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Is Glass a Thing of the Past?

As machine tools, inspection machines and materials handling equipment have evolved from rudimentary manual mechanisms to highly sophisticated automated systems, the internal positioning devices they feature have also evolved into more sophisticated products. One of the most commonly used measurement technologies today is the linear encoder.

It was 1968 when the first linear encoders, featuring glass scales, appeared on the market. While these open glass-grating devices proved to be effective for very fine work, they were found to be extremely problematic in harsher machining conditions. For example, they offered very little resistance to the ingress of workshop contaminants.

While attempts were made to rectify this in the late 1970s with the introduction of large enclosed glass-scale encoders, many of the problems initially associated with glass encoders, such as ineffective sealing, the fragility of glass and the difficulty associated with fitting the scales, persisted; indeed these problems continue to persist today with this type of technology.

To address these problems, engineers at Newall embarked on a research program dedicated to exploring the possibilities inductive technology presented. Initially, the objective was to develop a non-contact encoder capable of withstanding the harshest workshop environments. This led to the development of an inductive encoder based on the inherent accuracy of fine tolerance ball bearings.

Newall's engineers designed an inductive encoder comprising two main assemblies: a scale and a head. The scale is a length of stainless-steel tube housing a column of precision nickel-chrome ball bearings maintained under compression; the compression load being set during manufacture to calibrate the scale. The head, which fits around the scale and slides along it, features a rectangular aluminum casting containing a coil assembly and electronics.

Unlike the linear glass encoders, the ability to permit any head to be used with any scale has delivered a high degree of flexibility for end users and reduced maintenance costs.

These ball bearing-based encoders, originally developed to withstand the harsh manufacturing environments of the machine tool retrofit industry, have since proved to be ideal for installation by OEMs (Original Equipment Manufacturers) and end users in a wide range of modern factory and process machinery. Machine tools, automation systems and measuring machines all benefit tremendously from using ball bearing-based linear encoders; even when operating under such adverse conditions as sub-aqua applications and nuclear caves -- long considered no-go areas for glass.

Unlike glass-scale devices, ball bearing-based encoders are IP67 rated. Completely sealed against oil, dust, coolant and other workshop contaminants, their design guarantees unbeatable levels of reliability and performance for end-users.

End-users further benefit from the fact that it shows none of the wear characteristics usually associated with glass encoders, removing the need for costly replacements while also negating the need for cleaning and maintenance. By increasing machine reliability, OEMs are able to add considerable value to their products.

The benefits this technology presents to OEMs are particularly salient given the highly competitive nature of today's market. The robust construction of ball bearing-based encoders, with no fragile or easily damaged components, means they can be easily mounted without the fear of damage associated with glass products. Their high resistance to shock and vibration also means they can be fitted in almost any environment, no matter how demanding, while continuing to deliver accuracy and repeatability.

The IP67 rating of ball bearing-based encoders also removes the need for air-purging and cleaning, allowing OEMs to make additional savings in both time and production costs, while self-aligning fixing brackets allow virtually effortless scale mounting, with a single drill hole required for each set of brackets. The design of the encoder also allows alignment in almost any position; unlike glass-scale encoders that usually have to be installed with the lip seal facing downwards to prevent contamination.

With the exception of a dial indicator, used to align the reader head, installing ball bearing-based encoders requires no need for additional electronic devices. The self-contained design also removes the need for additional electronic connections.

The present trend in modern machinery and automation systems is for increasingly higher accuracy, repeatability and resolution, increased reliability and speeds, and more efficient working ranges. The boom in telecommunications in the late 1990s resulted in the development of cost-effective digital signal processors (DSP) alongside high-speed analogue to digital conversion ICs. This has provided the technological foundation to allow for the signal processing capabilities required for direct integration with OEM systems to be realized.

Although the latest digital ball bearing-based encoders feature the same coil assembly, the drive (or induction) coil is driven from an amplifier at 10kHz as opposed to the 1kHz fundamental frequency of its analogue predecessor; allowing for the raw signals generated within the encoder to be directly translated to industry standard quadrature signals. Newall has also enabled users of control systems that demand 1Vpp or 11µA signal output from encoders, by developing an in-line matchbox interface, thus covering the market requirement of 90% of all systems.

An increasing number of machine-tool builders, system integrators, machine designers and engineering companies are managing to cut costs and increase productivity with the help of ball bearing-based digital encoder technology. The continued application of new, appropriate technologies will guarantee that ball bearing-based linear encoders will meet the needs of OEMs for many years to come.