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1. Power Consumptiom mW Time ms 5 Application Examples The interface is compatible with most of the common types of sensors used in WSN applications For this work the sensors have been selected so as to be LPLV compliant Another important characteristic is the sensor cost in these applications where a large number of nodes will be spread over large areas low cost is a prerequisite The selected resistive sensors are an NSL 19M51 LDR Light Dependent Resistor from Silonex 21 a H25K5A humidity sensor from Sencera 22 and an NTC 11 The NTC undergoes relatively small changes in the value of their resistance while the LDR and the humidity sensor present a high output span As a voltage output sensor an A1391 linear Hall effect sensor from Allegro 23 has been selected The chosen current output sensor is an S8265 photodiode from Hamamatsu Photonics 24 whose output variations are very small Each sensor has its own electronic datasheet where variables such as model parameters or operating ranges are stored Once the sensor application is selected the maximum Vmax and minimum Vm n Sensor output is known So the gain and the offset can be calculated according to the following equations offset Vum 8 gain A 9 MAX Vin To show the performance of the proposed system its operation when applied to conditioning the aforementioned NTC a RH sensor a LRD sensor a linear Hall sensor and a photodiode is as follo2. Ripr AL 13 where constants A and a experimentally established are A 43 783 Q lux and a 0 683 The basic conditioning circuit for an LDR is a resistive divider Rsense POT2 connected between VCC and gnd similar to the circuit used to condition the NTC resistor with a resistance value of 2 7 KQ programmed in POT2 From these values the proper TEDS field values are determined for a suitable sensor conditioning acquisition and measure recovery Figure 13 shows the LDR value as a function of the incident light ranging from 10 to 2 000 lux and the conditioned output voltage recovered by the master microcontroller Figure 13 LDR behavior x and output voltage obtained from the master uC after the application of the DCM to the frequency coded signal provided by the STIM o 12000 d 10000 eee pentane en cual H a6 ca SE TE eg EE E TEE E 2 5 a a ct oe n E LENTE deuce AEE essa etcetera aucune E 15 8 D gt HM k ATIC e E E S E 2 G 0 500 1000 1500 20g luminance flui Sensors 2011 7 1 9028 5 4 Linear Hall Sensor The application of the proposed interface to a voltage based sensor has been tested using an A1391 linear Hall sensor from Allegro This sensor has been designed to deliver a voltage proportional to the distance to a magnet in a range of some centimeters providing 5 10 um accuracy The input output characteristic is given by V Ad B 14 Coefficients A and B are obtained experime
3. NTC configuration Sensor configuration Figure 12 shows similarly to the NTC example the results of applying the proposed interface to this sensor at 26 C for relative humidity ranging from 40 to 90 Figure 12 Humidity sensor behavior at 26 C a Experimental characterization b Conditioning interface scheme c Resistive divider output voltage function of the corresponding sensed temperature and d Conditioned output sensor range fit to a common 0 Vpp range x and VFC output 0 Resistance kilo ohms oO M uo 60 0 30 Relative Humidity 10 Temperature C a offset Vmin OV Vopb gain 1 52 Vmax VMN b Sensors 2011 7 1 9027 Figure 12 Cont 18 0 45 1 6 statetetetetece siete fetetecieteteted p Te ee Oe ee oe a 036 Wh 4 ees tai gana ieee ye RE 2 ER 2 E on sn A Pewee eae a Se Sn eg ise sees pete nse Ree sewnn ys eer res passe es om a D o25 i malta a 5 gt _ C pA COES EEE EEEE EETA AS E OA rf 2 E O LF re a ne SENEE i ae oes 0 15 2 0 2 fen 2 foo eb fe fee eee pe eee bo 40 s0 60 70 80 90 Go 50 60 70 BO 940 Relative Humidity Relative Humidity c d 5 3 Light Dependent Resistor Sensor LDR The third sensor connected to the proposed interface to test its performance is a light dependent resistor NSL 19MS51 from Silonex Its resistance to light dependence is exponential according to
4. System VFC System and drops to a low power mode Under these conditions the full one STIM interface current consumption is 4 uA When the master uC wakes up and detects through changes in current consumption that a new interface has been connected an I2C address request is sent All the connected STIM microcontrollers wake up to verify the instruction returning to a low power mode and only the new device sends its pre defined address and the sensor information stored in the flash memory completing the plug amp play operation and returning to a low power state until a measurement process is requested When a STIM is unplugged from the host device the system detects a reduction in current consumption the next time the host will address each one of the STIM interfaces only the connected devices will give an answer The sensor node will delete from its database the I2C address of the disconnected device thus updating the interface list Note that when a connected interface is replaced by a different STIM it is necessary to wait for two detection phases for a correct operation 2 5 2 Host Device In order to detect a new hardware connection the Vpp line that provides power to the STIM devices is monitored each time the sensor node is awakened thus detecting any changes in the current flow Current detection is performed by an LMP8645 precision Current Sense Amplifier CSA 16 as shown in Figure 1 The CSA is connected to the bias line through
5. a 30 shunt resistor Thus when a new interface is connected to the bus the current in the bias line is increased 4 uA due to the quiescent current of the STIM uC in low power mode the rest of the electronics remain unbiased as explained Sensors 2011 7 1 9018 previously With a 330 kQ gain resistor placed in the output of the CSA to amplify the voltage across the shunt resistor the increase in the bias line current results in a voltage rise of 8 mV for each connected STIM at the CSA output with an initial offset of 15 mV which is read by one of the ADC lines of the master uC This ADC is a 10 bit converter with an offset of 2 bits so it is able to detect the considered voltage increments and hence whether a new device is connected Before the master uC sends the I2C address request and the connected STIMs wake up increasing the current consumption a switch in parallel with the CSA shorts the 30 shunt resistor to avoid the voltage drop and provide a suitable Vpp Figure 1 In order to minimize the effects of noise the analog to digital conversion is performed by using an ADC noise reduction operation mode available in the host microcontroller In addition the STIM interface includes filtering capacitors in the supply lines In the case of noisy environments the shunt resistor can be increased to obtain a greater voltage in the CSA output thus reducing the system sensitivity to noise 3 Software Design After designing the hardwa
