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Linear encoder does more with less

April 3, 2003

There are a growing number of applications, especially in the photonic and semiconductor industries, that need fast and accurate motion-control systems using high-quality signals with periods of 4 microns for reliable interpolation. High speeds are usually attained by using high accelerations and low-mass components. And this is increasing the demand for small and lightweight encoders.

New, small LIF 481 encoder with glass scale (center) and Zerodur scale (front) compared with the larger LIF 181 (back).

This graph shows how Supradur graduations on the LIF 481 are affected by various contaminants.

To keep up with this demand engineers at Heidenhain Corp., Schaumburg, Ill. (www.heidenhain.com), reduced the size of the scanning head glass scale of its new linear encoder, Model LIF 401, to 26 X 16 X 16.5 mm. And for movements less than 1 meter, the cross section is only 10 X 5 mm. The encoder includes a built-in optical limit and homing functions, thus making it even more compact, lightweight, and easier to install.

The encoder uses incremental and reference-mark scanning, which is based on the interferential measuring principle and is characterized by large scanning areas, single-field scanning, and a hysteresis-free optical reference mark. The limit-switch, homing signals, and reference mark are generated optically from one track. A homing signal determines direction of travel when the reference mark is being traversed.

Wear particles or PCB dust can lead to light-absorbing and light-dispersing contamination. This reduces signal amplitude in encoders using interferential measuring with single-field scanning and large scanning areas.

Contamination, such as fingerprints, can also impair diffraction characteristics of phase-grating scales. It disturbs the grating characteristics, reduces interpolation quality, and -- in the worst case -- leads to encoder failure.

To combat both forms of contamination, the Model LIF 481 encoder gratings are coated with Supradur, a transparent dielectric that covers the grating's peaks and valleys and guarantees planarity of the scales. With it, liquid and oily contaminants can only distribute themselves evenly over the surface of the sturdy and accurate grating, leaving the encoder unaffected.

The encoder's scale is available on glass with an expansion coefficient of approximately 8 ppm/K, and in a Zerodur version with a coefficient of about 0 ppm/K. The accuracy grade of the scales is ±3 µm. Measuring lengths range from 70 to 220 mm, and will be eventually upgraded to 1,000 mm. Speeds are 1.2 m/sec at -3 dB and 1.6 m/sec at -6 dB. The scanning head can withstand vibrations of 400 m/sec and g shocks, and it operates at temperatures from 0 to 50°C.