AUTOMATED

STORAGE/RETRIEVAL SYSTEMS

CHAPTER 6
 1. INTRODUCTION

1.1. Storage Systems

Function – to store materials (e.g., parts, work-in-
process, finished goods) for a period of time and
permit retrieval when required
• Used in factories, warehouses, distribution centers,
wholesale dealerships, and retail stores
• Important supply chain component
• Automation available to improve efficiency
Performance measures for storage systems:
• Storage capacity - two measures:
– Total volumetric space
– Total number of storage compartments (e.g., unit loads)
• Storage density - volumetric space available for storage
relative to total volumetric space in facility
• Accessibility - capability to access any item in storage
• System throughput - hourly rate of storage/retrieval
transactions
• Utilization and availability (reliability)
 Storage Location Strategies
Two strategies: 1. Randomized, and 2. Dedicated
Randomized storage
. Incoming items are stored in any available location
. Usually means nearest available open location
Dedicated storage
Incoming items are assigned to specific locations in the
storage facility
– Typical bases for deciding locations:
• Items stored in item number sequence
• Items stored according to activity level
• Items stored according to activity-to-space ratios
 Conventional Storage Methods

• Bulk storage - storage in an open floor area

– Problem: achieving proper balance between storage density
and accessibility
• Rack systems - structure with racks for pallet loads
– Permits vertical stacking of materials
• Shelving and bins - horizontal platforms in structural
frame
– Steel shelving comes in standard sizes
– Finding items can be a problem
• Drawer storage - entire contents of each drawer can
be viewed
Bulk storage arrangements:
(a)high-density bulk storage provides low accessibility,
(b)bulk storage with loads forming rows and blocks for
improved accessibility
(c)Low cost per sq meter
Pallet loads placed on
racks in multi-rack
structure
(a)Low cost
(b)Good stroge density
(c)Good accessibility
Drawer Storage
• Contents easily
visible
• Good accessibility
• Relatively high cost
Small items (tools,
repair parts, etc.)
 1.2 Automated Storage Systems

Mechanized and automated storage equipment to

reduce the human resources required to operate
a storage facility
• Significant investment
• Level of automation varies
– In mechanized systems, an operator participates in
each storage/retrieval transaction
– In highly automated systems, loads are entered or
retrieved under computer control
 Objectives and Reasons for Automating
Storage Operations