6. output of resistive sensors and sensors with voltage current output to a quasi digital signal compatible with the electrical levels of the digital input ports of the low power uC in the sensor node thus allowing easy reading 5 This electronic interface system can be reprogrammed according to the electrical characteristics of the connected sensor so as to achieve an optimum sensor reading performance This is done by the software module implemented into a small auxiliary uC which adjusts the programmable electronics to optimize the conditioning circuit operation and coordinates the measurement process managing the resources involved in the operation The information to properly configure the hardware module and recover the value of the measured magnitude from the sensor reading is contained in a small flash memory in this auxiliary uC In addition the proposed interface is plug amp play P amp P containing all the required information for configuration when it is plugged into the master uC of the sensor node self configuring its operation without user interaction The paper is organized as follows Section 2 describes the proposed smart transducer interface design Section 3 explains the software design for the conditioning and communications processes Sensors 2011 7 1 9011 including the final frequency to code conversion performed in the master uC Section 4 shows the system implementation and analyses power consumption in a wireless sensor
7. these values Table 2 TEDS for the sensors applied in the work Shaded values are not sent to the master uC NTC parameters RH parameters LDR parameters Hall parameters Photodiode parameters Ro 4 705 Q Type l Type 2 Type 3 Type 4 To 25 C Ro 4 730 Q A 43 783 Q lux A 2 8187 V m A 130 lux mA B 4007 To 23C a 0 683 B 0 0417 V B 2 8419 luxes Twin 20 C B 4004 Lux 10 XMIN 0 0178 LUXMIN 10 TMax 80 C JOMIN 40 Luxyax 2 000 XMAX 0 0192 Luxax 2 000 Rs 2 7009 PMax 90 File Hume POTI 0 kQ POTI 0 kQ POTI 0 kQ POTI 0 kQ POTI 0 kQ POT2 0 kQ POT2 0 kQ POT2 2 7 KQ POT2 0kQ POT2 39 KQ Gain b22 Gain 1 52 Gain 1 30 Gain 45 1 Gain 5 0 Offset 0 15 Offset 0 Offset 0 50 Offset 0 Offset 0 MUX s 0x05 MUX s 0x6B MUX s 0 x 79 MUX s 0x 49 MUX s Ox7B Figure 10 a shows the NTC experimental characterization Figure 10 b depicts the block diagram of the proposed interface applied to the NTC sensor at temperatures ranging from 20 to 80 C The NTC R resistive divider provides the output voltage function of the corresponding sensed temperature as shown in Figure 10 c The output is driven to the IA programmed to reduce the offset and vary the gain so that within our 20 C 80 C operating range the output voltage signal spans the entire range 0 3 V of the supply voltage optimizing the input range to the VFC and maximizing the system sensitivity Figure 10 d shows the 0 3 V output voltage read by the master uC and the output frequency also
8. SO8KA8 from Freescale 14 It is a 20 pin low cost uC specially designed for low power applications It handles the communications with the sensor node master uC based on the I2C protocol and manages the interface electronics This device allows the implementation of plug amp play technology using Transducer Electronic Data Sheets TEDS in a similar way to the IEEE 1451 4 standard 7 This is achieved thanks to its small memory unit containing the information about the different sensor characteristics necessary for the plug amp play functionality such as operation ranges conditioning parameters resistors values etc The STIM uC has two different data storage elements a volatile random access memory RAM 256 bytes fast access and a non volatile Flash memory 8 Kbytes non volatile and slower than RAM The STIM uC stores all the electronic datasheets of the connectable sensors in the non volatile flash memory thus avoiding being deleted if the microcontroller is turned off At the STIM power up time the connected sensor is recognized and its datasheet preceded by the NTC datasheet which is included to perform temperature calibration are mapped into the RAM memory to Sensors 2011 7 1 9016 allow a faster access at runtime and not slow down the process This datasheet information is then used to properly configure the electronic interface of the NTC followed by that of the sensor Figure 4 shows the structure of the STIM RAM memory w
9. Sensors 2011 11 9009 9032 doi 10 3390 s 110909009 Sensors ISSN 1424 8220 www mdpi com journal sensors Article A Programmable Plug amp Play Sensor Interface for WSN Applications Sergio D Vera Alberto Bayo Nicol s Medrano Bel n Calvo and Santiago Celma Group of Electronic Design Aragon Institute for Engineering Research 13A Facultad de Ciencias Pedro Cerbuna 12 50009 Zaragoza Spain E Mails svera unizar es S D V bayo unizar es A B becalvo unizar es B C scelma unizar es S C Author to whom correspondence should be addressed E Mail nmedrano unizar es Tel 34 976 761 240 Fax 34 976 762 143 Received 4 August 2011 in revised form 6 September 2011 Accepted 15 September 2011 Published 21 September 2011 Abstract Cost reduction in wireless sensor networks WSN becomes a priority when extending their application to fields where a great number of sensors is needed such as habitat monitoring precision agriculture or diffuse greenhouse emission measurement In these cases the use of smart sensors is expensive consequently requiring the use of low cost sensors The solution to convert such generic low cost sensors into intelligent ones leads to the implementation of a versatile system with enhanced processing and storage capabilities to attain a plug and play electronic interface able to adapt to all the sensors used This paper focuses on this issue and presents a low voltage plug amp pl
