Smart Shelf Technology: Wireless Inventory Control [622951]

Smart Shelf Technology: Wireless Inventory Control
System Implemented with Weight Sensors

7230fe2a -fba3-11e8-8eb2-f2801f1b9fd11,2

1) Department of Applied Electronics 2) Department of Research and Development
Technical Universi ty of Cluj -Napoca,Romania Zollner Elektronik AG Satu Mare,Romania
7230fe2a -fba3-11e8-8eb2-
f2801f1b9fd1 @yahoo.com www.zollner.ro

Abstract — The world’s fourth largest retailer – METRO
Group is using RFID (Radio Frequency Identification)
technology in t heir stores. Their smart shelves have
readers whose purpose is to inform store’s staff when
shelves need replenishment.This RFID readers are placed
at the bottom of the smart shelves and communicate with
warehouse or with the main RFID products control
system. When an items is removed or added on the shelf,
the inventory in the system is updated, all products being
automatically registered, preventing inadequate depositing
and out -of-stock situations. It is assumed that in the near
future these smart shelve s will have more options such as:
sending alert/warning signals, replenishment depending on
goods availability in the warehouse, urgency, duration of
out-of-stock.

Keywords -strain gauge,wheatstone bridge,load cell,PGA.

I. INTRODUCTION
Nowadays, technology is proliferating in almost every
industry, including the retail one. Shopping has become more
easier and comfortable because people have so many choices.
Also, they are more sophisticated having many claimes. In order
to fulfill people’s requirements and to solve one of the major
problems this industry is currently facing, meaning the loss of
customers due to out -of-stock items, to keep up with the
technology advancements and to face the stiff competition,
retailers must come with effective ideas. They hav e to think
about ways to optimize their business by enabling innovative
technologies like Smart Shelves technology. Currently, they
assign employees to verify the store shelves for depleted
products and to replenish them, but with this shelf inventory
system a lot of time would be saved for both customers and store
staff. The owners of the business are able to keep a real time
tracking on what products are being t aken from the store shelves
and what is remaining [1][2]. The concept behind the smart
shelves is to make the business run more efficiently and to
enable the inventory/store executives to :
➢ Inform warehouse and/or the shipment vendor
for immediate replenishment;
➢ Refill stocks from stock room as they get
depleted from the store shelf;
➢ Intimidate the customer about the demand and
out of stock situation.

The objective of this project is the implemetation of a system
that uses weight sensors (load cells), whose main purpose is to
monitor the shelf ’s stock. The sensors will detect the shelf ’s
weight and will transmit this information to a microcontroller.
With the help of a Wi -Fi module the received data will be
transmited to a computer. In this way, the person responsible
for securing the required stock of components will notice the
current stock . For a good communication, the distance
between the system and the computer will not exceed 150
meters.
Figure 1 represents the general work flow that is
taking place on the production line. There are different boxes
with components stored on shelves. The load cells will
monitor the stock of the shelf and will signal the stock
reduction. This signal is transmitted to the computer and the
person responsible for supplying the necessary components
will be notified.

Figure 1. Representation of the work flow

II. STRAIN GAUGE

The strain gauge is the sensitive element in load cells and it
cannot be expressed as a unit. They are used in a large variety
of physical measurements. Accelereration, pressure, tension
and force can be measured with the help of these strain
gauges. Typically, it is a metallic foil glued to a bar of metal
or a tensionable material. The strain gauge is positioned in
such a way that when the cell is loaded, the gauge is strained,
changing its resistanc e. By measuring the tension, the weight
applied to the load cell can be determined. Normally, if a piece
of metal is stretched, it becomes longer, hence the electrical
resistance will increase. In the case of strain gauges, it will be
shortened, meaning a lower resistance. Generally, strain
gauges use metal wires, continuous/discontinuous metal films,
semiconductors and cermets. The effect of the change in
electric resistance of these elements as a result of a strain is
usually named elastoresistance or pie zoresistance.

