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Beam quality test technology and devices of electron beam welding
Vacuum 86 (2011) 261e266
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Beam quality test technology and devices of electron beam welding Y. Peng a, K.H. Wang a, *, Q. Zhou a, Y.J. Wang b, P.F. Fu c a
School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, 210094 Jiangsu, PR China Beijing Institute of Aeronautical Materials, Beijing, PR China c Beijing Aeronautical Manufacturing Technology Research Institute, Beijing, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 January 2011 Received in revised form 14 April 2011 Accepted 27 June 2011
The electron beam four-dimensional quality test system was developed targeted at the 5e100 mA electron beam current of a high-voltage electron beam welding machine. The system includes the control module, sensor module, driver module, and analysis software, which is based on the complex programmable logic device (CPLD). The quality test system obtains the data of quasi-instantaneous power density distribution (QIPDD) of the electron beam by controlling the beam periodically, scanning the surface of a Faraday cup. Fixing the Faraday cup at an X- and Y-axis coordinate position under constant conditions, the three-dimensional graphic of QIPDD of the beam about the data of a single cross-section was showed by using computer reconstruction technology. Several QIPDD of the electron beam cross-section at different working distance in axial direction reconstructed the four-dimensional graphics of the beam. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Electron beam welding Quality Quasi-instantaneous power density distribution
1. Introduction Electron beam welding (EBW) is a high power density welding method that has a number of advantages over conventional techniques. The main advantages of this technique are large depth-towidth ratio, narrow heat-affected zones (HAZ), insignificant deformation and pure chemical composition of weld in a vacuum, so it is suitable for the welding of refractory and thermal sensitive materials, precision welding and other special requirements for welding. The process of EBW is influenced by a huge number of process parameters [1]. The actual manufacture and design theory of an EBW electronic gun differ. The degree of stability of an electron gun power supply system and vacuum changes could affect the electron beam diameter and power density distribution. The geometry and quality of the weld is strongly influenced by the process parameters such as accelerating voltage, beam current, welding speed and focus current. Therefore it is necessary to study the characteristics of the electron beam precisely, measure the electron beam quantity quantitatively, such as focus position, beam diameter and power density distribution [2,3]. As the extremely high power density is of about 107 W/cm2 at the focus of the beam, it can melt any refractory material, which makes measurement very difficult. DIABEAM is an electron beam system developed in the Welding and Joining
* Corresponding author. Tel.: þ86 25 84315776. E-mail addresses: [email protected], [email protected] (K.H. Wang). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.06.017
Institute of RWTH Aachen University which provides measurement and the three-dimensional display of the power density distribution [4]. A diagnostic tool, which utilizes a variation of the slit detection system, has been developed at Lawrence Livermore National Laboratory to provide quantitative information on the properties of electron beams used in welding [5]. The two systems just mentioned can only get the density distribution of a single cross-section rather than the four-dimensional data. For 5e100 mA electron beam current of high-voltage EBW machine, the electron beam four-dimensional quality test system was developed. An engineer can measure the electron beam quantity with our system, such as focus position, beam diameter and four-dimensional QIPDD. In the idea case the power density distribution should have the form of a symmetrical Gaussian shaped curve [2], but the actual distribution may be caused by cathode adjustment, cathode distortion, and variation of the vacuum level and of electrical system parameters. If measured distribution is asymmetrical, it could result in the asymmetrical welded joint. The system uses high-speed magnetic deflection technology based on CPLD to drive the electron beam scanning quickly on the Faraday cup sensor. The special waveform generator here takes advantage of the computational advantages of CPLD. The CPLD provides a new approach to Application Specific Integrated Circuit (ASIC) implementations that feature both large scale integration and user programmability [6e8]. High efficiency of signal transmission, good electromagnetic compatibility (EMC) and low manufacturing costs have made CPLD technology popular for rapid system prototyping. After reconfiguring the CPLD, the new system can varied
262
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the function for the high-speed scanning of electron beam without changing any hardware circuit, so it is highly flexible. 2. Principle and structure of the system The Industrial computer (IPC) communicates with the electron beam scanning control system based on UART (Universal Asynchronous Receiver Transmitter). According to the test parameter IPC sends an 8-bit binary code to the scanning control system, and then the control system drives multi-polar magnetic deflection coils to produce a high-frequency alternating magnetic field. A high power density electron beam will change the direction of movement by electromagnetic force as it passes through the uniform magnetic field. Before and after the test, the electron beam must be deflected to the energy absorbing device. The signal sensor of the electron beam current is based on the principle of the Faraday cup. There is a fixed pinhole on the surface of the cup. Part of the electron beam goes into the pinhole when it is scanning on the sensor and passes through the current/voltage conversion circuit. The weak voltage signals will be amplified by the multi-stage amplifier circuit in the sensor, converted into a digital signal and then finally entered into the IPC for storage and processing. The electron beam signal of a single cross-section depicts the threedimensional graphic of the QIPDD of the beam. Without changing the conditions of the test parameters, it can obtain the QIPDD of different sections by adjusting the axial position of the sensor. The system structure is shown in Fig. 1. 3. System realization 3.1. Electron beam scanning control module Firstly, The main function of the electron beam scanning control module based on the CPLD is to drive a multi-polar magnetic deflection coils to produce an alternating magnetic field so that electron beam scans in high-speed on the surface to prevent destroying the sensor with converting and amplifying circuit. Secondly, the module also sends a sampling clock and trigger signal to the data acquisition card installed in IPC.
