Time is Money: Save Both by Using Laser Seam Finding Systems for Demanding Robotic Arc Welding
Servo-Robot Inc. Posted 04/16/2010
The new non contact seam finding technology increases the flexibility of arc welding cells. Servo-Robot compact laser sensing systems allow robotic welding jobs to be quickly programmed. The number of defective parts is drastically reduced while the productivity is increased. This is done by giving blind robots eyes and a brain, very similar to human welders, thus allowing them to adapt to inconsistent part preparation and fit-up.
The defects caused by unrepeatable fit-up and part preparation (Fig.1) are the major obstacles to achieving maximum flexibility for robotic welding applications. In particular, for low-volume applications where building expensive tooling and using laser cutting for proper part preparation is typically prohibitive, an alternative is necessary. Touch sensing and through-arc sensing are possible alternatives, however they can only be used for a small number of joint types (thick plate fillet, large groove), processes (mostly GMAW and FCAW) and materials (ferrous). They both have limitations which reduce the maximum welding travel speed possible and the percentage of arc-on time.
High-Speed Smart Seam Finding Systems offer the best solution to cope with these problems and optimize the welding process. Using either a laser-spot or a laser-stripe sensor allows one to precisely locate the joint position and to modify the welding path of the welding robot.
Fig. 2 shows the principle of the seam finding and path correction processes using a laser-spot or a laser-stripe. After making one or more measurements, the welding path trajectory is recalculated and the weld is placed at the right place even if the joint is not exactly at the location the welding process was programmed for.
The advantage of the laser-stripe system over the laser-spot system is the ability to find the joint using a single snapshot without having to move the laser-camera with the robot across the joint. The SF/D system is the first on the market to incorporate the image processing in the sensor itself. It runs powerful software containing a joint library that allows one to find almost any joint type including Tee Fillets, Lap Fillets (down to 0.8mm thickness), tight Butt joints, Vee and flare Vee grooves, etc.
Another benefit of these systems is the ability to modify the welding parameters (current, voltage, travel speed, weaving amplitude, etc.) based on the acquired measurements. Figure 3 shows how the profiles acquired by the Smart Seam Finding System are used to adjust the weaving amplitude or any other welding parameter so the amount of material and energy used is minimized.
The Servo-Robot Sense-I/D-H (Fig. 4) includes an embedded 2D color video camera for remote programming and monitoring.
Excellent examples of applications that benefited tremendously from the use of laser vision seam finding are explained in the following Case Histories.
Case Histories - Farm Component
The farm tiller component shown in Figure 5 is produced in a robotic Gas Metal Arc Welding (GMAW) cell. There are a total of about 60 welds per part having an average length of 35mm.
The productivity and quality levels were reasonable, but in the new economic environment this company finds itself in, more parts per hour and reduced repair rates are required from every piece of equipment they operate. The biggest place to improve in this particular robotic welding station was to reduce the time spent doing seam finding which totaled about 4.2 minutes. The touch sensing was used to try to handle the stack-up of part and fixture variability. The reason the touch sensing took so long was that the sequence required first finding the bottom of the Tee joint and then going over to find the upper part of the joint. This search routine was done at a very slow rate of 0.2 meters/minute because at any faster speed the search would be very inaccurate due to overshoot and wire bending. In addition, due to distortion concerns the part was welded in sections. This meant that after touch sensing the robot would weld a section of the tiller component and then move over to the wire cutter so that the wire end was sharp to improve touch sensing performance. This extra wire cutting operation wasted even more time.
With the laser line sensor, this 4.2 minutes of searching was reduced by 2/3 which resulted in a big time savings. In addition, the laser searching was much more precise which put the wire exactly in the joint which allowed the travel speed to be increased 20% and the reject rate reduced 10%. All these improvements added together resulted in the ability to produce 70 more parts per week from the same robot cell. The payback on the installed laser vision system was less than 6 months.
Case History – Mining Shovel Boom
Robo-Find can also be used in conjunction with high deposition rate welding processes used to fabricate mining shovel components made from heavy steel plate involving large groove weld joints. Real time seam tracking can be used for this application, but when one does not expect major distortion and one wants to reduce the cost of the vision system, seam finding can be used. The seam finder takes “snapshots” at a few places along the boom joint and then does a “calculated adapt” adjustment to the welding process to make sure the weld joint is perfectly filled. This precise fill provided by the Robo-Find SF/D sensor saves on consumables (wire, gas), reduces finishing time and minimizes distortion. If desired, the system can also determine if the joint preparation is out of tolerance and stop the welding process before it is even started.
Servo-Robot sensors can also be of great benefit for welding related processes, such as cutting, beveling, etc. For example, they are successfully implemented in the measurement of the steel coil prepared edge used in spiral pipe welding applications. This measurement immediately detects any problem with the beveling process which can adversely affect the quality of the formed weld joint.
All Servo-Robot systems offer a variety of interface types (Ethernet, serial, analog, digital, etc.), so that almost any type and vintage of robot can be used. Their rugged housing and easy to use software ensures reliability even in the toughest environments.
The power, speed, precision and flexibility of Servo-Robot laser-vision based measurement systems give fabrication shops the flexibility needed for increased productivity and quality.