Systems of control and monitoring for welding

And if a computer, such as a welder, could capture an image of where is the electric arc and infer what are the appropriate welding characteristics?

And if an engineer could detect a problem with a cell of welding in a city through your computer to another?

These questions exemplify areas where work is underway in the development of the welding technology. A system of welding Automation includes a set of components that have the function capture the characteristics of the process and compare them to average values, default, by adjusting these characteristics in real-time when necessary.

Once certain parameters of the welding is necessary to ensure that they remain inside of acceptable limits throughout the process. To do this, the use of sensors that read the parameters referrals, depending on the type of sensor, the type of parameter to measure, control and adjustment of the parameters by controllers incorporated into the automated system is used.

How to operate, the sensors can be classified into sensors that come into contact with the piece, either mechanical or electrical, and sensors without contact, for example, the optical and inductive.

For the most part of welding processes, parameters monitored and compared with reference values are electrical quantities, such as intensity of current and voltage, temperature, speed of welding, distance between the torch and the piece, acoustic and light emissionthe force applied to the electrodes and other geometric parameters that characterize the welding process. Other phenomena that occur during the welding can be monitored to better control the quality of welding, such as distortions of material caused by the thermal cycles and the stability of the keyhole in the processes of high energy density. The adjustment of the monitored parameters is carried out by the control systems incorporated in the installation. These systems may be based on mathematical algorithms relating parameters of the process with the characteristics of the obtained strings, fuzzy logic or neural networks.

The main difficulty in the control and monitoring in welding is to ensure a response in real time of the equipment to a certain deviation in relation to the optimal parameters of welding.

In the following paragraphs, after a description of the main types of sensors to monitor the processes of welding of industrial awareness, is in a systematized way existing information concerning monitoring and control systems applied to welding technology.

• sensors for monitoring of Board
• sensors for control of the arc length

• sensors to control the welding penetration

Sensors for monitoring of Board

In this technique, it is essential that the system captures and processes the information received concerning the Board values to be welded and to get adapt to variations throughout the process. Various sensors, which include the touch sensors, Ultrasonic sensors, sensors for monitoring journey, inductive sensors, vision systems of structured light and laser guidance systems they may be used for the control of this technique.

The touch sensors guide the electrode through contact with the pieces to be welded. The simplest are the docks, guided by wheel sensors. The Ultrasonic sensors determine the distance between the workpiece and the torch, by measuring the time it takes to broadcast and receive an ultrasonic wave after reflected in the piece, known velocity of ultrasonora frequency in the propagation medium. A provision of three Ultrasonic sensors allow by locating a point in space, this being its principle of operation in the control and monitoring in welding. The role of detect variations of current and voltage in arc sensors through arc. In the Monitoring Board, as the torch progresses along the edge, this can suffer detours in relation to the edge. These diversions are associated with a variation of tension of a small electric arc between the sensor and the edge, being the position of the torch corrected according to the information received.

Inductive sensors evaluate the drop in an electromagnetic field caused by instantaneous variation of welding current. These sensors have the advantage of not being influenced by corrosion, ink, or humidity, and can be used for any thickness of material. As a limitation of these sensors, it highlights the fact that they must be stored at low temperatures and can not get dirty with splashes [1].

[2] Structured light vision systems use a CCD camera (initial in English Charged Coupled Device, or device of load transfer.) (It is a semiconductor that transforms the signals of light, e.g. images into electrical signals) to capture an image of this light, as you can see in Figure 1.

A band-pass filter has been used to avoid interference with the light emitted by the electric arc. These systems were expensive and had a time slow response, but the integration of multiple, best and most compact sensors, lowered costs. For thicker parts where it is necessary to use welding in several passes, the system of follow-up to the Board is responsible for correct positioning of the torch in the first pass, and the following, ensures that the cord of welding is placed in a correct position in relation to the previous strings.

Guide laser [3] systems were used for the first time by the defence industry and formed part of the military equipment that is known today as "smart weapons". The basic principle of operation of these systems is the catchment for a camera of a beam of inert laser, reflected by the white. The sensor captures the reflected light in white and guide the missile towards it with precision. In Figure 2 you can see some of these sensors at the end of the missile.

Sensors for control of the arc length

It is advisable during the welding process that the length of the arc will not suffer from variations [4, 5]. In the simplest process is operation is carried out through the control of the tension, in welding of self-regulatory for welding machines, since the tension of the arch clearly indicates its length. Flat welding machines, control of the length of the arc should be made by the current. Another way to control the length of the arc is through sound sensors [6, 7], as sound pressure increases as it increases the length of the arc, in the range of wavelengths characteristic of electrical arcs in welding.

Photoelectric sensors consist of an infrared transmitter and a receiver. These sensors have the advantage of having a low cost, be robust and not to be affected by the instability of the arch.

Light and spectral radiation [8, 9] sensors are based on the fact that the intensity of the total light of the arc increases in a linear fashion in relation to the length of this. These sensors have a high cost.

