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Factors
to Consider When Selecting an LVDT
With the myriad of linear position sensors available on today's market, selecting the right LVDT (Linear Variable Differential Transformer)for an application involves two high level choices, based on interfacing to the LVDT, as well as some lower level choices, based on the LVDT's performance specifications and the application environment. First, an engineer should be concerned about the mechanical interface, followed by the electrical input/output (I/O). Key factors in making these choices are shown in the charts below. After high-level choices have been made, lower level choices must be made based on an LVDT's performance specifications and environmental ratings. Environmental ratings for either an AC-LVDT or a DC-LVDT are typically fairly easy to interpret. However, the performance characteristics of an LVDT often require a more detailed explanation. The following five terms and parameters often cause the most confusion. Nominal Linear Range The basic variable in LVDT selection is the maximum range of core motion, which produces an analogue output of specific linearity. Full-scale displacement is the distance a core can travel from its null position in this linear region. Since the core can be displaced from null toward either end, the linear operating range is twice the full-scale displacement. When stated as plus or minus (+/-) full-scale displacement, it is referred to as the nominal linear range. When stated without a "polarity", it is called the LVDT's full range, or full stroke, or total stroke. The nominal linear range of any LVDT varies somewhat with frequency. When the LVDT is used with the correct core for the specified frequency, the actual linear range will always equal or exceed the nominal value. When optimum linearity is not essential in an application, the practical operating range may extend well beyond the specified nominal linear range. Linearity Error As LVDT output is a nominally linear function of core displacement within its linear range of motion, a plot of output voltage magnitude versus core displacement is essentially a straight line. Beyond the nominal linear range, output begins to deviate from a straight line into a gentle curve. From a statistically best-fit straight line versus core displacement within an LVDT's nominal linear range, the maximum deviation of LVDT output is defined as the linearity error or the non-linearity of the LVDT. Linearity error is typically expressed as +/- a percentage of full-range output (FRO) or in terms of an error band width that envelopes the straight line and deviations. The statistically best-fit straight line is usually determined by applying the method of least squares to a series of calibration readings. The proper interpretation of the linearity error specification for an LVDT depends on the ultimate application on the LVDT in a measuring system. Some users use non-linearity as a measure of system "accuracy" as it is often the largest error. Sensitivity, Scale Factor, and Full Scale Output 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 millivolts output per thousandths of an inch core displacement per Volt of excitation (mV/mil/Volt) or as Volts output / inch /Volts input. Sensitivity varies with excitation frequency, which must also be specified. Sensitivity mostly affects the gain required of the LVDT's signal conditioning electronics. Resolution 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. Repeatability The ability of a sensor to reproduce the same output for repeated trials of exactly the same input under constant operating and environmental conditions are 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. Macro Sensors Div of Howard A. Schaevitz |
Director
of Technology|
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