Sensor Network with Data Transfer over Power Supply Wires

Transcription

1 Sensor Network with Data Transfer over Power Supply Wires Vašek V., Dolinay J. and Dostálek P. Abstrat This artile desribes system whih allows reating sensor networks where sensors ommuniate with a entral unit using only two wires the power lines. The system an be used in wide range of appliations in whih it is needed to ollet data from several sensors onneted by only two wires. The wires provide power to these sensors and at the same time transfer the data. Keywords sensor, miroontroller, power line ommuniation, data transfer, HCS08, PWM. P I. INTRODUCTION ROCESS measurement is one of the most important parts of any ontrol system. It follows from the fat that ontrol auray depends on how preisely the measuring hain works. These days there is number of devies available performing data aquisition tasks standard ards for PCI or ISA bus whih are suitable for standard personal omputers and its industrial versions and modules for industrial automation usually equipped with RS485, CAN and other interfaes [4]. Independent ategory is formed by smart sensors inorporating sensor, onverter to unified signal and data aquisition devie in one embedded system with very ompat dimensions and low power onsumption. They have number of advantageous features, suh as automati diagnosti and alibration, high auray and immunity against eletromagneti interferene due to short signal paths. Suh sensors an be found in inreasing number of appliations, inluding automotive and airraft industry, where small dimensions and low weight are ruial. Rather often it is neessary to measure data in terrain where This work was supported by the Ministry of Eduation, Youth and Sports of the Czeh Republi under the Researh Plan No. MSM and by the European Regional Development Fund under the projet CEBIA-Teh No. CZ..05/..00/ ). Vladimír Vašek is with the Department of Automation and Control Engineering, Tomas Bata University in Zlin, Faulty of Applied Informatis, nám. T. G. Masaryka 5555, 7600 Zlín, Czeh Republi ( vasek@fai.utb.z). Jan Dolinay is with the Department of Automation and Control Engineering, Tomas Bata University in Zlin, Faulty of Applied Informatis, nám. T. G. Masaryka 5555, 7600 Zlín, Czeh Republi ( dolinay@fai.utb.z). Petr Dostálek is with the Department of Automation and Control Engineering, Tomas Bata University in Zlin, Faulty of Applied Informatis, nám. T. G. Masaryka 5555, 7600 Zlín, Czeh Republi ( dostalek@fai.utb.z). it is not possible to use standard omputer equipped with DAQ ard. In this ase it is very advantageous to use laptop omputer or other devie with suitable ommuniation interfae equipped with portable sensor network. The main intention for our work is to develop simple sensor network minimizing number of hardware omponents of the system, its onfiguration omplexity and deploy time of the measurement system. II. SYSTEM OVERVIEW The following requirements were kept in mind when designing the system: Use only wires whih both provide the power and arry the data. Transfer data seurely, detet and disard damaged data. Make the devies as heap as possible. The system onsists of one or more sensors (transmitters) whih are all onneted to one two-wire line starting from a entral unit (reeiver). All sensors are powered from this line. Transmitting data from the sensor is done by lowering the voltage of the power line (by swithing on a load in the sensor). The reeiver monitors the voltage and interprets the hanges as logial zeroes and ones in the transmitted digital data. The ommuniation is only possible in one way from the sensor to the entral unit. This ould be onsidered a drawbak, but for intended appliation of olleting data from e.g. temperature or humidity sensors, this one-way ommuniation is suffiient. It has the advantage of simpliity both in software and hardware of the sensor. We onsidered also master-slave model with the entral unit being the master whih requests data from the sensors (slaves) only when needed. This would mean better use of the bandwidth sensors would transmit data only when requested. However, on the other hand, if we deide to require that the sensors are also able to reeive data, it would require extra iruitry in eah sensor whih inreases the prie and moreover it ompliates the setup of suh system address of eah sensor must be defined in the entral unit with the polling interval et. After onsidering all pros and ons we deided for one-way ommuniation solution. In this solution eah sensor transmits data with predefined period. This period an be fixed (e.g. 0 Issue, Volume 6, 0 8

