Magnetic field sensors for high precision surface scanners

Abstract:
The new technical paradigm of Industry 4.0 has been pushing the sensor technology, instrumentation, and measurement science to continue the progression of achieving the control and efficiency over the entire mass production process. Therefore, the development of smart and autonomous systems has led to a high demand of sensors for the monitorization of angular and linear positioning among the industrial, automotive and robotics market.

The main limitations of the existing magnetic sensing technology based on state-of-art anisotropic magnetoresistive and hall effect sensors for incremental and absolute positioning systems rely on a low reading distance and a minimum pole pitch size down to 500 µm, which leads to a lower resolution and accuracy when compared to optical based encoders.

The recent developments on magnetoresistive sensor technologies provide a competitive and reliable solution for the next generation of motion systems based on magnetic encoders, where the high signal-to-noise ratio and the enhanced spatial resolution of tunnel magnetoresistive sensors (TMR) can promote the detection of sub-100 µm pole pitch dimensions, driving to a performance similar to low-level optical encoders in terms of accuracy but with a lower production cost, enhanced working distance, reduced mounting tolerances and higher robustness to harsh environments.

Therefore, following the technological trend observed in the hard disk devices, the development of the new generation of incremental magnetic encoders based on the TMR sensing technology focused on the (i) enhancement of the device reading distance beyond a gap equivalent to 1x pole pitch dimension and on the (ii) improvement of the accuracy employing (a) commercial elastomer bonded ferrite linear scales from a sub-100µm to a 5 mm pole pitch range and (b) microfabricated thin film magnetic scales based on a pole-groove pattern with a sub-500 µm pole pitch range.

The optimization of the sensing technology, the sensor arrangement and packaging method delivered a successful outcome for a future short-term breakthrough on the available encoder solutions by achieving an accuracy around 3 µm at a reading distance of 1x for a pole pitch of 80 µm. On the other hand, a reading distance range around 3x was achieved for standard pole pitch dimensions, being able to reduce the mounting requirements in non-accurate applications.