Position Indicators VARIABLE OUTPUT DEVICESLinearVariableDifferentialTransformers(LVDT)A device which provides accurate position indication throughout the range of valve or controlrod travel is a linear variable differential transformer (LVDT), illustrated in Figure 6. Unlike thepotentiometer position indicator, no physical connection to the extension is required.Figure 6 Linear Variable Differential TransformerThe extension valve shaft, orcontrol rod, is made of a metalsuitable for acting as the movablecore of a transformer. Moving theextension between the primary andsecondary windings of atransformer causes the inductancebetween the two windings to vary,thereby varying the output voltageproportional to the position of thevalve or control rod extension.Figure 6 illustrates a valve whoseposition is indicated by an LVDT.If the open and shut position is allthat is desired, two smallsecondary coils could be utilized ateach end of the extension’s travel.LVDTs are extremely reliable. Asa rule, failures are limited to rareelectrical faults which cause erraticor erroneous indications. An openprimary winding will cause theindication to fail to somepredetermined value equal to zerodifferential voltage. This normallycorresponds to mid-stroke of thevalve. A failure of either secondary winding will cause the output to indicate either full openor full closed.Rev. 0 Page 9 IC-05
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