Comparison Of Waspmote And Arduino

Published Date: 02 Nov 2017

Chapter 1

Concern over health issues and environmental pollution health issues has driven legislation over the past two decades and significant development and research efforts have been undertaken to look into the environmental issues. Worldwide research for many years in the field of gas sensors has been pushed by the demand to minimize gases emissions from various on-road engines and industrial sources. In particular, oxygen (O2) sensors have its prominent role in pollution control via biological and food processing plants, vehicle engine management and control of chemical processes. Based on the number of sensors in operation, the predominant use of oxygen sensors is in the control of air-fuel mixture in the combustion engine of the automobiles and is an integral part of the ‘on board diagnostic’ (OBD) of the exhaust emission control system.

On the other hand, Ozone sensors are in need to monitor the pollution caused by the use of ozone nowadays. The use of ozone gas is on the growth rapidly in a broad variety of applications, such as food processing, odor control and water treatment. Commercial ozone generators mimic these natural processes to produce large amounts for environmental and industrial treatment processes or drip a little ozone liquid to the air for its brisk effect and refreshing scent. However, this similar high oxidizing potential induces ozone to damage respiratory tissues and mucus in animals as well as tissues in plants with reaching above concentrations of approximately 100 parts per billion (ppb). This makes ozone a pollutant and potent respiratory hazard near ground level. On the other hand, tropospheric ozone (or low level ozone) is an atmospheric pollutant. It is not emitted directly by industrial operations or by automobile engines , but formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that react to form ozone directly at the source of the pollution or many kilometers down wind. Because the ozone generators have been widely in use, gas leak detection is certainly in a dire need near ozone application points or the generators.

1.2. Objectives

The objective of carrying out this project is to build a data acquisition system to measure the parameters of the concentrations of gases O2 and O3. Then, we get to construct and write programming thereby familiarizing programming C and Labview. Subsequently, another objective has to be attained in such a way that the output data is converted into the desired parameters through Labview and easily apprehended by the end users. Last but not least, all in all, the system built throughout the project has to be tested for its reliability.

1.3. Overview of Project

I carry out the data acquisition of the concentration of Oxygen (O2) and Ozone (O3) with the use of SK25 Oxygen (O2) sensor and MiCS-2610 Ozone (O3) sensor with the Waspmote gases sensor board manufactured by Libelium. In additional, the auxiliary parameters are also concerned to be analyzed with the assistance of 808H5V5 humidity sensor and MCP9700A temperature sensor.

C programming language has to be generated in order to communicate Waspmote sensor board with the computer via USB port connection. C has been applied in this project as the main communication medium as it was designed to be compiled using a relatively straightforward compiler, to provide low-level access to memory, to provide language constructs that map efficiently to machine instructions, and to require minimal run-time support. The C language is then executed through Waspmote IDE which is a platform to input the instructions from the computer to the Waspmote sensor board.

Last but not least, the output data received through the USB port connection is then converted into the desired parameters through Labview and easily apprehended by the end users. Labview is a graphical programming language that was developed to make it easier to collect data from laboratory instruments using data acquisition systems. LabVIEW is used in this project to process, filter and transform data as well as analyze the output data of the project.

Chapter 2

Literature Review

2.1. Measuring Methods for Gases O2 and O3

Nowadays, there are numerous useful sensors available in the market to capture the concentration of gases oxygen and ozone for many industrial usages and environmental issues as mentioned in the previous chapter. Therefore, in this paper, we have made up our minds to select SK25 oxygen sensor and MiCS-2610 ozone sensor to probe into our doubts.

(Honeywell, June 2008) While there exists other oxygen sensor products having the similar competing standards with the mentioned above have previously been put on the list. KGZ-10 Series oxygen sensor by Honeywell applies the dynamic sensing principle allowing failsafe operation. Electronics necessary to operate the sensor can be incorporated into the customer’s own electronic circuits. High resistance to corrosion allows the sensor to be used in aggressive, harsh environments. Figure 2.1 shows the KGZ-10 series of oxygen sensors produced by Honeywell.

Figure 2.1: KGZ-10 series oxygen sensor

The dissolved oxygen sensor DO1200 and DO1200TC by Sensorex feature dependable galvanic technology. No warm up period is required and thus the results can be obtained right after everything is set. Sensors are supplied with fast responding high-density polyethylene (HDPE) membrane caps for portable or lab use. Optional Polytetrafluoroethylene (PTFE) membrane caps are available for on-line or heavy UV exposure applications. Fill solution is replaceable when required. Figure 2.2 shows the DO1200 and DO1200TC oxygen sensor produced by Sensorex.

