HARDWARE- WSN application in aggriculture

8/7/2019 HARDWARE- WSN application in aggriculture

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HARDWARE DESCRIPTION:

Keeping in mind the above factors the system was developed with the following

hardware components. It was necessary that best platform to be selected for the

project.

1. Sensor node:

The sensor node is a device that is capable of performing some processing,

gathering sensory information and communicating with other connected nodes in

the network.

For the TinyOS application there were compatible nodes. But the main features

that were to be considered are:

y Low power consumption

y High distance communication range

y Low cost

y Accessibility of support

y Resistance to rough weather

The best platform was tinynode 584 developed by Stockfish

According to tinynode user¶s manual version V.1.3 26.01.2011 PCA

The key elements of the node are discussed below:

y Microcontroller:

The microcontroller used in tinynode is MSP430 by Texas Instruments.

The MPS430 is ultra low power microcontroller. It has 10 kB of RAM,



48kB of flash and 128 B of memory. It has a powerful 16 -bit RISC CPU

with 16-bit register. The microcontroller has two 16-bit in built timer, a fast

12-bit A/D Converter, dual 12-bit D/A converter, universal serial

synchronous/asynchronous communication interfaces(USART), l2C, DMA

and 48 I/O pins.

y Transceiver:

The Xemics 1205 transceiver is used. It is low power transceiver and

operates at 868MHz. The date rate ranges from 38 kBps to 153.2 kBps

(76kBps default). The transceiver enables to adapt transmission power

from 5dBm upto 12 dBm, which reduces the consumption when nodes are

placed near each other. The node will be placed at 2 meter above the

ground so that RF line of sight is reached.

y Antenna:

The tinymode is supplied with a ¼ wave monopole wire antenna thatgives good performance when the wire is kept straight or bent with

enough distance from electrical mass. The antenna should not be bent

any closer than 20mm to PCB board.

y SMA antenna connector:

The back side of the tinynode PCB allows soldering a SMA connector for

connection of an external antenna or board to board connection of the RF

signal.

y External Flash:

Tinynode has a 4-Mbit flash (Atmel AT45DB011) for external data and

code storage. The flash holds 512kB of data. Ths flash shares SPI

communication with the XE1205 transceiver.

2. Sensors:

The sensors used in the project will collect two different types of information

about the water content in the soil:

Volumetric moisture content or volumetric water content with soil moisture

sensor.

Soil matrix potential with watermark sensors (associated with temperature

sensor)

Both will be measured at two depths below the ground: 10cm and 30cm.

i. Soil moisture sensor:

Volumetric moisture content is defined as the fraction of water volume

contained in the total volume.

VMC= Vw/ (Vw + Vs)

Where Vw= volume of water

Vs= volume of soil



The sensor selected for this application was EC-5 sensor from Ech20. It

measures the dielectric constant of soil in order to find its volumetric water

content. Since the dielectric constant of wate r is much higher than of air or

soil, the dielectric constant of soil is sensitive measure of water content.

The sensor gives output related to the dielectric constant. The ADC is 12-bits

and Vcc is 2500 mV, the actual output voltage is given by V= output*2500 /

(2^12) mV. To obtain VWC from this voltage, there are equations for different

types of soil. Example, for all types of mineral soil having electrical

conductivities from 0.1 dS/m to 10 dS/m the following equation is used:

VWC=11.9*10E-4*V ± 0.401 where V is actual voltage output.

ii. Watermark sensor:

The volumetric moisture content gives information about water content in the

soil. But it does not give information how easily that water can be used by the

plants. Beneath the soil surface, there is soil particles, water, air, plant roots.

The water cannot move freely as on the surface. It is affected by soil

structure. Water is naturally attracted by the soil partic les by the phenomenon

called capillarity. The smaller the capillary between soil particles is , the

harder it will be for the plant to extract the water from it. Thus, when the soil is

wet, all the large capillaries inside the soil are filled with water, w hich makes it

very easy for the plant to extract. On the contrary, when the soil is very dry,

the remaining water is contained in the very small pores and is therefore very

difficult for the plants to extract.

The phenomenon is quantified by the parameter called soil matrix potentialand is defined as the force required to remove water from soil per surface

unit.

The SMP is measured using resistive sensors. A resistive sensor is porous

block made of gypsum or fibreglass. The block contains two electrodes

connected to a conductive wire. When buried in the soil the water is free to

move in and out of the sensor, until it is equilibrium with the soil moisture. The

electrical resistance of gypsum block then depends on soil matrix potential.

Watermarks are basically ameliorated gypsum blocks. The granular matrix is

surrounded by a synthetic membrane for protection against deterioration. The



watermark sensor output is electrical resistance. The resistance depends on

the water contained in the pores of gypsum block of sensor.

The watermark sensor used in the project was irrometer watermark soil

moisture sensor

Two steps are required to be able to interpret the Soil Matrix Potential (SMP)from the Watermark sensor. The first step consists in reading the electricalresistance of the Watermark sensor; the second one is the conversion fromthe electrical resistance (in kOhm) into the actual SMP value (in kPa).A non-linear equation has been developed to convert the electricalresistance (in kOhm) of the Watermark sensor into Soil Matrix Potential (in

kPa). This conversion equation is the one used by the Watermark ele ctronicreader from Irrometer.

SMP is the Soil Matrix Potential in [kPa], R is the Watermark resistancevalue in [kOhm] and TS is the estimated or measured soil temperature in [°C]near the probe.

However, the coefficients of this equation can be soil -specific; these that areappropriate for a clay soil may not be appropriate for a sandy soil. For a veryaccurate application Watermark sensors should be calibrated in order to findthe optimum coefficients. The conversion equation takes into account theeffect of temperature. As a matter of fact, as temperature increases theWatermark resistance decreases. Thus, variations in soil temperature canaffect Soil Water Potential readings by 1 to 3% per °C. Moreover, the dryer the soil is, the larger the effect of temperature becomes. It is thereforenecessary to take into account the effect of temperature.



3. Sensor board:

The tinynode needs an interface board that provides a power source, circuits and

connectors to the sensors.

Two sensorboard that was use, was not as same provided by shockfish

because both the sensors are interconnected at the excitation line. So the valuechange in one sensor causes change in the other.

The sensorboard for the watermark sensor is shown below:

The sensorboad has

o Two connectors and driver circuit for EC-5 soil moisture sensor

o Two connectors and driver circuit for watermark sensor

o Socket for AA lithium 3.6V battery.

Tinynode socket:

The socket schema having I/O leds, sensors, power source and ground is

shown below:



Sensor Driver circuit:

The circuit enables simple DC voltage application on the probe and then to

sensor

Driver circuit for EC-5:

Driver circuit EC-5 1

Driver circuit EC-5 2



Driver circuit for watermark sensor:

The watermark sensor required AC supply voltage, for this purpose we used a

regular 555 timer.

Watermark sensor driver circuit 1

Watermark sensor driver sensor 2

4. Battery:

The power source used in the node was AA 3.6V battery( Model:ER14505H)

which could be fitted in the socket. Moreover the battery has 2700 mAh.



5. Box:

The box is necessary as the system will be used in the rural areas with robust

environmental condition. The conditions such as humidity and temperature may

cause short circuits which can damage the circuit. For this purpose, polycarbonate

case from FIBOX was used. This box is dust tight and water proof. Pressure

equalizer plug was also included to avoid condensation inside the system, which

can be harmful to the system.

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HARDWARE- WSN application in aggriculture