1.1 - SensMiLi: Optical Absolute Position-Encoder by Single-Track, Q-ary Pseudo-Random-Sequences for Miniature Linear Motors

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings OPTO 2011
O1 - Components for Sensing and Detection
D. Wibbing, J. Binder - Festo AG & Co. KG, Esslingen (Germany), W. Schinköthe - IKFF, Stuttgart Universität (Germany), C. Pauly, C. Gachot, F. Mücklich - Universität des Saarlandes (Germany)
16 - 21


In Industry sectors like the semiconductor-, electric light assembly or the biomedical-industry, in which work is carried out in clean environments, actuators are needed that can move small parts with an acceleration of several hundred m/s2 and a precision in the one-digit micrometer range. Miniature linear motors have the dimensional and dynamic characteristics to optimise these processes. However there has not been a sufficient solution for an absolute position encoder capable of being integrated into such motors till now.

Coding with Diffraction
State-of-the-art linear absolute position encoders use pseudo-random coded absolute tracks in combination with a parallel or overlaid incremental track to improve their resolution. The micrometersized scale structures have to be resolved by a lens-system which makes the sensor very sensitive to changes in distance between the scale and readhead. The optics get out of focus quickly as a result. Instead, by coding the position information by diffraction gratings, the small-sized scale structures do not have to be resolved by imaging optics anymore. The depth of focus can be decoupled from the resolution by illuminating the scale by a relatively large spot of light and decoding information on the scale-position by detecting the position of the diffraction-spots on a sensor-matrix.

It can be shown for the first time that the highly efficient pseudo-random-sequences can also be implemented by diffraction-gratings. Normally the information about the order of the simultaneously illuminated diffraction-gratings of a code-word is lost if their diffraction-patterns are observed in the far-field. However by cyclically varying the gratingperiod across the gratings, this information can be retrieved.

Direct Structuring of the Scale in Steel
The manufacturing process as well as the thermal characteristics of the motor's stainless steel pistonrod can be improved if the scale can be directly written onto it. A method to realize the micrometersized structures is the Focused-Ion-Beam-Writing.