US Patent 7521921 Displacement sensor

Sensors and methods for measuring displacement are disclosed. In one embodiment, among others, a resistive element is configured to receive an alternating voltage between a first electrical terminal and a second electrical terminal, the first and second electrical terminals defining a length of the resistive element. A signal pickup is capacitively coupled to the resistive element and is configured to be moved along the length of the resistive element at a substantially fixed distance from the resistive element and without contact between the resistive element and the signal pickup. A shielded cable, such as a coaxial cable, electrically connected to the signal pickup carries a signal having an amplitude proportional to the position of the signal pickup with respect to the first and second electrical terminals.