A Novel Mixed Current and Voltage Control Scheme for Inverter Arc Welding Machines

Transcription

1 A Novel Mixed Current and Voltage Control Scheme for Inverter Arc Welding Machines Young Min Chae, Yungtaek Jang, Milan M. Jovanović, Jae Suek Gho and Gyu Ha Choe Delta Products Corporation KonKuk University Electrical Engineering Power Electronics Laboratory 93-1 Mojin-Dong Kwangjin-Gu, 511 Davis Drive, Research Triangle Park, NC 2779 Seoul, Korea, Abstract - In this paper, a novel mixed current and voltage control scheme for an inverter-controlled arc welding machine is proposed. The proposed control scheme uses both a closed-loop current controller and an open-loop voltage controller to optimize the output current and voltage waveform d on the metal transfer procedure. An experimental comparison is made between the proposed and the conventional control scheme for the full output-current range. I. INTRODUCTION Generally, Gas Metal Arc Welding (GMAW) Machines are widely used in industrial applications for joining or cutting materials. They can be classified into various types depending on the welding objectives, shielding gases, or electrodes. An integral part of most welding machines is an AC/DC rectifier, or an AC/DC/DC converter that produces a constant, low DC voltage, typically in the 2-V to 7-V range. One of the most widely employed implementation of the welding machines is the thyristor-controlled arc welding machine, which is composed of an input side transformer and thyristor rectifier. Although this type of welding machines is simple in structure and cost effective, it is bulky and heavy because it utilizes a source-side line-frequency transformer. In addition, it has an undesirable low frequency output voltage ripple caused by the thyristor rectification. The size and weight of a welding machine can be significantly reduced by employing high-frequency PWM inverter techniques. In addition, these techniques improve the welding performance due to a stable DC output voltage. However, the major drawback of the conventional inverter arc welding machines that use a simple open-loop output voltage control method is that the metal transfer procedure is determined by the magnitude of the output DC voltage and wire feeding rate [1]-[3]. In case of GMAW, the metal transfer procedure occurs either by the short-circuit metal transfer procedure (SMTP), or globular metal transfer procedure (GMTP) depending on the magnitude of the output current. In the low-output-current region, which is typically less than 25 A, the SMTP mainly occurs. Since in SMTP the arc state and short-circuit bridge state occur irregularly these transient changes of metal transfer procedure result in poor welding performance such as a heavy spatter generation and a poor bead state. On the other hand, during the GMTP, which occurs at high output currents, the metal transfer procedure is performed in the arc state without forming a short-circuit state. Because in the arc state the output voltage and current are maintained constant, the GMTP exhibits much less spatter generation and a good bead state. As a result, most of today s research in the GMAW area is focussed on the improvement of the welding performances of the SMTP. Generally, the welding performance is mainly determined by the spatter generation since excessive spatter generation prohibits continuous operation during the automatic welding process, and results in a poor predictability of the whole automatic welding system. It is well known that most spatter is generated at the instant when short-circuit bridge or arc reignition state is formed. Therefore, the spatter generation is even more pronounced in the instantaneous short-circuit metal transfer procedure (ISMTP), introduced in [1], [4] and [5]. The welding performance can be improved by the proper control of the inverter. Among the control methods proposed are the output-current slope control method and the pulsed output-current control method. Since these control methods basically ignore the welding condition of the metal transfer procedure itself, the welding procedure is performed under non-optimum conditions so that the reduction of spatter generation is limited, as described in [2] and [6]. A much better welding performance can be achieved by the instantaneous output current control, which controls the output current instantaneously d on metal transfer procedure by using the feedback current control method. However, the practical implementation of this control is complex since this control method requires the optimum output current reference waveform and also has difficulty in adjusting the gain of the current controller [3]. In this paper, a novel simple, mixed current and voltage control scheme for inverter arc welding machines is proposed that controls the output power d on the metal transfer procedure. The proposed control scheme uses both the feedback current controller and the open-loop voltage controller and its digital implementation employs 8C196KC microprocessor /1/$1. (C) 21 IEEE

