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Inspection Technology Review for Automotive Bead Dispensing Process

2017-05-18 10:03:06

Zhenhua Huang

Bead dispensing becomes a more and more common process for automotive manufacturing.  In automotive body frame and chassis, aluminum is gaining popularity, as manufacturers seek to improve fuel efficiency without sacrificing vehicle integrity or performance.  Thin gage aluminum part per se is difficult to weld, not to mention joining two dissimilar materials between aluminum and steel.  That is why structural adhesive is becoming an industry trend.  Right amount of structural adhesive is applied at the right location is crucial to the rigidity of the vehicle body frame.  In powertrain and vehicle final assembly, significant amount of FIPG (Form-In-Place-Gasket) is applied to ensure the sealing performance desired.  Neither “too-much” or “too-few” 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 at where.  And given the dynamics (viscosity variation, temperature variation, bubbles introduced by barrel change, etc.) of the dispensing process, a more direct in-line 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: 2D (two-dimensional) and 3D (three-dimensional).  2D 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, 2D solution can only provide 2D information, which is the bead width.  It cannot provide 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 rate is common for 2D solutions in real production environment.  It can also cause 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 inline bead inspection.  There is a big variety of 3D sensing technologies such as laser triangulation, stereovision, structured lighting, time-of-flight, laser interferometry, and 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 the multiple laser lines stationary around the nozzle (eg. Coherix Predator3D).  This sensor design ensures inspection of the bead with no blind corner for any arbitrary dispensing direction.

Predator3D

3D View

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 which is directly related to its functionality; and manufacturing engineering is empowered to better control and optimize the expensive dispensing processes. 


Other aspects to review an 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, etc.  Coherix Predator3D has proven field success on these aspects.  


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