Fault detection in Flexible Assembly Systems using Petri net

Following the success of flexible manufacturing systems (FMSs), their counterpart, flexible assembly systems (FASs), have been developed to automate factories further. As in a FMS, the cellular approach is considered as the most feasible and flexible configuration for a FAS by researchers. Because of the differences between manufacturing and assembly operations, the logic of cell formation between a FMS a FAS is not the same. This paper discusses the similarities and differences between FMSs and FASs, and assembly cell design problems. Since the time for assembly operation is usually relatively short, the transfer time is thus very crucial for the performance of assembly systems. Therefore a good assembly cell should be constructed in a way that the flows of products within this cell are mostly in sequence. More in-sequence flows usually have the advantages of smaller flow distance, easier material-handling problems and more efficient production. In order to achieve these advantages, we propose a...

Flexible Assembly Systems - Hardware Components and Features: Basic components of a FAS.- Classification of flexible assembly systems.- Examples of industrial installations.- Issues in Design, Planning and Scheduling of Flexible Assembly: FAS design issues.- Network design for material flow systems.- Design for assembly.- Assembly planning.- Planning and scheduling.- Loading and Routing Decisions in Flexible Assembly Systems: Description of a flexible assembly system.- Optimization of station workloads and product movements.- Design and balancing of flexible assembly lines.- Numerical examples.- Simultaneous loading and routing.- Sequential loading and routing.- Loading and Routing Decisions with Assembly Plan Selection: Simultaneous loading, routing and assembly plan selection in a general flexible assembly system.- Simultaneous loading, routing and assembly plan selection in a flexible assembly line.- Loading and Scheduling in Flexible Assembly Cells: Loading and scheduling in a robot assembly cell.- Loading and sequencing in printed circuit board assembly.- Production Scheduling in Flexible Assembly Lines: Flexible assembly line with limited intermediate buffers.- Flexible assembly line with no in-process buffers.- Global lower bounds for flexible flow lines with unlimited buffers.- Just-in-time scheduling of flexible assembly lines.- Multilevel scheduling of flexible assembly lines with limited intermediate buffers.- Machine and Vehicle Scheduling in Flexible Assembly Systems: Dispatching scheduling.- Machine and vehicle scheduling - a multi-level approach.- Machine and vehicle scheduling - a single-level approach.

Components of the Design Process of Flexible and Reconfigurable Assembly Systems

Global competition and the rapid pace of technological change now require the almost continual introduction of product upgrades by any manufacturer. Thus, such a manufacturer is likely to market older and newer versions of a product simultaneously, not to mention niche-specific editions of any product upgrade. An increasingly successful response to this product proliferation is the implementation of flexible assembly systems. In the context of a flexible assembly system (FAS), the ability to estimate the impact of various product and process options on the maximal level of system output becomes crucial to managing the ever-changing product mix. This paper presents a tool for such impact estimation that can facilitate concurrent development and engineering. Experience with an actual FAS is the basis for the reported results. The tool is a specialized combination of discreteevent computer simulation, experimental design, and regression analysis. Application of the tool assumes FAS use with a cellular manufacturing philosophy. Thus, uncluttered process flow for a family of products in the sense of group technology places the focus on potential bottlenecks. The new tool here models the impact of process and product options on bottleneck and, hence, FAS behavior.

The successful implementation of a Flexible Manufacturing/Assembly System can be an effective means of reducing labor, resources and work-in-process inventory. It can increase equipment utilizations, improve quality and result in increased throughput. Simulation is an effective tool for analyzing critical system components and their inter-relationships; consequently, ensuring systems design meets throughput requirements at reduced costs. This paper presents an approach to the design, specification and implementation of a Flexible Assembly System (FAS) using simulation. It presents a practical approach for designing manufacturing systems involving uncertainties. Two simulation models are discussed:(1) a GPSS/H model and(2) a PCMODEL model.The GPSS/H model was used during the early FAS design and specification stage. The PCMODEL model was used to sell the proposed FAS configuration to management. The FAS has been successfully implemented.

Recently a major trend in flexible and precision assembly automation, and in automation in general, has been in robots that can safely work next to people. Increasingly, as part of collaborative automation, such robots are also designed to collaborate as teams with other robots. These intelligent machines are called cobots, or collaborative robots, and are reducing the cost and time to install robots in many applications. Flexible and precision assembly refers to an assembly system that can build multiple similar products with little or no reconfiguration of the assembly system. It can serve as a model for some of the emerging applications in flexible and precision automation. Precision assembly implies assembly with precise motions and operations. Recently, machine learning interacting with the robots’ sensors enables robots to assemble parts and components with precision that exceeds their own motion precision. A truly flexible assembly system would enable precision assembly, include flexible part feeding, grasping, and fixturing, as well as a variety of mating and fastening processes that can be quickly added or deleted without costly engineering. There is a limited science base for how to design flexible assembly systems in a manner that will yield predictable and reliable throughputs. The emergence of geometric modeling systems (as part of CAD) has enabled work in geometric reasoning in the past few years. Geometric models have been applied in areas such as machine vision for object recognition, design, and throughput analysis of flexible part feeders, dynamic simulation of assembly stations and assembly lines. Still lacking are useful techniques for automatic model generation, planning, error representation, error prevention (avoidance), and error recovery. Future software architectures for flexible and precision automation should include geometric modeling and reasoning capabilities to support autonomous, sensor-driven systems.

