Using Hall-Effect Sensors For Contactless Rotary Encoding and Knob Applications

Abstract

Dials and knobs for user interfaces traditionally use a rotary encoder or potentiometer to determine the change of rotation or absolute angle. These methods have internal metal contacts that can wear out over time and provide a point-of-failure in long-life applications.

Reducing the number of electromechanical contacts in a system reduces the points of failure and results in a more reliable design. Dials that use electromechanical contacts may have a shorter life span compared to those that use other contactless implementations such as Hall-effect sensors and magnets.

There are different ways to implement a dial with a magnetic sensor, but using a sensor with an integrated coordinate rotation digital computer (CORDIC) calculation can provide angular position data through register reporting, reducing the need to process data externally and simplifying the design process while still providing accurate results. Otherwise, sensors that just provide the magnetic field strength must have an MCU perform calculations to determine the angle of the magnet. Hall-effect sensors also offer a variety of different full-scale measurement ranges to enable the use of various magnets.

This white paper briefly introduces magnetic sensors, describes their use in a contactless dial application, and explains the benefits of a contactless method.

1 Problems With Mechanical Knobs and Rotary Encoders

A common way to implement a dial in a human machine interface (HMI) system is to use a potentiometer or rotary encoder. Both implementations may have internal contacts that change to provide the rotational output.

Potentiometers have a resistive element and a sliding contact that moves along the element. Depending on the rotation of the potentiometer, its resistance changes, which makes it possible to determine the rotational change. These are relatively cheap devices and generally only require three contacts to implement.

Rotary encoders either measure the absolute angle or the incremental angle change. Electromechanical rotary encoders are built using tracks on a printed circuit board and contact brushes that move as the encoder rotates. Rotary encoders can be implemented with both electromechanical and contactless based sensing, which leads to a variation in cost due to different technologies.

But both potentiometers and electromechanical rotary encoders have a significant problem: wear and tear. As the contacts move over other electrical elements, they can break down over time, leading to a change in performance or eventually a loss of operation altogether. Any loss in functionality can cause products with electromechanical rotary encoders and potentiometers to require repair or replacement. Performing rotational sensing using magnetic, inductive, or optical methods eliminates potential failure modes that can reduce product lifetimes but these implementations may cost more due to the additional components required. Magnetic rotational sensing requires a magnet and a sensor to determine the change in rotation. One option for this is a Hall-effect sensor that measures the strength of the magnetic field.