• To increase storage capacity

• To increase storage density
• To recover factory floor space currently used for WIP
• To improve security and reduce pilferage
• To reduce labor cost and/or increase productivity
• To improve safety
• To improve inventory control
• To improve stock rotation
• To improve customer service
• To increase throughput
Types of Automated Storage System
1. Automated Storage/Retrieval System (AS/RS)
– Rack system with mechanized or automated crane
to store/retrieve loads
2. Carousel Storage System
– Oval conveyor system with bins to contain
individual items
Unit load on pallet AS/RS with one aisle
• AS/RS Applications
1. Unit load storage and retrieval
– Warehousing and distribution operations
– AS/RS types: unit load, deep lane (food industry)
2. Order picking
– AS/RS types: miniload, man-on-board, item retrieval
3. Work-in-process storage
– Helps to manage WIP in factory operations
– Buffer storage between operations with different
production rates
– Supports JIT manufacturing strategy
– Kitting of parts for assembly
Carousel Storage Systems
• Horizontal
– Operation is similar to overhead conveyor system
used in dry cleaning establishments
– Items are stored in bins suspended from the conveyor
– Lengths range between 3 m and 30 m
– Horizontal is most common type
• Vertical
– Operates around a vertical conveyor loop
– Less floor space required, but overhead room must be
provided
• Horizontal Carousel Storage System
Manually operated horizontal carousel storage system
• Carousel Applications
1. Storage and retrieval operations
– Order picking
– Kitting of parts for assembly
2. Transport and accumulation
– Progressive assembly with assembly stations located
around carousel
3. Work-in-process
– WIP applications in electronics industry are common
4. Unique applications
– Example: time testing of electrical products
 1. 3. AS/RS
• AS/RS are means to high density hands free buffering of
materials in distribution and manufacturing environments.
AS/RS is a complete system designed to transport,
stage/store, retrieve, and report on every item in any
industrial inventory with up-to-the minute accuracy.
• An automated storage/retrieval system (AS/RS) can be
defined as a storage system under which a defined
degree of automation is to be implemented to ensure
precision accuracy and speed in performing storage and
retrieval operations.
• AS/RS are computer-controlled systems for storing and
retrieving products in warehouses or manufacturing
facilities without manual labor.
• These automated storage and mechanized systems
eliminate human intervention in performing basic sets
of operations that includes :
- Removal of an item from a storage location
automatically
- Transferring the above item to a specific processing
or interface point
- After receiving an item from a processing or
interface point, it is automatically stored at a
predetermined location.
• Objectives for Installing an Automated Storage
System in a Factory
 2. AS/RS COMPONENTS AND
TERMINOLOGY
• An AS/RS consists of one or more storage aisles that are
serviced by a storage/retrieval (S/R) machine.
• The stored materials are held by storage racks of aisles.
• The S/R machines are used to deliver and retrieve
materials in and out of inventory.
• There are one or more input/output stations in each
AS/RS aisle for delivering the material into the storage
system or moving it out of the system.
• In AS/RS terminology, the input/output stations are
called pickup-and-deposit (P&D) stations.
Generic Structure of as AS/RS
• Storage Space: It is the three-dimensional space in the
storage racks used to store a single load unit of material.
• Storage Racks : This structural entity comprises storage
locations, bays and rows.
• Bay : It is the height of the storage rack from floor to the
ceiling.
• Row : It is a series of bays placed side by side.
• Aisle: It is the spacing between two rows for the machine
operations of AS/RS.
• Aisle Unit: It encompasses aisle space and racks adjacent to
an aisle.
• Storage Structure: It is the rack framework, made of
fabricated steel that supports the loads contained in the AS/RS
and is used to store inventory items.
• Storage/Retrieval Machine : It is used to move items in and out
of inventory. An S/R machine is capable of both horizontal and
vertical movement. A rail system along the floor guides the
machine and a parallel rail at the top of the storage structure is
used to maintain its alignment.
• Storage Modules: These are the unit load containers used to hold
the inventory items. These include pallets, steel wire baskets and
containers, pans and special drawers. These modules are generally
made to a standard base size capable of being stored in the
structure and moved by the S/R machines.
• Pickup and Deposit (P/D) Stations: P/D stations are where
inventory are transferred into and out of the AS/RS. They are
generally located at the end of the aisles to facilitate easy access
by the S/R machines from the external material-handling system.
The location and number of P/D stations depends upon the
origination point of incoming loads and the destination of output
loads.
Courtesy of Stöcklin Logistik AG
 3. TYPES OF AS/RS
• Unit Load AS/RS : The unit load AS/RS is used
to store and retrieve loads that are palletized or
stored in standard-sized containers. The system is
computer controlled. The S/R machines are
automated and designed to handle the unit load
containers. Usually, a mechanical clamp
mechanism on the S/R machine handles the load.
However, there are other mechanisms such as a
vacuum or a magnet-based mechanism for
handling sheet metal. The loads are generally over
500 lb per unit. The unit load system is the
generic AS/RS.
• Mini Load AS/RS: This system is designed to
handle small loads such as individual parts, tools,
and supplies that are contained in bins or drawers
in the storage system. Such a system is applicable
where the availability of space is limited. It also
finds its use where the volume is too low for a
full-scale unit load system and too high for a
manual system. A mini load AS/RS is generally
smaller than a unit load AS/RS and is often
enclosed for security of items stored
• Deep-lane AS/RS : This is a high-density unit load