10. Wireless Personal Area Networks Enabling Wireless Sensors with IEEE 802 15 4 IEEE Press Hoboken NJ USA 2007 Bayo A Antolin D Medrano N Calvo B Celma S Early detection and monitoring of forest fire with a wireless sensor network system In Proceedings of the Eurosensor XXIV Conference Linz Austria 5 8 September 2010 Volume 5 pp 248 251 Silonex NSL 19M51 Datasheet Silonex Inc Montreal Canada 1997 Sencera Co Ltd H25K5A Resistance Humidity Sensor Specification Datasheet Sencera Co Ltd Taipei Taiwan 2002 Allegro Microsystems the A139x family Micro Power 3 V Linear Hall Effect Sensor ICs with Tri State Output and User Selectable Sleep Mode Datasheet Allegro Microsystems Inc Worcester MA USA 2010 Hamamatsu Si photodiode S8265 Datasheet Hamamatsu Photonics Hamamatsu city Japan 2001 2011 by the authors licensee MDPI Basel Switzerland This article 1s an open access article distributed under the terms and conditions of the Creative Commons Attribution license http creativecommons org licenses by 3 0
11. ay reprogrammable interface capable of adapting to different sensor types and achieving an optimum reading performance for every sensor The proposed interface which includes both electronic and software elements so that it can be easily integrated in WSN nodes is described and experimental test results to validate its performance are given Keywords embedded microcontroller plug amp play sensor interface smart sensors TEDS wireless sensor networks Sensors 2011 7 1 9010 1 Introduction The ever increasing reduction of sensor size has favored their integration in embedded sensing applications This fact together with the recent advances in mobile communications has made it possible to use low cost low power sensor networks which interact in widely diverse environments by means of wireless communication protocols 1 In this way a broad range of innovative applications arises such as environmental monitoring military sensor networks healthcare applications networks for detecting chemical biological or radiological risks traffic sensor networks manufacturing automation forest fire detection etec Numerous applications of Wireless Sensor Networks WSNs involve monitoring physical and chemical parameters over large regions thus needing a large number of sensor nodes In order to reduce the cost of these nodes it is customary to use low cost analogue sensors along with a programmable electronic interface capable of adapting ev
12. cy Set by yE AEAN 4 75 V to 5 25 V P A External System Clock Shift 8 Bit Serial In Parallel Out SN74HC594 2 0 V to 6 0 V 65 C to 150 C 0 2 mW shea Register f s d i i Shift Registers With Storage Switch ADG701 1 8 V to 5 5 V 40 C to 85 C lt 0 01 uW Rail to Rail Operation Sensors 2011 7 1 9022 Figure 8 Photograph of the sensor interface dimensions 76 x 46 mm At the system level to minimize the power consumption the interface should be active only when the node requests a configuration and measurement process and remain off the rest of the time Therefore an ADG701 switch from Analog Devices 18 controlled by the STIM uC is used to connect and disconnect the power supply from the whole STIM electronics Thus in sleep mode the conditioning electronics Sensor Platform Amplification System VFC System remain unbiased and the microcontroller is diverted to a low power mode When the STIM interface is in the configuration and measurement steps the microcontroller is awake and the power line is connected By means of the corresponding enable terminals of the components and suitable switches it is possible to power on only the required electronics at each step thereby reducing the power consumption Figure 9 shows the consumption levels of the STIM interface tested in a WSN mote that complies with the IEEE 802 15 4 standard 19 It consists of an XBee transceiver from Digi and an Atmegal281 from Atmel powered by t
13. d Sensors Handbook CRC Press Boca Raton FL USA 1999 9 Maxim Integrated Products MAX5413 414 5415 Dual 256 Tap Low Drift Digital Potentiometers in 14 Pin TSSOP Datasheet Maxim Sunnyvale CA USA 2001 10 Analog Devices ADG704 Low Voltage 2 2 4 Channel Multiplexer Datasheet Analog Devices Norwood MA USA 1999 11 Vishay BCcomponents NTC Thermistors Radial Leaded Standard Precision Datasheet Vishay Shelton CT USA 2011 12 Texas Instruments INA326 INA327 Precision Rail to Rail I O Instrumentation Amplifier Datasheet Texas Instruments Dallas TX USA 2004 13 Analog Devices AD7740 3 V V Low Power Synchronous Voltage to Frequency Converter Datasheet Analog Devices Norwood MA USA 2001 14 Freescale Semiconductor MC9RSOSKAS Series Reference Manual Freescale Semiconductor Inc Tempe AZ USA 2008 15 Texas Instrument SNS54HC594D SN74HC594D_ 8 bit Shift Registers with Output Registers Datasheet Texas Instruments Dallas TX USA 2003 16 National Semiconductor LMP amp 645 Precision High Voltage Current Sense Amplifier Datasheet National Semiconductor Corporation Santa Clara CA USA 2011 Sensors 2011 7 1 9032 17 18 19 20 21 22 23 24 Philips I2C Bus Specification and User Manual NXP Eindhoven The Netherlands 2007 Analog Devices ADG701 Low Voltage 2 2 SPST Switches Datasheet Analog Devices Norwood MA USA 2006 Gutierrez J Callaway E Barrett R Low Rate
14. ding resistive sensors and sensors with voltage and current output It provides a value of the measured parameter coded as the frequency of a signal compatible with the logic levels of the master uC that manages the sensor node resources offering high immunity to noise and signal interferences The frequency to digital value conversion can be easily performed in the master uC by means of the simple direct counting method allowing more than 12 bit accuracy for conversion times below to 16 ms Figure 16 shows the full scale error in the recovering of the sensor output magnitude compared to the value measured directly in the sensor output i e the error due to the electronic interface For those sensors without a linear relationship between the electric magnitude and the measured physical property NTC RH sensor and Hall the correlated error for the corresponding physical property recovered from the sensor reading is also plotted The obtained errors are compatible with the typical requirements of wireless sensor system applications which handle with overall errors in the range of 1 to 5 Sensors 2011 7 1 9030 Figure 16 Full scale errors associated with the interface electronics for a NTC b RH sensor c LDR d linear Hall sensor and e photodiode Pdeneeneseneeeersnerserseneaeneeeeseneenngenneaeeaeneeneseneseeeaereneeeeeaeeenesepeneneeeaneeenneeneeneaneeneeaenanseenguaneaereeneaereaeresesseneanreneranneseegnesanrsneesesenesereeneaane