Sensors for control of the penetration of the weld

The cord of welding penetration is one of the most important parameters to control because it is an indicator of the quality of the cord of welding and influences in decisive way in the mechanical behaviour of this. For the penetration of the weld at the root, the welding parameters are critical because it is necessary to ensure the total penetration, completely filling the opening of the root and the separation of the Board. A control system that uses infrared radiation can be used to do it and which is presented in Figure 3.

The infrared sensors were used in the 1950s in the defence industry with the emergence of the Sidewinder missiles, employing an infrared sensor to capture the thermal radiation from the target. These early sensors had little scope and could only be used in a straight line. Currently, these sensors incorporate the most varied teams and are used in the control of the penetration of the weld as illustrated in Figure 3.

Another problem lies in the fact that these systems have a low bandwidth, which means that the image takes some time to come and go and this leads to a shadow of this image appear some time in the Viewer after the image has disappeared. Recent cameras such developments lead to extremely resistant to dimmable appliances which can give similar results to the infrared cameras [12].

Another recent development refers to the possibilities of use of CMOS (CoMplementary Oxide Semi-conductor) cameras, that they have a dynamic range and a speed exceeding cameras CCD image acquisition. They also allow a better visualization of the bath of fusion, which improves the evaluation capacity of the overall quality of the cord during its execution.

Radiographic control has been used to verify the quality of the cord of welding. This technique is placed a source of X rays on top of the piece, while the camera and the image intensifier are placed below the piece. This system can detect a large number of defects of welding, such as an incomplete fusion and porosities, and offers an immediate of the interior of the welding. Systems controlled by computer to analyze the image and offer appropriate corrections are used. Such systems represent a large cost and are complex, so application is very restricted for matters related to the safety of the operation.

It is true that all objects reflect light that strikes them in greater or lesser extent. The intensity of the light indicates the color of the object, being this light focused on CCD sensors (for example) through a system of lenses. These sensors convert visible light into electrical signals. The CCD sensors can be of various sizes, depending on the resolution for each application. Smaller is the size of these sensors, and largest number, higher is the resolution.

The type of camera to use in a given application depends on the type of sensor that is fitted to the conversion of image and of the necessary resolution in each case. The source of lighting is extremely important because a good image is not obtained without sufficient light. The source of illumination is conditional, not only by the object to inspect but by its size, color, shape, reflectance and texture, among other things.

The computer, equipped with a software and a proper database, provides the key for the control of the welding monitoring, insofar as it carries built-in image capture boards and I/O boards.

Image capture boards together the camera to a computer to receive the video signal from the camera that is going to be digitized and converted into an array of individual values (one per pixel). These values represent the intensity of light at each point, or pixel of an image. In the case of a color camera, there are three signals, one for each primary color, still image capture plate responsible for digitizing the three signs at the same time, what are three values for each pixel. Converted into matrix (after being digitized) values will be compared with the pattern of values in the database, and depending on the conditions imposed by the programming, a signal is introduced in the I/O plates that communicate with the PLC, so that, together with the controller of the robot can be controlled efficiently and in real time.

Other optical image acquisition systems have a basic principle of operation similar to the one described, just introducing some changes in the characteristics and types of components.

The Board [14] follow-up already described above, is of extreme importance in fully automated systems, then by very well machined to be the pieces to be welded, always there will be some variations in their positioning. To start from the beginning of the process of welding phenomena of expansion and thermal contraction of the pieces due to the applied thermal cycle can cause deformations as well as movements of the pieces to the torch, being these most important both problems the more fine are the pieces to be welded.

Control of welding penetration is one of the most important and most difficult of control parameters. To this effect have been developed, among others, some systems using techniques of ultrasonic [15].

With regard to the transfer of metal, has been observed that this can be controlled, in the MIG/MAG process, through the analysis of light emissions during the detachment of drops of supplies threads [16].

The heat transmitted to the piece can be controlled through the variation of the speed of progress of the torch, so this heat to be scrapped and that allow you to control the thermal state of the bath of fusion and the surrounding material. An image of the thermal profile is obtained through an infrared sensor.

In regards to the control of quality [17], in critical applications are used normally radiographic control techniques by ultrasound to determine if the welding has no internal flaws. For less demanding applications, it is enough surface control by penetrating liquids or a visual inspection of the cord of welding profile.

The optical system presented in Figure 5 can be used to measure various parameters, such as the height of the cord, his reinforcement and the alignment of the pieces. Using this optical system with an appropriate image processing it is possible to get an answer in real time.

Hardware systems

Examples of interest in systems of hardware [18,19] dedicated to the collection of information for welding include the Weldcheck QA TWI, TWI AMV Weldcheck, Data Harvest Monarc and OIS PAMS V.

Typically, these systems comprise a single hardware unit together with a team of welding that establishes the required welding parameters (current, voltage, speed food, etc.) and presents the information collected.

The main advantage for the use of these systems of hardware is the Elimination of the development time and costs, as it is a system that, once purchased, are ready to use. These systems are more robust for industrial environments. They can protect themselves with ease in the high frequencies, allowing its use in welding processes that have a procedure of priming by high frequency.

