Variable inductance position sensors, known as LVITs (Linear Variable Inductance Transducers), have been around for decades. Recent advances in electronics and package designs have made LVITs cost effective for mainstream in-cylinder applications. This contactless technology offers many advantages for product life and long-term reliability. Equally important is the fact that LVITs can typically withstand greater shocks and vibration, such as those commonly found in heavy industrial and mobile equipment applications for cylinders.
LVITs operate by measuring the resonant frequency of an oscillator that uses a simple inductive probe. Its inductance is varied by the position of a conductive tube or gun-drilled rod that surrounds it. LVITs are typically offered in full-scale ranges from 4 to 36 in. (100 to 900 mm). These sensors produce an analog dc voltage or current output, with a digital output available for OEM applications.
Remote field calibration is a standard feature offered on LVITs. This feature lets a user scale the analog output of the sensor after it has been installed in the cylinder. By merely grounding a wire to set the zero and full-scale output points, the sensor will give the desired full-scale output over its newly set range, making it unnecessary to scale the unit in an actual control system.
LVITs in cylinders offer a contactless position sensing solution that does not require machining a cavity in the cylinder piston for a ring magnet or wiper contact. In fact, if an LVIT was installed to replace an existing magnetostrictive sensor, the magnet could be left in place in the cylinder rod end without interfering with the inductive sensor’s basic operation.
In the past few years, requirements for instrumented cylinders in subsea applications have dramatically increased. LVITs can be offered in a pressure-sealed version that allows a user to install the sensor and cylinder in a subsea environment in depths of 12,000 ft (3,650 m) with 3,000 psig of internal hydraulic pressure.
Another application for LVITs that is similar to cylinder position sensing is spool position feedback for two-stage hydraulic valves. Inserting a short range (<1-in.) port-mounted LVIT into a blind hole in one end of the main spool is often an easier and lower cost installation than a pressure-sealed LVDT. A pressure-sealed LVDT requires an isolation tube to seal off its windings from the valve’s pilot pressure, and needs relatively expensive electronics to operate it.
An LVIT has many advantages when used in a cylinder. Whether inserted into an O-ring port or embedded in the cylinder endcap, an LVIT features simplicity, needing just a deep hole in the piston rod. It offers excellent operating characteristics such as linearity, repeatability, stability and temperature coefficient. In addition, an LVIT is remarkably rugged and resistant to shock and vibration-induced failure. Furthermore, it has a long service life because it works without any physical contact between the sensor probe and the bore of the deep hole, or without using an electrical contact that can wear out. Altogether, these features make an LVIT a good choice for a broad range of fluid power position sensing applications, particularly for in-cylinder position sensing.
Alliance Sensors Group
Filed Under: TECHNOLOGIES + PRODUCTS