Introduction

Optical encoders are the most accurate instruments to measure angles (rotations of parts of a machine). Classical incremental encoders achieve very high resolutions, but the measurement is only incremental: i.e. it measure an angular distance from an arbitrary starting point. On the other hand absolute encoders solve the problem at the expenses of increased complexity and reduced resolution. The ROTOENCODER, with a rotating disk embedded in the structure, is able to measure absolute angles (i.e. does not require a zeroing procedure), has excellent resolution and cancels the intrinsic errors typical of optical encoder (i.e. does not need preliminary calibration).

Technical features

The device is based on a double bearing structure embedding a continuously rotating encoder, which is read by two heads: one integral with the fixed part of the device and a second integral with the moving part. The phase measurement between the two heads’ signals is proportional to the relative angle suspended between them (and, therefore, the angle between the fixed and the movable parts of the device). This allows to implement absolute reading with the resolution of an incremental encoder. A further advantage of this solution is to reduce the encoder graduation errors and to cancel the cyclic errors due to the interpolation of the encoder lines. On the basis of this invention, the national angle standard preserved at INRIM has been realized (TRL=9). It is named Rotating Encoder Angle Comparator, REAC.  A compact version of this (now at TRL = 4) can find applications in industry and research where absolute knowledge of the angle and high resolution are needed.

Possible Applications

• Replace absolute encoders where higher resolution and accuracy are required;
• Replaces incremental encoders where a zeroing procedure is to be avoided;
• Precision control of robots;
• Accurate angular metrology.

Advantages

• Absolute angular reading;
• Extremely high resolution;
• Extremely low intrinsic error;
• No zeroing procedure.