LVDT sensor has two components in its design; a fixed housing and a separate moveable 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. The 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 electronic components are housed separately in a controlled environment separated from the sensor. On the other hand, DC LVDT typically contains a printed circuit board (PCB) with signal conditioning electronics that generate output signals directly from the sensor. The DC LVDT maintains all the desirable properties of AC LVDT but is simplified with DC operation. Therefore, no calibration or special signal conditioners are required for DC LVDT.
While DC LVDT offers simplicity and the capability to generate digital outputs compatible with computer-based and buses system, AC LVDT still possesses its own set of benefits for some measurement applications.
Measurement in Extreme environment
AC LVDT contains no signal conditioning electronics, making it ideal for measuring in extreme or high-temperature environments where electronics could not survive. This can range from as low as -200C to as high as 500C. As an example, P-520 (AC LVDT from Newtek) is ideal to be used as a positioning system for downhole drilling in oil and gas applications where sensors are exposed to extreme pressure and temperature. Due to the absence of internal electronics, the sensor housing and core assembly can be designed as vented housing or hermetically sealed.
Access restricted areas where space is limited
AC LVDT commonly features a 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 separately from the sensors. The connecting cables can be as long as 30 meters and are required to power the sensors. They are also required to amplify and demodulate the output signals. Site engineers should carefully design the site to minimize the risks and issues due to cabling.
Wide range of choice of construction materials
AC-operated LVDT can be constructed from a wide range of different materials to achieve maximum performance. Since AC-operated LVDTs are known for measuring in extreme temperature environments, the sensors can be constructed from a 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 environments or even underwater .
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