storage system that is appropriate for storing large
quantities of stock. The items are stored in multi
deep storage with up to 10 items in a single rack,
one load behind the next. Each rack is designed for
flow-through, with input and output on the
opposite side. Machine is used on the entry side of
the rack for input load and loads are retrieved from
other side by an S/R- type machine. The S/R
machines are similar to unit load S/R machine
except that it has specialized functions such as
controlling rack-entry vehicles.
• Man-on-board AS/RS : This system allows storage
of items in less than unit load quantities. Human
operator rides on the carriage of the S/R machine to
pick up individual items from a bin or drawer. The
system permits individual items to be picked directly
at their storage locations. This provides an
opportunity to increase system throughput. The
operator can select the items and place them in a
module. It is then carried by the S/R machine to the
end of the aisle or to a conveyor to reach its
destination.
• Automated Item Retrieval System: This system is
designed for retrieval of individual items or small product
cartoons. The items are stored in lanes rather than bins or
drawers. When an item is retrieved from the front by use
of a rear-mounted pusher bar, it is delivered to the pickup
station by pushing it from its lane and dropping onto a
conveyor. The supply of items in each lane is periodically
replenished and thus permitting first-in/first-out inventory
rotation. After moving itself to the correct lane, the
picking head activates the pusher mechanism to release
the required number of units from storage.
Various system concepts for AS/RSs (Modified after Roodbergen and Vis 2009)
Some common types of stacker cranes in AS/RSs (Courtesy of Stöcklin Logistik AG)
 4. DESIGN OF AN AS/RS
1. Load Sizes Determination
- Load size determination is the most important element in the design
of an AS/RS and is based on work flow information. The movement
frequency of parts, tools, fixtures, pallets, and other supplies define the
overall work flow.
- Work flow is determined by variety and volume of part types and the
type of production system.
- The width, length and height of rack structure of the AS/RS aisle are
related to the unit load dimensions. The dimensions of the unit loads
with proper clearances provide the individual storage space dimensions
and account for the size of supporting beams in the rack structure.
- Storing unique items of unusual and complex shape are excluded
from the AS/RS design.
Weight of the unit load is also considered in the structural design
2. Calculating Individual Storage Space Dimensions
• Let l, b and h be the length, width and height of the unit load. The
length (L), width (W) and height (H) of the rack structure of the AS/RS
aisle are related to the unit load dimensions and number of
compartments as follows :
• L = ny (l + x)
• W= u (b + y)
• H = nz (h + z)
• where ny and nz are respectively number of load compartments along
the length and height of the aisle; x, y, and z are allowances designed
into each storage compartment to provide clearance for the unit load; u
is storage depth in number of unit loads. All the dimensions of rack
structure are in ‘mm’.
• The total storage capacity of one storage aisle is expressed as follows :
• Capacity per aisle = 2 x ny x nz
• Constant ‘2’ is multiplied because loads are contained on both sides of
the aisle.
• Example: In each aisle of an AS/RS, there are 70 storage
compartments in the length direction and 10 storage
compartments vertically. The dimensions of the unit load in
inches (in) are 50 (length), 45 (width) and 50 (height)
respectively. The allowances designed for each storage
compartment are : x = 8 inch, y = 7 inch and z = 10 inch. Storage
depth u in the number of unit load is 3. Determine the capacity
per aisle and the dimensions of the single storage system.
Solution
(a) Capacity per aisle = 2 x ny x nz = 2 x 70 x 10
= 1400 unit loads
(b) L = ny (l + x) = 70 (50 + 8) = 4060 in
W = u (b + y) = 3 (45 + 7) = 156 in
H = nz (h + z) = 10 (50 + 10) = 5000 in
3. Estimating Storage Spaces Number
• Dedicated and randomized storage policies are used to
determine the number of storage spaces in AS/RS.
• In dedicated storage policy, a particular set of storage slots is
allocated to a specific product. Hence the sum of the
maximum inventory levels for the entire products match with
the number of slots required to store the product.
• In case of randomized storage policy, any compartment in the
storage aisle is equally probable to be selected for transaction.
• Likewise, there is equal chance of each unit of particular
product to be retrieved when a retrieval operation is
performed.
• Thus, in a long run, maximum of the aggregate inventory level
of all the products is taken into account for determining the
storage space number.
4. Estimation of AS/RS Throughput and the
Number of S/R Machines
System throughput is defined as hourly rate of
S/R transactions (number of loads stored and
number of loads retrieved) that an automated
storage system can perform. A dual command
cycle is used to increase the throughput, since it
reduces travel time per transaction. Following
are the factors that influence system throughput :
• Velocity of S/R machine
• Single and dual command cycles
• System utilization per hour
• Arrangement of stored items
• Speed of AS/RS control system
• Speed and efficiency of the material handling equipment
The number of S/R machines can be determined as
follows :
Number of S/R machines = System Throughput / (S/R
machine capacity in cycles per hour )
5. Estimating the Size Parameters of the Storage and
Retrieval System
System length, width and height are vital to estimate the
size of AS/RS. For this purpose, it is required to
determine following parameters:
(i) Number of Rows and the Number of Bays in Each
Row of a System
Number of S/R machines used to store and retrieve
materials depends primarily on the system throughput
and the cycle time. S/R machines are used for one or
more aisles. Each aisle has two rows. Therefore, the
number of rows in case of one S/R machine per aisle is :
Number of rows in the system = 2 x Number of
S/R machines in the system

