IMPROVEMENT OF METROLOGICAL CHARACTERISTICS OF PORTABLE IMPEDANCE ANALYZERS ABSTRACT

Transcription

1 U (Pape): un:nbn:de:gbv:ilm1-2014iwk-132:4 58 th ILMEAU SCIETIFIC COLLOQUIUM Technische Univesität Ilmenau, Septembe 2014 U: un:nbn:de:gbv:ilm1-2014iwk:3 IMPOVEMET OF METOLOGICAL CHAACTEISTICS OF POTABLE IMPEDACE AALYZES B. Stadnyk 1) / T. Foehlich 2) / Y. homa 1) 1) Lviv Polytechnic ational Univesity 2) Ilmenau Univesity of Technology ABSTACT eed fo potable impedance analyzes has been poven in the pape. A novel concept of impovement of the metological chaacteistics fo potable impedance analyzes has been developed. It s based on minimizing the analog pat of the measuing channel and compensating fo eos using algoithmic coection and digital signal pocessing. Hadwae implementation of potable analyze based on digital signal contolle and modified autobalancing cicuit ae pesented in this aticle. Also single-point Fouie tansfom in combination with Blackman-Hais window has been poven as the best solution fo quadatue convesion of measuing signal ove a wide fequency band. Coection algoithms have been developed in ode to minimize dynamic eos, and thus to impove the systems' accuacy. Application of algoithmic coection allows to eliminate dynamic eos and extend the fequency ange. Index Tems impedance spectoscopy, potable impedance analyze, autobalancing cicuit, single-point Fouie tansfom, windowing, algoithmic coection 1. ITODUCTIO Impedance spectoscopy is a measuing technique which is widely used in vaious applications, such as biomedical measuements [1,2,3], mateials engineeing [4], paticulaly at the mico- and nanoscale [5], coosion monitoing [6], contol of batteies and fuel cells paametes [4,7]. In many cases studies ae pefomed on the objects located in non-laboatoy conditions. A good example is contol of coosion-esistant coatings on bidges, pipelines and othe steel stuctues [8]. This leads to the need fo a small-sized measuement devices with low powe consumption called potable impedance analyzes. owadays, most of the impedance measuement instumentation ae pesented with stationay devices, poduced by such leading manufactues as Agilent Technologies, Wayne e Eleconics, Solaton Analitical [9,10,11]. These devices ae multi-pupose, and can opeate not only as impedance analyzes, but also as spectum and netwok analyzes. Howeve, in most cases such wide functionality is edundant, and affects in high pice and big dimensions. Fo example, impedance analyze Agilent 4294 has a weight of 25 kg and powe consumption of 300 VA and costs moe than $ 45,000 [9]. Some companies poduce small potable impedance metes, but these devices pefom measuements on one o seveal fixed fequencies ove a limited band. One of the most advanced in this class is Motech MT-4080A LC-Multimete (Taiwan) with acceptable weight (0.47 kg) [12]. Howeve, the uppe fequency limit of the device is 100 khz. Theefoe, the aim of this pape is to design the stuctue of the potable impedance analyze and develop a concept fo impovement of its metological chaacteistics by using digital signal pocessing and algoithmic coection TU Ilmenau

2 2. POTABLE IMPEDACE AALYZE DESIG The esults of impedance spectoscopy ae data sets, which contain values of esistance and eactance in evey single point along the fequency sweep, thus it s impotant to achieve high dynamic pefomance of the device. Theefoe, the use of autobalancing method (modification of I-U method) fo impedance analyze design makes lots of sense, since this appoach povides elative high accuacy and speed. It also is simple in hadwae implementation (in compaison to bidge o auto-compensation cicuits), that means benefits in weight, costs and size [4,13]. Anothe fundamental equiement to potable analyzes is one-channel stuctue (also diectly elated to the size and cost). When designing impedance analyzes, the complex issue is to ensue the stable metological chaacteistics of the measuement channel acoss a wide fequency and measuing ange. The cuent investigation allowed to develop a novel concept to impove the metological chaacteistics of potable analyzes. This concept is based on minimizing the analog pat of the impedance analyzes measuing channel and on compensating fo eos using algoithmic coection and digital signal pocessing [14]. Accoding to the developed concept, using diect digital synthesis fo pobe and othogonal signals geneation in potable impedance analyzes makes a lot of sense. The existing types of diect digital synthesizes have vaious disadvantages. It makes thei application as signal souce in impedance spectoscopy poblematic. Theefoe we popose a new modified stuctue fo diect digital synthesizes, based on a combination of binay counte and phase accumulato. We have investigated the eos and spectal composition of this modified synthesize, and detemined its equied paametes. Figue 1 shows the hadwae implementation of potable analyze. It is based on the digital signal contolle STM32F4 (32-bits AM Cotex-M4F coe with Floating Point Unit). Two intenal 12-bit DAC (sampling ate up to 1 MHz) and thee intenal ADC (sampling up to 7 MHz) enables diect pocessing of analog pobe and measuement signals. PC communication and contol is pefomed via buit-in USB module [15]. Fig. 1. Block-diagam of potable impedance analyze TU Ilmenau 2

