Linear voltage differential transformer

The position can also be determined in a resistive or voltage mode. In the resistive mode, a current is sent to the center tap and to one of the end terminals, whereas the resulting voltage drop is measured by the transmitter. A potentiometer has an infinite resolution. Linearity for a precision rotary potentiometer can be as good as 0.25%. However, due to linkages and gears, the linearity for the entire assembly usually is about 0.5-1%. A linear variable differential transformer (LVDT) is mostly used in linear motion applications and also inside some pressure transmitters (Figure 3.131). 

Linear variable differential transformer (LVDT, Fig. 2) is a device that produces voltage proportional to the position of a core rod inside a cylinder body. It measures displacement or a position of an object relative to some predefined zero location. On tablet presses, LVDTs are used to measure punch displacement and in-die thickness. They generally have very high precision and accuracy, but there are numerous practical concerns regarding improper mounting or maintenance of such transducers on tablet presses. 

Linear position transducers The simplest form is a linear potentiometer, where the position of the slider is proportional to the output voltage. Linear variable differential transformers (LVDls) move a metal core between primary and secondary coils to produce a voltage proportional to core position. 

FIGURE 3.2 Illustration of the basic operating principle of linear variable differential transformer displacement sensors. When the core is positioned at the exact center between secondary coils 1 and 2, the differential voltage between the two secondary coils, V, is zero. Movement of the core creates an imbalance, increasing V. The signals at the primary coil, secondary coils 1 and 2, and the differential voltage Vare also shown. 

Transducer - dii-s9r (1924) n. A device that transforms the value of a physical variable into an electrical signal, usually voltage or current. Examples are thermocouple, pressure transducer, linear variable differential transformer (a motion transducer), tachometer generator, and force cell. 

One of the more popular devices for cryogenic use is the linear variable differential transformer (LVDT). The LVDT (see Fig. 8.3) is a transformer whose core moves in response to the displacement input. The primary is wound in an ordinary manner, but the secondary is split and wound in such a fashion that there will be no output when the core is exactly in the central position. As the core is displaced from the center, a voltage is generated across the terminals of the secondary, which is directly proportional to the displacement of the core. 

The differential transformer is a compact, highly. sensitive, linear device which is commercially available. The most satisfactory instrument. system for the differential transformer is a high-frequency oscillator-amplifier phase-sensitive demodulator carrier. system which provides the neces.sary sensitivity and stalrility and eliminates phase-motion ambiguity a.ssociatod with the null voltage. 

Impedance may be defined not only for discrete systems but also for arbitrary distributed systems as the Fourier transform of the differential equation defining the voltage response divided by the Fourier transform of the periodic current excitation Z( (d) = F v t) F i t). Here the F operator denotes a Fourier transform. However, Fourier transformation only reduces differential equations to simple Ohm s law-like form under conditions of linearity, causality, and stationarity of the system therefore impedance is properly defined only for systems satisfying these conditions. 

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