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MULTIPLE ELEMENT DC ELECTRO METER WITH REDUCED SPACING SENSITIVITY

IP.com Disclosure Number: IPCOM000024066D
Original Publication Date: 1979-Aug-31
Included in the Prior Art Database: 2004-Apr-02
Document File: 2 page(s) / 336K

Publishing Venue

Xerox Disclosure Journal

Abstract

The simple DC electrometer, because of its low cost and simplicity, would be an ideal sensor with which to measure the electrostatic potential of charged surfaces, but all known DC electrometer configurations exhibit some degree of spacing sensitivity. For most practical applications, the spacing tolerance is so critical that AC or mechanically perturbed electrometer designs must he used at significantly greater cost.

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XEROX DISCLOSURE JOURNAL

MULTIPLE ELEMENT DC ELECTRO~ Proposed Classification METER WITH REDUCED SPACING U.S. Cl. 355/3 SENSITIVITY Int. CI. G03g 15/00

Robert E. Grace

The simple DC electrometer, because of its low cost and simplicity, would be an ideal sensor with which to measure the electrostatic potential of charged surfaces, but all known DC electrometer configurations exhibit some degree of spacing sensitivity. For most practical applications, the spacing tolerance is so critical that AC or mechanically perturbed electrometer designs must he used at significantly greater cost.

The DC electrometer derives its signal from the coupling capacitance between the probe electrode and the test surface. If the surface potential is V, the signal is Q =

CV, where C is the coupling capacitance and the charge Q is converted to a low impedance output signal by means of a suitable electronic circuit. C is determined by the location and length of the electric flux lines connecting the probe to the test

surface. These lines are shown for a typical DC electrometer geometry in cross-

section in Figure 1.

A grounded shield with a slot is usually provided to limit the flux to a small region on the test surface. If the flux lines are strongly curved, as with a small probe diameter and wide slot, the proportional change in length is minimized and spacing sensitivity is lessened. If the flux lines are nearly straight, as with a large probe

diameter and/or narrow slot, spacing sensitivity is aggravated. In general, there is

a direct trade off between spacing sensitivity and spatial resolution.

If we construct an electrometer with two or more independent probe electrodes, each having a different spacing sensitivity curve but deriving their coupling flux from essentially the same area on the test s...