XVIII Summer School "Francesco Turco" - Industrial Mechanical Plants
New pick-to-light system configuration: a feasibility study
Andriolo Alessandro, Battini Daria, Calzavara Martina,
Gamberi Mauro, Peretti Umberto, Persona Alessandro,
Pilati Francesco, Sgarbossa Fabio
Department of Management and Engineering, University of Padua, Stradella San Nicola, 3, 36100, Vicenza – Italy
([email protected], [email protected], [email protected], [email protected],
[email protected], [email protected], [email protected], [email protected])
Abstract
Purpose
Warehouse order picking has often been described as the most labor-intensive operation in manual warehouses, as
well as the most expensive and time consuming.
These factors have inevitably become even more crucial due to manufacturing and warehousing recent trends,
requiring high flexibility and efficiency in processing orders that are always smaller and needed in very few time.
For this reason, in the recent years more efficient and performing systems have been proposed and developed,
employing various technological solutions that can support pickers during their work.
The aim of this study is to present a new pick-to-light design solution capable of driving a number of different
operators in their picking activities, preventing the possibility of human errors by a new real-time control and alert
system.
The new solution has been analyzed from a technical and economical point of view, developing also a cost and
benefit analysis in comparison with other existing solutions.
Design/methodology/approach
The most widespread techniques adopted to facilitate picking operations generally rely on led displays or digital
screens, voice-activated devices, RFID technology, wireless appliances or lighting systems.
As far as the latter are concerned, a pick-to-light system supports the picker during his activities by using different
kind of lighting devices that can send evident light signals.
In particular, the pick-to-light system here described drives an operator through the various storage locations he has
to visit with a set of different coloured lights. These lights can be turned on or off according to the picker’s picking
list. Furthermore, they are linked to a control system that it is able to recognize whether the operator is accessing the
right storage location or not and to alert him with a set of visual (or also acoustic) signals, preventing him from
completing the wrong picking action.
A prototype of the new pick-to-light system is under development and implementation too, in order to directly
understand its strengths and weaknesses.
Originality/value
The benefits that such support device can typically provide can be summed up in three main points:
- Increased accuracy: picking errors are strongly reduced, since pickers can quickly understand whether they are
collecting the right item or not
- Increased productivity: picking is made easier and more focused on other activities, and the time needed to look
for the right picking location or to remedy errors is reduced
- Reduced training: finding the various locations is more intuitive and immediate and pickers don’t have to learn to
use complex devices
The new presented pick-to-light configuration is designed to be easy-to-use, inexpensive, modular and with a low
energy impact.
Furthermore, the different coloured lights allow multiple operators to work simultaneously in the same area, as every
picker can follow the lights of a specific colour.
Finally, comparing this solution with other systems already available on the market, its economic sustainability
emerges, making it applicable in every kind of industrial environment.
Keywords: Warehouse manual picking, Pick-to-light, RFID system.
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1. Introduction
This feature generally helps to understand whether the
scanner has correctly read the barcode, but it could be
used also to notify that the product scanned is exactly
what the picker has to take. Such system can be combined
with paper picking lists, but picking lists can be also
integrated directly in the handheld: once an item has been
picked, the screen of the handheld shows the following
product to take.
Recently, handhelds RFID scanners are also available. The
operating principle is similar to what just presented,
except that the SKUs are tagged with RFID passive tags
instead of barcodes. The working frequency is LF (Low
Frequency) or HF (High Frequency), with small reading
distances of the handhelds.
In addition, and sometimes also alternatively, to such
systems other devices have been developed. They are
often referred to as Poka-Yoke solutions, because they
perfectly reflect the principle according to which, in order
to avoid mistakes, it is important to eliminate every
chance of their happening (Baudin et al., 2005). The most
widespread techniques are pick-to-voice and pick-to-light.
A pick-to-voice system is a voice-directed system that uses
speech recognition to allow warehouse operators to
communicate with the warehouse management system.
