A capacitive transducer for monitoring tridimensional submicrometric displacements

A new three-in-one capacitive transducer that operates with three transform er bridges at separate frequencies is here described. Three plane capacitor s result from the placing of a triangular right pyramid (the moveable eleme nt, being fixed to the point to be monitored) to face a reentrant right tri angular pyramid which was carved into one of the bases of a triangular pris m (the fixed element). Three out-of-balance signals result when the moveabl e element moves from the zero position, where each variable capacitance equ als the corresponding reference one. The three differences between the curr ents that flow through each series of capacitances are summed up in one shi elded 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 t he three bridge frequencies in sequence one obtains the three Cartesian com ponents of the vectorial displacement that the moveable element has undergo ne. The use of glass-ceramic material with a negligible thermal expansion c oefficient and the integration of reference capacitors in the body of the t ransducer, allow this transducer to operate within fairly large ranges of t emperature and humidity. The use of a current detector with low input imped ance allows this transducer to be connected to the bridges and to the pream plifier with some meters of coaxial cables, without any appreciable effects on the sensitivity. The various noise and uncertainty sources are also her e discussed. A approximate to 40 nm uncertainty with displacements in the /- 12 mu m range has been obtained in a thermal test that was carried out u sing four transducers applied to a cylindrical aluminum sample. (C) 2000 Am erican Institute of Physics. [S0034-6748(00)03803-X].