Multi-degree cyclic hoist modeling and optimization
抓鉤多元週期建模與優化
Student thesis: Doctoral Thesis
Author(s)
Detail(s)
Awarding Institution | |
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Award date | 15 Jul 2014 |
Link(s)
Permanent Link | https://scholars.cityu.edu.hk/en/theses/theses(26a60d0b-4fa9-45ac-854e-aaab959818f2).html |
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Other link(s) | Links |
Abstract
Automated manufacturing systems, which integrate processing machines and
material handling equipment, are commonly used in modern manufacturing
industries to improve system productivity and throughput. A good example of
an automated manufacturing system is an electroplating line used in the
production of printed circuit boards (PCBs)-PCBs are referred to as parts in
this thesis. Identical parts go through a number of sequential stages (operations)
following a specified process flow. To complete a stage, a part is processed in a
specific tank. The loading and unloading tank store raw and completed parts
respectively. In an electroplating line, hoists are usually used to transport parts
between processing tanks. There are no buffers between processing tanks.
Hence, operations in automated electroplating lines can be expressed in terms of
hoist moves which transport parts from one tank to the next. Cyclic scheduling
is commonly used to sequence hoist moves in PCB (parts) production. After
completing a required number of planned hoist moves (a cycle), the line returns
to its initial status. The number of parts inserted and completed in the line
during a cycle is referred to as the degree of the cycle.
The main challenge in an electroplating line is to improve its production
throughput done via a more effective planning of its hoist moves. This thesis
tackles this challenge by modeling and optimizing multi-degree cyclic hoist
moves.
This research begins with a basic line, where a single hoist transports parts
between successive tanks and there is one-to-one correspondence between stages
and tanks. Through a detailed analysis of operations in basic lines, a mixed
integer linear programming model is formulated to minimize the cycle time for a
line of a given degree. Scenarios in the model are solved using the commercially
available software ILOG CPLEX. Experiments on benchmark and randomly
generated examples have shown that multi-degree cycles can offer significantly
superior throughput.
The research then proceeds to consider reentrant lines with a single hoist. Parts
visit some tanks more than once in order to complete the process needed to be
executed in the reentrant line. In practice, different stages have to be completed
in exactly the same processing environment for each of the layers in a multi-layer
PCB. The mixed integer linear programming model for basic lines is then extended
to deal with reentrant lines. Numerical experiments on reentrant lines are carried
out to observe the changes in efficiency achieved by the model.
Parallel tanks are usually used for stages with much longer processing times
for balancing the line and improving the throughput. Therefore, a staged
operation rule accommodating parallel tanks is developed. It is found that the number of feasible schedules decreases when the schedule is optimized by
applying the operation rule. An extended mixed integer linear programming
model is formulated based on the model in basic lines. The results from the
extended model are illustrated via a numerical example.
Since all parts are transported by hoists between processing tanks, hoists
usually turn out to be the bottlenecks in automated electroplating lines. The
problem is usually avoided by using multiple hoists instead of a single hoist.
Multiple hoists also help balance the line and improve throughput.
When multiple hoists share the same overhead track, collisions among hoists
need to be preempted since the hoists are not normally allowed to cross over one
another. A hoist assignment principle without overlapping is adopted to avoid
collisions in this thesis. A mixed integer linear programming model is formulated
to determine the optimal schedules in multi-hoist environments. Results from a
number of randomly generated examples have confirmed the benefits of
employing multi-degree cycles. Moreover, the general scenario consisting of
multi-hoist lines without avoiding overlapping is represented in a mixed integer
linear programming model. Computational times associated with the model are
found to be substantially longer than those for multi-hoist lines where no
overlapping is allowed.
- Electroplating, Automation