Digitization of welding machine

Foreword

Digital signal processing has great advantages over analog signal processing in terms of high precision, flexibility, reliability, and ease of large-scale integration. Taking the recording system as an example, the signal-to-noise ratio of an analog recording system is generally around 60 decibels, and the signal-to-noise ratio of a 16-bit digital recording system can reach 96 decibels, which is more than 60 times higher than that of an analog recording system. Digital signal processing technology is therefore widely used in communications, speech processing, graphics/image processing, automatic control, instrumentation, medical electronics, military and cutting-edge technology, computers and workstations, consumer electronics and many other fields.

As a result of the combination of digital signal processing technology and arc welding technology, the emergence of digital welding machines has attracted wide attention from the industry. The focus of attention is first of all the concept and characteristics of digital welding, followed by the implementation of digital welding machines, and finally the role of digital welding machines in promoting the entire welding production process.

1. Digital welding machine and its characteristics

Because the digital welding machine appeared relatively late, for example, Fronius began to produce digital welding machines in 1998. So far, there has not been a unified definition of digital welding machines, which has been recognized by various parties. We understand that the so-called digital welding machine should refer to such welding machines. Their main control circuits are directly replaced by digital technology by traditional analog technology. The control signals in the control circuit also transition from analog signals to 0/1. Encoded digital signal.

There have been two key phases in the development of computational control techniques, the pioneering period and the direct digital control period. In the pioneering period, the computer system is slow, expensive, and unreliable. Therefore, the calculation control at this stage is mainly carried out in two ways. One way is that the computer prints the command to the operator, and the other way is that the computer modifies the analog adjustment. Set value of the device. After entering the direct digital control period, the task of the computer is no longer limited to sending instructions to the operator and modifying the settings of the analog regulator, but instead directly replacing the functions of the analog regulator.

It should be noted that there are also two stages similar to this in the development of digital welders. The single-chip controlled arc welding power source represented by 8098 or 80C196 is basically a product of the digital welding machine. Its main feature is that the main function of the single-chip microcomputer in the welding machine is to complete the management of the welding machine and the welding parameters. The constant pressure and constant current control in the welding process are completed by the simulated PI controller. Of course, this is not absolute. Not all single-chip controlled arc welding power supplies are only given by the single-chip microcomputer. In some SCM controlled thyristor welders, from the parameter setting, welding parameter feedback sampling, PI control, to the occurrence of thyristor trigger pulse is completed by the microcontroller. The arc welding power source controlled by the single chip in this case should belong to the product of the direct digital control period.

In a certain sense, only when the digital welding machine enters the direct digitization stage and truly realizes digitization, fully demonstrates the advantages brought by digital control. In general, digital control is superior to analog control mainly in several aspects such as good flexibility, strong stability, high control precision and good interface compatibility.

It goes without saying that digital control has good system flexibility. For analog systems, the configuration and gain of the system is determined by hardware parameters such as the RC network, and once determined, it is difficult to change. For digital systems, all this is just changing the software. For digital welding machines, flexibility means that the same set of hardware circuits can achieve different welding process control. Different control strategies and control parameters can be selected for different welding methods and different wire materials and diameters, so that the welding machine can be realized. At the same time of multi-functional integration, the process effect of each welding process will be greatly improved. Taking the waveform control of the CO2 welding short-circuit transition as an example, the current waveform during the short circuit determines the size of the weld spatter and the weld bead shape, and the optimal current waveform will be different for different wire diameters and welding process intervals. the difference. When using analog control, we often select the current waveform according to the principle of taking into account the entire process interval, which will inevitably lead to the unsatisfactory process effect of some sections. The flexibility of digital control is also reflected in the online upgrade of the control software for digital welders. At present, the more advanced digital welding machine in technology has gone from E2PROM to Flash in the selection of memory, and the online Flash programming function has also been added in the circuit design. Therefore, for the control program upgrade or online debugging modification of the digital welding machine, the insertion, extraction, ultraviolet cleaning, and programming writing of the E2PROM are no longer required, but the flash programming is simply performed through the universal RS232 serial communication interface.

The second advantage of the digital welder is its greater stability. In an analog system, the processing of signals is performed by an active or passive electrical network, and the setting of processing parameters is accomplished by selection of resistors and capacitor parameters. In this way, the tolerance and drift of the resistance parameter in the analog system will inevitably lead to the change of the controller parameters. On the one hand, the temperature stability of the analog control is poor, and on the other hand, the product consistency during the simulation control is difficult to guarantee. In digital control, the signal processing or control algorithm is implemented by software addition/subtraction, multiplication/division, so the stability is good and the consistency of the product is well guaranteed.

The digital welder has higher control accuracy. The accuracy of the analog control is generally determined by the error caused by the component parameter values ​​and the error caused by the non-ideal characteristic parameters of the operational amplifier (such as Ad, ACM, UOS, IOS, noise, etc.). Take the relative error of the amplification factor caused by the network of the inverting amplifier as an example, the amplification factor Relative error ,Here Is the relative error of resistors R1 and R2. If the relative error of R1 and R2 is ±5%, the relative error of the magnification is ±10%. The error of the actual analog amplifier circuit is much lower than this value. As long as R1 and R2 have the same relative error, that is, the ratio of R2 and R1 is guaranteed to be accurate, accurate magnification can be obtained. Even if R1 and R2 are not equal, some errors can be compensated for when the errors are both positive or negative. However, the analog control has a higher overall error due to the error accumulation of multi-stage processing and the step-by-step amplification of noise. The accuracy of digital control is only related to the quantization error of analog-to-digital conversion and the finite word length of the system. If a 10-bit analog-to-digital conversion is performed on a signal with 0~10V variation, the quantization error in analog-to-digital conversion is Therefore, digital control often achieves high precision.

The interface of the digital welder is well compatible. Since digital welding machines use a large number of digital chips such as single-chip microcomputers and DSPs, PCs and digital welding machines, digital welding machines and robots, and digital welding machines have power supply and wire feeders, power and water cooling devices, and power supplies and welding torches. The communication interface can be implemented very conveniently. It is believed that with the development and popularization of modern welding production network management, digital welding machines will inevitably play an increasingly important role with their good interface compatibility.

Digital welders have an undisputed advantage over traditional analog control welders. However, the slow processing speed and poor anti-interference ability are the main shortcomings of digital control. Therefore, in the implementation of digital welding machine, it is necessary to meet the minimum requirements of the arc welding process by reasonable control chip selection and overall design, and to take effective measures. The anti-interference measures enable the digital control circuit to adapt to the arc welding process environment of high radiation and strong electromagnetic interference.

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