15. e behavior for incident light from 10 to 2 000 lux and the voltage recovered by the master uC Sensors 2011 7 1 9029 Figure 15 Photodiode output x and output voltage obtained from the master uC after the application of the DCM to the frequency coded signal provided by the STIM o a ET aie T a i CL ia n ee ee 2 5 o 0 iS ic 5 5 ey eee ee re ee ree ee ee ETE Serre ee 9 6 D o LLI oe i 0 0 SOC 1000 1500 2 J00 luminance iuei Table 2 summarizes the information stored in the corresponding TEDS for all the sensors used in this work Shaded values are used only for programming both the Sensor Platforms Amplification and VFC Systems and so are not sent to the master uC in the first plug process of the STIM 6 Conclusions This paper presents a programmable conditioning interface for low cost sensors in WSN applications The proposed STIM can be connected to a sensor node by means of a standard I2C protocol and stores a reduced Transducer Electronic Data Sheet of the connected sensor to properly self configure the conditioning electronics thereby improving the output span increasing the sensor sensitivity and reducing the temperature drift in the sensor response For easy integration in the sensor node this STIM uses a plug amp play technique which allows connection with no previous operation The interface can be used with a wide variety of sensors and has been validated for a set of sensors inclu
16. ent of the STIM into the sensor node Tk D E Ln Ee b d E A T a a WRT 3 PRT 10 0MM s sf 2 24V Valor Medio Min Max Desv est 1M pts Frecuencia 5 577kH2 5 468k 5 253k 5 577k 186 1 The frequency to digital value conversion is performed in the master uC using the classical Direct Counting Method DCM 6 This method counts the number of pulses N of a signal of unknown period Tx in a temporal window defined by n periods of a signal of known frequency fo Figure 7 shows the timing diagram of the DCM Figure 7 Direct Counting Method timing diagram Sensors 2011 7 1 9021 The number of pulses into the counting window is given by I Nx Sng 6 where T is the period of the known signal The unknown frequency fy is calculated by the number of pulses into the counting window Nx Ix or 7 The bigger n is the greater the accuracy but at the cost of a longer calculus time In our case using a signal of period Ty 2 5 x 10 s 4 MHz n 65 535 provides a suitable tradeoff between accuracy and operation time giving an accuracy higher than 12 bits for measuring times of 16 ms Thus the master uC in the sensor node receives the frequency coded value and translates it into a digital value according to the DCM Before the data are sent by the sensor node to the WSN coordinator the recovered value is converted to the measured value of the physical magnitude by properly applyin
17. ery sensor output to the port requirements of the microcontroller uC embedded in the sensing node Such a reprogrammable sensor interface widens the range of applications and thus eases the marketability of the interface circuit sensing solution In the literature implementations of such systems have been recently reported e g designed for gas sensor arrays conditioning 2 or industrial environments 3 In 4 a portable general programmable sensor interface is presented based on a commercial System on Chip SoC This system allows connecting several sensor types including sensors with digital output and smart sensors and provides several standard communication protocols To allow all these capabilities the programming interface becomes complex thus requiring a specific development environment and some programming background while plug and play capability is achieved through specific detection and trigger lines increasing the required input and output resources of the interface to master module bus where the system is connected The goal of the present work was the design and experimental validation of a simple plug amp play programmable sensor to wC interface able to self configure its operation when adapting the output of different sensors optimizing every sensor span The proposed Smart Transducer Interface Module STIM includes both electronic and software elements The hardware module consists of an electronic system that transforms the
18. esolution a range of appropriate offset values from approximately 0 to Vpp 2 The appropriate resolution of the voltage values would not be possible to achieve considering only the potentiometer POT3 working as a resistive divider between Vpp and gnd due to the coarse voltage discretization in the potentiometer output associated to the maximum number of programmable levels 256 available Note that if the subtraction of an offset to the sensor signal is not necessary MUX4 allows the grounding of the IA input Vpm to make this possible the IA amplifier must also be rail to rail at its input In the case of differential amplification the positive sensor output Vsensg 1s driven from P1 to the non inverting A input Vm through MUX3 and the negative sensor output Vsgnsg 1s driven from P2 to the inverting IA input Vm through MUX4 The IA output voltage is then given by R Vour 2 POT Vises VsENsE 4 which as already said can be digitally controlled through the value of POT4 Sensors 2011 7 1 9015 The instrumentation amplifier used in the amplification block is a rail to rail input and output INA327 from Texas Instruments 12 MUX3 and MUX4 are ADG704 4 1 from Analog Devices 10 and potentiometers POT3 and POT4 are MAX 5415 from Maxim 9 with a nominal value of 100 kQ and 256 taps so this very number of possible gains and offset values can adjust the sensor characteristic The RC circuit R 100 Q C 1 uF located at
19. g the information stored in the corresponding TEDS previously sent to the master uC in its first connection Figure 5 left 4 System Implementation and Power Consumption Considerations The designed circuit has been implemented using Commercial Off The Shelf COTS components Since it targets an application using battery operated WSN nodes it must have reduced power consumption and accordingly at the circuit level the components used for its implementation must be compliant with the Low Power Low Voltage LPLV requirements The selected components are summarized in Table 1 Note that all these components have low power modes to allow a selective device enabling in order to optimize the operating power consumption except the VFC AD7740 13 whose typical power is 3 mW and no low power mode is available so a switch connects and disconnects this device from the power supply to implement a low power VFC mode Figure 8 shows a photograph of the proposed system Table 1 Component characteristics Commercial Single Supply Temperature Power Other Characteristics Name Range Consumption 4 1 MUX ADG704 1 8 V to 5 5 V 40 C to 85 C lt 0 01 uW Rail to Rail Operation With Enable Pin POT _MAX5414 5415 2 77 Vto 5 5V 40 C to 485 C 0 3 uW ee ee 256 Taps Positions True Rail to rail I O Excellent Long Term IA INA327 2 7 V to 5 5 V 40 C to 125 C 6 0 mW oe Stability With Enable Pin 3 0 V tort3 6 V or no P Full Scale Frequen
