A linear displacement sensor converts mechanical motion into electrical signals that can be measured. The motion, or displacement, is detected by a push rod or guided actuator that moves within the sensor transducer and generates an output quantity (usually a voltage) that correlates to the distance the object has moved from the initial position. The sensor is then used to monitor and control the movement of an object in a given direction.
A LVDT (Linear variable differential transformer) linear displacement sensor has a hollow metallic cylinder in which a shaft of smaller diameter, or pushrod, moves freely back and forth along the cylinder's long axis. The shaft's end carries a magnetically conductive core. Movement of the pushrod causes movement of the core within the coil assembly and produces an electrical signal that is proportional to the displacement of the pushrod. The sensor is then able to provide an output signal, usually in the form of a 4 to 20 mA current, that corresponds to the displacement measurement.
Linear displacement measurement sensors are often mounted to a fixture that will hold the object whose position is to be monitored or controlled. The mounting system must be robust, stiff, and thermally stable in order to avoid thermal expansion and contraction that can cause a shift in the position of the sensor or the object being positioned.
In addition, the non-contact displacement measurement sensor itself should be free of any foreign material that may interfere with its operation. A good example of this is the presence of metal chips in a machine tool that are generated during machining operations. These particles are invisible to eddy-current sensors and can introduce errors into the displacement measurements.