# A New Self-Balancing Assembly Line Based on Collaborative Ant Behavior
## 6 Discussion and Conclusions  
Once an assembly line is configured on the basis of the assembly line balancing (ALB) decisions, the system is operated in a static way where workers perform the pre-determined set of assembly tasks repetitively. The static decision often leads to low performance in dynamic and uncertain assembly environments. The idea of bucket brigades (BBs) has been applied to assembly systems to respond to system uncertainties. However, the naive BB-based assembly cell (NBB cell) suffers from some efficiency losses including hand-offs and blocking in most assembly flow lines. This paper presented an improved version (ABB cell) of the NBB cell. The performance of the new assembly cell is compared  
Figure 11: Efficiency losses caused by hand-offs and worker blocking with (a) buffer size of zero and (b) buffer size of one. Figure 10: Performance of assembly systems over different work speeds with (a) buffer size of zero and (b) buffer size of one. The notation at the \(x\)-axis, \((s1,s2,s3)\) indicates that the work speeds are \(s1\), \(s2\), and \(s3\) for the first, second, and third workers, respectively. with the other assembly systems under various assembly environments. From the numerical and experimental analysis, we have produced some interesting findings of the new BB-based assembly system (ABB cell) as follows:  
* The key finding of our study is that the ABB cell is robust in that it performs especially well under dynamic and uncertain assembly environments. As the variabilities increase, both the NBB and ABB cells provide worse performance. However, the performance of the ABB cells is less affected by the high variabilities than that of the NBB cells, with fewer hand-off and blocking delays. * The BB-based cell works well when \(n/m\) is large, where \(n\) is the number of assembly tasks and \(m\) is the number of workers in an assembly cell. As given in Equation (1), the hand-off losses are proportional to the \(n/m\) ratio. Parallel to this finding is that assigning a small number of workers to a BB-based cell is preferable. * Even a minimal buffer size, e.g., buffer size of one, in the ABB cell leads to a big performance increase in terms of production rate. Moreover, the buffer size has little effect on the production lead time in the ABB cells, unlike traditional assembly cells. Hence, the ABB cell can be a good assembly system when the lead time as well as the production rate is a key performance measure. * Existing research works about the BB-based system assume that the workers are sequenced from slowest to fastest to have a maximum capacity. Our results are contradictory to this assumption. In our BB-based system, the workers may be sequenced in the other way, i.e., from fastest to slowest. It is found that the reverse sequence can reduce hand-off delays and improve performance in some cases (see Figure 10). This finding is important because we can have the flexibility to deploy the workers along the line in the new assembly system. Although the new assembly system has some advantages over traditional assembly cells and naive BB-based cells, there are some considerations that should be taken into for the new assembly system to be successfully implemented. One of the important characteristics in the new assembly system involves a distributed and autonomous decision making instead of centralized decision making. Each worker in the production line performs the tasks possibly different from cycle to cycle by following a control logic without any predefined tasks assigned in advance. Hence, workers should be multi-functional and willing to work in collaboration with their colleagues. An appropriate training program for workers should be provided. Moreover, as argued in [38; 39], both the benefits and negative aspects of the dynamic work assignments should be taken into consideration together in selecting appropriate assignment strategies. Workers in the BB-based system perform their jobs commensurate with their capabilities. Skilled workers work more while un-skilled workers less.