Fig. 2. Sketch of the synchronous control signal and scan path.
In the electron beam quality test process, the electron beam scans in the direction of the arrow in Fig. 2a, and the corresponding control signals of the magnetic field are shown in Fig. 2b and c. The labels UX and UY are the control voltage of the magnetic field in the X direction and in the Y direction respectively. The notation Ts is the scanning period. In the forward-stroke scanning process of the electron beam (i.e., in the direction of the solid line in Fig. 2a), the strength of the magnetic field that makes the electron beam deflect in the Y direction is maintained and the corresponding field in the X direction is increased linearly. In the retrace scanning process (i.e., the dotted line in Fig. 2a), the magnetic field in both the X and Y direction is increased linearly. As noted above, the data is collected in the process of forward-stroke scanning but is not in the retrace scanning in order to ensure the special phase relationship of the data distribution.
Fig. 1. Schematic diagram of the beam quality test system of EBW： (1) Cathode; (2) Control electrode; (3) Anode; (4) Control coil; (5) Optical observation system; (6) Focusing coil; (7) Deflection coil; (8) Vacuum chamber; (9) Energy absorbing device; (10) Sensors; (11) Work piece; (12) Electron beam scanning control system; (13) IPC; (14) Display.
Y. Peng et al. / Vacuum 86 (2011) 261e266
Fig. 3. Sketch of the beam defection over sensor.
Typically, the focus diameter is about 0.1e1 mm under the working conditions that the rated voltage of electron beam welding gun is 30e175 kV and the electron beam current is 1000 mA or less. For large power welder beam spot diameter of the focus extends to 2 mm, while the beam diameter will work with the process parameters such as the axial working distance and focusing current. In order to accurately measure the QIPDD when the position of the focus is unknown, the contour outline of the electron beam must be guaranteed within the testing range so that the scanning range must be greater than 2 2 mm. However, because of the limit of the maximum deflection angle of the deflection coils, the maximum scanning range is 30 30 mm. The resolution of three-dimensional graphic reconstruction in Y direction of QIPDD depends on the distance DY between two adjacent scans. If DY is smaller, the distribution of the graphic reconstruction is denser and the graph is high-quality. Thus we could better describe the details of the beam power distribution by reducing the value of DY. Considering the deflection coils performance of the control system, the step-by-step number n of the electron beam scanning was set to the constant 100, corresponding to 100 cycles of the triangular wave in Fig. 2b. Once the test is completed, the electron beam is deflected to the surface of the energy absorption device. When the scanning area is 2 2 mm, DY is equal to 20 um, by the same token, when the range for 30 30 mm, DY is equal to 300 um. The relationship between the pinhole diameter and DY can be well explained in Fig. 3. We just change the DY in the Y direction to meet the diameter of beam spot.
