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LVITs, Linear Variable Inductive Transducers Cheat Sheet (DRAFT) by [deleted]

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Introd­uction

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.

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.

LVIT

Figure 1

While Figure 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?
In fact, LVITs can be used in place of tradit­ional gage heads primarily because, electr­ically, an LVIT offers the same resolution and repeat­ability and, mechan­ically, the same outer diameter and an external mounting thread, but with about half of the length of the gage head, making the stroke­-to­-length ratio of an LVIT substa­ntially better. And all of these features come at a markedly lower cost. Why utilize a 9-in. long sensor to measure 1 inch of travel when the same perfor­mance can be achieved with a 4-in. long LVIT sensor? LVIT-based gaging applic­ations in factory automation typically mirror those for tradit­ional gage heads, as shown below.
 

LVDT

Compared to LDT

When compared to LVDT pencil gaging probes, a spring­-loaded LVIT can satisfy many of the same applic­ations: automo­tive, medical and mil/aero test stands, robotic arms, part placement, and shop-floor dimens­ional gaging applic­ations to name a few. Pencil probes are typically selected for one of two reasons: resolution and repeat­abi­lity, or size. Pencil probes are smaller (either 8mm or 0.375 inch OD) than LVITs and have resolution and repeat­ability of 4 millionths of an inch. However a pencil probe requires a separate LVDT signal condit­ioner, making the cost per channel typically double the cost of an LVIT. If an applic­ation does not require the specific features of a pencil probe, a spring­-loaded LVIT is a much lower cost altern­ative.

Factory Automation Applic­ations

Some factory automation applic­ations that have been solved by proximity sensors can be better satisfied with LVIT techno­logy, which offers a propor­tional analog output, giving greater control flexib­ility than merely an NPN or PNP TTL switching signal. The spring­-loaded LVIT shown in Figure 2 has an 18mm thread on its housing, matching a thread commonly used by proximity sensor manufa­ctu­rers. With an LVIT’s short body length, the sensor can fit in places or mountings where there previously was a proximity probe.