Mikrodenetleyicili kapasite ölçme yöntemi

Abstract

Bu tez çalışmasında, kapasite ölçme ve kapasite diyotlarının Kapasite- Gerilim eğrilerinin çıkartılması ele alınmıştır. Esas olarak analog ve sayısal yöntemlerin birleşimine dayalı olan ve mikrodenetleyici ile kontrol edilen bir kapasite ölçme yönteminin teorisi ve pratiği incelenmiştir. İlk olarak, genel kapasite ölçme yöntemleri üzerinde durulmuştur. Ayrıca özellikle bu tezde kullanılan ölçme yönteminin özellikleri ve getirdiği avantajlar anlatılmıştır. Daha sonra temel olarak, kontrol işlemini gerçekleştiren mikrodenetleyicinin tanıtımı yapılmıştır. Sistemin tasarımı donanım ve yazılımı olmak üzere başlıca iki aşamada gerçekleştirilmiştir. Donanım kısmını, analog kapasite ölçme düzeniyle, analog ölçme sonuçlarını değerlendiren ve sistemin kontrolünü sağlayan mikrodenetleyici düzeni oluşturmaktadır. Yazılım kısmını ise mikrodenetleyici programı oluşturmaktadır. Bu kısımda sistem yazılımının mantığım ortaya koyan akış diyagramı üzerinde durularak bilgi verilmiştir. Ölçme aletinin, 12 fonksiyonlu tuştakımının yardımıyla, nasıl kontrol edildiği ve kullanıldığı ayrıntılı olarak ele alınmıştır.Oluşan sistemde kapasite ölçme ve kapasite diyodunun kapasite - gerilim (C-V) eğrisini çizdirme olmak üzere iki konum kullanıcı tarafından bu tuştakımı yardımı ile girilmektedir. Ayrıca çeşitli nedenlerle oluşan parazitik etkileri ortadan kaldırmak için sıfır ayan konumuda mevcuttur. Ayrıca her konumun içinde de 0-120 pF ve 0-1200 pF olmak üzere iki kademe daha mevcuttur.Kapasite ölçme konumunda kapasite değeri doğrudan göstergede verilir. Kapasite diyodu konumunda ise mikrodenetleyici yardımıyla üretilen gerilim adımlarına karşı ölçülen kapasite değerleri grafik veya tablo halinde göstergede verilir. Bütün bu kademe seçimleri ve kontrol işlemleri kullanıcı tarafından tuştakımından girilmektedir. Son olarak sonuçlar ve öneriler bölümünde cihazın beklenen özellikleri ne derece sağladığı ve bundan başka neler yapılabileceği konuları üzerinde durulmuştur. Ayrıca bir örnek ölçmenin sonuçlan tablo ve grafik olarak verilmiştir.