263
If the beam spot is bigger, DY is increased accordingly. For the deflection accuracy, we just make sure that DY is equal or larger than the pinhole diameter, otherwise there will be some overlap of the collected current between two adjacent scans (i. e., the overlap between the grey blocks A and B in Fig. 3). In order to make the electron beam stay for a very short period at the same position of the sensor to prevent depth of fusion on the sensor, we must control the electron beam to scan rapidly and to heat the sensor uniformly. The scanning experiment was carried on with 316 stainless steel sheets to test the lowest scan rate which a sensor can withstand. The experiment used the ZD150-15A type electron beam welding machine with the accelerating voltage Ua of 150 kV and the maximum power Pm of 15 kW. The thin steel plate size is 200 150 2 mm. The welding parameters are: voltage Ua ¼ 150 kV, current Ib ¼ 100 mA, focus current If ¼ 345 mA, working distance h ¼ 330 mm, etc. The experiment result indicated that if scanning frequency (Fig. 2, the frequency of the triangular wave) fs ¼ 1=Ts 3 kHz, there can be no depth of fusion on the plates, and therefore the max scanning frequency is set to 5 kHz limited by the power amplifier. In terms of hardware circuitry, the scanning control circuit consists of a CPLD, a serial communication circuit, an external clock circuit, a reset circuit, a power circuit, a digital-analog conversion (DAC) circuit, a filter circuit, an optical isolation circuit and a power amplifier circuit. A general view of the relationship between the components of the circuit is depicted in Fig. 4. The main part of the external clock is an oscillator of which clock signal passes through the frequency divider circuits of CPLD to produce a variety of clocks. These clocks are used for serial communication, controlling data acquisition card and generating the variable frequency waveform. IPC sends 8-bit binary code which contains the information about scanning start/stop and scanning frequency to the CPLD control circuit through the serial communication circuit. Triggered by the clock signal, the CPLD control circuit generates two digital waveforms according to the received binary code. The digital outputs are converted to analog waveforms through 12-bit DAC. The digital signal in the conversion process has great impact on the current generating a relatively large voltage drop on the transmission line and internal resistance of the source, so it’s necessary to use the low-pass filter circuit to filter the higher harmonic. The filtered signal passes through the bipolar optical isolation circuit that protects the pre-stage circuit against the impact of the rear power circuit. Finally the signal enters the constant current source power amplifier. Design and simulation of the control signal generator has been performed using Very-High-Speed Integrated Circuit Hardware Description Language (VHDL). Fig. 5 shows the Block diagram of the CPLD top-level file with clk_in as the clock signal, reset as the reset signal, rxdr as the input signal of serial communication, txd as the output signal of communication, txd_done as the signal that shows txd is completed, x_out [11..0] and y_out [11..0] as the control signal
Fig. 4. Sketch of control circuit.
Fig. 5. Block diagram of CPLD top-level file.
Fig. 6. Flow chart of the synchronous-waveform block.
Y. Peng et al. / Vacuum 86 (2011) 261e266
265
of coils, clk_sample and trigger as the control signal of acquisition card, wr as the write input of DAC. Fig. 6 is the flow chart of the control signals that are generated by the main synchronouswaveform block in the Fig. 5. 3.2. Signal sensing and acquisition module The sensor based on the Faraday cup can accurately measure the QIPDD of high-energy electron beam. The current entered into sensor produces a voltage drop Uout in the signal sampling resistor R. As the collected current is very small, R should be taken as large. If the electron beam welder is stable at work and the current density distribution of electron beam does not change over time, it can be assumed that both ends of the sampling resistor are connected to constant current source that the voltage drop Uout is “very stable” and does not change with the load. Under the above assumptions, if R is infinite, then Uout is also infinite. Therefore, the ideal constant current source is not allowed to be open-circuit. For the actual circuit, when R is large enough, the capacity of the voltage output would be insufficient, so the value of R should not be large. In the testing process R is taken to be the precise noninductance resistance of 1 kU. The voltage Uout is processed by a differential amplifier and PCI-1714 high-speed acquisition card, and then stored in the buffer for subsequent signal calculation and reconstruction of QIPDD. The calculation is shown by the following equations:
Uout ¼ Je Ah R
(1)
Je ¼ Uout =ðAh RÞ
(2)
Jp ¼ Ua Je ¼ Ua Uout =ðAh RÞ
(3)
where Ah is the pinhole size in the sensor, Je is the current density of the electron beam entered into the sensor, Jp is the power density of the beam, and Ua is the accelerating voltage. 4. Results and discussions The signal waveform collected by the beam quality test device is shown in Fig. 7a. The beam parameters of the signal are: voltage Ua ¼ 150 kV, current Ib ¼ 8 mA, focus current If ¼ 345 mA, working distance h ¼ 330 mm, scanning frequency fs ¼ 3 kHz, etc. Signal acquisition is taken in each forward-stroke scanning process. After zooming in the time axis of the collected waveform, a peak signal collected in the signal scanning process appears in the upper left corner of Fig. 7a. The peak indirectly indicates QIPDD of the beam in a single scan region. The voltage signal about a single cross-section that is collected by repeatedly scanning forms a matrix Amn as following equation:
0
Amn
a11 ¼ @ « am1
. aij /
1 a1n « A amn
(4)
where aij is discrete output voltage collected by the acquisition card:
aij ¼ Uout ¼ Je Ah R
Bmn
0 a Uout @ 11 ¼ « Ah R am1
(5) . aij /
1 a1n « A amn
Fig. 7. Flow chart of QIPDD reconstruction.