2 seonds) or it an be programmed into the sensor during initial setup, together with unique serial number. In the entral unit it is easy to hoose the period with whih the data from sensor are reeived and drop the extra measurements, whih are not needed. An example of the sensor network an be seen in the following piture. There are three sensor onneted to a entral unit, whih reeives and deodes the data from the sensors and passes them on to the laptop. In the laptop it is then easy to store the data, display their trends et. USB / RS3 Central unit Laptop Bus + - Power / Data Fig.. Sensor network blok diagram. III. MAIN PARTS OF THE SYSTEM The system onsists of three main parts: entral unit (reeiver), sensor unit (transmitter) and a programming unit. A. Central unit... Sensor Sensor Sensor 3 Sensor N This unit reeives data from all the sensors and provides these data to supervisory system. In our design this is a simple MCU-based devie whih handles the low-level operations and presents data to a personal omputer for storing, displaying et. Basially the unit provides power soure for all the sensors and it monitors the voltage of the power soure to reeive data from the sensors. Of ourse, it would be possible to reate a unit whih would also present the data on display or store them to storage devie. But for our purpose we prefer to keep the unit simple and let the user manipulate the data as needed in the supervisory omputer. This has the advantage of bigger versatility of the unit. Fig. Central unit hardware The Central unit hardware an be divided into six funtional bloks: High-pass filter eliminates DC omponent ( V supply voltage) from useful signal. Low-pass filter shape high-pass filter output signal to near retangular wave form. Amplifier amplifies signal before it enters to A/D onverter. Gain is seletable in 8 steps by miroontroller. Control logi utilizes same miroontroller as sensor unit Communiation interfae standard RS3 interfae Power supply provides stabilized +5 V for digital and +/-8 V for analog iruits (operational amplifier and multiplexer). Detailed desription of the unit inluding shematis an be found in the Hardware hapter below. B. Sensor unit The sensor unit is the transmitter, whih transmits data to the entral unit with ertain period. For example, this an be temperature sensor whih sends the measured temperature one every 30 seonds. In fat we an divide this unit into two parts: sensor itself (e.g. temperature sensor as an integrated iruit as available from semiondutor manufaturers, e.g. with PWM output) bus module, whih handles the ommuniation with the entral unit. In our design the sensor unit is equipped with simple 8-bit miroontroller whih handles both the ommuniation on the bus and with the sensor itself. Issue, Volume 6, 0 8

3 interpreted as one or zero depends on its length. Nominal length of 0.8 ms represents, length 0.35 ms represents 0, see figure below. The software allows for inauray in the length up to ertain limits these limits were experimentally set to +/- 0. ms. Voltage drops of a length whih does not fall into the tolerane are onsidered a noise and ignored. 0.8 ms Fig. 3 Sensor unit hardware Three sensor types are supported: sensors with analog output, sensors with PWM (or other digital) output, sensors with serial interfae (SPI, IIC). This allows onneting wide range of available sensors to the network. For example, the analog interfae an be used to onnet sensors with voltage (e.g. 0-0V) or urrent output (e.g. 4-0 ma). The interfae for onneting digital (PWM or other) signal allows onneting sensors smart sensors, suh as pre-alibrated temperature or humidity sensors. The serial interfae then allows onneting devies with IIC and SPI interfae, whih inludes even aelerometers, gyrosopes and so on. C. Programmer unit The programmer unit is devie whih is used for initialization of the sensor units. It is used only when the sensor network is set up or re-onfigured. It is not needed for normal operation of the system. The sensor units are assumed to have their miroontrollers programmed before assembly (soldering them to the PCB) and given the overall prie of the unit it is not neessary to allow for firmware hange in the miroontroller. However, it is possible to store some onfiguration data into the MCU, suh us unique serial number of the sensor, period of transmission and possibly also other parameters. The programmer unit makes it possible to write these data into the sensor unit. The programmer unit is a simple MCU-based devie, whih is onneted to the sensor unit to be programmed. It is also onneted to a PC, whih provides user interfae for setting up the parameters (there is no interfae on the programmer unit itself). The PC an provide additional servies, for example, store the database of sensor IDs. IV. COMMUNICATION PROTOCOL As mentioned above the sensor units transmit data to the entral unit by the means of lowering the voltage of the power soure, whih is monitored in the entral unit and deoded as logial ones and zeroes. It uses PWM enoding with period of ms; the pulse is the voltage drop. Whether this drop is ms Fig. 4 The pulse for log ms ms Fig. 5 The pulse for log. 0 Data are sent in pakets. Eah paket begins with 3 ms start pulse, delay of ms, then transmission of the data bits with ms period. The struture of the data an be seen in Fig. 6. Byte Byte Bytes (max. 55) Byte Sensor ID Length Data, Data,, Data N CRC 8 Fig. 6 data paket struture There is byte for the ID of the sensor, one byte with length of the data, the data itself and then byte CRC. The CRC is used for detetion of errors inluding ollision (if more than one transmitter transmits at the same time). V. HARDWARE OF THE SYSTEM Hardware of the system an be divided into the three types of units: sensors transmitting data to the bus, entral unit reeiving data from the bus and finally programmer unit whih is used to program onfiguration data to the sensor using simple TTL UART interfae. Sensor and entral unit hardware design is based on 8-bit miroontroller Freesale MC9S08SH8. It is a member of low-ost, general purpose, high-performane 8-bit flash-based miroontrollers with Von-Neumann arhiteture. Central proessor unit with enhaned HCS08 ore is fully upward ompatible with Freesale HC05 family. CPU arhiteture is Issue, Volume 6, 0 83

