Some features of an LVDT
The most significant parameters for LVDT are:
The linearity
This defines the maximum deviation between calibration points and the best straight line drawn through all calibration points; it is expressed as a percentage of the Full Scale Output (FSO). The absolute error is proportional to the displacement. Applications of LVDT’s are usually limited to 500mm.
The sensitivity error
This defines the difference between the theorical sensitivity and the real one determined by linear regression (Meggitt (Sensorex) uses the best straight line method).
The sensitivity is expressed in m V/V/mm. Associated electronics provide a span adjustment, so this error term can be avoided by the end user.
Thermal zero drift and sensitivity shift
From experience, the thermal zero drift is negligible. Therefore, the only thing to consider is the thermal sensitivity shift.
It is expressed in ppm/°C of FS (1ppm=1.10-6).
Excitation frequency influence
LVDT’s specifications are given at ambiant temperature and for a given excitation voltage and frequency (23° and 2.2Vrms at 3500 Hz for Meggitt (Sensorex) devices).
The transducers can be used at other frequencies. Nevertheless, for a given LVDT, some limits determine a change in linearity.
Generally, Meggitt (Sensorex) LVDT can be used with excitation frequencies from 1,5kHz up to 20kHz.
Magnetic field influence
Meggitt (Sensorex) technology allows the user not to take into consideration the influence of external magnetic fields.
Primary to secondary coil phase shift
A few degrees phase shift can be observed between primary and secondary voltage; it gives an error which is integrated in the specified linearity, sensitivity and drift.
Bandwidth
Its limitation (a few hundred Hz) is determined by the associated electronics or by the excitation frequency, not by the transducer itself.
Meggitt (Sensorex) is able to deliver special LVDT which can be used in other modes such as DIFFERENCE/ SUM (V1-V2/V1+V2).
This type of processing can minimize the influence of external parameters (temperature, excitation…)