Laser sensors in the tire industry.
As tire specifications become more stringent, tire manufacturers are challenging their suppliers to develop faster, more sophisticated and more accurate measurement and inspection devices for both on- and off-line applications.
What the manufacturers want are dependable devices that inspect tires with better throughput, higher sampling rates and longer stand-off distances, without sacrificing accuracy. Production and throughput mean everything to a tire manufacturer. They can afford little or no downtime, and back-up inventory must be kept to a minimum. Manual re-inspection of false rejects must also be minimized.
For these applications, non-contact laser measurement sensors have proven to be superior to traditional contact/ mechanical followers or even older capacitive sensors whose repeatability and outputs change with each measurement.
The biggest drawback to contact or mechanical followers are that these types of sensors need a clean path on the tire to measure consistently. The tire lettering or embossments will destroy the touch probes as the tire rotates at 60 revolutions per minute (rpm). Any lettering or embossments on the tire also severely reduce repeatability due to unwanted bounce.
In contrast, properly designed laser measurement sensors are not affected by surface texture, color, speed or different ambient light conditions. Compared to contact sensors, laser sensors are free of probe wear or bounce, and collect data at much higher rates.
Performance requirements for rubber applications
Since rubber is black and absorbs almost all the light falling on the surface, measurement devices need sufficient light power to obtain good quality laser spot images and very fast gain control to accommodate changes in surface reflectivity.
As a general rule, the best sensor performance also requires a laser with a smaller spot size and a very fast frequency response rate to obtain reliable data from interrupted surfaces such as tread profiles or sidewalls with their lettering and variety of complex shapes.
In-process rubber material, typically fresh and uncured from an extruder, calender or in the tire building process, is normally hot and sticky, with a black shiny surface that evaporates smoke or fumes. Data from the measurement must reflect the application's true shape or dimensions of the web or profile. It cannot be influenced by a hot, smoky environment or surface, the angle of measurement or the texture, brightness, slope, speed or temperature of material. When measuring a rotating tire on a TUO (tire uniformity) machine at high speeds, the non-contact sensor must provide fast samples to insure that a defect does not escape detection.
Non-contact laser measurements are ideal for these types of applications, since they rapidly respond to process variations, are capable of high speed measurement and measure consistently throughout the manufacturing process.
How triangulation works
Most non-contact sensors for the rubber and tire industry use the optical laser triangulation measurement method to accurately measure objects or surfaces.
With this technique, a beam of light is projected from the sensor to the surface being measured. This technique is sometimes referred to as structured light, and is the equivalent of an automated light section microscope. At the surface, the laser projects a spot of light at one point. At some angle to the laser beam, a lens is used to form an image or picture of the spot at an image plane where the position sensing detector is located, if the surface is further away from the sensor, the spot on the detector shifts to a different point. By determining the position of the imaged spot and calculating the angles involved, the distance from the sensor to the surface can be determined.
There are two main types of detectors used in non-contact triangulation sensors. Both are fully solid state, and are integrated circuit chips with rugged construction and reliable performance, even in hostile environments, when properly packaged in a sensor housing.
The first detector type is a PSD or position sensing detector; the second is the CCD or charged-coupled device.
The PSD is a single element detector that converts incident light into continuous position data. It is essentially an analog device. PSD sensors are used when extremely high data rates are required, such as is common in the rubber and tire industry. PSD sensors are designed for high frequency response, fast light power control and small spot size requirements. They provide very fast compensation for light level changes, an important feature for rubber applications.
CCD detectors are essentially a form of a digital camera, and come in both one and two dimensions. Typically, the single dimensional CCD array is used for single point measurements. The 2D version is used with laser line sensors, which can measure 2D profiles in a single image frame. The primary disadvantage of the CCD detector is related to speed of operation, which is typically less than can be achieved with a PSD.
PSD-based laser sensors are ideal for tire and rubber applications, and are replacing dial indicators, linear variable differential transformers, capacitive, inductive and ultrasonic sensors for these types of applications.