Pickers are equipped with a headset and a microphone to
receive instructions of the picking by voice, and to
verbally confirm their actions back to the system. The
warehouse operator reads back the last digits of the item
he has picked so that the system can check whether the
correct item has been selected, then it can give the
following instruction.
On the other hand, in a pick-to-light system operators are
guided by lights that are installed on the warehouse
shelving. Each stock location has one light that turns on if
the operator has to pick the corresponding product. In
order to complete every single pick, the picker has to
press the button of the interested stock location and, in
some cases, he has also to scan the barcode of the picked
item. If the simultaneous work of more than one picker in
the same warehouse area is needed, such system has to be
integrated with paper picking lists, with digital displays or
with handhelds, so that every picker can understand which
are the lights turned on for his order.
An example of pick-to-light using RFID has been
presented in 2011 in the RFID Journal (Friedlos, 2011). In
the reported test case RFID readers are installed in some
points beneath the conveyor belt, while RFID passive tags
are attached to the plastic buckets in which workers place
the products required to fulfil the orders. When the
bucket reaches a RFID reader point of the conveyor belt,
this sends the signals of turning on of the lights of the
required products, so that the operator can easily and
quickly identify them.
Some companies are also proposing automated pick-tolight configurations, in which picker’s activities are fully
assisted: also the progress and the stops of the picking cart
are guided by the composition of the order.
Another frontier for picking is represented by special
glasses worn by the operator reporting on the lenses all
the information he needs (Banker, 2013).
The evolution of customer orders, always more frequent
and smaller, has inevitably changed the way of their
processing performed by suppliers, which are facing the
need of being very rapid and flexible.
Consequently, such trend heavily affects the configuration
of picking warehouses and of the activities that have to be
carried out within them, with a large pick volume that has
to be satisfied in a short time window (De Koster et al.,
2007).
One of the priorities for warehouse managers is,
therefore, improving picking performances. According to
De Koster et al. (1998) paperless order picking systems
can be a useful strategy to obtain some benefits in this
sense. Paperless order picking can be via mobile,
handheld or with terminals and printers that are vehiclemounted. Pickers and warehousemen are connected online with the warehouse information system, warranting
updated stock information, immediate reaction to
particular situations and real-time monitoring of
operations status. As pickers don’t need to leave the
storage area to do their work, mistakes are reduced and
the overall productivity increases.
A new frontier of paperless picking is represented by the
use of important devices that have been developed to
speed picking activities and to avoid picking errors. Some
examples are led displays or digital screens, voice-activated
devices (voice picking), wireless appliances or lighting
systems (pick-to-light).
RFID (Radio Frequency Identification) is a technology
that has recently achieved great success in various
warehouse applications, above all as far as managing and
controlling the flow of products through the whole supply
chain is concerned (Lee et al., 2010).
This paper describes a new pick-to-light system that relies
on RFID technology. In particular, next section illustrates
the currently available technology able to support
warehouse picking activities and summarizes some
important concepts concerning RFID. In the third
paragraph the RFID pick-to-light system configuration is
presented, while in the fourth paragraph such system is
compared to other already existing solutions, highlighting
their strengths and weaknesses. Finally, in the paragraph
of the conclusions, some further general considerations
are emphasized.
2. Technological framework
2.1 State of the art of the systems supporting picking
activities
As warehouse manual picking is considered one of the
most critical warehouse activities, many support systems
have been developed, able to drive and control pickers
during their work. One of the first devices adopted to
facilitate picking process and one of the most widespread,
too, is the handheld barcode scanner. All the stock
keeping units are tagged with a barcode, that are scanned
by the operator during the picking of the particular SKU.
In this way, the picking information are immediately
communicated to the warehouse information system.
Handhelds are often able to emit acoustic signals, too.
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2.2 The RFID technology
Finally, Baudin et al. (2005) point out that the current
success of barcodes is going to end soon, exactly as RFID
is perfectly able to overcome barcodes limits. For
example, the amount of data that can be stored in a
barcode is much smaller than in an RFID tag and it
cannot be updated, too.
