The brainchild and creation of Alliance Sensors Group (ASG), a linear position sensing technology called Linear Variable Inductance Transducer (LVIT) forecasts an excellent price-to-performance ratio compared to other linear position sensing technologies such as LVDTs, linear potentiometers, and magnetostrictive devices. These components' overall length only increases by a relatively small percentage beyond their stroke. As a result, LVITs may offer better stroke-to-length ratio of any contactless linear position sensor.
Because LVITs are inductive, they are contactless devices with no friction issues or moving parts that can wear out (see figure 1). As is common for inductive sensors, they exhibit outstanding repeatability, with resolution limited only by the electronics used with them. LVITs offer the performance of contactless sensors like LVDTs or magnetostrictive sensors at a price comparable to industrial grade linear potentiometers.
Fig. 1: LVITs have no moving parts and are essentially contactless components.
They are DC-in/DC-out sensors constructed with built-in smart electronics that have relatively low power consumption, typically much less than 1W (see figure 2). Because the basic sensor of an LVIT is extremely repeatable, an on-board microprocessor can be used to linearize the sensor's output signal and apply temperature compensation to the system using a built-in temperature sensor. This linearization process yields typical linearity errors of ? 0.1% of full-scale output (FSO), depending on the number of data points used for the sensor's calibration.
 Fig. 2: Block diagram of an LVIT (Click image for larger version) |
An LVIT sensor uses a long, small-diameter probe coil that is connected to the resonant tank circuit of an oscillator found in the built-in electronics. A conductive tube moved over the coil changes its inductance and the resonant frequency of the tank circuit. Oscillator output is coupled to an integrated 12-bit microprocessor to measure its frequency. The digital output of the microprocessor then goes to a d/a converter that develops a voltage proportional to frequency. This voltage is conditioned to produce a high level analog dc output from the sensor. The 12-bit microprocessor also determines the resolution of an ASG LVIT position sensor, which is 0.025% of FSO.
One reason for the robustness of LVITs is that the probe coil is wound on a glass-filled thermosetting plastic rod which can survive large deflections without fracturing. Its single coil winding makes only two connections with the electronics, which substantially reduces the likelihood of a failure and enhances reliability.
The built-in electronics is completely potted within its housing, so these sensors can sustain high shock and vibration levels. This robustness, coupled with a normal operating temperature range of -20ºC to 85ºC, and an extended operating temperature range of -40ºC to +105ºC, permits an ASG LVIT to function very well in practically any industrial or commercial environment. In special cases where an LVIT might be required to operate in a higher temperature environment, the sensor's electronics may be located in a more benign environment a short distance away from the sensor's probe assembly. Another reason for an LVITs' robustness is that the sensors' housings are made of heavy wall anodized aluminum or stainless steel and are environmentally sealed to IEC IP-67 or IP-68. The LVITs' operating rods, also called spoilers, are also stainless steel and do not easily bend.
LVITs are available with industrial duty housings and operating rods having swivel rod ends for use in typical factory automation applications such as packaging machinery, gates for sorting chutes, press brakes, and automatic assembly test stands. Other possibilities include LVITs with extra heavy duty housings to operate in severe environments such as mining machinery, off-road equipment, rail or road bridge motion monitoring, road construction equipment, snow plows, and garbage haulers.
Other LVIT configurations include sensors specifically made for operation to 5,000 psig in hydraulic cylinders, either for port mounting externally or for embedding internally in the cylinder. For these in-cylinder applications, the cylinder rod or ram is gun-drilled with a hole in clearance of the probe coil, which then becomes the spoiler tube for the sensor. There is no need to machine a cavity in the piston for the magnet of a magnetostrictive device or for the spool of an in-cylinder potentiometer.
Other pressurized sensors can be built for subsea applications at depths of 12,000 feet in a PBOF environment. There are also short-stroke LVITs that operate in a pressurized fluid environment to provide spool position feedback in two-stage electro-hydraulic proportional or servo valves. In this application, the LVIT probe goes into a blind hole in one end of the main spool, which greatly simplifies installation and eliminates any need to mechanically couple a position sensor to the spool.
A unique feature built into ASG LVITs is a proprietary process called SenSet that gives a user the ability to match the end points of the sensor's analog output with the ends of the range of motion of a workpiece or the ram of a hydraulic cylinder to which the sensor is attached. This SenSet feature permits users to optimize the position measuring system's resolution over its full range of motion.
LVITs also offer various connector or cable terminations in axial or radial configurations and can measure full ranges from fractions of an inch to 40 inches with a resolution of 0.025% of FSO. Operating from dc-power from 5V to 24V, standard LVIT analog outputs include several high level dc voltages and 4 mA to 20 mA dc sourcing. Being digital devices, LVIT dynamic responses are shown as update rates, which nominally range from 250 to 500 Hz depending on the LVIT type.
Summary
Rounding things up, a comparison of related sensor technologies is in order (see figure 3). Once again, it's a matter of choosing the right part for the application.
Fig. 3: A comparison of various related sensor technologies including LVITs.
In summation, the answers to the initial question, what's so good about LVIT technology are:
- Excellent price-performance ratio compared to other common linear position sensors. LVDT performance at industrial potentiometer prices.
- Better stroke-to-length ratio compared to other contactless linear position sensors. Much better than LVDTs and better than magnetostrictive sensors (LDTs).
- Contactless, frictionless operation with no components to wear out and no ring magnet.
- Extremely robust to environments as a result of ruggedized construction and materials.
- Many sensor sizes and configurations for different position sensing applications.
- Pressure sealed sensors for various fluid power applications, including cylinder position. Less expensive than LDTs and more reliable than in-cylinder potentiometers.
- Excellent performance specifications: ? 0.1% of FSO non-linearity; 0.025% of FSO resolution.
- dc-in/dc-out operation with numerous analog output choices and low power consumption: 5- to 24-Vdc input, 0 to 5V, 0.5 to 4.5V, 0 to 10V, or 4 mA to 20 mA outputs.
- SenSet field programmability feature.
About the Author
Edward E. Herceg is currently Vice President and Chief Technology Officer of Alliance Sensors Group, a division of H. G. Schaevitz LLC. He is noted for his applications engineering expertise and for his technical innovations. Before joining ASG, he held senior technical and marketing positions at Everight Sensors Corp., AST Macro Sensors, Massa Products Corp., and Schaevitz Engineering (now part of Measurement Specialties Inc.), and is known as the author of the Schaevitz Handbook of Measurement and Control.
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