26-11-2012, 11:42 AM
Travel Time Analysis of a New Automated Storage and Retrieval System
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Abstract
In conventional Automated Storage and Retrieval
Systems (ASIRS), stacker cranes are used to deliver loads to the
ASIRS racks and to retrieve loads from the racks. Stacker
cranes travel simultaneously in the vertical and horizontal
directions. However, the stacker cranes are inadequate for
extra heavy loads. In this paper a new kind of StoragelRetrieval
(SIR) mechanism is presented to handle loads,
stacker cranes, the new SIR mechanism has one
platform and N horizontal platforms to serve N tiers of an
ASIRS rack. The wrlicsl platform provides the vertical link
among the tiers of the ASIRS rack, whereas the horizontal
platforms transfer loads to the Individual storage cells on a
given tier. The operations of the vertical Platform and the
horizontal platforms may be independent and ~on~uirentI.n
this paper, the design, operation and advantages of the new
travel-time model
under the dwell point policy that the platforms remain where
they are after completing a storageiretrievai operation. The
medel is validated by computer simulations. The show
that our model is reliable for the design and analysis of the new
kind of ASIRS. We also present guidelines for the optimal
design of a rectangular-in-lime ASiRS rack with the new S/R
mechanism.
Advantages of the new S/R mechanism
Compared with the traditional stacker crane, the new SIR
mechanism has many advantages.
(I) High lifting capacity. Because of its StNcNre, the
lifting capacity of a stacker crane is normally a few tons.
This is not enough to deal with extra heavy loads, such as
those used in heavy industries. While by separating the
vertical movement mechanism from the horizontal
movement mechanism, the lifting capacity of the new SIR
mechanism can increase quite a lot. This is the most
important reason for us to consider this new SIR mechanism.
(2) More flexible rack configuration. Using the new
design, it will be quite convenient to change rack
configurations to meet various performance requirements
from practice. such as to change the locations or numbers of
the VP, HP and 110 station.
(3) Splitting the vertical movement and horizontal
movement makes the mechanism of the ne*' ASIRS simpler
than the traditional one, and it also leads to easier
maintenance and reduced down-time ofthe ASIRS.
(4) Higher performance can be obtained. This is due to
the separate vertical movement mechanism can offer better
velocily. An example is shown in Section 3.3.
TRAVEL-TIME MODEL
We will analyze the travel time by using continuous
models that can considerably reduce the difficulty of the
subsequent analysis. We will then validate this by
comparing the results predicted by the with those
from comouter simulations.
4.1 Assumptions and notations
The following assumptions are madc throughout this
paper:
(I) The rack is considered to be a continuous rectangular
pick face.
(2) Platforms operate on single command basis.
(3) Unit loads are considered.
(4) Randomized storage is used, which means that any point
within the pick face is equally likely to be selected for
storage or retrieval.
(5) Specifications of the rack and the platforms are known.
The platforms travel at constant speeds.
In order to derive the analytical model, wc also assuinr
that there is no concurrent movement of VP and HPs Sor
different operations.
CONCLUSIONS
We have presented a kind of new SIR mechanism that
enables AS/RS to handle vely heavy loads. The advantages
of the new ASRS include high throughput, high lifling
capacity, more flexible ASIRS rack configurations and
reduced downtime of ASRS.
We have presented a continuous travel-time model for
the new ASIRS under the dwell point policy that the
platforms of the SIR mechanism stay where they are after
completing every storage or retrieval operation. The model
has been validated with the simulation results and it appears
to perform quite satisfactorily. The results of sensitivity
analysis on b and asuggest that interleaving storage
operations with retrieval operations will decrease the
expected travel time, and an optimal shape factor b is found
to be near 1. It is also observed that the global minimum of
the expected travel time is obtained around a =OS and
h=1.05.