How do Hall effect sensors work and where are they used in motion applications?

The Hall effect is an outcome of the Lorentz force at work.

When a thin conductor (or semiconductor) has a steady flow of current running through it and a magnet is placed so that its magnetic field runs perpendicular to this current, the magnetic field of the current reacts to the magnetic field of the permanent magnet, causing the electrons flowing through the conductor to be pulled to one side of the conductor, due to the Lorentz force. This creates a potential difference, referred to as Hall voltage, in the conductor. The magnitude of the Hall voltage is proportional to the strength of the magnetic field.

The Lorentz force is the force that a particle experiences due to electrical and magnetic fields.

The Hall effect is put to use in sensors, where the resulting Hall voltage can indicate the presence, absence, or strength of a magnetic field. Although Hall sensors operate by detecting a magnetic field, they can be used for sensing a wide variety of parameters, including position, temperature, current, and pressure.

Hall effect sensors are typically divided into two categories: digital Hall effect sensors — which include Hall effect switches and Hall effect latches — and analog Hall effect sensors.

Hall effect switches — also referred to as unipolar sensors — detect the presence (or absence) of a magnetic field as compared to a predefined threshold for magnetic flux. When a suitable magnetic field is detected, the switch turns on (closes), and when the field is removed, the switch turns off (opens). Proximity sensors are a common application for Hall effect switches.

The operation of a Hall effect latch — also referred to as a bipolar sensor — is similar to a switch, but a latch turns on (closes) when a positive magnetic field is applied and remains on even when the field is removed. Conversely, the latch turns off (opens) when a negative magnetic field is applied and remains off even when the field is removed. Hall effect latches are commonly used in brushless DC (BLDC) motors to detect rotor position for proper commutation.

Digital Hall effect sensors incorporate a Schmitt trigger — a circuit that adjusts the switching threshold to a slightly higher point on the rising edge of the signal and to a slightly lower point on the falling edge of the signal. The difference between these switching points is referred to as hysteresis, and ensures that the switch will not oscillate, or switch on-and-off, due to noise in the input signal.

Analog, or linear, Hall effect sensors produce a continuous output voltage proportional to the magnetic flux density (strength of the magnetic field), which makes them suitable for measuring position and movement. In fact, many magnetic rotary encoders use linear Hall effect sensors. However, the interaction of the flowing current and the magnetic field produces a Hall voltage that is very small, so linear Hall sensors typically incorporate an amplifier to increase the output voltage, along with other signal conditioning electronics to improve the sensor’s response and compensate for temperature effects.

Hall effect devices make excellent sensing elements, since they’re completely non-contact and have no moving parts, giving them a long operating life. And they can operate at high speeds and switching frequencies with excellent repeatability.

Feature image credit: Texas Instruments