Linearly variable differential transformers or LVDTs are a traditional, inductive method for measuring linear displacement. Their fundamental physics were first discovered by Michael Faraday in the 19th century, followed by rapid development of the technique during World War II for military aircraft and rockets.
Typically, an LVDT comprises a series of wire spools or bobbins arranged so that a metallic rod can move in and out of the spools. One of the wire spools (the primary) is energised with an AC frequency usually in the range 1-10kHz. As the rod displaces relative to the spools, a variable transformer is formed which couples energy in to secondary windings in proportion to the rod’s linear position. The LVDT produces two or more AC signals from the secondary windings. The ratio or differential of these signals is used to calculate absolute position of the rod.
LVDTs continue to be widely used in aerospace applications as they have an excellent reputation for safe and reliable operation in harsh conditions. High accuracy LVDTs are available but these require high precision winding of the wire spools. The transformer construction technique means that LVDTs tend to be rather bulky, heavy and expensive.
More generally, the use of LVDTs in industrial or automation applications is reducing as inductive linear encoders replace them. Inductive linear encoders are based on the same fundamental physics as LVDTs but use printed circuits rather than transformer constructions. This allows inductive linear encoders to enjoy the same advantages of robust, reliable, accurate operation as LVDTs but without the disadvantages of bulk, weight and cost.
A further advantage of inductive linear encoders is that they typically accept a DC power supply and generate digital data. This means that they are more readily integrated with modern control systems and more readily understood by modern engineers.