robots to carry a flexible between the spars with sufficient clearance. Concurrently, skin panel. The rib-insertion task and larger ladder assembly the spar-catcher robot aligns a temporary fixture (Figure 3) process (the focus of this paper) involve moving ribs (the with the spar cap and then slides along the spar cap until cross-wise pieces) into place between two spars (the long it detects the L-bracket (via a limit switch), at which point length-wise pieces). In our testbed, the spars are 10 feet long it engages a brake to provide a rigid target for the rib. The and accommodate 5 ribs; the ribs vary in length (the spars are rib carrier then approaches the target using a guarded move, not parallel) but average about 3.5 feet and weigh about 20 i.e., a move that stops when the “guard” is triggered. The pounds. The spars have L-brackets spaced every two feet, to “guard” sensor is an accelerometer attached to the target. which the ribs need to be attached. There are three 3/8 inch The accelerometer signal is processed by the spar-catcher diameter holes on either end of the rib that must be aligned robot, which communicates with the rib carrier to trigger with corresponding holes on the L-bracket. The alignment the “guard.” Passive compliance is used to handle the small tolerance is about a millimeter. amount of movement after contact, due to latency caused by The robots are 30-inch diameter, 3-wheeled omnidirec- the signal processing and communications. tional bases, designed at CMU. Each robot carries a manip- Having hit the target, the system uses a camera mounted on ulator module specialized for the role it plays in the assembly the rib carrier to determine where the rib is along the target task (examples of these mobile manipulators can be seen in (by calculating the percentage of the target not obscured by Figures 1, 2, 3, and 4).