A RFID (Radio Frequency Identification) system is a
wireless communication system in which the radio link
between the base station and the transponders are
provided by the modulated back-scattered waves. Initially
created for military purposes, almost since the beginning
of its development the RFID technology has been used
also in manufacturing and logistics applications (Raza et
al., 1999).
A basic RFID system consists of an antenna or coil, a
transceiver, and a transponder tag (Lee et al., 2010). Such
tags consist of an antenna and a chip, electronically
programmed with unique information, that are often
attached to objects in order to allow their identification. In
fact, they can store data related to the products but also,
more simply, a unique serial number that creates the
connection to actual data in a database.
According to their application, the transponders can be of
two types: active or passive. The first ones have an own
power supply (a battery) that enables them to transmit at
higher power levels, hence to be read and written at
greater distances (also over 100 m). Due to their
characteristics, they are typically larger and more
expensive. On the contrary, passive tags obtain their
energy from the electromagnetic field of the reading
device, so they are very small and economical.
A RFID system can differ in terms of the frequency range
in which it operates, too. In particular, there are three
worldwide established frequencies: Low Frequency (LF),
< 135 kHz, High Frequency (HF), 13.56 MHz and Ultra
High Frequency (UHF), between 850 and 960 MHz.
Every working frequency is more suitable for some
applications than for others: when a RFID project is being
developed it is important to perfectly understand what are
its needs. Low Frequency systems are well-suited to
industrial use, above all when working near metals and
water is needed. High Frequency systems are characterised
by greater ranges and higher reading speeds. The
simultaneous reading of multiple tags is possible, but it
could be influenced by the presence of metal objects. For
warehousing and goods tracking UHF systems are more
suitable. In fact, they enable very high data transfer rates
and long ranges (up to six meters), even if signals typically
do not pass through most of the materials.
The basic principle, however, is that full advantages of
RFID are obtained when the application, the
manufacturing process and the supply chain are
considered as a whole (Weinstein, 2005).
In warehousing and manufacturing passive tags and UHF
readers are the most widespread; this certainly happens
because passive tags are very cheap and versatile. They are
often used as an alternative to barcodes, but with better
performances. In particular, they have a high reading
capacity without needing of line-of-sight and a good
writing/modifying capacity for storing data. According to
Baudin et al. (2005) it is possible to obtain more benefits
in the contexts in which a high rate of scanning is needed,
hence, where warehouse operators or workers have to
scan a lot of tags in very few time, as in the case of
warehouse order picking.
3. New RFID Pick-to-light system
3.1 Presentation of the general configuration
The background idea of the solution presented in this
paper is the will to create a pick-to-light system able to
drive the picker through the locations he has to visit in a
smart as well as simple way.
In particular, the main objective is to exploit the benefits
of RFID technology, according to which there is no need
of direct contact between the reader and the tag to obtain
the information stored in the tag.
Another important aspect that has been considered is to
give the picker a RFID reader that doesn’t need to be kept
in hand, so that he can perform the picks using both
hands. To do this, a wearable RFID reader is needed. The
best found solution is to provide the operator with a
particular glove containing the RFID reader.
Recently, some examples of similar solutions have been
developed, for various applications. Among the first there
are the iGlove and the iBracelet, invented by the Intel
Research Seattle group (Fishkin et al., 2005). These
devices are able to interact with unobtrusively tagged
objects. Furthermore, the glove can also report whether
the grasp of the object is with the palm or with the
fingerprints. Medynskiy el al. (2007) use a wearable RFID
reader for gaming applications. Muguira et al. (2009)
propose the RFIDGlove system, consisting of a glove
with an integrated RFID reader, an organic micro display
and a communication system. They also highline the
usefulness of such device for inventory and warehousing
activities, as all the movements performed by warehouse
operators are completely traced.
Lee et al. (2010) created a wireless RFID glove for
interactive learning and for a meal aid system useful for
blind people. Most of the already existing devices can be
used with passive tags and work at high frequency (13.56
MHz), which means that the read distances cannot be
much larger than 1 meter.
