LVDT Linear Displacement Sensor
"The linear variable differential transformer (LVDT) (also called linear variable displacement transformer, linear variable displacement transducer,  or simply differential transformer) is a type of electrical transformer used for measuring linear displacement (position). A counterpart to this device that is used for measuring rotary displacement is called a rotary variable differential transformer (RVDT)." Source: Wikipedia.
What is a Linear Variable Differential Transformer?
An LVDT is an electromechanical sensor used to transform mechanical motion into a variable electrical signal (current or voltage) and is used for measurement of displacement. They function as actuators for automatic control systems or as mechanical motion sensors in measurement technologies.
LVDT is an acronym for Linear Variable Differential Transformer. LVDT linear position sensors can measure displacements ranging from few microns to centimeters, but are also capable of measuring positions up to ±1 meter.
In short, a LVDT sensor provides output quantity in voltage or current, related to the displacement or movement being measured. LVDT Transducers are prone to external electromagnetic interference.
A linear displacement transducer requires three to four connection wires for power supply and output signal delivery.
Physically, the LVDT Transducer construction is a hollow metallic cylinder in which a shaft of smaller diameter moves freely back and forth along the cylinder’s long axis. The shaft, or pushrod, ends in a magnetically conductive core which must be within the cylinder, or coil assembly, when the device is operating.
In common practice, the pushrod is physically attached to the moveable object whose position is to be determined (the measurand), while the coil assembly is attached to a fixed reference point. Movement of the measurand moves the core within the coil assembly; this motion is measured electrically.
Figure 1: LVDT Linear Displacement Sensor.