**1. Introduction**

In order to compete in a global market, nowadays companies need to offer a wide range of different products. The production technology most sensitive to this evolution is assembly, because of its position at the end of production processes, where the whole product variety is present. As a consequence, companies try to use adaptive flexible assembly systems [1]. As reported by Barbazza et al. [2], the basic requirements for the winning design and management of a flexible assembly system concern the optimization of:


The basic assembly production strategies are traditionally classified as manual assembly systems, flexible (automated) assembly systems, and dedicated (automated) assembly systems [3]. According to Heilala and Voho [4], the transition from a dedicated to a flexible automated assembly system and then to a manual assembly system is due to the increase in the requirements of flexibility and number of variants. On the other hand, pure manual assembly has different drawbacks. The accuracy of the tasks performed and the activity repeatability need improvements. Ergonomic problems, as well as occupational injuries, could occur [5]. At last absenteeism can create production lack and low efficiency, especially in the case of balanced assembly lines [6]; for these reasons a pure manual assembly system cannot be very competitive, especially in countries with high labor cost levels. As affirmed by Edmondson and Redford [7], the driving factor behind the definition of a suitable assembly system configuration is, first of all, economy. Automation is usually a way to lower the costs of labor in Western countries and the key issue is to achieve and optimize flexibility and a suitable degree of automation in a fast changing market situation [8]. Therefore, in the last decades, a subset of flexible automated assembly systems have been developed with the aim of increasing flexibility and decreasing production costs if compared with the traditional manual and automated assembly systems.

The traditional flexible assembly system (FAS) is typically composed of one programmable manipulator, which picks the parts from predefined positions, places them on the assembly station and perform the assembly task and one or more re-configurable feeding devices, typically vibratory bowl feeders, for each component needed to complete the production process. An evolution of the concept of the flexible assembly systems is presented in Reference [9], fully FAS (F-FAS). The F-FAS is a single-station robotized assembly system, able to perform mixed model assembly, where the unique feeding device is a fully flexible feeder, ensuring a higher level of flexibility than the traditional automated FAS. The feeder system, since it is composed by a bulk, a vibratory plane and a camera, can feed several different components without the need for reconfiguration. As far as this solution presents high levels of flexibility, it has a lower throughput than a traditional FAS, since image processing for shape recognition is time consuming and the feeding process is stochastic.

In order to bridge the gap between the presented solutions, the hybrid FAS (H-FAS) has been developed [10]. The H-FAS consists of one programmable manipulator, one flexible feeder and one or more re-configurable feeder devices like the vibratory bowl feeder. Lowering the number of parts deployed by the fully flexible feeder leads to a reduction in the complexity of the system, thus increasing the throughput. It should be noted that the H-FAS is able to reach performances, in terms of unit direct production costs, mix flexibility and volume flexibility, higher than those of the other systems presented. The advantages of using this type of solution can be noticed when the feeding process is optimized for the different components, as a function of the characteristics of each one.

The proposed work arises from a case study, which aims to improve the efficiency of an assembly kitting line. The assembly kitting process is widely used for different purposes, for example, sales kits or production kit. A typical application of the studied assembly kitting line is the production of sales kits of connectors, such as screws and bolts, ready to use in the product assembly process.

Such a process is performed by means of a traditional FAS. The production line taken as a reference is indeed composed by 8 bowl feeders, which through a conveyor belt and three hoppers delivers the components onto the inferior belt, which moves towards the packing machine. The line is depicted in Figure 1.

A literature review analysis has led to a limited number of studies on increasing the efficiency of an assembly kitting line. Bevilacqua et al., in Reference [11], present a case study in the pharmaceutical industry, where they applied new procedures based on the lean production approach; moreover, they evaluated the increase in efficiency based on the Overall Efficiency Equipment (OEE, [12]) value, an approach suitable also for our study. In Reference [13] the authors described the design process for an automated assembly line; it should be noted that the authors have carried out a redesign of the deployed components, a solution which could not be applied in our case study.

In order to increase the efficiency in our case study, we observed that the deployment of low quantity components by means of the bowl feeders leads to high inefficiency, due to the time required for retooling and for the feeding rate unbalance between the components. For such reasons this paper investigates the possibility of applying an H-FAS model on the considered process, since we believe that for its characteristics it is more suitable than traditional ones. The aim of this work is therefore to propose an alternative solution to the kitting line layout taken as reference from the case study. The study will be divided into two problems: the first will consider the feeding of the low quantity components and will be solved with the development of a flexible robotic workcell, the second will

deal with the feeding of the high quantity components and will lead to the improvement of the original feeding system.

**Figure 1.** Line layout.

With regard to the feeding system, a focus on the hoppers cascade system was considered advantageous in the proposed case study. No one, to the authors' knowledge, has studied the impact of a specific configuration of a hoppers cascade system and its parameters (e.g., opening/closing time, output feed-rate, etc.) on its performance. Similar studies [14] have focused on continuous processing with the need for control flow rates of materials and whose models were not suitable for the presented case study.

The aim of the study will therefore be:


The novelty of the presented work, besides suggesting two different improvements for an assembly kitting line, is the study of the behavior of a hoppers cascade system based on its configuration. In this way we can estimate the throughput of a feeding system and evaluate more convenient configurations. Moreover the proposed solution was applied in a case study, validating the proposed results. The presented work is organized as follows: Section 2 presents the systems used to improve the current assembly kitting line, divided in Section 2.1 for the robotics F-FAS and Section 2.2 for the hoppers cascade system. Section 4 shows the impact of the proposed solutions and Section 3 presents results provided by the application of the model to the case study. Lastly, Section 5 concludes the work.