(6)
where Bmn is the current density Je matrix of a single cross-section. Then using Equations (3) and (4), the Equation (6) is obtained by
which the three-dimensional graphic of QIPDD about a single cross-section is reconstructed in Fig. 7b. Fig. 7c is the projection of Fig. 7b in the XY plate. From Fig. 7b it is easy to see, that the system is able to measure and record very fine details of density
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Y. Peng et al. / Vacuum 86 (2011) 261e266
distribution and the diameter of the beam under h ¼ 330 mm is about 0.9 mm. Signal cross-section data can not express exhaustive spatial information such as the focus. Using several QIPDD of crosssection at different working distance in axial direction we can reconstruct the four-dimensional graph of the beam like Fig. 7d. Fig. 7e shows the ideal four-dimensional distribution. 5. Conclusion and perspectives The electron beam four-dimensional quality test system development was targeted at the 5e100 mA electron beam current of a high-voltage electron beam welding machine. The system includes the control module, sensor module, driver module, analysis software and so on, which is based on the CPLD. The main parameters of an electron beam scanning control module are: the scanning range is 2 2 mme30 30 mm, the stepby-step number n of the electron beam scanning is 100; the maximum scanning frequency is 5 kHz. The signal waveform is collected by the beam quality test device under the conditions of Ua ¼ 150 kV, current Ib ¼ 8 mA, focus current If ¼ 345 mA, working distance h ¼ 330 mm, scanning frequency fs ¼ 3 kHz and the diameter of the beam is about 0.9 mm. The development of a user-friendly software interface and the actual four-dimensional distribution of QIPDD are subject to further research. In addition to the software improvements, we need to
focus on the relationship between QIPDD and the parameters of beam. Acknowledgments This work was supported by NUST Research Funding (NO. 2010XQTR01). References [1] Arata Y, Tomie M, Terai K, Nagai H, Hattori T. Transactions of the JWRI 1973; 2(2):130e46. [2] Schultz H. Electron beam welding. Woodhead Publishing; 1993. [3] Zhou Q, Liu FJ, Guan Q. Hanjie Xue Bao/Transactions of the China Welding Institution 2004;25. 77e79þ84. [4] Dilthey U, Goumeniouk A, Böhm S, Welters T. Electron beam diagnostics: a new release of the diabeam system. Vacuum 2001;62:77e85. [5] Palmer TA, Elmer JW, Nicklas KD, Mustaleski T. Transferring electron beam welding parameters using the enhanced modified faraday cup. Welding Journal 2007;86:388-se98-s. [6] Qasim SM, Abbasi SA. A new approach for arbitrary waveform generation using FPGA and orthogonal functions. In: 6th IEEE international workshop on system on chip for real time applications, IWSOC 2006, 2006 december 27e29. Cairo, Egypt: Inst. of Elec. and Elec. Eng. Computer Society; 2006. p. 28e32. [7] Kolouch J. Templates for CPLD and FPGA designs. Radioelektronika. In: RADIOELEKTRONIKA ’09 19th international conference 2009; 2009. p. 211e3. [8] RoyChoudhury J, Banerjee TP, Nathvani A, Chowdhury RBR, Bhattacharya AK. Design methodology internal sub state observer using CPLD. Nature & Biologically Inspired Computing, NaBIC; 2009. pp. 1636e40.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Beam quality test technology and devices of electron beam welding Y. Peng a, K.H. Wang a, *, Q. Zhou a, Y.J. Wang b, P.F. Fu c a
School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, 210094 Jiangsu, PR China Beijing Institute of Aeronautical Materials, Beijing, PR China c Beijing Aeronautical Manufacturing Technology Research Institute, Beijing, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 January 2011 Received in revised form 14 April 2011 Accepted 27 June 2011
The electron beam four-dimensional quality test system was developed targeted at the 5e100 mA electron beam current of a high-voltage electron beam welding machine. The system includes the control module, sensor module, driver module, and analysis software, which is based on the complex programmable logic device (CPLD). The quality test system obtains the data of quasi-instantaneous power density distribution (QIPDD) of the electron beam by controlling the beam periodically, scanning the surface of a Faraday cup. Fixing the Faraday cup at an X- and Y-axis coordinate position under constant conditions, the three-dimensional graphic of QIPDD of the beam about the data of a single cross-section was showed by using computer reconstruction technology. Several QIPDD of the electron beam cross-section at different working distance in axial direction reconstructed the four-dimensional graphics of the beam. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Electron beam welding Quality Quasi-instantaneous power density distribution
