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Linear variable differential transformers (LVDTs) are a common type of linear position sensor widely used in electromechanical systems today. An LVDT consists of two basic elements: a stationary coil assembly and a movable core or armature. Because it’s a transformer, an LVDT is fundamentally an AC-in/AC-out device. However, some LVDTs have electronics built in to make them DC-in/DC-out devices. This gives rise to the terms “AC-LVDTs” and “DC-LVDTs.”
Furthermore, because an LVDT is usually connected differentially, it has a natural null point in the magnitude of its AC output. And because an LVDT typically has no end position stops, the null position, located in the middle of the range of motion of the LVDT’s core, is the “stake in the ground” for determining core position and is always specified on an LVDT data sheet.
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The process of selecting the right LVDT for an application from the myriad of products available on today’s market involves two high-level choices based on interfacing to the LVDT, and some lower level choices based on the LVDT’s performance specifications, as well as the application environment.
The first choice concerns the mechanical interface, while the second concerns the electrical input/output (I/O). Once the high-level choices have been made, lower level choices based on an LVDT’s performance specifications and environmental ratings can be made.
Environmental ratings for either an AC-LVDT or a DC-LVDT are typically straightforward to understand and fairly easy to interpret. However, the performance characteristics of an LVDT often require a more detailed explanation. This is true both when choosing an available LVDT or developing the specifications for one for an OEM application. The following five terms and parameters are ones that most often cause confusion when choosing an LVDT.
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These DC-operated LVDTs are designed for a wide range of position measurement applications. Built-in electronics provide frictionless operation and dynamic response, with the added convenience and simplicity of DC input and precalibrated DC output. |
• Nominal linear range. The basic variable in LVDT selection is the maximum range of motion of the core which produces an analog output of specific linearity. The distance the core can travel from its null position in this linear region is called full-scale displacement. Since the core can be displaced from null toward either end, the linear operating range is twice the full-scale displacement. When it is stated as plus or minus full-scale displacement, it is usually called the nominal linear range. When it is stated without a “polarity,” it is called the LVDT’s full range, or full stroke, or total stroke.
Nominal linear range of any LVDT varies to some degree 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. In applications where optimum linearity is not essential, the practical operating range may extend well beyond the specified nominal linear range. Nominal linear range is specified for a high-impedance load, typically 50 kilohms to 0.5 megohms. Low-load impedance can have a deleterious effect on linearity and nominal linear range.
• Linearity error. The output of an LVDT is a nominally linear function of core displacement within its linear range of motion, so a plot of output voltage magnitude vs. core displacement is essentially a straight line. Beyond the nominal linear range, this output begins to deviate from a straight line in a gentle curve. The maximum deviation of the output of an LVDT, from a statistically best-fit straight line applied to a plot of output vs. core displacement within an LVDT’s nominal linear range, is defined as the linearity error or the nonlinearity of the LVDT.
Linearity error is commonly expressed as plus or minus a percentage of full-range output, 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. Because it is often the largest error, some users use nonlinearity as a measure of system “accuracy,” even though authorities such as the National Institute of Standards and Technology regard accuracy as a qualitative, not quantitative, descriptor of a measurement or system.
Typical linearity errors of standard LVDTs are ±0.25 percent of full range output, but improvements beyond the ordinary specifications are possible by special construction techniques or by the use of on-board signal processing. Linearity error as low as ±0.05 percent 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 or DC-LVDTs.
• 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 comparator of AC-LVDTs of the same linear range is sensitivity. Sensitivity is usually specified in terms of millivolts output per thousandths of an inch core displacement per volt of excitation or as volts output/per 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. The smallest core position change that can be observed in the output of an LVDT is called resolution. Because an LVDT operates on the principle of magnetic coupling, its resolution is essentially infinite. This means that 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 output of the LVDT, which is called the signal-to-noise ratio of the system. With a properly designed LVDT measuring system, micro-inch resolution is not uncommon.
• Repeatability. The single most important factor for any sensor is the ability to reproduce the same output for repeated trials of exactly the same input under constant operating and environmental conditions. This parameter, called repeatability, 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.
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