The role of triangulation sensors in providing non-contact measuring solutions to tire makers.

Over three decades ago, Selective Electronic, known today as LMI Selcom, developed the first in a line of non-contact laser sensors that would transfigure quality control procedures in the tire industry. In the years to follow, the company honed its sensors' capabilities, creating more rugged products ideal for non-invasive measurement of rubber in numerous application areas.

A sensor line was created that was one of the first to measure fresh uncured rubber from an extruder or calender, characterized as one of the harshest environments for a sensor to consistently measure with any accuracy.

As the tire industry continues to become more competitive, LMI Selcom will continue to place increased importance on the need to enhance automation, lower material cost and continue reduction of waste.

Trends in the use of laser sensor measurement in a variety of rubber and tire applications have grown as the technology to support such products has evolved. Overall, reliability and accuracy of laser sensors have greatly increased and provide added benefits that can yield millions of dollars in savings over a long term quality control program.

Basic objective of measurement using laser triangulation 

For all on- and off-line applications, laser sensors are used in the rubber and tire industry to enhance competitiveness by improving productivity. Basic benefits of using laser sensors for quality control include:

* increasing yield and productivity,

* increasing quality by providing 100% product inspection,

* reducing scrap production and rejects, and

* improving work environment.

A facility cannot afford high rejection rates in tire uniformity that would result in waste and lost production time. By initiating a quality control program using laser sensors, problems with poor quality or unusable product can greatly be reduced.

Measurement principal of laser sensors

Sensors for the rubber and tire industry use the "optical laser triangulation measurement" method to accurately measure objects or systems. With this technique, a beam of light is projected from the sensor to the surface to be measured. This technique is sometimes referred to as a "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 this spot at an image plane known as a detector. If the surface is farther 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 on the surface can be determined.

It is important to understand that triangulation involves imaging of the spot (essentially taking a picture of the spot on the surface), and not just collecting reflected light from the surface.

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Spot size and surface effects

A general rule for best sensor performance is that the spot size on the surface must be small. For large, flat, uniform surfaces, this is not a major issue, but for small objects or surfaces of large curvature, spot size becomes very important. The spot size can be no larger than the feature to be measured. If a small feature is precisely positioned beneath the laser beam, the beam must be smaller than the feature. If, however, the sensor is to be scanned over a surface and a small feature found, a good rule of thumb is that the spot size must be less than one-tenth the size of the smallest feature to be measured.

The challenge for laser sensor manufacturers is to meet demands for the rubber and tire industry for longer stand-off distances without sacrificing accuracy. Stand-off distances of up to 400 mm are now available for rubber and tire applications.

In rubber and tire applications where spot size is important, the sensor should be mounted with the laser beam as close to perpendicular to the surface as possible. If the laser is at an angle to the surface, the spot size on the surface will be elongated, proportional to the angle away from normal.

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An important issue in proper sensor operation for a specific application relates to the surface being measured, and how its reflectivity may change. Since triangulation operates by imaging light reflected from the surface, a change in reflectivity will change the level or intensity of light reaching the detector. Reflectivity is influenced by several factors, including color and surface finish of the materials being measured.

Rubber measurement has often been a problem for laser sensors due to its inevitable characteristics as hot, sticky and smoking targets play havoc on accuracy. Recent technologies have evolved to ,handle these situations with a greater accuracy and reliability than in the past.

Detector types

Since triangulation requires finding the location at the center of the imaged spot, the imaging detector must be capable of detecting the spot location. While a number of types of detectors are available, two main types of detectors are used in 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.

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The first detector type is a PSD or position-sensing detector, also known as a lateral effect photodiode. 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. It offers advantages in several areas, particularly in speed of reading. Disadvantages of the PSD include lack of ability to display an image of the detector pattern.

CCD detectors are essentially a form of a television camera, and come in both one dimensional and two dimensional forms. The CCD detector has several advantages. First, the "video" output of the sensor can be viewed to display light levels, cleanliness of the image and to show any stray light effects, as well as the ability to apply filtering to the image to remove noise from the optical image. Disadvantages of the CCD detector are related to speed of operation, which typically is less than can be achieved with a PSD.

Comparison of laser sensors versus other devices

In general, laser sensors compared to contact devices are free of probe wear or bounce, and collect data at much higher rates. The sensors evaluated, versus capacitive or inductive devices, have a much larger standoff or distance to the material being measured. Thus, the sensor is most likely to receive less physical wear and is not as close to the actual environment measure, which could be harsh and unforgiving. Unlike most ultrasonic sensors, laser triangulation sensors are available with much higher resolution and accuracy, even with their longer stand-off distance.

Applications in the rubber and tire industry

Specific applications include measurement of product thickness, dimensional and cross-sectional profiles of on- and off-line extruded products, on-line thickness profiling of rubber and calendered rubber sheets, splice detection for rubber sheets, sidewall inspection and radial runout measurement.

Measuring extruded rubber

For example, fresh and uncured rubber from an extruder or calender has been proven difficult to measure with any reliability. Contact gauges tend to stick and are mostly non-applicable for this situation. Other gauges, like radioactive gauges, are normally sensitive to material and can cause health hazards. Since the extruded rubber creates a harsh environment, capacitive sensors that inherently have short stand-off distances are in danger of malfunction and reduced life expectancy.

Laser sensors are ideal for this type of situation due to their longer stand-off and durability. The sensors are capable of this measurement without regard to the speed or temperature of the extruded rubber.

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In measuring extruded rubber, the key variables measured and controlled include the weight per unit and the thickness of the sheet. Thickness measurement using laser sensors can either be performed in fixed tracks along the line or in a scanning system that can provide complete profile information.

A scanning system using two opposing sensors, one above and one below the material, is ideal for obtaining true thickness.

Measurement during tire production

On-line continuous tread profile measurement systems constantly monitor physical dimensions. This application ensures early detection of faults, reduced rework and fewer downgraded tires. Three techniques are often used:

* Adjustable measuring points - identify the thickness error at specific geometric points, such as the outside or inside shoulder, and can identify width errors.

* Statistical analysis - selects the appropriate set of profile measurements and uses software features to calculate and chart average or deviation. Variations due to changes in ambient environment, compound, processing equipment, line settings and operators will most likely be found.

* Porosity - tires with compounded rubber, where one compound contains a certain amount of trapped air bubbles, are measured for porous variations.

Summary

With increased competitiveness in moderately growing markets, the tire industry is relying on more increased quality control standards requiring reliable measurement solutions.

For this reason, laser sensors with their speed, accuracy and reliability are playing an increasingly important role. Laser sensor manufacturers are also responding to the tire manufacturers' demand for more stringent measurement criteria 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.