تجزیه و تحلیل عملکرد از سیستم های مونتاژ با ایستگاه highlift: روش عملی

One common problem associated with automatic assembly systems is that some assembly operations may have relatively long cycle times. As a consequence, the productivity, as determined by the operations with the longest cycle time, can be significantly reduced. Therefore, a special form of parallel workstation known as a highlift station was developed to improve the performance of an automatic assembly system. In this paper, we present a simple analytical model that can help engineers to estimate the performance of highlift stations. This model is particularly useful at the design stage, as only approximate results are necessary for selecting the configuration.

One of the problems associated with automatic assembly systems is that they are usually structured as serial production systems. As a consequence, it is not generally impossible to improve the productivity of an individual station in the line by using multiple parallel stations installed next to one other. However, highlift stations arranged in a serial form can serve the parallel processing purpose. With this design, several identical in-line parallel stations performing the same function are installed at one stage in the series, and the pallet is then transferred to the first idle highlift station encountered. While one of the highlift stations is working, additional pallets can be processed by another station. If an assembly has been processed by a highlift station, it will be allowed to pass through the others without further processing. In this way, a form of parallel processing can be achieved. Most of the researchers are interested in studying serial production systems (Buzacott and Shanthikumar, 1993 and Pike and Martin, 1994). However, the results of the performance characteristics of highlift stations using simulation have been examined by the authors (Lai et al., 1994, Leung and Lai, 1994a, Leung and Lai, 1994b and Leung and Lai, 1996). The underlying idea of the estimation is mainly based on the observation of the arrival sequence and the working cycles of the model. Three different working cycles were identified, namely the synchronization cycle, jam cycle and non-synchronization cycle.

The estimation can generally provide a good approximation of the productivity under different situations. However, a bad approximation may result if non-synchronization cycles are ignored when they have actually taken place. But most of the time, this effect will not significantly affect the estimation. However, as B2 (the buffer size between the highlift station and the last station) becomes larger, the period of non-synchronization also becomes longer and consequently reduces the occurrences of non-synchronization resulting from the non-synchronization cycles. Since, the main difference between simulation and the analytical result is the occurrence of non-synchronization due to non-synchronization cycles, the differences in the performance of the results becomes insignificant as B2 increases.