LVDT PDF

22 LVDT 22.1 Linear variable differential-transformer (LVDT) Linear variable differential-transformers are two types, translational and rotational. Excitation signal is sinusoidal voltage with 3 to 15 V rms amplitude and frequency of 50Hz to 20KHz. Measurement r...

22 LVDT 22.1 Linear variable differential-transformer (LVDT) Linear variable differential-transformers are two types, translational and rotational. Excitation signal is sinusoidal voltage with 3 to 15 V rms amplitude and frequency of 50Hz to 20KHz. Measurement range of commercially available LVDT is 125µm to 635mm. AC phase opposition AC output for this connection 1 22 LVDT The two identical secondary coils have induced in them, sinusoidal voltages of the same frequency as the excitation; however, the amplitude varies with the position of the iron core. When the secondary are connected in series opposition, a null position exists at which the net output 𝑒 is essentially zero. Motion of the core from null then causes a larger mutual inductance (coupling) for one coil and a smaller mutual inductance for the other and the amplitude of 𝑒 becomes a nearly linear function of core position for a considerable range either side of null. The voltage 𝑒 undergoes a 180 phase shift in going through null. The output 𝑒 is generally out of phase with the excitation 𝑒 however, this varies with the frequency of 𝑒. For each LVDT there exists a particular frequency (numerical value supplied by the manufacturer) at which phase shift is zero between 𝑒 𝑎𝑛𝑑 𝑒. Sensitivity: 𝑆 = (𝑒 /𝑒 )/𝑑 range 1 to 5 𝑉/𝑉/𝑐𝑚 Higher frequency of excitation gives more sensitivity. 𝑒 = −𝑁 ⟹𝑒 = −𝑁𝜙 𝜔𝑐𝑜𝑠(𝜔𝑡). But in the higher frequency core loss is increase very much.. 𝐻𝑖𝑠𝑡𝑒𝑟𝑒𝑠𝑖𝑠 𝑙𝑜𝑠𝑠 ∝ 𝑓 𝑎𝑛𝑑 𝑒𝑑𝑑𝑦 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑙𝑜𝑠𝑠 ∝ 𝑓 And smaller strokes usually have higher sensitivity. Range:±0.002𝑐𝑚 𝑡𝑜 𝑠𝑒𝑣𝑒𝑟𝑎𝑙 𝑐𝑚𝑠 Harmonics in the 𝑒 and stray magnetic and stray capacitive coupling between primary and secondary usually result in a small but non zero null voltage. Grounding reduces capacitance coupling effects. Output can be increased by increasing the primary voltage but this involve a larger primary loss and heating of primary and in consequence an increase in the primary resistance which in turn decrease the o/p. constant current fed to the primary is one solution to avoid these difficulties. A large primary voltage gives a distortion in the output as a consequence of its higher harmonic contents. RVDT is linear for limited rotation −40 < 𝜃 < 40 2