20. heduling the three blocks of the proposed sensor interface Sensor Platform Amplification System and VFC System In addition the plug amp play concept is used to self configure this interface when it is connected to a sensor node microcontroller Configuration data are stored in a small memory in the STIM uC similar to TEDS Transducer Electronic Data Sheet sensors 7 Communication between the STIM interface and the sensor node is carried out through two lines a clock line clk and a bidirectional data line DIO 2 1 Sensor Platform Figure 2 shows a schematic of the sensor platform It includes two pins to connect the sensors and basic reconfigurable conditioning electronics to adequately transform changes in voltage current or resistance to voltage variations that will then be amplified to fit the full output voltage span Thus each sensor is connected between terminals P1 and P2 driven by 4 1 analog multiplexers MUX1 MUX2 which allow setting the suitable basic conditioning scheme for each sensor type Figure 2 Sensor platform o PDig P3 R1 Resistive sensors Rsensg employ a resistive divider as basic conditioning electronics As shown in Figure 2 if Rsgensg 1s connected between terminals P1 and P2 a resistive divider POT1 Rsgnse or alternatively Rsense POT2 can be formed between Vpp and gnd properly configuring MUXI and MUX2 POTI and POT2 are programmable resistances implemented by linear digitally programmable pote
21. here the information is saved The first 16 bytes correspond to the NTC TEDS and the following 18 bytes to the sensor TEDS The latter number suitable for all the sensors considered here can be increased or decreased according to the characteristics of the sensors to be used The last TEDS position 15 for the NTC and 33 for the sensor are reserved to store the commands sent by the master uC to the STIM TEDS positions 34 to 38 then store the settings for conditioning and performing a reading of the NTC Positions 39 to 43 store these same parameters for the corresponding connected sensor Figure 4 details the specific information stored in each register block information and configuration for the NTC Figure 4 STIM wC s RAM memory with NTC information NTC information NTC command register Sensor information NTC configuration Sensor configuration The 8 bit shift register a Texas Instruments SN74HC594D 15 contains appropriate control signals for all MUXI to MUX4 multiplexers Its programming is performed as follows the STIM uC writes in series the 8 bit control values on the register once the register contains the eight suitable control signals to set the paths of the multiplexers the STIM uC uses the I O available resources to 1 control the fed switch 1 I O port 2 program the digital potentiometers three I O ports Sensors 2011 7 1 9017 3 enable the multiplexers and the instrumentation amplifier and co
22. m 0 to 3 V corresponding to the sensor node supply voltage So the gain and offset voltage used to suit the sensor signals allow obtaining maximum voltage resolution The circuit basically contains an instrumentation amplifier A and two programmable potentiometers POT3 and POT4 The sensor output voltage Vsensg can be driven from P1 or P2 to the non inverting IA input Vm through the 4 1 multiplexer MUX3 An offset signal Vogtset generated through the R3 POTS3 resistive divider can be driven to the inverting input Vm through MUX4 The voltage at the output of the IA is proportional to the difference of the voltages at its inputs and the ratio of resistors R4 and POT4 as described by the following equation R Vour 2 or vm Vyy_ 1 Sensors 2011 J 9014 By properly connecting Vsense to Vin and Voffset to Vm the IA output voltage is given by R Vour 2 por Voms 2 Vra 2 Therefore the amplifier gain can be digitally controlled through the value of POT4 configured as a variable resistor Figure 3 Amplification System MUX P1 P2 P3 VIN R5 POT4 Voo pe N C1 R4 VIN MUX4 P2 R3 POT3 According to Equation 2 the subtraction of the offset voltage is performed before the amplification This offset voltage is given by the simple equation of a voltage divider made up of R3 and POT3 POT V offset B Vop POT R 3 The value of R3 equals the nominal value of potentiometer POT3 to achieve with high r
23. node Section 5 presents the application of the proposed system as an interface for some low voltage sensors in particular for a temperature dependent resistor NTC a humidity dependent resistor RH sensor a linear Hall sensor a light dependent resistor LDR and a photodiode Finally conclusions are drawn in Section 6 2 STIM Electronic Interface The proposed sensor interface can accommodate resistive sensors and sensors with voltage current output In addition it is compatible with the needs and restrictions of the nodes of a wireless sensor network low voltage to be powered with low form factor batteries minimum power consumption to optimize the node life and low cost to minimize the total cost for WSN applications involving a high number of nodes spread out over large areas A simplified diagram of the interface circuit is shown in Figure 1 The different output voltage ranges provided by the different sensors connected to the sensor platform are converted into a common voltage span by means of the amplification system block which digitally adjusts its gain and offset voltage depending on the sensor signal characteristics This common voltage span is next converted through a voltage to frequency circuit VFC into a pulse signal whose frequency proportionally depends on the input voltage Frequency conversion is selected because frequency coded information shows much less sensitivity to interference 5 Furthermore to achieve the bes
