Rotational Speed Sensor Basics: Working and Types

This page explains the fundamental function of a rotational speed sensor, focusing on how it operates using a magnetoresistive (MR) sensor. Other speed sensors leverage inductive sensors and Hall effect sensors. These different types of sensors are used for rotational speed measurement, as detailed below:

• Magneto-Resistive Sensor
• Inductive Sensor
• Hall Effect Sensor

In the magneto-resistive approach, rotational speed is determined by counting ferro-magnetic markers. These can be passive gear wheel teeth or the number of magnetic elements on a magnetized ring.

Here are the advantages of magneto-resistive sensors over inductive and Hall effect sensors:

• The output signal level remains consistent regardless of rotational speed.
• MR sensors can measure very slow rotations, even down to 0 Hz.
• MR sensors generate a larger output signal compared to Hall-effect sensors. For example, an MR sensor provides about 20 mV/KA/m, while a Hall-effect sensor provides only about 0.4 mV/kA/m.

Speed sensor using MR sensor

How Rotation Speed Sensors Work Using the Magneto-Resistive Principle:

• The MR sensor remains stationary. Its output signal is produced by the bending of magnetic field lines as the target wheel changes position, as illustrated in the figure above.
• Because magnetic field lines bend even when the target is not moving, MR sensors are able to measure very slow rotations.
• MR sensors produce a large signal voltage, leading to better signal-to-noise ratio (SNR) and improved electromagnetic compatibility (EMC).
• The MR sensor allows for a larger air gap between the target and the sensor at higher field strength. This allows for larger tolerances during the design process, which ultimately reduces the overall cost of the speed sensor.
• Magneto-resistive sensors can utilize ferrite magnets, which are cheaper than the samarium cobalt magnets used in Hall-effect sensors.

Due to these advantages, magneto-resistive sensors are widely used in industrial and automotive applications for rotational speed measurement.