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Article from "EuroLaser" 1/98 by Franz J. Gruber (main editor):

The Precision of 3D Joints
 

Sensors for
          Joint Tracking
                 in Laser Welding
 
"The best sensor is the one that you don't need!" Who hasn't heard this much-quoted saying among die-hard welding practicians who, if possible, want to do without control systems in automation solutions. Yet, the need for high-precision joint tracking sensors is becoming increasingly evident, particularly in laser welding with industrial robots. SCOUT, a product of Dr. Barthel Sensorsysteme GmbH (Munich), is probably the most well-known sensor of this class. There is already the third generation of this system available on the market.
 
 
It first should be mentioned that the use potential of the Scout Sensor is not limited to the field of laser welding. The latest generation can also be used in MAG and MIG welding, as well as in inert arc welding with non-consumable electrodes. It has received wide acceptance in laser welding primarily due to the high precision requirements of this process, but also due to the fact that Dr. Barthel specifically focuses on this market. The being, that apart from the particular suitability of the sensor, there is much less competition in laser welding than for example in joint tracking systems for electric arc welding.
Scout's origins go back eight years. The system was conceived at DASA (Daimler-Benz Aerospace) in Munich, where the idea was born to develop a joint tracking system for three-dimensional laser processing based on an extremely powerful image processing board originally developed for other purposes. The idea suggested itself at that time, since laser portals for welding jobs in automobile manufacture were within the high-tech company's sphere of activity then.
DASA retreated from its manifold laser activities, but the survival of the sensor system was not in danger:
In October 1994, Dr. Barthel, who had already been responsible for Scout at DASA, initiated a "friendly and generous" separation from DASA together with four other collegues, established his own company and has since then been monitoring the development and marketing of the Scout sensor.

What is special about Scout?

The joint tracking system three-dimensionally positions industrial robots or portal systems with an accuracy of +/- 50 micrometers along the laser joint, facilitating the automation of welding, cutting, gluing or sealing processes at tool path feedrates of more than 20 meters per minute. The realtime 3D image recognition precisely follows the three-dimensional path of the joint. Dr. Barthel: "The accuracy and the possible feedrates are the two main advantages of our system. The powerful image processing board enables us to interpret the image data stream online. We have a result as soon as the picture is through."
The sensor head and the sensor computer are the primary components of Scout. The sensor head that is attached  to the portal or tool in a leading position transmits video sequences of the joint or edge to the computer, where they are processed according to the light-slit / gray image method. In every single video image, up to five measuring points are recognized, guaranteeing a high density of measuring points, good signal-to-noise ratio and the reliable recognition of starting and end points of the joint. The sensor computer is based on VME bus and therefore supports all standardized interfaces. In laser beam welding, the work laser can be controlled via analog as well as digital interfaces. One-dimensional applications in joint tracking with a separate axis have also been successfully tested.
The team at Dr. Barthel Sensorsysteme GmbH now has a total of eight co-workers and is continuing to grow. The new headquarters in Schwabing (Munich) could soon prove too small again. Besides two vacancies that are going to be filled there is another reason for the limited space: an industrial SEF robot is now installed in order to be at the technicians' easy disposal for experiments.
An extremely useful investment, considering the fact that the intelligent integration of Scout measuring data into machine or robot control systems is one of the main aspects of their daily work. Interfaces to the most widespread control systems, however, have already been completed (Siemens 840, Siemens RCM, VW VRS1, IBH acro 8000, Power Automation PA 8000) or are in preparation (Comau C3G 900, Kuka VKR Cl).
Still, special adaptations are constantly being made to meet customer demands. Varying levels of integration are called for, and, in addition, Scout is capable of more than just determining the position of the joint. For example, every change in the process (due to e.g. thermal fluctuations) is detected. By measuring the gap width, Scout can also help to control the amount of filler rod or the welding parameters. In electric arc welding, for instance, changes in quality can be determined by using data about the reinforcement of the weld, and the process controls can be adjusted accordingly. This alone brought Dr. Barthel and his team quite a number of orders for retrofits. "There were applications where the people said they didn't need the joint-guidance. But later, they saw the benefits of our system because after all it is interesting to know whether the gap changes."
More than 50 Scouts have been sold by now, with most of the customers coming from the automobile industry. The most common application is welding jobs in the manufacture of car bodies. The company has been doing very good business with Volkswagen and P.S.A. (Peugeot/Citroen) but is in contact with almost every automobile manufacturer in Europe. Additionally, there are customers from freight car or airplane manufacture, or from vessel construction for the food industry.
Since it was founded, Dr. Barthel's company has focused on one single product - the Scout. This will change, however. The topic "trailing quality check" in particular is intensively being worked on at the moment. Furthermore, there are projects in the works for a sensor system for MAG and MIG welding. Dr. Barthel: "It deals with solving a relatively simple measuring problem in the field of exhaust manifolds by using a designated sensor system. As you can see, we have a lot of ideas for products in other fields, the only restriction being that we exclusively do optical sensor systems."
However, the main focus is on Scout. After all, the company was swamped with inquiries and orders last year. The demand for sensor systems just is higher than some will admit in practical manufacturing. Dr. Barthel comments: "Our capacities were hardly sufficient to keep up in 1997" The current third generation of the sensor head is so compact that it is hardly comparable to the very first Scout. The next generation, that at present only exists on the drawing board, is to be even smaller. The goal is to completely hide the sensor head by integrating it into the tool holder. There, the upcoming Scout will do its work.