Design of on-line PH monitoring system based on ZigBee technology

Transcription

1 Internatonal Conference on Advanced Electronc Scence and Technology (AEST 216) Desgn of on-lne PH montorng system based on ZgBee technology a Lad Wang and Xul Zhang College of Electronc and Informaton Engneerng, Zhejang Wanl Unversty, Nngbo 3151, Zhejang,Chna Abstract. The ph of water s very mportant to human health and aquatc products breedng. Tradtonal PH detecton technology cannot meet the requrements of real-tme, dynamc and effcent montorng, ths paper desgns an on-lne PH montorng system based on ZgBee technology. The system s based on the ZgBee protocol, usng the SoC chp CC253 whch s completely compatble wth protocol 27 and manufactured by the TI company, DHT11 temperature and humdty sensor and the PH composte electrode, for real-tme acquston and montorng of PH value. System also uses a lnear regresson algorthm to deal wth PH value, n order to reduce the nfluence of temperature on PH measurement precson. Comparatve measurement datum after processng show that we elmnate the nfluence of temperature on the PH montorng wth temperature compensaton model. Keywords: ZgBee; wreless sensor network (WSN); CC253; PH montorng. 1 Introducton PH value nclude chemcal analyss, test paper analyss and potentometrc analyss. Both of the former cannot realze real-tme onlne measurement of PH value, and the potentometrc analyss method usually use composte electrode sensor, ths knd of nstrument s small, easy to use and the measurement data are accurate and relable. Combned wth wreless sensor network technology whch has been concerned n the current nternatonal, can real-tme montorng, awareness and acquston of all knds of water envronment nformaton, so as to take tmely and effectve measures to control water qualty, to guarantee the qualty of water, reduce the labor ntensty of workers have mportant practcal sgnfcance. Ths paper desgns the wreless sensor network water qualty montorng system based on ZgBee technology, ths system has low operatng cost, low power consumpton, hgh relablty and broad applcaton prospects. 2 ZgBee technology ZgBee s the standard of a seres of communcaton protocols for wreless communcaton whch defnes the short dstance, low data transmsson rate[2]. Its characterstcs consst of hgh relablty, low cost, low power consumpton, hgh safety and low data rate and manly used for short dstance wreless control system to transfer a small amount of control nformaton. The workng frequency of wreless network based on ZgBee s 868 MHZ (Europe), 915 MHZ (USA), 2.4 GHz (global), and the a Correspondng author : @qq.com 216. The authors - Publshed by Atlants Press 469

2 AEST216 maxmum data transmsson rate s 25kbps. ZgBee protocol s on the bass of the OSI reference model, combned wth the characterstcs of wreless network layered, IEEEE only defnes the ZgBee physcal layer (PHY) and meda access control layer (MAC), ZgBee protocol defnes the network (NWK) layer, applcaton support (APS) and the applcaton layer (APL) data transmsson specfcaton [3]. 3 The overall desgn of PH montorng system The overall framework of the PH water qualty montorng system s shown n Fgure 1. A pluralty of data acquston nodes are dstrbuted n the montored area, and each node form a self-organzaton network by adoptng the ZgBee protocol, each node collects the ph value and temperature regularly and sends t to the data snk node through the ZgBee network. Fnally, data snk node transmts the data to the montorng center computer n real-tme through the seral port. Accordng to the montorng center receves the data to calculate the ph value of each acquston node. Fgure 1. The framework of PH montorng system. 4 The hardware desgn of the system The hardware of PH montorng system s composed of ZgBee network and gatherng node, and ZgBee network s composed of many termnal acquston node, the man control part of the termnal acquston node and coordnator node adopts the same CC253 master control chp. Every termnal acquston node ncludes master control module, PH acquston module, the temperature and humdty acquston module and A/D converson module. 4.1 Master control module desgn Master control module ncludes RF chp, antenna, power supply, communcaton nterface and other parts, RF chp select CC253 as the core chp. CC253 has a hgh performance 2.4GHz drect sequence spread spectrum rado frequency transcever and a hgh performance, low cost 851 mcrocontroller core, to meet the requrements of low cost, low power consumpton of wreless sensor networks, and has a strong ant-nterference ablty, won't make the transmsson of data naccurate or dstorton [4].Master control module crcut s shown n fgure 2. 47