The Wheatstone Bridge was originally developed to measure
unknown resistance values and as a means of calibrating
measuring instruments, voltmeteres, ammeters, by the use of a
long resistive slide wire . In gen eral, the change of resistance
is small, hence the bridge has the main purpose to measure
these very low resistances values, therefore it is suitable for
measuring the resistance change in a strain gauge. Figure 23,
represents the configuration of the brid ge. The circuit consists
of four resistances connected in such a way to form a square
with two input and two output terminals. The Wheatstone
bridge is perfectly balanced when the output voltage is equal
with zero. If one of the four resistances is changin g, the output
voltage will also change. When all four resistances vary the
output wil l be linear and very sensitive[3 ].
Figure 2. Wheatstone bridge

The bridge’s branches are formed by the resistors R1, R2, R3,
R4. Usually, the nodes A(black) and B(blue) are connected to
the excitation voltage, therefore Vout will appear at nodes
C(white) and D(red). The output voltage value depends on the
relation (1).

(1)
When the bridge is perfectly balanced and

=
, the relation(2) is valid.

(2)
Under the assumption that the four resistors vary and the
resistance variation
is smaller than the resistance
, the
relation (3) is alw ays valid for metallic strain gauges.

(3)

III. LOAD CELL

One of the most used methods of converting weight into an
electrical signal is with a resistive load cell in a Wheatstone
bridge configuration. In this way, one or more resisto rs change
value in proportion to the load (weight) applied. To be able to
measure this effect, an excitation voltage source is applied
across the top and bottom of the bridge and the output signal is
measured as the differential voltage across the middle n odes.
Load cells are designed to measure the weight applied in one
direction, but there are many cases when the measurement is
done in other directions. In this case, the sensitivity of the
sensor differs, because some parts of the load cell which were
compressed are now in tension[4 ].

IV. PGA

A Programmable Gain Amplifier (PGA) is in most cases an
operational amplifier whose gain may be controlled by a
digital circuit. When it receives a signal, the analog to digital
converter quantizes that signal. It is ve ry important for most
communication systems optimizing the dynamic range of that
system. The PGAs which have fine programmable gain steps,
represent a good solution for reducing the noise and non –
linearity in CMOS proccesses[5 ].
The input analogic signal i s processed by cascaded
analog blocks and then is provided to the ADC.
In order to maximize the ADC dynamic range and
resolution, the input amplitude variation must be absorbed. In
this way, the PGA gain may be controlled with the helpof the
control word generated by the digital section.

V. Wi_Fi COMUNICATION

Wi-Fi (Wireless Fidelity) is a wireless networking technology
which is used to exchange informations between devices. It is
also called as local area network (LAN), allowing to
communicate without wir es. There are two ways to
communicate with Wi -Fi: through access point to the client
and second is client to client. The communication is made
using radio waves. They are transmitted from antennas and
routers, being picked up by Wi -Fi receivers which devic es like
computers and cell phones have. The Wi -Fi cards most
devices have will read the received signals and make an
internet connection between user and network. The speed of
the device wich is using this type of connection will increase if
the computer i s closer to the main source and also, the speed
will decrea se if the computer is further[9 ].

VI. CLIENT/SERVER
MODEL

The Client -Server model is represented by the architecure of
the Web. By dividing functions of sharing data into both client
and server, the application development time has been
minimized. The client is the one which is requesting an
information, while the server provides that service. Usually,

the data is processed by the server and the result is returned to
client.
Thus, the client -serve r model is defined as a software
architecture in which the client will request an information and
the server will pro vide that information. Figure 3 represents
the communication between the client and server. Also, they
can exchange data between them, each one having individual
functions.

Figure 3. Communication between client and server

VII. HX711 AMPLIFIER

When the load cell measures a certain weight, a load cell
amplifier is used in order to amplify the signal and convert it
into an output value. T he amplification is essential, otherwise
the signal is too small to be read. Figure 4 represents a way to
do this using the HX711 load cell amplifier.
This board has the HX711 IC which is used to read
the output of the load cell finding afterwards the m easured
weight. By connecting it to the microcontroller, the resistance
change will be read. In order to get an accurate measurement,
the calibration of the load cell is needed.
In order to obtain the resolution needed for the
measurement system, the rela tion ( 4) is used .

