1. Field of the Invention
The present invention relates to improvements in an interpolation device for a scale arrangement, and more particularly to an interpolation device for a scale arrangement which arrangement is used for detecting a rotational position of a motor, movement of a table in a machine tool and the like.
2. Description of the Prior Art
Various types of scale arrangements have been proposed and practically used in order to detect a position of a moving subject such as a rotational position of a motor or a movement of a table in a machine tool. A typical scale arrangement has a sensor such as a magnetic sensitive element which implements a readout of a scale in the form of electrical signals of a sine wave and a cosine wave, and obtains the moving direction and position by analyzing the detected signals.
However, it is difficult to make a scale arrangement so as to keep a distance and an angle between a scale T and a sensor S constant during the operation of the scale arrangement. For example, the sensor S generates a wave relative to the scale T as shown in FIG. 5, and the inclined angle of the sensor S relative to the scale T becomes different in the forward movement and the backward movement as shown in FIG. 6. Furthermore, Abbe's error occurs when the sensor S is not aligned parallel with the scale T in a non-contact type scale arrangement as shown in FIG. 7. Accordingly, a signal detected by the sensor S includes various errors. For example, a DC offset voltage O.sub.A, O.sub.B is included in the signal as shown in FIG. 8; a deviation of a gain level is generated as shown in FIG. 9; a gain of the sine wave signal and the gain of the sine wave signal becomes different as shown in FIG. 10; and a phase drift between the sine wave signal and the cosine wave signal is generated. Although these errors are negligible in a scale arrangement which is not required to have a high discrimination, these errors can not be negligible when a scale arrangement is required to have a high discrimination such as 0.5 .mu.m, 0.1 .mu.m or 0.05 .mu.m. Furthermore, such errors appear as a fatal error in a long-size scale arrangement requiring a high discrimination. Conventionally, the generation of such errors during movement of the sensor S has been prevented by the improvement of the mechanical structure. However, this mechanical improvement requires a fine machining degree in the structure, and therefore raises the production cost.