1. Introduction Electron beam welding (EBW) is a high power density welding method that has a number of advantages over conventional techniques. The main advantages of this technique are large depth-towidth ratio, narrow heat-affected zones (HAZ), insignificant deformation and pure chemical composition of weld in a vacuum, so it is suitable for the welding of refractory and thermal sensitive materials, precision welding and other special requirements for welding. The process of EBW is influenced by a huge number of process parameters [1]. The actual manufacture and design theory of an EBW electronic gun differ. The degree of stability of an electron gun power supply system and vacuum changes could affect the electron beam diameter and power density distribution. The geometry and quality of the weld is strongly influenced by the process parameters such as accelerating voltage, beam current, welding speed and focus current. Therefore it is necessary to study the characteristics of the electron beam precisely, measure the electron beam quantity quantitatively, such as focus position, beam diameter and power density distribution [2,3]. As the extremely high power density is of about 107 W/cm2 at the focus of the beam, it can melt any refractory material, which makes measurement very difficult. DIABEAM is an electron beam system developed in the Welding and Joining
* Corresponding author. Tel.: þ86 25 84315776. E-mail addresses: [email protected], [email protected] (K.H. Wang). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.06.017
Institute of RWTH Aachen University which provides measurement and the three-dimensional display of the power density distribution [4]. A diagnostic tool, which utilizes a variation of the slit detection system, has been developed at Lawrence Livermore National Laboratory to provide quantitative information on the properties of electron beams used in welding [5]. The two systems just mentioned can only get the density distribution of a single cross-section rather than the four-dimensional data. For 5e100 mA electron beam current of high-voltage EBW machine, the electron beam four-dimensional quality test system was developed. An engineer can measure the electron beam quantity with our system, such as focus position, beam diameter and four-dimensional QIPDD. In the idea case the power density distribution should have the form of a symmetrical Gaussian shaped curve [2], but the actual distribution may be caused by cathode adjustment, cathode distortion, and variation of the vacuum level and of electrical system parameters. If measured distribution is asymmetrical, it could result in the asymmetrical welded joint. The system uses high-speed magnetic deflection technology based on CPLD to drive the electron beam scanning quickly on the Faraday cup sensor. The special waveform generator here takes advantage of the computational advantages of CPLD. The CPLD provides a new approach to Application Specific Integrated Circuit (ASIC) implementations that feature both large scale integration and user programmability [6e8]. High efficiency of signal transmission, good electromagnetic compatibility (EMC) and low manufacturing costs have made CPLD technology popular for rapid system prototyping. After reconfiguring the CPLD, the new system can varied
262
Y. Peng et al. / Vacuum 86 (2011) 261e266
the function for the high-speed scanning of electron beam without changing any hardware circuit, so it is highly flexible. 2. Principle and structure of the system The Industrial computer (IPC) communicates with the electron beam scanning control system based on UART (Universal Asynchronous Receiver Transmitter). According to the test parameter IPC sends an 8-bit binary code to the scanning control system, and then the control system drives multi-polar magnetic deflection coils to produce a high-frequency alternating magnetic field. A high power density electron beam will change the direction of movement by electromagnetic force as it passes through the uniform magnetic field. Before and after the test, the electron beam must be deflected to the energy absorbing device. The signal sensor of the electron beam current is based on the principle of the Faraday cup. There is a fixed pinhole on the surface of the cup. Part of the electron beam goes into the pinhole when it is scanning on the sensor and passes through the current/voltage conversion circuit. The weak voltage signals will be amplified by the multi-stage amplifier circuit in the sensor, converted into a digital signal and then finally entered into the IPC for storage and processing. The electron beam signal of a single cross-section depicts the threedimensional graphic of the QIPDD of the beam. Without changing the conditions of the test parameters, it can obtain the QIPDD of different sections by adjusting the axial position of the sensor. The system structure is shown in Fig. 1. 3. System realization 3.1. Electron beam scanning control module Firstly, The main function of the electron beam scanning control module based on the CPLD is to drive a multi-polar magnetic deflection coils to produce an alternating magnetic field so that electron beam scans in high-speed on the surface to prevent destroying the sensor with converting and amplifying circuit. Secondly, the module also sends a sampling clock and trigger signal to the data acquisition card installed in IPC.