Figure 2.2: DO1200 and DO1200TC oxygen sensor

(J. Dittrich, 2005) The oxygen measuring system MF010-O-LC developed by J.Dittrich determines the oxygen content in gas mixtures up to a temperature of 250oC. It is particularly suitable for the automatic control of furnaces (exhaust gas temperature max. 250 oC) and for measuring oxygen in areas that are not easily accessible or in closed systems (ventilation pipes, containers, etc.). Standard measuring range is from 0.1 to 25 vol. % oxygen. The oxygen sensor is mounted in the head of the bar probe and is protected by a stainless-steel sintered disk which serves as flame back-flash stop. The water-proof plastic housing accommodates the electronics and is connected to the bar probe by mechanical means. Figure 2.3 shows the MF010-O-LC oxygen sensor developed by J.Dittrich.

Figure 2.3: MF010-O-LC oxygen sensor

(Futurlec, 2012) MQ-131 ozone sensor by Futurlec has been composed by micro Al2O3 ceramic tube. It is also equipped with metal-oxide semiconductor sensitive layer. Measuring electrode and heater are fixed into a crust made by nylon and stainless steel net. The heater provides necessary work conditions for work of sensitive components. The enveloped MQ-131 has 6 pins, 4 of them are used to fetch signals, and other 2 are used for providing heating current. This sensor is normally used in air quality control equipment for buildings and offices. Figure 2.4 shows the MQ-131 ozone sensor by Futurlec.

Figure 2.4: MQ-131 ozone sensor

Besides, O3E1 ozone gas sensor by Sensoric - G is an amperometric electrode sensor cell using an organic gel electrolyte, equipped with a Transmitter Board. This small device converts the raw sensor signal of the O3 electrochemical sensor cell into a standard 4-20 mA output or into a 40-200 mV voltage output. The sensor is calibrated by the manufacturer in the range 0 – 1000 ppb. (Michel Gerboles and Daniela Buzica, 2006) The sensor is normally used for environmental and indoor air monitoring. However it is way too pricey compared to the SK-25 sensor we use in this project. Figure 2.5 shows the O3E1 ozone gas sensor by Sensoric - G.

Figure 2.5: O3E1 ozone gas sensor

(Michel Gerboles and Daniela Buzica, 2006) The SENS3000 semi-conductor based ozone gas sensor (UNITEC - I) is a thick film solid state sensor placed in anodized aluminium case. The sensible surface of the sensor is a semiconductor oxide (the metal oxide type is not disclosed by the manufacturer) made of nano-particles of the size of 200 μm. The first reaction which happens on the surface of the sensor is the absorption of the atmospheric oxygen and the consequent charge transfer from the semiconductor to the oxygen molecule. The second reaction is related to the specific gas to be measured, which while linking to the oxygen molecule allows the electron to be released in the conduction band of the semiconductor. Taking the current signals from the sensor, the direct concentration of the specific gas in atmosphere can be measured. Selectivity and precision are reached using special semiconductor oxides with appropriate filters. The output analogic signal from the sensor needs to be converted into concentration using a known function. Figure 2.6 shows the SENS3000 semi-conductor based ozone gas sensor by UNITEC - I.

Figure 2.6: SENS3000 semi-conductor based ozone gas sensor

2.2. Embedded Systems

A microprocessor-based embedded system is incorporated into a device to control and monitor the functions of the components of the device. The embedded systems are used in many devices, ranging from a microwave oven to a nuclear reactor. Besides, the embedded systems are designed for performing specific tasks, unlike personal computers that run a variety of applications. (Parineeth M Reddy, 2002) An embedded system used in a device is programmed by the designers of the system and generally cannot be programmed by the end user, for instance, the embedded system in washing machine that is used to cycle through the various states of the washing machine.

(Dejan Milojicic, 2012) Embedded systems developed with all-in-one computer systems. They were at the forefront of the use and justification of computer products, the closest to the field. Flashing back to the past, most of the embedded development and research was identified with industrial and real-time systems settings. With wider transformations of computers, the embedded systems induce the increment in need. We can always spot this little tiny board in any aspects of our lives, no matter in most house appliances, tools, electrical devices, cars, and industrial devices.

LibeliumWaspmote

(Libelium, 2012) Waspmote is an open source wireless sensor platform uniquely focused on the performance of low consumption modes to enable the sensor nodes ("motes") to be completely autonomous and battery powered, offering a variable lifetime between 1 and 5 years depending on the duty cycle and the radio used. In addition, Waspmote is based upon a modular architecture which is to synthesize only the modules needed in each device to optimize costs. Because of this reason, all the modules (sensor boards in this case) plug in Waspmote through sockets. (Libelium, 2012) Besides, Waspmote works with different protocols (ZigBee, Bluetooth, GPRS) and frequencies (2.4GHz, 868MHz, 900MHz) which enables it getting links up to 12km. (Libelium, 2012) In addition, it counts with a hibernate mode of 0.7μA which allows to save battery when it is not transmitting. Not less than 50 compatible sensors already available and a complete open source IDE (API libraries + compiler) make it a quick start working with the platform. Figure 2.7 shows the main Waspmote components.

