Wireless Sensor Network, 2011, 3, 318-321
doi:10.4236/wsn.2011.39034 Published Online September 2011 (http://www.SciRP.org/journal/wsn)
Wireless Sensor Network for Monitoring Maturity
Stage of Fruit
Monai Krairiksh1, Jatuphong Varith2, Apichan Kanjanavapastit3
1Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Bangkok, Thailand
2Department of Agricultural and Food Engineering, Faculty of Engineering and Agro-Industry,
Maejo University, Chiangmai, Thailand
3Department of Telecommunication Engineering, Faculty of Engineering,
Mahanakorn University of Technology, Bangkok, Thailand
Received August 2, 2011; revised August 26, 2011; accepted September 5, 2011
Abstract
In this letter, we present a wireless sensor network for monitoring the maturity stage of fruit. A dual-polari-
zation coupled patch sensor, which is robust to environmental changes, was designed to operate at 2.45 GHz.
It was attached to a Durian fruit for a period of days to measure the magnitude of mutual coupling corre-
sponding mainly to the starch concentration of its pulp. Signal was transmitted from a sensor node, via tree
nodes, to a master node that displays the variations occurring in the period. The maximum mutual coupling
occurred at the maturity stage of 60% whereas the minimum occurred at 70%. These results demonstrate that
this wireless sensor network can enable fruit growers to harvest their Durians at an appropriate time, provid-
ing a reliable quality control for export.
Keywords: Fruit Sensor, Maturity Stage Monitoring, Wireless Sensor Network, Pre-harvest Control
1. Introduction
As climacteric fruits ripen, the starch in their pulp turns
into sweet-tasting sugar. It is necessary to harvest them
at an appropriate time, i.e., harvesting them too early and
they will no longer ripen properly, harvesting them too
late then they must be consumed within a few days be-
fore they are spoiled. Timing the harvest for sufficient
shipping time for export is absolutely crucial. Durio
zibethinus Murray (Durian) is a major export fruit that
brings into Thailand large amount of income every year.
The conventional method for monitoring the maturity
stage of Durian is by counting the number of days after
full-bloom. However, the results are often inaccurate due
to variations in the environment, e.g., temperature and
humidity [1]. Therefore, a number of research projects
have been conducted to develop sensors for monitoring
the maturity stage of Durian [2-4]. Yet, these are gener-
ally only used to monitor post-harvest products.
This work presents a wireless sensor network for
monitoring the maturity stage of Durian in the pre-har-
vest monitoring scheme.
2. Dual-Polarization Coupled Patch Sensor
It has been shown in [5] that starch and reducing sugar in
Durian pulp vary, respectively, proportionally and in-
versely proportionally to the days after full-bloom. The
peel is relatively unchanged. These changes gradually
decrease the dielectric constant of the pulp in the days
following full-bloom [6,7].
Due to severe environment during Durian-growing
season, which is highly humid with heavy rains, a
dual-polarization coupled patch was selected to ensure
that measurements were robust to environmental changes.
For the sensor, using coupled patch antenna, we simu-
lated a Durian fruit as a pulp covered by a prickle peel
with CST microwave studio software [8]. The Durian
was modeled as an ellipsoid, see Figure 1(a), with the
major and minor axis of 30 cm and 20 cm, respectively.
This is the standard size for export Durian. The thickness
of the prickle peel was 1 cm. Figure 1(b) shows the dia-
gram of the du a l - polarizati on co upled patc h sensor which
was fabricated on an FR-4 substrate 1.6 mm high. The
dielectric constant of the substrate was 4.4. The right

M. KRAIRIKSH ET AL.319
(a)
8.498 cm.
2.911 cm.
1.456 cm.1.456 cm.
0.728 cm.
0.728 cm.
0.728 cm.
4.716 cm.
0.633 cm.
1.238 cm.
0.728
cm
0.728 cm
0.728
cm
(b)
Figure 1. Durian model and coupled patch sensor (a)
Durian model; (b) Dual-polarization coupled patch sensor.
patch was used for transmitting a microwave frequency
of 2.45 GHz whereas the left patch was used for receiv-
ing the coupled signal. The probe along the same line
with the transmitting probe was used for receiving the
parallel polarization signa l whereas the one below it was
used for receiving the perpendicular polarization signal.
