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Ultrafine Displacement Detector and Balance

IP.com Disclosure Number: IPCOM000051186D
Original Publication Date: 1982-Aug-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 2 page(s) / 39K

Publishing Venue

IBM

Related People

Courtens, E: AUTHOR

Abstract

Displacements in the sub-angstrom range can be measured and compensated by a capacitance bridge arrangement which may find application, e.g., in the areas of thermal expansion and static elastic compliance measurement or scanning tunneling microscopy.

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Ultrafine Displacement Detector and Balance

Displacements in the sub-angstrom range can be measured and compensated by a capacitance bridge arrangement which may find application, e.g., in the areas of thermal expansion and static elastic compliance measurement or scanning tunneling microscopy.

A dielectric plate 1 carries four electrodes 2...5, two each on top and bottom which, together with counter-electrodes 6...9 arranged in a rigid casing 10, form four capacitors C1 through C4 connected in a bridge circuit 11. Displacement of plate 1 causes detuning of bridge 11 with the output signal being equal to: DV/V=V(C1/C1+C2-C4/C3+C4 approx. equal to V/4C(DC1-DC2+DC3-DC4).

The last equality assumes that all capacitances have approximately the same value C, with Ci=C + DCi and DCi/C much less than 1. Hence the voltage is proportional to the vertical displacement,
DV/V=Dl/l=DC1/C1=DC3/C3=- DC2/C2=- DC4/C4. and rather insensitive to lateral displacement and tilt of plate 1.

With the bridge voltage V=1 volt and l=0.1 mm, a D/2V . 1 MuV corresponds to dl=1 Angstroms, and with an electrode size of 5 cm/2/, the capacitance will be C(i) approx. equal to 50 pF, which gives a bridge impedance of 30 kOmega at 100 kHz.

Assume plate 1 is supported against a stack 12 of piezoelectric crystals with a body 13 whose thermal expansion (or shrinking) is to be compensated. The output signal of bridge 11, appropriately amplified by a phase-sensitive amplifier 14 using V as a reference, ma...