Figure 2.7: Main Waspmote components

Arduino

Arduino is a family of microcontrollers (tiny computers) and a software creation environment that makes it easy for us to create programs (called sketches) that can interact with the physical world. Inventions we make with Arduino can sense and respond to light, sound, touch, position, and heat. This type of technology is used in all kinds of things from the smartphones to automobile electronics systems. Arduino makes things become easy - even people with neither electronics nor programming experience to use this complex and rich complex technology. (Margolis, 2011) Arduino is based on the Atmel AVR microcontrollers, such as the ATmega328. The ATmega328 is an 8-bit MCU with on-chip memory in the form of 32K flash, 2K SRAM, and 1K EEPROM. Besides, the processor in the Arduino designs is generally clocked at 16MHz. The board can only control and respond to electricity, so particular components are appended to it to interact with the real world. These components can be sensors, which convert physical quantities to electricity so that the board can sense it, or actuators, which retrieve electricity from the board and transform it into something that make changes to the world. Examples of sensors include switches, accelerometers, and ultrasound distance sensors. Actuators are things like lights and LEDs, speakers, motors, and displays. (Margolis, 2011) On the other hand, the Arduino IDE is an open source cross-platform tool using Java language. This is just a set of function calls that gets translated into C, though we can program in the "Arduino language", so we can also program in C or C++. There are a wide range of Arduino-compatible boards produced by members of the community. The most common boards comprises a USB connector which is utilised to supply power and provide connectivity for uploading one’s own software onto the board. Figure 2.8 shows a basic board that most people start with, the Arduino Uno.

Figure 2.8: Arduino Uno basic board

National Instruments (NI) Single-Board RIO

(National Instruments, 2012) NI Single-Board RIO embedded acquisition and control devices synthesize the three main components of an NI Compact RIO system - field-programmable gate array (FPGA), processor, and Input/Output (I/O) - on a single printed circuit board (PCB) and peripherals such as USB, RS232, RS485, CAN, SD, and Ethernet, all on a single board.. This integration maintains the benefits of the proven reconfigurable I/O (RIO) architecture while minimizing costs. (National Instruments, 2012) NI LabVIEW FPGA Module graphical programming tools make less effort to customize the hardware and I/O with advanced control, custom timing, and inline processing. We also can use the LabVIEW Real-Time Module to create deterministic, reliable embedded applications for data logging, network communication, and floating-point processing. NI Single-Board RIO devices are designed for original equipment manufacturer (OEM) and high-volume embedded control and acquisition applications that require reliability and high performance. Figure 2.9 shows the NI Single-Board RIO.

Figure 2.9: NI Single-Board RIO

Comparison of Waspmote and Arduino

As NI Single-Board RIO devices are designed for original equipment manufacturer (OEM) and high-volume embedded control and acquisition applications that require reliability and high performance which are apparently more conducive for industrial usages, hence, we only compare the Waspmote and Arduino embedded system devices.

Firstly, Waspmote and Arduino platforms are compatible. Waspmote uses the same IDE (compiler and core libraries) with Arduino. (Libelium, 2012) For this reason the same code could be compatible and utilised in both platforms by just adjusting tiny commands like the pinout and the I/O scheme. An Arduino user may work with Waspmote in a transparent and easy way as the source code will be the same the learning curve does not exists. When talking about the costs, Arduino has an advantage in it. (Margolis, 2011) Arduino, so-called ‘inexpensive embedded systems’, have apparently been sold for use in prototyping and in a range of low-volume products. Waspmote has it a reason to be sold slightly higher in price due to its strengths in memory storage, battery consumption, etc. In term of memory storage, Libelium has provided its client a 2GB of external storage within a SD card while Arduino does not privilege its client from this aspect. In addition, Waspmote provides internal RAM of 8KB but Arduino, for instance Uno, only gives out 2KB in its board. One will see the memory value reduce as the size of the string is increased each time through the loop. If one runs the sketch long enough, the memory will run out. Writing code like this that creates a constantly expanding value is a sure way to run out of memory. This is another reason to make us resist Arduino. (Libelium, 2012) Last but not least, Waspmote has definitely beaten Arduino in the race of battery consumption where Waspmote only utilise constantly 9mA compared to Arduino which consumes up to 50mA. Moreover, one may set Waspmote to sleep mode and hibernation mode while the Arduino users have to suffer due to the absence of these functions.

3.1. Sensor Principle

(Jon S. Wilson, 2005) Sensors convert physical world into electrical signals. Due to this, sensors are part of the interface between the the world of electrical devices and physical world, such as computers. In short, a sensor receives a stimulus and responds with an electrical signal. In this section, we will talk about the theory behind and the operating principles of each sensor involved in this project.