2 II. INVERTER ARC WELDING MACHINE A. Circuit configuration An inverter controlled GMAW machine usually consists of a power converter unit, a wire feeder control unit, and a shielding gas flow control unit. The power converter unit serves to produce a stable DC output voltage, whereas the wire feeder unit controls the feeding rate of wire, which is directly related to the output current value. Finally the shielding gas flow unit is used to stabilize the welding performance by shielding the arc plasma from the air which causes oxidization. Typically, the power converter unit of an inverter controlled GMAW machine consists of a 3-phase rectifier, full-bridge inverter, transformer and output diode rectifier, as shown in Fig.1. In this paper, the focus is on the control method of power converter unit especially the inverter control method that improves the welding performance. B. Metal transfer procedure in GMAW Generally, the metal transfer procedure starts with the separate electrode and metal and ends when the two materials are combined by the electrical energy. Typical voltage and current waveforms for the SMTP, which exhibits the arc and short-circuit state, are shown in Fig 2. During the arc-state period, the output voltage is high (2-3 V) because the consumable electrode that is being melted by the arc plasma represents a high-impedance load (process step E in Fig. 2). Gradually, the molten electrode is enlarged and finally contacted to the metal, as shown in Fig. 2 (process steps E-H). As soon as the contact to the metal is made, a short-circuit bridge is formed that extinguishes the arc plasma (process steps H and A in Fig. 2). At the same time, the output voltage rapidly drops and the output current start to increase. Since the short-circuit current is limited by a small contact resistance the output current is large. This large output current makes the contacted area melt so that the short-circuit bridge is eventually broken and the arc plasma is ignited again, as shown in Fig. 2 (process step D and E). In the SMTP the transitions from the arc state to the shortcircuit state and vice-verse are continuously repeated, which generates a larger amount of undesirable spatter. The spatter generation can be observed and studied by a high precision camera. III. PROPOSED MIXED AND CONTROL SCHEME A. Conventional open-loop voltage control scheme In the conventional inverter controlled GMAW, the constant output voltage characteristics of the inverter is usually generated with an open-loop voltage controller. In this open-loop control, reference voltage signal v is obtained by a linear combination of the voltage and current setting signals vdial and i dial, as v k1vdial + k2i dial =, (1) where k1 and k2 are constants. Therefore, in this control, the duty ratio and the output voltage of the inverter are fixed by the magnitude of v. Recently, to improve the welding performance by preventing a rapid change of the output current, outputcurrent sensing signal i real is added to form the voltage reference signal v, as v k1vdial + k2idial + k3ireal Short-Circuit =. (2) Arc State Voltage Wire Feede Controller Gas Flow Controller Current Inverter Wire Feeder Control Signal Control Signal Welder Controller Fig. 1 Circuit configuration of inverter controlled GMAW. Fig. 2 Short-circuit metal transfer procedure in GMAW /1/$1. (C) 21 IEEE

3 Digital Controller Limiter PWM Limiter PWM V dial K1 PI V K 2 I dial Filter V C α i real K 3 K 3 i real + - V PI I Current Control Mux V Eq.(1) Filter V C α Short Arc Short Detector K Filter K V real I real (a) Control block diagram (a) Control block diagram Voltage Short Arc Voltage Short Arc Current Current I (b) Output voltage and current waveform time Fig. 3 Control block diagram and waveform of conventional GMAW. Figure 3 shows the output waveform and the control block diagram of the conventional control scheme, where k 1, k 2 and k 3 [2]. The major advantage of the open-loop voltage control scheme is the simplicity and low cost. However, since the voltage reference signal v does not contain the information about the metal transfer procedure, the desired spatter reduction cannot be achieved. B. Proposed mixed current and voltage control scheme To reduce the spatter generation, a new control scheme is proposed in this paper. The block diagram of the proposed control scheme which uses both the closed-loop current control method and the open-loop voltage control method is shown in Fig. 4(a). In this control scheme, the information about the metal transfer procedure, i.e., whether it is the arc state, or the short-circuit state, is obtained by sensing the output voltage. This information is then processed by the digital controller which generates a proper reference signal. Figure 4(b) shows the output voltage and current waveforms of the proposed control scheme. In Fig. 4(b), periods I and II represent the closed-loop current control period and the open-loop voltage control period, respectively. (I) T d (II) I : Current Control II : Voltage Control (b) Output voltage and current waveform time Fig. 4 Control block diagram and waveforms of proposed GMAW. During the arc state period, the open-loop voltage control method is used to achieve a stable output voltage and easy adjustment of controller, as it is done in the conventional inverter controlled GMAW. During the constant outputvoltage condition the electrode is melted and the molten globule is enlarged by the arc plasma heat. After the enlarged molten globule forms the short-circuit bridge, the closed-loop current controller is activated. And the output current is regulated at constant current I. Finally, with this control it is possible to reduce the spatter generation by suppressing the rapid change of the output current at the instances of the short-circuit bridge forming. To break the short-circuit bridge and also to ignite the arc plasma, the open-loop voltage controller is activated after a time delay. To achieve stable transitions between this two-control scheme and also to simplify the hardware implementation, a fully digitized controller is developed by using an 8C196KC microprocessor. With this digital control method, the optimum values for I and T d can be easily set by the software. IV. EXPERIMENTAL RESULT The characteristics of the proposed control scheme and it s welding performances are evaluated and compared with /1/$1. (C) 21 IEEE

