While typical linearity errors of ±0.25% of full range output are common with standard LVDTs, improvements to these specifications are possible with special construction techniques or by the use of onboard signal processing. Linearity errors as low as ±0.05% of full range output can be obtained in this manner. In some cases, improved linearity may also be obtained by using an AC-LVDT at less than its full range, or on only one side of null. Linearity error means the same for AC-LVDTs as well as DC-LVDTs.
For an AC-LVDT, full-scale output is the output of an LVDT with its core positioned at full-scale displacement and with its primary excited at a specified nominal input voltage. In most cases, though, a better way to compare AC-LVDTs of the same linear range is through sensitivity. Sensitivity is usually specified in terms of milliVolt output per thousandths of an inch core displacement per Volt of excitation (mV/mil/Volt). Sensitivity varies with excitation frequency, which must also be specified. Sensitivity mostly affects the gain required of the LVDT’s signal conditioning electronics.
For most DC-LVDTs, the comparable characteristic to sensitivity is scale factor, which is usually expressed as Volts DC output per inch of core displacement. Some legacy DC-LVDTs use a ratiometric configuration, which requires that they also use the same units as sensitivity, or have their scale factor specified for a particular DC input voltage. There are also DC-LVDTs whose output is into a 4-20mA current loop, so their scale factors are expressed as milliamperes per inch (mA/in) or milliamperes per mil (mA/mil).
Resolution is the smallest core position change that can be observed in LVDT output. An LVDT’s resolution is essentially infinite, as it operates on the principle of magnetic coupling. An infinitesimal change in core position will produce an output change. In practice, the limitation on system resolution is the ability of the associated electronic equipment to sense the change in LVDT output, which is called the signal-to-noise ratio of the system. With a properly designed LVDT measuring system, microinch resolution is not uncommon.
The ability of a sensor to reproduce the same output for repeated trials of exactly the same input under constant operating and environmental conditions is the single most important factor for sensor selection. Called repeatability, this parameter is the only irreducible and uncorrectable source of static error in any electromechanical measuring system. Repeatability error is the limiting factor in making any sensor-based measurement.
A well-made LVDT is so repeatable that overall transducer repeatability is affected only by the mechanical factors of the physical members or structures to which the LVDT’s core is attached, and to which the LVDT’s coil is mounted. Both repeatability and resolution contribute to overall measurement error, and are usually expressed as a percentage of full-scale output. These parameters apply equally well to AC-LVDTs and DC-LVDTs.
This article was written by Lee Hudson, Application Engineer at Macro Sensors, Pennsauken, NJ. For more information, visit http://info.hotims.com/40440-320.