LVIT technology in factory automation

May 24, 2016 // By John Matlack
What is a LVIT and where are they used? LVITs, Linear Variable Inductive Transducers, which have been around for more than 30 years, are relatively low cost, contactless position sensing devices that utilize eddy currents developed by an inductor in the surface of a conductive movable element that is mechanically coupled to the moving object whose position is being measured.

The common form of an LVIT uses a small diameter inductive probe surrounded by a conductive tube called a “spoiler”. Typical LVITs have full ranges from fractions of an inch to 30 or more inches.  Modern electronics utilizing microprocessors make possible outstanding performance, achieving linearity errors of less than ±0.15% of FSO and temperature coefficients of 50 ppm/ºF, along with either analog or digital outputs.  See Figure 1 for a cutaway view.

Figure 1

LVITs are used in many factory automation applications, including packaging and material handling equipment, die platen position in plastic molding machines, roller positioning and web tension controls in paper mills or converting facilities, and robotic spray painting systems. Being contactless, the basic measurement mechanism of an LVIT does not wear out over time due to rapid cycling or dithering like a resistive device. LVITs also offer a much lower installed cost than that of most other contactless technologies.

Figure 2

While Figure 1 shows a LVIT that is intended to be attached to the part it is measuring, LVITs can also be spring loaded, as shown in Figure 2. The natural question is: where does one use a spring-loaded LVIT sensor versus another spring-loaded technology such as an LVDT gage head?