Rubber and tire manufacturers are requiring more precision and more detailed information from tire inspection systems as inspection requirements become more stringent every year. One example of new requirements is in sidewall inspection, where safety and quality needs require very high reliability detection of smaller defects, wherever they occur. Sidewall bulge and depression measurement at final inspection require very high density data to properly detect bulges and dents, since the analysis software must first remove or filter out all points that relate to lettering, bar codes, and other acceptable variations in the surface. Bulges can be anywhere from 0.3-3.0 mm high and 5-7 mm wide. Customers are now asking to restrict heights to 0.2 mm since many bulges are not cord related but rather, are air blisters.
Suppliers are responding by developing faster, more sophisticated, and more accurate measurement and inspection devices for both on- and offline applications, for both in-process and final product inspection. The basic need is for dependable devices that inspect rubber products with better throughput, higher sampling rates, and increased accuracy, which are easy to install in both new inspection machines and in retrofit applications. Higher density data also improves the decision-making process, which minimizes expensive, time-consuming manual reinspection of false rejects.
Laser Sensors Used in Tire Manufacturing
Historically, tire inspection applications have relied on single point laser triangulation sensors that measure one point on the rubber surface, usually generating a single path or line of data as the product moves or rotates at high speed. Over 3,000 point sensors have been installed in tire manufacturing applications by our company, both for in-process monitoring and final tire inspection, and have proven to perform much more reliably and with greater precision compared to earlier capacitance and contact-based sensors.
High performance noncontact laser triangulation sensors have been successful in meeting the measurement challenges of tire manufacturers for decades. Noncontact operation measures rubber without deformation errors. Laser sensors have large standoffs (typically several hundred millimeters or more, depending on specific sensor type), with larger measurement ranges, and obtain reliable data even when not perpendicular to the surface. Large standoff moves the sensor farther away from the tire, reducing expensive-to-repair crashes if a tire is oversize or not properly positioned in the inspection station. With larger measurement range, a wider size range of tire models can be inspected without repositioning the sensors or tire location mechanism, improving flexibility and simplifying the mechanical system design. Capacitance sensors must be located close to and perpendicular to the measured surface, requiring complex, multiple axis positioning mechanisms, and need frequent recalibration due to sensitivity to material property changes.
Modern laser sensors do not require recalibration, and are insensitive to material property changes, unlike capacitance sensors. Properly designed laser sensors compensate automatically for changes in surface condition such as color, finish, or presence of bead lubricant on the surface.
Development of Laser Line Sensors for Sidewall Inspection
For sidewall inspection applications, increasing quality and safety of tires requires manufacturers today to detect smaller defects, covering a smaller area on the tire surface, while the tires rotate at very high speeds. With single point sensors, measurement of one or two tracks around the sidewall may not detect the smallest sized defects required, depending on their location. To resolve this issue, new generations of laser line sensors have been developed and are now used online.
The fundamental principle of laser line sensing is quite similar to point triangulation. The major difference is that the laser beam is optically expanded in one dimension to create a line of laser light on the surface to be measured. The output of each scan of the camera is a full profile of the surface with many data points across the laser line. As the tire rotates, a full 3D profile of the sidewall is provided, allowing detection of small defects wherever they are located.
The principle of laser line sensing has been used in other industries for measurement of surface profiles, but until recently, speed of operation has been too low to provide the high-speed data density required for sidewall inspection. Developments in high-speed digital camera chip technology, compact solid state lasers, and ability to provide high-speed image processing in the sensor head itself, coupled with new technologies for high-speed image synchronization from multiple cameras, have allowed the introduction of very high-speed laser line sensors for sidewall inspection, in cost-effective configurations.
The Class II Sensors That See EyeCon-2 laser line sensor from LMI Technologies is specifically designed for sidewall final inspection using laser line technology, with operation at data rates twice as fast as current sensors. Profiles are measured 2,000 times a second or more, providing very high data density to detect small defects as the tire rotates.
A unique feature of this sensor incorporates two cameras in one sensor housing, one on each side of the laser line. A single camera sensor may not see the laser line at a steep edge, and will not provide information close to the edge, not detecting the presence of a defect in this area. This can be a frequent error on tire sidewalls, which have large amounts of raised lettering, bar codes, and other geometric features. The dual camera design insures that one of the cameras will “see” the complete laser line, even at steep edges of raised or embossed geometric features, providing sidewall profile data right up to steep edges. This unique design insures that data dropouts or invalids are eliminated, providing full contour information.
This new sensor also incorporates “Smart Sensor” technology to internally process image data, linearize data and convert it to engineering units, outputting the measured 3D sidewall profile.
Application in Sidewall Final Inspection
In a tire sidewall inspection application, two EyeCon-2 sensors are used, one for each sidewall. A third sensor can be used to measure radial runout of the tread surface. A Scan Station concentrator combines data from multiple sensors and the Tire Uniformity Machine spindle encoder into a single synchronized data stream and distributes power. This total integration architecture simplifies installation and delivers real-time tire maps over one Ethernet connection in essentially real-time.
Other Applications of Line Sensors
With minor variations, the same laser line technology developed for sidewall inspection has been applied to other in-process measurement and control applications in rubber manufacturing. One example is the EyeCon-1™ line laser sensor for cross-section 3D profile scanning. Primary applications for the EyeCon-1 are on the preparation side of the plant, such as tire tread extrusion geometry monitoring. The laser line technology provides full cross-sectional contour measurement, without the need for mechanically scanning or other moving components, reducing complexity, cost, and maintenance. Image processing software operates inside the sensor, eliminating the need for external computing devices, and communications to external devices is via Ethernet for easy installation and integration.
The same technology is also used in other applications such as conveyor belt manufacturing, where the line sensors can continuously monitor both thickness and width or provide signals for edge position guiding.
Conclusion
High-speed laser line sensors with unique dual vision cameras are now available that provide higher density along the entire laser line. For sidewall inspection, these sensors detect small bulges, dents, or depressions on sidewalls by profiling the geometry of the tire across the line at frame rates of up to 2,000 profiles per second or more. As the tire rotates, a full 3D profile of the sidewall is provided, allowing detection of small defects wherever they are located. The same technology is also used for contouring geometry of extrusions and measurement of belts for in-process control.
Laser line sensing has dramatically improved inspection of sidewalls for defects, profiling of extrusions, and has enabled other applications such as monitoring geometry of belts.