Fig. 2. Sketch of the synchronous control signal and scan path.
In the electron beam quality test process, the electron beam scans in the direction of the arrow in Fig. 2a, and the corresponding control signals of the magnetic field are shown in Fig. 2b and c. The labels UX and UY are the control voltage of the magnetic field in the X direction and in the Y direction respectively. The notation Ts is the scanning period. In the forward-stroke scanning process of the electron beam (i.e., in the direction of the solid line in Fig. 2a), the strength of the magnetic field that makes the electron beam deflect in the Y direction is maintained and the corresponding field in the X direction is increased linearly. In the retrace scanning process (i.e., the dotted line in Fig. 2a), the magnetic field in both the X and Y direction is increased linearly. As noted above, the data is collected in the process of forward-stroke scanning but is not in the retrace scanning in order to ensure the special phase relationship of the data distribution.
Fig. 1. Schematic diagram of the beam quality test system of EBW： (1) Cathode; (2) Control electrode; (3) Anode; (4) Control coil; (5) Optical observation system; (6) Focusing coil; (7) Deflection coil; (8) Vacuum chamber; (9) Energy absorbing device; (10) Sensors; (11) Work piece; (12) Electron beam scanning control system; (13) IPC; (14) Display.
Y. Peng et al. / Vacuum 86 (2011) 261e266
Fig. 3. Sketch of the beam defection over sensor.
Typically, the focus diameter is about 0.1e1 mm under the working conditions that the rated voltage of electron beam welding gun is 30e175 kV and the electron beam current is 1000 mA or less. For large power welder beam spot diameter of the focus extends to 2 mm, while the beam diameter will work with the process parameters such as the axial working distance and focusing current. In order to accurately measure the QIPDD when the position of the focus is unknown, the contour outline of the electron beam must be guaranteed within the testing range so that the scanning range must be greater than 2 2 mm. However, because of the limit of the maximum deflection angle of the deflection coils, the maximum scanning range is 30 30 mm. The resolution of three-dimensional graphic reconstruction in Y direction of QIPDD depends on the distance DY between two adjacent scans. If DY is smaller, the distribution of the graphic reconstruction is denser and the graph is high-quality. Thus we could better describe the details of the beam power distribution by reducing the value of DY. Considering the deflection coils performance of the control system, the step-by-step number n of the electron beam scanning was set to the constant 100, corresponding to 100 cycles of the triangular wave in Fig. 2b. Once the test is completed, the electron beam is deflected to the surface of the energy absorption device. When the scanning area is 2 2 mm, DY is equal to 20 um, by the same token, when the range for 30 30 mm, DY is equal to 300 um. The relationship between the pinhole diameter and DY can be well explained in Fig. 3. We just change the DY in the Y direction to meet the diameter of beam spot.