4 fully optimized for C language ompilers. On the hip are integrated many of modules, for example []: On hip 8 KiB FLASH memory with in-iruit programming apability 5 B on-hip RAM Up-to 40 MHz CPU lok speed Internal lok generator Two -hannel 6-bit timer/pulse width modulator modules (TPM) Serial ommuniation interfae (SCI) Serial peripheral interfae (SPI) 0-bit Analog-to-digital onverter with -hannel analog multiplexer 7 general-purpose I/O pins, output only Wathdog system and other modules, see []. A. Central unit Central unit utilizes same miroontroller type as sensor unit. Its hardware an be divided to six funtional bloks: High-pass filter and over-voltage protetion Low-pass filter Amplifier Control logi Communiation interfae Power supply Bus voltage first enters high-pass filter with over-voltage proteting iruit eliminating DC omponent from useful signal. Then signal shape is orreted in the seond order Butterworth type ative low pass low-pass filter with Sallen Key topology implemented by operational amplifiers (Fig. 9). Filter parts was designed for utoff frequeny of 5000 Hz and gain of 0 db in the passband. Parts values were designed using proedure published in [7]. Computation is based on transfer funtion of the Sallen- Key nd order low-pass filter () where oeffiients a and b are equal to () and (3). A = A () + a s + b s 0 ( s) [ C ( R + R ) + ( A ) R ] a = ω () b 0 C ω RRCC = (3) Final transfer funtion of the nd order low-pass Sallen-Key filter is: A ( s) 0 = (4) [ C ( R + R ) + ( A ) R C ] s R R C C + ω + s 0 ω where ω is a utoff angular frequeny, A 0 is gain of the filter in the passband and a and b are filter oeffiients determining its properties. After the formulation of R from equation (3) and onstituting to () we obtain quadrati equation: A Fig. 7 Complete shematis of the entral unit Issue, Volume 6, 0 84

