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Miniature encoder uses a 4.2mm disc

01 June, 2002

Miniature encoder uses a 4.2mm disc

Stegmann has developed a new optical encoder system that uses a code disc just 4.2mm in diameter – one tenth the size of the discs used in most other high-performance encoders. The company says that the resulting encoders are so small that they will allow servo motor manufacturers to produce much shorter machines.

The miniature encoder (shown above) made its UK debut at last month`s Drives & Controls show, where Stegmann`s rival Heidenhain was also showing a tiny rotary encoder.

The LED light source for Stegmann`s patented Mini Disc system is mounted concentrically in the encoder shaft, resulting in a continuous illumination of the disc. This allows the system to sense the whole disc and to compensate for any eccentricities in the disc, the bearings or the shaft. The absence of these errors – which are present in most conventional encoders – results in high linearities and positional accuracies, according to Stegmann.

Because the disc is so small and is mounted at the centre of the axis of rotation, it is no longer a limiting factor for high-speed operation.

Stegmann is using the disc in an encoder with a Hiperface interface that can fit inside a 55mm brushless servo motor and screws conveniently onto the motor shaft. The 35mm diameter and 33mm long encoder offers a resolution of 128 sine/cosine steps per revolution, or 4,096 absolute steps. The encoder monitors its own absolute position so that any fault can be determined before the shaft moves.

As an alternative, Stegmann is offering a version of the encoder with a built-in bearing so that the encoder becomes the motor`s rear bearing. Although this configuration saves even more space, it will require motor manufacturers to change the way their build their motors.

Darren Pratt, Stegmann`s encoder product manager in the UK, says that the new encoders will be priced similarly to traditional 42mm diameter types. He adds that the new encoder is the first in a family of products based on the mini disc technology.

Heidenhain`s miniature encoders are 36.5mm in diameter and are available in single- or multi-turn absolute formats. Like the Stegmann encoders, they provide an absolute position value when they are switched on or after a power interruption. They are suitable for use with servomotors with axis heights as small as 20mm.

One model in the Heidenhain range (the EQN 1125) incorporates a mechanical gear transmission which, the company claims, avoids the interference and battery failure problems that can affect electronic revolution counters. The encoder transmits position information via a bidirectional EnDat serial interface with one revolution providing information on 8,192 positions.

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