Variable inductance linear position sensors for hydraulic cylinders
Recent advancements in electronics and flexibility of package designs have made Linear Variable Inductance Transducers or LVIT variable inductance sensors cost effective for mainstream in-cylinder applications. This contactless technology offers many significant advantages regarding product life and long-term reliability over some competitive technologies, and competes very favourably in performance with all competitors for specifications such as non-linearity, resolution, and frequency response, but at a significantly lower cost. Equally important is the fact that LVIT sensors can typically withstand greater shocks and vibration, such as those commonly found in heavy industrial and mobile equipment applications for cylinders.
Linear variable inductance sensors 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, as seen in Figure 1. LVITs are typically offered in full-scale ranges from 4 inches (100 mm) to 36 inches (900 mm). Both port-mounted and internally embedded packages are available, with either connector or cable terminations. Figure 2 shows examples of both packages. 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 LVIT variable inductance sensors. This feature permits a user to 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.
LVIT variable inductance sensors 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 sensor were 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.
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