The challenges from a design standpoint are the angle and relative distance between the gear’s teeth and sensor, and how to ensure sensitivity and responsiveness when considering the magnetic influence of the surrounding air. In order to proceed with an advance study like this that considers a precise geometry and material properties, an electromagnetic field analysis using the finite element method (FEM) is effective.
This Application Note presents the use of magnetic field analysis to evaluate the variation of the voltage signal of a magnetic speed sensor for a range of air gap distances.
Magnetic Flux Density Distribution
The figures show how the magnetic flux from the magnet flows through the yoke and reaches the rotating ring. When the rotating ring rotates, the permeance in the magnetic circuit changes due to the moving teeth. This changes the amount of magnetic flux that passes through the yoke, thus generating the electromotive force in the coil.
Voltage Signal at Different Gap Widths
Fig. 2 shows the voltage signal waveform at each gap width, and fig. 3 shows the peak value of the voltage signal as a function of the gap width.
When the gap width is large, the voltage is small because the permeance is smaller compared with a small gap width, and then the magnetic flux amount that flows through the yoke and the coil becomes smaller.