Gas Sensor

2598 IEEE SENSORS JOURNAL, VOL. 11, NO. 10, OCTOBER 2011

Energy Efficient and Low-Cost Indoor EnvironmentMonitoring System Based on the IEEE 1451 Standard

Anuj Kumar, I. P. Singh, and S. K. Sud

Abstract—An Indoor Environment Monitoring System (IEMS)for monitoring the concentrations of indoor air pollutant gases andindoor environmental parameters has been developed in compli-ance with IEEE1451.2 standard. The sensor array is implementedusing the electrochemical sensors. The smart transducer interfacemodule (STIM) is implemented using the PIC18F4550 microcon-troller. Network Capable Application Processor (NCAP) imple-mented in LabVIEW 9.0 is based on the IEEE 1451.1 standard.The NCAP is connected to the STIM via a USB 2.0 Transducer In-dependent Interface. The level of indoor environment parametersand information regarding the STIM can be seen on the graphicaluser interface (GUI) of the NCAP. Sensors are recalibrated usingthe potentiometer adjustment technique of signal conditioning cir-cuits. The IEMS is low cost, energy efficient, and portable.

Index Terms—Electrochemical gas sensor, environment mon-itoring, IEEE 1451, network capable application processor(NCAP), smart transducer interface module (STIM), transducerindependent interface.

I. INTRODUCTION

E NERGY and efficiency have now become an importantconcern for sustained growth and overall development

[1]. For a developing country like India, the situation is furthergrieved because major part of energy, to drive the economy, isimported [2]. Today it is widely accepted that human activitiesare responsible for high level of pollution and climate change.According to the Fourth Intergovernmental Panel on ClimateChange (IPCC) report, global greenhouse gas emissions from1970 to 2004, due to human activities rose by 70% [3]. UnitedNation Environment Program (UNEP) report states that build-ings are using the lion’s share (40%) of the available globalenergy and are responsible for one third of global greenhousegas emissions, both in developed and developing countries [4].

The main source of greenhouse gas emissions from buildingsis the energy consumption. Buildings are also major emittersof other non- greenhouse gases. The life cycle cost anal-ysis (LCA) approach reveals that over 80% of greenhouse gasemissions takes place during operational phase of buildings,where energy is being used for heating, cooling, ventilation,lighting, appliances, and other applications (normal life span

Manuscript received December 06, 2010; revised February 07, 2011 andMarch 12, 2011; accepted April 14, 2011. Date of publication April 29, 2011;date of current version August 24, 2011. The associate editor coordinating thereview of this paper and approving it for publication was Dr. Larry Nagahara.

The authors are with the Instrument Design Development Centre, Indian In-stitute of Technology, Delhi, India 110016 (e-mail: [email protected];[email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2011.2148171

of a building is considered to be 50–100 years) [1]. A smallerpercentage of emissions, roughly from 10% to 20% are dueto manufacturing of materials, construction, maintenance, ren-ovation, transportation, and demolition [1]. Major greenhousepollutant include , CO, , , suspended particulatematter (SPM), Lead aerosol, volatile organic compounds, andother toxics [5]. From different studies, it is well revealed thatwhen human beings comes in contact these chemicals/pollu-tants have adverse effect on human health [6], [7]. These chem-icals are responsible for disease like lung cancer, pneumonia,asthma, chronic bronchitis, coronary artery disease, and chronicpulmonary disease [6], [7].

To reduce greenhouse gas emissions and to provide betterindoor environment to the occupants it becomes necessity thatbuilt environment should be continuously monitored [8]. Studyshows that by constant monitoring and using commerciallyavailable technologies, it is possible to reduce carbon emissionsby 60% or more, which translates to 1.35 billion tones ofcarbon [9]. Built environment has become an important area ofresearch because of its influence on human health and energyconsumption profile. The inside unconditioned environmentaffects indoor physical environment, and subsequently healthand quality of life of its occupants [10]. Achieving occupantcomfort is the result of a combination of environmental con-ditions, such as air quality, indoor air temperature, relativehumidity, mean radiant temperature, air velocity, illumination,sound, etc. Of these, the most important factors are air qualityand thermal comfort of the built environment [11]. If this isignored discomfort will be felt, which, in turn, will lead tophysiological stress affecting human health [9], [12].

Hence, there is a growing demand for indoor environmentmonitoring and control systems [13]–[19]. In view of ever in-creasing pollution sources with toxic chemicals, these systemsshould have the facilities to detect and quantify the sources ofpollution rapidly [20], [21]. In this paper, a technique has beenproposed and based on the proposed technique an indoor envi-ronment monitoring system has been developed. It is fully oper-ational and follows international standards such as IEEE 1451,ASHRAE 55 – 2004, and ISO – 7730 and gives results withscientifically acceptable accuracy. It has added advantages suchas portability, low cost, fast response time, easy to operate, andlow-power consumption.

