Digital Signal Processing of Optical Encoder for High Resolution Angular Measurement of X-Ray Diffraction Goniometer

Transcription

1 Digital Signal Processing of Optical Encoder for High Resolution Angular Measurement of X-Ray Diffraction Goniometer Shrihari Shinde, Shirish Ghadigaonkar, Lalan Jaiswal, Abhishek Sakhare, Vaishali Bambole, H. Muthurajan National Centre for Nanoscience& Nanotechnology, University of Mumbai, Mumbai, India Department of Physics, University of Mumbai, India Abstract: X ray diffraction (XRD) is the instant chemical structure analyzing technique for the crystalline properties of compound materials. In XRD, X- ray source is the means of exposure to the crystalline material and X-ray detector detects the diffraction pattern from the sample material. XRD does the analysis of material from the diffraction pattern detected by the X- ray detector. Here goniometer is the key mechanical part of the XRD system, the function of the goniometer is to achieve the angular rotation of the X-ray source/detector/sample in order to get crystalline properties of the subjected sample. The angular movement of the X-ray source, detector and sample holder decides the configuration of the goniometer. Most widely used goniometer configurations are THETA:THETA and THETA:THETA. In THETA:THETA configuration sample holder is stationary in horizontal plane, the X-ray source and detector moves at the same time over the angle THETA with reference to the sample. In THETA:THETA configuration distance between sample to X-ray source and sample to X-ray detector is kept constant. In THETA:THETA configuration X-ray source is kept constant at one position and x-ray detector moves in angular plane at THETA and simultaneously sample moves in angular plane at THETA. The paper gives the brief introduction to XRD technique and technical application of the goniometer in XRD. In view of use of goniometer in the XRD, a simple and accurate angular measurement system is fabricated with available plastic based goniometer as prototype. High precision encoder is selected for the fabrication of goniometer; the encoder is mechanically fitted with the plastic goniometer. Digital output signal from goniometer is processed using microcontroller having our embedded programme, which precisely converts the digital pulsed signal in to angular movement. The system fabricate is explained with embedded algorithm, PC based data acquisition software algorithm is designed in VB (Visual basics) to lively display the angular reading of goniometer. Keywords: Digital Signal processing, Optical Encoder, Angular Measurement, X-ray diffractometer.. Introduction X-ray diffraction (XRD) permits analysis of the phases or compounds found in the samples. It is more often used as a qualitative tool in central or research laboratories [] as well as for quantitative determination for process control. XRD uses X-ray as means of energy source with X-ray detector. X- ray is high energy photon []. X-ray is produced when electrons from cathode is emitted with high speed and it get collide with anode. This collision emits an electron from inner shell of anode and which in turn gets vacant because of this electron, vacancy is filled by another electron from higher shell and during this transition it emits x-ray of different frequency and energy. The most important is K alpha, k beta x-ray which is emitted when electrons jump from L-shell to k-shell and M-shell to L-Shell. Earlier X-ray diffraction was widely known for determining the size of matter. But as time passes wide application of x-ray diffraction came to known. Wide application include such as chemical analysis and stress measurement, study of phase equilibria. It is also used in determining the orientation of one crystal or ensemble of orientation in polycrystalline aggregate. The main principle involved in working of XRD is bragg s law which states that crystal is made up of large numbers of parallel plane containing atoms. The peak detection in XRD is done with the help of reflection of light from one of the plane of crystal and constructive interference of the reflected light from the same plane [-5]. XRD is an instrument designed to collect data diffracted from crystal. XRD consist of x-ray source, goniometer for orientation of crystal and detector for collecting scattered x-ray [6]. To analyze sample in XRD, x-ray form the source is incident on the crystal say C, which is set at desired angle with respect to x-axis through origin O, center of spectrometer circle. Detector is place on 75