4 the conventional control scheme experimentally. The experimental conditions are summarized in Table I. It should be noted that in Table I, delay time T d and current value I that are the parameters in the proposed control scheme are set to achieve the least spatter generation. TABLE I EXPERIMENTAL CONDITIONS OF INVERTER CONTROLLED GMAW Item Conventional Proposed CASE I V out 19 [V] 19 [V] T d=2[ms] I out 15 [A] 15 [A] I =5[A] CASE II V out 22 [V] 22 [V] T d=1[ms] I out 15 [A] 15 [A] I =7[A] CASE III V out 27 [V] 27 [V] T d=2[ms] I out 15 [A] 15 [A] I =9[A] Shielding Gas : CO 2 Distance between electrode and metal : 2. cm Figure 5 shows the output voltage and current waveform of the proposed control scheme. This figure also shows the multiplexer output waveform, which is the input signal of the digital controller. From this figure, it is possible to distinguish the voltage control and current control regions, respectively. To observe the effect of different delay times in the proposed control scheme, the experiment was performed with 1ms and 3ms delay time. The time period of the SMTP is increased as the delay time is increased because the longer delay time requires more time to break the short-circuit bridge as shown in Fig. 6. Figures 7 and 8 show the output voltage and current waveforms of the welding machines with the conventional and proposed control scheme under the same output power conditions, respectively. In both figures it is possible to observe an increased number of time periods of the SMTP as the output current increases from 15 A to 25 A. Nevertheless, the comparison of these figures also shows that the proposed control scheme exhibits more regular output voltage and current waveform than the conventional control scheme. In addition, as can be seen from Fig. 8, the proposed control scheme keeps the peak value of the output current constant, whereas the peak of the output current varies in the conventional control, as seen in Fig. 7. Also, it should be noted that ISMTP, which has a time period less than 2 ms, is frequently observed in the conventional control scheme, Fig. 7. This causes a large amount of the irregular metal transfer procedure that results in an excessive spatter generation. The proposed control scheme improves the welding performance by providing stable SMTP, i.e., it reduces spatter by eliminating the irregular, instantaneous short-circuit forming phenomenon. Fig. 5 Output waveforms of proposed GMAW. (a) delay time 1ms (b) delay time 3ms MULTIPLEXER OUTPUT 1ms/div Fig. 6 Output voltage and current waveforms of proposed GMAW. 1[V]/div 1ms/div 1ms/div /1/$1. (C) 21 IEEE

5 1ms/div 1ms/div (a) output current 15[A] (a) output current 15[A] 1ms/div ISMTP 1ms/div (b) output current 2[A] (b) output current 2[A] 1ms/div ISMTP 1ms/div (c) output current 25[A] Fig. 7 Output voltage and current waveforms of conventional GMAW. (c) output current 25[A] Fig. 8 Output voltage and current waveform of proposed GMAW /1/$1. (C) 21 IEEE

6 Figure 9 shows the trajectories of the output voltage and current waveforms of the conventional and proposed control methods. Generally, during the short-circuit metal transfer procedure the trajectories are squares with a counterclockwise orientation. If the short-circuit metal transfer procedure were maintained consistently, than all trajectories would coincide, i.e., they would form a single square. Therefore, a correlation exists between the number of inner loops and the irregularity of the metal transfer procedure. The comparison of Figs. 9(a) and (b) implies that the proposed control method shows a more stable metal transfer procedure than the conventional control method. The comparison of the spatter for the conventional and proposed inverter arc welding machines in all the output current regions is shown in Fig. 1. The proposed control scheme reduces the spatter generation more than 3 % compared to the conventional control scheme. The maximum spatter reduction was recorded at the output current of 2 A, where the spatter reduction was about 5 %. (a) Conventional control scheme V. CONCLUSION To improve the welding performance of inverter controlled GMAW machines, a new mixed voltage and current control scheme is proposed. The proposed control scheme uses both the open-loop voltage and closed-loop current controllers to adjust for the metal transfer condition. By employing closed-loop current control during the delay time when the short-circuit forming occurs, it is possible to reduce the spatter generation by stabilizing the metal transfer procedure. The experimental evaluation the proposed control scheme shows that the proposed control improves the welding performance by reducing the spatter generation by 3-5 %. REFERENCES [1] J. F. Lancaster, The Physics of Welding, Pregon Present, [2] R.L. O'Brien, Welding Handbook, AWS, vol.2, [3] Y. M. Chae, G. H. Choe, et. al., "A New Instantaneous Output Current Control Method for Inverter Arc Welding Machine IEEE, PESC 99 Records, Vol.1, p. 521, [4] H. Yamamoto, "Recent advances in inverter controlled arc welding power sources and their application", Journal of Japan Weld. Soc., Vol.58, No.4, p. 273, [5] T. Mita, et. al., "Spatter Reduction-Power Source Considerations", Journal of the Japan Welding Society, August, 199 [6] H. Yamamoto, et.al, "The Development of Welding Current Control Systems for spatter reduction", Welding International, Vol.4, No.5, p. 398, 199 Spatter (g) (b) Proposed control scheme Fig. 9 Output current and voltage trajectories of GMAW. Generated Spatter Comparison Conventional Proposed 15A 2A 25A 27A Output current (A) Fig. 1 Generated spatter quantity comparison /1/$1. (C) 21 IEEE