The variation in the desired system height is in

between 30 to 90 ft.
(ii) Estimation of Bay Width, System Width, Rack
Length, System Length, Bay Depth and Aisle Unit
The length of a single storage space is added to the
centre-to-centre rack support width to calculate
bay width. Thus we get,
Bay width = Length of storage space + Centre-to-
centre rack support width
= l + x + x1
where x1 is the centre-to-centre rack support width
Rack length = bay width number of bays
System length = rack length + clearance for (S/R
machine run-out + P/D area)
Bay depth = width of the individual storage space + bay
side support allowance
= u (b + z) + x2
where x2 is the bay side support allowance
 Example
The single command cycle system of a XYZ
Inc. has cycle time per operation as 2 minutes.
Expected system throughput for the corporation
is 360 operations per hour. Number of storage
space per system height is 15 and total number of
storage spaces using a randomized policy is 9000.
Assuming storage and retrieval operation take
same time, determine :
(i) Number of S/R machines;
(ii) Number of rows; and
(iii) Number of bays in each rows.
Solution
6. Determination of Single- and Dual-command
Cycle Times for Unit Load AS/RS
• Single-command Cycle
It performs either storage or a retrieval operation. There
are certain steps that are followed in storage or retrieval
cycle to determine the cycle time. In case of storage
cycle, machine picks up a load, travels to the storage
location, deposits the load, and returns empty to the P/D
station. Similarly, in a retrieval cycle, the S/R machine
begins at the P/D station and travels empty to the
retrieval location. Thereafter, it picks up the load,
travels to the P/D station, and deposits the load.
• Dual-command Cycle
Cycle time is determined in case of dual-
command cycle when it starts its operation with
the S/R machine at the P/D station. The machine
picks up the load and travel to the storage
location to put down the load. Thereafter, the
machine travels to the retrieval location to
recover the load. Finally, it travels back to the
P/D station to deposit the load.
• simultaneous horizontal and vertical travel. For a
single-command cycle, the load to be entered or
retrieved is assumed to be located at the center of the
rack structure, as in Figure . Thus, the S/R machine
must travel half the length and half the height of the
AS/RS, and it must return the same distance. The
single-command cycle time can therefore be expressed
by
where Tcs is the cycle time of a single command
cycle; L is the length of the AS/RS rack structure; vy
is the velocity of the S/R machine along the length
of the AS/RS; H is the height of the rack structure;
vz is the velocity of the S/R machine in the vertical
direction of the AS/RS; and Tpd is the pickup-and-
deposit time. Two pickup-and-deposit times are
required per cycle, representing load transfers to
and from the S/R machine.
• For dual command cycles, the S/R machine is assumed
to travel to the centre of the rack structure to deposit a
load, and then to three quarters the length and height
of the AS/RS to retrieve a load. The total distance
travelled by the S/R machine is ¾ the length and ¾ the
height of the rack structure, and back. Cycle time is:

• where terms as defined above; and Tcd is the cycle

time for a dual command cycle.
• System throughput depends upon the above analysis
of single and dual command cycles; so that Rcs may
be set as the number of single command cycles
performed per hour, while Rcd may be set as the
number of dual command cycles per hour at a
specified or assumed utilization level. Thus, the
amounts of time spent in performing single command
and dual command cycles each hour, is:

• where U is the system utilization during the hour; The

right-hand side of the equation gives the total number of
minutes of operation per hour.. For this equation we need
to determine the relative proportions of both Rcs and Rcd;
• Assumptions about these proportions must be made.
When solved, the total hourly cycle rate is given by

• where Rc = total S/R cycle rate, cycles/hr. Note that

the total number of storage and retrieval transactions
per hour will be greater than this value unless Rcd =0,
since there are two transactions accomplished in each
dual-command cycle. Let Rt: the total number of
transactions performed per hour; then
 EXAMPLE 1 Sizing an AS/RS System