The dielectric constant of the peel was kept constant at
15 whereas the dielectric constant of the pulp was varied
from 62 to 22. The variations of the magnitude of mutual
coupling for parallel polarization are shown in [9]. They
are plotted together with those for perpendicular polari-
zation as shown in Figure 2. It can be seen that the mu-
tual coupling decreased as the dielectric constant de-
creased from 62 to 37, but then increased slightly after-
wards. Thus, the variation in mutual coupling can be
used as an indicator for monitoring the maturity stage of
Durian. It should be noted that the perpendicular polari-
zation showed higher mutual coupling due to the effect
of near-field region of the antenna.
3. Wireless Sensor Network
The architecture of the wireless sensor network is shown
in Figure 3. It shows sensor nodes attached to Durian
fruits sensing the magnitude of the mutual coupling of
the fruits and sending signal to a tree node installed on
each tree. The signals were then sent from each tree via
Dielectr ic constant of durian pulp
Figure 2. Simulation of parallel and perpendicular polari-
zation mutual coupling when dielectric constant of pulp was
varied (dielectric constant of peel was fixed at 15).
Figure 3. System architecture.
other tree nodes to a master node using a routing proto-
col similar to the directed diffusion method [10]. For a
large tree, in order to overcome the path loss on the tree,
it was possible to install several tree nodes on the tree.
Each tree node oversaw a number of sensor nodes and
communicated via other tree nodes to the master node. In
order to keep the cost low, readily available wireless
communication modules from local market were used for
both sensing and communications. The operating fre-
quency was 2.45 GHz. An experiment was set up to
measure the path loss on a tree and a link budget was
calculated for the system using a TRW2.45 communica-
tion module operating at 2.45 GHz. It had an output
power of +4 dBm and a sensitivity of –80 dBm. From the
measurement, the path losses at various positions on the
tree from the stem 1.5 m above ground were around 65
dB. It was evident that a sufficient margin of 27 dB was
obtained.

M. KRAIRIKSH ET AL. 320
4. Experimental Results
A dual-polarization coupled patch sensor was designed
as detailed above. It was equipped with two RF switches
as shown in Figure 4. The first RF switch (SW01) was
connected with the transmitting patch and a monopole
antenna for communications. The TRW2.4 5 was used for
transmitting 2.45 GHz to the sensor and the monopole
whereas the second RF switch (SW02) was connected to
a MAXIM MAX4004 power detector and the parallel
and perpendicular polarization ports of the receiving
patch. After converted to digital signal by a 12 bit ADC,
the signal was input into an MCS-51 microcontroller
which was used for controlling RF switches. Communi-
cations on a tree was tested on a 20-meter high Durian
tree at Nakornsrithamarat, Thailand, in July 2009.
Eight sensors were utilized for measuring the mutual
coupling of eight fruits of the same size. Measurements
were conducted during 10 am to 12 pm every day since
the humidity was low and the rain was not as heavy as
that in the other periods of the day. The averaged results
of coupling voltage of parallel and perpendicular polari-
zation from the ADC throughout the data collecting pe-
riod are plotted in Figure 5. It is appa rent that on the 94 th
day after full-bloom, the coupled voltage increased from
1.9 V and 2.2 V for the parallel polarization and perpen-
dicular polarization, respectively. It reached the maxi-
mum on the 97th day where the parallel polarization and
perpendicular polarization were 2.15 V and 2.4 V, re-
spectively. The coupled signals then decreased gradually
until the minimum was reached on the 100th d ay. Then, it
happened to increase again. Please note that, if only the
parallel or perpendicular polarization was used, there
might be an error since the coupled voltage for the paral-
lel polarization was increasing while that for the perpen-
dicular polarization was fluctuating, probably due to en-
vironmental changes. During the 94th - 97th days, the
variations in the parallel polarization were quite large
compared to those in the perpendicular polarization. On
TRW
2.45 GHz
Power
Detector
ADCMCS -51
SW02
SW01
2.45 GHz
Antenna
Sensor antenna
TX RX
Figure 4. Diagram of a sensor and communication system.