3 AEST216 Fgure 2. CC253 master control module crcut dagram. 4.2 PH acquston module PH value s a sgn of acd and alkal soluton, natonal standards for the dscharge of the ph value between 6~9. PH value s measured by the PH composte electrode, the ph value of the water can be calculated by the electrc potental whch s produced by the dfferent of the acdty and alkalnty of the water. Due to PH composte electrode s nfluenced by ts hgh resstance, so the output sgnal s small, n order to satsfy the nput range of the A/D analog dgtal converson crcut, the output sgnal must be amplfed. As shown n fgure 3, the op-amp dfferental amplfer crcut s used to acheve values n the range of ~ 3.3 V, and mprove the accuracy of the data acquston. The output of the crcut s connected to the P_7 pn of the CC253 master control chp. Fgure 3. PH measurement adjustment crcut. 4.3 Temperature and humdty acquston module Temperature and humdty acquston are realzed by DHT11 sensor. DHT11 s a knd of temperature and humdty sensor whch has been calbrated dgtal sgnal output, and the sensor has hgh relablty and long-term stablty. As shown n fgure 4, the P11 pn s used for the communcaton and 471

4 AEST216 synchronzaton between the mcrocontroller and DHT11, the two sdes are usng a sngle bus data format for nformaton transmsson, a communcaton tme 4ms. DHT11 wll not take the ntatve to collect nformaton, t wll begn by recevng from the termnal devce to start the sgnal. After fnsh collectng the data, t wll transton to the low speed mode. Fgure 4. Temperature and humdty acquston crcut. 4.4 A/D converson module The PH sensor n ths system needs to change the analog sgnal to dgtal sgnal. CC253 has an analog multplexer wth 8 confgurable channels. ADC supports 14 bt analog dgtal converson, and has as many as 12 bts of vald dgt. Regsters are ADCL (storage low part of the ADC converson results), ADCH (storage hgh part of the ADC converson results), ADCCON1, ADCCON2 and ADCCON3 (the three control regsters are used for ADC confguraton state, converson sequence, sequence of executon mode converson, reference voltage, samplng rate selecton etc.), the ADCCON1.EOC bt s the status bt, whch s 1 when the converson s fnshed; the ADCCON1.ST bt s used to start a AD converson; the ADCCON2.SREF s used to select the reference voltage. The system chooses the 3.3V as the reference voltage, and uses the 12 bt converson precson, that s to say, 3.3V corresponds to 496 of the converson value, the complete sgn of converson can be quered by scannng or nterrupt the way. 5 The system software desgn Software desgn of the system s realzed on the bass of the hardware platform, and t s an mportant part of the system functon. The system software ncludes 3 parts, ZgBee network software, termnal node software and coordnator software. Software development envronment s IAR Embedded Workbench (EW), ts C cross compler s a complete, stable and Easy to use development tools. At present, t can support at least 35 knds of ARM mcroprocessor archtecture. It has the characterstcs of a fully compatble wth standard C language, the correspondng chp program rate and nternal optmzer, memory mode selecton and effcent Pro Mable. The use of the debugger s SmartRF4EB whch s recommended by TI, and ts program download speeds up to 15Kb/s, and support the TI company's producton wth 8C51 kernel of ZgBee chp, can be seamless connecton wth IAR ntegrated envronment[5]. 5.1 ZgBee network software desgn ZgBee network protocol stack s Z-Stack, t's clear structure, easy to transplant, herarchcal s able to run n a mult taskng envronment n the code. The whole Z-Stack uses a layered software archtecture, ncludng the PHY layer, MAC layer, NWK layer, APS layer, APL layer, securty layer and operatng system abstracton layer. The man functon n ZMan, manly ncludes two parts, namely the system ntalzaton process and mplementaton of operatng system enttes. Among them, the "osal start system()" s the operatng system enttes code, t s a dead cycle, and contnuous query each task n whether there s any event, f there s an event, through comparng the prorty, executve correspondng processng functon; If no event occurs, then contnue to query the next task, so keep the cycle. 472

5 AEST Termnal node software desgn The termnal node s responsble for data collecton, processng and communcaton of the collected data. After start-up s ntalzed frst, then choose communcaton channels, automatc scannng network and jon. After the success of the network, the termnal wll be receved from the coordnator of the acquston request, and then start the tmer capture, nodes receve tmng acquston news(work_time_evt), to complete the correspondng data collecton, and then by callng the functon AF_DataRequest () to complete the transmsson of data. The termnal sends the data to the coordnator n the form of uncast, and the coordnator receves the nformaton n the form of broadcast. The program flow chart s shown n fgure 5. start devce ntalzaton detect the coordnator message jon network falure no no whether to receve the message whether to jon the network successfully yes yes record the data from the sensor transmt the data from the sensor enter a dormant state, wake up regularly Fgure 5. Flow chart of termnal node. 5.3 The coordnator software desgn Coordnator s qute mportant for network, on the one hand,t should be responsble for the formaton of the Internet and the management, on the other hand also receve data from the termnal node, and make them converge to the montorng center. After the start, frst of all, I/O port, seral port and RF system ntalzaton, and then select the correspondng channel to form a network, Then enter the OSAL schedulng system, OSAL wll regularly scan the message status table to see f the new message s receved, f have, OSAL wll call the correspondng processng program. The news whch need to deal wth conssts of RF receve messages(af_incoming_msg_cmd) and messages from the seral port control command (MT_SYS_CONTROL_MSG), Ths part of the software manly ncludes devce ntalzaton, coordnator network, node jon network, wreless data transcever and processng procedures, the program flow chart s shown n fgure