3.1.1. Galvanic Cell Typed Oxygen Sensor

The galvanic-cell oxygen sensor is a lead-oxygen battery which incorporates a lead anode, an oxygen cathode made of gold, and a weak acid electrolyte. The gold electrode is bonded onto a non-porous Teflon FEP membrane. Oxygen molecules enter the electrochemical cell through a non-porous fluorine resin membrane and are reduced at the gold electrode with the acid electrolyte. [15] A resistance and a thermistor for temperature compensation are connected between the cathode and the anode and therefore, the lead-oxygen battery is always discharged. The current which flows between the electrodes is proportional to the oxygen concentration in the gas mixture being measured. The terminal voltages across the thermistor (for temperature compensation) and resistor are read as a signal, with the change in output voltages representing the change in oxygen concentration. Figure 3.1 shows the structure of Galvanic cell typed Oxygen sensor. The following chemical reactions which take place in oxygen sensors:

Cathodic reaction : O2 + 4H+ + 4e- → 2H2O

Anodic reaction : 2Pb + 2H2O → 2PbO + 4H+ + 4e-

Total reaction : O2 + 2Pb → 2PbO

http://www.gs-yuasa.com/gyid/us/products/ke_series/img/structure_s.jpg

Figure 3.1: Structure of Galvanic cell typed O2 sensor

A small volume air bubble is contained inside the sensor body in order to compensate for internal influence from pressure changes. The sensor's electrolyte is primarily composed of acetic acid with a pH of approximately 6. Fluorine resin membrane affects the diffusion speed of oxygen molecules and, as a result, the response speed and life of the sensor. [16]

3.1.2. Metal Oxide Semiconductor Ozone Sensor

For more than four decades, it has been known that the electrical conductivity of semiconducting metal oxides varies with the composition of the gas atmosphere surrounding them. Since 1980´s, there have been many efforts done to develop ozone sensors based on metal oxide materials. According to the working temperature, such ozone sensors can be classified as either high-temperature or room-temperature ozone sensors. As the ozone sensing materials, In2O3, ZnO, SnO2 and CeO2 have been used.

The gas sensor is composed of sensing element, sensor base and sensor cap. The sensing element contains sensing material and heater to heat up sensing element. Depending on the target gas, the sensing element will utilize different materials such as Tin dioxide (SnO2). A heating element is integrated to regulate the sensor temperature for two purposes: to reactivate the gas sensor by gas desorption at high temperatures, and to detect different gas species as the sensors exhibit different gas response characteristics for different temperature ranges. Usually, at room temperature there is only a slow and not appreciable interaction of the surrounding gas with the surface.[17]

The boundary layer theory is well suited and widely accepted for the qualitative description of the processes and the prediction of the sign of the change in conductivity of metal oxide in the presence of a reducing or oxidizing gas. As shown in Figure 3.2, as a n-type metal oxide surface or the grain surface such as SnO2 is exposed to an oxidizing (reducing) gas, chemisorbed particles cause a localized energy level within the band gap of the metal oxide, acting as electron acceptor (donor).

Figure 3.2: Classical gas sensor with integrated heating system

This results in a charged layer of electron depletion (accumulation) at the surface leading to a compensating boundary layer which prevents carriers from moving freely and to the formation of surface potential Vs. Inside the sensor, electric current flows through the conjunction parts (grain boundary) of SnO2 micro crystals. The electrons must overcome the electronic barrier to go across the grain boundaries. [17] Figure 3.3 (a) and (b) indicate the surface charge of n-type metal oxide in the presence of an oxidizing and reducing gas while (c) and (d) refer to the corresponding band bending on the n-type metal oxide surface.

Figure 3.3: N-type metal oxide surface and energy value of surface barrier

Then, the conduction of the sensing layer can be approximated with the help of the Schottky model by

[3.1]

where is the conductivity of the semiconductor material and is the bulk conductivity, which depends on the temperature and geometric properties of the layer.

The relationship between sensor resistance and the concentration of deoxidizing gas can be expressed by the following equation over a certain range of gas concentration:

Rs = A[C] –α [3.2]

where Rs is the electrical resistance of the sensor, A is constant, [C] is the target gas (Ozone) concentration and is the slope of Rs curve.

