Digital linear encoder knows where it's at

A new absolute digital linear encoder delivers high accuracy in poor operating conditions.

Newall Measurement Systems has developed an innovative digital version of its versatile, rugged, well-proven Spherosyn linear encoder.

Capable of achieving an accuracy of +/-3um/m, sealed to IP67 and resistant to shock, vibration and RFI, the digital Spherosyn needs no "home" signal and can deliver real-time feedback at traverse speeds up to 20m/s.

By connecting a PC, the device can also be reprogrammed in-situ to change parameters such as the resolution.

Based on the proven technology underlying the analogue Spherosyn linear encoder, the absolute digital version features the same characteristics of being accurate, wear-free, and requiring no maintenance.

90 standard lengths are offered, giving travels ranging from 51 to 10,668mm, and the technology enables special units to be constructed with travels up to 27.75m.

If required, more than one reading head can be installed on each scale tube.

Housed within the stainless-steel tube is a column of precision preloaded steel balls.

The reading head uses a 10kHz drive coil to induce an electromagnetic field in the balls, and the variation in field is detected by a series of pickup coils.

Voltage from the pickup coils is analysed by the onboard electronics to indicate the position.

In addition, annular elements are inserted between the balls but without interfering with the ball-to-ball contact.

These inserts carry miniature magnets and are arranged in a defined order.

An array of Hall-effect sensors detects the pattern of poles (north, south or none) so that the absolute position can be determined immediately, with no need to reference a datum.

This configuration gives a maximum travel length of 9.25m without repeat, or a travel of 27.75m is achievable if the inserts are each fitted with two magnets.

Onboard electronics interpret the signals from the pickup coils (as for the analogue version of the encoder) and the Hall effect sensors, but the digital version incorporates a digital signal processor to convert the analogue positional value to an industry-standard differential quadrature output.

The output format can be any one of a large number of industry standards suitable for transmission via various fieldbus protocols or for direct input to, for example, servo-driven control systems.

Alternatively the digital signal can be fed directly to a PC, with no need for an interface card.

Each reading head has an option to be fitted with stainless-steel armoured cabling, and cable lengths of up to 25m can be used.

Compared with conventional glass-based linear encoders, the Spherosyn technology is far more robust.

The Spherosyn encoders are also easier to incorporate into machine designs, simpler to fit and align, and require no maintenance.

Furthermore, as length increases, the Spherosyn devices become even more cost-effective, while glass-based systems become significantly more expensive.

Typical applications for the digital Spherosyn encoders include machine tools, transfer lines, automation, packaging and printing machinery.

Some of the more extreme examples of successful harsh environment applications for Spherosyn technology include the positioning of fuel rods in nuclear caves, the measurement of fish length on trawlers at sea and the machining of graphite material.

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