263
If the beam spot is bigger, DY is increased accordingly. For the deflection accuracy, we just make sure that DY is equal or larger than the pinhole diameter, otherwise there will be some overlap of the collected current between two adjacent scans (i. e., the overlap between the grey blocks A and B in Fig. 3). In order to make the electron beam stay for a very short period at the same position of the sensor to prevent depth of fusion on the sensor, we must control the electron beam to scan rapidly and to heat the sensor uniformly. The scanning experiment was carried on with 316 stainless steel sheets to test the lowest scan rate which a sensor can withstand. The experiment used the ZD150-15A type electron beam welding machine with the accelerating voltage Ua of 150 kV and the maximum power Pm of 15 kW. The thin steel plate size is 200 150 2 mm. The welding parameters are: voltage Ua ¼ 150 kV, current Ib ¼ 100 mA, focus current If ¼ 345 mA, working distance h ¼ 330 mm, etc. The experiment result indicated that if scanning frequency (Fig. 2, the frequency of the triangular wave) fs ¼ 1=Ts 3 kHz, there can be no depth of fusion on the plates, and therefore the max scanning frequency is set to 5 kHz limited by the power amplifier. In terms of hardware circuitry, the scanning control circuit consists of a CPLD, a serial communication circuit, an external clock circuit, a reset circuit, a power circuit, a digital-analog conversion (DAC) circuit, a filter circuit, an optical isolation circuit and a power amplifier circuit. A general view of the relationship between the components of the circuit is depicted in Fig. 4. The main part of the external clock is an oscillator of which clock signal passes through the frequency divider circuits of CPLD to produce a variety of clocks. These clocks are used for serial communication, controlling data acquisition card and generating the variable frequency waveform. IPC sends 8-bit binary code which contains the information about scanning start/stop and scanning frequency to the CPLD control circuit through the serial communication circuit. Triggered by the clock signal, the CPLD control circuit generates two digital waveforms according to the received binary code. The digital outputs are converted to analog waveforms through 12-bit DAC. The digital signal in the conversion process has great impact on the current generating a relatively large voltage drop on the transmission line and internal resistance of the source, so it’s necessary to use the low-pass filter circuit to filter the higher harmonic. The filtered signal passes through the bipolar optical isolation circuit that protects the pre-stage circuit against the impact of the rear power circuit. Finally the signal enters the constant current source power amplifier. Design and simulation of the control signal generator has been performed using Very-High-Speed Integrated Circuit Hardware Description Language (VHDL). Fig. 5 shows the Block diagram of the CPLD top-level file with clk_in as the clock signal, reset as the reset signal, rxdr as the input signal of serial communication, txd as the output signal of communication, txd_done as the signal that shows txd is completed, x_out [11..0] and y_out [11..0] as the control signal
Fig. 4. Sketch of control circuit.
Fig. 5. Block diagram of CPLD top-level file.
Fig. 6. Flow chart of the synchronous-waveform block.
Y. Peng et al. / Vacuum 86 (2011) 261e266
265
of coils, clk_sample and trigger as the control signal of acquisition card, wr as the write input of DAC. Fig. 6 is the flow chart of the control signals that are generated by the main synchronouswaveform block in the Fig. 5. 3.2. Signal sensing and acquisition module The sensor based on the Faraday cup can accurately measure the QIPDD of high-energy electron beam. The current entered into sensor produces a voltage drop Uout in the signal sampling resistor R. As the collected current is very small, R should be taken as large. If the electron beam welder is stable at work and the current density distribution of electron beam does not change over time, it can be assumed that both ends of the sampling resistor are connected to constant current source that the voltage drop Uout is “very stable” and does not change with the load. Under the above assumptions, if R is infinite, then Uout is also infinite. Therefore, the ideal constant current source is not allowed to be open-circuit. For the actual circuit, when R is large enough, the capacity of the voltage output would be insufficient, so the value of R should not be large. In the testing process R is taken to be the precise noninductance resistance of 1 kU. The voltage Uout is processed by a differential amplifier and PCI-1714 high-speed acquisition card, and then stored in the buffer for subsequent signal calculation and reconstruction of QIPDD. The calculation is shown by the following equations:
Uout ¼ Je Ah R
(1)
Je ¼ Uout =ðAh RÞ
(2)
Jp ¼ Ua Je ¼ Ua Uout =ðAh RÞ
(3)
where Ah is the pinhole size in the sensor, Je is the current density of the electron beam entered into the sensor, Jp is the power density of the beam, and Ua is the accelerating voltage. 4. Results and discussions The signal waveform collected by the beam quality test device is shown in Fig. 7a. The beam parameters of the signal are: voltage Ua ¼ 150 kV, current Ib ¼ 8 mA, focus current If ¼ 345 mA, working distance h ¼ 330 mm, scanning frequency fs ¼ 3 kHz, etc. Signal acquisition is taken in each forward-stroke scanning process. After zooming in the time axis of the collected waveform, a peak signal collected in the signal scanning process appears in the upper left corner of Fig. 7a. The peak indirectly indicates QIPDD of the beam in a single scan region. The voltage signal about a single cross-section that is collected by repeatedly scanning forms a matrix Amn as following equation:
0
Amn
a11 ¼ @ « am1
. aij /
1 a1n « A amn
(4)
where aij is discrete output voltage collected by the acquisition card:
aij ¼ Uout ¼ Je Ah R
Bmn
0 a Uout @ 11 ¼ « Ah R am1
(5) . aij /
1 a1n « A amn
Fig. 7. Flow chart of QIPDD reconstruction.