As far as the new RFID pick-to-light system is concerned,
the lights that are installed on the shelving units to signal
the picker the locations he has to visit are of different
colours: every single operator can focus only on a
particular colour and pick the items his order is made up
by following that specific coloured lights, that are turned
on or off according to the picker’s picking list.
Red lights are used instead to alert the operator if he
makes some errors during his picking activities. If the
picker enters the wrong stock location and tries to pick
the wrong product, the corresponding red light is turned
on, in order to make him understand his mistake and to
give him the possibility to immediately collect the right
item.
The proposed configuration is composed of three main
units. The first one is a system of lights and tags installed
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on the shelving, so that every stock location has one
passive tag (or more, depending on the available storage
room and on the kind of product stored) that identifies
that particular stock location, one red light useful to alert
the picker if he enters the wrong location and as many
different coloured lights as the number of the pickers that
are working at the same time in the same picking area, so
that every picker follows only the lights of a particular
colour to make up his order. Even if there are several
pickers working in the same area, it is sufficient to have
only one red light in every stock location. This because
the red light turns on only if the corresponding location is
entered by a picker by mistake. In this way, the picker
understands unequivocally that who is wrong is exactly
himself.
The second unit consists of a wireless UHF RFID reader
that every warehouse operator wears thanks to an
appropriate glove. In this way, the picker has both hands
free and he can pick the items he needs in a better and
faster way. The system of lights installed on the shelving
and the reader are managed and controlled by the third
component of the configuration, a centralized control
system. It takes as input the various picking lists (one for
each picker) coming from the warehouse information
system and sends the signals to turn on and off the
appropriate lights of the shelving. Furthermore, such
system is able to monitor the pickers’ picking activities, in
order to highlight possible picking errors by turning on
the red lights.
Figure 1 shows a scheme of the proposed RFID pick-tolight configuration, useful to understand its general
operation.
Picking
list A
Picking
list B
Items to pick Items to pick
B-1
G-1
R-1
B-2
1
RFID
G-2
R-2
B-3
G-3
2
3
Tag 2
Tag 3
R-3
Wifi signal
(read tag)
A
Tag 1
B-4
B-7
G-4
R-4
B-5
G-5
R-5
B-6
G-6
4
5
6
Tag 4
Tag 5
Tag 6
G-7
R-7
B-8
G-8
R-8
B-9
G-9
7
8
9
Tag 7
Tag 8
Tag 9
Centralized Control
System
Lights turning on/
off signals
R-6
RFID
B
R-9
Figure 1: Scheme of the new RFID pick-to-light system configuration.
The centralized control system receives the picking lists
and it extracts the information of the products to pick to
send the signal of turning on of the lights, each colour
corresponding to a specific operator. According also to
the routing policy that is adopted in the warehouse, the
light of the first location the picker has to visit is turned
on, so that the picker can easily and immediately
understand where the item he has to pick is located. When
the picker, wearing the glove that includes the RFID
reader, reaches the particular location there are two
possibilities: he enters the right stock location or he enters
a wrong one. In both cases, the reader reads the tag
corresponding to the stock location and sends via Wi-Fi
the read code to the centralized control system, that acts
accordingly. In case of correct access, once the system has
verified that the code received from the RFID reader is
the right one, corresponding to the item the operator has
actually to pick, it sends the signal of turning off the
coloured light of that location (for example, the blue one)
and of turning on the coloured light of the following
location, corresponding to the following line of the
picking list. In this case, it could be useful to associate an
acoustic signal to confirm the correct picking. If the
picker mistakes the stock location and tries to pick the
wrong item, the centralized control system receives from
the RFID reader a code corresponding to a product
different from what it expects. Then, it sends the signal of
turning on of the red light of the wrong stock location, so
that the picker immediately realizes that he is not in the
right one. When the operator pulls out his hand from the
wrong location the centralized control system doesn’t
receive the wrong code from the RFID reader anymore,
so the red light is turned off.
It is important to underline that every picker can perform
his work independently. As every operator wears his own
RFID reader, the centralized control system can perfectly
recognize the different signals coming from the various
pickers. Hence, it is perfectly able to manage separately
the turning on and off of the different lights.