II. SENSORS

A sensor is a device that detects a physical quantity and re-sponds with an electrical signal [22]. A gas sensor is a transducerwhich converts input energy of one form to output energy of an-other form. It detects gas molecules and produces an electricalsignal with a magnitude proportional to the concentration of the

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KUMAR et al.: ENERGY EFFICIENT AND LOW-COST INDOOR ENVIRONMENT MONITORING SYSTEM BASED ON THE IEEE 1451 STANDARD 2599

gas [23]. Earlier there was no gas sensor which can be 100% sen-sitive to a particular gas. Recently, few researchers have reportedthe gas sensors with improved performance [14], [18]. There arefive commonly used technologies for gas monitors and these are:electrochemical, solid state, infrared, catalytic bead, and photoionization [18]. More details of these sensors including theirusage, lifetime, advantages, and disadvantages are given else-where [24]–[30]. The electrochemical gas sensors are capable ofdetecting different gases with high accuracy. These sensors havemany advantageous aspects such as minimum power consump-tions as compared to catalytic bead and semiconductor sensors,cost effectiveness, and miniature size. These sensors are beingextensively used in various applications like: automotive, con-sumer, commercial, industrial, and indoor environment moni-toring [31]–[33].

III. ELECTROCHEMICAL SENSOR

Electrochemical sensors are used to determine the concen-trations of various analytes in testing samples such as fluidsand dissolved solid materials. Electrochemical sensors arefrequently used in occupational safety, medical engineering,process measuring engineering, environmental analysis, etc.[33], [34]. Electrochemical sensors have electrode arrays withtwo, three or more electrodes, which are called auxiliary elec-trode, reference electrode, and working electrodes [35], [36].Electrochemical gas sensors are well known for detecting andquantifying toxic gases such as carbon monoxide, hydrogensulphide, nitrogen oxides, chlorine, sulphur dioxide, and thelike. The electrodes of an electrochemical sensor provide asurface at which an oxidation or a reduction reaction occurs toprovide a mechanism whereby the ionic conduction of an elec-trolyte solution in contact with the electrodes is coupled withthe electron conduction of each electrode to provide a completecircuit for the current [32], [37]. In a typical electrochemicalgas sensor, the gas to be measured typically passes from theatmosphere into the sensor housing through a gas porous or gaspermeable membrane to a working electrode where a chemicalreaction occurs [38]. Electrochemical sensors, such as PHsensors, ion selective sensors, and redox sensors, are equippedwith electrical conductors to allow electrical signals to betransmitted to and from electrodes contained within the sensor.An electrochemical sensor used for measuring PH, ORP, orother specific ion concentrations is typically comprised of threeparts: a specimen sensing ion electrode, a reference cell, andan amplifier that translates signal into useable information thatcan be read [39].

Electrochemical sensors require very little power to operate.In fact, their power consumption is the lowest among all sensortypes available for gas monitoring. Moreover, they are verylinear and have good selectivity, excellent repeatability, andaccuracy. For these reasons, these sensors are widely used inportable instruments that contain multiple sensors. They arethe most popular sensors in confined space environment appli-cations [40]. However, their disadvantages include; shortenedlifetime in very dry and very hot areas, sensitive to EMF/RFI,limited storage life, and maximum response time. These dis-advantages have been rectified by using the special strategies,further details could be found in [37]–[40].

Fig. 1. Basic electrochemical sensor [32].

IV. THEORY OF OPERATION

Electrochemical sensors operate by reacting with the gas ofinterest and producing an electrical signal proportional to thegas concentration. A typical electrochemical sensor consists ofa sensing electrode or working electrode, reference electrode,and a counter electrode separated by a thin layer of electrolyte[32], [37], as shown in Fig. 1.

Gas that comes in contact with the sensor first passes througha small capillary type opening and then diffuses through a hy-drophobic barrier, and eventually reaches the electrode surface.This approach is adopted to allow the proper amount of gas toreact at the sensing electrode to produce a sufficient electricalsignal, while preventing the electrolyte from leaking out of thesensor.

The gas that diffuses through the barrier reacts at the surfaceof the sensing electrode involving either an oxidation or reduc-tion mechanism. These reactions are catalyzed by the electrodematerials specifically developed for the gas of interest. With aresistor connected across the sensing electrode, a current pro-portional to the gas concentration flows between the anode andthe cathode. The current can be measured to determine the gasconcentration. Because the current is generated in the process,the electrochemical sensor is often described as an ampero-metric gas sensor [41]–[44].

