Historically, if you wanted to automate welding of a part, you needed two key factors: no gaps at the joint and consistency to ensure repeatability. However, in some situations it may be impossible to have one or both of those ingredients.
Paired with the proper software, adaptive welding sensors can drastically improve part quality and consistency while reducing downtime caused by adjustments to fixturing and/or robot programming. Sensors used specifically for robotic welding applications typically fall into four categories: touch, through-arc, laser and vision. Likewise, they have three primary functions: seam finding, seam tracking, and/or part scanning, which often can also be used for inspection. Each function features unique benefits depending on the part and expected outcome, and most technologies can be mixed and matched, where use is not redundant.
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Seam Finding vs. Seam Tracking
“What is the difference between seam finding and seam tracking?” and “How do I know when to use seam finding vs. seam tracking?” These are common questions our robotic welding experts are frequently asked. With that in mind, here are several things to consider when deciding how to proceed with your robotic welding process:
Seam Finding
For a robot to precisely locate a weld joint before welding begins, high-speed seam finding or joint finding is recommended. Work pieces will inevitably have some range of variation, but your goal is to minimize that variation with spec’d parts and fixturing, and be within the half-width of a weld wire into your joint seam. This process can be done in several ways via various technologies, enabling the robot to find the weld joint.
Once the seam is discovered by finding usually two or more known points on the part, the program path is shifted by the robot to complete the weld. The type of seam finding required is dictated by two primary factors: the expected cycle time and the type of joint.
Seam finding is one of the most popular welding functions, and is often achieved through the following tactile options:
Touch Sensing – Ideal for finding the orientation of parts with simple joints and geometries, this method, also known as “wire touch”, involves the physical touch of a weld wire from the end of the torch to detect the conductive surface of the part about to be welded. The slow speed of the robot and the eventual touch complete a circuit with a low amount of voltage fed through the wire. This can also be done with the nozzle of the torch in some scenarios. Completed through built-in features on a welding power supply designed for automation, systems like Yaskawa’s Touch Sense package use a low voltage circuit during a low-speed search to determine the best position for the weld joint.
Pros:
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Low complexity; Built-in pendant commands
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Works on all conductive material
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Easy to teach with macro jobs
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Does not interfere with joint access
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No external hardware is required on robot
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Performs multiple searches with one wire cut
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Locates most lap and fillet joint types; can also be used with V butt joints
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Offers a lower cost option
Cons:
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Requires a wire cutter/wire brake (optional)
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Limited to lap joint thickness (>3 mm)
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Slower vs. laser or camera
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Limited ability to detect joint gap
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Cannot find square butt joints
Wire Sensing – Similar to touch sense, where a wire from the torch makes tactile contact with the part, this option uses a servo motor in the torch to rapidly move the wire up and down while the robot moves across the part. This enables easy location of lap joints, and it can measure items like material height and gaps. Offered through Fronius, the Fronius Wire Sense software option provides great efficiency.
Pros:
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Can detect joints like butt joints that cannot be easily found using traditional static wire or nozzle touch sense
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Can be used for lap joints less than 3 mm
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Ability to measure part height offsets and gap width and depths
Cons:
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Requires specific hardware and software license from Fronius
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Slower vs. laser or camera
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Not available on all brands of welding power supplies
Laser Point Sensing – Two to five times faster than touch sensing, the use of a basic, laser dot sensor (that is mounted to the weld torch) captures the location and orientation of a part nearly as quickly as the laser fires, providing fast and accurate seam finding. Capable of working with any welding power supply, Yaskawa’s AccuFast™ non-contact laser sensing solution provides a cost-effective option between tactile and vision sensing solutions.
Pros:
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Low- to medium-complexity; Some training required with built-in commands
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Works for most materials
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Easy to teach with macro jobs
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Uses a non-contact sensor
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Faster search speeds and touch sensing
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Eliminates the need for a wire cutter
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Finds most joint types, detecting lap joints down to 1/16” thick
Cons:
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Sensor box is mounted adjacent to the torch
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Mounting bracket per torch type
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Limit in lap joint thickness (>1.5 mm)
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Limited ability to detect joint gap
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Cannot find square butt joints
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Highly reflective material requires evaluation
Laser Seam Finding – Capable of picking up more characteristics in a single scan over a laser dot sensor, the utilization of a profile laser interface, such as Yaskawa’s MotoEye™ SF, provides extremely fast joint measurement. This solution works well with a sensing device that uses 3D multi-laser range imaging optics to provide the needed measurements/joint gap data to the robot before welding begins. Options from SERVO-ROBOT’s i-CUBE™, ABICOR BINZEL/Scansonic and Wenglor work with Yaskawa’s MotoEye SF pendant interface.
