With the start of a new decade, some wonder whether arc welding has really changed much. One way to answer that question is to look to the equipment manufacturers, which have increased the speed of processors in power sources, incorporated feedback to adjust waveforms in microseconds and added capability with servo-controlled wire feeding into torches.
Moreover, gas metal arc welding (GMAW) can now weld materials that are less than 1 mm thick and boasts cosmetics that rival the “layered penny” beads of gas tungsten arc welding (GTAW). The capability for power sources to connect to external devices has also advanced, prompting robot manufacturers to refine robotic welding tools. From a robotic controls standpoint, this has inspired Yaskawa to develop a new Universal Weld Interface (UWI).
Universal need
Welding is performed around the world, but there are regional differences in how it is approached. For example, in Asia, a robot is integrated with weld settings being part of the robot program. In Europe, operators store weld settings in power source files and reference these files/jobs when they start the arc. And similar to how it is managed in Asia, North America tends to use the robot control to store weld settings in the robot program.
As a global company with a worldwide customer base, Yaskawa recognized these differences and that the advanced sophistication in power sources required a different approach. This, combined with the increase of information available from power sources as well as customer preference to access this information strengthened Yaskawa’s commitment to providing a common interface.
While they have differences, modern power sources have some common aspects, including an internal database of processes for wire types, sizes and gas mixtures. They have weld settings to change amperage and voltage, but these may affect waveform settings to achieve it. A power source will either accept weld settings sent from the robot or store the weld settings in files inside the power source. Keep in mind that a modern-day interface needs to be flexible to utilize a variety of weld settings with different names and units.
Process selection
Manufacturers define processes and have many control parameters they set in a laboratory. This is often done with instrumentation and a high-speed camera to observe arc phenomenon and set control waveforms to ideal conditions. These are for a specific wire type, size and gas mixture, and they usually reside in the power source as a library that users reference to set up their plant conditions.
The UWI has a process selection table where the specific processes for a plant can be registered from the power source. This is important because the processes may have different control variables, even from the same manufacturer. The selection table provides filters, so a specific wire type and size can be designated, and it allows the user to see all the processes available for selection in the power source. A job shop can set up different material combinations in the process table to be referenced by arc instructions.
The interface should also be flexible enough to accommodate new process variations as they become available. This might occur by updating power source software with a new library or adding a custom waveform developed by the manufacturer.
Process variables
Whether a weld is being made manually or with a robot, a Weld Procedure Specification (WPS) should be followed. AWS/ANSI B2.1 Specification for Welding Procedure and Performance Qualification outlines the essential electrical characteristic variables as Heat Input (J/in.) = (Volts x Amps x 60)/ Travel Speed (in./min). This is based on the output of the power source, and few power source manufacturers use variables directly related to the output. Key process parameters may include:
- Wire feed speed – This is directly proportional to weld current output. Weld process waveforms will vary peak and average currents so that a given wire feed speed produces different amperages with different processes. This can generally be set in in./min. or m/min. Wire feed speed can be closely regulated and controlled to ±1 to 2 ipm with tachometer feedback. Amperage can vary based on process type or contact tip to work distance.
- Arc voltage/arc length – The introduction of pulse welding made arc length a characteristic independent of arc voltage. Most manufacturers use a “trim” value for arc length that will make synergic adjustments as wire feed speed is changed. Manufacturers use different nominal values and ranges for arc length trim. The variable for arc length may have different names for the process; voltage if constant voltage, arc adjust, trim, arc length correction, etc. The UWI allows the user to program with the specific name for the weld setting along with the unit and allowable range for the process selected.
- Other weld settings – With digital controls, many power source manufacturers exposed more variables for advanced processes. These may vary the inductance, which was used on older constant voltage machines to reduce spatter or adjust pulse parameters to vary the width of the arc cone. Added settings allow advanced users to optimize specific arc characteristics without changing detailed waveform data. These settings have default values; many users do not need to alter them. While they typically have a minor effect on the output of the machine, they can impact the spatter level or bead cosmetics. The UWI was designed to expose up to six variables for weld settings.
- Weld files/JOB mode – A power source may have the ability to store weld parameters internally where the robot references the desired file number when turning on the arc by turning on a group of outputs to designate a file number (often in a separate instruction). The UWI has been designed to integrate this functionality into the robot ARCON command. The file nomenclature used by the power source manufacturer is included into the weld instruction TAGs to display like any other weld settings. This allows power sources with weld files to use the same instructions and weld functions as power sources with remote control.