24. nstrumentation amplifiers To properly perform the differential amplification the positive terminal is connected to P1 and the negative to P2 Since many commercial sensors are greatly dependent on temperature a NTC sensor has been included within to check the internal temperature and apply thermal compensation so that accurate readings of the physical quantities can be taken 8 A low cost NTC is chosen as a temperature sensor as it has a known calibration curve and requires as a conditioning circuit a simple resistive divider as shown in Figure 2 Its output pin P3 is directly connected to the amplification system Finally PDig pin is a digital output to drive the control input of some commercial analog sensors in order to enable low power modes To implement this sensor platform the selected POT and POT2 potentiometers are MAX 5414 from Maxim with a nominal value of 50 kQ and 256 taps or digitally controlled different positions for their mobile terminal 9 Analog multiplexers MUX1 and MUX2 are ADG704 4 1 from Analog Devices 10 NTC and NTCI1 are 4K7 resistors at 25 C from Vishay 11 and R1 2 7 KQ 2 2 Amplification System Figure 3 shows the schematic of the proposed amplification block It mainly consists of a programmable voltage adapter circuit that performs the subtraction of an offset voltage and the multiplication of the signal by a programmed value to convert the different sensor output ranges to a common voltage range fro
25. ntally In this case A 2 8187 V m and B 0 0417 V Since this sensor is active it must be connected to Vpp and gnd to be properly biased In addition it presents and additional low power mode selection pin that is connected to PDig Figure 2 to reduce energy consumption The sensor output can be directly connected to the amplification system without additional resistors We have configured the sensor platform to measure sensor displacements up to 2 mm with 10 um accuracy Figure 14 shows the sensor behavior and the voltage recovered by the master uC as a function of the distance to a permanent magnet in a 2 2 mm span Figure 14 Hall sensor behavior x and output voltage obtained from the master uC after the application of the DCM to the frequency coded signal provided by the STIM o i 29 4B 215 2 5 a4 21 3 m 46 E 2 05 3 5 45 5 D 2 Ad 1 95 g 0 5 1 5 2 25 distance mili metres 5 5 Photodiode Sensor Photodiodes are light sensors that have a faster response than LDRs to incident light variations making them suitable for use in systems where fast response times are required Their output current depends on the incident light according to L Al1 B 15 Coefficients A and B were obtained experimentally A 130 lux mA and B 2 8419 lux In this case the sensor is connected in series with potentiometer POT2 between Vpp and gnd POT2 is configured to a resistive value of 39 kQ Figure 15 shows the photodiod
26. ntiometers whose value is adjusted depending on the sensor characteristics In addition the platform includes a grounded Negative Temperature Coefficient NTC resistor NTC1 to adequately Sensors 2011 7 1 9013 condition a low cost resistive humidity RH sensor This thermistor is used to self compensate the RH temperature output drift Impedance Z1 is actually a socket so it can be replaced with any particular resistance or component according to the specific conditioning characteristics of other sensors used in the platform Isense Current output sensors are usually conditioned employing a resistor in series which converts the current into voltage Typically these sensors need to be fed and their basic conditioning scheme is similar to those of resistive dividers Then with Isgnsp connected between P1 and P2 MUX1 MUX2 will be configured to form the series connection Isgnsg POT2 between Vpp and gnd In this way Vpp supplies the current sensor and current variations are transformed through an adequate value of POT2 to voltage to be further processed Voltage output sensors Vsgensg do not require a specific conditioning step but usually need to be fed So Vsensg will be connected between P1 and P2 and these pins directly through the MUX1 MUX2 lines to Vpp and gnd Voltage differential sensors are directly connected between the unconnected MUX1 MUX2 lines so that the voltage difference is amplified in the next stage with the use of differential i
27. ntrol the PDig pin 3 I O ports 4 create the VFC frequency reference signal 1 I O port 5 communications with the sensor node 2 I O ports and 6 drive the shift register gnd Vpp and two ports reserved to program the microcontroller complete the 17 available I O STIM uC ports Therefore the 8 bit shift register enables to virtually increase the number of microcontroller I O pin number by performing a series parallel conversion 2 5 Plug amp Play Implementation Plug and play is a feature in which both the device to be connected as well as the host device must show some specific characteristics To allow this functionality the host system needs to be able to detect the connection at one of its ports of the P amp P device in order to start the identification and configuration protocol conversely the connected device needs to include the required information and be able to send it to the host device when requested Although this protocol can be fully software implemented this increases the requirements of the STIM uC as well as its cost Thus a mixed hardware software technique has been selected in this work 2 5 1 Interface Device Figure shows the four lines that connect the host device i e the master uC and the STIM Vpp gnd clock clk and a bidirectional data line DIO When a new STIM interface is connected to these lines the STIM uC turns off the power line of the conditioning modules Sensor Platform Amplification
28. ources of the interface to master module bus where the system is connected to implement the plug amp play capability thus leading to more efficient implementation Acknowledgments This work was supported in part by MICINN and FEDER RYC 2008 03185 PET2007 0336 PET2008 0021 TEC2009 09175 DGA La Caixa GA LC 033 2009 DGA PI 113 09 and the I3A Fellowship Program References 1 Krishnamachari B Networking Wireless Sensors Cambridge University Press New York NY USA 2005 2 Bissi L Placidi P Scorzoni A Elmi I Zampolli S Environmental monitoring system compliant with the IEEE 1451 standard and featuring a simplified transducer interface Sens Actuat A Phys 2007 137 175 184 3 Pal S Rakshit A Development of network capable smart transducer interface for traditional sensors and actuators Sens Actuat A Phys 2004 112 381 387 4 Mattoli V Mondini A Mazzolai B Ferri G Dario P A universal intelligent system on chip based sensor interface Sensors 2010 10 7716 7747 5 Kirianaki N V Yurish S Y Shpak N O Deynega V P Data Acquisition and Signal Processing for Smart Sensors John Wiley amp Sons Chichester UK 2002 6 Meijer G C M Smart Sensor System John Wiley amp Sons Chichester UK 2008 7 Mark J Hufnagel P The IEEE 1451 4 Standard for Smart Transducers IEEE Standards Association Piscataway NJ USA 2004 8 Webster J G The Measurement Instrumentation an