5 R ( C + C A C ) 0 C C arcc ω + b 0 = ω. (5) Its solution is equation for omputation of R part value (6), R part value an be omputed by (7). R b = (6) RCC ω acc ω + R = ( a C C ω ) 4C C ω b ( C + C A C ) Cω C 0 (7) Fig. 9 high-pass and low-pass filters In order to obtain non-negative value under square root in (7), apaitor values must fulfill (8). C 4b A0 + a A0 a C (8) a A0 Pratially the easiest way is to hoose first apaitors C and C manufatured usually in E6 series and then ompute resistor values. Exat resistor value an be reahed by onneting more resistors in parallel or in series. After amplifiation to the suitable level in the seletable gain amplifier (Fig. 8) onsisting of parts IC4C and IC6 is signal diretly brought to the analog input of the miroontroller. Communiation interfae is in basi version standard RS3 realized by TTL to RS3 level onverter MAX3 (Fig. 0) onneted to MCU UART interfae. There is an option to use USB interfae utilizing USB UART interfae FT3BM integrated iruit [3]. In this ase omputer detets entral unit as standard USB serial port devie. Fig. 8 Seletable gain amplifier B. Sensor unit Fig. 0 Communiation interfae Sensor unit hardware an be divided to the three funtional bloks: Power supply Modulator Control logi The omplete shematis of the sensor unit an be seen in the Fig.. Input voltage from the bus enters power supply iruit whih is based on three terminal positive voltage regulator 78L05 with output voltage of 5 V and 00 ma output urrent apability. It is used for supply digital parts suh as miroontroller and sensor onneted to sensor unit I/O interfae. Diode D onneted before regulator protets sensor units from polarity reversal and separates filtered supply voltage from the bus too. Without this diode it would be impossible to effetively modulate bus voltage when data transfer takes plae due to high-apaity filter apaitors in eah sensor module. From the non-stabilized filtered bus voltage is supplied operational amplifier LM358 whih is used in modulator and bus voltage sensing feedbak iruit. All sensor unit funtions are ontrolled by miroontroller Freesale MC9S08SH8 (IC). System lok is generated by Piere rystal osillator with output frequeny of khz (Q) onneted to EXTAL and XTAL pins of the MCU. External referene lok is by internal MCU s PLL iruit inreased to 40 MHz resulting in internal bus lok frequeny of 0 MHz. Modulator iruit onsists of operational amplifier IC3A and MOSFET IRF630 (T) ating as onstant urrent Issue, Volume 6, 0 85

6 load swithed with PTC3 pin of MCU to on or off state leading to the bus voltage variations when data are transmitted. Before and during data transmit MCU is sensing bus voltage whih is to appropriate level for A/D onverter adapted using voltage divider R5, R6, R7 and operational amplifier IC3B. Bus voltage sensing is used for ollision detetion and monitoring of modulator funtion. Sensor is onneted to the 4 pin sensor I/O interfae SV providing two analog inputs omplete SPI interfae (signals MISO, MOSI, SPSCK), two general purpose digital inputs and outputs and regulated 5 V supply voltage. Sensor unit onfiguration an be updated by TTL UART interfae onneted to 3 pin pinheader SV3. BDM interfae onnetor SV is used for devie firmware updating respetively for debugging in SW development stage. Fig. Power supply and modulator shematis VI. SOFTWARE OF THE SYSTEM The software onsists of programs for the MCUs in the entral unit, sensor unit and programmer (embedded software) and the programs for PC whih allows ommuniation with the entral unit and the programmer unit (PC software). A. Embedded Software The embedded software is written in C language. It is developed in Freesale Code Warrior IDE, whih is the development tool provided by Freesale for their miroontrollers. First version of the program is implemented in plain C language but for future development we intend to use real-time operating system RTMON [6] to make the development easier even with more features implemented in the program. The program for the transmitter performs the following tasks (see the figure below): Chek if there is a request from the programmer unit for setting up the parameters of the sensor unit. If there is suh a request the unit enters speial programming mode in whih it reeives onfiguration parameters from the programmer unit and saves them into internal flash memory. This ommuniation is arried out via serial interfae (UART). During normal operation the transmitter unit reads data from the sensor. The exat way of doing this depends on the type of the sensor whih is onneted. This type of sensor is defined by parameter in the devie memory. For example, if the onneted sensor has analog output, reading the data involves performing analog-to-digital onversion on the built- Fig. Complete sensor unit shematis Issue, Volume 6, 0 86

