Development of 2-d Displacement Sensors by Using 2-d Angle Grid and 2-d Angle Sensors

This paper presents a newly developed displacement sensor system. Interferometers or linear encoders are normally used to monitor the motion of precision stages. However, these sensors operate in one dimension only. Hence, if the device has multiple degrees of freedom, the number of sensors required to monitor its motion must be the same as the number of degrees of freedom. This adds complexity when designing the machine layout. In this study, we propose a two-dimensional displacement sensor system. This system consists of two-dimensional angle sensors and a two-dimensional angle grid. Because of the angle independency of the x and y directions, it is possibly to monitor the x and y components of motion from the angle sensor outputs. This system can also improve its capability by using in-situ self-calibration to calibrate the sensors and angle grid. Two-dimensional angle sensors were developed for the proposed system and the basic performance was tested. The sensors could clearly detect the two-dimensional angle grid. The results of a displacement measuring experiment showed that the proposed system can monitor stage movement within a 2-micron error with in-situ self-calibration compensation. INTRODUCTION Precision stages that can be positioned within the sub micron order are a fundamental component of precision machines. Displacement sensors play a highly important role in ensuring the accuracy of such precision stages. Interferometers or linear encoders are normally used to monitor the motion of precision stages. However, these sensors operate in one dimension only, so it is impossible for them to monitor two-dimensional stage motion directly. For example, one linear encoder cannot monitor the two-dimensional motions of a precision stage driven by a linear motor or walking drive. If interferometers are applied to detect these motions, two interferometers and scales, which are assured its profile accuracy, are required. Moreover, when there is a need to detect pitching motion or rolling motion of a stage, the number of sensors must be further increased. This adds complexity when designing the machine layout and significantly raises costs. Recently, a two-dimensional encoder has been developed to detect plane motion using image processing of a grid target. This system is used for calibration of NC machining systems. However, it does not handle angular motion because it depends on images of grid shapes. In this study, we propose a two-dimensional displacement sensor system. This system consists of two-dimensional angle sensors and a two-dimensional angle grid. Because of the angle independency of the x and y directions, it is possibly to monitor the x and y components of motion from the angle sensor outputs. This system can detect pitching and rolling motion as well simply by adding angle sensors, because it detects displacement from the angle outputs. This system also can improve its capability by using in-situ self-calibration to calibrate the sensors and angle grid. The in-situ self-calibration method only involves repeated tiny variations of the calibrated value, and does not require any other external reference for calibration. This paper describes the principles of the proposed system, the development of the two-dimensional angle sensors, and the results of basic performance experiments.