29. possible to send two different commands to be executed in parallel Although this is not currently implemented the STIM for example could configure the conditioning electronics according to the sensor TEDS and send the output signal to the master uC while it updates some of the TEDS information of the embedded NTC sensor according to a new calibration thus minimizing the runtime Figure 6 shows a chronogram of a first connection of the proposed STIM to the sensor node after identification followed by a measurement process For a better display of the data in the bidirectional DIO line we have split it into two different channels SDA from master uC to STIM and SDAI from STIM to master uC Clock line SCL is only active for I2C communications between both systems The process shown in the figure presents several steps after the STIM is recognized by the master uC the interface sends the TEDS of the connected sensor to the node a the master wC then sends a Receive Address Instruction b the STIM interface is then fully configured performing a Sensors 2011 7 1 9020 measurement process for the NTC embedded in the interface c after that another Receive Address Instruction d configures the STIM to perform a measurement of the connected sensor e Once the process 1s finished the STIM returns to the low power mode waiting for the next interrupt to start the measurement process again Figure 6 Chronogram of a first plug and measurem
30. properly selects the inputs and the corresponding gain and offset interface configuration selects the path lines to send the measured data as a frequency coded signal output configuration and the STIM uC returns to the sleep mode low power At that moment the master uC receives the sensor data for its digitalization transferring the results to the sensor network coordinator by a wireless protocol Once the data is digitized the measurement information can be recovered from the TEDS information sent to the master uC Sensors 2011 7 1 9019 Figure 5 Basic STIM uC operations flow for left first connection and right measurement request Measurement Interrupt Wake up Address no match l2C Request Vefiry 120 Interrupt address Address match Wake up Previously connected Receive Address Instruction y Sensor NTC Send 12C address Configure Platform and Configure Platform and System from sensor System from NTC Receive Address TEDS TEDS Instruction send TEDS Measure and Send TEDS Frequency Transmission Mode with Sleep with low consumption low consumption The use of two different memory positions for commands storage in the STIM uC one for the embedded NTC instructions and the other for the connected sensor instructions makes
31. re interface the STIM uC must be programmed to manage the whole electronics of the interface and the communications protocol with the WSN node optimizing the resources to obtain an efficient implementation in WSN systems where power is provided by small form factor batteries it is a priority to minimize the power consumption to extend the operational battery life Thus in these applications the electronic system is set into a low power mode most of the time waking up to perform the interface programming measurements frequency to digital conversions and the final radiofrequency data transmission then returning to the sleep mode where the power consumption is minimal Figure 5 presents the STIM uC operating diagram When the interface is connected the STIM is taken out from the sleep mode by a master uC I2C address request interrupt communications between the STIM and node microcontrollers are based in the I2C protocol 17 and sends the I2C address corresponding to the own STIM uC Then in the corresponding memory address it receives the command to be executed position 15 in RAM for the NTC and 33 for the sensor Figure 4 The first time the STIM uC receives an interrupt the request is to send the electronic datasheet of the connected sensor to the master uC The following times the STIM uC receives an interrupt the requested operation is to configure the interface and the output to perform a measurement process Therefore the control system
32. recovered in the master uC Sensors 2011 7 1 9025 Figure 10 NTC behavior a Experimental characterization b Conditioning interface scheme c Resistive divider output voltage function of the corresponding sensed temperature and d Conditioned output sensor range fit to a common 0 Vpp range x and VFC output 0 an S mo 8 amp S Resistance kilo ohms on 10 iui a Seas aimee S a Toa offset Vmn 0 15V i i Won Vmax VMN gain 60 8 100 120 140 Temperature C a b bo 0 20 3 3 0 45 PA one eet eee Serre heen enn errr eee PA si ee ee eee eee e T 0 35 r w T ee ee Pen eet Seen eee Senne se 5 D 1 rr rrr ren MMMM MMMM i B z E Samir ea na lt r a a Ca ace E 1 Gp eenochonnnnnnnndnneg lonccbooneenenedporenoeees 0 25 gt ay a C TTE A occ ccc setdne sce R ee eh er ree QO 6 0 15 Oli sends ect E E E A E ness oE E ETT See eee u VMN l l Do 0 20 4 amp 80 100 120 140 Bo 0 20 40 60 ag OS Terperature C Temperature C c d 5 2 Humidity Sensor The H25K5A humidity sensor used in this work is a resistive element whose resistivity depends on both the relative humidity and the temperature according to a complex expression Thus the sensor operation is not usually given by an equation but rather by a table in which the sensor output value is provided for different ambient humidity and temperature value
33. s However the table size is too big to be stored in the sensor node memory Thus the final network processing system usually a computer that receives all the WSN information from a coordinator node must keep it in a file describing this sensor behavior and to properly process the output signal the name of this file is stored in its corresponding electronic datasheet and sent to the master uC in the first STIM connection process Then the first time the node is connected to the network coordinator it sends the name of the file By using the corresponding data file the network processing unit is able to recover the measured RH value from the temperature and Rsgnsg values sent by the corresponding STIM The conditioning circuit for this sensor is a resistive divider Rs pnse NTC1 between Vpp and gnd while the temperature value is known thanks to the NTC embedded into the interface So once the value of the sensor resistance is calculated and the temperature value is known the table gives the RH estimation Figure 11 shows the memory map of the parameters stored in the TEDS for this sensor and Table 2 shows the stored values It includes the parameters of NTC1 the sensor RH operating range and the name of the characteristic data file Sensors 2011 71 9026 Figure 11 STIM wC s RAM memory with RH sensor information Sensor code ID Data file name Sensor command register NTC information Sensor information resistive RH
