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.