What is a Linear Variable Inductive Transducer (LVIT) and where is it used? LVITs have been around for more than 30 years. These components are relatively low-cost, contactless position sensing devices that employ 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 using microprocessors make outstanding LVIT sensor-performance possible, achieving linearity errors of less than ±0.15% of FSO and temperature coefficients of 50 ppm/°F, along with either analog or digital outputs.
View of a typical LVIT position sensor that is intended to be attached to the part it is measuring.
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.
LVITs Versus Gage Heads
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 2igure 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?
A spring-loaded LVIT position sensor.
In fact, LVITs can be used in place of traditional gage heads. This primarily due to two reasons:
- Electrically: an LVIT offers the same resolution and repeatability as traditional gage heads.
- Mechanically: LVITs have 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.
Additionally, all of these features come at a markedly lower cost. Essentially, why use a 9-inch long sensor to measure 1 inch of travel when the same performance can be achieved with a 4-inch long LVIT sensor?
LVIT-based gaging applications in factory automation typically mirror those for traditional gage heads. 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 8-mm 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.
Some factory automation applications that are solved by proximity sensors can be performed much better 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 18-mm 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.
All LVIT products offered by Alliance Sensors Group also feature the company's proprietary SenSet field programmable scaling, which allows a user to adjust for any mechanical variations after installation in the application simply by pushing a button or grounding a single connection. This SenSet feature reduces setup time and total cost of ownership. As a typical example, consider a 6-inch range LVIT used to measure the position of a process valve stem that moves 5.5 inches. By using SenSet to scale the output of the sensor after it's installed, the full 4-mA to 20-mA output can be obtained over the 5.5 inches that the stem actually moves.
About the Author
Harold Schaevitz, President, Alliance Sensors Group, is the third generation in the sensor industry. He started his career in 1981 at Schaevitz Engineering as an eastern regional sales manager. He founded Schaevitz Intertech in 1990 for contract manufacturing. In 2006 he founded HGSI The Sensor Connection marketing and distributing EGT sensors and linear positions sensors. In 2009 he founded Sensing Solutions, specializing in marketing American-made linear position, rotary, and tilt sensing products to the Far East.