Sealing bead inspection system relies on laser profile sensor

An innovative high-speed inspection system has been developed for monitoring the quality of polyurethane (PU) beads as they are applied to glass during vehicle assembly. The BeadMonitor measurement system, developed by Oxford Sensor Technology, measures the height and width of the PU bead as it is applied to the edges of the vehicle glass. The sensor head is mounted on the robot arm, directly behind the applicator nozzles. The system is based on Micro-Epsilon's scanCONTROL 2700 laser profile sensor.

Anthony Williams, Managing Director of Oxford Sensor Technology, comments: "BeadMonitor provides a cost-effective solution for vehicle assemblers and systems integrators who need to inspect PU beads before the robot places the glass into the vehicle. BeadMonitor enables much tighter control of safety-critical procedures.

"Typical cycle times for inspecting adhesive bead and for assembling glass into the vehicle are around 60 seconds, depending on the number of robots employed in the line. The bead measurement process is critical because if a glass windscreen is incorrectly fitted to the vehicle due to little or no adhesive being applied at a certain point on the glass edge, the result could be a water leak for the driver. Worse still, for many vehicles today, the windscreen has become an integral part of overall vehicle strength and rigidity, so any weaknesses in the glass bonding could pose a safety risk to the passengers.

"Conversely, if too much adhesive is applied, the vehicle assembler considers this wasteful and an increased cost to the operation. This scenario may also result in a degradation of the vehicle's aesthetic appeal to the customer, particularly if excess adhesive is visible around the glass edge."

Machine vision for the automotive industry

Established in 1989, Oxford Sensor Technology (OST) develops, installs and commissions machine vision systems for a wide range of industrial applications, most of which are for the automotive sector. Such systems typically comprise custom-developed software, innovative sensing technologies and third-party hardware, all integrated into a single, easy-to-use package. Customers include Ford, Jaguar Land Rover, Honda, Fiat and Ferrari.

BeadMonitor provides the vehicle assembler with a simple interface that shows a depiction of the vehicle glass, with the adhesive bead marked on the screen in green, orange or red, depending on the measured width and height when compared to preset tolerances. Two separate X and R bar charts show the consistency of the bead width and height over time.

Williams states: "When we decided to develop BeadMonitor, we needed to select a suitable supplier of a non-contact laser profile displacement sensor. We looked at five suppliers, three of which made it onto our shortlist. We then had to rate each sensor against specific technical criteria. It was very important that the sensor being offered was technically capable of doing the job.

"The sensor scanning speed and image quality were also critical. If you apply a bead at high speed, say 210mm per second, and inspect at 70Hz, in effect you are taking measurements of the bead at every 3mm, which is borderline satisfactory in terms of sensor performance for most car assemblers. Ideally you want to be taking measurements every 1mm.

"Image quality is important too, as you need to be confident that the sensor can cope with detection of shiny black surfaces in order to reliably detect the bead. During product evaluation, we were 100 per cent confident that the scanCONTROL 2700 sensor was providing our software with reliable data."

About the sensor

Micro-Epsilon's scanCONTROL 2700 is a non-contact, high-performance laser profile displacement sensor. Unlike conventional laser line scanners, the scanCONTROL 2700 features an integrated controller in the sensing head, which makes the complete sensor package very compact and lightweight. The user also requires no other components to evaluate the measurement data and the unit is easy to set-up, configure and adapt for different applications.

According to Micro-Epsilon, competing profile sensors with integrated controllers are either very bulky - often two to three times larger - or their measurement performance is inferior due to the restriction in space for the processing electronics. The scanCONTROL 2700 offers multiple interface options for ease of use with a wide variety of software platforms. The measurement range is programmable by the user from 10mm, 25mm and 100mm up to as high as 600mm.

Mounting of the sensor to the robot was also a consideration. Williams says that the sensor needs to be positioned approximately 1mm after the bead nozzle so that the system can still take measurements when moving around bends or corners. He states: "We liked the scanCONTROL because it does not have flying leads, just connectors directly integrated to the sensor housing. With robots twisting and flexing continuously, we would have required robot-grade cabling. Other sensors we looked at had half a metre of cabling that would have needed upgrading."

Williams says that sensor compactness and weight were also key selection criteria: "With the scanCONTROL 2700, we get an integrated controller rather than a separate one. We therefore do not have to worry about what is located next to the air knives or other nozzle cleaning systems on the robot. Also, the weight is important because, as a designer, you do not want to place too much weight on the end of the robot arm, otherwise your system speed or flexibility may suffer."

Those suppliers whose sensors did match BeadMonitor's technical specification were invited to meet at OST's offices to discuss the technical aspects and commercial terms. Williams says he considered commercial aspects such as price and delivery but these were of secondary importance to the technical capabilities of the sensor: "We decided to work with Micro-Epsilon because the scanCONTROL 2700 sensor technically met all our criteria. In addition, during sensor trials the technical support provided by Micro-Epsilon was excellent. Nothing was a problem to the engineers there and the company showed great willingness and openness during our discussions."

User interface

BeadMonitor is built around the scanCONTROL 2700 sensor but has a user interface and custom software program developed by OST. Williams says: "BeadMonitor presents the measurement results pictorially to the vehicle assembler using graphs and a simple green-orange-red tolerance warning system. The software also enables the user to look at the average bead width and height over a given time period, similar to a statistical process control system. The system also provides full traceability, which is critical to the automotive industry."

BeadMonitor is currently undergoing trials with a major car manufacturer in Italy.

Follow the link for more information about Micro-Epsilon's scanCONTROL 2700 laser profile sensor.