Assembly is the last phase in the process of production and involves hundreds and even thousands of different parts. Even the omission of a single part is enough to make the product defective. Therefore, an inventory is made for each part type. Acquiring, handling and managing a multitude of inventories is a significant burden with significant cost, both of which are much higher in cases where multiple product versions are assembled on the same assembly line. In today's market, the intense competition drives producers to cater to different market segments by offering a larger variety of products. This growth in product variety is broad, but applies strongly to the assembled products. Changes in demand, volume and product mix, as well as the appearance of new product models and components, require a proper, flexible assembly system. Therefore, successful assembly systems design and management must deliver efficient, flexible systems that can deal with product variety and changes in product volume and mix. A crucial part of such a system is the part supply and part feeding. With an increasing product variety, thousands of different parts need to be delivered just-in-time (JIT) to a multitude of assembly stations. Some of the issues which have to be addressed are: high number of parts managed, mixed-model production, variety of parts’ shapes and sizes, limited space at assembly stations and manipulation of a wide variety of part types during the assembly. Forming efficient part-supply and part-feeding mechanisms presents an important challenge faced by today’s assembly systems. These are crucial factors in making an assembly business sustainable and competitive. The logistics of parts feeding is dictated by the three-level hierarchical configuration of assembly systems: assembly line, assembly-line segment and assembly station. The first level of the logistics problem is a macro-level problem related to feeding the parts to the assembly system from outer locations (and/or different suppliers). The second level is related to delivering the parts within the facility to the assembly stations. The third level of the logistics is related to feeding the parts within a station directly to the precise assembly location. This third stage may be considered as the micro-level. These three levels are different but interdependent aspects of the problem: part feeding from the warehouses to the assembly line, parts delivery to the stations and part feeding inside the assembly station from the stocking units to the product. Each of these aspects requires optimal design and management in terms of organization, logistics and industrial automation. Part feeding from the warehouse to the assembly line includes macro-logistic issues that are partly determined or affected by external factors, such as: the transportation costs, inventory-holding costs, production pace, product mix fluctuations, lead times, safety inventory levels, quality procedures and part acceptance procedure. Part delivery within the assembly line facility to the right assembly stations includes shaping the logistic strategy and depends on strategy or policy decisions regarding: the central inventory-holding configurations (i.e. a supermarket system, warehouse implementation or a kitting shop); the space available for part inventory, both at the central inventory location and the destination stations; the material-handling and stock-keeping units definition (i.e. station stocking, kanban and kitting); and the delivery discipline of bringing the right part to the right station (i.e. tots, milk runs and periodic stocking and automatic feeding). Part feeding inside the assembly station includes micro-logistic issues and factors related to bringing and aligning the parts to the product. These issues and factors could be separated into two main categories: station design (for example, space design, workplace ergonomics, flexible automation and part feeding mechanism); and station operational factors (for example, quality control, material positioning, separation and orientation, part/material identification, sequencing and impact of part types on the assembly load). Due to the high costs of part/material shortages in the assembly process, reliable and robust supply of parts to the assembly station is indispensable. On the other hand, keeping high parts stocks for all the parts translates to an enormous amount of stock across the assembly line. The holding cost for large assembly lines is significant, and the costs of increasing the inventory levels may be prohibitive. Moreover, loading a station with parts typically decreases the system productivity. It clutters the station and takes essential volume with a negative effect on the ergonomics for manual assembly, the parts recognition and the pick-and-place times for automated assembly. The strong interdependence between the macro, facility and micro levels of the logistics problem requires a strong motivation to tackle the problem as a whole from an integrated approach. On the other hand, due to the complexity of the problem, the current trend in the literature (as demonstrated by many contributors) is to focus on just one level, frequently dealing with only one aspect of the problem. One consistent direction in improving assembly systems is enhancing flexible automated technologies. Automating the logistic process of parts supply could save a lot of money. Thus, some efforts should be made in developing innovative, flexible automated part-delivery and part-feeding technologies. The traditional isolated automated flexible assembly systems (composed of programmable manipulators and feeder subsystems) are no longer sufficient, as they do not address the macro and facility levels of part supply. Moreover, they are insufficient even on the micro level and typically inadequate for dealing with new parts, and present serious drawbacks in terms of station flexibility. For example, in the case of component changes, such systems require new bowl-feeder tooling, additional bowl-feeder tops and expensive time-consuming setup activities. Recent developments in automated assembly seek to overcome the flexibility limitations by integrating innovative technologies in robotic and control systems, and adding vision hardware/software. This is especially effective for handling a high number of different part types and short product life cycles.
Bibliographical notePublisher Copyright:
© 2015, The Author(s).