May 15, 2016
In recent times, a major challenge position sensor customers have encountered is the need for a short to medium range sensor that is rugged and does not wear out, but at the same time is not unduly expensive. The low cost issue drives these customers to resistance potentiometers, but because pots are not very rugged and utilize an internal sliding contact, they wear out pretty quickly. In fact, in motor sports applications, it is fairly common to observe pots being discarded after just one race. In factory automation applications, pots are often used because of their lower initial cost, but they also introduce significant down time on the production line when they must be replaced because of wear.
On the other hand, customers found that contactless sensors like DC-LVDTs last much longer, but are substantially more costly and are relatively long compared to their range of measurement. This poor stroke-to-length ratio can be a major installation problem, especially in automation applications. Thus, many customers often found themselves trapped by price-to-performance and reliability-to-cost issues, or with difficult installation problems.
A solution for these customers focuses on sensors using Linear Variable Inductance Transducer (LVIT) technology. LVIT position sensing technology offers the ruggedness and reliability of contactless operation, but with pricing competitive to resistance pots, and similar stroke-to-length ratios as well. Like most common linear position sensors, LVITs are DC in, DC out devices with built-in electronics. There is a wide variety of LVITs available compatible with most industrial and mobile applications.
For example, LVITs are very attractive for cost-sensitive mobile hydraulic applications such as on the hydraulic actuators used on snow plows for both highways and airports to measure blade height. Extra heavy duty LVIT sensors are used to monitor bridge expansion or contraction, as well as railroad track shift and rail “sun kink” to warn of a potential derailment. For such civil engineering applications, the ruggedness and reliability of these heavy duty LVITs take precedence over their size. For motor sports applications, smaller and lighter LVIT sensors with swivel rod eye ends are available as virtually exact mechanical replacements for the linear potentiometers previously used, but do not need to be thrown away after a race because they do not wear out and can survive the shock and vibrations particularly well. Because of their very favorable stroke-to-length ratio, LVITs for measuring up to 1 meter are only about 5 cm longer than their nominal measuring range.
Besides these LVITs, there are high-pressure-sealed LVIT position sensors for operation inside a hydraulic cylinder that can be inserted into an O-ring port in the end cap of a cylinder, or be embedded inside the cylinder itself. In these applications, the cylinder rod has an 8 to 11 mm diameter gun-drilled hole that serves as the moving part of the sensor. These LVIT in-cylinder position sensors can provide an analog feedback signal at comparable cost to an in-cylinder potentiometer, without the wear out issue associated with pots. Furthermore, in-cylinder LVITs cost substantially less than magnetostrictive sensors that may have been chosen for hydraulic actuator applications where reliability and service life are the dominant factors. In addition to these in-cylinder LVITs, long stroke LVIT sensors that can be mounted externally onto the housing of a hydraulic actuator are also available.
For automation and dimensional gaging applications, there are spring-loaded LVITs that can supplant spring-loaded DC-LVDTs of similar ranges for about half the price and that have much shorter overall lengths, which can help to minimize sensor installation problems due to excessive length.
The designs for all of these LVIT products were driven by the need in the marketplace for an improved position sensor with a better price-to-performance ratio. Users wanted the advantages of a contactless but rugged sensor at an acceptable price compared to the rest of the components in their system. This issue was particularly important to OEMs that had been using DC-LVDTs in their automation systems in order to get maximum reliability compared to pots, but who were quite dissatisfied with the product cost.
Two key features of the built-in electronics of an LVIT that make it a particularly suitable replacement for a DC-LVDT, magnetostrictive sensor, or resistance potentiometer are its microprocessor, which permits linearizing the analog output to a high degree of precision, and an ambient temperature sensor that functions in conjunction with the microprocessor to effect wide range temperature compensation. When these features are combined with the exceptional robustness and superior stroke-to-length ratio of LVITs, they compete very effectively with the high precision associated with magnetostrictive sensors which can sometimes encounter reliability problems due to the fragility of their internal waveguide under shock and vibration conditions.
This fragility was exemplified by a customer in a lumber mill using magnetostrictive sensors to control position of the blade in the rough sawing process, in which very heavy logs were being loaded into feed cradles that received large shocks as the logs dropped into the cradles. Many failed magnetostrictive sensors were stacked like cord wood nearby. When the blade position sensing hardware was changed to LVITs, those shock-induced sensor failures were totally eliminated.
The conclusion is that sensors using LVIT technology can solve a wide variety of linear position sensing applications at low cost because they offer:
• Exceptional price-to-performance factors
• Contactless operation with no wear out
• Very good stroke-to-length ratio
• Extreme robustness to shock and vibration
• Temperature compensation over industrial temperature range
• Excellent linearity and repeatability
• Diverse application-oriented packages
• Pressure sealed versions for in-cylinder use