Three electrode sensors require an external driving voltage. Itis important to have a stable and constant potential at the sensingelectrode. In reality, the sensing electrode potential does not re-main constant due to the continuous electrochemical reactiontaking place on the surface of the electrode. It causes deterio-ration of the performance of the sensor over extended periodsof time. To improve the performance of the sensor, a referenceelectrode is introduced. The reference electrode is placed withinthe electrolyte in close proximity to the sensing electrode. Afixed stable constant potential is applied to the sensing elec-trode. The reference electrode maintains the value of this fixedvoltage at the sensing electrode. No current flows to or from thereference electrode. The gas molecules react at the sensing elec-trode and the current flow between the sensing and the counterelectrode is measured and is typically related directly to the gas


Fig. 2. IEEE 1451 standard family structure [45].

concentration. The value of the voltage applied to the sensingelectrode makes the sensor specific to the target gas.

In a three electrode sensor, there is normally a jumper whichconnects the working and reference electrodes. If it is removedduring storage, it will take a long time for the sensor to stabilizeand be ready to be used. Three electrode sensors require a biasvoltage between the electrodes and two electrode sensors do notrequire any bias voltage. For example, Oxygen sensors do notrequire a bias voltage [37], [41].

V. IEEE 1451 STANDARD AND FAMILY STRUCTURE

The IEEE 1451, a family of Smart Transducer InterfaceStandards, describes a set of open, common, network-inde-pendent communication interfaces for connecting transducersto microprocessors, instrumentation systems, and control/fieldnetworks. The IEEE 1451 standard makes it easier for trans-ducer manufacturers and system designers to develop smartdevices and to interface those devices to networks, systems,and instruments. The standard is comprised of seven parts andeach of them has different aspects of the interface standard, asshown in Fig. 2 [45]. In this work, we have used IEEE 1451standard to develop the IEM system. The standard IEEE 1451.1is used to design the network capable application processors.We also used IEEE 1451.2 standard to develop the smart trans-ducer interface module with electrochemical gas sensors andimplemented transducer electronic data sheet. The transducerindependent interface is implemented according to the IEEE1451.7 standard.

VI. DEVELOPED IEM SYTEM

Indoor environment monitoring system (IEMS) is a completereal-time monitoring and data recording system. It automati-cally measures and records the air quality and environmentalparameter. The developed IEMS can measure and analyze theconcentration of major air pollutant gases such as CO, ,

, , and , along with the temperature and relative hu-midity. The STIM is linked to a Network Capable ApplicationProcessor (NCAP) PC through Transducer Independent Inter-face (TII). The detailed block diagram and photograph of devel-

Fig. 3. Detailed block diagram of the IEM system.

Fig. 4. Photograph of the developed IEM system (laboratory setup).

oped IEM system is shown in Figs. 3 and 4, respectively. Thedeveloped IEM system has ten channels out of which eight areused for developed sensing module, while the other two chan-nels are open for further processing.

VII. IMPLEMENTATION OF SENSOR ARRAY AND STIM

A. Sensor Array

The selected sensor has several advantages such as low-powerconsumption, low cost, high accuracy, and capable of detectingdifferent gases. The detailed explanation of the selected electro-chemical sensors including their usage, advantages, and disad-vantages are given in [33] and [46]–[54].

The low-power consumption and low-cost electrochemicalsensors with additional temperature and humidity sensors aresuitable to use as an array for cost effective and energy efficientindoor environment monitoring system to measure the air pollu-tant gases with oxygen and environmental parameter. The spec-ifications of the sensors used in the indoor environment moni-toring system are given in Table I. All these sensors have high


TABLE ISPECIFICATIONS OF THE SENSORS USED IN THE IEMS

TABLE IIIMPLEMENTATION OF SENSOR MODULE

sensitivity and selectivity to detect gas with improved propertysuch as relative insensitivity to fluctuations in relative humidity,EMF/RF noise, low-power consumptions, high response time,and long lifetime [47]–[54].

The sensors are connected to signal conditioning circuits.The signal conditioning circuit is based on potentiostatic circuit.The designing of a potentiostatic circuit with a high input biascurrent amplifier and without precision will impact the sensorsensitivity resulting in increased sensor to sensor variation.Hence, the precise, ultra-low input bias current amplifier (lessthan 5 nA), such as LMP7721, OP90, and OP296 are used todesign the potentiostatic circuits. The inbuilt amplifiers, OP90and LMP7721 improve the circuit performance and allowthe electrochemical sensor to detect low gas concentrationwith high accuracy. The input bias current of these ampli-fiers LMP7721 and OP90 is 3 fA and 4 nA, respectively, atroom temperature (25 C) [42]–[44]. The power consumption,response time, sensing range, and operating voltage of theimplemented sensor module are given in Table II.