34. sneoacoeneeaey i s Resistance ni ee ry ss ini f IE i SS ae Humidity i ooh I a i a i A ai 3 3 it g H 1 ii k H a i 20 10 0 10 20 30 40 50 60 70 80 40 50 60 70 80 90 Relative humidity Temperature C a b 6 5 5 5 4 5 4 3 5 _ 3 S25 5 a 2 1 5 1 0 5 0 0 5 H H H H H 4 f H H H H H H H f H 0 0 5 1 1 5 2 2 5 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Distance mm IIluminance lux c d 0 1 SE aenea manan aaas aaneen ea sanaan ee 0 08 e a a ONORE a A 0 06 4 f 4 EO SOE so A SET 0 04 Amm EEE IE OEN TAREE EON ER A ee BERERE ESON ee EERTSE sm O02 S pm S S S SSS S A See LER EL ENE OE oS W O ene SEESE ERG DE EEEPAESE AEEA DEES TEE E seessesseensceeeeeed 0 04 Am ee ee 1 ne A P scisesesieunienen anrnrnrnrornens ee i ssassessereereeneseeed 0 08 penn d 0 250 500 750 1000 1250 1500 1750 2000 Illumination lux e By properly managing the interface electronics the average power consumption in a measurement process of the conditioning electronics is compatible with the power requirements of portable sensing applications Compared to previously proposed general purpose hardware interfaces based on the IEEE 1451 4 standard for intelligent sensors 4 the designed system is simpler and do not need to Sensors 2011 7 1 9031 increase the required input output res
35. t performance in the subsequent frequency conversion to digital values the sensor common voltage span must cover the complete 0 Vpp supply voltage The quasi digital signal provided by the VFC is read directly by the sensor node master uC using a single digital input output port The master uC then digitizes the data using the Direct Counting Method DCM 6 and transfers the results to the sensor network coordinator by a wireless protocol Figure 1 Complete scheme diagram of the proposed sensor interface and communications e 9eeeeeee eeeedwsteeoee e eeeeeedvseee e e Sensor Smart Transducer Interface e R d LLE Module STIM e e ei NTC Sensor 2 X Master uC i S D ER pe O E X ADCO e Po U C S e V s e System mE a z STIMHC y ctrl ODD p Programming lt e R gt DIO l e e n a lt _ e clk 2 o e ai Shift Register e gnd x p R i e ae a eee o e e s VFC e System Se E e e Sensor Data 5 Sensor node J gt gt OO OOOO 0 0 06 Ry Sensors 2011 7 1 9012 The STIM uC is responsible for sc
36. the IA output filters the signal as recommended in the amplifier datasheet 12 2 3 Voltage Frequency Conversion System The output of the previous amplifier block is a voltage signal Vour whose value ranges from 0 to 3 V The next conditioning block consists of a voltage controlled oscillator VFC which performs the transformation to the frequency domain according to the expression y Four Ol pr fon 5 DD where fc 1s a reference clock frequency provided by the STIM uC to the VFC and set to 500 kHz and Vpp is the supply voltage 3 V in our case Therefore the output frequency ranges from 50 kHz to 450 kHz which are adequate values to be afterward processed in the master uC driven at 4 MHz In addition the VFC output signal is fully compatible with the logic levels of the digital input output ports of the master uC Therefore the VFC data output signal is driven directly to a digital I O port of the master uC which performs the digitalization using the DCM 6 Results are then transferred to the sensor network coordinator The selected commercial VFC is an AD7740 from Analog Devices 13 This is the only commercial VFC found by the authors that complies with the supply requirements 3 V of WSN applications 2 4 Control System The control block is responsible for scheduling activating and configuring all the interface electronics It consists of a microcontroller and an 8 bit shift register The selected STIM uC is an MC9R
37. wo 1 5 V 1 500 mAh LRO6 batteries 20 Over a complete measurement cycle and depending on the interface state the system presents different power consumption values the lowest level below 12 uW corresponds to the sleep mode i e the STIM uC is in low power mode and the whole STIM electronics are turned off using the mentioned power switch The second power level 8 7 mW 2 ms corresponds to the embedded NTC interface configuration step giving the suitable values to the potentiometers POT1 POT2 gain offset and the multiplexers The third level 18 3 mW 16 8 ms corresponds to the NTC output configuration and measurement biasing the sensor and enabling the multiplexers the A and VFC System The next two power consumption levels 8 7 mW 2 ms and 18 mW 16 8 ms are again interface configuration and output configuration and measurement levels but for the specific connected sensor which in the case of Figure 9 is again the NTC Power consumption level 2 remains unchanged for every sensor but this last power consumption 18 mW can increase or decrease depending on the connected sensor power requirements Data are transmitted to the sensor node uC while the measurement process is performed by the STIM controller After the measurement process is complete the system returns to the initial low power state Sensors 2011 7 1 9023 Figure 9 STIM power levels 1 Sleep Mode Interface Config Output Config amp Measurement
38. ws 5 1 NTC NTC is a resistive sensor Rsense whose resistance value depends on temperature Tsense reducing its value with any increase in temperature Its dependence is exponential according to the equation Fore a 10 Rense o e Sensors 2011 J 9024 where B is a characteristic constant and Ro is the sensor resistance value at a known temperature To usually 298 degrees Kelvin For this work both values were experimentally obtained from a complete NTC characterization resulting B 4 007 K and Ro 4 705 Q at 278 K The NTC conditioning circuit is a resistive divider Rsense R between Vpp and GND so that resistance variations are converted to voltage variations R sense R 1 Vour Vpp R sense The R value is calculated according to the conventional criteria of having a maximally linear response within the temperature operating range as given by the following expression 7 B 2T BET 12 1 where Tc is the operating range central temperature and Rc is the value of Rsgnsg at Tc For the typical range of temperature measurement in environmental applications 20 C 80 C Tc 30 C and thus the value of R is about 2 7 KQO All these values Ro To B Tyan Tmax R are stored in binary format in the NTC TEDS in conjunction with the binary words to properly configure the conditioning and reading electronics of the STIM Figure 4 shows the corresponding memory map and Table 2 summarizes all
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