Heidenhain Technical Data

HEIDENHAIN encoders use gratings on various carriers (such as glass, ceramic, solid steel or steel tape) as their measuring standard. The accuracy and thermal behavior of the encoder can be optimized by the choice of carrier. In contrast to the earlier method of visually reading the graduations of measuring devices, today's encoders are scanned photoelectrically.

Photoelectric scanning through the projected light principle.

Photoelectric Scanning
When light falls onto the graduation of a measuring standard it is diffracted. If the grating period is much larger than the wavelength of light, the diffracted beam components become insignificant. When collimated light is directed through such graduations, it projects an image of the grating pattern. If however the grating period approaches the wavelength of light, this generates a complex pattern of superposed light beam components: an interference pattern.

Large gratings are scanned essentially according to the projected light principle illustrated above. Moving the measuring standard relative to an index grating produces a periodic fluctuation of light intensity, which is detected by photovoltaic cells.

For finer grating periods, graduations of appropriate structure are used to evaluate phase shifts in interference patterns. The figure below illustrates this principle as it is realized in the LIP 401 linear encoder. Interferentially scanned encoders are employed whenever very fine measuring steps (down to a few nanometers) necessitate small grating periods of typically 8 µm, 4 µm and less.

Photoelectric scanning through diffraction and interference.

Reference marks
Photoelectric scanning of a single periodic graduation track results in an incremental, i.e. counted, measurement. Since most applications require some absolute reference, incremental HEIDENHAIN encoders have at least one reference mark, which is likewise photoelectrically scanned. In this way, the absolute reference position is permanently assigned to exactly one measuring step.

The reference mark must therefore be scanned in order to establish an absolute reference. In some cases this may necessitate traversing large lengths of the measuring range. To speed and simplify the referencing procedure, many HEIDENHAIN encoders have distance-coded reference marks. Next to the incremental track is a track with several reference marks individually spaced according to a mathematical algorithm. With encoders featuring distance-coded reference marks, the absolute position value can be re-established after traversing two successive reference marks, or at most 20 mm (0.8 in.) for LS linear encoders, 80 mm (3.1 in.) for LB linear encoders, and 20° for angle encoders.

By evaluating several encoded tracks simultaneously, logical conclusions can be made from the pattern of the tracks to find the absolute position value. In this way the absolute position value is available from the very beginning. Such measuring systems, used for digital measured value output, are called absolute encoders.