How Does Laser Vision Seam Tracking System Work?

In today’s highly competitive market, manufacturers must stay both lean and efficient. Taking advantage of the different seam tracking options available for your welding projects is one way to gain an edge.

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Here at Bancroft Engineering, we understand the importance of optimizing your welding process.

Tactile Seam Tracking

Tactile seam tracking systems track the seam using a sensor with a tip that physically touches along the length of the seam about 2-3” in front of the arc. When a change in location or alignment is detected, the system responds and adjusts to the correct position. It can be used in almost any welding process.  Tactile seam tracking is very versatile and works well on any material type. This option offers a great balance between cost and flexibility.

One disadvantage to tactile seam tracking is that the torch is adjusting in real time according to where the probe is touching not where the torch is welding.  If the probe is 3” in front of the arc and a quick drastic change to the seam is detected the arc may actually move outside of the seam. To solve this problem the tracker can be paired with another type of control system to delay the correction in relation to the travel speed of the weld head or part.

Through Arc Seam Tracking

 Through arc seam tracking is a type of 2-axis real-time tracking.  The tracking occurs as the weld is being deposited and adjusts by monitoring the voltage across the arc.  In order to shift along 2 axes’ with through arc tracking weaving is required.  As the tip to work distance changes so does the voltage across the arc.  This change in voltage is monitored and used to adjust the path of the torch, maintaining proper weld position in the joint.

Laser Vision Seam Tracking

Laser based vision tracking, also referred to as optical tracking, involves a laser beam device which hits the welding surface, reflects off and bounces back to the sensor. With this technology, the sensor then recognizes exactly where the seam is located. Laser vision seam tracking is highly reliable and great for high speed welding applications for both small and large parts.

Touch Sensing

Touch sensing, or wire touch sensing, is the physical touching of the weld wire to the welding surface to detect the joint. Most commonly used in robotic welding applications, this type of seam tracking uses low voltage through the welding wire or torch tip to search and detect the welding surface and seam.  The seam needs to be detected prior to welding so the cycle time can be double or more compared to that of other methods.

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.

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:

• Low complexity; Built-in pendant commands

• Works on all conductive material

• Easy to teach with macro jobs

• Does not interfere with joint access

• No external hardware is required on robot

• Performs multiple searches with one wire cut

• Locates most lap and fillet joint types; can also be used with V butt joints

• Offers a lower cost option

Cons:

• Requires a wire cutter/wire brake (optional)

• Limited to lap joint thickness (>3 mm)

• Slower vs. laser or camera

• Limited ability to detect joint gap

• 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:

• Can detect joints like butt joints that cannot be easily found using traditional static wire or nozzle touch sense

• Can be used for lap joints less than 3 mm

• Ability to measure part height offsets and gap width and depths

Cons:

• Requires specific hardware and software license from Fronius

• Slower vs. laser or camera

• 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:

• Low- to medium-complexity; Some training required with built-in commands

• Works for most materials

• Easy to teach with macro jobs

• Uses a non-contact sensor

• Faster search speeds and touch sensing

• Eliminates the need for a wire cutter

• Finds most joint types, detecting lap joints down to 1/16” thick

Cons:

• Sensor box is mounted adjacent to the torch

• Mounting bracket per torch type

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• Limit in lap joint thickness (>1.5 mm)

• Limited ability to detect joint gap

• Cannot find square butt joints

• 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:

• Works on different materials in all lighting

• Easy to teach with macro jobs

• Provides joint gap data

• Long focal length; mount away from arc

• Locates 2.5D; offset and depth

• Compact and self-contained

• I/O interface can be retrofit to older controls

Cons:

• Medium- to high-complexity; Training on vision system suggested

• May restrict access into part/tooling

• 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:

• Low complexity

• Reliable sensor and easy to support

• Passover function restricts sensor error

• Phase Compensation calibrates weld cirucit

• Can track lap joints 1/8 in. or 3 mm thick

• Supports dual robots and coordinated motion

• Offers a lower cost option

Cons:

• Requires weaving and thicker material

• Limited by arc/weld physics

• 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:

• Reliably tracks thin gauge lap joint

• Supports high travel speeds (>100 IPM)

• Weaving Motion with tracking possible

• Tracking is not affected by weld settings

• Supports coordinated motion

• Ethernet interface available

• Camera hardened against welding arc

• Adaptive welding function; speed and weld settings

Cons:

• High-complexity and often high cost; Training on vision system required

• Torch-mounted sensor restricts joint access

• Tracking radii is limited to 40-60 mm

• 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.

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