Abstract: This paper introduces the application of laser tracking measurement technology in welding fixture assembly and adjustment, as well as API's new generation of laser tracking measurement technology, including ADM absolute ranging technology, Intelliprobe intelligent probe technology, and a coordinate system fitting method based on a digital model, along with application examples of these functions. Laser trackers, as high-precision portable coordinate measuring machines, are not new to the global automotive manufacturing industry. In Europe, Peugeot Citroën and Renault have extensively adopted Tracker II Plus laser trackers manufactured by Automated Precision, Inc. (API) to replace other field measurement equipment such as articulated arms. In China, Shanghai GM has also selected two Tracker II Plus laser trackers from API for tooling inspection and measurement of body-in-white and stamped parts on the production line. Today's trackers have overcome the limitations of early trackers, such as bulky appearance, frequent calibration, complex operation, and susceptibility to environmental influences, becoming a widely used measuring instrument in modern automotive manufacturing. The Tracker II Plus laser tracker, manufactured by API Corporation in the United States, weighs only 8.3 kg, and even with the matching lightweight tripod, the total weight does not exceed 20 kg, making it easy to install on the production line. A typical application of laser trackers is for the assembly and adjustment of tooling fixtures in welding workshops. Body welding is one of the most important processes in the entire automobile manufacturing process. The quality of welding not only affects the subsequent final assembly but, for cars, can also affect the strength and safety of the entire body. Welding quality directly depends on the positioning accuracy of the welding fixtures. In the past, the initial installation and positioning of tooling fixtures on automobile production lines typically involved manufacturing a prototype vehicle. The accuracy of the prototype was ensured using a fixed coordinate measuring machine (CMM). The prototype was then placed at the workstation, and the positions of the chucks were adjusted to ensure that each working surface was properly aligned with the prototype. In effect, this prototype served as a transfer gauge for measurement. After adjusting the positions of the chucks using the prototype, a relatively low-precision on-site measuring device such as an articulated arm was used for re-measurement and verification. Because articulated arms have a very small measurement range, it's impossible to measure the original positioning reference surfaces (hole, slot, and three other positioning surfaces) of the vehicle body in a single measurement station. Therefore, the initial coordinate system often needs to be established through one or two station relocations. This inevitably introduces relocation errors into the established coordinate system, directly limiting the accuracy of remeasurements and making it impossible to guarantee relative positional accuracy over a large range. Furthermore, as is well known, articulated arms have a relatively short service life due to their inherent limitations. After a period of use, or even after the equipment is idle, wear on the bearings in the joints and deformation of the arm body will lead to a significant and irreparable decrease in accuracy. For these reasons, the positioning accuracy of the welding fixture is essentially highly dependent on the accuracy of the prototype vehicle. However, the most fatal flaw of using a prototype as a template is its susceptibility to deformation. The prototype vehicle itself is merely a high-precision body-in-white, a shell component. Although it may pass inspection by a fixed coordinate measuring machine, deformation due to temperature changes and external forces during transport to the site and subsequent assembly and adjustment is unpredictable. This error directly translates into the positioning error of the welding fixture. If a laser tracker is used, firstly, no station relocation is required within a large measurement range, ensuring a high-precision reference coordinate system (accuracy of ±0.025mm within 5m). With this accurate reference coordinate system, after importing the mathematical model, the vehicle body model can be directly used as a comparison reference to adjust the fixture positioning surface, as shown in Figure 2. Because the theoretical model of the fixture positioning surface coincides with the theoretical model of the positioning surface on the vehicle body, there is no need to create a separate model for fixture measurement. After establishing the reference coordinate system, the target ball is placed on the fixture positioning surface (positioning pin) to be adjusted. The software calculates the projected distance from the center coordinates of the target ball to the model of the positioning surface (positioning pin), and calculates the direction and value of the 3D deviation based on the radius of the target ball. This is displayed on the computer screen in real time, allowing observation and adjustment of the deviation within the tolerance range. Using this measurement process eliminates reliance on the accuracy of the prototype vehicle, reduces intermediate error transmission links, and saves on the cost of manufacturing and measuring prototype vehicles. The advantages are obvious. Although articulated arms can also perform the above process in terms of measurement methods and software, their measurement range and accuracy cannot meet the process requirements. The laser tracker uses its built-in laser interferometer as the length standard and the grating code disk as the angle standard. Through a series of calibration procedures, it can maintain a high-precision working state throughout the entire product lifecycle. Typically, API laser trackers used and stored in fixed workplaces can require self-calibration for several months. In contrast, the self-calibration time for a laser tracker is less than ten minutes. Like all coordinate measuring machines, the laser tracker offers a variety of coordinate establishment methods. In addition to traditional three-point coordinate establishment, point-line-surface coordinate establishment, and multi-point fitting coordinate system methods, it also provides a unique complex fitting coordinate establishment method. This method allows the use of arbitrary curved surfaces, planes, positioning holes, and positioning points on the workpiece as a reference for coordinate establishment, fitting and calculating the workpiece coordinate system. First, appropriate surface points on the reference surface (corresponding to the positioning surfaces on the tooling) and the center of the reference hole (corresponding to the positioning pins on the tooling) are selected on the imported workpiece mathematical model. Then, the positions of these reference elements on the workpiece are measured sequentially to obtain the fitting calculation results. If the positioning datum is over-positioned, analyzing the results data can reveal conflicts between datums, allowing for a decision on whether to discard potentially problematic datums (achieved by abandoning certain fitting calculation constraints). This complex fitting coordinate system allows laser trackers to replace various mechanical tooling fixtures for inspecting body-in-white and stamped parts. Specifically, the part is first fixed, and the positioning elements (including positioning surfaces and pins) used during tooling fixture inspection are selected as the reference datum for coordinate system establishment. The fitted workpiece coordinate system accurately reproduces the workpiece coordinate system used during mechanical fixture inspection. The measured workpiece deviation matches the deviation data detected by the mechanical fixture, accurately reflecting the actual state of the workpiece. This is the so-called "electronic tooling" technology. The principle for inspecting body-in-white, welding fixtures, and gauges is that the coordinate system datum must be consistent with the actual workpiece's installation positioning datum or working datum to minimize errors caused by the measurement coordinate system. Using this mathematical model-based "electronic tooling" technology saves the high cost of manufacturing mechanical tooling and also eliminates the human resources required for its inspection and maintenance, significantly improving production efficiency. Furthermore, laser trackers can also be used in automotive exterior design. New car models designed by artists on wooden molds need to be precisely converted into mathematical models in computers. Previously, this required an expensive, high-precision, large-scale coordinate measuring machine (CMM). Now, a Tracker II Plus laser tracker, along with its powerful measurement software, can complete this complex reverse engineering process. The laser tracker's diverse dynamic scanning modes can accurately reverse engineer the mathematical models of free-form curves and surfaces in space, significantly reducing the manufacturing cost of stamping dies. Simultaneously, laser trackers can also be used for the inspection of these stamping dies and workpieces. Laser tracker measurement technology is still in its early stages in my country's automotive production online inspection field, and its widespread application in the automotive manufacturing industry has broad prospects.