Pros:
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Works on different materials in all lighting
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Easy to teach with macro jobs
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Provides joint gap data
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Long focal length; mount away from arc
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Locates 2.5D; offset and depth
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Compact and self-contained
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I/O interface can be retrofit to older controls
Cons:
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Medium- to high-complexity; Training on vision system suggested
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May restrict access into part/tooling
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40 mm FOV may require multiple searches for large offsets
Seam Tracking
Often simplifying programming, this option uses innovative technology to equip the robot to track the weld position in real time, during the welding process. Seam tracking is popular for applications where distortion can occur while welding a part or for heavy cast parts, and it is commonly performed using the following methods:
Through-the-arc Seam Tracking – Best for parts with long or curved seams, varying from part to part, a though-the-arc seam tracker, like Yaskawa’s ComArc LV (low voltage), utilizes a solid-state sensor mounted near the welding power supply to actively measure arc characteristics during the weld sequence. This determines variations between a robot’s taught path and the actual seam path.
Pros:
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Low complexity
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Reliable sensor and easy to support
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Passover function restricts sensor error
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Phase Compensation calibrates weld cirucit
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Can track lap joints 1/8 in. or 3 mm thick
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Supports dual robots and coordinated motion
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Offers a lower cost option
Cons:
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Requires weaving and thicker material
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Limited by arc/weld physics
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Requires a pre-weld search to find the weld joint
Laser Seam Tracking – Suggested for thin material with varying seams that demand the fastest cycle time possible, this method combines a high-performance laser with a high-speed controller to find the seam and part location in real time while the part is being welded. A dedicated program compensates the path, as well as adapts to welding parameters for seam location and variation. Yaskawa’s MotoEye LT or SERVO-ROBOT’s DIGI-I/Power-cam products work well for this.
Pros:
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Reliably tracks thin gauge lap joint
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Supports high travel speeds (>100 IPM)
Contact us to discuss your requirements of Laser Tracking Seam Control. Our experienced sales team can help you identify the options that best suit your needs.
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Weaving Motion with tracking possible
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Tracking is not affected by weld settings
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Supports coordinated motion
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Ethernet interface available
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Camera hardened against welding arc
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Adaptive welding function; speed and weld settings
Cons:
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High-complexity and often high cost; Training on vision system required
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Torch-mounted sensor restricts joint access
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Tracking radii is limited to 40-60 mm
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Limited to two robots on one system
Dual Laser Seam Tracking – To optimize cycle time, sometimes, two robots are equipped with seam tracking technology to work in unison. This utilizes the same interface and technology mentioned before, but can cut cycle time in half and reach more weld joints on larger or complex parts than a single robot.
What Sensor Makes Sense for Your Application?
Every automated welding process has its own unique challenges. Components are not absolutely identical, positioning during clamping is not always the same and heat from the process can cause material distortion. With manual welding, an experienced welder can compensate this directly without any problems. A welding robot or automatic welding machine needs the "eyes" of a human in a different way in order to recognize the individuality of each component and create the perfect weld seam.
In this e-book, we present different approaches to automated welding in order to produce a perfect weld. You will learn about "seeing" and "thinking" methods, understand their principles and deepen your knowledge of optical seam tracking. You get a direct comparison of optical seam tracking with one line and with three lines. You will learn what makes a good seam tracking sensor and when a corresponding investment is worthwhile for you. A sample calculation illustrates the cost-benefit factor.
Using various practical applications, you will learn about the wide range of possible applications, which in some cases allow completely unexpected additional optimizations.
So if you are thinking about the topic of optical seam tracking, this e-book is just the right introduction for you!
Optical seam tracking is used to recognize the joint to be welded and to follow its actual course. The sensors used for this have adaptive control. This means that they detect geometric deviations such as edge offset, gap width, seam width and seam volume and pass this information on to the system, which in turn directly adjusts the welding parameters. The seam properties are recorded using laser triangulation (light section method), transmitted to the guide machine and adjustments are made almost in real time.
