A Two-degree-of-freedom Linear Encoder for Measurement of Position and Straightness

INTRODUCTION Displacement measurement is an essential technique in the field of precision metrology and precision manufacturing. The linear encoder is the most popular displacement sensor for measurement of the position of a moving stage along the axis of motion. Many commercially available linear encoders, whose principles are based on interference and diffraction, can achieve nanometer-level accuracy and subnanometer level resolution over the range of more than tens of millimeters [1]. Compared with laser interferometers, linear encoders are more robust to influences of air pressure, air temperature and relative humidity because of the short optical length in the sensor system. In addition to position measurement, straightness measurement, which is defined as the displacement along the axis vertical to the axis of motion, is also an essential task for a precision stage [2]. Conventionally, it is necessary to employ a different sensor, such as the straightness measurement kit of a laser interferometer or the combination of a capacitance-type displacement sensors and a straightedge, to accomplish the straightness measurement. This makes the measurement system complicated and expensive. To solve these problems, we propose a twodegree-of-freedom (two-DOF) linear encoder that can measure not only the X-directional position but also the Z-directional straightness of a linear stage. In this paper, principle of the twoDOF encoder and a prototype encoder are introduced. Basic performances of the prototype encoder are also reported.