3.1.3. Capacitive Humidity Sensor

Capacitive relative humidity (RH) sensors are widely used in industrial, commercial, and weather telemetry applications. Humidity sensors based on capacitive effect consists of a hygroscopic dielectric material sandwiched between a pair of electrodes forming a small capacitor. Most capacitive sensors use a plastic or polymer as the dielectric material, with a typical dielectric constant ranging from 2 to 15. In absence of moisture, the dielectric constant of the hygroscopic dielectric material and the sensor geometry determine the value of capacitance. [18]

At normal room temperature, the dielectric constant of water vapor has a value of about 80, a value much larger than the constant of the sensor dielectric material. Therefore, absorption of water vapor by the sensor results in an increase in sensor capacitance. At equilibrium conditions, the amount of moisture present in a hygroscopic material depends on both the ambient temperature and the ambient water vapor pressure. This is true also for the hygroscopic dielectric material used on the sensor. By definition, relative humidity is a function of both the ambient temperature and water vapor pressure. Therefore there is a relationship between relative humidity, the amount of moisture present in the sensor, and sensor capacitance. [19] This relationship governs the operation of a capacitive humidity instrument. Figure 3.4 shows the basic structure of capacitive type humidity sensor.

A capacitive humidity sensor changes its capacitance based on the relative humidity (RH) of the surrounding air. As the relative humidity increases the capacitance also increases. Relative humidity is the percentage of actual vapor pressure (P) compared to saturated vapor pressure (Ps). [19]

[3.3]

Figure 3.4: Basic structure of capacitive type humidity sensor

3.1.4. Temperature Sensor

A Thermistor is an economical means of precisely sensing heat over a limited range of temperatures. It is also a metal oxide whose change in resistance is an inverse function of the change in temperature. An excitation current is passed across the sensor and the voltage, which is proportional to the resistance, is measured and converted to units of heat calibration. Thermistors, like RTDs, vary their resistance as the ambient temperature is changed. Unlike RTDs, which use pure metals, the material used in a thermistor is generally a ceramic or polymer.

A thermistor is an electrical device made of a solid semiconductor with a high temperature coefficient of resistivity which would exhibit a linear voltage-current characteristic if its temperature were held constant. When a thermistor is used as a temperature sensing element, the relationship between resistance and temperature is of primary concern.[20] The approximate relationship applying to most thermistors is

[3.4]

Where, R0 = resistance value at reference temperature T0 (K), Ω

RT = resistance at temperature T (K), Ω

= constant over temperature range, dependent on manufacturing process and construction characteristics (specified by supplier)

There are two general types of Thermistors. The positive temperature coefficient (PTC) type has a resistance that increases with increasing temperature while negative temperature coefficient (NTC) thermistors will show a decrease of resistance with increasing of temperature. [21] Figure 3.5 shows the common types of thermistor sensors.

Figure 3.5: Common types of thermistor sensors

3.2. Data Acquisition Systems

Data acquisition is the process of sampling signals that measure real world physical conditions and converting the resulting samples into digital numeric values that can be manipulated by a computer. The components of data acquisition systems include sensors that convert physical parameters to electrical signals. Besides, it also consists of the signal conditioning circuitry to convert sensor signals into a form that can be converted to digital values. Last but not least, the Analog-to-digital converters, which convert conditioned sensor signals to digital values. Data acquisition applications are controlled by software programs developed using various general purpose programming languages such as BASIC C, C++, Java.

3.2.1. Signal Conditioning

In order to get information from a sensor into a computer, the signal from the sensor must first be sent to an interface device of some form and from there to the computer. However, in order to be useful to the interface device, the signal from the sensor must often undergo some form of conditioning. Almost all interface devices designed to allow interfacing of sensors to computers are designed to accept a voltage signal in the range of 0 to 5 volts and to digitize this. This means that the aim of the signal conditioning circuit is to take whatever output is available from the sensor, whether voltage or resistance, and convert it to a 0 to 5 volt signal. This process generally involves a combination of one or more simple processes: converting a resistance to a voltage, dividing a voltage, amplifying a voltage and shifting a voltage. [22]

(A) Voltage Divider

In sensors, a voltage divider (also known as a potential divider) is a linear circuit that produces an output voltage (Vout) that is a fraction of its input voltage (Vin). Voltage division refers to the partitioning of a voltage among the components of the divider. Most input transducers (sensors) vary their resistance and usually a voltage divider is used to convert this to a varying voltage which is more useful. The voltage signal can be fed to other parts of the circuit, such as the input to an IC or a transistor switch. The process of converting a resistance to a voltage involves the use of a circuit known as a voltage divider. A voltage divider uses two resistors in series to divide the input voltage by the ratio of the resistances as shown in Figure 3.6. [23] The division of the voltage is according to the following formula:

Formula for dividing a voltage [3.5]

Figure 1: Basic Voltage Divider Circuit

Figure 3.6: Basic schematic voltage divider

If one of the resistors in the circuit is replaced with a variable resistance, then the output voltage is proportional to the change in resistance of the variable resistor. As a resistive sensor operates as a variable resistor, the resistive sensor can be used to replace one of the resistors, giving a voltage output which is proportional to the resistance of the sensor. The most suitable value for the other resistor in the circuit can be determined using the voltage divider equation, the minimum and maximum resistance values for the sensor and the input voltage (commonly 5V). [23]