25
30
35
40
50 60 70 80
Maturity stage (%)
% Dry-weight
1.5
1.7
1.9
2.1
2.3
2.5
2.7
93 94 9596 97 98 99100101102103104
Day after full - bloom
C
o
u
p
l
i
n
g
V
o
l
t
a
g
e
f
r
o
m
A
DC
(
V
o
l
t
)
y = 0.088x-6.08
R2 =0.678
y = -0.185x+20.3
R2 =0.979
y = 0.145x-12.7
R2 = 0.748
y = -0.191x+20.94
R2 = 0.905
y = 0.078x-5.92
R2 = 0.128
y = -0.179x+19.52
R2 = 0.985
y = 0.078x-5.33
R2 = 0.477
y = 0.069x-4.58
R2 = 0.297
y = 0.212x-19.48
R2 = 0.873
co-polarization
cross-polarization
averaged
Figure 5. Measurement results.
the other hand, during the 100th - 104th days, the parallel
polarization was increasing whereas the perpendicular
polarization was decreasing and then increasing slightly.
When the results of the parallel and perpendicular po-
larizations were averaged, the measurement reliability
increased. The averaged results of both polarizations
provided more reliable results than that from each single
polarization measurement. Together, the maximum and
minimum coupled signals on the 97th and 100th days after
full-bloom were a good indicator of the maturity stage of
Durian.
During this data collecting period, three Durian fruits
of the same age were sampled every three days to inves-
tigate their physical properties, i.e., starch concentration,
reducing sugar concentration, color, etc. This informa-
tion was used to evaluate the maturity stage o f Durian by
comparing it with the conventional method in [2]. The
horizontal axis of Figure 5 shows the maturity stage
corresponding to the days after full-bloom whereas the
vertical axis shows the coupling voltage from the ADC
and% dry-weight of Durian pulp. Figure 5 shows that
the% dry-weight of 29% and 32% corresponds to the
days when the maximum and minimum coupling took
place. It also shows that the maturity stage of 60% - 70%
was indicated by the % dry-weight of 29% - 32%. Hence,
the transition of the couplin g voltage from the ADC from
the maximum to the minimum represents the maturity
stage of 60% - 70%.
It is recommended that sensors be installed on the
Durian fruits that are representatives of Durians of the
same season around the 94th day after full-bloom, which
corresponds to 50% maturity stage. Coupling voltage
should be measured every day. The maximum voltage
indicates the maturity stage of 60%. The results of this
experiment show that the maturity stage increases from
60% to 70% within three days. It should be pointed out

M. KRAIRIKSH ET AL. 321
that a harvesting date can also be predicted by calculat-
ing the slope of the measured mutual coupling. And
when the slope changes from negative to positive, it is
time that Durians should be harvested.
According to the conventional method of counting the
days after full-bloom, Durian is at the maturity stage of
75% around 120 days after full-bloom [1]. However,
Figure 5 shows that, actually, on the 101st day, the ma-
turity stage was 75% already. Therefore, under some
conditions, it is quite possible that fruit growers may
harvest Durian at the wrong time. The proposed sensor
may be a good alternative fo r pre-harvesting q uality con-
trol of this fruit.
5. Conclusions
A dual polarization coupled patch antenna for sensing
maturity stage of Durian was proposed. The averaged
mutual coupling signal can indicate the appropriate time
for harvesting Durians, which is absolutely essential for
controlling its quality for ex port. Th is work demonstrates
an application for Durian quality control. Future work
will apply to other climacteric fruits.
6. Acknowledgments
This work was supported by the National Research
Council of Thailand (NRCT) and the Thailand Research
Fund (Grant No.RTA 5180002). The authors ap preciate P.
Sooksumrarn, T. Limpiti, P. Yoiyod, P. Leekul and
T.Tantisoparak for implementing the system and data
collection.
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