2 opposite of source to detect reflected x-ray and this detector can be rotated at any angle for desired angular position. Crystal is positioned so that it makes an angle (theta) with incident beam and D is set at corresponding angle theta. Intensity of diffracted beam is measured and its wavelength is calculated from bragg s law. This procedure is repeated for various angles to get the final analysis data [7-]. A goniometer is an instrument that either measures angle or allows an object to be rotated to a precise angular position. It is necessary to satisfy Bragg's law for crystallographic analysis in XRD. X-rays striking a substance are scattered by the electrons in that substance. For crystal,the scattered X-rays are reinforced at certain angle(θ),each of which depend on the wavelength (λ) of the radiation and the spacing(d) between the planes of atoms in the crystal causing reinforcement at the angle. This reinforcing phenomenon and angle at which it occurs is often called a reflection. The relationship between λ, θ and d is the Bragg equation []. The diffractometer circle in fig. is different from the focusing circle. The diffractometer circle is centered at the specimen, and both the x-ray source and detector lie on the circumference of circle.the radius of diffractometer circle is fixed. The diffractometer circle is also referred to as the goniometer circle.the goniometer is the central component of an x-ray diffractometer and contains the specimen holder. It has arms to which the x-ray source and the detector are mounted. In most powder diffractometers, the goniometer is vertical as well as horizontal mounted. Goniometer play major role in XRD [4]. Figure : Diffractometer Circle. System Development In the view of application of goniometer in XRD, a sample model of goniometer is fabricated with available plastic goniometer and same has been modified to fit the optical encoder module and stepper motor. The optical encoder is the integral part of goniometer and act as an angular sensing and feedback element to the controller for controlling the angular movement of the goniometer arms. Optical encoder provides the closed loop control feedback for the goniometer system. Minimal angular movement that can be achieved by the goniometer is full depends on the resolution of the optical encoder used in the system. The system block diagram is depicted in figure followed by the description of the same. Figure : Goniometer System diagram 000PPR optical encoder is attached to the available plastic goniometer assembly. Encoder digital signal is connected to the digital signal processing controller, the signal is processed by the controller and decoded signal is send to the PC via RS interface. Angular reading received in the PC is captured and displayed by the application software written in visual basics.. Digital Signal Processing Hardware Digital signal from optical encoder has to be analyzed to precisely detect the degree and direction of angular motion. To get the exact reading, the digital signal processing hardware is designed and fabricated with PIC8F0 microcontroller.. Optical Encoder Encoder available for angular motion sensing is of two types that are optical and magnetic encoder. Optical encoder used here has a circular plastic/glass/metal disk with patterns of one or two lines deposited on it [-4]. The assembly uses incremental type of encoder, in which LED source is used with one/two detectors, the line pattern on 76

3 4 5 International Journal of Engineering Technology, Management and Applied Sciences the disk blocks and allows the light from LED source to reach the detector. The black portion of line on the disk block LED light to reach to one of the detector and clear portion of the disk will allow the light to reach at detector. Two detectors are used to sense clockwise and anticlockwise angular rotation respectively. Direction of angular movement can be governed by processing and comparing the output signal pattern from both the detectors. Rotary quadrature encoder with index of 000PPR, it is having 5 terminals for interfacing to microcontroller. Because of 000PPR encoder used in the goniometer will provide the minimum angular resolution of 0.8 degree. It also provides one single pulse at each rotation. Figure (a) shows the circular disk used in incremental optical encoder and Figure (b) shows the possible digital output signal pattern from optical encoder. The digital signal output shown in figure (b) is from terminal A, terminal B and terminal Z respectively. Each falling or rising edge of digital signal at terminal A or B indicates the 0.8 degree angle of rotation for goniometer and both output A and B are quadrature of each other. So minimum angle can be measured with the 00PPR encoder is 0.09 degree. pin. The figure 4 shows electronics hardware schematics for goniometer. Figure 5 shows the working model of the digital signal processing hardware for goniometer and figure 6 shows the goniometer attached to rotary encoder with digital output connector from encoder. J CON U RA0/AN0 RA/AN/LVDIN 4 RA4/T0CKI 5 MCLR/VPP/RA5 6 VSS/AVSS 7 RA/AN/VREF 8 RA/AN/VREF+ 9 RB0/AN4/INT0 RB/AN5/TX/CK/INT R PIC8F0 0.UF C4 0.UF 0K J CON5 8 RB/CCP 7 RB/INT 6 OSC/RA7 5 OSC/RA6 VDD/AVDD 4 RB7/PGD RB6/PGC RB5/PGM/KBI 0 RB4/AN6/RX/DT C C 0.UF C 0.UF U 8 RIN ROUT 9 RIN ROUT 4 0 TIN TOUT 7 TIN TOUT C+ 4 C- 5 C+ C- 6 V+ V- MAX Figure 4: Encoder signal processing circuit J CON (a) (b) Figure : (a) Optical inside assembly (b) Digital signal from optical encoder. Electronics Hardware Electronics hardware is designed and fabricated to interface the optical encoder output signals for processing. The hardware is fabricated using PIC8F0 microchip fabricated microcontroller. Angle information decoded by the microcontroller is sent to the PC via RS interface and can be displayed in designed application software. Here on chip interrupt resources available with the microcontroller are used for real time detection of the digital signal from the optical encoder. The on chip interrupt is configured to raise the interrupt on rising edge of the digital input signal at the interrupt Figure 5: Inside view of goniometer signal processing unit 77