Each aisle of a four-aisle AS/RS contains 60

storage compartments in the length direction and
12 compartments vertically. All storage
compartments are the same size to accommodate
standard-size pallets of dimensions: x = 42 in
and y = 48 in. The height of a unit load z = 36 in.
Using the allowances a = 6 in, b = 8 in, and c =
10 in, determine (a) how many unit loads can be
stored in the AS/RS and (b) the width, length,
and height of the AS/RS.
• Solution:
(a) The storage capacity is given by Equation:
Capacity per aisle = 2(60)(12) = 1,440 unit loads.
With four aisles, the total capacity is
AS/RS capacity = 4(1440) = 5,760 unit loads
(b) From Equations, the dimensions of the storage
rack structure can be computed as:
W = 3(42 + 6) = 144in = 12 ft/aisle
Overall width of the AS/RS = 4(12) = 48 ft
L = 60(48 + 8) = 3,360 in = 280 ft
H = 12(36 + 10) = 552 in = 46 ft
 EXAMPLE 2: AS/RS Throughput Analysis
Consider the AS/RS from Example 1, in which an
S/R machine is used for each aisle. The length of the
storage aisle = 280 ft and its height = 46 ft. Suppose
horizontal and vertical speeds of the S/R machine are
200 ft/min and 75 ft/min, respectively. The S/R
machine requires 20 sec to accomplish a P&D
operation. Determine
(a) the single-command and dual-command cycle
times per aisle and
(b) throughput per aisle under the assumptions that
storage system utilization = 90% and the number of
single-command and dual-command cycles are equal.
 Carousel Storage Systems
• Storage Capacity. The size and capacity of a carousel can
be determined with reference to below figure. Individual
bins or baskets are suspended from carriers that revolve
around an oval rail with circumference given by
C = 2(L - W) + pi W
where C = circumference of the oval conveyor track, m (ft);
and L and W are the length and width of the track oval, m
(ft).
• The capacity of the carousel system depends on the
number and size of the bins (or baskets) in the system.
Assuming standard-size bins are used, each of a certain
volumetric capacity, the number of bins can be used as the
measure of capacity.
• The number of bins hanging vertically from each
carrier is nb and nc = the number of carriers around
the periphery of the rail. Thus,
Total number of bins = nc * nb
• The carriers are separated by a certain distance
so that they do not interfere with each other
while traveling around the ends of the carousel.
Let sc = the center-to-center spacing of carriers
along the oval track. Then the following
relationship must be satisfied by the values of sc
and nc:
Sc * nc = C
• where C = circumference, m(ft); sc = carrier
spacing, m/carrier (ft/carrier); and nc = number
of carriers, which must be an integer value.
Throughput Analysis. The storage/retrieval cycle time can be
derived based on the following assumptions.
• First, only single-command cycles are performed; a bin is
accessed in the carousel either to put items into storage or to
retrieve one or more items from storage.
• Second, the carousel operates with a constant speed vc;
acceleration and deceleration effects are ignored.
• Third, random storage is-assumed; that is, any location around
the carousel is equally likely to be selected for an S/R
transaction. And
• fourth, the carousel can move in either direction.
• Under this last assumption of bidirectional travel, it can be
shown that the mean travel distance between the load/unload
station and a bin randomly located in the carousel is C/4. Thus,
the S/R cycle time is given by
• Where Tc = S/R cycle time, min; C= carousel
circumference, m (ft); vc : carousel velocity,
m/min (ft/min); and Tpd = the average time
required to pick or deposit items each cycle by
the operator at the load/unload station, min.
• The number of transactions accomplished per
hour is the same as the number of cycles and is
given by the following:
• Bozer and White (1984) derived an expression for cycle time
based on following assumptions :
- Randomized storage of loads in the AS/RS
- Horizontal and vertical velocities of the S/R machines are
constant
- Rack openings are of single-size
- P/D station is located at the base and at the end of the aisle
- Simultaneous horizontal and vertical travel of S/R machine
The storage space dimensions help to determine the length (L1)
and height (H1) of an AS/RS aisle and it is given as follows :
L1 = n (l + x)
H1 = m (h + z)
• where n and m are the number of bays and storage spaces
per system height.
• Time required travelling full horizontal length and vertical
height of an aisle is given by

• where Vh and Vv are the average horizontal and vertical

speeds of S/R machines.
- For single-command cycle, cycle time is given as :

- For dual-command cycle, cycle time is :

• where Tsc = single-command cycle time,
• Tdc = dual-command cycle time,
• T = max (Th, Tv),
• M = min (Th / T, Tv / T),
• Tpd = time to perform either a pick up or deposit,
• Th = time taken to traverse full horizontal aisle
distance, and
• Tv = time taken to traverse full vertical aisle distance
7. Estimating the Utilization of S/R Machines
• The performance evaluation of an automated storage
and retrieval system is based on the percentage
utilization of S/R machines.
• Suppose there are N number of S/R machines in the
system and each aisle is served by one S/R machine.
Then the number of transaction per S/R machine per
hour is

• where ST is the system throughput for an AS/RS.

• System throughput depends on the relative numbers
of single and dual command cycles performed by the
system. Let Anpha be the number of single command
cycles performed per hour, and Beta be the number of
dual command cycles per hour, at a specified or
assumed utilization level.
• Then an equation can be formulated for the amount of
time spent in performing single command and dual
command cycles each hour and is given as :
• BOZER, Y. A., and J. A. WHITE (1984), Travel-Time Models for
Automated Storage/Retrieval Systems, IIE Transactions, 329-
338.
• Groover, M. P. (2001), Automation, Production Systems, and
Computer-Integrated Manufacturing, 2nd Ed”, Pearson
Education: Singapore