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Hall Effect and Variable Reluctance Sensors

November 17th, 2017

These sensors have many different uses. A common application for either sensor is to measure the speed of a spinning object, such as a wheel or gear. The application can be analyzed in FEA to determine the effectiveness of the sensor.

Hall Effect Sensor

The hall effect sensor is a digital chip that reads the flux density in a small area across the wafer. It will typically turn "On" at the presence of a strong enough field, and turn "Off" when the field is no longer present or too low. This allows a simple logic circuit to know if a field is present. If the sensor only activates in the presence of a North pole, a speed sensor can be designed. This type of sensor requires an external source of the magnetic field.

Below is an analysis showing the B-field around a theoretical sensor created by a spinning wheel with alternating North and South poles. The sensor will output a "True" when the North face of the field is present and a "False" when the South face is present. Since it takes a certain level of B, measured in Gauss or Tesla, to activate the chip, the signal output will not be symmetrical. The signal will be "False" slightly longer than "True". This is irrelevant however, as we can measure RPM of the wheel by knowing the number of North poles on the face of the wheel and incrementing a counter whenever the chip changes from "False" to "True".  Average RPM = (Counter/Total North poles)/Time in minutes the counter has been running.

FEA mappings of flux density in two sensor types.

Hall effect sensor with bonded spinning multi-pole wheel. Wide view.

Hall effect sensor close up.

Hall effect sensor side view.

Variable Reluctance Sensor

Unlike the Hall Effect sensor, a VR sensor does not require an external magnetic field. It is an analog sensor that reads a change in field. The sensor typically has an integrated magnet with a coil wrapped around it. As the field changes, a current is induced in the coil, giving an analog signal. A lack of external magnet can be an attractive option when comparing the sensors.

A speed sensor can be designed from a VR sensor by placing it near a soft magnetic gear (even para-magnetic materials will cause small field changes, but preferably the gear is ferromagnetic). As a gear tooth comes closer to the sensor, the field across the magnet increases and causes a potential in the coil. Reading this voltage will create a signal that spikes when a gear tooth is present. An A2D convertor can be used to digitize the signal and increment a counter when the tooth is present. Similar to the Hall Effect sensor, now that our signal is digital, we can calculate RPM of the gear if we know the number of teeth. Average RPM = (Counter/Number of Teeth)/Time in minutes since the counter was started.

Below is an analysis of a VR sensor while a steel gear is spinning in the vicinity.

Variable reluctance (VR) sensor with steel gear.

Variable reluctance (VR) sensor. Side view.