(6)
where Bmn is the current density Je matrix of a single cross-section. Then using Equations (3) and (4), the Equation (6) is obtained by
which the three-dimensional graphic of QIPDD about a single cross-section is reconstructed in Fig. 7b. Fig. 7c is the projection of Fig. 7b in the XY plate. From Fig. 7b it is easy to see, that the system is able to measure and record very fine details of density
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distribution and the diameter of the beam under h ¼ 330 mm is about 0.9 mm. Signal cross-section data can not express exhaustive spatial information such as the focus. Using several QIPDD of crosssection at different working distance in axial direction we can reconstruct the four-dimensional graph of the beam like Fig. 7d. Fig. 7e shows the ideal four-dimensional distribution. 5. Conclusion and perspectives The electron beam four-dimensional quality test system development was targeted at the 5e100 mA electron beam current of a high-voltage electron beam welding machine. The system includes the control module, sensor module, driver module, analysis software and so on, which is based on the CPLD. The main parameters of an electron beam scanning control module are: the scanning range is 2 2 mme30 30 mm, the stepby-step number n of the electron beam scanning is 100; the maximum scanning frequency is 5 kHz. The signal waveform is collected by the beam quality test device under the conditions of Ua ¼ 150 kV, current Ib ¼ 8 mA, focus current If ¼ 345 mA, working distance h ¼ 330 mm, scanning frequency fs ¼ 3 kHz and the diameter of the beam is about 0.9 mm. The development of a user-friendly software interface and the actual four-dimensional distribution of QIPDD are subject to further research. In addition to the software improvements, we need to
focus on the relationship between QIPDD and the parameters of beam. Acknowledgments This work was supported by NUST Research Funding (NO. 2010XQTR01). References [1] Arata Y, Tomie M, Terai K, Nagai H, Hattori T. Transactions of the JWRI 1973; 2(2):130e46. [2] Schultz H. Electron beam welding. Woodhead Publishing; 1993. [3] Zhou Q, Liu FJ, Guan Q. Hanjie Xue Bao/Transactions of the China Welding Institution 2004;25. 77e79þ84. [4] Dilthey U, Goumeniouk A, Böhm S, Welters T. Electron beam diagnostics: a new release of the diabeam system. Vacuum 2001;62:77e85. [5] Palmer TA, Elmer JW, Nicklas KD, Mustaleski T. Transferring electron beam welding parameters using the enhanced modified faraday cup. Welding Journal 2007;86:388-se98-s. [6] Qasim SM, Abbasi SA. A new approach for arbitrary waveform generation using FPGA and orthogonal functions. In: 6th IEEE international workshop on system on chip for real time applications, IWSOC 2006, 2006 december 27e29. Cairo, Egypt: Inst. of Elec. and Elec. Eng. Computer Society; 2006. p. 28e32. [7] Kolouch J. Templates for CPLD and FPGA designs. Radioelektronika. In: RADIOELEKTRONIKA ’09 19th international conference 2009; 2009. p. 211e3. [8] RoyChoudhury J, Banerjee TP, Nathvani A, Chowdhury RBR, Bhattacharya AK. Design methodology internal sub state observer using CPLD. Nature & Biologically Inspired Computing, NaBIC; 2009. pp. 1636e40.