In order to complete the robot axle hole assembly task in an unstructured environment, an adaptive impedance control algorithm based on joint space force and position double feedback is proposed, and the assembly process adjustment strategy is designed to complete the assembly task. By installing a torque sensor on the joint of the six-axis cooperative robot, the control model of joint force position with double feedback is formed. By combining the model reference adaptive control (MRAC), operational force planning, and operational space motion planning, an assembly system based on joint space flexibility control is finally designed. According to the task of axle hole assembly, the geometric constraint analysis and force analysis are carried out in the axle hole assembly process, and the corresponding adjustment strategy is designed. The assembly experiments were carried out for holes with different gaps, and the pose parameters, contact force, and control adaptability parameters during assembly were analyzed experimentally. The experimental results show that the flexible assembly system can complete the shaft hole assembly well, and the assembly torque is controlled in a small range, which meets the assembly requirements.

This paper proposes a rule-based dynamic scheduler for a flexible assembly system. We first introduce a flexible control system developed by the Intelligent Robot and Automation Laboratory in National Taiwan University. Based on that control system, the relationship between the control system and the scheduler is characterized. With focus on realization, hardware limitations such as long computation time and excessive memory-space usage are relaxed by incorporating several heuristic measures. The present work is applied to the simulated robotized flexible assembly system in our laboratory.

This paper proposes and analyzes an efficient robot task sequence for a flexible assembly system that consists of multiple 3D printers, post-processing, assembly, inspection machines, and a material handling robot. The flexible assembly system, which has been built in many large cities in Korea, has been designed to fabricate customized products for start-ups or individuals. We consider cyclic scheduling of the system in which the robot repeats a certain sequence to produce identical products. We propose a robot task sequence to minimize the cycle time. The system behavior with the proposed robot task sequence is modeled with a timed event graph (TEG), and the optimality of the sequence is proved by analyzing the circuits in the TEG. The results are extended for producing multiple types of products simultaneously.

Flexible Assembly Systems (FASs), which are different automatic assembly product types, are finding increasing use in today's industry. In this article, an ordered-entry array of unit stations of Generalized CSPSs, as a simple Flexible Assembly System is designed and studied. The problem will be addressed as follows: given an objective cost function, design a simple FAS to meet (as closely as possible) the given specifications. The design variables are divided into input/output variables and buffer variables. A methodology of two-stage method is introduced and proposed in order to design the system. After some considerations in the first stage of the design method, the objective function has been minimized, using the independently-computed minimum cost of each workstation (WS) by a complex-type method, at the second stage. In addition, the proposed method is compared to the GA optimization method. Based on the results from an optimization standpoint, the two-stage method seems to produce reasonable engineering results and is superior to the GA, in computational time.

The paper presents integer programming formulations and an approximative lexicographic approach for a bicriterion loading and routing problem in a flexible assembly system. The problem objective is to determine an assignment of tasks to assembly stations for a set of products so as to balance station workloads and minimize total interstation transfer time. In the approach proposed, first the station workloads are balanced using a linear relaxation-based heuristic and then assembly routes are selected based on a network flow model. An illustrative example is provided and some results of computational experiments are reported.

Hybrid particle swarm optimization algorithm for scheduling flexible assembly systems with blocking and deadlock constraints

Car Latch (CL) has very complex mechanical structure, requiring strict technological process, and it is difficult to realize automatic manufacturing. In allusion to above problem, a kind of embedded flexible assembly system (FAS) for CL based on laser identification (LAI) has been put forward according to wide investigation and analysis for current CL assembly technology. In this paper, FAS idea is firstly brought into CL manufacturing. So kinds of CL can be assembled according to different production demands and flexibility of this system can be fully presented. CL structure and its technological processes are analyzed. By the introduction of LAI, Latch piece location and key outline identification problems are resolved in process of its assembly. Embedded micro process unit (EMPU) acts as a main controller, and closed loop control system, which has abundant control functions is implemented to realize control task for the whole assembly process. The idea of modularization is adopted, and that makes this system have strong augmentability and multiplexing ability to meet different assembly demands. A number of industry field experiments proved that this system locates exactly, runs stably, and increases the production efficiency of CL.

Flexible Assembly Systems through Workplace-Sharing and Time-Sharing Human-Machine Cooperation (PISA)

This paper describes the development and early use of a test bed for Just‐In‐Time GIT) control of Flexible Assembly Systems (FAS). JIT has, up to now, been used in repetitive manufacturing systems. However, increasingly, companies involved in batch production who frequently use Materials Requirements Planning (MRP) approaches, are seeking to achieve the benefits of JIT, and to use Kanban type control at the operational level. The test bed simulation can be configured by the user to model a range of assembly systems each of which can, within limits, be configured by the experimenter. It is built using the SLAM II simulation package. Kanban cards are modelled as resources, as are the individual assembly stations and the transportation devices which carry work between the assembly stations. The test bed can be used to experiment with batch sizes, set‐up times, assembly station breakdowns, various Kanban sequences, varying purchasing lead times, etc. It allows experimentation with the application of the Kanban system to manufacturing systems which are not purely repetitive in nature and more closely resemble batch manufacturing systems.