Back to Basics: Non-contact laser measurement
Manufacturers in a range of industries can use non-contact laser encoders to accurately measure the length and speed of products and optimize process control. Tutorial follows with diagram, photos, and links.
As manufacturers are driven to become more efficient and improve product quality, their ability to measure length and speed with increasing accuracy has become essential. Central to improving process control automation is manufacturers' ability to accurately measure the length and speed of product during production.
Traditionally, length and speed measurements were obtained from mechanical measurement methods such as rotary encoders, tachometers, and drive encoders. Contact measurement methods can have mechanical and calibration problems that cause costly measurement errors.
Non-contact, laser-based encoders are quickly becoming the standard measurement technology for many common manufacturing applications, such as measuring continuous product length and speed, controlling cutting systems, regulating differential product speed, and positioning products. Non-contact encoders are being used on production lines for manufacturing paper, film, and foil products; packaging; non-woven and textile materials; building materials; metals; and other industrial products. The high measurement accuracy of non-contact laser encoders is enabling manufacturers to realize significant product quality improvements, production savings, and a fast return on investment (ROI).
Contact versus non-contact
Typically, product length and speed measurements are made using a tachometer connected to a roller or wheel that’s in contact with the product. These mechanical systems are subject to slippage and calibration changes caused by variations in the diameter of the roll or wheel due to dirt build-up or wear. The measurement error of mechanical systems will change with the material and production line conditions, requiring the plant operator to continuously check the length or speed accuracy using manual or weight methods. The operator then adjusts the production line to keep product lengths or line speeds within specification.
Using a mechanical contact encoder with a high amount of mechanical complexity, product length or speed inaccuracies may be as much as 2% or more, depending on the application. A 2% inaccuracy on a large amount of product translates into a substantial amount material scrap, waste, money, and unnecessary expense.
Non-contact encoders use a unique, laser-based measurement technique that does not make contact with the product. This measurement system is permanently calibrated and has no moving parts to wear out. It works by projecting a unique pattern on the surface of the product.
As the product moves, light is scattered back to the encoder. This information is translated into product speed, and pulses are produced to determine the product length, independent of shape, surface, or color. The pulse output (pulses/unit) is then sent to a control system, such as a PLC, to trigger actions such as length counting, product positioning, cutting control, printing, and other tasks. Length and speed measurements are captured with better than +/-0.05% accuracy and +/-0.02% repeatability.
The following illustrates some typical applications using non-contact laser measurement to control production processes.
A manufacturer of corrugated boxes needed to accurately control the speed of product during normal production and “tail-out” situations. Slippage from contact encoders and "out-of-calibration" pull rolls created variances in the cut that ultimately resulted in material scrap. A non-contact laser encoder can accurately measure the board’s speed as the product enters the cutting knives; the speed signals from the unit provided the control system with precise pulse counts to control the cross cutter. This accuracy helped to significantly reduce scrap and increase product quality. The company saved more than $200,000 per year.
Non-woven, glass-, stone-wool
A glass wool manufacturer was experiencing product length and speed inaccuracies of up to 1% during cut-to-length operations. The contact wheel encoders would slip and lose contact with the surface due to dust and debris from the production process and the soft composition of the glass wool mat. Increasing contact pressure of the wheels also caused product damage. The measurement inaccuracies threw off the cutting and marking systems, causing synchronization issues between the fed glass wool mat and the transverse movement of the chopper. Implementing a laser encoder provided a non-contact and slip-free method to measure the length and speed of the glass wool with near 0.05% accuracy. This non-contact method improved cut-to-length control, reduced waste, and enabled the production of higher quality products.
A lumber company used mechanical wheel encoders at the saw mill to measure the length and speed of product during cut-to-length operations. Wheels of the mechanical encoder would slip because the surfaces of the boards were not homogeneous and would be laden with dust and debris. To minimize this slip, plant personnel would increase the contact pressure of the wheels. This approach damaged the surface of the boards and also quickly wore out the encoder wheels.
Inaccurate length and speed information threw off the accuracy of the cutting and marketing systems by as much as 1%, causing synchronization issues between the fed boards and the transverse movement of the saw. Replacing the mechanical wheel encoders with a non-contact laser measurement system enabled the plant to directly measure the true length and speed of the boards before they were cut. This data is updated at a rate of 1,000 times per second and then transferred at a frequency of up to 250 KHz to control the cutting system.
While the non-contact laser encoder has proved itself in a range of manufacturing applications, it has also been successful in other industries. For instance, it is used to control flying splice operations in paper manufacturing, flying saw applications in automotive brake tube production, cut-to-length of automotive rubber tire liners, rubber and plastic profiles, and other automation control challenges.
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