Smart laser line sensors for inspection.

Thousands of laser measurement sensors are in use monitoring dimensional quality in tire manufacturing, both for in-process and final inspection applications. Applications range from tire tread extrusion profiling and width measurement over a roller, to tire build measurement inspection, finished wheel alignment and measuring the volume of material on a conveyor belt. Almost all of these non-contact sensors are single point sensors, which measure at a single point on the rubber surface. They have been proven superior to traditional contact/mechanical followers and capacitive sensors.

As tire specifications become more stringent, tire manufacturers require faster, more sophisticated, accurate measurement and inspection devices with higher density data for both on- and off-line applications. By providing more information, higher data density sensors can detect local geometric changes often missed by single point sensors.

To meet these requirements, a new line of digital imaging smart laser line sensors has been developed. These sensors provide extremely high resolution three-dimensional cross-sectional profile scanning of up to 150 points or contours across a horizontal measuring line in a single frame.

Laser line sensing

Laser line sensing (figure 1) is similar to point triangulation. The main difference is that an expanded laser beam with optics illuminates a line across a horizontal surface. A 2D digital camera image detector is used to view an image of the line at standard frame rates of up to 30 frames per second with a camera exposure time of 15 milliseconds.

[FIGURE 1 OMITTED]

Key features include on-the-spot measurement of material geometry variations across multiple dimensions, increased operator control for greater throughput, and reduced waste and tighter production control for optimum uniformity and product quality.

Development of these laser line sensors, branded under the name EyeCON, is the result of various recent advances in digital technology, image processing and laser line projection. These new sensors process more data than many other scanning systems.

The primary use of these sensors is in the preparation side of the plant, such as tire tread extrusion width and profile monitoring, tire building and measurement of calendered rubber.

Laser line sensor specifications

The compact sensor package (figure 2) includes the laser, 2D camera detector, imaging optics and internal processing to operate the sensor. Internal processing converts the data to engineering units for output, and is also capable of internally processing the data. The laser line, optics and 2D camera detector are calibrated together during manufacturing using Cartesian Coordinate Algorithms for advanced imaging and enhanced quality control. 3D data are achieved when an encoder is applied to moving material and used in the calculation.

[FIGURE 2 OMITTED]

Packaged in a rugged IP65/NEMA 4 industrial housing for maximum precision, the sensor is ideal for a multitude of applications. For maximum reliability and stability, the sensor does not use internal moving mirrors, and can operate in temperatures from 0[degrees]C to 50[degrees]C (32[degrees]F to 122[degrees]F). Dimensions are 359 x 70 x 70 mm (14.13 x 2.75 x 2.75 in.).

Total depth of view/measurement (DOV) of the sensor is 287 mm (11.3 in.). Field of view (FOV) measurement is 104 mm (4.09 in.) at the close end to 179 mm (7.04 in.) at the far end. Accuracy is [+ or -] 0.1% of the measuring distance, and resolution is 0.01 mm (0.0004 in.).

The laser line sensor's eye-safe visible laser is laser safety Class II, simplifying installation requirements. In the past, Class IIIb laser sensors were required to adequately gather enough data for many applications. Class IIIb sensors have much more stringent laser safety requirements, adding to system cost and complexity.

The laser line sensor has two connectors, a power in connector to power the sensor and an ethernet connector for communicating with the sensor, located on the sensor housing. The sensor's ethernet connectivity allows use within any standard PC environment and runs on Windows 95 or higher. Power required is +8 to 32 volts DC @ 10 W maximum.

A diagnostic software package, provided with the sensor, allows taking measurement and image snapshots for monitoring the system during installation, normal production and trouble-shooting for optimum quality control and greater uniformity. Installation is easy at any line location for total quality verification.

Point sensor vs. line sensors

Point triangulation based sensing has historically been very successful in tire measurement since it offers extremely high data rates (16 to 32 kHz), both analog and digital outputs for simple interfacing and resolution as low as 25 microns. In contrast, laser line sensors take data over multiple surface points and are slower in data accumulation.

Selection of the best sensor for a given application depends on specific performance requirements. An important consideration is the data density required. This is directly related to the data rate or frame rate of the sensor. Sensor type selection depends on the application, as well as the capability of data analysis software.

If a high sensor measurement data rate is required to inspect tires reliably without limiting production rates, laser line sensors are recommended to not only take data at high speed, but also effectively communicate with a PC or other device. The new laser line sensor family comes equipped with ethernet connectivity.

Most point sensor implementations in the past communicated data by analog methods, making data subject to noise pickup in the plant environment. Recently, point sensors have been developed with a high speed streaming RS-422 capability, making them able to transmit digital measurement data at 460 kbps.

Smart sensors with applications inside

Most sensors in tire monitoring applications have been "smart sensors," with the software and hardware for controlling sensor operation running inside the sensor head. These software functions include linearization, conversion of measurement data to engineering units (digital output), automatic gain control to insure data are accurate even when the object surface reflectivity varies rapidly, low pass filtering and analog to digital conversion in some cases. Sensor operating parameters for specific types of applications are also stored inside the sensor head.

Recent developments in smart sensor technology have expanded the processing capacity inside the sensor to allow additional processing ability (generated by the sensor manufacturer, the systems integrator or the end user) to reside in the sensor head, along with sensor operating software.

Single point sensors were recently introduced with new smart sensor technology (application software inside), integrating sensor data collection and control software functions within the sensor head for high efficiency and reduced system costs by eliminating the need for an external controller (PC, PLC or other device). The technology also simplifies software routine programming and sensor control.

This smart sensor technology, with application specific analysis software operating inside the sensor housing, is also available with newer laser line sensors.

Implementation examples

Applications for laser line sensing are primarily in the preparation side of the plant. They range from tire tread extrusion width measurement and profiling, width measurement over a roller, lateral and radial runout for uncured tires, tire build measurement inspection, finished wheel alignment and measuring the volume of material on a conveyor belt.

Thickness contouring/edge tracking

Laser line sensors are ideal for thickness profiling and edge tracking. For tread profiling, laser line sensors eliminate transverse scanning frames for smaller parts and enhance data collection during a sweep for higher automation intelligence and control. Line sensors are also useful to customers to replace expensive "smart cameras" with complicated lighting packages and high engineering cost.

A single sensor (figure 3) can provide both thickness profile data and edge location tracking. The main advantage is that the sensors do not require a light bar or reflective strip underneath the web, which reduces accuracy over time due to dirt buildup. The sensors are positioned above the material and are not affected by debris falling from the line.

[FIGURE 3 OMITTED]

The use of two sensors for width measurement (figure 4) allows profiling over the edge of the material for optimum quality control.

[FIGURE 4 OMITTED]

Conclusion

For in-process and finished product applications, laser sensors are used in the rubber and tire industry to enhance competitiveness by improving productivity. New laser line sensors provide much higher density data to provide a high level of quality, productivity and safety, with instantaneous measurement data to meet exact customer requirements and to improve manufacturing quality and productivity.

Walt Pastorius and Mike Snow, LMI Technologies