Fig. 5. Detailed block diagram of the developed STIM.

The relationship between the output voltage and the gasconcentration in ppm can be expressed by the followingexpressions:

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

where – Concentration of CO gas in ppm, –Output voltage of CO-CF sensor module in volts, –Concentration of gas in ppm, – Outputvoltage of sensor module in volts, – Con-centration of gas in ppm, – Output voltageof sensor module in volts, – Concentration ofgas in %, – Output voltage of sensor module involts, – Concentration of gas in ppm, –Output voltage sensor module in volts, – Indoor envi-ronment temperature in , – Output voltage oftemperature module in volts, RH – Indoor relative humidity in%, – Output voltage of humidity module in volts,

– Concentration of gas in ppm,– Output voltage of -BF sensor module in volts.

In Fig. 5, the sensor module blocks are connected to the se-quential channel (ADC1-ADC8) and two channels (ADC9 andADC11) are open.

B. Smart Transducer Interface Module (STIM)

The development of a smart transducer interface modulewith electrochemical gas sensors has to be designed accordingto IEEE 1451.2 standard. The STIM must be capable of han-dling the actuator interface, supporting TEDS, communicatingwith NCAP, and supporting TII interface. A microcontroller


is selected to support all above functions. PIC 18F4550 mi-crocontroller has been chosen to develop the STIM. Thedeveloped STIM includes all the above mentioned facilities.The power consumption of the developed STIM was observedas 42.3467 mW. The detailed block diagram of the developedSTIM is shown in Fig. 5.

1) MMC Interface Module: The PIC 18F4550 microcon-troller and MMC interface module has been developed. TheMMC is a flash memory storage device designed to providehigh capacity, nonvolatile, and rewritable storage in a small size.These devices are being frequently used in many electronic con-sumer goods such as cameras, computers, GPS systems, mobilephones, and PDAs, etc. The capacity of the MMC can be in-creased at any time for further use. Presently, the available ca-pacities of these memories lie in the range of 128 MB to 32 GB.

The MMC can be interfaced to microcontroller using two dif-ferent protocols: the SPI (Serial Peripheral Interface) protocoland SD (Serial Digital) protocol. As the SPI protocol is beingwidely used, it is preferred over SD protocol in this module. Thestandard MMC has nine pins and these pins have different func-tion depending on the interface protocol. The function of eachpin in both the SD and SPI modes of operation are given in [55],[56].

Operation of the MMC in SPI Mode: The SPI bus is a syn-chronous serial bus standard, introduced by Motorola, that op-erates in a full duplex mode. During operation, the SPI bus isoperated in the master–slave mode where the master device ini-tiates the data transfer, selection of a slave, and provides a clockfor the slaves. The selected slave responds and sends its data tothe master at each clock pulse. The SPI bus can operate with asingle master device and it can operate one or more slave devicessimultaneously. Such kind of interface is called a four wire in-terface. In this module, the master sends out data on line masteroutput, slave input (MOSI) and received data on line masterinput, slave output (MISO). During the operation of MMC de-vice in SPI mode, The MMC can use only six pins. Among thesesix pins; two pins may be used for power supply, whereas otherfour pins are fixed to be interfaced with the microcontroller. Thedescriptions of interfaced pins are given as: power ( ),ground, chip select, CLK, data-out, and data-in.

At power-up, the MMC automatically responds to the SD busprotocol. The card is switched to SPI mode if the chip select(CS) signal is asserted during reception of the reset commands.But when the card is in SPI mode, it only responds to SPI com-mands. Hence, the host may reset a card by switching the powersupply OFF and then ON again.

An MMC has a set of registers that provides the informationabout the status of the card. The operation of the card in SPImode is given by following registers: Card identification reg-ister (CID), Card specific data register (CSD), SD configurationregister (SCR), and Operation control register (OCR) [55].

Structure of Transducer Electronic Data Sheet: The MMCin SPI mode can support single block and multiple block opera-tions. In this paper, we have chosen the multiple block operationaccording to the requirements of the system. Initially, a mul-tiple block file (MYFILE.TXT) is created in MMC separatelyfor each parameter viz. CO, , - , Temperature, Rel-ative Humidity, , -BF, and . In the multiple block

Fig. 6. Memory map of the micro SD card.

reading operation, the card sends data to each block having itsown CRC check which is attached to the end of data block.While in multiple block writing operation, the host sends thedata of each sensor viz. CO, , - , Temperature, Rela-tive Humidity, , -BF, and which is saved in separatefile (.TXTFILE), simultaneously. Once developed the system isswitched ON, the save button on the front panel of “LabVIEW”software is pushed (click ON) to save data in MMC in TXTformat. After clicking ON the save button the system will start,saving data in real time. When the memory card is full the frontpanel of LabVIEW will display the “time out error.” At thispoint, it is required that the data should be transferred in notepadand to make the MMC free for further use. The transferred datain notepad has flexibility to save it in EXCEL sheet and otheranalysis software.

