Inspection Technology Review for Automotive Bead Dispensing Process
Written by Dr. Zhenhua Huang, General Manager of the Americas at Coherix in Ann Arbor, Michigan.
Bead dispensing becomes an increasingly common process for automotive manufacturing. In automotive body frames and chassis, aluminum is gaining popularity, as manufacturers seek to improve fuel efficiency without sacrificing vehicle integrity or performance. Thin gage aluminum parts are difficult to weld per se, not to mention joining two dissimilar materials between aluminum and steel. That is why structural adhesive is becoming an industry trend. Applying the right amount of structural adhesive at the right location is crucial to the rigidity of the vehicle body frame.
In powertrain and final vehicle assembly, a significant amount of Form-in-Place-Gasket (FIPG) is applied to ensure the desired sealing performance. Neither “too much” nor “too little” is acceptable. The state-of-the-art bead dispensing systems are typically equipped with flow meters or shot meters to measure the total volume of the material dispensed. However, this method cannot tell how much is applied specifically, and where. And given the dynamics (viscosity variation, temperature variation, bubbles introduced by barrel change, etc.) of the dispensing process, a more direct inline inspection technology is truly needed for bead dispensing.
With the tight cycle time and the gel-like nature of the bead, inline non-contact optical inspection is the most suitable solution for good quality control of the dispensing process. Generally, there are two main categories of inline optical bead inspection solutions: two-dimensional (2D) and three-dimensional (3D).
Two-dimensional solutions are based on contrast (in other words, color or intensity difference) in images acquired to locate and measure the bead. There are several limiting factors to this solution. First, the 2D solution can only provide 2D information, which is the bead width. It cannot provide a cross-section profile, which is what really matters to functionality – wet-out for structural adhesive bonding and sealing for FIPG. Second, performance of 2D solutions is heavily impacted by part color or ambient lighting. High false reject rates are common for 2D solutions in a real production environment. This can also cause a missing bad part when the nozzle scuffs the part surface and leaves a thin film of bead marks without sufficient material to “fool” the 2D system. This happens often when the part vibrates or the robot program is not fully optimized.
With the limitations mentioned above, 3D is the trend the industry is leaning toward for the inline bead inspection. There is a variety of 3D sensing technologies such as laser triangulation, stereovision, structured lighting, time-of-flight, laser interferometry, etc. Given the inline high-speed continuous imaging requirement, laser triangulation is the technology suitable for bead dispensing application. However, the general-purposed single-laser line sensor is not sufficient because of the arbitrary dispensing direction and limited space around the dispensing nozzle. One option is to implement a motorized single laser line to trail the arbitrary dispensing direction. However, the modern dispensing process can reach 400mm/s to 1,000mm/s for high productivity. With this high speed, it is very difficult to have the motor respond quickly and reliably enough to any sharp dispensing direction change. The other option is to keep the multiple laser lines stationary around the nozzle (e.g. the Coherix Predator3D). This sensor design ensures inspection of the bead with no blind corner for any arbitrary dispensing direction.
The Predator3D inline bead inspection technology enables a possible industry paradigm change. With this instantaneous 3D visualization and inspection capability on the bead, product development is empowered to specify the bead in a manner that is directly related to its functionality; and manufacturing engineering is empowered to better control and optimize the expensive dispensing processes.
Other aspects of reviewing inline 3D inspection technology for production bead dispensing process include minimum space required for mounting and plant floor implementation, compatibility with different robots and dispensing equipment, minimum added complexity to robot programming, tolerability of robot speed change, support for high dispensing speed, robustness, traceability, ease of setup, and use. Coherix Predator3D has a proven field of success in these areas.