(B) Signal Amplifying

Signal amplification performs two important functions, increasing the resolution of the input signal, and increasing its signal-to-noise ratio. Sometimes a sensor will produce a small output voltage, perhaps with only a few mV of range. In this case it is necessary to amplify the voltage so that it covers as much of the range of the analog-to-digital converter as possible. The easiest way to achieve this amplification is to use an Op-amp (Operational Amplifier). [24] An operational amplifier is an amplifier circuit with very high open loop gain and differential inputs which employs external feedback for control of its transfer function or gain. [25] Figure 3.7 shows the circuit required to use an op-amp chip as a voltage amplifier. This circuit amplifies the voltage by a factor which is determined by the ratio of the resistors as follows:

http://www.sensorwiki.org/lib/exe/fetch.php/tutorials/voltage_amplifier_formula1.png?w=&h=&cache=cache [3.6]

Figure 2: Basic Voltage Amplifier Circuit

Figure 3.7: Basic schematic op-amp

3.2.2. Signal Digitization

Digitization is the process of converting information into a digital format. In this format, information is organized into discrete units of data (called bits) that can be separately addressed (usually in multiple-bit groups called bytes). This is the binary data that computers and many devices with computing capacity (such as digital cameras and digital hearing aids) can process. [26] Digitizing information makes it easier to preserve, access, and share. Analog signals are continuously variable, both in the number of possible values of the signal at a given time, as well as in the number of points in the signal in a given period of time. However, digital signals are discrete in both of those respects – generally a finite sequence of integers – therefore a digitization can, in practical terms, only ever be an approximation of the signal it represents. Digitization occurs in two parts, discretization and quantization. Discretization implies that the reading of an analog signal A, and, at regular time intervals (frequency), sampling the value of the signal at the point. Each such reading is called a sample and may be considered to have infinite precision at this stage. [27]

Generally, the embedded system uses the Analog-Digital Converters (ADC) as its digitizer. An analog-to-digital converter is a device that converts the input continuous physical quantity to a digital number that represents the quantity's amplitude. The result is a sequence of digital values that have converted a continuous-time and continuous-amplitude analog signal to a discrete-time and discrete-amplitude digital signal. The conversion involves quantization of the input, so it introduces a small amount of error. An ADC has an analog reference voltage or current against which the analog input is compared. The digital output word tells us what fraction of the reference voltage or current is the input voltage or current. So, basically, the ADC is a divider. An n-bit ADC has a resolution of one part in 2n. Here is an example of a 3-bit A/D converter. Because it has 3 bits, there are 23 = 8 possible output codes. The difference between each output code is VREF/23. The Resolution of an A/D converter is the number of output bits it has (3 bits, in this example). [28]

3.2.3. Universal Serial Bus (USB)

Just about any computers that we buy today come with one or more Universal Serial Bus (USB) connectors. Compared to other ways of connecting devices to our computers including parallel ports, serial ports and special cards that you install inside the computer's case, USB devices are incredibly simple. USB is an industry standard developed in the mid-1990s that defines the cables, connectors and communications protocols used in a bus for connection, communication and power supply between computers and electronic devices. Then how data is sent across the USB? This happens when the software requires data transfer to occur between itself and the USB, it sends a block of data called an I/O Request Packet (IRP) to the appropriate pipe, and the software is later notified when this request is completed successfully or terminated by error. Other than the presence of an IRP request, the pipe has no interaction with the USB. In the event of an error after three retry attempts, the IRP is cancelled and all further and outstanding IRPs to that pipe are ignored until the software responds to the error signal that is generated by sending an appropriate call to the USB. How exactly this is handled depends upon the type of device and the software. As suggested by the name Universal Serial Bus, data transmission in the bus occurs in a serial form. [29] Bytes of data are broken up and sent along the bus one bit at a time, with the least significant bit first. As illustrated by Figure 3.8, it indicates the serial transmission of the binary number 11010010.

Figure 3.8: Serial transmission of the binary number 11010010

The actual data is sent across the bus in packets. Each packet is a bundle of data along with information concerning the source, destination and length of the data, and also error detection information. Since each endpoint sets, during configuration, a limit to the size of the packet it can handle, an IRP may require several packets to be sent. Each of these packets should be the maximum possible size except for the final packet. The USB host has a built in mechanism so that the software can tell it when to expect full sized packets. [29]

4.1. Experimental Set-up

In this part, we will discuss about the experimental set-up applied in this project and the detailed information of the main components used. This research mainly focuses on data acquisition system and hence, the set-up would be explained from this aspect. Through the system built in this project, we are able to acquire the data on the concentration of oxygen (O2) and ozone (O3) existing in surrounding. Moreover, the auxiliary parameters such as ambient temperature and relative humidity will be part of the monitoring results of this project as well. Figure 4.1 shows the block diagram of the data acquisition system.

