Elektrokimyasal ölçümlerde kullanılma amacına yönelik işaret üreteci
A Function generator for electrochemical measurements
- Tez No: 39360
- Danışmanlar: DOÇ.DR. HAKAN KUNTMAN
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1992
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 51
Özet
ÖZET Bu çalışmada kimyasal çözeltilerin elektriksel özelliklerinin ve MOS kapasitelerin C-V eğrilerinin çı kartılmasında kullanılan olcum düzenlerinde kullanılan analog işaret Üreteci yerine, özellikleri ayarlanabilen özel bir işaret üretecinin sayısal olarak gerçekleştirilmesi amaçlanmıştır. Gerçeklenen cihazın Üzerinde bulunan tuş takımından, elde edilmek istenilen işaretin ± değerleri, iniş ve çıkış eğimleri, tepelerde bekleme sureleri, Üretilecek işaretin karakteristik verileri olarak girilirken, aynı anda göstergeden de izlenebilmektedir. İşaretin Üretilmesi ya bilgisayarla, ya da ayrı bir başlama tuşu ile başlatılabilmaktedir. Bilgisayarla başlatılması halinde, Üretilen çıkış geriliminin 12 bitlik sayısal değeri ve Üzerinde olcum yapılan düzenekten elde edilen analog çıkış değeri bilgisayara seri iletişim yolu ile gönderilmektedir. İşaret periyodik olmayıp, bittiğinde istenirse aynı özelliklerle yeniden Üretilebilmektedir. Farklı karakteristiğe sahip bir işaretin Üretilmesi istenirse tuş takımından yeni verilerin girilmesiyle, yeni işaret elde edilmektedir. Ayrıca cihazın çalışması sırasında Üretilen işaretin ve üzerinde olçüm yapılan düzenekten elde edilen değer cihaz Üzerindeki göstergelerden de izlenebilmektedir. ?iv-
Özet (Çeviri)
SUMMARY A FUNCTION GENERATOR FOR ELECTROCHEMI CAL MEASUREMENTS Electronic ling the potential rent in the cell» electrode is contr that the actual working electrode electrode follows held constant at i of electrochemical potentiostats are used for control - in an electrochemical cell. The cur- from a counter electrode to a working oiled by means of the potentiostat so potential of the cur rent -carrying relative to a current- free reference a predetermined control voltage and is ts instantaneous values irrespective processes. CFiq l.D. potentiostat,; cr '. Block diagram ci measurement sys: err. for electrochemical measurements Potentiostants in their electronic action are i- dentical with difference operational amplifiers whose power output provides voltage and current for the cell in the feedback loop. Unlike the usual application of operational amplifiers in which the feedback loop is de termined by separate electrical components, potential control by potentiostats includes an electrochemical cell in the control loop whose impedance varies greatly -v-with potential and time. Both the conductivity and the double layer capacitance of the boundary layer where the potential is being cont oiled vary over many decades as the potential changes. Information on the electrical replacement circuit of the control loop is often avail able only after experimental results with controlled po tential have been obtained. It is essential therefore that the potenti ostat provides suffiently accurate and stable control of the ceil potential over an extremely wide range of cell currents for extensive variations in the usual complex feedback circuit and in addition ope rates within a sufficiently short interval of time. In general the system operates by controlling the potential of a cur rent -carrying electrode., the wor king electrode or test electrode, relative to a refe rence electrode which carries no current. It may, ho wever, be necessary, e.g. in investigating ion exchange phenomena through a diaphragm, to control the potential difference between two reference electrodes, neither of which carries any current. 9^-^=HM Fig £. Block diaaran of POTEMTI OSTAT Potenti ostat consists of four main blocks, as seen Fig 2. Block 1 connected as a empedance converts'" is a unity gain amplifier. This block transfers the voltage of the reference electrode to potential output. -vi -Every potentiostat can be used as a gal variostat by modifying the connections to the electrochemical ceil Instead of controlling the potential difference in the cell, the instrument controls the current through the cell indepently of electrochemical reactions within it. This form of control is achi ved by inserting an external resistance R into the return line from the wor king electrode to the potentiostat.. thus producing a voltage drop at R to ground as the cell current flows through it. This drop is then applied to the inverting input of the operational amplifier and compared with the control voltage at the non-inverting input. Function generators have a wide range of appli cations in communications, telemetry, bioengineering, mechanical and chemical studies and process controlc In many cases, IC function generators can provide a low cost alternative to conventional discrete units. The basic function generator is a device that generates a stable, well-defined perodic output signal that may be controlled externally. A typical function generator consists of three sectionsş C1D an oscillator that generates a periodic waveform C2D a wave shaper that converts the output of the oscillator into the de sired waveform C usually sine or triangle!) and C3D an output buffer amplifier to enable the generator to drive the required load. The general performance characteristics of a function generator are determined by the performance of each of the sections the comprise the complete system. The oscillator determines the stability and linearity of a triangle output signal, as will be discussed later, and the wave shaper usually determines the distortion of the sine wave output. General ly, microcomputer controlled function ge nerators are implemented in two ways: In the first met hod, signals are generated by means of microcomputer. For standard waveforms Csine wave, triangular wave, squ are wave, trapez and pulse D, look-up tables are - constituted in EPROM which are adressed by a counter. The counter clock, which defines the signal frequency, -VI 1is derived from a separate clock generator. The clock is the core element of the generators, since its perfor mance directly affects the output signal waveform. It must ensure good frequency stability and be easily prog- rammble via the microcomputer. The clock is based on a Phase Locked Loop CPLLD, which for this application can be simple» due to the modest settling time requirement for the clock frequency. The amplitude of arbitrary waveform are either computed by the microcomputer or entered by the user. In the second method, function generator is imp lemented with discrete elements and controlled by the microcomputer. There are several advantages of this way compared to the first one such as low-cost, operating a- bility at high frequencies and easy implemantion. The basic building blocks of the system are key boards, displays, microcontroller system, digital -to- a- nalog and analog-to-digital converters, external memory and amplifiers. The microcomputer system that controls all the system is designed around an 8098 microcontroller. Microcomputer system consist of a 8098 microcontroller, an 2764 EPROM. display and keyboard. Although the 8098 is a single-chip microcontrol ler, it still requires several external connections to make it work. Power must be applied, a clock source provided, and some form, of reset circuitry must be pre sent. Power for the 8098 flows through six pinsş they are: three positive voltage pins -VccCdigi talD, VrefCPort O digital I. O and A. D power D, VpdC power down mode} and three common returns -two Vss pins and one ANGND pin. All six of these pins must be connected on the 8098 for normal operation. The Vac, Vref and Vpd pins should be tied S volts. The two Vss pins and the ANGND pin must be grounded. When the analog to diaital converter is -vi li -used it may be desirable to connect- the Vref pin -to a seperate power supply, or at least. a separate power supply line. Digital signal C16-bitO which is obtained from port cf HSO. 2 is converted to the analog signal by means of low-pass filter. The value of this signal is between O V and 5 V. For this reason this values must be trans ferred to -5 V and 4-5 V. Power stage is used to increase the output- rent of the signal obtained from port of HSO. 2 cur - The current which flows from counter electrode to working electrode is converted to the voltage and is applied to the A. D converter on 8098. The A. D converter shares pins with port 0. Analog input voltage is converted to a 10- Bit binary number. -IX-
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