Design of Feed Control System for Micro EDM Machine Tool Liao-yuan ZHANG, Yue GENG, Da-lin JIANG, Ming-wei SHANG and Yue-long WANG

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1 2016 International Conference on Electrical Engineering and Automation (ICEEA 2016) ISBN: Design of Feed Control System for Micro EDM Machine Tool Liao-yuan ZHANG, Yue GENG, Da-lin JIANG, Ming-wei SHANG and Yue-long WANG School of Mechanical Engineering, Shenyang Ligong University, Shenyang , Liaoning, China Keywords: Micro electrical discharge machining (MEDM), Motion control card, Data acquisition card, Wear compensation. Abstract. This paper designs the closed-loop feedback control system of MEDM, which is based on the motion control card and data acquisition card. To ensure the accuracy of the process, the gap can be regulated through the analysis of discharge condition and the analysis and treatment of the gap voltage, and then a servo control scheme on small holes machining is presented. Through this way, the electrode loss compensation scheme can be solved, which can improve the efficiency and practicality of MEDM. Introduction With the development of the scientific research and top national defense industry, the new high-tech products progress towards the direction of functional integration, ultra precision and small shapes, which makes the size of parts gradually miniaturization. Accordingly, the effective design of the MEDM control system not only can enhance working efficiency, precision, and the degree of automation, but can greatly increase the application of that. We should lay emphasis on the manufacture of the MEDM device, but pay more attention to its control system. Thereby the way to improve the processing precision and efficiency can be found, which provides theoretical guidance for the evolution of the MEDM technology. Design of Processing Device The processing device is shown in Figure 1, reduce the impact of the discrete capacitor of the device to processing, the base portion of the device is composed of granite, three axis are driven by the stepper motor realize synchronization and single axis basic motion, the major rotating spindle is driven by the DC servo motors. RC pulse power is used for providing the high frequency pulse needed by MEDM. Electrodes are mounted on head of the major axis, so that the rotation motion of MEDM can be made. Figure 1. The hardware structure of micro EDM control system.

2 The motion control card is Altair PCI1020 as the motion control system in this paper. It s a kind of PCI but independent 4 axis stepper motor drive motion control card. It can ensure that the error of pulse output frequency is small, the motor speed, rotation angle and acceleration can be accurately controlled. It can realize the position control of the motor, arc, linear interpolation and acceleration and deceleration control, etc. At the same time, it also has a large number of software library function, the users can have a secondary development when using Visual C or other software. Data acquisition card AltairPCI8602 is chosen as the core signal of the control system to collect and process the device. Through divide voltage, isolation and filtering, the discharge gap voltage changes into an electrical signal which is acceptable for the data acquisition card, and after the AD converter converts, the analog signal converts into the digital signal which is discernible by the motion control card. The signal is delivered into the PC machine by data acquisition card, and the PC machine starts the real-time data operation and issues the corresponding instructions, through the servo drivers to control servo motor, thus the gap can be regulated. Design of Servo Mechanism EDM is a non-contact processing. When processing, the discharge gap is between the tool electrode and the work-piece electrode. If the gap is too large, the pulse voltage can t breakdown the insulating medium in it, and EDM can t be made, the tool electrode should go downwards to reduce the gap. The design of EDM servo control system, in this paper, is composed of the measuring link, comparing link, transform amplifying link and perform link, its block diagram is shown in Figure 2. Figure 2. Servo control system block diagram. (1) Measuring procedure. In the EDM process, it is difficult and unnecessary to measure the distance and variation of the discharge gap directly. The study found that a strictly monotonic relation between the size and the gap voltage of discharge gap. Therefore, discharging gap voltage is the only one to be measured. (2) Comparing link. In this procedure, the measured signal is compared to the one which is given, in order to judge the condition of the electrode, feed, back, or remain stationary. Due to the different processing rules, the set value may be changed to meet the processing requirements. (3) Transforming amplifying procedure. The signal obtained in measuring procedures is too small to drive this device. In order to ensure the driving power, the amplifier is used to amplify the signal. Attention that the amplification ratio of the amplifier should be selected appropriately to ensure the stability of processing. (4) Execution procedure. The executive procedure of EDM usually refers to the servo motor, which adjust the feed rate of the tool electrode through the distance of the control signal, to ensure that the discharge gap is in an appropriate range. Voltage values measured through the EDM have two functions: one is to serve as an important basis for the regulation of the parameters of pulse power, another is to judge the gap discharge state, thereby controlling the motor to make the corresponding motion.

Basic Function Design of the Control System Design of Manual Control Function Manual control includes single axis point control and single step incremental control.

