Development of a New Magnetic Scale Based on Magnet Width Modulation Method

Abstract We propose a new method for scaling magnets in magnetic encoders. Magnetic encoders often use a position detection method based on changes in pulse output caused by magnetic pole switching of the magnetic scale. However, increasing the number of magnetic poles on the magnetic scale to improve the position detection accuracy decreases the signal strength due to the small magnet volume. In this study, we propose introducing the modulation method utilized in optical encoder scales to the magnetic scale to achieve high position detection accuracy while maintaining the signal strength. A three-dimensional model was created in which the width of the permanent magnets that make up the magnetic scale gradually changes. The magnetic flux density waveforms were evaluated using the finite element method. The width-modulated magnetic scale produces a sinusoidal waveform with distortion. Although the unwanted signal decreased with the number n of divisions of the modulation, the fast Fourier transform revealed the presence of harmonic components regardless of n. We found that the harmonic component could be reduced by improving the design with a function that cancels it. By using the width modulation method, a high signal output and position detection performance can be achieved.