3.2 Prototype design
In order to properly assess the potential of such
configuration and to test the various technologies that
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have been used, a prototype of the new RFID pick-tolight system is being developed.
Such prototype is designed to manage two operators that
work simultaneously. It is composed of a shelving with
nine stock locations, each of which has three leds, one
green led for the first operator, one yellow led for the
second one and one red led to indicate the picking errors.
All the leds are connected to a microcontroller that sends
the signals of turning on and off of the lights.
Every stock location has also one passive tag, in which the
code of the product stored in that stock location has been
wrote.
The UHF RFID readers are composed of a wireless
reader unit, of a linear polarized UHF antenna and of a 12
V lithium battery. This device is worn by the operator
through a glove, where the antenna is on the upper part of
one of his hands and the reader unit is on his forearm.
The battery, instead, can be hooked, for example, at his
waist. The data read by each reader are sent to a personal
computer via Wi-Fi.
The PC represents the centralized control system: in
addition to receive the information of the RFID readers it
has a Labview project that has been developed to manage
all the control logic. In particular, the program is able to
read two different picking lists from an Excel file and to
send the appropriate signals to turn on the green and
Ease of
use
yellow leds according to the order lines. The RFID
readers are interfaced with the Labview project through a
TCP/IP Socket, that receives the information of the read
tags as a Wi-Fi input and transmits them to the rest of the
Labview project. The remaining logic is necessary to
compare the tags read by the RFID readers to the order
lines of the picking lists, in order to understand whether
the picker is picking the right product or not. If the
operator picks the right item the system sends to the
microcontroller the command of turning off the led
corresponding to that product and of turning on the one
that refers to the following line of the picking list.
Otherwise, it makes turning on the red led of the stock
location the picker has erroneously entered.
4. Analysis of the solution
Table 1 reports a qualitative comparison of some of the
supporting devices used for manual picking already
described in Section 2: handhelds, pick-to-voice system,
traditional and fully automated pick-to-light system and
the new RFID pick-to-light system presented in this
paper. The criteria used for the analysis are focused on the
usability of the devices, as well as their efficiency and
effectiveness.
Picking
Cheapness Flexibility Modularity
time
Reading
distance
Pickers
Environment
simultaneity
influence
Errors
interception
Handheld and
barcodes
High
Medium
Medium
High
High
Few
centimetres
Possible
Medium
After barcode
scanning
Handheld and
RFID
High
Medium
Medium
High
High
Up to 20
cm
Possible
Low
After tag
scanning
Low
High
High
Not
applicable
Possible
High
After code
communication
Medium
Medium
Medium
Medium
Not
applicable
Difficult
Low
At the end of
picking
Fully automated
Medium
pick-to-light system
Short
Very low
Very low
Medium
Not
applicable
Not possible
Low
Immediate
RFID pick-to-light
system
Short
High
High
High
Up to 2 m
Possible
Medium
Immediate
Pick-to-voice
system
Traditional pick-tolight system
Medium Medium
High
High
Table 1: Comparison of some manual picking supporting devices.
As the new RFID pick-to-light configuration does not
require for the operator the using of any particular device,
it proves to be very easy to use. In fact, the RFID reader
is directly integrated within the glove, so the picker has
only to focus on the physical picking of the products he
needs, without scanning barcodes or, as in the case of the
pick-to-voice, without reading aloud the product code.
This implies that the picking time is short, because the
picking action is just limited to take the required item and
to put it in the picking cart. In other systems, instead,
every pick is associated to other operations, for example
the scanning of the barcode or of the tag. It derives that
using the new RFID pick-to light system improves
warehouse overall performances.
The new RFID pick-to-light system is inexpensive, too.
This is specially due to the fact that in this case instead of
tagging or barcoding all the SKUs, as in the case of
handhelds or other picking systems, the RFID tag are put
only on the stock locations. Furthermore, the UHF RFID
reader is a device commonly available on the market, and
the proposed configuration requires just one reader for
every picker, as with handhelds and pick-to-voice.