3 In the poposed stuctue, most measuing convesions (except impedance-to-voltage convesion) ae implemented in digital fom. In addition, the stuctue of the measuing channel includes no analog devices, such as filtes, due to the hazad of uncontolled phase shifts. This digitized stuctue of measuing channel enables to each potentially bette metological chaacteistics as the eos ae caused mainly by analog measuing convete. Howeve, the impact of these eos can also be educed by means of DSP and algoithmic coection on the stage of measuing data post-pocessing [14]. 3. IMPOVEMET OF ACCUACY OF DIGITAL BLOCS It is ecommended to pefom quadatue convesion of measuing single (pocedue that etuns impedance quadate components esistance and eactance) on the base of singlepoint Fouie tansfom (SPFT). SPFT is a special case of the discete Fouie tansfom and can be descibed by the following equation: ( f ) u i cos i j sin i, (1) Z Z U i 0 whee u Z (i) i-sample of the measued signal; numbe of samples used in the single-point Fouie tansfom (one section size); ν atio of obsevation time to the peiod of the pobe signal; U Z (f) spectal intensity of the measued signal at the fequency f; cos(2πνi/) and sin(2πνi/) digital othogonal signals [16]. The advantages of the SPFT algoithm ae simultaneous calculation of esistance and eactance, simple softwae/hadwae implementation and noise eduction. Also it s indiffeent to all kinds of DC offsets and paasitic hamonics in measuing signal. Howeve, the main disadvantage of the single-point Fouie tansfom is its sensitivity to spectal leakage, which significantly educes accuacy of impedance measuements. The most suitable DSP solution, which allows to ovecome this issue is windowing. Fig. 2. esistance (left) and eactance (ight) leakage eos afte application of Blackman- Hais window TU Ilmenau 3

4 The efficiency of vaious window functions has been investigated, but the best esults wee achieved fo the Blackman-Hais window. In this case leakage eos become negligible (less than ± 0.1 %) fo the signals with the peiod two times smalle than the obsevation time T O /T 2 (see Fig. 2). Thus, window functions allow to impove accuacy of impedance measuement ove a wide fequency ange. Howeve, this appoach equies time edundancy (inceasing of the obsevation time). 4. IMPOVEMET OF ACCUACY OF AALOG MEASUEMET COVETE As it was mentioned above, measuing convete emains analog and makes the biggest contibution to the total eo of the impedance analyze. The application of autobalancing cicuits as measuing convetes have been poven advantageous. This appoach allows to each high accuacy and esolution, good dynamic chaacteistics and lineaity, stable cuent and voltage conditions on the object unde test, as well as simple (one-channel) stuctue of measuing channel, small dimensions, low weight and low powe consumption. To pevent unwanted effects (fist of all self-oscillation and paasitic inductions) autobalancing cicuit has been modified as shown on the figue 3 [17, 18]. ẎZ I H I L U L U H Measuing conveto U + U - s 1 s U P U Z Active cable. OpAmp s. 1 s 1 OpAmp DiffAmp Fig. 3. Modified autobalancing cicuit The most citical impact on the system s accuacy has limited amplifies bandwidth (small gain on the high fequencies), what esults in dynamic eos. Theefoe algoithmic coection has been poposed fo elimination of these eos: H H H H H H T H H 1 H 1 H H T H (2a) (2b) whee H, H aw measuing esults of esistance and eactance coespondently;, TU Ilmenau 4