3 Basic Function Design of the Control System Design of Manual Control Function Manual control includes single axis point control and single step incremental control. The motion of each axis is driven by pulse signal. Point control can be realized through continuous pulse output and single step incremental control can be realized through quantitative pulse output. The quantitative pulse driving usually refers that the motion control card outputs a fixed number of pulse with the fixed speed, acceleration and deceleration. Acceleration / deceleration quantitative driving speed curve is shown in Figure 3. The speed reduction will occur when the remainder of pulse output is less than the acceleration pulse, and stop before the fixed pulse is outputted Figure 3. Quantitative pulse driving Figure 4. Four axis synchronous motion velocity curve. The four axis synchronous motion speed curve is shown in Figure 4. (the speed of axis Y is same to that of axis Z, therefore, their curves overlap). Design of Contact Sensing Function Contact sensing function is the special function of the EDM CNC machine tools. The principle is: when the voltage is applied between the tool electrode and work-piece electrode, if the distance is far, the voltage is close to the applied voltage, the tool electrode is gradually close to the work-piece, when the gap distance is reduced to a certain degree, the short circuit voltage occurs. The voltage value decreases rapidly. Through the voltage value obtained by the data acquisition card, judge the gap distance. The initialization includes the parameter settings. If the initialization fails, exit after the error handling, if the initialization succeeds, read the voltage value between the electrode and the work-piece timely. If the voltage value is within that of the short circuit range, then stop. Otherwise control the control motor, move the electrode to the work-piece, until it reaches the short circuit condition and stop. The contact sensing is made. Design of Discharge Machining State Detection Function (1) Signal sampling module The real-time voltage waveform is obtained by the data acquisition card, typically, the open circuit voltage of the MEDM discharge gap is high; the short circuit voltage is very low and the amplitude range of the voltage is fluctuant. When gathering the gap voltage, the normal discharge voltage and the short circuit voltage in the middle are of not much difference if the voltage is just reduced proportionally, and some high-frequency interference waves are probably mistaken as short circuit condition. Therefore, gap voltage sampling module adopts "break off both ends" type, as shown in Figure 5. Only the middle part of the voltage waveform, can improve the sampling accuracy of the gap voltage. The stabilivoltvd2 controls the upper limit voltage value VH, and then stabilivoltvd1 controls the lower voltage value VL.

4 Figure 5. Collection voltage waveform. Figure 6. State recognition module block diagram. The voltage value gathered is higher, so it is needed to be controlled at about 10V in order to facilitate and compare the subsequent circuit, the resistance R1 and R2 are used to divide voltage. Take the voltage value V2 of R2, and the range of the voltage value is limited through the value ratio of two resistances. And: R 2 VZmax VH VL R 1 R 2 (1) The high-frequency wave of the EDM can interrupt the recognition of the state therefore, the loss pass filter is chosen to filter out the high frequency signal produced by the discharge and short circuit condition, remove the interference signal, so that the divided voltage signal becomes gentle, thus the accuracy of the discharge gap condition can be improved. (2) Condition recognition module Compare the average gap voltage value with the set threshold, judge the EDM discharge gap condition. The block diagram is shown in Figure 6. The comparison circuit was mainly used to compare the average gap voltage value with the open circuit threshold Vref1 and the short circuit threshold Vref2, according to the magnitude relation, judge the discharge condition. Photoelectric isolation circuit is mainly used for anti-interference and level translation. The decoding circuit is to decode pulse width signal tr of comparison circuit, the light signal td and non-short circuit signal te into the EDM discharge gap condition correspondingly. The truth table is shown in Table 1. Table 1. Logic truth table of gap status. tr te td Gap state Short-circuit condition Ts Spark condition Te No-load condition Td Design of Electrode Loss Compensation Control The electrode loss is a very serious problem when using micro electrode to process holes. This paper records the data of the EDM discharge condition experiment on a small hole, uses the statistics of open circuit times of discharge condition in the process to estimate the electrode wear rate. Then according to the measurement results of the axial loss of the electrode, propose the electrode loss compensation scheme, which is an important part of the MEDM control system. The process is shown in Figure 7. Processing Experiment Figure 8 is the micro-pore processed out of the MEDM machine tool closed-loop feedback control system, which regulates the electrical parameters. 16 groups are divided to process in table 2, to verify the practicability of the processing control system. Use the 0.4mm electrode, apply reversed polarity processing, the electrode material is hard alloy, the speed of machine tool is 0 ~ 800rmp; the work-piece material is Ti-Mg alloy plate in 1mm, and the medium is spark oil, process a hole in diameter 0.4mm.

5 Figure 7. Electrode loss compensation process. Figure 8. Processing examples. Table 2. The experimental data and results. Open-circuit voltage U/V Peak current I/A Pulse width T/µs Pulse interval T/µs Machining diameter mm Conclusion Through the research and analysis of the MEDM processing principle, the MEDM machine tool closed-loop feedback control system is designed. The motion control card and data acquisition card controlled by PC machine are the cores of this system. Finish the design and development of the hardware and the software, combine the discharge gap voltage sampling condition and pulse power. When the gap condition changes, regulate it through the interaction of the servo control system and the pulse power parameters, and the analysis of the processing experiments confirms the applicability and feasibility of this device in the control system. References [1] Di Shichun, Huang Ruining, Chi Chi, Zhao Wansheng. Key technologies of micro electrical discharge machining. [J]. Journal of mechanical engineering, 2006, 42(1): [2] A.K. Garber, A.M. Arsenkin, K.V. Grigorovich, S.S. Shibaev, A.V. Kushnarev, Yu. P. Petrenko. Analysis of various versions of the deoxidation of rail steel at OAO NTMK [J]. Russian Metallurgy (Metally) (7).

6 [3] M. Kh. Liberman, M. Sh. Shermeister. The plan of the scientific organization of labor at the blooming mill of NTMK [J]. Metallurgist (12). [4] Zhang Yong, Zhao hang, Zhang Guangyu, Wang Zhenlong, Zhao Wansheng. Micro electro discharge machining system and its technology [J]. China Mechanical Engineering, 2008, 19(5): [5] Philip T. Eubank, Mukund R. Pate L., Maria A. Barrufet, et al. Theoretical models of the electrical discharge matching process. III. The variable mass, cylindrical plasma model. Texas A&M University [J]. J. Appl. Phys. 1993, 7 3(11):

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