Its main disadvantage is its lack of flexibility, as they were designed specifically to record average values of the parameters of the process and not signals in waveform. In general, each system is projected for a specific application, which is why not adapt easily to alternative processes, such as for example, a sampling of large numbers of parameters simultaneously, or incorporate an extensive information processing or a feedback control. Once chosen a system specific it is difficult to modify or improve and if possible, would have a great cost. As additional disadvantages, is the initial cost and the costs of maintenance of the system, since both will be higher than in a software system.

Two presented systems are commercially available in the market (QA Welcheck and the Weldcheck AMV). The first is portable, compact and robust equipment for the monitoring of the current and the voltage. Welding information is presented in a digital display. This unit provides protection against the frequencies and high tensions and is compatible with the Weld Validator software, used for the control of the welding parameters.

The Weldcheck AMV is a portable, compact model and a robust machine for the monitoring of current, voltage, speed of feeding, transverse velocity, temperature and electric arc welding gas flow. Welding information is presented in a liquid crystal display. This unit is autocalibra and provides protection of the tensions and high frequencies, and is also compatible with the Weld Validator software.

Software systems

Software [20] systems integrate commercially available hardware systems in which the capabilities of the software system depend on the capabilities of the system chosen hardware, so they depend on the ability of the software to take advantage of the full potential of the hardware.

The advantage of a software system is its flexibility, as to the parameters in the form of wave to reach a rate sufficiently high can be recorded and analyzed small fluctuations. Assuming that the chosen hardware is fully programmable by software, the software can be as expensive as necessary. It can be adjusted to start the hardware, configure input parameters and control the process of recruitment. Another advantage is the ability use the potential of the computer (processor, disk...) to extensively process information and use the high resolution of the monitor to display the information graphically or in text.

The flexibility of the system depends on the ease with which it can be maintained and improved. The majority of the catchment boards are compatible with many software applications, allowing that same plate may be used in various software applications. Similarly, most of the available software is projected so that the same application can be used with the widest available range of plates of hardware according to specific implementation criteria.

The main drawback of these systems is time and labor required to develop it. A software application itself, it would need to be physically maintained and perfected with new operating systems than they appear. Depending on the complexity of application and use the production of a manual on employment may be necessary (to minimize maintenance time).

Rather than developing the software manually, a fourth generation, such as the LabVIEW programming language can be used [21] or HP VEE, in order to simplify the design and development procedures. Using Visual objects and flowcharts, the programmer can develop programs without losing time to write low-level code. Figure 8 shows a system using LabVIEW.

Another disadvantage of software systems is that they are not as robust as the systems of hardware, being therefore less suitable for severe industrial environments.

Control systems based on fuzzy logic have been used successfully in a large number of areas. These systems are based on rules that incorporate variables of fuzzy linguistics to guide the decision-making process. They can be operated with heuristic rules, which often occur in complex systems of welding. Recently, such systems have been adapted successfully to welding. The signal is obtained with a camera that functions as a sensor and a digital image is processed. Fuzzy logic learning process and adapting the parameters to different conditions for welding, by sensing of weld bath.

[24] Genetic algorithms are search algorithms based on the mechanism of natural genetics, which has the capacity to quickly find approximately optimal solutions to complex systems. These algorithms have efficiency associated with fuzzy logic systems. [25] Neural networks are used for digital industrial process modeling [26]. These networks were recently studied for welding applications and proved that they were able to distinguish the metal transfer modes and control of processes of welding with a satisfactory reliability. Work that uses neural networks for the control of the length of the arc, for follow-up of the Board and for the control of the penetration of the weld have already been submitted. Neural networks are an important tool of computer simulation and control of industrial systems.

With modern electronic circuits and existing sophisticated equipment, it is possible to control operations in real time with the required quality.

Control of welding penetration is one of the aspects required more frequently by which continues the development of techniques for this purpose.

The catchment of the thermal image sensors and laser sensors are extremely efficient, but represent a high cost. The optical sensors are the most versatile and intelligent but are also the most expensive. Other systems, used for the control of the characteristics of the arch, are efficient in many applications and are available at a low price.

In the market there are systems of hardware and software very varied and suitable for monitoring and control of welding applications. The present compilation of data allows to conclude that currently on the market there is a wide range of products for the control and monitoring of the welding.

The user seeking a solution to a specific case should be understood that an effective system for the control and monitoring of a given application of welding technology requires integration, for example, for a certain type of sensor with a welding equipment and a system of positioning and monitoring between the torch and the piece. The knowledge needed for the selection and integration of the various components of a system of this kind are not always accessible. Companies often opt to ask for advice to the supplier of the equipment, which in many cases leads to the implementation of the most appropriate solutions. It should not forget that in cases of greater complexity or which require high investments can make recourse to the Council of institutions with proven experience in this field.

One of the needs of the users of these systems is the development of standard control processes of the different monitoring equipment available in the market so that the integration of diverse origin teams as possible and taking into account price/efficiency reasons. It is expected that the use of fuzzy logic, neural networks and genetic algorithms allow an evolution in this sense.

The Internet is a useful tool in the process of obtaining technical information detailed and updated by the manufacturers themselves or distributors of sensors and equipment for the monitoring of the institutions that possess the necessary knowledge in this field. A search can be started with the following keywords:

sensor + control + monitoring + welding robots + simulation software.

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