4 Figure 6: Goniometer assembly 4. Software Development To processes and decode the digital signal data from optical encoder there is need to design an embedded algorithm. The simple embedded algorithm with proper interrupt handling is the need for getting the accurate angular control of goniometer. Embedded algorithm is designed and implemented in embedded C programming language. Figure 7 shows the embedded algorithm. Interrupt INT0 of PIC8F0 is used to detect the terminal A digital signal of encoder. INT0 is configure for rising edge of input signal, once rising edge is detected at INT0 pin interrupt subroutine is execute to note the anticlockwise of clockwise angular rotation of goniometer. Counting of each rising edge at INT0 add or subtract the 0.8 degree value in the final reading according to the logic level of terminal B. Final angular reading of the goniometer will be sent to the PC via RS on chip interface present in PIC8F0. Figure 7: Embedded Algorithm for Goniometer 5. Conslusion The proof of concept and simple model of goniometer is fabricated using available plastic goniometer. In view of mechanically connecting optical encoder to the plastic goniometer, it is modified in such a way that the one arm of the goniometer rotates along with hollow shaft of optical encoder. Embedded hardware for the digital signal processing of optical encoder is designed using PIC8F0 microcontroller chip. Embedded algorithm is designed to efficiently use onchip available interrupt module for real time detection of angular movement, the angular reading of the goniometer can be sent to the PC via RS interface. Form the designed hardware setup it is practically observed that angular resolution obtained is 0.8 degree. The application software for real time angular data acquisition has been designed in VB programming. Future work will be to integrate the electrical actuation means to both arm of goniometer along with encoder to each arm and effort will be made to achieve the more angular resolution by incorporating high PPR rated encoder or customized encoders. Referances [] R. Yellepeddi, Applications of an Integrated XRF- XRD Spectrometer, Journal de physique iv, Volume 4, July 996 [] Jan felix steinbreiner, X-ray diffraction microscopy: computational methods and scanning types experiments, Stony brook University, Thesis, August 00. [] M A Wahab, solid state physics structure and properties of material Second edition, Narosa publication. [4] S.O. Pillai, Solid State Physics, sixth edition, New Age International Publishers. [5] B.D Culity, Element of x-ray diffraction, University of Notre Dame, Addision-Wesley Metallurgy Series,956. [6] Zakia Moore, Application of X-ray Diffraction Methods and Molecular Mechanism Simulation to structure Determination and cotton Fiber analysis, University of New Orleans, Theses, 008. [7] Catherine louise Fleck, Magnetism in complex cobaltates Y -X Sr x CoO and Ca Co O 6, University of warwick, Thesis, October 0. [8] Barbara L Dutrow, X-Ray powder diffraction, louisiana state university, Christian M clark, Eastern Michigan University, Thesis. 78

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