To increase the system autonomy the system is made com-patible to use 1 GB memory capacity MMC. Here, the 1 GBMMC is divided into ten blocks; out of which eight blocks areof 125 MB each for channel 1 to channel 8. One block is of44 Bytes is used as register bank and remaining space equal to25165780 Bytes is kept blank. The memory map of multipleblocks in MMC is shown in Fig. 6. All channels are configuredas to specify all the mandatory information. Subsequently, oneseparate channel is allotted to each parameter. Table III consistsof enumeration of the channels defined in the STIM.

VIII. IMPLEMENTATION OF THE TRANSDUCER

INDEPENDENT INTERFACE (TII)

A parallel port interface between STIM and NCAP based onthe IEEE 1451.2 standard has been developed and discussed indetail using SPI data transfer protocols [14], [16], [57].

In this investigation, the USB 2.0 based TII between theSTIM and NCAP PC has been used. The TII and data transferprotocols used are based on the IEEE 1451.7 standard [45].One side of TII is connected with the STIM and the other sideis interfaced with NCAP PC (USB port). The USB is a fourwire interface with two data lines and two power lines. Here,the STIM receives power from the USB, hence no external


TABLE IIIENUMERATION OF THE CHANNELS DEFINED IN THE STIM

power supply is required and the data transfer rate supportedby USB 2.0 may be up to 480 Mb/s.

The data transferred on a USB bus can be represented in fourways: bulk transfer, interrupt transfer, isochronous transfer, andcontrol transfer and all these are based on the IEEE 1394 USBstandard [58]. In this study, the interrupt transfers have beenused. Interrupt transfers, are used to transfer small amounts ofdata with a high bandwidth, where the data are to be transferredas quickly as possible without delay. In USB, if a device requiresthe attention of the host, it must wait until the host polls it beforeit can report its need for urgent attention. An interrupt requestis queued by the device until the host polls USB device askingfor data. The interrupt packets can be in the size of 1 to 8 Bytesat low speed, 1 to 64 Bytes at full speed, and up to 1024 Bytesat high speed.

The USB 2.0 has many advantages such as no extra powersupply required, high speed as compared to other interface, au-tomatic device detection, hot-pluggable, reliability, low cost andlow-power consumption. The maximum power available to anexternal device is limited up to 100 mA at 5.0 V. If the powerrequirement is more than 100 mA, a separate power supply isrequired.

Some of the PIC18 series microcontroller can be directlysupported by USB interface [58], [59]. For example, the PIC18F4550 microcontroller contains a full-speed compatible USB2.0 interface that allows communication between a host PC andthe microcontroller. In this study, the PIC 18F4550 microcon-troller has been used with pins RC4 (pin 23) and RC5 (pin 24)connected to the and pins of the USB connector, re-spectively. The library function of PIC 18F4550 microcontrollerhas been explained through “micro C” language and is given asfollows.

• HID_Enable: This function enables USB communicationand requires two arguments such as the rear-buffer addressand the write-buffer address. It must be called before anyother functions of the USB library, and it returns no data.

• HID_Read: This function receives data from the USB busand stores it in the receive-buffer. It has no arguments butreturns the number of characters received.

• HID_Write: This function sends data from thewrite-buffer to the USB bus. The name of the buffer(the same buffer used in the initialization) and the lengthof the data to be sent must be specified as arguments to thefunction. This function does not return any data.

• HID_Disable: This function disables the USB datatransfer. It has no arguments and returns no data.

The developed NCAP module is based on LabVIEW 9.0.In this paragraph, we will discuss about the communicationmethod to access instrument and control supported by Lab-VIEW. There are various communication methods betweenNCAP and STIM which include: GPIB (General PurposeInterface Bus), Serial Communication, VXI (VME Extensionfor Instrumentation), LXI (LAN Extension for Instrumen-tation), VISA (Virtual Instrument Software Architecture),DDE (Dynamic Data Exchange), OLE (Object Linking andEmbedding or Automation), TCP/IP (Transmission Controlprotocol/Internet Protocol), Data Socket, DAQ (Data Acqui-sition), NI-DAQmx (Data Acquisition for MAC), File I/O(Input/Output), and CIN (Code Interface Node).

