Magnetoresistive Sensors for the Condition Monitoring of High-frequency Spindles

The magnetoresistive (MR) sensing principle was first applied industrially in the read-heads of hard disc drives. However, in the past decade the MR effect has also been used successfully for sensors to measure position, current and magnetic fields. MR sensors offer a number of benefits compared to other technologies used for condition monitoring of machine spindles. They are more compact and offer a higher bandwidth than inductive or capacitive sensors. They are more robust than optical encoders, being largely insusceptible to oil, water and other contaminants [1]. MR sensors can be applied over a wider temperature range and with less demanding assembly tolerances. This makes them particularly well suited to applications with limited available space and demanding operating conditions, as is often the case in machine tool sensing applications. This paper explains the basics of magnetoresistive sensor technology and outlines the advantages compared to other sensor principles. Furthermore the results of a joint R&D project between Sensitec GmbH and the PTW Institute of the Technical University Darmstadt, funded by the Arbeitsgemeinschaft industrieller Forschungsvereinigungen “Otto von Guericke e.V. (AiF), to investigate the application of MR sensors to the condition monitoring of high-frequency air spindles will be described [2]. Position sensors based on the giant magnetoresistive (GMR) effect have been specially adapted to measure directly the radial and axial displacement of the spindle shaft. Importantly only minor modifications have been made to the spindle shaft to enable this measurement and the sensors are completely integrated within the spindle housing. Tests demonstrate that this solution can resolve axial and radial displacements of less than 0.5 μm with high repeatability under typical operating conditions.