7 in AD onverter of the MCU. If the sensor type is PWM or other digitally oded type, reading data involves proessing the digital input on the MCU input pin. The data from the sensor are then filtered and transformed to required range (as defined by the onfiguration parameters). Last part of the proess is transmitting the data to the entral unit. This is done using PWM modulation as desribed in the hapter about ommuniation protool. In priniple the MCU uses its output pin (binary output) to open MOSFET transistors whih applies load to the power supply and thus reates a small voltage drop on the line. pulse is valid, i.e. if its length falls into the limits set for logial 0 or logial ; it means new bit has just been reeived. This bit is stored into a variable at the appropriate position, whih depends on the reeived bits ount. One whole byte is reeived, it is passed to the main loop of the program for further proessing. If at any time invalid bit is reeived (i.e. there is a pulse deteted with length whih does not fall into limits for neither logial 0, nor logial ), the reeived bit ount is reset and also the program resets it internal state and waits for new start pulse. This means the whole paked is disarded. TIM interrupt The program for the entral unit (reeiver) ontinuously monitors the voltage on the power line. The hardware of the reeiver amplifies the voltage differenes and provides this amplified voltage to the AD onverter in the miroontroller. The MCU samples the analogue input and detets the pulses whih enode the data. The flowhart of evaluation of the pulses an be seen in figure below. It depits the part of the program whih validates the reeived pulse (this is done in interrupt handler). If the START Chek for programming request (parametrization) Pulse on input deteted? YES Pulse length within limits for log. 0? NO NO YES Store log. 0 Read data from the sensor Proess the data (filtering, transformation) Pulse length withing limits for log.? NO YES Store log Transmit the data Disard urrent data paket (invalid data) Wait for speified time Return Fig. 3 Flowhart of the transmitter program Fig. 4 Flowhart of the reeiving interrupt handler Issue, Volume 6, 0 87

8 B. PC Software The PC software is still under development. It is planned that there will be graphial user interfae for setting up the network and for setting the parameters in the sensor units. This program will allow for preparing the network and for basi visualization and storage of the data obtained from the sensor. For proessing the data by 3 rd -party programs there should be open interfae (e.g. a DLL library or some ommuniation protool) whih will allow users to use the data in their own programs. We also plan to reate drivers for ommonly used tools suh as Matlab or Control Web, whih will allow easy integration of this sensor network with these programs. Currently a terminal program is used on PC for ommuniating with the entral unit and programmer. The standalone GUI appliations whih will allow more omfort in ontrolling the units should be implemented in future. VII. CONCLUSION This paper desribed system for olleting data from sensors using just two wires. The same wires are used both for powering the sensors and for ommuniation between the sensors and the entral unit whih ollets the data. The system an be used in appliations whih require monitoring several quantities in a number of plaes, suh as, for example, monitoring the temperature and humidity in a building or monitoring various quantities in a tehnologial proess. The omponents of the system are based on 8-bit miroontrollers, whih allow them to be low-ost and simple to implement. Currently, the system development is still in progress, more features will be added in future. REFERENCES [] Freesale Semiondutor. MC9S08SH8 Data Sheet rev.3, 008. Available from WWW: < Aessed: [] Freesale Semiondutor. CPU08 Central Proessor Unit, 00. Available from WWW: < Aessed: [3] Future Tehnology Devies International. FT3BM USB UART IC Data Sheet, 00. Available from WWW: < Aessed: [4] Dolinay, J., Dostálek, P., Vašek, V., Kolomazník, K. & Janáčová, D. Modernization of Control System for Enzymati Hydrolysis, WSEAS Transations on Systems and Control, 009, pp [5] Morton, T. D., Embedded Miroontrollers, Prentie Hall, 00. [6] Dolinay, J., Dostalek, P., Vasek, V., Implementation and Appliation of a Simple Real-time OS for 8-bit Miroontrollers, In proeedings of the 0th WSEAS International Conferene on Eletronis, Hardware, Wireless and Optial Communiations (EHAC'), Cambridge 0, pp.03-06, ISSN [7] Texas Instruments. Op Amps for Every One - Design Referene [online]. 00. [it ]. Available on WWW: < Issue, Volume 6, 0 88