In this study, we have used VISA communication method forthe USB based most recent system. VISA is a standard appli-cation programming interface (API) for instrument I/O com-munication. VISA is a means for talking to GPIB, VXI or se-rial instruments. VISA is not LabVIEW specific, but is a stan-dard available in many languages. When a LabVIEW instrumentdriver uses VISA write, an appropriate driver for the type ofcommunication being used is called [60]. This allows the sameAPI to control a number of instruments of different types. A VIwritten to perform a write operation to an instrument will notneed to be changed if the user switches from a GPIB to a se-rial device. Only the resource name must be modified whereinstrument unlock is used. Another benefit of using VISA isthat it is platform independence. Different platforms have dif-ferent definitions for items, like the size of an integer variable.VISA will perform the automatic conversion of the size of aninteger variable. The main work in a VISA application is in theinitialization. GPIB communications require the address stringto be passed when ever a driver is called. In a large applica-tion changes in the instrument, like using a serial instrumentinstead of GPIB instrument require considerable changes. Anapplication using VISA would require changing only the inputto the VISA open VI. The resulting instrument reference wouldstill be valid for the VISA drivers, requiring no change. VISAdrivers present flexibility. The VISA drivers VIs are located inthe instrument I/O section of the function palette. The VISAsubpalette contains a wide range of program functions. Mainpalette contains standard VISA driver VIs. These VIs allowsopening communication session, reading and writing data, as-serting a trigger, and closing communications. In addition to thestandard VISA VIs, there are a number of advanced VISA func-tions. These are contained in the VISA advanced subpalette andthree subpalettes on the advanced palette.

The first subpalette on the VISA advanced palette is the bus/interface subpalette. It contains VIs used to deal with interface-specific needs. There is VIs to set the serial buffer size, flushthe serial buffer, and send a serial break. The VISA VXI Cmdor Query VI allows sending a command or query, or receiving aresponse to a previously sent query based on the mode input.


Fig. 7. (a)–(d) STIM kernel flow chart.

The next subpalette is the event handling palette. The VIsin this palette acts on specified events. Examples of events aretriggers, VXI signals, and service requests. Finally, the register

access subpalette allows reading, writing, and moving specifiedlength words of data from a specified address. The low-levelregister access subpalette allows peeking and pokes specified


bit length values from specified register addresses. The VISAdriver performs three functions such as configure an instrument,take a measurement, or check the status.

First of all, initialization and configuration of the STIM setupwas carried. The initial configuration can allow to the STIMsoftware to work successfully for the desired measurement ortesting process. The STIM software has been used to take mea-surement, read specific data, and save the data in MMC from thesensor modules. Using the STIM software, the real-time graph-ical waveform and digital output of the sensor modules are dis-played in NCAP PC (LabVIEW front panel).

IX. GENERAL PROTOCOLS

The data transfer functions have been implemented using theprotocols described in the IEEE 1451.2 [20]. The active con-trol of the developed system is handled by the NCAP. A datatransport frame begins by the NCAP sending an address to theSTIM. The complete address specifies whether the data shouldbe written or to be read in the MMC from the STIM and whichchannel with corresponding function is involved. Then, the datais transferred from the NCAP to the STIM via and .Thus, the whole system is controlled through the NCAP andprovides the power to the STIM. The reset condition of the sen-sors is handled manually through JFET.

X. THE STIM KERNEL – MAIN CONTROL PROGRAM

The STIM kernel and flow chart has been illustrated inFig. 7(a)–(d) and the STIM program has been developed in Clanguage. The STIM kernel program has three main functions:trigger, data transport, and interrupt. Each of these functionshas special tasks and works cooperatively with the NCAP. Thedeveloped STIM kernel program can perform only for the eightsensing module. If the extra sensor module are to be connectedin the future, modification in the STIM kernal program isrequired. The STIM kernel program has been divided into eightparts like in the sensor module. Each part of these eight sensormodule have been further divided into three blocks such as:i) read analog input and then send it to USB; ii) write on file;and iii) open file (MMC) and save data with date and time.

After receiving power, the STIM kernel executes all initial-izations routines including the TII inialization, memory clearingprocess, loading the TEDS in MMC, setting the channel databuffers, and status registers. Subsequently, it enters into a per-manent loop (a loop that executes for ever) and goes throughthe processes, as shown in the Fig. 7(a). The developed soft-ware modules have been saved in the 32 kB flash/EE programmemory of the PIC18F4550.

XI. THE NCAP PROGRAM

The developed NCAP is in accordance with IEEE 1451.1standard [21]. Its logical componets are included in two groups:support and application. The components of support are thetransducer interface, the network interface, and the operatingsystem. The transducer interface block encapsulates the detailsof the transducer hardware implementation with a program-ming model when the NCAP is connected to a STIM. Thenetwork interface block encapsulates the details of the different

Fig. 8. NCAP program flow chart.