(4)
where Vexc is the load cell excitation voltage and n is the
ADC number of bits.

Figure 4 . Internal structure o f HX711 load cell amplifier

The multiplexer is used to select the differential
input, either channel A or B, to the programmable gain
amplifier (PGA). Channel A can be programmed with a gain
of 128 or 64, while channel B has a fixed gain of 32. The
clock input is from the on -chip oscillator which does not
require any external components. All controls to the HX71 1
are through the pins.

Figure 5. Data output, input/gain selection timing/control

The input channel and gain are controlled by the number of
CLK input pulses. Channel A may be configured for a gain of
128 or 64, and channel B for a gain of 32. The t ransmission of
clock pulses must be done correctly in order to have a reliable
communication.
The output 24 bits of data is saturated at minimum 800000h
and maximum 7FFFFFh.
The CLK (PD_SCK) signal has the main purpose to power
down the HX711 IC. When it i s low, the chip will work in
normal mode. If it is high, will remain so for about 60 µs and
then the HX711 is powered down. After the CLK turns back
to low, the chip will be reset, being again in normal operation
mode.

Figure 6. The slope graph

The out put 24 bits of data is in 2’s complement format. When
input differential signal goes out of the 24 bit range, the
output data will be saturated at 800000h (MIN) or 7FFFFFh
(MAX), until the input signal comes back to the input range.

VIII. EXPERIMENTAL
RESULTS

Based on the Wheatstone bridge and strain gauge
theories, Figure 56. ilustrates the tested load cell. The voltage
source V1 is actually the excitation voltage that is applied to
the load cell. The output voltage is represented by the positive
output sign al, Signal+ and by the negative signal, Signal -. The
two resistances noted as Ro -Rf, are tensioned strain gauges,
while the other two resistences noted as Ro+Rf, are
compressed strain gauges. When compression takes place, the
resitances of the two Ro+Rf de creases. In the same time, the
resistances of the two Ro -Rf will decrease because they are
stretched.

Figure 7. The unamplified load cell

The output of the load cell is influenced by the weight applied,
the excitation voltage and its rated output. For TAL220 load
cell, the rated output, also called the full scale output is 1
mV/V. In other words, for each volt of excitation voltage
applied at full scale, the output voltage is 1 mV. When 10 kg
are applied on the load cell, with the excitation voltage of 5V,
the output voltage will be equal with 5mV.
Figure shows the linear behaviour of the load cell when the
excitation voltage is a simple 5V source. The x -axis represents
the load applied and y -axis is the output voltage.

Figure 8. Linear characteristi c of the output voltage

0 1 2 3 4 5 6 7 8 9 100.0mV0.5mV1.0mV1.5mV2.0mV2.5mV3.0mV3.5mV4.0mV4.5mV5.0mV5.5mVV(signal+)-V(signal-)

In order to obtain the slope value, two cases were analysed:
1. In the first case, the slope was calculated for a load of
200g and 1200g as shown in relation(5 ).

(5)

2. In the second case, the slope was calculated for a load
of 200g and 1000g as shown in relation(6 ).

(6)

where 8494257, 8279334, 8451277 are the biggest load value
from HX711 amplifier expressed in decimal which result after
performing several readings.
The offset value was calculated using the relation(7 )
as follows:

(13)

IX. CONCLUSION
For the last years, the world has seen staggering
technological changes. These have a huge impact on our
everyday lives, being implemented in almost every aspect of
them and also in every industry. As all the othe r industries,
the retail one must keep up with the unprecedented
technological development that has taken place from the
beginning of the century until nowadays. With the
technologies changes, the humanity has changed too. We
have become bigger and pretent ious consumers. In order to
satisfy the customers needs, people from the retail industry
have to find ways to improve the in -store shopping
experience.
This paper had the aim to implement an innovative
shelf stock monitoring system named “Smart Shelf”. Th is
system has been introduced in the last years by some of the
biggest retailers in the world and some factories. For both, it
is very important to avoid the out -of-stock cases. By
implementing this system, a lot of time and human/money
resources are saved .

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