Figure 4.1: Block diagram of the data acquisition system

Before kicking off the data acquisition system, the instructions and commands have to be fed into the gases sensor board through the Waspmote IDE (compiler and core libraries) applying C programming language. The four sensors utilized in the system are then stimulated with the physical inputs from the environment and convert the inputs into electrical signals. These analog signals will then be conditioned and amplified in the gases sensor board. Then the signals proceed to next stage where Waspmote, the embedded system, plays the role to digitize the signals with the assistance of ADC converter. The digital signals are eventually outputted to the computer via USB transport for further data analysis processes and parameters conversion using Labview.

4.1.1. Sensors

(A) Figaro SK-25 Oxygen Sensor

The Figaro Oxygen Sensor SK-25, as in Figure 4.2, is a unique galvanic cell type oxygen sensor. Its most notable features are no influence from CO2, good linearity up to 30% Oxygen, and excellent chemical durability. This feature makes the sensor ideal for oxygen monitoring in various applications such as the biochemical field, food industry, and domestic safety applications. [30]

Figure 4.2: Figaro SK-25 O2 sensor

This oxygen sensor comprises a lead-oxygen battery consisted of a lead anode, an oxygen cathode made up of gold and a week acid electrolyte. The gold electrode is bonded onto a non-porous Teflon FEP membrane. A small amount of oxygen permeating through the membrane is reduced electrochemically at the gold electrode. A resistance and a thermistor for temperature compensation are connected between the cathode and the anode and therefore, the lead-oxygen battery is always discharged. The current which flows through the resistance and the thermistor is proportional to the oxygen concentration (strictly speaking, Oxygen partial pressure) of the ambient atmosphere in contact with the Teflon FEP membrane. Figure 4.3 shows the graph of output voltage versus oxygen which gives a linear relationship.

Figure 4.3: Graph of output voltage versus oxygen

(B) e2v MiCS-2610 Ozone Sensor

The MiCS-2610 ozone sensor, as in Figure 4.4, is a resistive sensor that allows to measure the variation of the O3 concentration between 10ppb and 1000ppb. Its resistance varies between 11kΩ and 2MΩ approximately. Unlike the MiCS-2710, this sensor is powered through a 2.5V voltage regulator, with consumption of approximately 34mA. The sensor’s resistance in air, as well as its sensitivity, can vary between different units, so it is recommended to calibrate each one of them before finally inserting them in the application. [31]

Figure 4.4: e2v MiCS-2610 O3 sensor

The gas sensor is composed of sensing element, sensor base and sensor cap. The sensing element contains sensing material and heater to heat up sensing element (eg. 400℃). Depending on the target gas, the sensing element will utilize different materials such as Tin dioxide (SnO2). Figure 4.5 represents the sensor response to O3 in air. The sensor resistance RS is normalised to the resistance under 100 ppb of O3 (R100ppb) where RS/R0 is indicated as a function of gas concentration at 50 % RH and 25 °C.

Figure 4.5: Graph of RS/R0 versus oxygen gas concentration

(C) Sencera 808H5V5 Humidity Sensor

The 808H5V5 humidity sensor is a capacitive sensor. That makes it easy to be used as a component in various humidity measuring and controlling situations for its delicate body. This is also an analog sensor which provides a voltage output proportional to the relative humidity in the atmosphere. As the sensor’s signal range is outside of that permitted to the Waspmote’s input, a voltage divider has been installed which converts the output voltage to values between 0.48 ~ 2.34V. [32] They consist of a substrate on which a thin film of polymer or metal oxide is deposited between two conductive electrodes. The sensing surface is coated with a porous metal electrode to protect it from contamination and exposure to condensation. The substrate is typically glass, ceramic, or silicon. The incremental change in the dielectric constant of a capacitive humidity sensor is nearly directly proportional to the relative humidity of the surrounding environment. Figure 4.6 represents the relationship between the voltage output and relative humidity of the ambient air which seems to be nearly linear.

Figure 4.6: Relationship between voltage output and RH for the 808H5V5

(D) Microchip MCP9700A Temperature Sensor

The MCP9700A is an analog temperature sensor that converts temperature to analog voltage. It is a low-cost, low-power sensor with an accuracy of ±2°C from 0°C to +70°C while consuming typically 6 μA of operating current. Besides, it does not require an additional signal-conditioning circuit. Therefore, the biasing circuit development overhead for thermistor solutions can be avoided by implementing this low-cost device. The voltage output pin (VOUT) can be directly connected to the ADC input of a microcontroller. Figure 4.7 shows the MCP9700A Temperature Sensor by Microchip.