If you want to use an optical system, it is important that the joint has clear geometric features. These features must be aligned within the view of the sensor. Any feature, such as a butt joint must have a gap or mismatch (hilo) greater than the resolution of the sensor. A square edge butt joint with a very small feature is a challenge for an optical seam tracking system, but can often be worked with if the joint has a small break or radius on one edge.
Contamination or damage to the workpiece surface may lead to measurement inaccuracies. The same happens with highly reflective surfaces that resemble a mirror. More on this coming up.
How laser triangulation works
Laser triangulation is a process for detecting, measuring and analyzing the surface of a workpiece. A laser projector in the sensor head projects a laser line, usually red, onto the workpiece. The light of the laser line is reflected by the surface of the component. The CMOS camera – Complementary Metal Oxide Semiconductor – in the sensor head records the reflected light. This camera is arranged to view the laser line from a special angle (triangulation angle) and records its shape and position. Depending on the distance between the sensor head and the workpiece surface, the reflection arrives at a different position in the camera image. The position and shape of the laser line are measured and analyzed in the camera image using algorithms in the sensor. This information is transmitted to the robot controller or welding machine and used to control it.
If necessary, the welding path of the robot torch is corrected. The great advantage of this is that the measurement is continuous. This makes laser triangulation very suitable for use for tracking all the way along the seam.
Laser triangulation can be used on all materials such as structural steel, stainless steel, aluminum, titanium, copper and brass, and even on non-metallic materials such as plastic and the like. However, it depends on their reflection behavior. Problems only arise with very highly polished surfaces that resemble a mirror. The reflections then scatter so much that the camera is no longer able to clearly distinguish what it is seeing.
The sensor must be setup correctly, starting with a good mounting position so it can see the seam clearly and then telling the sensor where it needs to track to produce the perfect weld. We will go into a little more detail below.
Mounting the sensor
The first step is to mount the sensor in front of the welding torch. The position of the sensor must be chosen so that:
- the sensor can see the seam clearly
- the sensor is measuring just in front of the welding torch, so it can measure the actual position of the seam just before it is welded
Setting the parameters
For each weld the sensor is to track, the sensor needs to be set up so that it knows what to measure and what the joint is expected to look like.
For the joint:
- Settings to optimize the sensors view of the surface
- Shape of joint (joint type, angles, curves, bends, etc.)
- Where the welding wire should be positioned
The robot controller
As for all robot welding systems, the robot controller needs to have an accurate Tool Center Point (TCP). When using an optical seam tracking sensor, the robot controller also needs to know the mounting position of the sensor in front of the welding torch. This is done by a special sensor calibration routine in the robot controller and a calibration plate. The robot moves the sensor over the calibration plate and uses the sensor measurements to calculate precisely where the sensor is mounted relative to the welding torch.
Transferring the data
The sensor and robot controller talk to each other using an Ethernet connection using the TCP/IP protocol. The robot controller tells the sensor when to make measurements and which joint is about to be welded. The sensor then sends to the robot controller the position of the seam under the sensor, as well as additional information on the geometry of the seam. The robot controller uses the measurements of the seam position from the sensor to calculate the correct path over the seam to produce a good quality weld. It can also use the geometric information to adapt the welding process, if required.
Challenge: look ahead distance and field of vision
Online seam tracking systems work with a look ahead (preview) distance between the sensor measurement and the welding torch. The sensor look ahead distance is necessary in order to:
a) give the robot reaction time for path correction and
b) to avoid looking directly into the weld pool and welding arc.
The challenge here is that the look ahead distance must not be too long so that the measurement can take place as close as possible to the TCP. There also has to be enough time to process the measured data. For certain welding processes, such as submerged arc welding and processes with additional wire feed (TIG with wire feed) the sensor is usually mounted further away from the welding point to be clear of the flux or wire feed.
However, it is important that the joint remains within the sensor's field of view. Depending on the bend of the joint, this can be more challenging. The programmed robot positions must ensure that the sensor has a good view of the seam at all times.
The main arguments for three lines are:
- It allows the sensor to make additional measurements, such as orientation of the part to the seam
→ Fact is: It is not possible to send sensor data to the robot using any of the standard robot interfaces, such as those from ABB, Fanuc, KUKA, Yaskawa, etc. The welding robots cannot use these additional information at all. This means that the robot is not able to use any additional information.