PSD-based triangulation sensors have a wide variety of applications in the tire industry, both in-process and online. Their small spot size, smaller than most all other sensing technologies, makes them an ideal choice for measuring small changes and interrupted surfaces. Here are some of the in-process applications.
Thickness measurement of calendered rubber
Most thickness applications are performed in a fixed position with two opposing lasers, one above and one below the material. With differential sensor output, a precise and accurate measurement of thickness variation for any type of material in the sheet is provided. Because of the high sample rate and small spot, any pass line variations or vibration of the material does not affect the thickness measurement value.
In some cases, the two sensors are mounted on a mechanical slide and "C" frame, which scans back and forth to monitor thickness variations across the material width. The frame must be rigid to prevent vibration from introducing measurement errors.
Other applications require use of one or more fixed sensors against a reference surface, such as a reference roller. Although simpler and more straightforward, the accuracy of this approach depends on the type of precision roller used. Compensation must be made for bearing wear and dirt build-up on the reference roll. There is also the risk in high-speed applications of ballooning of the material away from the reference point.
Extrusion profiling and guiding
Laser sensors are ideal for this type of measurement due to their longer stand-off and durability. They are also capable of measurement without regard to the speed or temperature of the extruded rubber.
Extruded rubber through a die forms a specific shape, such as the tread portion of the tire. In-process measurement of the profile during extrusion allows control of the process by correcting parameters such as thickness, width and profile to maintain the proper shape. Surface features such as ridges, centerlines and edges can also be monitored. Basically, the information gives the operators an idea of how the die is wearing and when to change it.
Profiling of tread extrusions can normally be done by mechanically scanning across the extrusion. Many applications are requiring even higher speed acquisition than that provided by mechanical scanning. To meet this need, some manufacturers have developed high-speed optically scanned point triangulation sensors. One of the sensors just developed is unique in its ability to rapidly adjust laser power along each point of a 2D scanned line. This insures proper exposure for each data point.
Overlap and splice detection
A common cause of a non-uniform tire is an incorrect ply and profile splice in the tire lay-up operation. By in-process monitoring on the tire building machine, for example, errors and trends are quickly detected, which avoids scrapping a large number of cured tires. In this application, laser sensors have proven to be more accurate than other types of sensors.
Monitoring radial and lateral runout of green tires
To reduce scrap, runout has to be detected early in the manufacturing process and measured as the tire is being built; it also has to provide the information necessary to implement corrective action. When measuring green tires in tire building machinery, splice overlap in the liner, sidewall and/or thread can be analyzed and quantified. By identifying and correcting any problems before curing and testing, better uniformity of the final product can be assured. The accuracy and repeatability provided are key factors for creating a good base of measurement for statistical calculations such as roundness and harmonics.
Final inspection applications
Tire sidewall inspection
A sidewall inspection process must detect all suspect products such as bulges and dents, while minimizing false rejects or classifying a good tire as defective. This process can involve a lot of hands-on manual inspection that costs a lot of time and money.
Many tire manufacturers are influenced by the measurement limitations of sensor manufacturers, and therefore had to over-sensitize their sidewall bulge and dent measurement systems, resulting in costly manual inspection requirements. Some measurement systems cannot even distinguish between a bulge or dent.
State-of-the-art measurement precision and advanced software analysis overcome most of these limitations. PSD-based laser triangulation measurement sensors, for example, provide accurate and reliable data with very high resolution at high speed.
Tire bulges indicate weak points in the tire construction due to a poor splice of the sidewall material. Basically, the splice does not have enough overlap, so when the tire is inflated to pressure, the tire blows out like a balloon. The bulge can be anywhere from 0.3 mm to 3.0 mm high and 5.0 mm to 7.0 mm wide.
PSD-based laser sensors have the ability to detect a bulge and other deformities quickly on a tire rotating at 60 rpm and measure bulge accurately to better than [+ or -] 0.0254 mm. Bulge heights of significance have been spaced around 0.3 ram. Customers are now asking to restrict heights to 0.2 mm, since many bulges are not cord related, but are air blisters.