6 AEST216 start devce ntalzaton Whether to buld the network successfully no yes watng to receve data request to jon the network nodes judge the receved data collected sensor data assgn a network address to a request node va a seral port to the PC send start command to network nodes Fgure 6. Flow chart of Coordnator. 6 The data processng method Because of the montorng system nstalled n damp envronment for a long tme, temperature on the PH measurement results had a greater nfluence. In order to reduce the nfluence of temperature on the output sgnal of PH sensor, after extractng the orgnal data from the sensor, the temperature compensaton s needed to mprove the accuracy of PH detecton. From the lterature 6 we can see that when the temperature s kept constant, the ph value and the PH composte electrode potental bascally meet the lnear relatonshp. A lnear regresson analyss method s used to compensate the temperature of the sensor, assumng that the output voltage of the sensor s x, the theoretcal value of the lqud PH s y, to make regresson analyss for y = ax + b, n whch a and b are the regresson coeffcent[6-7]. In order to determne the regresson coeffcents a and b, least square method s usually used to acheve the mnmum value of Q = [ y ( ax + b)]. Accordng to the prncple of extreme value, a and b satsfy the followng equaton: 2 Q = 2 [ y ( ax + b)]( x) = a Q = 2 [ y ( ax + b)]( 1) = b (1) (2) 474

7 AEST216 Soluton a = ( x x)( y y) ( x x) 2 b = y ax n whch x= x / n y = y / n The regresson coeffcents a and b of the three knds of temperature are calculated by monadc lnear regresson analyss method, and the results are shown n Table 1. Table 1. Values of regresson coeffcent a and b n three knds of temperature. Temperature / C Slope (a) Constant (b) Analyss of the change of the slope a n the table, temperature 1 C per change, a value changes are.97,.15, b value changes are.688,.715,the average value of a and b are.11,.715, from whch can calculate the slope change rate of the system s Δ a =.11 V / C (PH), the rate of change of b s Δ b =.715 / C. Fnally, the temperature compensaton model of PH sensor s obtaned: PH = ( T ) E +.715T Expermental test results analyss In the laboratory condton, I selected three concentratons of solvents (4.,6.86,9.18) and measured them. As shown n table 2, Compared wth measurement of ph value and standard value, t can be seen that the error s very small. It shows that the measurement accuracy s better, and t s proved that the lnear regresson analyss s effectve when the temperature compensaton of the PH sensor s processed. Table 2. Expermental results. PH standard value Temperature / C PH measured value before processng Error before processng PH value after temperature compensaton Error after temperature compensaton Concluson Nowadays, the rapd development of wreless sensor networks has become a hot research topc n the feld of scence and research. Ths paper s based on the characterstcs of PH montorng network, analyss the desgn requrements of montorng nodes, through the n-depth study of types of sensor 475

8 AEST216 and wreless RF chp, On the bass of CC253 and Z-Stack protocol stack, an onlne ph montorng program based on ZgBee s proposed. Compared wth other methods, the method has the advantages of low cost, hgh data precson, relablty and so on. At the same tme, the lnear regresson algorthm was used to analyze the measurement ph value, and the temperature compensaton model s deduced wth the combnaton of the temperature varaton. The expermental results show that the effect of temperature on PH montorng was elmnated after the temperature was processed wth temperature compensaton model. References 1. Standards for drnkng water qualty [EB/OL], [ ]. html 2. Zhong Jang, Dan Lu. ZgBee technology and realzaton course [M]. Bejng: Tsnghua Unversty press(214) 3. Xaoqang Wang, Ouyang Jun, Lnnng Huang. ZgBee wreless sensor network desgn and mplementaton [M]. Bejng: Chemcal Industry Press(212) 4. Shu Wang, Yuje Yan, Fupng Hu. Theory and applcaton of wreless sensor networks [M]. Bejng: Bejng Aeronautcs and Astronautcs Unversty Press(27) 5. Chun Jn, Zuqu Luo. ZgBee techncal foundaton and case analyss [M]. Bejng: Natonal Defence Industry Press(28) 6. Xaowen Chen, Xnde Ba, Xang Xue. Potental ph equlbrum dagram and ts applcaton n corroson studes of nuclear materals [J]. Tsnghua Unversty Journal, 42,5 (22) 7. Yao Chen, Yueju Xue.PH sensor temperature compensaton model [J]. Journal of sensng technology, 25,8(212) 476