# A New Self-Balancing Assembly Line Based on Collaborative Ant Behavior
## 4 A New Control Method of BB-Based Assembly Cell  
We examined efficiency losses caused by hand-offs and blockings in the BB-based cells in the previous section. To handle efficiency problems in the existing BB-based assembly cell, this paper presents an improved version. (For comparison, the existing naive BB-based assembly cell and the newly proposed BB assembly cell are called NBB and ABB cell, respectively). The idea for the new model was obtained from the BB-based system in [17], where a BB-based order picking and a zone picking were integrated to reduce the hand-off and blocking losses in the order picking system. Figure 5 illustrates a three-worker ABB cell to assemble a product type with 12 assembly tasks. The major difference between NBB cell and ABB cell is twofold: (1) there is a limited set of tasks that each worker can perform in ABB cell, and (2) there may be buffer space at the end of each worker except the last worker. Each worker is not allowed to perform assembly tasks out of the assigned set. In Figure 5, worker 1 is allowed to perform assembly jobs between task A1 and A6, worker 2 between A2 and A9, and worker 3 between A3 and A12.  
In an ordinary situation, the workers follow a naive control logic of the NBB cell. The new control logic comes into effect when a worker finishes all the tasks within the assigned tasks but his immediate downstream worker is not available. For example, if worker 1 (W1) finishes all the tasks up to task A6, but finds worker 2 (W2) busy, then W1 cannot start performing a new assembly task beyond A6 but waits until W2 is available to take his job. (Note that in the NBB cells, W1 will attempt to start A7 assembly tasks.) When W2 is available, [PERSON] hands over his semi-finished assembly to W2 and returns back upstream to the beginning of the assembly line to start the first assembly task for a new product. If buffer space is allowed at buffer B1, then [PERSON] puts the semi-finished assembly at  
Figure 4: Effect of blocking over different assembly time variabilities in terms of (**a**) blocking time percentage over total assembly time and (**b**) production rate.  
Figure 5: Task assignment of workers in the proposed BB assembly (ABB) cell. B1 and B2 indicate the buffers.  
buffer B1 after finishing all the tasks assigned to him and returns to the beginning of the assembly line. When ready for the next assembly job, W2 moves upstream and checks if there is any semi-finished product in buffer B1. If there exists at least one workpiece at B1, W2 starts the assembly task with it. Otherwise, as in the NBB cell, W2 goes upstream to take over the assembly job from W1. Worker 3 (W3) completes assembly jobs by performing task A12 and puts the finished product at output storage and moves upstream. If there is a semi-finished assembly at buffer B2, W3 starts the assembly task with it. Otherwise, W3 goes upstream to take a job from W2. Note that if there is a workpiece in a buffer, it means that the upstream worker does not experience blocking and the downstream worker does not experience a hand-off time loss. Hence, efficiency losses caused by blocking as well as hand-offs are expected to be reduced with the new system. With each worker assigned to limited work contents, they perform assembly tasks far from each other, possibly resulting in less blocking frequencies.