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British balls pose absolute challenge to glass scales

01 March, 2003

British balls pose absolute challenge to glass scales

A British encoder manufacturer is challenging Heidenhain`s dominance of the absolute linear encoder market, with a magnetic-based technology which, it claims, out-performs glass-scale technologies in most ways, and is cheaper.

Leicester-based Newall Measurement Systems has spent Ł500,000 developing the new Spherosysn Absolute technology and setting up new production and calibration lines. The system is based on Newall`s Spherosyn inductive technology, developed in 1973, which uses a series of magnetically identical nickel-chrome steel balls mounted in a stainless steel tube. A sealed reader head containing a coil assembly and electronics senses the balls to determine linear positions to a high accuracy.

The new system (above) sandwiches coded markers between the balls to determine positions instantaneously to a resolution of 0.1µ, and to a selectable accuracy of between ±3µ and ±10µ. It can operate at read speeds of up to 63m/s. According to Newall`s technical director, Dr Mark Hudman, the technology has been made feasible and affordable by using digital signal processors originally developed for use in mobile phones.

The big attraction of absolute encoders is that they allow true, precise positions to be acquired, even after emergency stops, power interruptions or overnight breaks, without having to recalibrate the encoder by returning to a datum point. Until now the market for absolute linear encoders has been dominated by systems using glass measurement scales.

According to Hudman, the inductive technology has several advantages over glass-scale systems. It is more resistant to shocks and vibration, it is better protected against the effects of dust, dirt and immersion in water, it can operate over continuous lengths of up to 12m compared to the 3m limit typical for glass scales, and, unlike glass, it expands at the same rate as the iron or steel materials typically being machined. Also the inductive technology uses AC signals which, says Hudman, are less prone to thermal drift problems than the DC systems used in glass-scale encoders.




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