These systems, in which each warehouse operator has his
own device, warrant great flexibility as well as great
modularity, too.
According to the configuration, the reading distance varies
from the few centimetres of the barcodes to the two
metres of the UHF RFID pick-to-light system presented
in this paper. RFID handhelds can read the tags only up
to 20 cm as they generally work at LF or HF.
The new RFID pick-to-light system allows also
simultaneous work of more pickers in the same
warehouse sector, because every stock location has as
many different coloured lights as the number of operators
working in such area. This is possible also for barcode and
RFID handhelds and for pick-to-voice systems. Working
at the same time is quite hard, instead, in a traditional
pick-to-light system, as for the pickers it is difficult to
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understand which are the lights that are turned on for him
and which that are not.
In some cases operators’ activities can be influenced by
the environment they work in. This is particularly true for
voice picking, as surrounding noises could prevent a
correct communication between the system and the
operator of the code to pick and of its following
confirmation. Besides, for such systems a wrong
pronunciation of the numbers could cause a useless delay
in picking activities. Also the scanning of barcodes could
have some problems: it requires a clean, high-contrast
environment, and often more than one attempt (Baudin et
al., 2005). On the contrary, pick-to-light is generally not
affected by its context of application. For the RFID pickto-light system the only issue concerning the environment
of application could derive from the interference of RFID
waves with the shelving, the products stored and with the
body of the operator. It is therefore fundamental to
carefully study the configuration of the whole system, in
order to prevent some side effects and to exploit some
other ones.
Another great strength of the new RFID pick-to-light
system is the immediate interception of errors. In fact, the
picker can understand right away whether he has picked
the right product or not. In case he puts his hand in the
wrong stock location, the red light turns on in order to
prevent him from completing the wrong picking action.
The same behaviour is performed also by the fully
automated pick-to-light system, but requiring at the same
time the installation of a complex as well as expensive
hardware. In some other configurations, instead, there is
the risk for the picker of discovering picking errors only
once the order is complete, or, even worse, when the
order is delivered to the customer. According to Frazelle
(1989) and Tolliver (1989), a computer-aided system can
be used for manual order picking to simplify the tasks of
human pickers; furthermore, it has been estimated that a
light-directed picking system with automated data entry
can reduce human errors of 95% as well as increase
productivity of 10%.
As the new pick-to-light system uses RFID technology
and it is real time connected to the centralized control
system, it is quite easy to obtain useful data about all
pickers’ activities, for example the number of picks per
hour, that could be used as a starting point for possible
improvements of the whole system.
When the RFID reader reads the tag the signal is sent via
Wi-Fi to a centralized control system that checks whether
the picker has taken the right item or not and that sends
the signals of turning on and off of the appropriate lights.
Thanks also to the creation of a prototype, that is still
under development, such system has been compared with
other manual picking supporting devices, showing its
potentialities as well as its economical sustainability.
Next steps of the project will concern the setup of the full
prototype. Subsequently, smart ways to communicate the
quantity to pick of every item to the operator will be
investigated. A solution could be integrating a display in
the glove (Muguira et al., 2009) showing the number of
pieces to pick.
6. References
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5. Conclusions and further research
Warehouse manual picking is one of the most critical
activity in a warehouse: every improvement made on this
area can lead to great results in terms of time and costs
saving (De Koster, 2007). In this sense, it could be
interesting the development of devices able to support
and help the pickers during their work.
In this paper a new RFID pick-to-light system
configuration has been presented, that combines the
benefits of RFID to the simplicity and effectiveness of
pick-to-light.
Every warehouse operator wears a glove in which a UHF
RFID reader is installed, while every stock location has a
RFID tag to identify the corresponding product stored.
Muguira, L., Vazquez, J.I., Arruti, A., Ruiz-de-Garibay, J., Mendia,
I., Renteria S. (2009). RFIDGlove: a wearable RFID reader.
IEEE International Conference on e-Business Engineering, 475-480.
Raza, N., Bradshaw, V., Hague, M. Applications of RFID
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