Figure 4.7: Microchip MCP9700A temperature sensor

The MCP9700A temperature coefficient is scaled to provide a 1°C/bit resolution for an 8-bit ADC with a reference voltage of 2.5V. When measuring relative change in temperature from +25°C, an accuracy of ±1°C (typical) can be realized from 0°C to +70°C. In addition, this family is immune to the effects of parasitic capacitance and can drive large capacitive loads. [33] Figure 4.8 represents the linear relationship between output voltage of the sensor and the ambient temperature.

Figure 4.8: Graph of the output voltage against temperature for MCP9700A

4.1.2. Gases Sensor Board

The Waspmote gases sensor board has been designed to monitor environmental parameters such as temperature, humidity, atmospheric pressure and 14 different types of gases. It allows the inclusion of 6 gases sensors at the same time, the regulation of their power through a system of solid state switches and the amplification of the output signal of each one of them through a non-inverting amplification stage of a maximum gain of 101 controlled by a digital potentiometer configurable through the Inter-Integrated Circuit Bus, I2C. The gases which can be monitored are carbon monoxide (CO), carbon dioxide (CO2), oxygen (O2), methane (CH4), hydrogen (H2), ammonia (NH4), isobutene (C4H10), ethanol (CH3CH2OH), toluene (C6H5CH3), hydrogen sulphide (H2S), nitrogen dioxide (NO2), ozone (O3), volatile organic compounds (VOC’s), and hydrocarbons. There is a dedicated socket for each of the following sensors: temperature, humidity, air pressure, CO2 (socket 1A) and O2 (socket 1B). There can be just one of the following sensors in one gas board: NO2, VOC and O3 (socket 2B). The VOC and O3 sensors require a hardware modification in the gas board, which is done in Libelium. Sockets 2A, 3 and 4 are available for the rest of the sensors while CO and NH3 sensors can only be placed in the sockets 3 or 4. For its electrical characteristics, the board power voltage can be either 3.3V or 5V which are both provided on the board. The sensor power voltage can be up to 5V. Moreover, the maximum admitted continuous current can be 200mA while the maximum admitted current at its peak can be up to 400mA. [34] Figure 4.9 displays the Waspmote gases sensor board.

Figure 4.9: Waspmote gases sensor board

4.1.3. Embedded System

Waspmote embedded system is a sensor device developers oriented. It works with different communicaction protocols (ZigBee, Bluetooth and GPRS) and frequencies (2.4GHz, 868MHz, 900MHz) and create links up to 12Km. Using the hibernate low power mode (0.7uA) it can save battery energy when not transmitting and be able to work even for years. Waspmote is compatible with more than 50 sensors and its open source IDE makes really easy to start working with it. Programming with Waspmote has a fast learning curve. The programming language used is C + +, widely known by developers of embedded devices. In addition, Waspmote offers an Open Source API to program using high level functions without having to deal with hardware-specific commands. The last point for configuring a sensor network is to decide the manner in which to access the data. With Waspmote, there are 3 alternatives: Waspmote Gateway, Waspmote GPRS/ GSM and Meshlium. Waspmote Gateway connected by USB to a computer. This is recommended for applications of few nodes that are indoors or for testing while configuring the network. Besides, Waspmote GPRS/ GSM can be used in small networks placed in difficult access locations, when using a single point of data collection alone or in combination with the other two options, using the GPRS radio only to send SMS alert messages. Meshlium, the multiprotocol router with wireless (2.4GHz, 5GHz), ZigBee, 3G/GPRS, Bluetooth and Ethernet. This is the most recommended option for outdoor networks and / or larger, since it allows to distribute the analysis of the data collected by the nodes. [10] Figure 4.10 and 4.11 show the main Waspmote components from top and bottom views.

Figure 4.10: Main Waspmote components (top side)

Figure 4.11: Main Waspmote components (bottom side)

4.2. Programming and Software

A programming language has to be utilised to write commands into Waspmote IDE to communicate and interface the Waspmote with computer via USB port. C has been chosen to succeed this procedure. Figure 3.12 shows the flowchart created to implement the commands in programming language C.

c flowchart.bmp

Figure 4.11: Flowchart for programming C

First of all, the USB port of the computer used to communicate the Waspmote has to be initialized, following that real time counter (RTC), accelerometer as well as gases sensor board. Then, the next step involved is to configure gain and resistance for sensors used in this project. After that, turn all the sensors on to run the system. However, a 30 seconds time interval has to be slotted as a heat-up process for the sensors before putting them into operation. Subsequently, the infinite loop is generated to retrieve the values and readings from the RTC, accelerometer and sensors used. The loop runs forever until the power has been casted off.