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A capacitive transducer for monitoring tridimensional submicrometric displacements

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3 Author(s)
Cabiati, F. ; Istituto Elettrotecnico Nazionale G. Ferraris, Strada delle Cacce 91, 10135 Torino, Italy ; Giaretto, Valter ; Miraldi, Elio

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A new three-in-one capacitive transducer that operates with three transformer bridges at separate frequencies is here described. Three plane capacitors result from the placing of a triangular right pyramid (the moveable element, being fixed to the point to be monitored) to face a reentrant right triangular pyramid which was carved into one of the bases of a triangular prism (the fixed element). Three out-of-balance signals result when the moveable element moves from the zero position, where each variable capacitance equals the corresponding reference one. The three differences between the currents that flow through each series of capacitances are summed up in one shielded output cable. One transconductance preamplifier converts the current signals into three superimposed voltage signals. By selecting the reference frequency to be sent to a lock-in amplifier, one can monitor the movement of the selected face of the moveable element, that is, by choosing one of the three bridge frequencies in sequence one obtains the three Cartesian components of the vectorial displacement that the moveable element has undergone. The use of glass–ceramic material with a negligible thermal expansion coefficient and the integration of reference capacitors in the body of the transducer, allow this transducer to operate within fairly large ranges of temperature and humidity. The use of a current detector with low input impedance allows this transducer to be connected to the bridges and to the preamplifier with some meters of coaxial cables, without any appreciable effects on the sensitivity. The various noise and uncertainty sources are also here discussed. A ≈40 nm uncertainty with displacements in the ±12 μm range has been obtained in a thermal test that was carried out using four transducers applied to a cylindrical aluminum sample. © 2000 American Institute of Physics.

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Review of Scientific Instruments  (Volume:71 ,  Issue: 4 )