Laser sensors can also measure on slopes without loss of data or the necessity to reposition the angle of attack. They provide an X/Y plot of bulge, dent, depressions and location on the sidewalls, and can measure through black lettering, lube oil or any other obstacles. Sensors that have a small spot size also can filter out high frequency signals and still detect and measure the flow frequency of bulges and dents.
False positive tests, generally referred to as alpha misses, are also minimized with laser sensors. The alpha miss rate is higher with capacitive probes or on tires that do not have a clear path to the sidewall for geometry testing. Totally unacceptable are tires that have a bulge that was not detected that could result in a failure or recall for that lot by the manufacturer.
Industry is also interested in line lasers that provide full-tire coverage and allow effective analysis of sidewalls. The disadvantage of this is the higher costs and complexity.
Tire uniformity machines
Many sensors are used in tire uniformity machines (TUO). Although some TUOs, such as the TTOC-II and TSOS system, use lasers, most systems use capacitance probes that have a very large spot size and can sometimes be the cause of false rejects. Other machines use touch probes that bounce over the surface of the tire as it spins, creating errors or low end laser sensors that do not perform as well as the high end lasers with high sample rates and sophisticated light control circuits.
It has been found that laser sensors used in conjunction with the TUO machines normally provide far greater accuracy and quicker cycle times than traditional contact measurement and capacitive sensor systems. Some microprocessor-based systems, for example, project over a 6.0 mm spot compared to a 0.02 mm spot for precision laser sensors (figure 1). The larger spot size affects the accuracy of the reading and limits the application. Some CCD sensors require additional cycle time to process extraneous data due to the slow measurement sampling rate.
[FIGURE 1 OMITTED]
In a typical TUO application, non-contact sensors are mounted on an aluminum c-frame arm. The TUO system monitors the signals from the sensor and identifies the type of depression, how wide it is at the base, dimension of the slopes and other geometric parameters. With the tire spinning at 60 revolutions per second, over 4,000 readings can be made with each profile. This compares to up to five profiles with a standard uniformity test.
Tire uniformity systems are being continually improved to inspect higher efficiency tires and new sidewall styles. With some systems, test cycle time has been reduced to 17 seconds, with further reductions necessary to meet the specifications desired by the tire manufacturers.
Radial runout sensors are also available that measure out of roundness in tires. Data processing software for this application is designed to eliminate pin vents, flashing and grooves, and only measures on top of the tread to produce a very accurate roundness analysis. The result is a filtered representation of the tire's runout that essentially reduces or removes all the groove depths and high frequency elements that cause undeterminable output.
To keep up with the stringent demands of the tire manufacturers, some sensor manufacturers are developing sensors with an even higher frequency data acquisition system. This would even further increase the speed of the sensor.
Tread wear analysis
Non-contact laser-based measurements provide a complete wear profile of the tire, as well as quantification of irregular wear. This is important for road noise implications and identifying local wear phenomena such as heel/toe wear, recessed lugs, diagonal wear, shoulder wipe and center wear.
Non-contact laser-based tread measurement is also an effective tool for diagnosing manufacturing problems with lateral and radial run-out displacement clearly visible from high precision surface measurement of a new tire.
Tire manufacturers require accurate wear data of prototype tires to improve the design. Automotive manufacturers use this analysis to provide comparative measurements to select the best tire design.
With increased competitiveness in moderately growing markets, the tire industry is relying on increased quality that requires reliable measurement solutions and limits downtime.
For this reason, laser sensors with their speed, accuracy and reliability are playing an increasing role. Laser sensor manufacturers are responding to these needs by developing sensors that can meet or exceed expectations.
As a result, rubber and tire manufacturers have turned to increased quality control to reduce manufacturing costs and remain competitive in the marketplace.
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|Title Annotation:||Process Machinery|
|Date:||Jan 1, 2004|
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