LVDT sensor has two components in its design; a fixed housing and a separate move-able core. The core assembly consisted of a primary and two secondary coils, supplied with current of constant frequency. The housing is mounted to a fixed component while the core is linked to the measured objects like an extendable arm. Movement of the objects changes the voltage output which is proportional to the distance. Two types of LVDTs are available; AC LVDT and DC LVDT.
AC LVDT contains no signal conditioning electronics in the sensor body. The electronics components are housed in a separate housing in a controlled environment separated from the sensor. On the other hand, DC LVDT contains typically a printed circuit board (PCB) with signal conditioning electronics that generates output signals directly from the sensor. The DC LVDT maintains all the desirable properties of AC LVDT but is simplified with DC operation. With this configuration, no calibration or special signal conditioning devices are required for DC LVDT on-site.
While DC LVDT offers simplicity and capability to generate digital outputs compatible with computer-based and buses system, AC LVDT still possess their own set of benefits for some measurement applications.
AC LVDT contains no signal conditioning electronics which makes it ideal for measuring in extreme or high temperature environment where electronics could not survive. This can range from as low as -200oC to as high as 500oC. As an example, P-520 (AC LVDT from Newtek) is ideal to be used as positioning system for downhole drilling in oil and gas applications where sensors are exposed to extreme pressure and temperature. Due to the absence of the internal electronics, the sensor housing and core assembly can be designed as vented housing or hermetically sealed.
AC LVDT commonly features subminiature design with low mass core assembly. This makes the sensor ideal for measuring position in an area where space is restricted. Remote monitoring is not an issue as the electronics are housed in separate locations from the sensors. The connecting cables can be as long as 30 meters and are required for powering the sensors. They are also required to amplify and demodulate the output signals. Site engineers should carefully design the site to ensure that the risks and issues due to cabling are minimized.
AC-operated LVDT can be constructed from a wide range of different materials to achieve maximum performance. Since AC-operated LVDT are known for measuring in extreme temperature environment, the sensors can be constructed from selection of materials compatible with this environment, such as stainless steel, Inconel, Titanium, Monel or Hastelloy. Materials can be freely selected to accommodate for higher pressure, corrosive environment or even underwater area.
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