Rotational Speed sensor basics | Speed sensor working,types
This page describes basic function of rotational speed sensor. It mentions working of rotational speed sensor using magnetoresistive sensor. The other speed sensors are based on inductive sensors and hall effect sensors.
The different types of sensors are used for rotational speed measurement as mentioned below.
• Magneto-Resistive Sensor
• Inductive Sensor
• Hall Effect Sensor
In magneto-resistive type rotational speed is measured using counting ferro-magnetic marks e.g.
passive gear wheel teeth or no. of magnetic elements of magnetized ring.
Following are the advantages of magneto-resistive type over other two sensors
(i.e. inductive and hall effect type).
• Output signal level does not vary with rotational speed.
• MR sensors measure slow rotations down till 0 Hz.
• MR sensors produce large output compare to hall-effect type. MR sensor provides about 20 mV/KA/m while halleffect sensor provides 0.4 mV/kA/m.
Rotation Speed Sensor working using magneto-resistive principle
• The MR sensor is static. The output signal of MR sensor is generated by bending of magnetic field
lines as per target wheel position. This is shown in the figure-1 above.
• Magnetic field lines can also bend even when target is not moving, they are used to measure very slow rotations.
• MR sensors produce large signal voltage and hence will have better SNR as well as improved EMC.
• The MR sensor has large air gap between target and sensor at higher field strength.
Higher tolerances are acceptable in the design and hence it reduces overall cost of the speed sensor as per MR concept.
• As ferrite magnets can be used in this speed sensor type based on magneto-resistive concept the sensor is cheaper compare to hall-effect sensor. This is due to the fact that hall-effect uses samarium cobalt magnets.
Due to advantages of magneto-resistive sensors they are widely used in industrial and automotive applications for rotational speed measurement.
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