In this master thesis, therotical and practical applications of small capacitance measurement, based on both digital and analog control methods, which is controlled by a microcontroller have been investigated. Presently, there are a lot of methods which are used for measuring capacitance. Previously, only analog methods were used, but now digital techniques are being widely used. In this thesis, analog and digital techniques have been used for realising a measurement system. Static capacitance has been measured by AC bridges, by reasonance methods and recently by LCR meters which are based on synchronous detection thecniques. The C-V converter proposed in these thesis is based on a negative feedback thecnique. The capacitor to be measuered is placed in the feedback path of an operational amplifier, therefore the unavoidable stray capacity incidental to the lead wires and the test fixture affects measurement only minutely. Also tile effects of stray capacitance can be canceled easily with a subractor before the measurement. Subsequently, a high resolution, and therefore, a converter capable of measuering small capacitances can be obtained. Since the negative feedback loop holds the voltage across the capacitors under test at a constant level, two major advantages are realized. The first one is the ability of testing the capacitor at a specified driving voltage and the second one is that the AC-DC coverter does not cause any conversion errors. This method can also provide a good linearity and stability when measuring small This device, consists of two parts, hardware and software. Hardware is based on a circuit controlled by a microcontroller, an analog measurement circuit, a keyboard and an LCD. The software part of the device consists of a microcontroller program. The central processing unit of the microcontroller control card is TMS 77C82 microcontroller. The analog measuring circuit consist of constant voltage source Vg, an integrator, a DC-AC converter a reference capacitor Cs, an operational amplifier which operates as a capacitance sensing circuit, an AC-DC converter and an vioperational amplifier as a subractor. When the circuit balances, integrator provides a DC voltage Vi. The DC-AC converter generates a staircase sinusoidal wave Vi(t) with a amplitude of Vi. The test signal Vj(t) is applied to the input operational amplifier which operates as a capacitance sensing circuit. The input impedance is a capacitor which is used as the referance capacitor. The output voltage of the operational amplifier V0(t) is, Vo(t) = [c,/(Cx + Cp)]Vi(t) (1) where Cs and Cx are the values of the reference capacitor and the unknown capacitor, respectively. The additional capacitance in the feedback path of the operational amplifier, including stray capacitance is represented by Cp. The AC signal V0(t) is also the voltage across the capacitor to be measured. When rectifying and smoothing V0(t), a DC voltage V0 is obtained equivalent to the amplitude of V0(t). The difference between this DC voltage Vs is applied to a summing integrator. The integrator operates so as to satisfy the equation VB-Vo = 0 (2) From (1) and (2), Vi=(Vi/CiXc5t+Cp) (3) is obtained. As shown in (3), the output voltage of the integrator is proportional to the capacitance which exists in the feedback path of the operational amplifier, in the balancing loop, AC-DC converter (rectifier) operates at a constant voltage level Vs. This means that the nonlinearity error of the AC-DC converter does not cause measurement error. The effect of the additional capacity Cp can be cancelled easily by using another operational amplifier as a subractor with a variable gain Kp. The output voltage of the subractor becomes, Vx=(V8/Cg)(cx+Cp)-KpVg (4) Before connecting capacitor Cx to the test fixture gainKp should be adjusted so that, V,=(V./C,)Cp-KpV. = 0 (5) Then with Cx connected, the subractor output voltage becomes, vuVX = (V,/C,)CX (6) Now a DC voltage proportinal to the unknown capacitance Cx is obtained If we can measured capacitance of a varactor instead of the unknown capacitance Cx, the circiut construction must be changed. Because there must be a DC voltage to apply the varactor. DC voltage Vx is converted to digital signal by using an ADC (MC 145053) to send microcontroller. From here, the value of capacitance is translated to the LCD. When a varactor is measured in mis case a DC staircase voltage which is between 0.8V and 30V is produced by using microcontroller card and program. Also a D AC and an operational amplifier are used to produce the signal. This DC voltage is applied to the varactor. Then Vx voltage value of capacitor is read by microcontroller and it is drawn in form of graphic mode on the LCD. For all this values, the same procedure is applied and finally capacitance-voltage curve is obtained. Besides, by using a keyboard, voltage and capacitance values are obtained in form of a table on the LCD. In the analog capacitance measuring circuit construction, the summing integrator and the subractor are typical DC operational amplifier circuits. For these operational amplifier are used LM 3900 and LM 324 respectively. The DC-AC converter converts the voltage Vi suplied by the integrator into an AC voltage of amplitude Vi. This conversion is done by the use of an 8 bit multiplying digital to analog converter (DAC 08), an operational amplifier (TL 074) used as a current to voltage (I-V) converter, and a erasable programable read only memory (2764 -EPROM) driven by a 5 bit binary counter (CD 4040). The AC-DC converter is driven by a constant voltage level Vs. Therefore its nonlinearity error does not cause conversion error. A simpler diode rectifier has been used. In addition capacitance sensing circuit is composed of an operational amplifier and a reference capacitor Cs. Cs is placed between the output DC-AC converter and inverting input of the operational amplifier. The capacitor to be measured is placed between the inverting input and the output terminals. Therefore the stray capacitance incidental to the lead wires and terminals does not caused significiant error on the measurement of unknown capacitance. There is also a relay which is used to pass a range to an other range in the analog measurement circuit As mentioned earlier, the TMS 77C82 microcontroller has been used for the automation of the system. This 8 bit controller requires peripheral devices in order to complete the system. The system consists of a 6264 RAM two 8255 PIOs, a keyboard and an alphanumeric graphic display. A 5V power supply and 5.0688 MHz clock frequency is also required for the system to work. VlttThe system program is written into EPROM of TMS 77C82 microcontroller, having a 8K capacity. Besides a 8K 6264 RAM and two 8255 PIO's are used for the system. RAM is located >2000->3FFF adresses of the memory map of the circuit controlled by the microcontroller. The first 8255 PIO is located >6000->7FFF adresses and second 8255 PIO is located >8000- >9FFF adresses of the memory map. EPROM exist between >E006 and >FFFF adresses. Two 74LS138 decoders are used for memory and I-O decodings. For memory and I-O decoding, A13, A14, A15 adressses are connected to the inputs of 74LS138 decoders. MC 145053 and DAC 08 are used respectively as analog to digital converter at the C port of the first 8255 PIO and digital to analog converter at the a port of the second 8255 PIO. ADC which is 10 bit and serial one, is connected to the fourth bit. The other bits are used to control ADC. It converts the analog output voltage of the capacitance measurin circuit. DAC 08 is a 8 bit paralel interface and used to generate staircase DC voltage that is applying to the varactor. Dot matrix LCD ( LMG 64240 ) is connected to the B port of 8255 PIO. The last four bits at C port is used to control the LCD. LMG 64240 is a 240 dot X 64 dot graphic and alpha-numeric display. In this system, a standart telephone keyboard (4 X 3) is usedKeyboard is connected to the B and C ports of the second 8255 PIO. The last four bits at C port are used for lines and the first three bits at B port are used for columns, for using this keyboard a program has been written. Measurement and display control, range selection and the other options are made by keyboard. In the measurement system, there are two range which are 120 pF and 1200 pF for capacitance measumerent In the first one capacitances can be measured until 120 pF. In the second one, the capacitance values which are greater then 120 pF are measured. Also there are four ranges which are 30 pF, 60 pF 120 pF and 1200 pF for measuring capacitance of a varactor. For all this ranges either graphical or table values are indicated on the LCD. As a result, the advantages of measurement system are the following: A DC voltage to apply varactor is easly applicable. Since the AC-DC rectifier is driven by a constant voltage level, its nonlinearity error does not cause error on the measurements. The effect of stray capacitance can be easily cancelled and, therefore, the converter can be used to measure a small capacitance. It can be said that, using this prototype measurement device very good conversion IXlinearity has been obtained in measuring small capacitances and capacitances of varactors. After completing the hardware and software sections, various measurements have been made in order to check the results. Expremental results have been included, which support the theory.