Which sensor involves a magnet and a coil moving relative to each other?

The velocity sensor is the correct answer because it operates on the principle of electromagnetic induction, where a magnet and a coil move relative to each other to generate voltage proportional to the velocity of the displacement. When an element, such as a magnet, moves past a coil, it induces an electrical signal as the magnetic field interacts with the coil. This induced voltage is directly related to the speed at which the magnet moves, allowing the sensor to measure the velocity of the object in question accurately.

In contrast, an accelerometer typically measures acceleration through piezoelectric materials or capacitive changes, rather than through a moving magnetic and coil arrangement. A proximity probe is used to detect the presence of an object and often relies on eddy current principles rather than a direct relative motion of a magnet and coil. A shaft rider, while having components that may involve magnetic technologies, primarily serves to monitor rotational position and does not utilize the specific principle of relative motion between a magnet and coil to generate its output. Thus, the velocity sensor is characterized distinctly by its reliance on this electromagnetic interaction.