- The measurements are faster with 3 measurements per image than with one measurement per image
→ Fact is: Most robot interfaces can only transmit approx. 15–20 measurements per second to the robot controller. So even with 1 line the sensors are already faster than most robot controller can accept measurements.
- With 3 measurements per image there is a high degree of reliability with regard to the correctness of the measurements
→ That means: Having 3 measurements in each picture means that these 3 measurements can be checked against each other to ensure that they are selfconsistent
→ Fact is: Software with 1 line compares every measurement with previous measurements and makes sure that they are within acceptable limits. Very good sensors with 1 laser line even have the ability to specifically check for regions with tacks and block any incorrect measurement when detecting a tack.
Typical questions for comparison
Why do sensors with 3 lines exist?
These sensors were developed primarily for applications where speed is the main focus. Here, sensors with 3 or 5 lines can have advantages.
Did the developers of sensors with 1 line develop this type of sensor because it is easier?
No, a 1-line sensor is the better choice for general welding applications.
3 lines give more information about the seam orientation.
In principle, that’s correct. However, the interface between sensors and the standard industrial robots like ABB, Fanuc, KUKA, Yaskawa, etc. for seam tracking don’t let you make use of this information.
Measuring is faster with 3 lines.
With 3 lines you can potentially make 3 measurements with every camera picture. However, most robot controllers can only process 15–20 measurements per second. Two additional lines such as those of a 3-line sensor can therefore not be processed by the robot controller. Since both a 1-line sensor and a 3-line sensor work with more than 30 measurements per second, a faster measurement brings hardly any advantages.
With 3 lines and so 3 measurements in each image, you can check whether the measurements are consistent before they are transmitted to the robot controller. But: Good 1-line sensors have a software filter that checks whether the measurements are consistent with previous measurements and uses this to filter out any inconsistent measurements. Some sensors also have a specific software filter to help stop spurious measurements when going over tack welds.
Needing all 3 lines over the seam means that using the sensor to measure the position of smaller features is not reliable. You have to have all 3 lines on the part to get a measurement at all. If the scan runs over a tacking, there is no seam tracking at this point. Since all 3 lines are also needed to detect the start and end of the joint when measuring with a 3-line sensor, this does not work as expected in some control systems.
High tolerances
A Tier 1 supplier for the automotive industry welds commercial vehicle ladder frames, among other things. Due to the tolerances in both the position and fit up of the raw parts, it was necessary to use very wide weld seams with a lot of filler material to ensure a secure connection. This resulted in a high cycle time and high consumption of filler material (welding wire). A cost-saving solution was sought.
With the help of a seam tracking sensor, it is now possible to precisely locate and measure the fit up of the welding joint.
With precise positioning of the weld seams and measurement of the part fit up, the cycle time has been reduced by around 1/3 and production requires significantly less filler material. This saves on cycle time, welding wire use and also on the weight of the product, as well as savings in gas and energy use.
Complicated welding torch position
A leading manufacturer of solutions for water heating, water supply and water treatment equipment produces water tanks, among other things. As there are both lateral and vertical variations in the seam position, different tank diameters require different weld positions, to maintain the required torch alignment. These issues often led to leaks. The company manufactures 4 hot water tanks of this type with different diameters.
For the welding process, the welding torch is mounted to a set of y/z-axis slides which allows the sensor to keep the torch positioned correctly in the horizontal and vertical axes. These y/z slides are mounted to another set of x-axis slides that automatically adjusts the x-position (welding direction) for different tank diameters to position the weld at the correct position (12 degrees) relative to the top center of the tank.
Once a seam tracking sensor has been implemented, the system control unit now adapts to a different tank diameter. To do this, the operator selects the tank diameter to be welded on the user interface of the sensor system. The x-axis slide assembly then moves so that the y/z slide assembly remains correctly positioned at 12 degrees from top center for that diameter and the sensor system continuously tracks the actual seam position using the y/z axis.
The x-axis slide assembly is not controlled directly by the seam tracking sensor, but by pre-programmed movements in the sensor system. This is unique. This solution is now to be introduced in other branches of this company, so they can also benefit from less scrap, less leaks and lower warranty claims.
The company is the world’s best High-Resolution Laser Seam Tracking supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.
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