Fig. 9. Front panel of indoor environment monitoring system.

networks protocols implementations behind a set of commu-nication methods. Whereas the operating system providesan interface with applications. The use of NCAP includes aPC with USB connection as the hardware component and asoftware component fully developed in LabVIEW 9.0.


Fig. 10. Block diagram of CO module.

Fig. 11. Block diagram of �� module.

The NCAP program has two main subprograms: controllingof the STIM and providing the Graphical User Interface (GUI).The STIM controlling program excutes data transport and in-terrupt request functions. In addition, it also supports the TIIthrough the USB 2.0. The GUI displays the STIM information,the output of the sensor module in digital and graphical wave-forms, and the status of the MMC. The real-time data are savedon the basis of set value of sample/s. Hence, NCAP GUI canbe used for the samples in the range of 1–100 s. Moreover, italso provides the facility to add the user interaction to triggerthe STIM and send functional address to the required channel.The flow chart of the LabVIEW program and a front panel ofthe NCAP GUI are shown in Figs. 8 and 9, respectively. Thefront panel of the indoor environment monitoring system han-dles function, input and outputs, while the flow chart performsthe work of NCAP. The front panel has a knob for setting thetime interval, start button, data save/start button in MMC, stopbutton, and a digital and graphical output. Here, the developedNCAP module can perform for the eight sensing module. A

Fig. 12. Block diagram of �� module.

Fig. 13. Block diagram of � module.

LabVIEW program is executed by pressing the arrow or therun button located in the palette at the top of the front panelwindow. While the VI is executing, the Run button (broken ar-rows) changes to a black color, as depicted in Fig. 9.

All coding in LabVIEW is done as given in the block dia-grams from Figs. 10–17. The block diagram accompanies thefront panel, as shown in Fig. 9. In the block diagram, the outerrectangular structure represents a while loop and the inner rect-angular structure is represented by the conditional structure andcontrolling structure of the VI. The controlling structure whichis accessible from the structure palette is shown in Figs. 10–17.

The controlling and conditional structure operation for thesensing module is given in Figs. 10–17. Initially, a delay (waitfunction timer) of the input signal is added through the USB.The timer will now delay the execution of the loop by ten timeson the setting of the sample rate knob at the front panel in 1 s. Ifthe knob is set at one, the sample rate will be 1 s ( ).The software in the loop must now be set up to send and receive8 bit data to the STIM via the USB port. The flags of VI are set


Fig. 14. Block diagram of �� – � .

Fig. 15. Block diagram of Extra (�� -BF) module.

on the basis of STIM such as the temperature sensor module ofthe flag 97

is set on the conditional structure and other sensor module suchas humidity sensor module is the flag 98 [ (connectedsensor module in ADC 2)] is set on the conditional structure.The data of the developed eight sensor module is saved in MMCfor selected flag 108

with corresponding date and time. The collected data (8 bits)may be passed through LabVIEW as a string, an array of inte-gers. Finally, the application involves converting the 8 bit data tothe units of the measured parameters. This is done with a con-version factor such as multiplication, subtraction, adding, anddivision related to convert 8 bit data from the USB into decimal

Fig. 16. Block diagram of temperature module.

Fig. 17. Block diagram of Relative Humidity module.

value. The orange line represents the data being passed from thecontrol into the VI. Function palettes and structure palettes aresame for Figs. 10–17.

XII. CALIBRATION OF THE SENSORS

The sensors are already calibrated by manufacturer and thesame have been used in the developed IEM system. But due tothe requirement of high precision in the measurement at lowconcentrations, the field calibration of the sensor is needed sothat more accurate results can be achieved by the developedsystem [61], [62].

In the field calibration, the comparison between the resultsof the developed IEM system to the available standard systemfrom known manufactures has been carried out. The differencesare then adjusted so that the reading matches with the standardsystem reading within the allowed calibration tolerance. Theseadjustments are made by applying a simple offset ( ) inthe sensor module using a two-point calibration at the extremesof the expected range by utilizing special electrical currentreadings and potentiometer ( ) adjustments. The commonschematic diagram of all electrochemical sensor calibration


Fig. 18. Schematic diagram of sensor calibration circuit.

TABLE IVSENSOR MODULE ACCURACY (AFTER CALIBRATION)

circuit is shown in Fig. 18, whereas the accuracy of the eachcalibrated sensor is given in Table IV.

