As their name implies, Position Sensors detect the position of something which means that they are referenced either to or from some fixed point or position. These types of sensors provide a “positional” feedback.

One method of determining a position, is to use either “distance”, which could be the distance between two points such as the distance travelled or moved away from some fixed point, or by “rotation” (angular movement). For example, the rotation of a robots wheel to determine its distance travelled along the ground. Either way, Position Sensors can detect the movement of an object in a straight line using Linear Sensors or by its angular movement using Rotational Sensors.

One type of positional sensor that does not suffer from mechanical wear problems is the “Linear Variable Differential Transformer” or LVDT for short. This is an inductive type position sensor which works on the same principle as the AC transformer that is used to measure movement. It is a very accurate device for measuring linear displacement and whose output is proportional to the position of its moveable core.

It basically consists of three coils wound on a hollow tube former, one forming the primary coil and the other two coils forming identical secondaries connected electrically together in series but 180o out of phase either side of the primary coil.

A moveable soft iron ferromagnetic core (sometimes called an “armature”) which is connected to the object being measured, slides or moves up and down inside the tubular body of the LVDT.

A small AC reference voltage called the “excitation signal” (2 – 20V rms, 2 – 20kHz) is applied to the primary winding which in turn induces an EMF signal into the two adjacent secondary windings (transformer principles).

If the soft iron magnetic core armature is exactly in the centre of the tube and the windings, “null position”, the two induced emf’s in the two secondary windings cancel each other out as they are 180o out of phase, so the resultant output voltage is zero. As the core is displaced slightly to one side or the other from this null or zero position, the induced voltage in one of the secondaries will be become greater than that of the other secondary and an output will be produced.

linear encoder
Linear encoders consist of precision ball bearings contained in a tube. This tube passes through a set of inductive coils that takes advantage of the regularity and uniformity of ball bearings to create a series of sinusoidal signatures. Using a proprietary algorithm, micron-level resolution and repeatability can be obtained. This is perfect for metal fabrication environments, which are subject to metallic debris and cutting fluids which make glass scale approaches very problematic and expensive to install and maintain. What’s more, the sensing elements can all be well sealed to at least an IP67 level for longevity and reliability in the harshest environments.