Two-sided assembly line balancing with operator number and task constraints: a hybrid imperialist competitive algorithm

Two-sided assembly line balancing problems usually occur in plants producing large-sized high-volume products such as buses, trucks, or cars. Comparing to traditional simple assembly lines, the two-sided assembly line can shorten the length of line, enhance the utilization of tools and fixtures, and improve the production rate of operators, etc. Whereas in a two-sided assembly line, due to a couple of constraints such as the operation direction constraint and the sequence-dependent finish time constraint, the balancing procedure is much more complex than for a simple assembly line. In this paper, besides some fundamental constraints of the two-sided assembly lines, we put forward additionally the restriction of operator number at each workstation, and take into account concurrently more constraints such as the positional constraints, zoning constraints, and the synchronous task constraints. A novel hybrid imperialist competitive algorithm (ICA) combined with the late acceptance hill-climbing (LAHC) algorithm is employed to solve the multiconstraint problem. For the evaluation of the proposed algorithm, the performance of the hybrid ICA is examined over benchmarks from the literature and compared with the LAHC algorithm and the lower bounds of the instances. Furthermore, a practical two-sided assembly line balancing problem from an engineering machinery company is solved with the proposed approach. The results confirm that the hybrid ICA can effectively shorten the line length and reduce the amount of operators for the existing two-sided assembly line.