5 measuing esults afte coection; Δ, Δ coection values; amplifies open-loop gain on fequency of measuing signal; T amplifies time constant. Figue 4 pesents impedance measuement eos befoe and afte coection. The pesented esults have demonstated the high efficiency of the coection algoithm. Application of algoithmic coection allows to extend device s bandwidth fom 5 khz to 200 khz. Fig. 4. Dynamic eos of impedance analyzes befoe (cuve 1) and afte (cuve 2) coection 5. COCLUSIOS Applications of the potable impedance analyzes, as well as issues associated with thei metological chaacteistics ae pesented in the aticle. The cuent investigation allows to develop a novel concept to impove the metological chaacteistics of potable analyzes. This concept is based on minimizing the analog pat of the impedance analyzes measuing channel and compensating fo eos using algoithmic coection and digital signal pocessing. Stuctue of autobalancing measuing cicuit has been impoved in ode to pevent selfoscillations and inductions. Equations fo algoithmic coections of dynamic measuing eos have been developed. Applying algoithmic coection allows to eliminate these eos and thus to impove the accuacy and extend the fequency ange. Advantages of quadatue convesion of measuing signal based on single-point Fouie tansfom in combination with windowing have been poven. Application of Blackman-Hais window allows to educe leakage effect and thus to impove pecision of quadatue convesion damatically. Designed system is able to measue impedance in ange fom 10 Ω to 100 kω and pefom sweep in fequency band fom DC to 200 khz with step 0.1 Hz. Measuements can be pefomed only if eactance and esistance vaies less than 10 times. Fo measuements on the fequencies highe than 1 khz algoithmic coection should be pefomed. Total system accuacy is 1 %. Fo example othe single-chip potable impedance analyzes (mostly based on impedance-to-digital convete AD5933) have accuacy appoximately of 2 % o moe. EFEECES [1] Gimnes S., Matinsen O.G. Bioimpedance & Bioelecticity Basics. Second Edition: Academic Pess, Elsilve, p. [2] Matinez F.S. Electical Bioimpedance Ceebal Monitoing: Fundamental Steps towads TU Ilmenau 5

6 Clinical Application: Thesis fo the degee of docto of philosophy / Chalmes univesity of technology, Götebog, Sweden, p. [3] Jaffin M.Y., Moel H. Body fluid volumes measuements by impedance: A eview of bioimpedance spectoscopy (BIS) and bioimpedance analysis (BIA) methods // Medical Engineeing & Physics 30 (2008).- p [4] Basoukov E., Macdonald J.. Impedance spectoscopy: theoy, expeiment and application. -.Y.: Willey, p. [5] O Haye., Lee M.,. Pinz F.B, alinin S.V. Fequency-Dependent Tanspot Imaging by Scanning Pobe Micoscopy // Scanning Pobe Micoscopy. Spinge, р [6] Caullo A., Pavis M., Vallan A. Fast impedance analyse fo coosion monitoing // VI IMEO Wold Congess 2000, Vienna, Austia 2000, Vol. VI, TC-10, p [7] aden E., Bulle S., De Doncke.W. A method fo measuement and intepetation of impedance specta fo industial batteies // J. Powe Souces Vol. 85. P [8] Goja J., Lentka G. A family of new geneation miniatuized impedance analyzes fo technical object diagnostics // Metology and Measuement Systems, Vol., (2013), o. 1, p [9] Agilent 4294A Pecision Impedance Analyze 40 Hz to 110 MHz. Technical Oveview / [ [10] LC Metes 4260, 4265, Wayne e Eleconics Catalogue / [ [11] Impedance/gain-phase Analyze Solaton Analytical Bochues / [ [12] 100 Hz LC Mete МТ 4080А / [ [13] Agilent Impedance Measuement Handbook. A guide to measuement technology and techniques: 4-th Edition. Agilent Technologies, Inc. Pinted in USA, (2009), p. [14] Stadnyk B., homa Y., Impoving the Accuacy of the Single Chip Impedance Analyze fo Senso Applications // Sensos and Tansduces, vol.150,.3 Mach 2013 p [15] STM32F405xx/07xx, STM32F415xx/17xx, STM32F42xxx and STM32F43xxx advanced AM-based 32-bit MCUs. M0090 efeence manual. [16] Lyons, ichad G. Undestanding digital signal pocessing. 3-d ed. Peason Education, Inc., Ann Abo, Michigan, p. [17] Sean Bennan, Measuing a Loudspeake Impedance Pofile Using AD5933// Analog Devices Application ote A-843. ( [18] Analog Devices, 1 MSPS, 12-Bit Impedance Convete, etwok Analyze AD5933, Data Sheet, pp ( COTACTS Pof. D.-Ing. habil. B. Stadnyk Pof. D.-Ing. habil. T. Föhlich D.-Ing. Y. homa stadnyk@lp.edu.ua thomas.foehlich(at)tu-ilmenau.de khoma.yuiy@gmail.com TU Ilmenau 6