Evaluation of Position Error of Terahertz Polarimetric Encoder By Ray-Tracing Method


Encoder has been widely used for detecting position change in various motion and systems. Optical encoders are the most commonly used in industrial applications, but in severe environment dust and dirt can result in a loss of measurability of optical signal. Magnetic encoders have resistance to such environment, but the working distance between a sensor and a scale is limited since the magnetic field intensity follows the inverse cube law. Our motivation is to realize an encoder with environmental resistance and wide gap tolerance, as a position sensor for motion systems and mobile body systems in severe environment.
To this end, we focus to use THz bands, since THz wave has the robustness to the ambient condition [1]. To achieve the wider gap tolerance than the conventional encoders, which detect position based on the intensity information, our idea is to use the polarization information. Fig. 1 (a) shows the schematic diagram of the proposed THz polarimetric encoder [3]. The encoder consists of a THz sensor head and a periodic linear array of polarizing plates. Here, we define that the polarization parallel to the moving direction as “H” and vertical to the moving direction as "V". When we scan the received signal intensities of two orthogonal beams along the scale, we obtain the two signal traces IH and IV'(Fig. 1 (b)), which can then be converted into differential signal IH - IV as shown in Fig. 1 (c). The position can be determined by extracting the change of sign in the differential signal. In this paper, we simulated polarization dependent reflection signals using the ray-tracing modeling. Position errors for the intensity-based detection, like conventional encoders, and the proposed polarization-based detection were compared under various conditions.