XIII. RESULTS

The main aim of this research paper is to develop an indoorenvironment monitoring system (IEMS), which is proficient inthe measuring common indoor air pollutant concentrations andthe environmental parameters by a sensor array of electrochem-ical sensors. The system is based on the IEEE 1451, ASHRAE55–2004, and ISO 7730 standards. Having developed theelectrochemical sensor array system, the standard transducerinterface module (STIM), the transducer independent interface(TII), and the network capable application processor (NCAP)program have also been successfully developed. The STIM,TII, and NCAP modules were developed using the guidelinesprovided by the IEEE 1451.2, IEEE 1451.7, and IEEE 1451.1standards. The developed IEM system has three main parts:i) STIM; ii) TII; and iii) NCAP. Though the NCAP is dependenton self powered PC or Laptop it does not consume any power.Thus, total power consumption of the developed IEM Systemis the sum of power consumed by STIM and TII module only.

Fig. 19. Measure and record the real-time indoor environment parameters (CO,�� , �� -BF, �� -� , �� , � , T, and RH).

TABLE VINDOOR AIR QUALITY WITH THERMAL PARAMETER

MEASUREMENT DATA IN SITU

The total power consumption of the IEM system is found to be45.3675 mW.

The sensors were recalibrated through the “field calibration”;by comparing the results of the developed IEM system with theavailable standard instruments from known manufactures. Thiswas achieved to verify the accuracy of the developed system.The current indoor air pollutant and environmental parameterlevels can be directly read from the NCAP GUI. Online datais saved on a memory card (MMC) to be used for furtherprocessing.

These sensors are highly sensitive to EMF/RFI, and due carewas taken to shield the system to prevent the degradation ofthe sensor performance by using appropriate PVC holders andmulticore PCB. The recommended temperature and humidityspecified by the manufacturer for proper functioning of thesesensors is within a temperature range of 21 C to 27 C andrelative humidity of less than 50% at 21 C to 27 C.

A set of real-time field measurements of the indoor gasessuch as CO, , , -BF, , and - sensorswere recorded in a normal laboratory environment. Fig. 19shows their concentration levels together with temperature andhumidity levels in the Electronics Lab (WS129) for durationof 1 min on 25 June 2010. Table V shows the minimum,maximum levels, and 1 h mean of the indoor air quality withthermal parameter data in situ.


XIV. CONCLUSION

The indoor environment monitoring system has been success-fully developed in compliance with the IEEE 1451, ASHREA55–2004, and ISO 7730 standards and consists of the followingfunctional blocks: the STIM, the TII, and the NCAP. The mainaim of the IEEE 1451 standard is to provide an industry standardinterface to efficiently connect transducers to microcontrollersand to connect microcontrollers to a network.

The STIM driver (which forms a part of the NCAP Model)uses the network communication capabilities of the LabVIEW9.0. The ten “smart” transducers have plug and play capability:the STIM can be moved from one NCAP to another.

The electrochemical sensors have been successfully used inreal-time monitoring target gas concentrations and environ-mental parameters. The usage of these sensors adds severaladvantages to a system such as low-power consumption, lowcost, fast response, ability to produce online measurement, etc.The calibration of the sensor with the appropriate accuracy isbeneficial for the energy efficiency in the building automation.

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Anuj Kumar received the M.Sc. degree in physics(electronics) from M.J.P.R University, Bareilly,India, in 1998, the M.Phil. degree in instrumenta-tion from the Indian Institute of Technology (IIT)Roorkee, Roorkee, in 2000, and the M.Tech. degreein instrumentation from NIT Krukshetra, India, in2004.

Currently, he is a Senior Research Student in theIDD Centre, IIT Delhi. His area of research is in smartsensing system, intelligent system, and instrumenta-tion electronics.

I. P. Singh was born in Allahabad, India, in 1947. Hereceived the B.E. degree in mechanical engineeringfrom REC Durgapur, Durgapur, India, in 1969, theM.Tech. degree in design engineering and the Ph.D.degree in solid mechanics from the Indian Instituteof Technology (IIT), Delhi, in 1971 and 1978,respectively.

Currently, he is an Associate Professor at the In-strument Design Development Centre at IIT Delhi.His current interests include microcontroller applica-tions, instrumentations, and composite materials.

S. K. Sud was born in Lahore, India, in 1946. Hereceived the B.E. degree in electronics and communi-cation and the M.E. degree in applied electronics andservomechanisms from the University of Roorkee(now the Indian Institute of Technology Roorkee),Roorkee, in 1967 and 1969, respectively.

Currently, he is working as Chief Design Engineerat the Instrument Design Development Centre,Indian Institute of Technology (IIT), Delhi. Hisfield of interests includes electronic instrumentation,microprocessor and microcontroller applications,

hybrid electric vehicles, and microturbine controllers.

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