- Travel speed – This essential variable is controlled by the robot and can vary as a global parameter in a file or a local setting in the motion program itself. The formula for heat input shows that travel speed has an inverse relationship, which means a small change in travel speed can have a large effect on heat input.
Robotic weld settings
Even if a plant doesn’t have a written WPS, robots are programmed to apply the same parameters consistently. The robot program determines torch angles and travel speed, which are part of WPS essential variables. Robot controls generally have the ability to store weld settings as global weld files or in weld instructions local to the specific program.
Arc files are set up for specific joint conditions with amperage and arc length settings to “burn” the wire with minimal spatter for a range of travel speeds. The travel speed can be changed to determine the weld size. Arc files often allow for more detailed settings such as parameters to start the arc and build a weld puddle before traveling for larger welds. Arc end files may have secondary settings to fill the crater and burn back the wire. As global weld settings, arc files can be reused in different weld locations and programs where the WPS would apply. Changes to a global arc file may affect multiple weld locations and programs that have been saved and reloaded.
Weld instructions with local settings in the program allow a programmer to see the specific details for that weld location. This may lead to inconsistent settings applied across the part as a technician may tweak them. It may also be advantageous for a problem location that varies in fit-up and requires frequent adjustments without altering more universal weld file settings. Some companies may prefer the inline setting of instructions because it provides documentation right in the robot program and is not affected when programs are saved and reloaded.
The UWI was designed to display process-specific weld settings whether arc files or inline instructions are used. The weld settings are displayed with tags that reference manufacturer nomenclature for that setting. Graphic help screens display the allowable range, and the software prevents invalid settings. This helps users when they communicate with the power source manufacturer regarding their application as referenced settings can be entered into the robot as specified.
Document for quality
Companies that produce safety critical welds maintain WPS documentation and require welds be requalified if parameters are changed beyond WPS limits. The robot control has several features that can aid the administration of a WPS:
- Security – Weld settings and program editing can be restricted and logged by activating a security level to limit the personnel that can make changes.
- Weld data displays – The robot control likely has a method to display data necessary to document a WPS, including torch angles for the weld location, weld lengths, travel speed, and amperage and voltage.
- Remote displays – The robot control often acts as a gateway to display the weld data on other devices. Programmable logic controllers (PLCs) are used in plants for cell control with large displays for operators to see information related to the cell. Robot data, such as travel speed, is also available to display/monitor.
- Remote monitoring – As part of the Industry 4.0 revolution, software is available that can collect weld data and chart it along with trends and issue alerts. This data can be effective to monitor overall equipment effectiveness, but is not collected fast enough to perform statistical arc monitoring.
- Arc monitoring – Welding manufacturers are integrating arc feedback and adjusting welder output tens of thousands of times per second. The UWI includes instructions to support the arc monitoring capability supplied by the manufacturer. The robot reports results to a PLC HMI for operators to judge which welds to inspect when the part is completed. Some auto manufacturers have used more stringent requirements to contain parts that do not pass arc monitoring. Power source-based arc monitoring is best for judging weld quality and should include a plan to deal with production that is flagged as outside of limits. Arc monitoring cannot detect good versus bad welds, but it can identify welds out of process tolerance. Follow-up inspection is required to make a final determination of the quality of these “suspect” welds.
Digital communication
As power sources move to digital communication networks, the wiring is simplified, but troubleshooting tools can still be helpful. A UWI utility allows users to search the network for a power source and assign the IP address of the located device to the robot. This is particularly useful when the workcell has multiple robots with multiple welders and different IP addresses.
Of course, the digital connection is monitored continuously to verify the health of the interface. If the power source doesn’t behave as expected, the UWI includes a logging function that records the digital traffic between the robot and the welder. It provides a downloadable file that users can email to Yaskawa support services to aid diagnosis.
This digital interface provides flexibility for different network platforms. Ethernet is common in North America while ProfiNet is popular in Europe. The UWI has a flexible interface module that can be edited by Yaskawa divisions around the world to adapt to the network or add new welders used in a specific region.
As the current industrial revolution continues to advance, the control capability between the robot and power source will also, including:
- The capability for a power source to switch modes while welding to create an alternating waveform (this capability is available in the UWI as a dual-pulse function for ripple bead cosmetics).
- Improve productivity with faster welding (the Dynamic Starting feature eliminates hesitation at the start of the weld or new processes in power source libraries).
- More data from their robotic welding workcell(s). As always, a strategic plan for using data should be in place, ensuring the right information and frequency of samples matches operational goals.
The development of a robotic weld interface requires close collaboration with the power source manufacturer. The goal is to allow the user to take full advantage of the capabilities they have designed in for a wide variety of applications and industries.
